Facts on File DICTIONARY of EARTH SCIENCE

The Facts On File DICTIONARY of EARTH SCIENCE

Revised Edition

The Facts On File DICTIONARY of EARTH SCIENCE

Revised Edition

Edited by Jacqueline Smith The Facts On File Dictionary of Earth Science Revised Edition

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Compiled and typeset by Market House Books Ltd, Aylesbury, UK

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MP 10987654321

This book is printed on acid-free paper PREFACE

This dictionary is one of a series designed for use in schools. It is intended for students of earth sciences, but we hope that it will also be helpful to other science students and to anyone interested in science. Facts On File also publishes dictionar- ies in a variety of disciplines, including astronomy, chemistry, biology, computer science, physics, mathematics, forensic science, weather and climate, and marine science.

This book is based on an edition first published by the Macmillan Press in 1976 and it was last revised in 2000. This second Facts on File edition has been extensively revised and extended. The dictionary now contains over 3500 headwords covering the terminology of modern earth science. A totally new feature of this edition is the inclusion of over 1,500 pronunciations for terms that are not in everyday use. An ap- pendix lists the chemical elements and their properties and includes a list of Web sites and a bibliography. A guide to using the dictionary has also been added to this latest version of the book.

We would like to thank all the people who have cooperated in producing this book. A list of contributors is given on the acknowledgments page. We are also grateful to the many people who have given additional help and advice.

v ACKNOWLEDGMENTS

Editors (previous editions)

Stella Stiegeler B.Sc. John O. E. Clark B.Sc.

Contributors

B. M. Abbott B.Sc., Ph.D., F.G.S. Anna Clyde B.Sc. Andrew Hill B.Sc., Ph.D., F.G.S. I. P. Joliffe B.Sc., M.Sc, Ph.D., C.I.C.E., F.C.S., F.R.G.S. R. B. Lanwarne B.Sc. P. A. Smithson B.Sc., Ph.D., F.R.Met.S. T. J. Speechley B.Sc. F. A. Sultan B.Sc., M.Sc., D.I.C. S. D. Weaver B.Sc., Ph.D., F.G.S. A. C. Wornell B.A.

Pronunciations

William Gould B.A.

vi CONTENTS

Preface v

Acknowledgments vi

Guide to Using the Dictionary viii

Pronunciation Key x

Entries A to Z 1

Appendixes The Chemical Elements 383 Web Sites 385

Bibliography 387

vii GUIDE TO USING THE DICTIONARY

The main features of dictionary entries are as follows.

Headwords The main term being defined is in bold type:

abyssal rock A type of intrusive igneous rock formed deep within the Earth’s crust.

Variants Sometimes a word has a synonym or alternative spelling. This is placed in brackets after the headword, and is also in bold type:

bed (stratum) The smallest division of stratified sedimentary rocks, consisting of a single distinct sheetlike layer of sedimentary material ... .

Here, ‘stratum’ is another term for ‘bed’. Generally, the entry for the synonym consists of a simple cross-reference:

stratum See bed.

Abbreviations Abbreviations for terms are treated in the same way as variants:

Antarctic bottom water (AABW)A cold and extremely saline water mass, the most dense water of the global oceans, that forms in the Southern Ocean along the coast of Antarctica ... .

The entry for the abbreviation consists of a reference:

AABW See Antarctic bottom water.

Multiple definitions Some terms have two or more distinct senses. These are numbered in bold type

aggregate 1. Mineral fragments and/or rock particles that combine to form a hard mass. 2. (in soil science) A cluster of soil particles that have bonded together through aggregation.

viii Cross-references These are references within an entry to other entries that may give additional useful information. Cross-references are indicated in two ways. When the word appears in the definition, it is printed in small capitals:

ablation cone A cone of FIRN, ice, or snow covered in rock debris caused by differential ABLATION.

In this case there are cross-references to the entries for ‘firn’ and ‘ablation’.

Alternatively, a cross-reference may be indicated by ‘See’, ‘See also’, or ‘Compare’, usually at the end of an entry:

alpine A type of climate found in mountainous areas above the timberline but below any permafrost level. Grass and other low-growing plants make up the typical vegetation. See also alp.

Hidden entries Sometimes it is convenient to define one term within the entry for another term:

anemometer ... An instrument for measuring wind speed and often wind direction. The most common type, the cup anemometer, has three cups mounted symmetrically about a vertical rotating axis ....

Here, ‘cup anemometer’ is a hidden entry under ‘anemometer’, and is indicated by italic type. The individual entries consists of a simple cross-reference:

cup anemometer See anemometer.

Pronunciations Where appropriate pronunciations are indicated immediately after the headword, enclosed in forward slashes:

abiotic /ay-bÿ-ot-ik/ Describing a nonliving factor in an ECOSYSTEM.

Note that simple words in everyday language are not given pronunciations. Also headwords that are two-word phrases do not have pronunciations if the component words are pronounced elsewhere in the dictionary.

ix PRONUNCIATION KEY

A consonant is sometimes doubled to prevent accidental mispronunciation of a syllable resembling a familiar word; for example, /ass-id/ /acid/,rather than /as-id/ and /ul-tră- /sonn-iks// /ultrasonics/, rather than /ul-tră-son-iks/. An apostrophe is used: (a) between two consonants forming a syllable, as in /den-t’l/ /dental/,and (b) between two letters when the syllable might otherwise be mispronounced through resembling a familiar word, as in /th’e-ră-pee/ /therapy/ and /tal’k/ /talc/. The symbols used are:

/a/ as in back /bak/, active /ak-tiv/ /ng/ as in sing /sing/ /ă/ as in abduct /ăb-dukt/, gamma /gam-ă/ /nk/ as in rank /rank/, bronchus /bronk-ŭs/ /ah/ as in palm /pahm/, father /fah-ther/, /o/ as in pot /pot/ /air/ as in care /kair/, aerospace /air-ŏ- /ô/ as in dog /dôg/ spays/ /o/ as in buttock /but-ŏk/ /ar/ as in tar /tar/, starfish /star-fish/, heart /oh/ as in home /hohm/, post /pohst/ /hart/ /oi/ as in boil /boil/ /aw/ as in jaw /jaw/, gall /gawl/, taut /tawt/ /oo/ as in food /food/, croup /kroop/, fluke /ay/ as in mania /may-niă/ ,grey /gray/ /flook/ /b/ as in bed /bed/ /oor/ as in pruritus /proor-ÿ-tis/ /ch/ as in chin /chin/ /or/ as in organ /or-găn/, wart /wort/ /d/ as in day /day/ /ow/ as in powder /pow-der/, pouch /e/ as in red /red/ /powch/ /ĕ/ as in bowel /bow-ĕl/ /p/ as in pill /pil/ /ee/ as in see /see/, haem /heem/, caffeine /r/ as in rib /rib/ //kaf-een/,/ baby /bay-bee/ /s/ as in skin /skin/, cell /sel/ /eer/ as in fear /feer/, serum /seer-ŭm/ /sh/ as in shock /shok/, action /ak-shŏn/ /er/ as in dermal /der-măl/, labour /lay-ber/ /t/ as in tone /tohn/ /ew/ as in dew /dew/, nucleus /new-klee-ŭs/ /th/ as in thin /thin/, stealth /stelth/ /ewr/ as in epidural /ep-i-dewr-ăl/ /th/ as in then /then/, bathe /bayth/ /f/ as in fat /fat/, phobia /foh-biă/, rough /u/ as in pulp /pulp/, blood /blud/ /ruf/ /ŭ/ as in typhus /tÿ-fŭs/ /g/ as in gag /gag/ /û/ as in pull /pûl/, hook /hûk/ /h/ as in hip /hip/ /v/ as in vein /vayn/ /i/ as in fit /fit/, reduction /ri-duk-shăn/ /w/ as in wind /wind/ /j/ as in jaw /jaw/, gene /jeen/, ridge /rij/ /y/ as in yeast /yeest/ /k/ as in kidney /kid-nee/, chlorine /klor- /ÿ/ as in bite /bÿt/, high /hÿ/, hyperfine /hÿ- een/, crisis /krÿ-sis/ per-fÿn/ /ks/ as in toxic /toks-ik/ /yoo/ as in unit /yoo-nit/, formula /form- /kw/ as in quadrate /kwod-rayt/ yoo-lă/ /l/ as in liver /liv-er/, seal /seel/ /yoor/ as in pure /pyoor/, ureter /yoor-ee- /m/ as in milk /milk/ ter/ /n/ as in nit /nit/ /ÿr/ as in fire /fÿr/

x A

aa /ah-ah/A LAVA with an extremely ments left at the side of a glacier following rough spinose surface. Compare block ABLATION. lava; pahoehoe. ablation till TILL material formerly pre- AABW See Antarctic bottom water. sent on the surface of glaciers and ice sheets and subsequently deposited as a result of Aalenian /ah-lee-nee-ăn/ The earliest part the melting of the ice beneath. These de- of the Middle JURASSIC Period. posits tend to contain relatively little fine material because this may be removed by abandoned channel The channel cut meltwater prior to deposition. Compare by a stream that has now ceased to flow lodgment till. through it. This may result from the stream forming a new channel; e.g. an oxbow (see Abney level /ab-nee/ A simple surveying oxbow lake) or cutoff, in which the curve instrument for measuring slope angles. The of a meander becomes cut off when the level is held by hand, objects being viewed stream breaks through to form a new chan- through a sighting tube. The observer may nel. stand at the top or bottom of a slope, but must sight onto a point at a height equiva- abandoned cliff See cliff. lent to his or her eye-level. Once an object is sighted, adjustment is made to a tilting abiotic /ay-bÿ-ot-ik/ Describing a non- spirit level, an image of the bubble being living factor in an ECOSYSTEM. Light, rain- visible in the sighting tube. When the bub- fall, soil type, and temperature are all ble image coincides with the object sight abiotic factors. Chemical residues from ar- the angle of slope can be read off an at- tificial fertilizers and pollution constitute tached scale, which is provided with a harmful abiotic factors. See also biotic. vernier to permit reading down to ten minutes of arc. The instrument is not very ac- ablation /ab-lay-shŏn/ The disappear- curate, but is small and quick to use. The ance of snow and ice by melting and evap- instrument is named for the English engi- oration. This can refer to ice crystals or neer and photographic chemist Sir William snow flakes in the atmosphere but is most de Wiveleslie Abney (1843–1921). commonly used for glacier ice and surface snow cover, when it can also include abnormal erosion (accelerated erosion) avalanching. The rate of ablation is pri- Erosion acting faster than normal as a re- marily controlled by air temperature but sult of the removal of the vegetation cover sunshine, rainfall, humidity, and wind by human agencies. It is abnormal in the speed also affect the process. sense that it is superimposed upon natural processes. See also blowout; human influ- ablation cone A cone of FIRN, ice, or ence on geomorphology. snow covered in rock debris caused by differential ABLATION. abrasion /ă-bray-zhŏn/ The wearing away of rocks by an agent of transportation ablation moraine Debris of rock frag- charged with a load of already eroded ma-

1 abrasion platform terial, which acts as a tool for cutting, logical purposes the absolute temperature grinding, scratching, and polishing. All the is taken as the Celsius temperature plus major transportation agents (running 273°, so that the freezing point of water is water, wind, moving ice, and sea waves 273° and boiling point 373°. Formerly and currents) can abrade so long as they measured in degrees absolute (°A), it is carry debris. Abrasion by water and ice now measured in KELVINS. characteristically produces rounded forms, and abrasion by ice also produces stria- absolute vorticity See vorticity. tions, while the sand-blasting effect of wind abrasion is most effective in a narrow absorption 1. The process by which en- zone just above ground level, resulting in ergy is converted into another form of en- undercut features. As abrasion takes place, ergy when a waveform is transmitted the corresponding reduction in size of the through a medium (e.g. the loss of energy initial load debris is known as ATTRITION. A by a seismic wave in the form of heat). distinction is sometimes made between 2. (in meterology) The conversion of short- abrasion and CORRASION, which refers to or long-wave radiation to a different form the erosional processes that result in of energy by gases in the atmosphere. Ab- abraded rock surfaces. sorption is highly selective in terms of wavelength; some wavelengths are entirely abrasion platform See wave-cut plat- absorbed and others are totally unaffected. form. The main atmospheric gases, oxygen and nitrogen, are not very important as ab- absolute age The age of a fossil, rock sorbers of radiation, but minor gases such formation, or individual rock, usually as carbon dioxide, water vapor, ozone, and stated in years. Such ages are usually deter- nitrous oxide have a significant effect. See mined by DENDROCHRONOLOGY or RADIO- also atmospheric window. METRIC DATING. See also relative dating. abstraction 1. The fraction of PRECIPI- absolute drought In British climatol- TATION that does not immediately run off. ogy, a prolonged period (usually at least 15 It is absorbed, evaporated, stored, or tran- days) during which no more than 0.25 mm spired. of rain falls on any day. 2. The union of two streams resulting from erosion of the land (DIVIDE) between them. absolute humidity The mass of water It generally occurs in gullies and ravines. vapor in a unit volume of air (usually stated in units of g m–3 or kg m–3). It de- ABW See Arctic bottom water. pends on the temperature and pressure of the air. See humidity. abyss /a-biss/ See deep. absolute instability See instability. abyssal hill /a-biss-ăl/ A large dome- shaped submarine hill on the ABYSSAL absolute plate motion The motion of a PLAIN. Such hills rise to heights of 1000 m LITHOSPHERIC PLATE with respect to a fixed in up to 6000 m of water and may be sev- frame of reference, such as a hot spot or eral kilometers wide. There are many along pole of rotation. Compare relative plate the MID-ATLANTIC RIDGE and in the Pacific motion. See also plate tectonics. Ocean. absolute stability See stability. abyssal plain An extremely smooth portion of the deep-sea floor. The gradi- absolute temperature A temperature ents across abyssal plains fall within the scale based on the coldest temperature that range of 1:1000 to 1:10 000, which means is physically possible. This absolute zero of that variations in depth amount to only a temperature is –273.l5°C, but for meteoro- few meters. This remarkable degree of flat-

2 accretion ness has come to light because of deep-sea acanthite See argentite. photographs and high-precision sounding techniques. In the Atlantic Ocean, abyssal Acanthodii A class of extinct fishes, plains range between 200 and 400 km in sometimes known as the spiny sharks, that width, but they can be several hundred included the earliest-known jawed verte- kilometers wide. They tail upwards into brates. The Acanthodii appeared during the continental rise and frequently, in a the Upper Silurian Period, some 400 mil- seaward direction, merge into ABYSSAL lion years ago, and lasted through to the HILLS. Their sediments are varied; while Permian Period. most are thinly veneered with pelagic sediment, perhaps globigerina ooze and red accelerated erosion See abnormal erosion. clay, they also display sediments and plant and animal remains that normally charac- acceleration of free fall (acceleration due terize shallow-water environments. The to gravity) The acceleration (g) of a body reason for this may be the operation of falling freely in a vacuum in the Earth’s TURBIDITY CURRENTS. gravitational field. The standard value is 9.806 65 m s–2, although actual values de- abyssal rock A type of intrusive igneous pend on the distance from the Earth’s cen- rock formed deep within the Earth’s crust. ter of mass. abyssal zone A zone of greatest ocean accessory mineral A mineral that is depth, lying seaward of and deeper than present in small quantities in a rock and the BATHYAL ZONE of the continental slope, does not affect the overall character of that that is below a depth of some 1000 m, and rock for classification purposes. Compare including the deeper parts of the oceans essential mineral. and the deep-sea trenches. Lying between the abyssal and bathyal zones is the CONTI- acclimatization The process by which NENTAL RISE, which is often bordered on its humans become adapted to living in a seaward flank by abyssal plains and markedly unfamiliar climatic regime. This abyssal hills. The pelagic–abyssal environ- normally refers to a change to hot and ment (see pelagic (def. 1)), which is not humid or especially cold climatic condi- reached for at least several hundred kilo- tions. If acclimatization has not taken meters from the coast, globally represents place, severe physiological stress may re- an area of 250 × 106 sq km, i.e. roughly sult. The full process may take up to ten half the area of the Earth. Few organisms years. live in these depths, where pressure is high /ă-kor-dănt/ (conformable) and light does not reach, and deposition of accordant Describing a drainage pattern that is con- sediment is very slow. trolled by the structures over which it flows. Actual patterns vary greatly, de- Acadian orogeny A phase of mountain pending on the nature of the structures or building that affected the North Ap- lithologies. Compare discordant (def. 2). palachian region of North America, extending from what is present-day New accordion folding /ă-kor-dee-ăn/ A type York to the Gaspé region of Canada. It of folding in rocks, in which the beds of the took place during the late Devonian and hinge area are markedly thickened and early Mississippian Periods, spanning sharply folded, while on the limbs the beds about 50 million years, and resulted from are straight and of uniform thickness. the westward movement of the Avalon ter- rane onto the continental landmass of Lau- accretion /ă-kree-shŏn/ 1. (in meteorol- rentia, closing a part of the proto-Atlantic ogy) The process of ice crystal growth by (Iapetus) Ocean. collision with water droplets in clouds.

3 accretionary wedge

This is one of the mechanisms by which acicular /ă-sik-yŭ-ler/ Describing a crys- minute cloud droplets achieve sufficient tal having a needle-like habit. See crystal size to give rainfall. habit. 2. See nucleation. acid brown soil A type of soil found in accretionary wedge (accretionary prism) the BROWN EARTH zone on base-deficient A collection of sediments that accumulates parent materials. Such soils are strongly at a PLATE BOUNDARY, on the landward side acid, with a moder humus, and although of an ocean TRENCH. It consists of intensely the B horizon is more clearly differentiated deformed sediments that have been by color than in the true brown earth there dragged down on the subducting oceanic is no appreciable eluviation of clay or iron oxides. With time it seems likely that these plate then scraped off onto the overriding soils would become podzolic brown earths. continental plate to form the wedge. Series of wedges ride up, one upon the other, with acid lava Slow-moving viscous LAVA repeated thrust faults. See also plate tec- containing a high proportion of silica. Pro- tonics; subduction zone. duced by so-called acidic volcanoes, it generally solidifies very quickly. See also basic accumulated temperature A method lava. of indicating the excess or deficit of temperature with reference to a specified value acid rain (acid precipitation) Any PRE- for a specified period. Two temperature CIPITATION (including fog, rain, sleet, and bases that have been frequently used are snow) that is acidic due to the presence of 6°C for the commencement of plant sulfur dioxide and nitrogen oxides in the growth and 16°C for heating requirements. atmosphere. Most of these pollutants de- The accumulated temperature is calculated rive from the burning of FOSSIL FUELS. Acid by taking the number of hours in a specific rain kills trees, poisons rivers and lakes, period the temperature was above or and corrodes buildings. Runoff of acid rain below the set value and multiplying by the affects the mineral composition of the soil. mean temperature during this time to give the number of degree hours. More com- acid rock The dominant chemical con- monly, daily or monthly values of accumu- stituent of igneous rocks is silica, SiO2, lated temperature are obtained from daily which typically ranges from 35–75% (by or mean values rather than hourly ones. weight). Arbitrary divisions are made as > follows: acid – SiO2 66%, intermediate – accumulation 1. The total amount of SiO2 55–66%, basic – SiO2 45–55%, and ultrabasic – SiO %t;45%. These strict di- PRECIPITATION that gathers on a snowfield 2 visions have been largely abandoned but or glacier. the general descriptive terms acid, basic, 2. The overall result of all processes that etc., remain. In current usage, an acid rock add mass to a snowfield, glacier, or ice floe, contains in excess of about 10% free including snow from avalanches, snowfall, QUARTZ, e.g.: granite, granodiorite, rhyo- and windblown snow. lite. achnelichs /ak-nĕ-liks/ Fragments of acid soil A soil with a pH value of less glassy smooth PYROCLASTIC ROCK derived than 7.0. Soil acidity is measured on the from basaltic LAVA that has sprayed in the pH scale, which is related to the concentra- air while molten and solidified. Pele’s tears tion of hydrogen ions in the soil (a neutral are achnelichs. soil is given a pH value of 7.0; if the value is greater than this it is termed an alkaline achondrite /ay-kon-drÿt/ A stony METE- soil). Thus there is an inverse relationship ORITE that does not contain CHONDRULES. between pH value and the concentration of Compare chondrite. hydrogen ions. Acid soils develop where,

4 adamellite for some reason, there is a lack of the ex- gii) but have narrow bases and are sup- changeable bases in the soil, such as cal- ported by thin fin rays. The Actinopterygii cium and sodium. The bases have been also have a single dorsal fin. The group in- largely replaced by the two cations hydro- cludes the modern TELEOSTEI. See also Os- gen and aluminum, which control soil teichthyes. acidity. Possible acid-forming factors are leaching, organic matter containing few active continental margin (active mar- bases, and an acid parent material. Acid gin) A margin of a continent that co- soils are therefore particularly common in incides with the margin of a LITHOSPHERIC the humid tropics and the humid temperate PLATE where earthquakes, volcanic activity, lands. Examples of typical acid soils are and mountain building are taking place, podzols, brown earths, and latosols. From and that is often bordered by an oceanic the agricultural viewpoint most cultivated trench offshore. Compare passive conti- crops will thrive on mildly acid soils. nental margin. acmite /ak-mÿt/ See aegirine. active layer See permafrost.

2 acre A unit of area equal to 4840 yd actual evapotranspiration (AE) The 2 (equivalent to 4046.86 m or 0.4047 true amount of water vapor returned to the hectares). atmosphere through evaporation from a ground surface and transpiration from its acre foot A unit of volume equal to an plant cover where the water supply is lim- area of 1 acre to a depth of 1 foot (equiva- ited. Compare potential evapotranspira- lent to 43 560 ft3 or 1233.5 m3). It is used tion. for expressing the volumes of lakes and reservoirs. actualism /ak-chû-ă-liz-ăm/ See uniformitarianism. acritarchs A group of organic-walled microfossils of various forms in the size actuopaleontology /ak-chû-oh-pay-lee- range of 5–200 µm across (most are on-tol-ŏ-jee, -pal-ee-/ The branch of paleon- 20–150 µm across). It is not known what tology in which investigations into modern these organisms were although they are believed to be derived from phytoplankton organisms, including their effects and re- (the name acritarch means of uncertain ori- mains in modern environments, are di- gin). Acritarchs range from Precambrian to rected toward the understanding of fossil Holocene and are especially useful in the analogs. Based upon uniformitarian princi- biostratigraphy of the Proterozoic Eon and ples (see uniformitarianism), actuopaleon- Paleozoic Era. tology provides a means of relating various TRACE FOSSILS to the particular animals actinolite A monoclinic AMPHIBOLE. from which they are derived and of understanding the changes that an organism un- actinomycetes /ak-tă-noh-mÿ-seets/ Aer- dergoes between death and fossilization obic bacteria that have a filamentous habit (see taphonomy; thanatocoenosis). BIO- as do fungi. Optimum conditions are a STRATONOMY is one aspect of actuopaleon- moist well-aerated soil with a pH between tology. 6.0 and 7.5. They are more prolific than other bacteria and fungi in drier areas and adamantine /ad-ă-man-tin, -teen, -tÿn/ are important also in that they can decom- Describing a mineral that has a brilliant di- pose the more resistant soil organic matter. amondlike luster.

Actinopterygii /ak-tă-nop-tĕ-rij-ee-ÿ/ adamellite /ad-ă-mel-ÿt/ A variety of Ray-finned bony fish in which the paired granite consisting of about equal propor- fins are not fleshy (compare Crossoptery- tions of potassium feldspar and sodic pla-

5 adiabatic gioclase feldspar together with one or more advection /ad-vek-shŏn/ The horizontal ferromagnesian minerals. component in the transfer of air properties. For example, the heat and water vapor adiabatic /ad-ee-ă-bat-ik/ Denoting an content of the air at the Earth’s surface atmospheric process in which there is no varies appreciably and by the wind systems exchange of heat between the system and these properties are transferred to other its environment. In the more rapid ex- areas. With winds from tropical latitudes changes, such as thermals rising from the there is advection of warm air, and from ground surface, this is a reasonable ap- polar latitudes, advection of cold air. proximation. In these circumstances, the change in temperature of rising air is deter- advection fog Fog formed by the hori- mined by the physical properties of the air zontal transfer of moist air over a cold sur- and the external pressure. As air pressure face, which sufficiently cools the lower decreases with height above the ground layers of the atmosphere to give saturation surface, rising air expands, and exerts me- and condensation. In summer it occurs chanical work on its environment. This ne- over cool seas, such as the North Sea, the cessitates a loss of heat energy from the Labrador area, and off the coast of Cali- rising air and its temperature falls. The rate fornia, frequently affecting the adjacent of fall of temperature in an adiabatic coasts. In winter the advection of warm process is constant for our atmospheric moist air over a cold snow-covered ground composition, being 0.98°C/100 m, and is can also produce this type of fog. known as the DRY ADIABATIC LAPSE RATE. AE See actual evapotranspiration. See also saturated adiabatic lapse rate. aegirine /ee-jeer-ÿn, -een, -in/ (acmite)A adit /ad-it/ A horizontal or slightly up- sodic clinopyroxene. See pyroxene. ward-sloping closed-ended passage into a mine, usually constructed to intersect a aegirine-augite See pyroxene. seam of coal or vein of mineral. The slope ensures that any water drains out of the aeolian erosion See eolian erosion. adit. aeolian form See eolian form. adjacent sea See marginal sea. aeolian transport See eolian transport. adobe /ă-doh-bee/ A mixture of silt and clay, common in Mexico and the south- aerial photography Photography of western USA. It has long been used for the Earth’s surface from airborne plat- making bricks because it dries to a hard forms, such as satellites, spacecraft, and weatherproof mass. See also loess. aircraft; it is the oldest method of REMOTE SENSING. The photographs may be vertical adret /ad-ret/ The sunny or, in the N or oblique (high-oblique or low-oblique) hemisphere, south-facing slope of a valley. and are frequently taken in runs of over- It is the side favored for farming, as in the lapping prints, which can be assembled European Alps. into mosaics (print lay-down). Three- dimensional images can be viewed when adsorption (in soil science) The process pairs of photographs taken with a stereo- by which compounds, molecules, or ions scopic camera are viewed through a stereo- are taken up and attached to the surface of scopic viewer (stereoscope). Aerial a particle (typically clay or humus) by photography has widespread applications chemical or physical means. within the earth sciences, e.g. in map- making and the revision of existing maps, adularia /aj-ŭ-lair-ee-ă/ A variety of al- and the production of three-dimensional kali FELDSPAR. models of the land surface; in land-use

6 age planning and mapping; and in providing ization of gas molecules takes place; the data on topography, geology, hydrology, lower limit for these processes to occur is and vegetation and soils. Color infrared about 30 km. As the relationships between film, which is sensitive to near-infrared the upper and lower atmosphere are as yet wavelengths and visible wavelengths, is little understood, there has been consider- used, for example, in vegetation and land- able research recently into the possible ef- use studies. fects of these upper levels in tropospheric weather. aerodynamic roughness An index of the nature of airflow near the ground sur- aerosol A particle of matter that is face. A surface is aerodynamically smooth larger than a molecule but small enough to if there is a layer of air immediately above remain suspended in the atmosphere. it that has laminar flow. However, in me- Aerosols may be solid or liquid and play an teorological terms, nearly all surfaces are important part in many atmospheric aerodynamically rough, producing turbu- processes, such as precipitation formation, lent flow down to the ground surface, even atmospheric electrification, radiation bal- for the lightest winds. ances, and visibility. The origins of aerosols are diverse. Over sea areas, sea aerolite /air-ŏ-lÿt/ A stony METEORITE spray provides large salt nuclei and over made up of silicate minerals. land, weathering dusts of clay particles are probably the major source. aeromagnetic survey Measurement of variations in the Earth’s magnetic field car- afforestation /ă-fô-rĕ-stay-shŏn/ The ried out using an instrument known as a large-scale planting or replanting of trees. magnetometer that is attached to an air- They may be planted to prevent soil ERO- craft (airplane or helicopter). The aircraft SION, to act as WINDBREAKS, or as a source may be flown at a fixed height above sea of timber (usually for wood pulp). level or at a constant height above the ground surface along flight lines that are a aftershocks A series of minor shocks or fixed distance apart. The magnetometer vibrations that follow the main series of usually records the total intensity of the EARTHQUAKE shocks. In general they origi- magnetic field along the flight lines. Differ- nate at or near the focus of the main earth- ences between the measurements obtained quake as a result of the readjustment of compared to theoretical values represent rocks that have over-reacted during the changes in the type of rock, or the thick- main period of movement. Depending ness of the rock, below the ground surface. upon the size of the earthquake they can continue for a few days or months. aeronautical chart A form of map produced essentially for air navigation or pi- agate /ag-it, -ayt/ An extremely fine- lotage. In addition to showing the relevant grained type of QUARTZ consisting of alter- topographical features, such as contours, nate bands of colored CHALCEDONY. It may vegetation, roads, and cultural detail, the be white, gray, red, brown, or black, and map shows supplementary information for can be dyed other colors. It generally oc- specialized use, such as detailed vertical curs in rock cavities. It is used for making obstruction information, flight areas, air ornaments and as a semiprecious gem- corridors, etc. stone. See also moss agate; silica minerals. aeronomy /air-on-ŏ-mee/ The science of age An interval of geologic time in the the upper atmosphere of the Earth, or Chronomeric Standard scale of chronos- atmosphere of other planetary bodies. It tratigraphic classification (see chronos- includes study of the chemical composi- tratigraphy). The equivalent Stratomeric tion, physical properties, motions, and Standard term, indicating the body of rock effects of radiation. Dissociation and ion- formed during this time, is the STAGE. Ages

7 ageostrophic wind may be grouped together to form EPOCHS aggregate 1. Mineral fragments and/or and may themselves be comprised of sev- rock particles that combine to form a hard eral CHRONS. mass. 2. (in soil science) A cluster of soil particles ageostrophic wind /ay-gee-ŏ-stroff-ik/ that have bonded together through aggre- The vector difference between the actual gation. wind and the geostrophic wind. Without an ageostrophic component there would be aggregation 1. (in meteorology) The no changes in pressure systems and atmos- process by which ice crystals grow to form pheric circulation would be a perfect bal- snowflakes through collision and clumping ance between the pressure gradient and the together. CORIOLIS EFFECT. 2. (in soil science) The process by which soil particles bond together to form soil ag- agglomerate /ă-glom-ĕ-rayt vb., <-glom-ĕ- gregates. The presence of organic matter rit, -rayt n./ A mixture of coarse angular aids the process. fragments of rock and finer-grained material formed during a volcanic explosion. agmatite /ag-mă-tÿt/A MIGMATITE in See pyroclastic rock. which melanocratic material occurs as angular inclusions in a leucocratic granitic agglutinate See pyroclastic rock. host, giving an overall appearance resembling a breccia.

long profile Agnatha /ag-ă-th’ă/ Primitive jawless of stream fish, from which all more advanced vertebrate types have presumably evolved. height of spillway = Modern species are few and represent new base level groups unknown as fossils. The diverse ex- SEA tinct forms can be grouped together as the (ultimate base level) OSTRACODERMI, which were abundant in aggradation in the Silurian and Devonian and are used in this stretch stratigraphic CORRELATION. Compare erosion downstream Gnathostomata.

Aggradation in a dammed stream agonic line /ă-gon-ik, ay-/ A line joining all points on the surface of the Earth where aggradation /ag-ră-day-shŏn/ The rais- the ANGLE OF DECLINATION is zero. Along ing of the level of the land surface through such a line, magnetic north and true north deposition, e.g. as a result of fluvial, ma- coincide. See also isogon. rine, eolian, or slope processes. The cause of aggradation in a river is incompetence Agulhas Current A warm surface cur- (see competence) or incapacity (see capac- rent that flows in the S Indian Ocean from ity) of the river to transport its load, lead- the South Equatorial Current (see equator- ing to deposition. This may arise through ial current) and is additionally fed by the an increase in the volume of load supplied Mozambique Current. It flows south-west- to the river, a loss of speed or volume of ward off the E coast of southern Africa to flow, or most commonly a rise in base join the eastward flow between Africa and level. Another common cause of aggrada- Australia. It is one of the fastest flowing tion is the damming of a stream, artificially ocean currents with a maximum flow ve- creating a new base level for the upper locity of some 5 knots off the SE coast of reaches of a river. Originally graded to sea South Africa. level, the upper reaches will aggrade to the new base level, the height of the dam’s A horizon The uppermost layer of soil, spillway. See also degradation. or topsoil. It consists of fine soil particles

8 air pollution and HUMUS. Some soluble material will main source areas for air masses. Away have been dissolved out and passed down- from their source areas, air masses undergo ward to the B HORIZON beneath. See also modification by coming in contact with horizon (def. 1). different surfaces with the result that in a short period of time they can become in- distinguishable. As most parts of the world represent modification zones rather than COMPOSITION OF AIR source areas, air mass terminology is less (% by volume) frequently used than formerly. There have been many attempts to classify air masses, nitrogen (N2) 78.08 but the most frequently quoted is that by oxygen (O2) 20.95 the Swedish meteorologist Tor Bergeron carbon dioxide (CO2) 0.03 (1891–1977). Two basic air masses are argon (Ar) 0.93 identified on thermal properties – polar (P) × –3 neon (Ne) 1.82 10 and tropical (T), and two by moisture cat- × –4 helium (He) 5.24 10 egories – maritime (m) and continental (c). × –4 methane (CH4) 1.5 10 The temperature of the air mass relative to × –4 krypton (Kr) 1.14 10 the surface over which it is passing is in- × –5 xenon (Xe) 8.7 10 cluded as warm (w) or cold (k) to give a × –5 ozone (O3) 1 10 wide range of combinations – mPw, cTk, × –5 nitrous oxide (N2O) 3 10 etc. The American meteorologist Hurd C. water (H2O) variable, up to Willett (1903–92) modified this system 1.00 slightly to add stable (s) or unstable (u) to hydrogen (H ) 5 × 10–5 2 indicate the likelihood of precipitation in the system. Other classifications distinguish equatorial, monsoon, and Arctic (or Antarctic) but difficulties can arise in iden- air The mixture of gases in the atmos- tification. phere. Its composition is almost uniform throughout the troposphere and is shown Substances (e.g. gases, in the table below. Only water vapor and air pollution carbon dioxide vary appreciably, the for- aerosols, and particles) released into the at- mer in relation to evaporation and precipi- mosphere, by natural or human activity, tation and the latter through plant that can cause harm to animals and plants. photosynthesis. The combustion of fossil fuel is a major source of air pollutants: exhaust from air-fall deposit A sediment composed gasoline and diesel engines contribute of fallout from a cloud of airborne material carbon monoxide, volatile organic com- from an erupting volcano, as opposed to pounds, and nitrogen oxides; electricity- LAVA (which generally flows from a vol- generating plants and industrial plants cano). burn fossil fuels releasing sulfur oxides; ground-level ozone is formed in a photo- air mass An area of the lower atmos- chemical reaction when sunlight acts on ni- phere with similar properties of tempera- trogen dioxides and volatile organic ture and moisture in the horizontal field. compounds close to the ground. Particu- At the margins of the air mass, temperature late matter also contributes to air pollu- gradients become steep at a transition zone tion, e.g. emissions of soot particles from known as a FRONT. The uniformity of the smokestacks of power plants and parti- properties is achieved by prolonged con- cles released during industrial processes tact with the underlying surface and little and fires. The presence of aerosols and horizontal or vertical mixing. These re- gaseous pollutants above an urban or in- quirements are experienced in areas of high dustrial area may occur in sufficient quan- pressure or anticyclones, which are the tities to form SMOG.

9 air stream air stream A flow of air that is not nec- albedo /al-bee-doh/ An index of reflec- essarily homogeneous but has a distinctive tion comprising the ratio of reflected radi- origin. Air streams are therefore distin- ation to the total incident radiation. guished by their direction of approach Usually this value is expressed in a percent- rather than assuming any specific thermal age form for visible wavelengths. Typical or stability properties. The mid-latitude values for surface albedo are: forest westerlies can be regarded as being a mix- 5–10%, wet soil 10%, sand 20–30%, grass ture of slightly baroclinic air streams 25%, old snow 55%, concrete 17–27%, bounded by sharp frontal zones. fresh snow 80%. Water surfaces vary from about 5% with high sun and calm seas to Airy’s hypothesis of isostasy /air-eez/ 70% at low elevation and rough seas. The The English astronomer George Biddell planetary albedo of the Earth measured Airy (1801–92) proposed that in order for from artificial satellites is approximately isostatic equilibrium to exist, mountain 34%, which means that over one third of ranges must have roots proportional to the Sun’s radiation is returned to space their height, i.e. the highest mountains without a change of wavelength. have the deepest roots. These roots are composed of sialic material and displace an Alberta clipper (Alberta low) A fast- equivalent volume of sima, thereby causing moving low-pressure system that forms or the gravity anomalies present near moun- redevelops, generally in winter, to the lee of tain chains. the Canadian Rocky Mountains, in or near Alberta. The depressions track eastward to bring cold snowy weather and strong gusty Aitken nucleus /ayt-kĕn/ See nucleus. winds around the Canadian–US border, into the plains states of the N USA and Aitoff’s equal-area projection /ÿ-toff/ across the Great Lakes. Sometimes the de- A CYLINDRICAL PROJECTION of a hemi- pressions travel as far as the Maritime sphere. The major axis, the Equator, is Provinces of Canada and the NE USA twice the length of the minor, central where they may intensify on meeting rela- meridian, axis. The projection is bounded tively warm moist Atlantic air off the east by an ellipse. The main characteristics of coast and bring heavy snow over land. this projection are that it is an equal-area projection and landmasses near the center albite /al-bÿt/ A sodic plagioclase FELD- of the area covered are of quite good shape, SPAR. although the distortion increases towards the east and west limits of the projection. It Aleutian Low /ă-loo-shăn/ The mean is named for the French cartographer low pressure center in the N Pacific Ocean. David Aitoff (1854–1933). See also map It represents a statistical average of pres- projection. sure value and location, which in turn are determined by the tracks of the depressions AIW See Antarctic intermediate water. and the point at which they reach their lowest pressure. It is most marked in the alabaster A fine-grained white, some- winter. times translucent, variety of GYPSUM, used for making ornaments. alexandrite /al-ig-zan-drÿt/ A transparent emerald-green type of CHRYSOBERYL, Alaska Current A comparatively warm used as a semiprecious gemstone. It has the surface oceanic current that flows counter- unusual property of turning red in artificial clockwise in the Gulf of Alaska. It results light. from a northward extension of part of the North Pacific Current where this is di- alfisol /al-fă-sôl/ One of the twelve soil verted on meeting the North American orders of the US SOIL TAXONOMY, covering continent. pedalfers that are equivalent to the gray-

10 alkali basalt brown podzolic, gray-wooded, gray-forest, more sophisticated types of alidade.) The sol lessivé, degraded chernozem, and ruler edge is placed against the point planosol soils of the old American classifi- marked on the table over which the appa- cation. They are found in the humid re- ratus is standing, and detail to be fixed is gions of the world under deciduous sighted onto. A ray is then drawn on the woodland or grassland vegetation. The plane table sheet along the ruler edge. Al- dominant soil-forming process is leaching, though either side of the ruler may be used which is more intense in these soils than in initially, once one ray has been drawn, only the inceptisols but less than in the spo- that side may be used until that sheet is dosols. They are productive soils and favor completed. the more common agricultural crops. alkali /al-kă-lÿ/ (alkaline) (in petrology algae /al-jee/ A group of largely aquatic and mineralogy) Denoting igneous rocks organisms formerly classified as plants. It and minerals that have high contents of the includes both microscopic forms, such as alkali metal oxides, Na2O and K2O. For a the DIATOMS, as well as the multicellular given silica content such rocks are rela- seaweeds, which may grow to a large size. tively richer in sodium and potassium and Algae are subdivided into different phyla poorer in calcium compared with calc-al- that are now usually placed in the kingdom kaline rocks. See alkali basalt; alkali gab- Protista (or Protoctista). The only geologi- bro; granite; ijolite; nephelinite. cally significant algae are those having hard parts, which may form bioherms, ei- alkali basalt A basic undersaturated ther by trapping sediment or by secreting volcanic rock that is the fine-grained equiv- massive laminated structures of calcium alent of ALKALI GABBRO. The essential min- carbonate. Such structures commonly con- erals of all basalts are plagioclase feldspar stitute lower Paleozoic REEFS. Compared of labradorite-bytownite composition and with bacteria and fungi, algae are relatively pyroxene. In alkali basalts the pyroxene is unimportant in soils, but they are often pi- augite or titanaugite and olivine is present oneers in colonizing new ground and may in abundance. Olivine is frequently number as many as 100 000 per gram of rimmed or pseudomorphed by the alter- dry surface soil. ation products iddingsite and/or serpen- algal bloom /al-găl/ A sudden increase tine. Small amounts of alkali feldspar and/or feldspathoid (nepheline or analcite) in the numbers of ALGAE in a body of water such as a sea, lake, or river. It may be may be present. Alkali basalts are typically caused by an increase in the amount of ni- porphyritic. Basalts containing large pla- trates, phosphates, and other nutrients. gioclase phenocrysts are referred to as Forecasting systems to provide advance feldsparphyric or big feldspar basalts. warning of harmful algal blooms have Those rich in olivine and augite are termed been established in some locations (e.g. picrite basalts. Oceanite and ankaramite Karenia brevis blooms in the Gulf of Mex- are varieties in which olivine and augite re- ico) using sources that include satellite im- spectively have become concentrated. agery, field observations, and data from Alkali basalts are usually holocrys- buoys. See eutrophication. talline and have ophitic or intergranular textures. Nodules of gabbro and peridotite alidade /al-ă-dayd/ A surveying instru- are often found in alkali basalt lavas. With ment used for sighting onto objects of de- an increase in the amount of nepheline to tail and for defining the rays to be drawn to greater than 10%, alkali basalts pass into them in PLANE TABLING. The alidade is basi- basanites (olivine-bearing) and tephrites cally a ruler of metal or wood with a verti- (olivine-free). In some rocks, the place of cal slit sight at the observer’s end and a nepheline is taken by analcite or leucite and vertical stretched wire sight at the other. (A such terms as analcite-basanite and leucite- telescope is fixed parallel to the ruler in tephrite are appropriate. See also basalt.

11 alkali feldspar alkali feldspar A member of a series of allivalite /al-ă-vă-lÿt/ A gabbro consist- minerals with composition varying being of olivine and plagioclase feldspar of tween the two end-members albite bytownite–anorthite composition. (NaAlSi3O8) and orthoclase (KAlSi3O8). See feldspar. allochem /al-ŏ-kem/ A discrete calcareous particle that has usually been trans- alkali gabbro Alkali gabbros and ported at some stage, including FOSSILS, syenogabbros are basic plutonic rocks con- ooliths (see oolite), INTRACLASTS, and PEL- taining, in addition to the normal gabbro LETS, found in LIMESTONES; the terminology mineralogy, alkali feldspar and/or felds- used in the petrographic description and pathoids. Syenogabbros contain approxi- classification of limestones is based upon these constituents and the matrix in which mately equal amounts of alkali and they are set (see micrite; sparite). Thus a plagioclase feldspar with TITANAUGITE, limestone composed of fossil fragments set ANALCIME, and/or NEPHELINE plus or minus in a micrite matrix is a biomicrite; a pellet olivine. Having more sodium and potas- limestone with a sparite matrix is a sium than gabbros, syenogabbros are, as pelsparite. the name implies, related to SYENITES. Essexite consists of labradorite, ti- allochthonous /ă-lok-thŏ-nŭs/ Denot- tanaugite, and olivine together with small ing an isolated mass of rock displaced over amounts of nepheline and/or analcime. Al- a considerable distance from its original kali feldspar, apatite, and ilmenite may source by tectonic processes such as low- also be present. Teschenite and crinanite angle thrusting. The mass of rock is known are analcime-bearing varieties from which as an allochthon. Compare autochtho- nepheline is excluded. Theralite is a nous. nepheline-bearing gabbro containing no analcime. Kentallenite is a saturated rock allogenic /al-ŏ-jen-il/ (allothigenous) De- containing augite, olivine, biotite, la- scribing rock constituents that were bradorite, and orthoclase and is equivalent formed at some distance from their present to olivine-monzonite. location, particularly minerals or rock The names of the plutonic rocks are ap- fragments derived from existing rocks. plied to the medium-grained varieties Compare authigenic. of doleritic aspect but with the prefix ‘micro-’. The volcanic equivalents include allotriomorphic /al-ŏ-trÿ-mor-fik/De- basanites, tephrites, and trachybasalts. scribing a rock in which the majority of crystals are euhedral. Compare hypid- Teschenite and theralite are found in dif- iomorphic; idiomorphic. ferentiated sills and dikes. allotrope A form of a chemical element alkaline soil A soil with a pH value that differs (usually in its crystal structure) greater than 7.0. Compare acid soil. from another form of the element. Graphite, diamond, and buckminster- allanite /al-ă-nÿt/ (orthite) One of the fullerene, for example, are allotropic forms EPIDOTE group of minerals. of carbon.

Allerød /al-ĕ-rohd/ A phase of warming alluvial cone A steep alluvial deposit of about 1000 years during the period of that forms where a fast-flowing stream de- deglaciation after the Würm/Weichsel/Wis- bouches onto a lowland plain. Apart from consin ice age. In many parts of NW Eu- its steepness, it in other respects resembles rope this was followed by a sudden cooling an ALLUVIAL FAN. from 8800 to 8300 BC. The type-site from which the period takes its name is in Den- alluvial fan A fan of material deposited mark. by a stream where it debouches from a

12 alpine glow

the basal portions of alluvial valleys, and may have originated at the end of the PLEIS- TOCENE period under PERIGLACIAL condi- distributaries MOUNTAIN tions; in some rivers they are attributed to deposition within the channel in conditions stream PLAIN in which severe scour removed the fine material. Silts, sands, and gravels are more FRONT important, constituting 75% of the Missis- sippi alluvium. Sands constitute the point bars formed on the inside of meander contours of fan bends, and are an important constituent of levées. Silts and clays are deposited in the Alluvial fan lee of point bars or as overbank deposits in times of flood, covering the far reaches of mountain front onto a plain, with the apex the floodplains with backswamp deposits. of the fan at the point of emergence from the mountains. In its mountain tract, the almandine /al-măn-deen, -din/ A light stream will have been confined into a sin- red to red-brown member of the GARNET gle flow in a rock-cut gorge, but on reach- group of minerals, Fe3Al2(SiO4)3. Deep red ing the plain this control is lost, and it crystals are valued as semiprecious gem- breaks up into a number of distributaries. stones. This increases the wetted perimeter, or area of contact between water and land, alnoite /al-noh-ÿt/ A basic or ultrabasic and hence friction increases; with more of dike rock composed largely of melilite and its energy used in overcoming friction, less biotite with subordinate pyroxene, calcite, is available for sediment transport, so de- and olivine. Alnoite is found in ijolite-car- position occurs, in the form of a fan. This bonatite complexes. is the direct landborne analogy to DELTA formation, and alluvial fans are character- A region of grassland on a high istic landforms of arid and semiarid envi- alp ronments. mountainside. Above the timberline and covered with snow in the winter, alpine grassland may be used as pasture for graz- alluvium The products of SEDIMENTA- ing animals in summer. It is named after TION by rivers, sometimes including deposits in estuaries, lakes, and other bodies the Alps in west-central Europe. of fresh water. Alluvium includes material of a wide range of PARTICLE SIZES, usually alpine A type of climate found in moun- restricted to the silt size fraction of tainous areas above the timberline but 0.006–0.02 mm. below any PERMAFROST level. Grass and There is a marked decrease in the size of other low-growing plants make up the typ- alluvial material down-valley, with finer ical vegetation. See also alp. material in the lower reaches. This may be due to SORTING, which leaves coarse ma- Alpine folding A recent type of large- terial behind and carries finer material scale FOLDING that created such mountain down-valley, or it may be due to progres- ranges as the Alps, Andes, Himalayas, and sive wear of the material as it travels down- Rockies. stream. Another cause may be that smaller valley-side slopes downstream supply a alpine glow (alpenglow) A sequence of finer caliber of material to the stream. colors seen on snow-covered mountain Within the FLOODPLAIN, alluvium (in the tops when the Sun nears the horizon at looser sense) varies in size from the finest sunset. The mountain tops are first yellow- clay and silt-sized material through sands orange, then pinkish, and finally purple. to coarse angular gravels. The gravels form The series of colors, reversed, also occurs

13 Alpine–Himalayan chain at sunrise on mountain tops facing the ris- and orographic clouds where the atmos- ing Sun but is less common. phere is essentially stable, but the uplift is forced by mountains. Altocumulus clouds Alpine–Himalayan chain A predomi- generally occur as globular masses in nantly east–west trending orogenic belt, bands across the sky. extending from Spain through Europe to S Asia. It was formed mainly in the altostratus /al-toh-strat-ŭs/ A grayish Oligocene and Miocene Epochs by the clo- cloud sheet normally composed of a mix- sure of the former Tethys Sea, mainly as a ture of ice crystals and water droplets. It is result of the northerly drift of the African distinguished from cirrostratus by lying at continental plate. It consists of a series of lower levels, being somewhat thicker, and igneous, metamorphic, and deformed sedi- not exhibiting halo phenomena. The Sun mentary rocks, which now form the Alps may be seen through the thinner parts of and Himalayan Mountains. the cloud. It is frequently followed by rain because the approach of a warm front is altiplanation /al-ti-plă-nay-shŏn/ A pro- heralded by this cloud type. cess associated with a PERIGLACIAL environment in which terraces or benches are cut alum 1. In general terms, any double salt in solid rock in hillside or summit loca- consisting of the sulfates of a trivalent tions. The weathering and erosion of the metal and a monovalent metal, with 24 rocks is achieved by a combination of molecules of water of crystallization; its FROST shattering, SOLIFLUCTION, and CON- general formula is A2(SO4)3.B2SO4.24H2O, GELITURBATION. Terrace formation is prob- where A is the trivalent metal and B is the ably initiated by the production of a monovalent metal. NIVATION HOLLOW in hard rock, the back- 2. Specifically, aluminum potassium sul- wall of which then retreats, causing an en- fate, Al2(SO4)3.K2SO4.24H2O, also known largement of the flat or gently sloping as potash alum. It occurs naturally as the terrace in front. Altiplanation terraces may mineral kalinite. have a cover of debris, which can exhibit PATTERNED GROUND. alumina The mineral form of aluminum oxide, Al2O3. altiplano /ahl-ti-plah-noh/ A high plateau that is surrounded by mountains. A typical aluminum silicates There are three alu- example is the South American Altiplano minum silicates with the composition between two extensions of the Andes, Al2SiO5: the polymorphs ANDALUSITE, mainly in Bolivia and southwestern Peru. It KYANITE, and SILLIMANITE. They are found contains volcanic material and eroded sed- in metamorphic rocks, mostly of argilla- iments, with evidence of ancient lakes in ceous composition. Each has a different yet the form of salt basins. closely related crystal structure and is stable over a different range of pressures and altithermal See hypsithermal. temperatures. Andalusite is orthorhombic and usually altitude A measure of height, usually pink or white in color. The variety chiasto- taken to be the height above mean sea lite shows a regular arrangement of impu- level. Sometimes the altitude of a hill or rities in the form of a cross. Andalusite is mountain is given in terms of its vertical characteristic of the low-pressure and high- height from base to summit. temperature conditions associated with contact metamorphism around igneous in- altocumulus /al-toh-kyoo-myŭ-lŭs/A trusions. Kyanite is triclinic and often blue- type of cloud that indicates some form of green to white in color. It is stable at higher vertical motion at medium levels in the at- pressures than andalusite and is found in mosphere. It includes a wide range of cloud intermediate- to high-grade regionally origins from genuine convection to billow metamorphosed rocks. Sillimanite is or-

14 amphibole thorhombic, usually white in color, and ter to over 2 m in some species. The animal commonly occurs as acicular crystals, inhabited the terminal, and most recently hence the alternative name fibrolite. It is formed, of these. The chambers were con- stable at higher temperatures than kyanite nected by a sometimes discontinuous cal- or andalusite and is found in the highest careous tube known as a siphuncle. The grades of thermally and regionally meta- septa met the inner wall of the shell at in- morphosed rocks. The orthorhombic alu- tersections called suture lines, and the minum silicate mullite has a composition character of these in fossil shells is used Al6Si2O13. It is found in argillaceous xeno- in taxonomic classification. The Am- liths (buchites) in basic igneous rocks. monoidea had relatively complex suture lines compared with those of the NAU- alum shale A claylike rock containing TILOIDEA; early ammonoids had angular potash ALUM, formerly much used as a folded suture lines (see goniatite) but in the source of alum for industry. It forms from later AMMONITES the suture lines were shales when sulfides they contain decom- highly convoluted and crinkled. Am- pose to produce sulfuric acid, which reacts monoids probably evolved from the nau- with mica to form aluminum sulfate. tiloids and the earliest are known from rocks of Silurian age. They reached their amazonite /am-ă-zŏ-nÿt/ (amazonstone) peak of development in the Mesozoic, for A type of microcline FELDSPAR, blue-green which they are important ZONE FOSSILS, or green in color, sometimes used as a and became extinct at the end of this era. semiprecious gemstone. amorphous Having no regular atomic amber Yellow translucent fossilized structure; noncrystalline. resin once exuded by trees and often enclosing insects that have been trapped prior Amphibia /am-fib-ee-ă/ The first class to hardening. of vertebrates to colonize land, evolving from crossopterygian fishes in the late De- amblygonite /am-blig-ŏ-nÿt/ A greenish vonian. These fish had a bony skeleton that or white lithium-containing mineral, could provide support out of water; they (Li,Na)Al(PO4)(F,OH). It crystallizes in also had lungs and their fleshy paired fins the triclinic system, occurs in PEGMATITES, evolved into the more substantial limbs of and is used as a source of lithium. the Amphibia. Amphibian eggs remain un- protected and must be laid in water, where amethyst A transparent purple variety the animals, too, spend their early life as of QUARTZ, used as a semiprecious gem- tadpole larvae before undergoing meta- stone. The color is due to impurities of iron morphosis into the terrestrial adults. The oxide. See silica minerals. Amphibia were abundant in Carboniferous swamps, some reaching the size of modern ammonite /am-ŏ-nÿt/ One of the more crocodiles, but the class was in decline by advanced mollusks of the subclass AM- the end of the Paleozoic. Modern represen- MONOIDEA, whose shells had extremely tatives include newts, salamanders, toads, convoluted crinkled suture lines. They are and frogs. Amphibians were the ancestors known from the Permian, are valuable as of the reptiles. Mesozoic zone fossils, and became extinct at the end of the Cretaceous Period. amphibole /am-fă-bohl/ Any member of a group of rock-forming minerals that have Ammonoidea /am-ŏ-noi-dee-ă/ An ex- (Si,Al)O4 tetrahedra linked to form a tinct subclass of marine mollusks of the double chain. PYROXENES are a similar class CEPHALOPODA. Ammonoids had exter- group but with a single chain structure. nal shells, usually coiled in a plane spiral The general amphibole formula is and divided by septa into chambers. The X23Y5Z8O22(OH)2, where X = Ca, Na, K, shells ranged from about 25 mm in diame- Mg, or Fe2+, Y = Mg, Fe2+, Fe3+, Al, Ti, or

15 amphibolite

Mn, and Z = Si or Al. The hydroxyl ions is an index mineral of the glaucophane may be replaced by F, Cl, or O. Most am- schist facies. phiboles are monoclinic but anthophyllite and gedrite are orthorhombic. Subgroups amphibolite /am-fib-ŏ-lÿt/ A metamor- are based on the dominant cation occupy- phic rock consisting predominantly of am- ing the X position. phibole. See also metamorphic facies. 1. anthophyllite-cummingtonite subgroup: anthophyllite amphidromic system /am-fă-drom- 2+ (Mg,Fe )7(Si8O22)(OH,F)2 ik/ A type of tide or tidal system in which gedrite the high water rotates around a central 2+ point (the amphidromic point). It is one (Mg,Fe )6Al((Si,Al)8O22)(OH,F)2 cummingtonite consequence of the modifying influence 2+) that the Earth’s rotation has on a standing (Mg,Fe 7(Si8O22)(OH)2 grunerite oscillation. The range of the tide is nil, or 2+ 2+ very small, at the amphidromic point itself (Fe )4(Fe ,Mg)3(Si8O22)(OH)2 2. hornblende subgroup: but increases outward from the point. The times of low and high water progress in a tremolite Ca2Mg5(Si8O22)(OH,F)2 actinolite counterclockwise or clockwise direction 2+ around the amphidromic point. In the N Ca2(Mg,Fe )5(Si8O22)(OH,F)2 hornblende hemisphere, high water rotates counter- 2+ 3+ clockwise round the central point. NaCa2(Mg,Fe ,Fe ,Al)5((Si,Al)8O22) (OH,F) 2 See annual maximum series. edenite AMS NaCa (Mg,Fe2+) (Si AlO )(OH,F) 2 5 7 22 2 amygdale /ă-mig-dayl/ A spheroidal or hastingsite ellipsoidal VESICLE within a lava, filled with NaCa (Fe2+,Mg,Al,Fe3+) (Si Al O ) 2 5 6 2 22 deuteric or secondary minerals often in a (OH,F) 2 zonal arrangement. Typical amygdaloidal kaersutite minerals include calcite, zeolites, and Ca (Na,K)(Mg,Fe2+,Fe3+) Ti(Si Al O ) 2 4 6 2 22 quartz. (O,OH,F)2 3. alkali amphibole subgroup: anabatic wind /an-ă-bat-ik/ An upslope glaucophane breeze often developing when mountain Na2,Mg3,Al2(Si8O22)(OH,F)2 slopes are heated by the Sun during calm riebeckite conditions. As turbulence is greater during 2+ 3+ Na2,Fe3 Fe2 (Si8O22)(OH,F)2 the day it is more often suppressed than the richterite night-time equivalent, the KATABATIC WIND. 2+ 3+ Na2Ca(Mg,Fe ,Mn,Fe ,Al)5(Si8O22) (OH,F)2 anafront /an-ă-frunt/ Any frontal sur- katophorite face at which the warm air is rising. As air 2+ 3+ Na2CaFe4 (Fe ,Al)(Si7AlO22)(OH,F)2 cools on rising, condensation and precipi- The angle between the prismatic cleav- tation are more extensive with this type of ° ages of amphiboles is 124 , the corre- front than on a KATAFRONT where air is de- sponding angle for pyroxenes being 87°. scending. Some amphiboles occur in fibrous forms; asbestos is a fibrous form of actinolite. analcime /an-al-sim, -sÿm/ (analcite) See Members of the anthophyllite-cumming- feldspathoid. tonite subgroup together with tremolite, actinolite, and hornblende occur in meta- analogs Similar patterns of the surface morphic rocks. Hornblende is also com- atmospheric pressure field that occur at mon in igneous rocks. The alkali different times or different places. The amphiboles occur in alkali igneous rocks basic assumption in analog forecasting is with the exception of glaucophane, which that if two pressure situations are identical

16 anemometer then the weather sequences that followed and are associated with basalts and rhyo- the first occasion will also follow the sec- lites in island arcs and orogenic regions. ond. However, since analogs are never Members of the alkaline basalt–trachyte identical in all important features, only suite containing dominant oligoclase and general trends can be deduced. andesine are called mugearite and hawaiite respectively. These rocks may be distin- anatase /an-ă-tayss/ A brown to black guished from andesites by their basic gen- tetragonal polymorph of titanium dioxide, erally undersaturated compositions, higher TiO2, found in vein deposits and peg- olivine contents, and differing field associ- matites. See also brookite; rutile. ations. Classification of volcanic rocks is often very difficult in petrographic terms anatexis /an-ă-teks-iss/ The partial melt- because of the fine grain size, and divisions ing of rock, which can then change compo- are made on a chemical basis. sition by being contaminated, by mixing with other rocks, or by MAGMATIC DIFFER- Andesite line A boundary that delimits ENTIATION. See also assimilation. those parts of the Earth’s surface that are of true oceanic structure as compared with anauxite /an-awks-ÿt/ A claylike alumi- true continental structure. The line can be nosilicate mineral, Al2(SiO)7(OH)4. It re- drawn throughout the Pacific Ocean: on sembles kaolinite in composition (see clay the ocean side of it are volcanic rocks that minerals). are entirely basic in character, but no in- situ volcanic rocks of the continental type. andalusite /an-dă-loo-sÿt/ A hard pink, In the case of the W coast of North and gray, or brown mineral form of aluminum South America, the Andesite line runs par- silicate, Al2SiO5. It crystallizes in the or- allel to and comparatively close to the thorhombic system and occurs in meta- coast. morphic rocks such as gneiss and schist. It is used as a refractory and as a semi- andisol /an-dŏ-sôl/ A soil that has devel- precious gemstone. See also aluminum sili- oped recently on base-rich volcanic materi- cates. als; one of the twelve soil orders of the US SOIL TAXONOMY. The upper A horizon of andesine /an-dĕ-zin/ A variety of plagio- these soils is dark owing to the presence of clase FELDSPAR. organic matter that is well mixed by earthworm activity. This is a low-density friable andesite /an-dĕ-zÿt/ The fine-grained layer and it overlies a more compact B volcanic equivalent of DIORITE, oversatu- horizon in which there has been little ap- rated or saturated in composition (see preciable change in the content of clay. silica saturation). These rocks are charac- With increased duration of soil formation terized by the presence of plagioclase in the these soils become deeper and profile dif- range oligoclase-andesine, often occurring ferentiation becomes more apparent. See as strongly zoned phenocrysts. The mafic also inceptisol. minerals, augite, hypersthene, hornblende, biotite, and olivine, occur both as phe- andradite /an-dră-dÿt/ A yellow, green, nocrysts and in the groundmass. Small or brown type of GARNET, Ca3Fe2(SiO4)3. It amounts of quartz and alkali feldspar may occurs in metamorphosed limestones and also be present. With an increase in the is used as a semiprecious gemstone. amount of quartz, andesites pass into DACITES and with an increase in the pro- anemometer /an-ĕ-mom-ĕ-ter/ An in- portions of alkali feldspar, into LATITES strument for measuring wind speed and (trachyandesites). Most andesites are por- often wind direction. The most common phyritic and have pilotaxitic or trachytic type, the cup anemometer, has three cups textures. Andesites are the intermediate mounted symmetrically about a vertical ro- members of the calc-alkaline volcanic suite tating axis, so that the rate of rotation is

17 aneroid barometer proportional to the wind speed. Wind can angle of magnetic inclination See dip also be measured by the pressure it exerts (def. 2). on a plate or in a tube, or by the degree of cooling of a hot wire. angle of reflection The angle between a light ray reflected from a surface and the aneroid barometer /an-ĕ-roid/ The normal to that surface at the point of re- most commonly used instrument for mea- flection. Other types of waves may also be suring air pressure. In its simplest form it reflected. consists of a corrugated metal box from which the air is evacuated. The top and angle of refraction The angle between bottom of the box are separated by a a refracted light ray and the normal from the surface at which the ray is refracted at spring and changes of atmospheric pres- the point of refraction. Other types of sure produce movements in the distance waves may also be refracted. apart of the faces. The changes are then amplified by a series of levers and can be angle of rest (angle of repose) The char- recorded on a chart. See also barometer. acteristic maximum angle of slope at which a pile of unconsolidated material is stable, angiosperms /an-jee-oh-spermz/ The the angle depending on the particle size of flowering plants. They differ from the the material. Above that angle, the slope GYMNOSPERMS in that their seeds are pro- will be unstable and subject to slides and tected by an outer casing known as a flow, which will reduce the angle of slope carpel. Angiosperms are thought to have until it is again stable. arisen in the Triassic; they became com- At rest, the particles interdigitate with mon during the Cretaceous and by the end each other, so that the weight of each one of this period had replaced the gym- is balanced against the one below. As the nosperms as the dominant land plants, a angle becomes too steep, the particles up- position they still hold today. They occupy slope tend to lack support at the base and a great variety of terrestrial and freshwater slide over the particles in front, moving environments and range in size from small downslope until the debris pile is again bal- herbaceous plants to giant trees. An- anced. Larger material has a higher angle giosperms form an important source of of rest than fine material; in most rocks ° food and habitats for both birds and mam- and sands the angle is 30–35 . Where the mals, reflected in the fact that these groups, material forming a slope is unconsolidated, too, began their main evolutionary radiation during the Cretaceous. The evolution and spread of the grasses, probably during the Miocene, had a particularly important effect on the environment, and therefore on animal life. There are two main divisions of angiosperms, the classes Monocotyledoneae talus slope and Dicotyledoneae. (constant slope, colluvial slope) angle of declination (magnetic declina- rock wall tion) The angle between geographical north and the direction of the magnetic meridian. angle of incidence The angle between a angle of rest scree or alluvium light ray arriving at a surface and the normal to that surface at that point. Angle of rest

18 annual maximum series its angle of rest determines the angle of anhedral /an-hee-drăl/ Describing crys- slope; such slopes are called talus slopes, tals having irregular boundaries, and hav- constant slopes, or colluvial slopes (see col- ing no crystal faces. Compare euhedral; luvium). subhedral. anglesite /ang-glĕ-sÿt/ A white mineral anhydrite /an-hÿ-drÿt/ An orthorhom- form of lead sulfate, PbSO4. Generally bic mineral form of calcium carbonate, formed by the oxidation of GALENA, it crys- CaSO4. It is usually white or colorless, oc- tallizes in the orthorhombic system. It is a curring in massive or fibrous forms in common secondary mineral, used as a evaporite deposits. See also gypsum. source of lead. anisotropic /an-ÿ-sŏ-trop-ik/ Having dif- angular momentum The product of ferent physical properties when measured the linear velocity of a point on the Earth’s in different directions. Crystals belonging surface, as it moves from west to east to systems other than cubic are optically owing to the Earth’s rotation, and the per- anisotropic and light is transmitted at dif- pendicular distance of that point from the fering velocities along different directions Earth’s axis of rotation, a line through the within each crystal. When a ray of light en- north and south poles. Its value is greatest ters an anisotropic crystal it is split into at the Equator, where the perpendicular two rays, the ordinary and extraordinary distance is at a maximum, and zero at the rays, which travel in different directions poles. and are polarized in perpendicular planes. Such crystals are said to exhibit double refraction or BIREFRINGENCE. angular unconformity A plane of erosion, generally related to earth movements, ankaramite /ank-a-ră-mÿt/ A basaltic marking the boundary between two rock rock containing a high proportion of units of different age. The beds above and augite phenocrysts. Compare oceanite. below the plane of erosion have different angles of dip, the lower beds generally dip- ankerite /ank-ĕ-rÿt/ A gray to brown va- ping more steeply. See also unconformity. riety of the mineral DOLOMITE that contains Compare disconformity. iron, Ca(Fe,Mg,Mn)(CO3)2. See also carbonate minerals. angular velocity The rate of turning of a rotating body, expressed in degrees, radi- Annelida /a-nel-ă-dă/ The phylum of an- ans, or revolutions per unit of time. In me- imals containing the true worms, such as teorology, it can be used to indicate the the common earthworm, whose bodies are horizontal rate of rotation around an area divided into a number of segments. An- of low pressure or, on a large scale, the ro- nelids of one group secrete calcareous tation of the Earth. As the Earth makes one tubes, and fossil specimens of these exist, complete rotation in a day, its angular ve- but most annelids are soft bodied and not locity is 15° longitude per hour, or 7.29 × often preserved after death. Certain small 10–5 radians per second. siliceous or chitinous remains known as scolecodonts, occurring in rocks of Or- angulate drainage /ang-gyŭ-lit, -layt/A dovician age onwards, are thought to be drainage pattern in which streams are de- annelid jaw parts. The effects of worms are veloped equally in two directions, like the often seen in rocks as TRACE FOSSILS, trails, RECTANGULAR DRAINAGE pattern, but in this burrows, or other characteristic distur- case the streams meet at an acute angle bances of the sediment, features present rather than right angles. The pattern re- even in some Precambrian rocks. sults from structural guidance by two sets of joints or faults in an area with homoge- annual maximum series (AMS) (in hy- neous strata. drology) A series of data that consists of

19 anomaly the event with the largest observed magni- along the sea floor, below other water tude (e.g., the largest observed flow of a masses. See bottom water. See also Antarc- river) that occurred in each year. tic intermediate water; Arctic bottom water. anomaly 1. (in climatology) The departure of any climatic element from the long- Antarctic Circle The line of latitude period average for that location. The 66°30′S. In December there is continuous spatial distribution of an anomaly is an in- daylight for 24 hours and in June continu- teresting feature because it helps provide a ous darkness for 24 hours along this line. greater understanding of the interaction between the atmosphere and the surface. A Antarctic high See polar high. good example of a temperature anomaly is the area of NW Europe where mean annual Antarctic intermediate water (AIW) A temperatures are much higher than the lat- cold and moderately saline water mass that itudinal average by as much as 12°C in forms around Antarctica between 45° and parts of coastal Norway. 55°S and flows north to the Antarctic con- 2. (in geophysics) The difference between vergence zone where it sinks below the less observed and computed geophysical val- saline Subantarctic surface water. It can be ues, especially with regard to the thermal, traced at depths of between 700 and 1000 gravitational, and magnetic properties of m in the North Atlantic. the Earth. See gravity anomaly. antecedence /an-tĕ-see-dĕns/ A type of anorthite /an-or-th’ÿt/ A calcic plagio- DISCORDANT DRAINAGE, originating when a clase FELDSPAR. preexisting drainage pattern cuts down into a rising fold, or a series of rising folds, anorthoclase /an-or-th’ŏ-klayss/ A vari- in its path. The cause of the folding will be ety of alkali FELDSPAR. tectonic, and in the best developed examples, the rate of uplift of the folds is anorthosite /an-or-th’ŏ-sÿt/ A type of equaled by the rate of downcutting of the coarse-grained leucocratic igneous rock streams, eventually leaving them in deep that occurs as vast intrusive bodies or as CANYONS. As an explanation of discor- members of layered basic complexes. Pla- dance, it is less commonly used than SUPER- gioclase feldspar constitutes over 90% of IMPOSITION, but is often used to explain the anorthosite and is generally of labradorite origin of streams cutting through moun- composition but anorthosites composed of tain ranges in deep gorges, e.g. the streams plagioclase ranging from andesine to anor- that cut through the Himalayas. thite are known. Minor mafic minerals include augite and hypersthene and with an anthophyllite /an-th’off-ŏ-lÿt/ An ortho- increase in the amount of mafic minerals, rhombic mineral of the AMPHIBOLE group. anorthosites pass into gabbros. Anthozoa /an-thŏ-zoh-ă/ A class of the Antarctic bottom water (AABW)A phylum CNIDARIA that includes the sea cold and extremely saline water mass, the anemones and CORALS. The corals are geo- most dense water of the global oceans, that logically the most important anthozoans forms in the Southern Ocean along the because many of them possess hard parts coast of Antarctica, especially in the Wed- that may be fossilized. Their evolution may dell and Ross Seas, during deep winter con- be used in BIOSTRATIGRAPHY and the colo- vection. Cooling and freezing of the nial forms are important reef-builders. surface sea water to form pure ice releases Typically, corals secrete a cup (corallum), salt into the water beneath the ice. This from whose wall (theca) supporting plate- cold dense water sinks, carrying oxygen like septa project radially toward the cen- and nutrients down to the bottom of the ter. The Tabulata (tabulate corals) are Antarctic Ocean, and flows northward known from the Ordovician Period to the

20 anticyclonic gloom

Jurassic, although the group had greatly anticyclone /an-tee-sÿ-klohn/ (high)An declined by the end of the Paleozoic. They area in the air of higher pressure than the were colonial forms having only rudimen- surrounding air, with a closed isobar of ap- tary septa or none at all. The walls were re- proximately circular form at its center. inforced by tabulae, calcareous plates Winds circulate in a clockwise manner extending across each coralite. The Ru- around the high pressure center in the N gosa, or Tetracoralla, (rugose corals) are hemisphere, and counterclockwise in the S first known from the Ordovician and are hemisphere, but are generally light. Once confined to the Paleozoic. Their septa established, the anticyclone moves only occur in multiples of four and are strength- slowly and normally is a significant feature ened by many small plates known as dis- of surface pressure charts for a much sepiments. The group included both longer period than cyclones. Anticyclones solitary and colonial species. The Sclerac- can be subdivided into two categories, cold tinia, or Hexacoralla, includes the modern and warm. Cold anticyclones are shallow reef-building corals, whose septa are in features produced by strong radiational multiples of six. They range from the Tri- cooling at the surface or in the cold air be- assic Period onwards. hind a depression. The tropopause tends to be low above a cold anticyclone. Far more anthracite /an-thră-sÿt/ Coal that has a important are the warm anticyclones, very high fixed carbon content and a low which are characterized by warmer tem- amount of volatiles. peratures throughout a deep troposphere, with a cold stratosphere above. Their most anthraxolite /an-thraks-ŏ-lÿt/ A hard frequent locations are in the subtropical type of BITUMEN that occurs in sedimentary high-pressure belts, but they do move pole- rocks. It is often found associated with OIL ward and block the normal westerly cir- SHALES. culation producing temperature and precipitation anomalies in these areas. De- anthropomorphic soil /an-thrŏ-pŏ-mor- scending motion is characteristic of anticy- fik/ An intrazonal soil that has been clones. They represent the parts of the formed as a direct result of human activity. atmosphere in which air slowly subsides Farming practices through the ages have, from higher levels, warming and drying as in some cases, produced distinctive soils or it does so. This has the effect of stabilizing distinctive surface horizons. An important the atmosphere within an anticyclone so anthropomorphic soil is the paddy soil. that rain is infrequent. In the surface layers, This is similar to a gley and its features divergence of airflow takes place to main- have been formed by the process of alter- tain the continuity of subsidence. Compare nate wetting and drying, which has been cyclone. controlled by farmers. Many soils have anthropic EPIPEDONS (surface horizons) or anticyclonic gloom /an-tee-sÿ-klon-ik/ A plaggen epipedons. The former have a high condition of low visibility; it occurs under phosphate content and the latter a high or- anticyclonic conditions when the layer of ganic matter content. air near the ground undergoes little descent and, if moist, can easily become saturated anticline /an-tee-klÿn/ An arch-shaped to give a uniform layer of cloud. This is a fold into which rock strata have been com- stable situation with the cloud slowly gain- pressed, the oldest rocks occurring in the ing in thickness through radiational cool- core. See fold. ing of the layer aloft. Above this level the air is much warmer due to descent; a tem- anticlinorium /an-tee-klÿ-nor-ee-ŭm,-noh perature INVERSION and stability are main- -ree/ A large-scale regional feature, many tained. At the surface, little light penetrates kilometers in diameter, consisting of an an- the cloud and pollution can be trapped to ticlinal structure with several minor folds give poor illumination. If prolonged, these on its limbs. circumstances can give high pollution con-

21 antiform centrations, which may reach dangerous apatite /ap-ă-tÿt/ A hexagonal phos- levels. phate mineral of composition Ca5(PO4)3- (OH,F,Cl), the commonest phosphorus antiform /an-tee-form/ An upward-clos- mineral. It may be formed as an accessory ing structure, the precise stratigraphic rela- mineral in igneous rocks of most composi- tionships of whose strata are not known, tions. Apatite also occurs as a detrital min- i.e. the core need not be the oldest rocks. It eral in sedimentary rocks and in results from complex folding in orogenic metamorphic rocks. areas. See fold. aphanitic /af-ă-nit-ik/ Denoting an ig- antigorite /an-tig-ŏ-rÿt/ A mineral form neous rock that is so fine-grained that indi- of hydrated iron-containing magnesium vidual crystals cannot be resolved with the silicate, (Mg,Fe)3Si2O5(OH)4, a variety of naked eye. Compare phanerocrystalline. SERPENTINE. It occurs as fibers or undulating plates. aphelion /ă-fee-lee-ŏn/ The point on the orbit of a planet that is farthest from the antiperthite /an-tee-per-th’ÿt/ An inter- central axis of rotation. Compare perihe- growth of orthoclase and albite feldspar in lion. which the orthoclase occurs as patches in the albite host. See also exsolution; aphotic zone /ay-foh-tik/ Ocean depths perthite. below the maximum depth at which photosynthesis takes place because of lack of antithetic fault A minor normal fault light, stretching from 200 m below sea associated with a major fault but whose level to the sea floor. See also disphotic planes dip into the main fault plane. zone; euphotic zone. antitrades /an-tee-traydz/ Originally con- aphyric /ă-fi-rik, -fÿ-/ Denoting an ig- sidered to be the returning flow in the neous rock that is not PORPHYRITIC. upper atmosphere for the surface TRADE WINDS, antitrades now more generally indi- /ap-lÿts/ Leucocratic acid igneous cate the airflow above the NE or SE trade aplites winds, blowing in a general westerly direc- rocks of medium to fine grain size occur- tion at heights of about 1800 m. ring as thin veins and dikes around granite intrusions. Aplites consist largely of quartz antitriptic wind /an-tee-trip-tik/ A wind and alkali feldspar and have micrographic in which the forces controlling air move- or SACCHAROIDAL textures. They are ment are the pressure gradient force and thought to have crystallized from residual friction. The CORIOLIS EFFECT and cen- liquids lower in volatiles than pegmatites. tripetal acceleration are neglected either because of the small scale of the system, apophyllite /ă-poff-ă-lÿt, ap-ŏ-fil-ÿt/A such as a sea breeze, or because the Corio- rare mineral of composition KFCa4(Si8O20). lis effect is negligible, as it is near the Equa- 8H2O found in AMYGDALES in basalts. tor. apparent dip The angle between a anvil A CUMULONIMBUS cloud in which structural surface, e.g. a bedding plane, the top reaches the tropopause where, as a and the horizontal, measured vertically in result of a temperature inversion, or strong any direction except that which is at 90° to wind shear, the main cloud updrafts are de- the strike of the surface. This results in a flected horizontally causing the ice crystals value of dip that is less than that of true DIP. at that level to spread out. From the ground, the whole cloud has the appear- appinite /ap-ă-nÿt/ A melanocratic vari- ance of a blacksmith’s anvil. ety of DIORITE.

22 Arctic bottom water applanation /ap-lă-nay-shŏn/ The pro- Archean /ar-kee-ăn/ See Precambrian. cesses that reduce the heights of land features and cause an area to become more archipelagic apron /ar-kă-pe-laj-il/ The like a plain. It results from the erosion of moatlike rock filling that may surround a high areas and the deposition of sediments group of volcanic islands or seamounts. in low areas. The moat filling is usually gently sloping and smooth, the upper surface being called apron A deposit of unconsolidated frag- an archipelagic plain. These groups of is- ments that forms a broad extension in lands or seamounts probably depress the front of a glacier or at the base of a moun- Earth’s crust to the extent that a moat or tain. depression is developed, but the depression may be concealed by a sedimentary over- apron reef See reef. lay. Part of the Hawaiian Islands display a moatlike form surrounding them. aquamarine A transparent blue-green variety of BERYL that occurs, sometimes as A group of islands in fairly very large crystals, in pegmatite. It is val- archipelago ued as a semiprecious gemstone. close proximity. The term is sometimes used to describe sea areas that contain nu- aquatic Describing any organism that is merous scattered islands. found in water. In an aquatic environment temperatures generally vary little, dehydra- Archosauria /ar-kŏ-sor-ee-ă/ A subclass tion is virtually impossible, and the water of the Reptilia consisting principally of the provides physical support for plants and extinct DINOSAURS and PTEROSAURIA and the invertebrate animals. modern crocodiles. Primarily carnivorous, with simple pointed teeth, they show a ten- aquifer /ak-wă-fer/ Any water-saturated dency to bipedalism involving develop- rock horizon that has sufficient porosity ment of the hind limbs, a corresponding and permeability to yield economic sup- reduction of the fore limbs, and a well-de- plies of groundwater, either as springs or in veloped tail for balancing the body. Ar- wells. chosaurs became the dominant form of terrestrial life during the Jurassic and Cre- aragonite /a-ră-gŏ-nÿt/ A white or gray taceous periods; some became secondarily mineral form of calcium carbonate, quadrupedal and vegetarian. CaCO3. It crystallizes in the orthorhombic system and occurs near the surface in sedi- Arctic bottom water (ABW) A deep mentary rocks and in deposits in caves and water mass in the Arctic Ocean. It origi- from hot mineral springs. It also occurs in nates partly from the Arctic shelf regions; PEARLS and in the shells of some inverte- the freezing of sea water to form sea ice re- brate marine animals. See also carbonate leases salt to the water below. The in- minerals. creased density of this water resulting from the salinity causes it to sink to the seafloor. Archaeopteryx /ar-kee-op-tĕ-riks/ The earliest known bird, whose fossils come It mixes with deep water that forms during from late Jurassic strata. Good fossil winter in the Greenland Sea when cooling preservation reveals that Archaeopteryx of surface water leads to vertical convec- had feathers but in its skeletal structure it tion. Submarine topographic barriers, such resembled the archosaur reptiles from as the Bering Sill, restrict the water mainly which it had evolved. The wings were to the bottom of the Arctic Ocean but some primitive and the three separate fingers had does intermittently flow through narrow claws. Other reptilian features included channels to enter the Atlantic from around teeth, a long tail, and solid bones (compare Greenland. See bottom water. See also Aves). Its flying ability was probably poor. Antarctic bottom water.

23 Arctic Circle

Arctic Circle The line of latitude arc-trench gap At a destructive PLATE 66°30′N. In June there is continuous day- BOUNDARY, a region 50–400 km wide be- light for 24 hours and in December contin- tween an ocean trench and a volcanic arc, uous darkness for 24 hours along this line. often associated with an ISLAND ARC. Gen- erally the wider the gap, the faster is the Arctic high See polar high. rate of convergence between the two tectonic plates involved. Arctic oscillation Periodic fluctuations in atmospheric pressure that occur in the arcuate delta /ar-kyû-it,-ayt/A DELTA mid-latitudes and Arctic above about with a rounded convex margin. 45°N. In a ‘positive phase’ of the oscillation there is a strengthening of the counter- arenaceous /a-rĕ-nay-shŭs/ Describing a CLASTIC sedimentary deposit or rock in clockwise polar vortex extending from the which the constituent fragments are of lower stratosphere to the surface. The Arc- sand grade in size. This includes particles tic surface air pressure decreases while from 0.06 mm to 2 mm in diameter. Are- there is higher pressure at mid-latitudes. naceous rocks are also referred to as SAND- The North Atlantic storm tracks bring rain STONES. The grains are often fragments of and mild temperatures to N Europe while crystals, although lithic sandstones are conditions are drier in the W USA and known (see graywacke). Mediterranean. In the ‘negative phase’ the strength of the vortex decreases and there arenite /a-rĕ-nÿt/ 1. A type of SAND- is comparatively high pressure over the STONE that has little or no matrix material Arctic region and low pressure in the mid- binding the grains together. latitudes. Cold air from the Arctic extends 2. A sedimentary rock that consists of down into the USA and Europe and Asia. sand-sized grains (up to 2 mm across). The During the1980s and 1990s the positive grains need not be silica or silicates, and phase was dominant. The Arctic oscillation may include fragments of carbonate rocks. is closely related to the NORTH ATLANTIC OSCILLATION. arête /a-ret/ A sharp ridge bounded by steep slopes and found in glaciated or for- Arctic sea smoke A type of fog formed merly glaciated areas. These forms seldom by evaporation from a relatively warm sea develop in isolation and headwall erosion surface into cold air aloft. Condensation of two adjacent cirques causes the forma- takes place in the cold air, but unless an in- tion of an arête. Continued headwall reces- version develops, further mixing prevents sion will result in the destruction of the the extensive development of fog. In the arête and the cirques will merge. Aleutian Islands, fog depths up to 1500 m argentite /ar-jĕn-tÿt/ A mineral form of may occur under suitable conditions. silver sulfide, Ag2S. It crystallizes in the cubic system but is stable only at high tem- Arctic warming The marked warming peratures; below 180°C it changes to the of the Arctic area that took place between monoclinic mineral acanthite. It is the most the l920s and 1950s. The ice-free period in- important ore of silver. creased and mean annual temperatures ° rose by about 4–10 C. It was primarily the argillaceous /ar-jă-lay-shŭs/ Describing result of the more northerly tracks of the a CLASTIC sedimentary deposit or rock in Atlantic and Pacific depressions carrying which the constituent fragments are of SILT moist warm air toward the poles. Although or CLAY grade in size. This includes parti- the amount of warming has decreased con- cles smaller than 0.06 mm in diameter. siderably, Arctic temperatures are still These consist of finely ground rock as well higher than pre-1920 and may be rising as the various clay minerals that have been again. produced in the course of weathering of the

24 artesian basin parent rock. SILTSTONES and MUDSTONES arkose /ar-kohss/ An ARENACEOUS sedi- are rocks formed of sediment in this size mentary rock that includes more than 25% range. FELDSPAR in its composition. argillite /ar-jă-lÿt/ A slatelike sedimen- Armorican orogeny /ar-mor-ă-kăn/A tary rock that is formed from shale or mud- period of mountain-building during the stone by the effects of pressure and Upper Paleozoic affecting W Europe, cementation. Unlike slate, however, it has named after Armorica (Brittany). It is part no distinct cleavage planes. of the more complex VARISCAN orogeny but characterized by a roughly northwest- argon An inert gas comprising about southeast trend. 1% of the atmosphere by volume. It has no meteorological significance. arrival time The first recording of a seismic disturbance. Close to an earth- arid climate A climate that experiences quake, there is little difference in the arrival a moisture deficiency sufficient to inhibit times of primary (P) waves and secondary but not prevent natural vegetative growth (S) waves. Farther away, the faster P waves and a mean annual rainfall usually taken as arrive first. The distance to the earthquake below 250 mm. Attempts to produce a pre- can be calculated from this difference in ar- cise definition of aridity involve an assess- rival times. See primary wave; secondary ment of the efficiency of precipitation. Arid wave. climates are found in areas of semipermanent anticyclones, where cold ocean cur- arroyo /ă-roi-yoh/ An ephemeral stream rents stabilize the lower atmosphere, of the semiarid USA and Latin America. within extensive mountain ranges, and in Arroyos originate as discontinuous gullies locations at enormous distances from the on a hillside where the vegetation has been sea. See also desert. locally weakened by trampling, grazing, or fire. Headward erosion during periods aridisol /ă-rid-ă-sôl/ One of the twelve when the gullies contain water leads to co- soil orders of the US SOIL TAXONOMY, including saline and alkaline mineral soils of alescence, and the formation of a continu- desert areas. They are characterized by low ous gully, or arroyo. See also abnormal organic matter contents and a horizon of erosion; gully erosion. calcium or sodium accumulation within 1 m of the surface. They are infertile because arsenopyrite /ar-sĕ-noh-pÿ-rÿt/ An or- of lack of moisture, coarse particle size, thorhombic arsenic mineral of composi- and their susceptibility to erosion due to tion FeAsS, found in hydrothermal veins. lack of vegetation. In some profiles, groundwater may concentrate sodium to artesian /ar-tee-zhăn/ Describing water toxic proportions, forming a salic horizon that has moved underground from its origin which sodium salts exceed 2% of the inal source. This may occur by percolation mineral matter. The aridisols include true upward along a sloping AQUIFER with the desert soils, sierozem, solonchak, and result that the artesian water is above the solonetz. level of the water table. aridity index /a-rid-ă-tee/ An assess- artesian basin A SYNCLINE that has a ment of the degree of dryness of a climate. layer of permeable rock between two im- There are a variety of such indices, the best permeable layers. Water can be obtained known being devised by the American cli- by drilling boreholes into the permeable matologist–geographer C. Warren Thorn- layer. If this layer is below the water table thwaite (1899–1963) in 1948. Most in nearby hills, the water may flow up the involve the relationships between tempera- borehole under hydrostatic pressure, like a ture, total rainfall, and humidity. fountain, forming an artesian well.

25 Arthropoda

Arthropoda /ar-throp-ŏ-dă/ The phy- down the side of a volcano. The resulting lum of animals having jointed limbs and a sediment may be partly blocky, but also segmented body protected by a chitinous contains tuffs where shards of glassy lava, exoskeleton. The Arthropoda is the largest carried by the flow, have become welded phylum in the animal kingdom and its six together. most important classes are the CRUSTACEA, Arachnida (spiders and scorpions), Myri- aspect The direction in which a slope or apoda (centipedes and millipedes), Mero- surface faces, expressed usually in compass stomata (king crabs and the extinct directions such as degrees from north in a eurypterids), INSECTA, and the extinct clockwise direction. TRILOBITA. Fossils of arthropods represent mainly aquatic benthonic forms, which asphalt /as-fawlt, -falt/ A type of brown lived in the most favorable environments or black BITUMEN consisting mainly of car- for preservation; such arthropods are bon disulfide (CS2) and hydrogen. It varies thought to have been recognized in the from a thick viscous liquid to a tarry solid. Cambrian, perhaps even the Precambrian. It occurs in oil-bearing rocks and probably represents an early stage in the formation artificial satellite See satellite. of PETROLEUM from buried marine organisms. A similar substance remains after the Artiodactyla /ar-tee-oh-dak-tă-lă/ The distillation of crude oil. order of herbivorous hoofed mammals comprising those with an even number of assimilation /ă-sim-ă-lay-shŏn/ The in- toes, such as antelopes, cattle, deer, and corporation of rock into a body of magma. pigs. Originating in the Eocene and at first During intrusion, magma forces its way outnumbered by early perissodactyls, they along joints and cracks in the country or are now the more successful group. The wall rock so that large blocks (XENOLITHS) most primitive members are the pigs, may become detached and sink into the which still possess four toes. In the rumi- magma. This process, known as stoping, nants the limbs are developed for fast run- may occur on a large scale when large vol- ning with the loss of all but two of the umes of magma are emplaced. The com- digits, and the teeth are modified to deal plex interactions taking place between the with a coarse vegetable diet. Compare Perissodactyla. magma and solid rock are known by the term syntexis. Melting of the rock and chemical reaction with the magma may asbestos A fibrous variety of AMPHI- result in the complete digestion or assimi- BOLE, usually tremolite or actinolite. The SERPENTINE mineral chrysotile is used in the lation of the incorporated material. Xeno- manufacture of commercial asbestos. liths survive only when assimilation is incomplete. Contaminated magma that has aseismic plate /ay-sÿz-mik/ Any of the assimilated large quantities of country rock large areas of the Earth’s crust above the is said to be hybrid. Hybridization also oc- asthenosphere, within which there are rel- curs when two different magmas are atively few earthquakes. mixed. That such a process has taken place may be indicated by the occurrence of cor- aseismic ridge A ridge on the seabed roded XENOCRYSTS within an igneous rock. that, unlike a MID-OCEAN RIDGE, has no volcanic activity along it. asterism /ass-ter-iz-ăm/ An optical phenomenon displayed by some crystals that ash See pyroclastic rock. produce starlike flashes of light. Caused by tiny needle-shaped inclusions, it may occur ash flow deposit A volcanic deposit re- in transmitted light (as with some forms of sulting from a NUÉE ARDENTE, in which an mica) or in reflected light (as with star sap- avalanche of glowing ash flows rapidly phire).

26 atmospheric window asthenosphere /ăs-th’en-ŏ-sfeer/ A zone the stratosphere to about 50 km, decrease within the Earth’s upper MANTLE in which through the mesosphere reaching values of the velocity of seismic waves is consider- about –90°C between 80 and 90 km, and ably reduced. Movement between the finally there is the thermosphere or ionos- Earth’s outer lithosphere and inner meso- phere where absorption of ultraviolet radi- sphere is thought to take place along this ation of shorter wavelengths than that zone, which is capable of prolonged plastic absorbed by ozone causes a rise of temper- deformation. It is thought to be composed ature to values of 1500 to 2000°C at be- of partly molten peridotite, with a liquid tween 300 and 400 km. Gases become fraction having the composition of basalt. ionized at these high levels, which is of vital It is developed between 50 and 240 km be- importance in radio communication. See il- neath the Earth’s surface. lustration overleaf. The distribution of water vapor in the astrobleme /ass-trŏ-bleem/ An ancient atmosphere is very variable. It depends crater on the Earth’s surface resulting from the impact of an extraterrestrial body. upon the amount of evaporation from the surface and the amount of moisture ad- astrogeology /ass-troh-jee-ol-ŏ-jee/ The vected from elsewhere. In continental inte- application of terrestrial geology to study- riors the value is very low. It is important ing the origin and history of extraterres- as an absorber of long-wave radiation, so trial objects in the Solar System. that moist air reduces the rate at which the ground surface cools at night by increasing asymmetric fold /ay-să-met-rik/ A fold counter-radiation. Moisture is also re- in which the axial plane is not vertical, quired in the atmosphere for precipitation, with the result that the two limbs have dif- although the absolute amount is less im- ferent angles of dip. See diagram at FOLD. portant than other factors favoring precipitation formation. Atlantic period The period from 5000 The atmosphere acts as a protection to to 3000 BC, within the present interglacial, the ground surface by burning up most cos- when the climate of much of NW Europe mic particles, which can be seen as meteors was warm and moist, achieving the highest in the night sky. See also air; general circu- mean annual temperatures since the last lation of the atmosphere. glaciation. It is also known as a CLIMATIC OPTIMUM. atmospheric pressure The pressure at the Earth’s surface that results from the atmophile /at-mŏ-fÿl/ An element that weight of the ATMOSPHERE. At sea level, occurs naturally as a gas, such as helium in standard atmospheric pressure is 760 mm some uranium ores and the various gases in of mercury, 101.3 kilopascals, or 1013.25 the ATMOSPHERE. See also chalcophile; millibars. It decreases with increasing alti- lithophile; siderophile. tude, and is measured using a BAROMETER. atmosphere The mixture of gases surrounding the Earth, which we breathe and atmospheric window The region in the which provides our weather. The atmos- radiation spectrum containing wavelengths µ phere can be divided into a number of lay- between 8.5 and 11 m, which are not ab- ers on thermal and lapse-rate properties. sorbed to any great extent by atmospheric The troposphere is the lowest, extending gases. In the absence of cloud, terrestrial up to approximately 10 to 15 km. radiation of this wavelength is lost to Throughout this layer, there is a decrease space, enabling the cooling of the Earth to of temperature with height, and it is the take place. Other narrower wavebands source of all precipitation and most of our also do not absorb long-wave radiation, weather phenomena. Above this layer, but these are of less importance in this con- temperatures gradually increase through text. See also absorption.

27 400 –

300 –

ionosphere

200 – 0.000001 height/km

°C re atu per tem

100 – 0.01 mesopause

mesosphere

50 – 0.78 stratopause – stratosphere 30 – – 55 tropopause 10 – 265 troposphere – 1013 – – – – sea level pressure/mb –100° 0° +100° +200° (millibars) temperature/°C

Atmosphere

28 autotrophic atoll A ring-shaped reef, island, or is- southern lights). The displays are most fre- lands that surround or nearly surround a quent around the geomagnetic poles, with lagoonal area of water, in which detrital a daily occurrence maximum about mid- material may collect. The surrounding rim night. There is some evidence of a seasonal may itself lie in shallow water, in which maximum about the time of the equinoxes, case the central area of water is rather but the major maximum follows the 11.2 deeper, sometimes very deep (the average year solar activity cycle. depth according to one authority is 45 m). The aurora is caused by the interaction Not all oceanic atolls are formed entirely of of the Earth’s tenuous upper atmosphere coral, and certain calcareous algae may and charged particles streaming from the constitute the bulk of the reef material. Sun. The solar particles are deflected by the Atolls are based on some kind of platform, geomagnetic field and so only occur in lim- often an eroded platform that subsided at ited parts of the upper atmosphere, at a some stage and on which coral growth or height of around 100 km. other organic growth later occurred. An atollon is a small atoll lying on the flank of autecology /aw-tĕ-kol-ŏ-jee/ A branch a larger atoll. See also reef. of ECOLOGY that studies the interaction between a single species and its environment. attenuation The loss of energy of electromagnetic waves as they pass through authigenic /aw-th’ă-jen-ik/ (authigenous) the atmosphere. It is caused by ABSORPTION Describing rock constituents that were and SCATTERING by the molecules and par- formed in situ, coming into existence dur- ticles of the atmosphere. In the ionosphere, ing or after the formation of the rock in free electrons absorb kinetic energy from which they lie. Compare allogenic. the radiation and lose it in subsequent collisions. autochthonous /aw-tok-th’ŏ-nŭs/De- scribing rocks that are still in their place of attrition The reduction in particle sizes formation and have not been displaced by of sediment by rubbing and grinding action thrusting, e.g. a folded sequence of rocks (see abrasion) during transport. whose roots are still connected. Compare allochthonous. augen /aw-jĕn/ Large eye-shaped crystals, commonly of feldspar, that have sur- autolith /aw-toh-lith/ An igneous INCLU- vived the intense shearing of schist and SION in an igneous rock. It consists of ma- gneiss taking place during the formation of terial that has crystallized from magma a MYLONITE. and is thus genetically related to the surrounding rock. See also xenolith. augite /aw-jÿt/ A monoclinic PYROXENE. automatic weather station A meteor- aulacogen /aw-lak-ŏ-jĕn/ A long depres- ological station that is equipped with in- sion on the edge of a continent bounded by struments to automatically make and FAULTS, which did not develop into an ac- record observations (e.g. air temperature, tive rift as two continental plates split relative humidity, solar radiation, baro- apart. See also rift valley. metric pressure, wind speed, and wind direction) without the necessity for an aurora A transient optical phenomenon observer to be present. The data may be usually seen in the polar skies at the time of stored at the station until retrieved by a vis- solar flare activity. The displays of the au- iting observer, transmitted at intervals, or rora can vary from shimmering rays to col- transmitted continuously to a base station. ored corona effects occupying large parts of the sky. In the N hemisphere it is called autotrophic /aw-tŏ-troff-ik/ Describing the aurora borealis (or northern lights) and an organism that produces its own ‘food’ in the S hemisphere the aurora australis (or directly from inorganic substances.

29 autumnal equinox

Chlorophyll-containing plants and algae flightless forms – the ostrich, emu, etc. – are typical autotrophs; they produce or- often of large size. ganic compounds from carbon dioxide and water during photosynthesis, using the en- axial modulus (in geophysics) The ratio ergy of sunlight. Some types of BACTERIA of stress to strain, when in the presence of are also autotrophs. laterally confining forces. autumnal equinox See equinox. axial plane A plane that passes through the successive hinge lines of the beds in a auxiliary mineral Any relatively rare FOLD. Different types of fold are character- light-colored mineral that occurs in an ig- ized by different angles of inclination from neous rock. Such minerals include APATITE, this plane. CORUNDUM, FLUORITE, and MUSCOVITE. axial plane cleavage Cleavage planes avalanche A rapid movement of snow that are parallel to the axial plane of a en masse down steep slopes, which must FOLD. Generally the cleavage is related to ° usually have an angle greater than 22 . It minor fold axes, but occasionally it may be may consist of fresh powdery snow, sliding more closely related to the region’s fold over the contact surface of compacted trend. older snow; of slabs of consolidated snow, which roll down the slope; or of the whole axial rift zone See median valley. ice and snow cover of a slope, in which case much rock material is also carried axial trace The intersection of a FOLD with the fall. If avalanches are a frequent AXIS with the Earth’s surface, reflecting the occurrence in any one location, they can be trend of the fold. (See diagram at FOLD.) important from the geomorphological point of view, because they maintain an ex- axinite /aks-ă-bÿt/ A lilac-brown tri- posed bedrock surface, which is therefore clinic mineral with the composition susceptible to continued weathering. See Ca (Mn,Fe2+)Al BO (Si O )OH, pro- also mass movement. 2 2 3 4 12 duced during the boron metasomatism of calcium-rich sediments and igneous rocks. aventurine /ă-ven-chŭ-rin/ A spangled translucent form of quartz or feldspar, used as a semiprecious stone. Its appear- axis of the Earth A line that joins the ance is caused by inclusions of particles of North and South Poles, about which the hematite, mica, or other mineral. Earth rotates every 24 hours.

Aves /ah-veez/ The birds: a class of ver- azimuth /az-ă-mŭth/ 1. (in surveying) A tebrates distinguished from the REPTILIA by horizontal angle measured clockwise from the presence of feathers. They probably true north (true azimuth) or magnetic arose from primitive archosaur reptiles (see north (magnetic azimuth) to another point. Archaeopteryx) and most of their evolu- 2. (in astronomy) The angle between the tion is connected with adaptation for plane of the meridian of the observer and flight. The wings are formed from the the vertical plane passing through a heav- whole forearm and three fused fingers enly body. (compare Pterosauria). They are warm- blooded and have short tails, a large breast azimuthal equal-area projection /ă-zim- bone for the attachment of flight muscles, ŭ-th’ăl/A MAP PROJECTION differing from and light hollow bones. All modern birds the other azimuthal projections in that the are toothless but teeth were present in their spacing of the parallels decreases with in- Mesozoic ancestors. Because of the fragile creasing distance from the center of the nature of the skeleton, fossils are few. projection, producing the equal-area prop- There have been a number of secondarily erty.

30 azurite azimuthal equidistant projection A azonal soil /ay-zoh-năl/ Soil lacking a B MAP PROJECTION that is the same as the az- horizon owing to insufficient time for com- imuthal projection in that the straight lines plete pedogenesis. Profiles of azonal soils radiating from the center of the projection therefore mainly reflect the influence of all have their true bearings, but it has the parent material and show an A horizon additional property that the distances slightly darker than the C horizon because along these lines are true to scale. It is not of additions of organic matter. The three an equal-area projection, nor is it confor- main groups are lithosols, regosols, and al- mal. luvial soils. In the US SOIL TAXONOMY azonal soils are classified as ENTISOLS. azimuthal projection (zenithal projec- Azores high (Azores anticyclone) The tion)A MAP PROJECTION constructed as though a plane were placed at a tangent to semipermanent anticyclone centered in the Atlantic near the Azores Islands. It is part the Earth’s surface and the portion of the of the subtropical high-pressure systems of Earth covered by it were projected onto the the N hemisphere. Its mean latitudinal po- plane. The result is that all points have sition oscillates, being farther south in win- their true compass bearings. The tangent ter and farther north in summer, with plane is not always drawn at the pole; it high-pressure cells occasionally drifting can be constructed anywhere on the sur- northeastward from the main center to face of the globe, the point where the tan- form blocking anticyclones within the gent touches the Earth being the center of westerly circulation. the map. azurite /azh-û-rÿt/ A deep blue mineral Azoic /ă-zoh-ik/ Designating PRECAM- form of basic copper carbonate, BRIAN rocks that were deposited before the Cu3(CO3)2(OH)2. It crystallizes in the origin of life. In practice, however, it is dif- monoclinic system, and occurs in oxidixed ficult to distinguish such rocks and the copper deposits, often associated with term is little used. It is never used for un- malachite. It is a copper ore and also a fossiliferous PHANEROZOIC strata. semiprecious gemstone.

31 B

back-bent occlusion See occlusion. autotrophic bacteria, which obtain their energy from the oxidation of mineral sub- backing In meteorology, a counter- stances and their carbon mostly from car- clockwise change in wind direction, such bon dioxide; heterotrophic bacteria, which as from easterly to northeasterly. See also obtain their energy and carbon directly veering. from the soil organic matter. Certain en- zymic transformations in the soil are dom- backshore The part of a beach that lies inated by bacteria. Autotrophic bacteria above the level of normal high spring tides. are much less abundant than heterotrophic Only when exceptionally high spring tides but because they include the bacteria re- occur, or severe storms take place, does sponsible for nitrification and sulfur oxida- this zone come under the influence of wave tion they are of paramount importance. action. Cliffs or sand dunes behind beaches See nitrification; nitrogen fixation; sulfur are usually included as part of the back- oxidation. shore. By their action of breaking down organic matter, bacteria are probably instru- backsight See leveling. mental in the formation of oil. They are also involved in inorganic reactions, such backwash The return of water down a as those leading to the origin of iron ore de- beach, under the influence of gravity, fol- posits. There is evidence indicating that lowing the breaking of a wave and the bacteria were present in PRECAMBRIAN associated SWASH. The difference in load- times: structures called STROMATOLITES, carrying capacity between the swash and which are formed by the action of bacteria, backwash of prevalent waves determines notably blue-green bacteria (cyanobac- whether a beach will aggrade or degrade. teria; formerly called blue-green algae), are Whereas flat waves tend to produce a present in Precambrian rocks. strong swash, steep waves, which break vertically onto the beach, result in a pow- badlands An eroded furrowed land- erful backwash and cause a net seaward scape in a dry region, such as parts of Ne- movement of beach materials. braska and South Dakota in the American West. There is little or no vegetation and so bacteria Single-celled microorganisms any rainwater runs off quickly along the that are fundamental to soil productivity. short steep slopes, further eroding any ex- Where oxygen is lacking they account for posed or soft rocks. most of the biochemical changes in the soil. Environmental controls such as moisture, bahada /bah-hah-dă, -th

32 bar which rapidly moves northward at this defined; they may be concentric or take the time as the upper westerlies move to the form of wavy lines. See also onyx. north of the Himalayas. banded iron formation (BIF) SEDIMEN- bajada /bah-hah-dă, -th

33 barbed drainage beach; bay bar; longshore bar; offshore cause the rate of advance of the dune is in- bar; spit; tombolo. versely proportional to its height, so the 2. A unit of pressure. See millibar. lower sides extend downwind faster than the center; at their tips they end in a dwin- dling of the sand pile to nil, which is blockage reached at the point where the slip face upwind ceases to shelter the wings, and they become subject to wind erosion. Barchans typically occur in belts up to 300 km long m ea str and over 12 km wide.

barite /bair-ÿt/ See barytes.

baroclinic /ba-rŏ-klin-ik/ 1. (in meteorology) Describing a state of the atmosphere where the surfaces of constant Barbed drainage pressure (isobars) intersect into surfaces of constant density (isopycnics). This state is barbed drainage A drainage pattern in the result of large horizontal temperature which a stream suddenly bends back on it- gradients and is believed to be responsible self and flows in an almost reverse direc- in part for the formation of mid-latitude tion, often due to glacial blocking, river depressions. capture, or local tectonic movement in its 2. (in oceanography) Describing a state of path. the ocean where the surfaces of constant pressure intersect into surfaces of constant density. Compare barotropic.

unidirectional _ barometer An instrument for measur- wind ing pressure, invented in 1643 by the Ital- ian scientist Evangelista Torricelli (1608–47). The standard instrument, the mercurial barometer (Kew pattern or ᭝ summit Fortin barometer), measures the height to 30 m (max.) wings which a column of pure mercury can be

400 m (max.) supported by the atmospheric pressure exerted upon it. Corrections have to be made _ to this value to allow for variations in grav- slipface ity, the purity of the mercury, and the am- (at angle of rest of sand) bient temperature. See also aneroid

_ 400 m (max.) _ barometer.

Barchan barotropic /ba-rŏ-trop-ik/ Describing the state of the atmosphere or ocean when barchan /bar-kahn/ A crescentic dune the surfaces of constant pressure are paral- characterized by an oval shape upwind, lel to the surfaces of constant density. Al- with a slip face and two wings spreading though this is a useful concept in out downwind. Barchans occur in desert theoretical studies, such an ideal state areas with a unidirectional wind regime. rarely, if ever, occurs in the atmosphere. The slip face is formed by erosion of ma- Compare baroclinic. terial from the upwind side, and its transport to the summit where it builds up until barrage A large engineered structure of unstable, at which point it slumps forward concrete or soil and rocks, constructed to its angle of rest. The wings develop be- across a river. It dams the flow to create a

34 basalt lake, generally for use in the irrigation of barytes /bă-rÿ-teez/ (barite) An or- adjacent farmland. A barrage across an es- thorhombic mineral form of barium sul- tuary also prevents exceptionally high tides fate, BaSO4. It is usually white but impure from causing flooding. varieties are colored. Barytes has a high density, which distinguishes it from calcite. barrier beach An elongated accumula- A continuous chemical series exists be- tion of sand, shingle, or in-situ rock lying tween barytes and celestite (SrSO4). Both roughly parallel to the coast but separated minerals are found in hydrothermal veins from it by a channel, lagoon, or other with galena, sphalerite, fluorite, and calcite water area. The distance offshore may vary as well as in cavities in limestone. from a few meters to several kilometers. Some lie across bay mouths or estuaries. basal conglomerate A CONGLOMERATE Off low coasts, barriers are often in the at the base of a sequence of sedimentary form of barrier islands, the lagoonal areas rocks. It consists of a mixture of well- behind being of the mangrove type in sorted coarse particles, usually deposited warm sea regions and of the salt-marsh as a thin layer by an encroaching sea. type or even devoid of vegetation in the case of temperate or cold sea regions. basal sapping Any of a range of Many barriers, such as those off the E coast processes that act to remove debris from of the USA and the Gulf of Mexico coast, the foot of a slope. Basal sapping is impor- carry dunes. Most sand and shingle barri- tant in the creation and maintenance of ers are destined to overwash, even breach- FREE FACES, leading to near vertical or steep ing on occasions during severe storms. The slopes that are often unstable, hence caus- more common sandy barriers are formed ing MASS MOVEMENTS. For this reason it by the continued activity of CONSTRUCTIVE constitutes the major explanation of PAR- WAVES, which can progressively build up ALLEL RETREAT of slopes, because the debris material above water level. Shingle barriers at the foot is constantly removed and the are formed by storm waves, the main input steepness of the slope is maintained, thus of material resulting from longshore move- preventing slope decline. Types of basal ments. sapping include wave action at the foot of a sea cliff, lateral erosion by a river at the barrier island See barrier beach. foot of a bluff, headward erosion of streams issuing at the foot of a slope, and barrier reef An elongate accumulation mass movements produced by heterogen- of coral, extending upward from the sea eity of rock strata. floor to about low-tide level, parallel with the coast but separated from it by a lagoon. basalt /bă-sawlt, bass-awlt/ A basic vol- A reef may lie between several meters and canic rock, the fine-grained equivalent of several kilometers offshore. They are be- GABBRO. Its essential constituents are calcic lieved to have been formed as a result of plagioclase, generally in the range the submergence of a flat surface as the labradorite-bytownite, and pyroxene. postglacial sea level rose. The growth of Olivine may not always be present but coral organisms kept pace with the rising magnetite and apatite are invariable acces- water so that great thicknesses could de- sories. Basalts range from undersaturated velop. The best-known example is the to oversaturated compositions (see silica GREAT BARRIER REEF. saturation) but two extremes are recognized, named ALKALI BASALT and tholeiite. Barrow zone See zone (def. 1). Alkali basalts are undersaturated and contain abundant olivine both as phe- barysphere /ba-ră-sfeer/ All of the nocrysts and in the groundmass. The py- Earth’s interior beneath the lithosphere, roxenes of alkali basalt are calcium-rich i.e. the asthenosphere, most of the upper augite or titanaugite. Tholeiites are satu- mantle, and the outer and inner core. rated or slightly oversaturated in composi-

35 basanite tion. They contain sufficient silica to partly pressed as a percentage of this theoretical or completely convert olivine to orthopy- figure to express the degree of base satura- roxene and are characterized by this reaction. A high base saturation, especially tion relationship, which is indicated by the with calcium, generally promotes a neutral mantling of olivine by calcium-poor pyrox- soil, with good structure, aeration, and fer- ene (hypersthene or pigeonite). When pre- tility. A low base saturation represents an sent, olivine occurs only as phenocrysts acidic soil. Different bases dominate in dif- and is absent from the groundmass. ferent environments: calcium is common- Tholeiites typically have intersertal tex- est in temperate soils, sodium in saline and tures and contain silica minerals in the alkaline soils of the arid environments. groundmass. A third type of basalt is now widely rec- base construction line The line drawn ognized, namely high-alumina basalt (or at right angles to the CENTRAL MERIDIAN of calc-alkali basalt) but mineralogical cri- a map projection, from which the other teria alone are insufficient to distinguish meridians of the map are established. the different basalt types and chemical criteria are applied. Even so, the three basalt base level An imaginary line running types, alkali basalt, high-alumina basalt, from SEA LEVEL under the landmass, rising and tholeiite, are gradational in composi- slightly above the horizontal. It constitutes tion. High-alumina basalt is distinguished the controlling level down to which, but by an Al2O3 content in excess of 17% in not below which, a river can cut its valley. aphyric rocks. The Na2O + K2O content Local base levels may replace sea level lies between those of the other two types as the controlling lower limit of erosion for but mineralogically, high-alumina basalt certain parts of streams, or in certain re- has a strong affinity with tholeiite. Three gions; for example, a dam across a stream, volcanic associations corresponding to the a hard rock band, or a lake will constitute three basalt types may be considered. the local base levels for the upstream sec- 1. alkali olivine–basalt–mugearite– tion of a river. In inland drainage basins, trachyte/phonolite the base level will be the lake to which the 2. high-alumina basalt–andesite–dacite– streams flow; the Lake Eyre basin in Aus- rhyolite (calc-alkaline suite) tralia has a local level 14 m below sea level. 3. tholeiite–tholeiitic andesite–rhyolite Members of the alkali basalt associa- No river ever actually reaches base level, tion are found on oceanic islands and con- except at its mouth, because it must retain tinents. Calc-alkaline volcanic rocks are some gradient in order to flow. characteristic of island arcs and orogenic Sea level change, tectonic movement, or belts. Tholeiites occur on the continents, the removal of ice pressure from a formerly especially along continental margins. Most glaciated area can all change the position of the plateau or flood basalts of the world of sea level (the ultimate base level) relative are of tholeiitic composition. The basalts to the land. Base level, along with climate of the ocean floor generated at the MID- and geology, is one of the independent OCEAN RIDGES are tholeiites with extremely variables that control the processes in a particular cycle of erosion. Changes in base low K2O and TiO2 contents. level therefore produce fundamental basanite /bass-8abreve;-nÿt/ A type of changes in the geomorphological system, olivine BASALT containing augite, plagio- to the extent that a major base level change clase, and a FELDSPATHOID mineral. initiates a new cycle of erosion. If the land is raised relative to the sea, base (in soil science) The neutral or al- base level movement is negative; if vice kaline constituent in the soil, notably cal- versa, it is positive. The former tends to cium, magnesium, sodium, and potassium. increase erosional activity, the latter to ini- Each soil has a theoretical maximum con- tiate deposition. See also eustasy; rejuvena- tent of bases and the actual content is ex- tion.

36 bathyscaphe baseline A line that is measured very ac- basin 1. A large sediment-filled depres- curately as part of a TRIANGULATION sion often present in cratonic areas; it may scheme. Great accuracy is required because be circular or elliptical. the location of all the points within the tri- 2. A synclinal structure having a plunged angulation area is based upon this one depression, i.e. two directions of plunge at measured line, although the largest 180° to each other but plunging toward schemes may use two or three baselines. each other. They are traditionally measured using long 3. See drainage basin. tapes or wires, supported above the ground. Several corrections must be ap- bastnaesite /bast-nă-sÿt/ A yellow to red-brown mineral, (Ce,La)CO (F,OH), plied to the taped lengths to allow for such 3 which occurs in alkaline igneous rocks factors as slope, thermal conditions, and such as CARBONATITE. It is used as a source sag. The use of electronic instruments in of rare-earth elements. surveying allow the measurements of long baselines without the need for tapes: both batholith /bath-ŏ-lith/ A large body of the GEODIMETER and TELLUROMETER pro- intrusive igneous rock consisting of several vide a rapid means of measuring a baseline plutons joined at depth and occupying very accurately. many thousands of square kilometers. They are generally composed of granite base map A map or chart used as a base material and are associated with mountain to which different types of information can belts. be added or overprinted. It usually contains basic information, e.g. major political bathyal zone /bath-ee-ăl/ The CONTI- boundaries, drainage, and coastlines. The NENTAL SLOPE zone within the oceans. It base is used when several maps of an area embraces depths of some 200 m out to are required, each covering a different depths of about 1000 m and lies between topic, such as geology, vegetation, popula- the relatively shallow NERITIC zone, within tion distributions, etc. which there is greater sediment deposition, and the deep ABYSSAL ZONE. Its upper limit basement The level below which sedi- corresponds approximately with the SHELF- mentary rocks do not occur. Rocks below EDGE zone and its lower limit is quite arbi- this level are generally igneous or meta- trary, because the depth of 1000 m does morphic. not mark an abrupt change in topography or other factors. Light reaches only to the top of this zone but animal life is quite basic lava A type of dark-colored LAVA containing basic ferromagnesian minerals abundant and varied. Globally the bathyal environment covers approximately 40 mil- and less than 50% silica. It flows freely lion sq km. from a volcano’s crater or fissures, quickly spreading across the terrain. It becomes bathyorographical /bath-ee-ô-rŏ-graf-ă- BASALT when it solidifies. Weathered basic kăl/ Describing a map showing the lava forms a rich soil, much used for farm- height of the land and depth of the sea, ing. See also acid lava. usually by layer-coloring. basic rock An igneous rock with a silica bathyscaphe /bath-ă-skayff/ A spherical content of 45–55% (by weight). Such or sausage-shaped vessel designed to en- rocks consist largely of relatively silica- able investigators to descend into the sea poor minerals such as olivine, pyroxene, and to observe, through portholes or other and calcic plagioclase feldspar. Typical observation windows, the marine environ- basic rocks are basalt, dolerite, and gab- ment. The first of such devices to achieve bro. See acid rock; intermediate rock; ul- useful underwater work were the Ameri- trabasic rock. can biologist William Beebe’s (1877–1912)

37 bauxite bathysphere and the American engineer tion of eroded solid rock (see wave-cut Otis Barton’s benthoscope. By about 1953, platform); they usually form a smooth pro- improved bathyscaphes were reaching file that is gently convex and is sometimes depths in excess of 3000 m during experi- interrupted by BEACH RIDGES. The response mental trials. During 1960, the bathy- of a beach to the activity of waves and scaphe Trieste landed on the floor of the other agents of erosion and deposition Mariana Trench (10 911 m). Bathyscaphes largely depends upon the nature and thick- can be equipped with grab-sampling, cor- ness of the deposits, sand and shingle being, and other equipment. having very differently. bauxite /bawks-ÿt/ An aluminum ore beach cusp See cusp. formed by the weathering of aluminum- rich, relatively iron- and silicon-poor rocks beach ridge An upstanding linear accu- (mainly syenites) under tropical condi- mulation found on shingle beaches (an actions. The breakdown of aluminosilicate cumulation developed in sand is known as minerals and the removal of silica by leach- a BERM). Whenever CONSTRUCTIVE WAVES ing leaves a residue composed mainly of are active on a shingle beach, a small ridge boehmite (AlO(OH)), diaspore (HAlO2), will be produced at the limit of SWASH for and gibbsite (Al(OH)3). each high tide. As maximum tide level falls from spring to neap level, a series of ridges bay An inlet or indentation in the shore will be produced; these will be destroyed of a lake or the sea. Bays are commonly again on tidal rise to spring level, when just formed by DIFFERENTIAL EROSION, in which one ridge remains at high-water spring softer rocks are worn away faster than the level. Much larger shingle ridges can be harder rocks surrounding them. produced in connection with severe storms. These develop far above the swash bay bar A barrier of sand or shingle that limits of normal waves (see storm beach). extends across the entrance to a bay, effectively straightening the coastline. The most beach rock Consolidated and erosion- likely causes of a bay bar are an enlarge- resistant sand-forms, often reeflike, that ment of a single SPIT in one direction or the develop along some intertidal shores in independent growth, and subsequent coa- tropical and subtropical regions. Beach lescence, of two spits that increase their rock has also been found in Portugal, lengths toward each other. Once closed off Hawaii, Morocco, and certain temperate by a bay bar, the bay will gradually fill up regions. Its origin has provoked a great with sediment. deal of argument and speculation. In general, it appears to develop on account of ce- bayou /bÿ-oo/ A marshy area of an estu- mentation or lithification processes, the ary or where a lake outflows, as occurs typ- cement being of a calcareous type (often ically in the southeastern states of the USA. calcite or aragonite). Hence, geologists The water flows only sluggishly or may be classify beach rock as a calcarenite. Its stagnant. rapid consolidation may be aided by the presence of microorganisms such as bacte- beach An accumulation of unconsoli- ria and unicellular algae, and certainly by dated materials found in the zone of inter- high temperatures. Beach rock, which section between land and sea. For most tends to form as small outcrops, is not al- practical purposes the beach can be consid- ways found on all beaches in the same lo- ered to extend from the highest point of se- cality. Once formed, however, it tends to vere storm-wave activity (see storm beach) resist the action of waves and may act as a down to the point at which waves ap- barrier to sediment movement. proaching the coast first start to cause extensive movement of seabed materials. beaded esker An ESKER comprising al- Beach materials are located on a founda- ternate wide and narrow segments, which

38 bed

BEAUFORT SCALE

Speed/knots (km/hr) 0 Calm Smoke rises vertically <1 1 Light air Smoke or leaves indicates movement, otherwise almost calm 1–3 (1–5) 2 Light breeze Wind felt on face, leaves rustle, etc. 4–6 (6–11) 3 Gentle breeze Flag extended; leaves and twigs in constant motion 7–10 (12–19) 4 Moderate breeze Small branches moved; dust and litter raised 11–16 (20–28) 5 Fresh breeze Small trees begin to sway 17–21 (29–38) 6 Strong breeze Large branches in motion; whis- tling in telephone wires 22–27 (39–49) 7 Near gale Whole trees in motion; inconveni- ence experienced in walking 28–33 (50–61) 8 Fresh gale (gale) Twigs broken off; walking impeded 34–40 (62–74) 9 Strong gale Slight structural damage experienced 41–47 (75–88) 10 Whole gale (storm) Widespread damage to trees and buildings 48–55 (89–102) 11 Storm (violent storm) Very rarely experienced inland, severe damage results 56–63 (103–114) 12–17 Hurricane Very rarely experienced inland, severe damage results >64 (>117)

reflect differential input rates of flu- Beaufort scale /boh-fert/ A scale of vioglacial sediments. The narrow sections wind speed based on easily observable in- were formed during colder periods, when dications such as tree movement and there was less meltwater and hence less smoke. The scale ranges from 0 to 17 sediment, whereas the wider parts were de- (numbers 13 to 17 were added by the US posited in warmer periods. These segments Weather Bureau in 1955) with the num- may mark successive winter and summer bers and indicators as shown in the table. The scale is named for the British hydrog- accumulations as an ice front (or glacier rapher Sir Francis Beaufort (1774–1857). snout) retreated. Beaded eskers are comparatively rare. bed (stratum) The smallest division of stratified sedimentary rocks, consisting of A horizontal angle measured bearing a single distinct sheetlike layer of sedimen- clockwise from a known direction, usually tary material, separated from the beds north, to another point. If the initial direc- above and below by relatively well-defined tion is true north then the angles are true planar surfaces called BEDDING PLANES, bearings. Angles measured with reference which mark a break in sedimentation. A to magnetic north, such as those obtained bed may be part of a MEMBER (see also with a prismatic compass, are known as marker bed). See lithostratigraphy; stratig- magnetic or compass bearings. raphy.

39 bedding bedding The parallel layering of SEDI- been surveyed extremely accurately by the MENTARY ROCKS. The different layers may national surveying body of the country have different compositions or structures. concerned. bedding plane Any of the planes that Benguela Current /ben-gwel-ă/ A sea separate each of the layers or BEDS (strata) current that flows northward off the coast in a sedimentary rock formation. There are of southwest Africa. Part of the region in generally differences of color, composition, which it flows is important because of the or structure on each side of such a bedding existence of an UPWELLING of cold water plane. and a zone of marked divergence. The upwelling water comes from subsurface bedding-plane slip The displacement water perhaps only two or three hundred of one bed of rock over another, taking meters deep. The most marked flow in the place along the bedding plane between ad- Benguela Current, which can be up to jacent beds. It is usually associated with about 16 million cubic meters per second, folding, where in extreme cases it can lead occurs between the Cape of Good Hope to DÉCOLLEMENT. and latitude 18°S. bed load (bottom load; traction load) Benioff zone /ben-ee-off/ The inclined The material that is carried along the bed seismic zone within the Earth’s lithosphere of a stream or the sea by moving water, or extending down at an angle of usually along the ground by the wind. It consists of around 45° from the base of an ocean particles that are too large to be trans- trench to the asthenosphere. It is typical of ported in suspension (see suspended load). destructive PLATE BOUNDARIES where one plate overrides another as a result of sea- bedrock The solid unweathered rock floor spreading. These are zones where that lies below soil or unconsolidated ma- ocean floor is consumed. The shallowest terial, or is exposed at the surface. earthquake foci along the zone occur near the base of the trench, the foci becoming beef Thin beds and veins of fibrous CAL- progressively deeper with distance along CITE. See carbonate minerals. the Benioff zone from the trench. The zone is named for the American seismologist belemnite /bel-ĕm-nÿt/ Any mollusk of Hugo Benioff (1899–1968). the extinct subclass BELEMNOIDEA. Belem- nites are important fossils from rocks of benitoite /be-nee-toh-ÿt/ A silicate min- the Mesozoic Era. eral of barium and titanium, noted for its strongly dichroic nature. It varies in color Belemnoidea /bel-ĕm-noi-dee-ă/ An ex- from deep sapphire blue to colorless, de- tinct subclass of marine mollusks of the pending on whether it is viewed by re- class CEPHALOPODA. With squids and octo- flected or transmitted light. puses, they are classified as Coleoidea (or Dibranchiata). They had an internal bullet- benmoreite /ben-mor-ÿt/ A basic ig- shaped shell with simple septa dividing it neous rock composed of an alkali into chambers; this shell is usually the only FELDSPAR, olivine, and pyroxene. See tra- part of the animal to be fossilized. Belem- chybasalt. nites are known from Lower Carbonifer- ous rocks and extend to the beginning of benthic /ben-th’ik/ Describing an organ- the Paleogene, but they reached their max- ism that lives on the floor of the sea or a imum development in the Jurassic and Cre- lake. See benthos. taceous Periods. benthos /ben-th’os/ Plant and animal bench mark A mark indicating a point marine life, both small and large, living of known position and height, which has within the waters of the sea or on the sea

40 billow cloud bed (see also pelagic (def. 2)). They are a open bergschrunds, where cold air can cir- fundamental part of the complex food culate freely, will there be any appreciable chains that operate within marine environ- attack on the rock of the headwall. ments. Benthos exist in numerous forms, the greatest variety occurring within the berm /berm/ An accumulation of ma- shallow-water environments associated terial found a little way above the mean with continental shelves. In the photic high-water mark on sand beaches. The (light-penetrating) zone, the benthos feed berm is usually flat (although it may slope on both phytoplankton and zooplankton; gently landward), is of variable width, and they are especially well developed in plank- is characterized by a marked break of slope ton-rich areas and often poorly developed at the seaward edge. Berms are created by in less fertile areas. The benthos help to the action of CONSTRUCTIVE WAVES but their make available to larger marine creatures growth is not a very rapid process. A beach the tiny food particles that they consume or of coarse sand, having a steeper gradient produce. Vagrant benthos are capable of than one of finer material, will develop a active movement on or within the sedi- berm more rapidly, since wave action is re- ment; sessile benthos remain attached to stricted to a narrower width of beach. See the sea floor. also beach ridge. bentonite /ben-tŏ-nÿt/ Clay formed by beryl A hexagonal mineral of composi- the alteration and weathering of tuffs and tion Be3Al2Si6O18 found in granitic rocks volcanic ash. See clay minerals. and PEGMATITES. Beryl is white to pale green in color. The semiprecious gem vari- Bergeron–Findeisen theory /bair-zhĕ- ety aquamarine is bluish green. The bright rawn fin-dÿ-zĕn/ (Bergeron theory)A green variety is EMERALD, found mainly in theory of precipitation formation proposed metamorphic rocks. by the Norwegian meteorologist Tor Bergeron (1891–1977) in 1933 and sub- B horizon The second highest layer of sequently modified by the German meteor- soil, or upper subsoil, immediately below ologist Walter Findeisen (1909–45). It the A HORIZON. It has less humus and con- suggested that ice crystals and supercooled tains less weathered material, but may con- water droplets could exist together at tain chemicals washed down from above. certain levels in the clouds where the tem- See also horizon (def. 1). perature was between –15° and –30°C. Be- cause the saturation vapor pressure for ice BIF See banded iron formation. is lower than that for a water surface, there would be preferential deposition of mois- bifurcation ratio The quantitative rela- ture on the ice crystals, causing them to tionship between STREAM ORDERS, ex- grow sufficiently for snowflakes or rain- pressed as the number of streams of one drops to result. order divided by the number of streams of the next highest order. Values are com- bergschrund /berk-shrûnt/ A deep nar- monly between 2.5 and 3.5, but are higher row chasm frequently found in CIRQUES in in long thin drainage basins dominated by which the upper part of the CIRQUE GLACIER one master stream. has become separated from the rock headwall. They were formerly believed to be the billow cloud Cloud found in a series of most favorable sites for FREEZE-THAW activ- regular bands with clear areas between, ity, which causes headwall retreat. How- usually of similar width to that of the ever, more recent work has shown that cloud. They occur most frequently at a fluctuations of temperature within berg- height of about 6–8 km, and appear to be schrunds are infrequently of sufficient due to a strong increase of wind speed with magnitude or rapidity to cause any rock height when the airflow is stable, but the breakdown. Only in the upper parts of precise mechanism is still in doubt.

41 bimineralic bimineralic /bÿ-min-ĕ-ral-ik/ Describ- cent bioherms are produced by CORALS, but ing a rock consisting of only two kinds of in the past important reef-building organ- minerals, e.g. websterite. isms have included sponges (see Porifera) and certain algae. A bioherm may include bioclastic rock /bÿ-oh-klas-tik/ A sedi- a hard skeletal structure secreted by the or- mentary rock that contains the fragmen- ganism itself, the remains of animals that tary remains of once-living organisms, such live in the environment it produces, and as the shell fragments that make up some trapped sediment. They are mainly marine. kinds of limestone. See also biogenic rock. biological weathering A mechanism in biocoenosis /bÿ-oh-see-noh-sis/ (life as- which rocks are broken down by the action semblage) An assemblage of fossil organ- of plants and animals. For example, plant isms associated and occurring in the same roots can enter cracks in rocks and break position as they occupied in life. Some bio- the rocks as the roots grow and expand. herms and reefs approximate to this condi- Bacteria, worms, mollusks, and other in- tion but such assemblages are usually rare, vertebrates can also split rocks or wear because most organisms suffer damage and them away. See also weathering. transport after death. Compare thanatocoenosis. See also taphonomy. biomass /bÿ-oh-mass/ The total organic matter – plant and animal – in a particular biogenic rock /bÿ-ŏ-jen-ik/ A type of area. It is usually stated as the dried weight rock directly created by living organisms, per unit area, such as kg m–2. Most of the their remains, or activities. Examples in- biomass is derived from plants. Biomass clude coal, coral reefs, and limestones com- varies widely in different regions of the posed of the shells of mollusks. See also world, from less than 0.02 kg m–2 in polar bioclastic rock. regions to as much as 45 kg m–2 in tropical rainforests. biogeochemical cycle The pathways and methods by which chemical elements biome /bÿ-ohm/ A major ecological are moved and circulated within the eco- community of plants and animals that sphere from living organisms to the physi- share the same climate and vegetation. It is cal environment and back again. The the largest such community recognized by chemicals may be in gaseous, liquid, or ecologists and may include several HABI- solid form and form different chemical TATS. Examples of biomes include desert, compounds at various stages in the cycle. grassland, savanna, taiga, temperate rain- Biogeochemical cycles have been docu- forest, tropical rainforest, and tundra. mented for many elements, e.g. carbon, oxygen, nitrogen, phosphorous, sulfur, and biosome /bÿ-oh-sohm/ An accumulation water. A biogeochemical cycle in which the of sediment that has been deposited under elements are trace elements essential to life constant biological conditions. is known as a NUTRIENT CYCLE. See also carbon cycle; nitrogen cycle. biosphere /bÿ-ŏ-sfeer/ The region of the Earth’s crust and atmosphere in which life biogeography /bÿ-oh-jee-og-ră-fee/ The exists; it contains a number of HABITATS. scientific study of the distribution of plants The biosphere extends from about 3 m and animals around the world. It includes below ground to 30 m above it. It also in- studies of soil (pedology), climate, ecology, cludes aquatic zones extending to about and ecosystems. 200 m deep. bioherm /bÿ-oh-herm/A REEF of un- biostratigraphy /bÿ-oh-stră-tig-ră-fee/ stratified limestone formed by organic The branch of stratigraphy that utilizes in- processes and usually having the shape of a formation from fossils in the CALIBRATION dome rather than a linear feature. Most re- of stratigraphic sequences of rock in which

42 Bivalvia they occur. The evolution of organisms is birefringence /bÿ-ri-frin-jens/ The dou- believed to be a continuous unidirectional ble refraction of light entering an aniso- process and it therefore provides an impor- tropic crystal, when it is split into two tant means of correlating and comparing plane-polarized rays, the ordinary and ex- separate rock sections. The fundamental traordinary rays. The two rays vibrate in biostratigraphic division is the ZONE. Some mutually perpendicular planes and travel geologists make no distinction between through the crystal in different directions biostratigraphy and CHRONOSTRATIGRAPHY and with different velocities (there is a fast and include the biostratigraphic zone ray and a slow ray) and thus have different within the chronostratigraphic hierarchy refractive indices. This may be illustrated of terms. Others consider biostratigraphy by the well-known experiment of viewing a to be simply one method of calibrating the single spot on a piece of paper through a essentially separate chronostratigraphic piece of Iceland spar. Two images of the scale (see chronozone). spot are seen representing the ordinary and extraordinary rays. biostratonomy /bÿ-oh-stră-tonn-ŏ-mee/ When an anisotropic crystal is viewed The branch of paleontology concerned under a petrological (polarizing) micro- with the processes by which the remains of scope, bright polarization colors are seen organisms become embedded in rock. This and the crystal is said to exhibit birefrin- is important in interpreting the significance gence. The two rays produced by the min- of fossil assemblages, many of which are eral are recombined in the microscope and THANATOCOENOSES. See actuopaleontol- their optical interference produces the po- ogy; taphonomy. larization colors. These colors are of great use in the identification of minerals. biostrome /bÿ-ŏ-strohm/ A structure similar to a BIOHERM but not swelling into a bittern The brine that remains when mound or lenslike body. It more closely re- useful more soluble minerals (such as car- sembles a broad sheet of sediment consist- nallite and polyhalite) have been removed ing of a large quantity of organic remains. from sea water by evaporation. Bittern can itself be used as a source of bromides and biotic Describing the living factors in an iodides. ECOSYSTEM. They result from the activities of animals and plants, and include compe- bitumen A semisolid or solid flammable tition and feeding. See also abiotic. substance that consists mainly of hydrocarbons, particularly those occurring in PE- biotite /bÿ-ŏ-tÿt/ One of the major forms TROLEUM. See also asphalt. of MICA which forms dark brown or black shiny slabs that split easily into transparent bituminous coal A black or dark flakes. It is found widely in many kinds of brown COAL with a high carbon content, igneous, metamorphic, and even sedimen- the type most commonly used as fuel. It tary rocks. burns with a yellowish smoky flame and is used for making degassed coal (smokeless bioturbation /bÿ-oh-ter-bay-ʃhobreve;n/ fuel) and coke. See also anthracite; lignite. The reworking and further degradation of sediment by the action of organisms mov- Bivalvia /bÿ-val-vee-ă/ (Lamellibranchia; ing through it and feeding on it. Pelecypoda) A class of the phylum MOL- LUSCA whose members are characterized by birds See Aves. a two-valved shell protecting the body. The valves are secreted by the mantle and meet bird’s-foot delta An elongated form of at a hinge line, where they are joined by an DELTA, with sediment deposited in a finger- elastic ligament. The hinge is aligned and like pattern following the courses of the strengthened by a system of interlocking various distributary streams. teeth and sockets, which are used in the

43 black alkali soil taxonomic classification of the group. The tains dark heavy minerals such as ILMENITE shell has growth lines and often ornamen- and MAGNETITE (containing iron) and RU- tation. The animals are bilaterally symmet- TILE (containing titanium). rical, with the plane of symmetry passing between the valves, unlike the BRACHIO- black smoker See hydrothermal vent. PODA, which are symmetrical across the valves. Some bivalves have secondarily lost blanket bog See organic soil. this symmetry and the valves are unequal. Bivalves are aquatic (mostly marine), bleaching See coral bleaching. occupying a great variety of habitats, and include free-swimming, sessile, and bur- blende /blend/ Any sulfide mineral, es- rowing forms. They range from the Cam- pecially one used as an ore. The best brian Period to the present day and have known is zinc blende (see sphalerite). been used biostratigraphically as ZONE FOS- SILS, particularly in the Upper Carbonifer- blizzard Heavy snow falling or drifting ous and Paleogene. during strong winds. black alkali soil See solonetz. block-faulting A series of normal faults that separate an area into a series of HORSTS blackband ironstone A SEDIMENTARY and GRABENS. See fault. ROCK, containing iron, that occurs in association with coal deposits. It contains iron blocking The situation arising when carbonate (SIDERITE) and is used as an iron anomalous pressure patterns develop in the ore. zone of the mid-latitude westerlies. It more commonly refers to anticyclone formations black-body radiation A perfect black that act as a block to the normal depression body is one that absorbs all radiation tracks, which then move toward the north- falling upon it and emits, for any tempera- east and southeast around the anticyclone ture, the maximum amount of radiant en- to give spells of quiet and dry weather near ergy. No real substance achieves this the blocking high. The two favored sites degree of physical perfection, but the inside for the location of blocking highs are over of a sphere coated with carbon black ap- NW Europe and the NW Pacific. The pre- proaches it. Some substances can act as cise reasons for these positions are not un- black bodies for certain wavelengths only. derstood but they are believed to be due to An extreme example of this is snow, which the interaction between the westerly circu- has a high albedo for visible light, but ef- lation and the north–south oriented moun- fectively acts as a black body for wave- tain ranges such as the Rockies. lengths greater than 1.5 µm. block lava A LAVA the surface of which black earth See chernozem. is broken into large retangular blocks. Compare aa; pahoehoe. black ice (glazed frost) A smooth transparent thin coating of ice formed on as- block mountain A steep-sided moun- phalted road or other surfaces, having a tain or plateau that has been uplifted be- black almost invisible appearance. It is tween a pair of parallel FAULTS. formed when drizzle or light rain falls onto a surface that is at a temperature below block stream A pile of rock debris that 0°C. has accumulated at the head of a ravine, probably as a result of glacial action. It is black jack See sphalerite. also called a rock stream. black sand A type of sand found on blowhole A vertical fissure in the roof beaches or in alluvial deposits that con- of a sea cave or cliff through which sea

44 Bonne’s projection water is forced as a jet at high tide. It is having traveled through the Earth’s inte- formed when wave action causes erosion rior. Body waves include primary (P or lon- along a joint in the rock. gitudinal) and secondary (S or shear) waves.

bog A waterlogged area of land, result- SEA ing from poor drainage, in which vegetation becomes partly decomposed. The WIND gully or ground is spongy and wet, and eventually destroyed transverse vegetation the vegetation forms an acid PEAT. See also dune organic soil.

Fig. 1 Fig. 2 bog soil See organic soil.

blowout bole Thin red weathered horizons (parabolic progressive dune) enlargement formed between basalt lava flows. The presence of bole indicates subaerial extru- advance sion and a tropical climate. See laterite.

bolson /bohl-sŏn, bohl-sohn/ See inland Fig. 3 Fig. 4 basin.

Blowout formation bomb (volcanic bomb) See pyroclastic rock. blowout A break in the crest of a sand dune, caused by vegetation destruction or Bonne’s projection /bon-ĕ/ A type of gullying. It leads to increased sand mobility conical MAP PROJECTION, with the central and wind speed at the break point, with the meridian straight and truly divided, the sand being blown downwind as a PARA- parallels all concentric circles, and the BOLIC DUNE bowing out from the transverse other meridians composite curves joining line. By the time stage 3 is reached, unless divisions on the correctly divided parallels. practical measures are taken to break wind The scale along the parallels is kept true. speed and reinstate the vegetation, the The shapes of landmasses are distorted blowout becomes progressively enlarged with distance from the central meridian by advancing downwind and widens itself but the projection is equal-area. The pro- through undercutting of the vegetation on jection is named for the French hydrogra- either side. blue ground See kimberlite. blue john See fluorite. • center of curvature pole bluff (river cliff) A steeply sloped river 80° bank on the outside of a MEANDER. It is 60° caused by erosion by the faster-flowing water on the outside of the bend. It may be 40° as much as 100 m in height. Any other headland or inland cliff may also be called a bluff. 20° body wave A seismic wave, generated either by an earthquake or explosion, which reaches a recording station after Bonne’s projection 45 bora pher and cartographer Rigobert Bonne trending WNW–ESE toward the colder (c.1727–c.95). See conical projection. continental interiors. bora /bor-ă, boh-ră/ A cold, dry, and Boreal forest See taiga. gusty northeasterly wind that sweeps through the mountain gaps of the W Boreal period The period from 7500 BC Balkan Peninsula to the Adriatic coast. It is to 5000 BC, which was characterized by an strongest in winter, with mean speeds of improving climate following the deglacia- about 27 knots (50 km/hr), and is the result tion of the last ice age. It was named Boreal of high pressure over central Europe and a because the vegetation at this time in NW deep depression to the southwest. The Europe, where the first investigations were presence of the mountains helps the fun- conducted, was predominantly coniferous neling of the wind and its rapid descent as in the Boreal areas at present. prevents adiabatic warming. See also mistral. borehole logging A method of record- borax /bor-aks, boh-raks/ A mineral ing the physical characteristics of materials below the ground surface by drilling wells form of hydrated sodium borate, Na2B4O7. or boreholes. Observations may be made 10H2O. It occurs as deposits and on the surface of the ground in dry regions, by examining the cuttings of material re- formed by the evaporation of alkaline moved during the drilling process, by low- lakes or hot springs. The transparent crys- ering instruments to measure different tals can lose water of crystallization to be- physical properties into the resultant well come tincalconite, a chalky white mineral. or borehole, or by studying the core that is It is used as a source of boron. extracted. Borehole logging is used, for example, in geophysics in the identification bore A tidal phenomenon that occurs in of ores and fossil fuels, in stratigraphy, and certain rivers or estuaries. The tidal range in hydrology to study groundwater flow. is usually fairly large and may lead to a See also electrical logging. tidal flood wave with a high sharply developed front. The frontal area is often char- borer See marine borer. acterized by subsidiary waves or furrows. With the initial flooding of the tide, a surg- bornhardt /born-hart/ A dome-shaped ing inflow may develop and travel rapidly mound of rock; a large INSELBERG. some kilometers upstream. Well-known bores occur in the Amazon, the Qiantang bornite /born-ÿt/ (peacock ore) A red- River in China, the Hooghly River in India, brown mineral form of copper iron sulfide, and the Bay of Fundy. The large bore that Cu FeS , with a characteristic purplish iri- travels up the Qiantang River has a fast- 5 4 descent tarnish. It occurs associated with moving abrupt front up to 4 m in height, other copper ores. necessitating the prior removal of boats from the river. borolonite /bŏ-rol-ŏ-nÿt/ A nepheline- Boreal climate /bor-ee-ăl, boh-ree/ A cli- containing type of SYENITE. matic type in Köppen’s classification system (see climatic classification) charac- boss A roughly circular igneous intru- terized by cold snowy winters and warm sion that has a diameter of less than 25 km. summers, giving a large annual range of Compare stock. temperature. Precipitation totals are small. It is found over the American, European, botryoidal /bot-ree-oi-dăl/ Describing a and Asian continents between latitudes mineral consisting of spheroidal aggregates 40°N and 65°N with the S boundary being resembling a bunch of grapes. Compare farther north on the warmer W coasts and reniform.

46 boundary layer bottom current A current that flows boudinage /boo-dăn-ahzh/ Pillow-like close to the sea floor or within the lower- structures formed in rock beds. When a most layers of water in deep-sea areas. The competent bed of rock is enclosed by in- term has also been used to describe the cur- competent material and is then stretched or rents over shelf areas that arise owing to squeezed, the competent bed breaks in a se- density-driven flow (see density current), ries of short sections, because it cannot de- tidal flow, or wave-induced flow. In the form plastically to the same extent as the case of deep-water bottom currents, flow incompetent material. As the competent velocities have been calculated theoreti- layer deforms, it thins locally and PINCH cally, others have been measured using cur- AND SWELL structures develop. On further rent meters or special floats. Bottom flow stretching this thinning continues until necks are formed; finally the layer breaks. in part of the Antarctic bottom current can They were originally described as boudins attain 3 cm/sec but is usually much less. by French geologists, to whom they resem- The speed of the bottom current on the W bled black sausages. flank of the Atlantic Ocean is, theoretically at least, between 0.5 to 2 cm/sec. Bottom Bouguer anomaly /boo-gay/ A gravity topography may significantly hinder the anomaly that takes into account the effect flow of bottom water, for example the of topography but not isostatic compensa- transverse ridges in the South Atlantic, tion, first observed by the French physicist which the Antarctic bottom current has to and mathematician Pierre Bouguer (1698– negotiate. Deep gashes or gaps through 1758) in 1735. such features are important in channeling bottom flow. Some sub-Arctic bottom boulder clay See till. water has its progress southward off the Labrador coast partly blocked by the boundary current A fairly fast-moving Labrador submarine rise. current that flows near the edge of an ocean, usually along its W flank. Theoreti- bottom load See bed load. cal models of oceanic flow, particularly that of the American oceanographer Henry bottomset bed See delta deposit. Stommel (1920–92), reveal that there should be a fairly localized current flowing bottom water The lowermost layer of at depth, in a southward direction, along water in the sea, particularly the water the W flank of the Atlantic. The necessity masses that move slowly in deep ocean for such currents arises from a considera- areas. Bottom water is relatively cold and tion of pressure gradients within the dense. Most of the North Atlantic bottom oceans and the forces associated with water has a temperature around 1–2°C. Earth rotation. Stommel’s theory suggests that there should also be a current along The Polar ANTARCTIC BOTTOM WATER origi- the W flank of the Pacific Ocean, although nates in the surface layers of the Antarctic not in this case everywhere directed south- and later flows as a deep-water layer. Dur- ward. Field measurements have demon- ing the fall and early winter, the very cold strated that in the case of the Atlantic, shelf water sinks owing to convection, and there is a south-flowing current under- slides down the continental slope to the neath the Gulf Stream. abyssal depths. This happens especially in the Weddell Sea area. The deep water then boundary layer The layer of the atmos- spreads outward, especially eastward into phere in which movement is closely deter- the Southern Indian Ocean, although influ- mined by the presence of the ground enced by bottom topography. In contrast, surface. In practice, this layer is subdivided the bulk of North Atlantic bottom water into the surface boundary layer, extending originates outside the Arctic. See also Arc- to approximately 100 m, and the planetary tic bottom water. boundary layer, with an upper limit of

47 boundary wave about 600 m above the ground. In the for- taining a current of water for respiratory mer, the influence of the surface is para- and feeding purposes. Brachiopods show mount, whereas in the latter layer, it variations in shell morphology and orna- remains significant but not dominant. mentation that are valuable in taxonomic classification. boundary wave See internal wave. The earliest fossil brachiopods are known from Lower Cambrian rocks; they bourne A stream, especially one in chalk flourished throughout the Paleozoic Era districts such as S England. When the water and were especially abundant and diverse table falls in summer, many bournes dry in the Silurian and Devonian Periods. up. Many became extinct at the end of the Pa- leozoic. There was a revival of a few Bowen ratio /boh-ĕn/ The ratio of the groups in the Mesozoic but brachiopods transfer of sensible heat from the ground are relatively insignificant today. They surface to that of latent heat effected by the have been used in the stratigraphic classifi- processes of turbulence and conduction. cation of the rocks in which they occur. Over oceans, the value of this ratio can be as low as 0.1 indicating that 90% of the brackish water Water that is partly heat flux from the surface is in the form of fresh and partly saline. Areas of such water latent heat. Desert areas represent the con- occur, for instance, behind coastal barriers verse, with no latent heat component be- that partly inhibit tidal penetration, hence cause all heat transfer is in the form of the incursion of saline water, but still allow sensible heat. The ratio is named for the the incursion of fresh water from streams American astronomer Ira Sprague Bowen and rivers. (1898–1973). braided stream A stream (or river) that boxwork The reticulated outward ap- divides into two or more smaller channels, pearance of the ceiling of a cave, resulting which further branch and separate before from the deposition of hydrated iron ox- rejoining farther downstream. The result is ides in fractures and cavities (from which a network of islands, sandbars, and chan- material has been eroded away). nels. It occurs when the volume of flow of Brachiopoda /brak-ee-op-ŏ-dă/ A phy- the stream varies widely from time to time lum of invertebrate animals having a body (as in dry regions) and there is plenty of BED enclosed by two valves of a chitinous or LOAD. It also occurs in front of a melting calcareous material. Brachiopods are bilat- glacier. See also braiding. erally symmetrical, but unlike the BIVALVIA the plane of symmetry passes across the braiding The division of a stream into a valves, not between them. The valves are complex pattern of several small channels thus dissimilar but each is symmetrical divided by mid-channel bars. After straight about its left and right halves. The phylum and meandering channels, braiding is the is divided into two classes, the Inarticulata third major channel pattern a river can and the Articulata. In the latter the shells have. It is often due to lack of COMPETENCE are connected by teeth and sockets along a or CAPACITY of the river to carry the load hinge line. The shells of the Inarticulata are supplied to it. As a result the excess load is held together only by muscles. Brachio- dumped in mid-channel to form the char- pods are marine benthonic animals at- acteristic mid-channel bars that divide the tached to the substrate by a pedicle, which channel into many separate subchannels. commonly protrudes through an opening In this form, it is often seen in the in the larger valve (the pedicle valve). The periglacial environment (see river terrace). other valve is called the brachial valve. It Easily erodible banks and variable dis- has an internal brachidium, which sup- charge are also said to favor braiding. Me- ports the structure responsible for main- andering patterns may change to braided

48 brown clay patterns if the stream gradient or discharge brine A solution of common salt is increased. (sodium chloride, NaCl) as occurs in sea water and in salt lakes. Naturally occur- breaker A mass of turbulent water mov- ring brines are important sources of salts ing landward, as a result of the breaking of and other chemicals. a wave on approaching the coast and reaching shallower water. The decrease in brittle failure The breaking of a rock water depth causes a reduction in velocity while still within the elastic range. of the wave, an increase in height, and a decrease in length. The orbital paths of water brittle strength The stress applied to a particles within the wave form also change, rock at the point at which it breaks within from open circles to open ellipses, until the the elastic range. water under the crest is moving faster than the wave form, at which point the wave broad A small shallow lake in the East breaks. Whether a breaker results in net ac- Anglia district of E England, formed on the cumulation or erosion of beach materials site of peat extraction several centuries depends largely upon the nature of the ago. Broads are prone to silting up and pol- breaking. The crest of a plunging breaker lution. falls vertically into the trough, producing little SWASH, but considerable BACKWASH. bronzite /bron-zÿt/ An orthorhombic PY- Spilling breakers, however, break forward ROXENE. and the associated stronger swash aids constructive activity. See also constructive bronzitite /bron-ză-tÿt/ A monominer- wave; destructive wave. alic ultramafic rock consisting wholly of bronzite. breaker zone See surf zone. brookite /brûk-ÿt/ A brown to black or- breccia /brech-ee-ă, bresh-/A RUDACEOUS thorhombic polymorph of titanium diox- sedimentary rock in which the constituent ide, TiO , found in hydrothermal vein clasts or fragments are angular. Compare 2 deposits. See also anatase; rutile. conglomerate. brown calcareous soil A type of calci- brickearth Fine-grained unconsolidated deposits (sand, silt, and clay) associated morphic soil, typically found in the humid with periglacial conditions in the Pleis- temperate regions of Europe and America tocene. There are two types of brickearth. under base-rich vegetation. Soil develop- Aeolian loess consists of very well sorted ment is more advanced than in a rendzina, material of predominantly silt size, picked producing a soil with an A (B) C system of up from glacial outwash, frost-shattered horizons. The soil is deeper, though rarely debris, or the floodplains of rivers, and more than 70 cm, owing to its development transported by cold dry winds in steppelike over a parent material with a greater in- conditions. The deposit is largely quartz, soluble residue. The upper part of the soil but with a mineralogy reflecting the rocks has a mull humus and is dark red/brown in of origin, and forms a sheet over the preex- color with a neutral or slightly acid reac- isting landscape when deposited in a wedge tion. It has a crumb structure and is rich in that thins away from the source. It is com- organic matter. With depth the A horizon monest in E Europe, but thin veneers reach merges into an ill-defined B horizon, which E and SE England. is often lighter in color and contains many Overbank material deposited on flood- fragments of parent material. They are plains by Pleistocene rivers is also termed good agricultural soils and fall into the brickearth. This type has a lower silt con- MOLLISOL order of the US SOIL TAXONOMY. tent than the wind-lain type, and is less well sorted. brown clay See red clay.

49 brown coal brown coal See lignite. data, such as harvests. He distinguished alternating warm and dry spells and cold and brown earth A type of soil ranging wet spells, but the cycle is not well defined from a true brown earth, which exhibits and has too small an amplitude to have any only mild leaching, to a PODZOL, which ex- forecasting value. hibits extreme leaching. Brown earths are found to the south of the pozdol zone brunizem /broo-nă-zem, broo-nă-zyom/ where precipitation is in excess of evapora- See prairie soil. tion. True brown earths lack the distinctive horizonation of a podzol. Organic matter Bryozoa /brÿ-ŏ-zoh-ă/ (Ectoprocta)A is rapidly formed owing to the deciduous phylum of small aquatic colonial inverte- forest vegetation being decomposed by the brate animals including the moss animals more abundant soil fauna. There is a thor- and sea mats. A bryozoan colony, known ough mixing of the mull humus into the A as a zooarium, is made up of numerous horizon. Carbonates are completely polyps, each of which inhabits a small tube leached from the soil but there is no move- of calcareous or chitinous material. The ment of the sesquioxides, as is shown by colonies show great diversity in shape: the constant silica:sesquioxide ratio down many resemble seaweeds and are attached the profile. Consequently there is a weakly to the sea floor; others are encrusted on the developed B horizon. These soils cover hard parts of other animals or rocks. Fos- large areas of the middle latitudes, often sils of the Bryozoa are known from rocks forming on deposits of the Pleistocene of the Cambrian Period onward and the glaciations. With their good crumb struc- group is still flourishing today. Bryozoans ture, mild acidity, and free drainage they have a potential use in micropaleontologi- are important agriculturally. See also acid cal stratigraphy and correlation. brown soil; brown podzolic soil; sol lessivé. Buchan spell /buk-ăn/ A period of the year when temperatures were anomalously brown podzolic soil /pod-zol-ik/ A soil cool or warm as deduced by the Scottish that is transitional in location and proper- meteorologist Alexander Buchan (1829– ties between BROWN EARTHS and PODZOLS. 1907), working on 50 years of records An Ea (eluvial) horizon is still lacking but prior to 1867 for SE Scotland. The periods they are more acid in nature and have a of below-average temperature were Feb. lower base status than a brown earth. An 7–14, April 11–14, May 9–14, June increase in free iron oxide results in a loose 29–July 4, Aug. 6–11, and Nov. 6–13, and crumb structured B horizon. These soils of above-average temperatures July 12–15, are typical of the northeastern USA where Aug. 12–15, and Dec. 3–14. Although they are found to the south of the true pod- there is some evidence of such periodicities zol zone. They fall into the SPODOSOL order from year to year they are not sufficiently of the US SOIL TAXONOMY. persistent or clearly defined to specific dates and so Buchan’s spells are largely of brucite /brooss-ÿt/ A white or greenish climatological curiosity only. mineral with a layered structure consisting of magnesium hydroxide, Mg(OH)2. It Buchan zone See zone (def. 1). commonly occurs as a hydration product of periclase in thermally metamorphosed buchite See contact metamorphism. dolomites. bulk modulus The ratio of compressive Brückner cycle /brûk-ner/ A climatic stress in rocks to the resulting change in cycle of about 35 years, which was de- volume. duced by the German geographer and climatologist Eduard Brückner (1862–1927) buoyancy (in meteorology) The temper- in 1890 on the basis of non-instrumental ature difference between parcels of warm

50 bytownite air and the surrounding cooler environ- butte A small upstanding mass of rock, ment. Because air density is inversely pro- usually consisting of resistant capping ma- portional to air temperature, parcels or terial overlying some softer though pro- thermals of warm air have an upward ac- tected rock type. It has steep sides and is celeration through the cooler surroundings produced as a result of long-continued just as a cork rises through water because back-wearing of a MESA. of its lower density. Where parts of the Earth’s surface are heated differentially, Buys Ballot’s law A law stating that if the warmer parts will heat the air in con- an observer stands with his or her back to tact, which will then possess buoyancy rel- the wind, then pressure will be lower on ative to its surroundings. This is the basis the left-hand side than to the right in the N of natural convection within the atmos- hemisphere. For the S hemisphere the con- phere. verse is true. This means that winds will blow in a counterclockwise manner Burgess shale /bur-jĕss/ A bed of black around a depression in the N hemisphere, Middle-Cambrian shale in British Colum- bia, Canada. It is the site of significant dis- and vice versa in the S hemisphere. The law is named for the Dutch meteorologist coveries of the FOSSILS of invertebrate animals, which occur as films of carbon be- Christoph Hendrik Diederik Buys Ballot tween the BEDDING PLANES of the rock. (1817–90). buried topography A preexisting land- bysmalith /biz-mă-lith/ A steep-sided scape that has been subsequently buried by vertical igneous body, roughly cylindrical younger strata. in form and following a steeply inclined fault. It arches up the overlying country bush An area of scrub in an isolated wild rock or becomes exposed at the surface. landscape, as occurs in parts of Australia, New Zealand, southern Africa, and the bytownite /bÿ-town-ÿt/ A calcic plagio- USA. clase FELDSPAR.

51 C

caatinga /kah-ting-gă/ A type of thorny calcicole /kal-să-kohl/ (calciphile) A plant scrub that occurs in northeastern Brazil. that grows best on chalky (i.e. alkaline) The plants include acacias, cacti, and other soils. See also calcifuge. drought-resistant vegetation (there is very little rainfall in winter). calcification (in soils) The redistribu- tion and accumulation of calcium carbon- cadastral map /kă-dass-trăl/ Usually a ate in soils. The process is especially large-scale map, showing the boundaries of characteristic of arid and semi-arid areas subdivisions of land. It is used for record- where the natural vegetation is grassland ing ownership of land. and the soil parent material contains calcium carbonate derived from limestone. Cainozoic /see-nŏ-zoh-ik, sen-ŏ-/ See Carbonic acid reacts with the calcium car- Cenozoic. bonate and the products remain dissolved in solution as ions. This solution may be calamine 1. See hemimorphite. carried down to the B horizon where it is 2. In the UK, another name for SMITH- precipitated, or carried upward as a result SONITE. of capillary action from water below. The accumulation of calcium carbonate may calc-alkaline /kal’k-al-kă-lÿn, -lin/De- form a calcic horizon. scribing igneous rocks that have relatively lower sodium and potassium contents and calcifuge /kal-să-fyooj/ (acidophile)A higher calcium contents for a given silica plant that grows best on acid (i.e. lime-free) percentage than do ALKALI rocks. The calc- soils. See also calcicole. alkaline volcanic suite is represented by the association of basalt, andesite, and rhyolite calcilutite /kal-sil-yŭ-tÿt/ A clastic lime- in orogenic regions and island arcs. stone of average grain size less than that of CALCISILTITES, i.e. less than 0.004 mm in di- calcarenite /kal-ka-rĕ-nÿt/ A clastic ameter. LIMESTONE of average grain size between that of CALCISILTITE and CALCIRUDITE. It is calcimorphic soil /kal-să-mor-fik/An composed of particles between 0.06 mm alkaline soil occurring within broad cli- and 2.0 mm in diameter. Calcarenites are matic zones where parent material is the subdivided into: very fine, between 0.06 dominant factor in soil formation. The and 0.12 mm; fine, from 0.12 mm to 0.25 parent material is calcareous and its ex- mm; medium, from 0.25 mm to 0.5 mm; treme nature resists any development to- coarse, from 0.5 mm to 1.0 mm; very ward acidity. Such soils are usually dark, coarse, having diameters from 1.0 to 2.0 organic-rich, and abundant in soil fauna. mm. Two main soil types can be recognized belonging to this group: the RENDZINA and calcareous /kal-kair-ee-ŭs/ Describing the BROWN CALCAREOUS SOIL. rocks or soils that contain CALCIUM CAR- BONATE. For example, limestone and chalk calcirudite /kal-sir-yŭ-dÿt/ A clastic lime- are calcareous rocks. stone of average grain size larger than that

52 Cambrian of CALCARENITES, i.e. greater than 2 mm in lation with another continuously operating diameter. undirectional non-reversible process such as EVOLUTION, using the fossils contained in calcisiltite /kal-siss-ăl-tÿt/ A clastic lime- the rocks. stone of a grain size intermediate between that of CALCILUTITES and CALCARENITES. It is caliche /kah-lee-chay/ See calcrete. formed of particles between 0.004 mm and 0.06 mm in diameter. Those of grades up caliper log A subsurface logging tech- to 0.03 mm are fine calcisiltites; those nique, which records the variations with above this are coarse calcisiltites. depth in the diameter of an uncased bore hole. calcite /kal-sÿt/ The principal and most stable mineral form of calcium carbonate, calorie A unit of heat now superseded CaCO3. It is the chief component of lime- by the joule. It was based on the amount of stone and marble, and occurs in the shells heat required to raise the temperature of 1 of mollusks. It forms colorless, white, or gram of water by 1°C. Because this amount gray hexagonal crystals, sometimes col- depended on the initial water temperature, ored by impurities. See also carbonate min- the standard calorie was taken as that at erals. 15°C, raising the water temperature from 14.5°C to 15.5°C. 1 cal (15°C) = 4.1855 calcium carbonate A white solid joules. (CaCO3) that occurs naturall in two forms: CALCITE and ARAGONITE. It is largely insol- calving The production of icebergs by uble in water but is soluble in water that the splitting off of large slabs of ice from a contains dissolved carbon dioxide. See also glacier or ice sheet at the edge of the sea. carbonate minerals. cambering An apparently increased dip calcrete /kal-kreet/ (caliche) Crusts and toward valley bottoms found within hori- nodules of LIMESTONE precipitated at or zontally bedded solid rocks located above near the surface of the ground in semiarid weaker unconsolidated rocks, such as regions. It results from evaporation of clays. No clear explanation has yet been moisture from the soil and may be associ- discovered, for this phenomenon. ated with deposits of gravel and sand. Cambrian /kam-bree-ăn/ The earliest caldera /kal-deer-yă, -dair-/ A volcanic period of PHANEROZOIC time and of the PA- crater whose diameter exceeds 1 km and LEOZOIC Era. Rocks laid down during this can reach up to 20 km, generally resulting period are the first to show an abundance from the collapse or explosive removal of of fossils, which consist of primitive repre- the top of a volcano. See also supervol- sentatives of most of the invertebrate ani- cano. mal phyla known today. The Cambrian began about 570 million years ago, follow- Caledonian orogeny /kal-ĕ-doh-nee-ăn/ ing the PRECAMBRIAN, and was succeeded The Lower Paleozoic orogeny in which the about 505 million years ago by the OR- Caledonian mountains, extending from DOVICIAN. The Cambrian System is often Ireland via Scotland to Scandinavia, were divided into Early, Middle, and Late. The formed. It resulted from the closure of the name Cambrian is derived from the ancient Proto-Atlantic Ocean, between the Baltic name for Wales (Cambria), where rocks and Canadian Shields. containing the earliest fossils were first studied. Cambrian rocks occur across all calibration (in stratigraphy) The deter- continents, the most complete being in mination of a rock sequence in relation to North America and Siberia, and are pre- an independent timescale. This scale may dominantly sedimentary in origin. Many be determined radiometrically or by corre- Cambrian rocks show evidence of deposi-

53 Campbell–Stokes recorder tion in or near shallow areas during marine lack of precipitation hinders weathering of transgressions into continental areas, and the sides and hence their degradation. oceans extended over most of what is now Many canyons are formed by erosion of North America during the period. horizontally bedded rock alternations, Cambrian fossils represent the animals which produce a stepped valley cross-pro- that lived in the seas of this period. Trilo- file, owing to their differential resistances bites were especially abundant and are to erosion. Others are said to have been used in the stratigraphic subdivision of the formed by cavern collapse or by the frozen system. Other important groups include ground and blocked valley phenomena of the brachiopods (inarticulate species being the Pleistocene. See also submarine dominant), gastropods, primitive echino- canyon. derms, and ostracods and there is evidence from TRACE FOSSILS of a variety of worms. capacity 1. (stream capacity) (in geol- Bivalves and graptolites had appeared by ogy) The maximum amount of sediment of the end of the period. a certain size that a stream can carry as bed load. This decreases as the grain size of the Campbell–Stokes recorder See sun- sediment becomes larger, but increases as shine recorder. stream gradient becomes steeper or discharge becomes greater. Since the trans- camptonite /kamp-tŏ-nÿt/ An alkaline porting surface is the stream bed, the wider LAMPROPHYRE. it is the more it can transport; to cancel this out capacity is usually expressed as a Canadian Shield A large area of Pre- weight of sediment per unit width of bed. cambrian rock that occupies 5 million sq Even so, capacity is still very much a func- km of Canada. Most of the shield consists tion of bed width, since for a given slope of GRANITE and banded GNEISS; the remain- and discharge, the velocity at the bed is der includes volcanic rock and some sedi- greater in a wide shallow stream than a mentary deposits. There are extensive narrow deep one. If the grain sizes being deposits of metal-bearing minerals, includ- transported are a fair mixture of large and ing those of copper, gold, iron, nickel, and small, capacity varies with the third power silver. of the velocity, with a lower power if the material is mostly coarse, and with a higher cancrinite /kan-kră-nÿy/ See feldspath- power if it is mostly fine. oids. 2. (carrying capacity) (in ecology) The maximum BIOMASS a region can support, or cannel coal /kan-ĕl/ A fine-grained lus- the maximum number of animals it can trous bituminous coal which burns with a sustain during the harshest time of the bright smoky flame; the common fuel coal. year. If the capacity is exceeded, there will Geologically, it is an example of a car- be insufficient resources, such as food, for bonaceous rock. See also coal. the population. The number of animals must fall, through starvation, failure to re- canyon A deep steep-sided section of a produce, or migration. river valley, the depth of which considerably exceeds its width, normally found in capillarity /kap-ă-la-ră-tee/ (in soil sci- arid or semiarid regions. Canyons are usu- ence) The mechanism whereby capillary ally produced where rivers have been water moves vertically up the soil profile deeply incised owing to ANTECEDENCE, or from the groundwater table or moist sub- as a result of downcutting by rivers whose soil; it is a process typical of arid and semi- sources are located in areas of greater pre- arid zones where evaporation of water cipitation than those in which the canyons exceeds precipitation. It is more effective are formed. This continuous external on claylike than sandy soils, with a maxi- source of water enables efficient down- mum height of rise of 2.5 m in clays, 0.7 m ward erosion to proceed, while the local in sands. As the water rises it brings with it

54 carbonate minerals dissolved salts and these are precipitated at inal material from which planets were the point where the capillary ‘current’ fi- formed. nally dries out, which may be at the soil surface or within the profile, forming a salt carbonate minerals A group of miner- 2– accumulation, e.g. in SOLONETZ and SOLON- als in which the anion (CO3) is the fun- CHAK soils. In arid zones subject to irriga- damental unit. The common rock-forming tion, excessive watering artificially carbonate minerals fall into three sub- accelerates the capillary currents, and for- groups as follows: merly fertile areas can be ruined by the vast 1. calcite – CaCO3, magnesite – MgCO3, salt deposits that result. siderite – FeCO3, rhodocrosite – MnCO3. capillary water /kap-ă-lair-ee/ (in soil 2. dolomite – CaMg(CO3)2, ankerite – science) Water held in the small pores CaFe(CO3)2. within the soil, existing as a film around 3. aragonite – CaCO3, strontianite – soil particles. It is this water that is mostly SrCO3, witherite – BaCO3. taken up by plant roots for plant growth, The calcite and dolomite minerals have as opposed to the gravitational water that trigonal symmetry. Dolomite results from rapidly flows through the soil and removes the substitution of calcium ions in calcite plant nutrients during wet periods, and hy- by the divalent cations Mg2+ and Fe2+, and groscopic water, which is the water that re- there is continuous replacement of Mg2+ mains in the soil even after air drying and by Fe2+ between dolomite and ankerite. is not available to plants. Calcite and aragonite are polymorphs of calcium carbonate, the latter being or- capillary wave A water wave whose thorhombic and the higher-pressure form. length is less than 2.5 cm. Its speed of prop- Calcite is usually colorless or white with a agation is largely determined by the mea- hardness 3 (see Mohs’ scale of hardness) sure of surface tension that exists in the and cleaves into perfect rhombs. Its ex- water, a force that tends to restore the treme double refraction (see birefringence) water surface to a horizontal position. As is apparent in crystals of the transparent with all sea waves, the generation of capil- variety Iceland spar. Calcite effervesces lary waves requires the transference of en- strongly in cold dilute hydrochloric acid, ergy from wind flow to the surface water in other carbonates reacting weakly unless the sea. Capillary waves may develop, for the acid is warmed. Most limestones con- instance, from the action of dropping a tiny sist largely of calcite, which may be a pri- stone into a pond. mary precipitate or in the form of fossil shells. Calcite also occurs as a secondary cap rock 1. A layer of SHALE or other im- cementing material in sediments. Veins and pervious rock that overlies porous rocks beds of fibrous calcite are called beef. Dur- containing deposits of oil or natural gas. ing metamorphism, a pure limestone re- 2. A layer of anhydrite, gypsum, calcite, crystallizes to form marble but if impure and sulfur that forms a hard covering on calcite reacts it produces such minerals as top of a SALT DOME. In the Gulf Coast re- diopside, wollastonite, and grossular gar- gion of the USA, cap rock is a major source net. Calcite occurs in hydrothermal veins, of sulfur. in amygdales, and as a primary magmatic mineral in carbonatites. carbonaceous chondrite /kar-bŏ-nay- Magnesite is usually white or colorless shŭs/ A stony METEORITE that contains and occurs as an alteration product of CHONDRULES in a claylike matrix of sili- magnesium-rich rocks under conditions in cates. These meteorites get their name be- which carbon dioxide is available. cause of their carbon content – up to 3% Rhodocrosite is pink and is found in meta- mainly in the form of hydrocarbons. Some somatic veins and pegmatites. Siderite is experts believe that they resemble the orig- usually brown and is a major constituent of

55 carbonation bedded ironstones, an important iron ore. sheets in ijolite complexes and are often It is also found in hydrothermal veins. surrounded by zones of FENITIZATION. Such Dolomite is colorless, white, or gray alkaline complexes are confined to stable and may form as a primary precipitate. continental regions, particularly those that Dolomitization of calcite and aragonite have been subject to rifting. takes place by reaction with magnesium- bearing sea water or by the permeation of carbon cycle The series of chemical re- magnesium-rich solutions along cracks and actions that circulate carbon through the joints in limestones. During metamor- global ECOSYSTEM. Plants take up carbon phism, dolomite breaks down to produce dioxide during PHOTOSYNTHESIS, convert- periclase and brucite. Yellow-brown ing it to carbohydrates and releasing oxy- ankerite is found with dolomite in hy- gen. The plants use the carbohydrates as drothermal veins and carbonatites. ‘food’ (or animals eat the plants), oxidizing Aragonite is colorless or white and is them during respiration to release carbon metastable at normal temperatures and dioxide. The burning of FOSSIL FUELS and pressures. Many shells are formed of arag- forest trees also releases carbon dioxide onite, which in time undergoes recrystal- into the atmosphere, contributing to the lization to calcite. Aragonite occurs as a GREENHOUSE EFFECT. See also carbon sink. primary precipitate and in amygdales. Strontianite and witherite are usually col- carbon-14 dating See radiocarbon dat- orless, white, or yellow, occurring in hy- ing. drothermal veins and carbonatites. The basic copper carbonates, malachite carbon dioxide A gas occupying only a (Cu2(OH)2CO3) and azurite (Cu3(CO3)2- small proportion of the atmosphere (OH)2), are bright green and bright blue (0.05% by weight) but having very impor- respectively. Malachite has a banded bo- tant consequences. It has the property of tryoidal form and occurs with crystals of absorbing radiation in wavelengths similar azurite in the oxidized zones of copper de- to those emitted by the Earth, thus it pre- posits. vents the loss of much terrestrial radiation and maintains, together with water vapor carbonation /kar-bŏ-nay-shŭn/ A pro- and some other trace gases, a higher tem- cess of chemical weathering that involves perature for the Earth than would other- the dissolving of soluble rocks and miner- wise occur (see greenhouse gas). It is vitally als by weak carbonic acid, formed by the important in plant growth for the process combination of water and carbon dioxide of PHOTOSYNTHESIS. The combustion of fos- derived from the atmosphere or from soils. sil fuel, such as coal and oil, releases car- Carbonation is most effective on lime- bon dioxide and the amount in the stones, in which calcium carbonate is con- atmosphere is now some 30% higher than verted to calcium bicarbonate and is before the Industrial Revolution. It is be- removed in solution. lieved that this could have the effect of increasing radiation absorption and so raise carbonatite /kar-bon-ă-tÿt/ An igneous the Earth’s mean temperature. However, rock composed chiefly of the carbonate the oceans and the terrestrial biosphere minerals calcite, dolomite, and ankerite. (plants and soil) are a great store of carbon Apatite and magnetite are common acces- dioxide and absorb part of the increase (see sories together with the silicate minerals, carbon sink). See greenhouse effect. alkali feldspar, nepheline, melilite, biotite, melanite, and sodic pyroxene. Certain ele- Carboniferous /kar-bŏ-niff-ĕ-rŭs/ The ments, particularly niobium, barium, period of the PALEOZOIC Era that followed strontium, and the rare earths, are concen- the DEVONIAN and preceded the PERMIAN. It trated in carbonatites, which are often of began about 360 million years ago and economic importance. Carbonatites occur lasted until about 286 million years ago. as central intrusive masses, dikes, and cone North American geologists divide this unit

56 Carnivora of time into two periods, the MISSISSIPPIAN tains fossils of freshwater bivalves, which and PENNSYLVANIAN, corresponding ap- are used stratigraphically. Fish remained proximately to the time represented by the abundant and amphibians became more Lower and Upper Carboniferous respec- common, some of them evolving into rep- tively. The lower part of the Carboniferous tiles at the end of the period. System, sometimes referred to as the Di- nantian, is formed of two divisions, the Carboniferous Limestone The lowest Tournasian and Viséan Stages. The upper lithological division of the CARBONIFEROUS part, the Silesian, consists of the Na- System in Britain. It corresponds to the murian, Westphalian, and Stephanian Lower Carboniferous (or Dinantian). Stages. Evidence suggests that most of the con- carbonization /kar-bŏ-ni-zay-shŭn/ The tinents of today formed two superconti- process that leads to the preservation of nents in the Carboniferous Period: FOSSILS as thin films of carbon in sedimen- Gondwanaland (present-day Africa, South tary rock. As sediments are laid down America, India, Middle East, Australia, under water, organic matter decomposes and Antarctica) was located in the S hemi- with the release of hydrogen, oxygen, and sphere and Laurasia (present-day North nitrogen, leaving only the carbon. Plants, America, Greenland, and N Europe) in the arthropods, and other invertebrates, and N hemisphere. By the end of the period fish have all been preserved in this manner. these continental plates had collided, clos- See also Burgess shale. ing the Tethys Sea; the mountain-building episodes of the Variscan and Allegheny carbon sink A pool or reservoir that can orogenies are attributed to these move- absorb or take up carbon from another ments. Large tropical swamps, from which part of the CARBON CYCLE. The atmosphere, the coal deposits of the later Carboniferous terrestrial biosphere (plants and soils), originated, extended across what is now oceans, and sediments can be regarded as North America, Europe, and Siberia. The carbon sinks. rocks of the Lower Carboniferous or Mis- sissippian are characterized by limestones cardinal points The four main points or formed in shallow seas of the continental directions of the compass: north, south, shelves, although deeper-water facies of east, and west. shales and sandstones were also formed. (In Britain limestone is so characteristic of carnallite /kar-nă-lÿt/ A white mineral this division that it is often called the Car- salt, a double chloride of potassium and boniferous Limestone.) The shallow seas magnesium, KMgCl3.6H2O. It occurs in supported a diverse fauna of Foraminifera, underground salt deposits derived from sea corals, Bryozoa, brachiopods, crinoids, water, and is used in fertilizers as a source blastoids, and other invertebrates, some of of potassium. which contributed to reef building. The Upper Carboniferous saw the alternating carnegieite /kar-nay-gee-ÿt/ See feld- transgression and regression of seas over spathoid. coastal swamps and facies mark a return to more terrestrial and freshwater conditions, carnelian /kar-neel-yăn/ (cornelian)A being composed of cyclothems of deltaic translucent red to brown type of CHAL- sandstones and shales with coal seams. The CEDONY, which gets its color from inclu- coal is especially prominent in the upper sions of hematite. It is used as a part of the division, being formed from semiprecious gemstone. vast forests of primitive land plants, such as ferns and horsetails. (In Britain the Carnivora /kar-niv-ŏ-ră/ The order of Upper Carboniferous is often divided into the class MAMMALIA that includes cats, two units: the Millstone Grit and the Coal dogs, bears, weasels, etc. Carnivores have Measures.) The Upper Carboniferous con- teeth modified for eating flesh and a skele-

57 carnotite tal structure suited to rapid locomotion folds developed on the limb of a major and the capture of prey. They evolved from fold, as a result of gravity collapse. the INSECTIVORA during the Paleocene Epoch. Early Cenozoic forms, classified as cassiterite /ka-sit-ĕ-rÿt/ A brown to the Creodonta and now extinct, included black mineral form of tin dioxide, SnO2, the largest carnivores known, some having found in hydrothermal veins and metaso- skulls up to one meter in length. Modern matic deposits associated with acid igneous carnivore groups evolved during the late rocks and in alluvial deposits. Eocene and Oligocene. cast A type of fossil consisting of a carnotite /kar-nŏ-tÿt/ A yellow mineral PSEUDOMORPH in which the skeletal parts of containing uranium and vanadium, the organism have been dissolved and the K2(UO2)2(VO4)2.3H2O. It occurs in sand- resulting space replaced by a secondary stone or near petrified wood, is highly ra- material, producing a replica of the origi- dioactive, and used as a source of uranium. nal form. In some cases a cast may preserve features of only the outer or inner surfaces carrying capacity See capacity (def. 2). of a structure such as a shell. These are known respectively as external and inter- cartography The technique and science nal casts. Compare mold. of representing spatial relationships by means of maps (including geographic castle kopje /kop-yay/ A type of INSEL- maps, plans, charts, and globes). This in- BERG developed by DEEP WEATHERING and cludes working from original surveys, pho- EXHUMATION in a rock divided by evenly tography, and other COMPILED MAPS, spaced vertical and horizontal joints. covering every aspect of map production Stacks of these fairly regular blocks give a from carrying out surveys to final printing castlelike stepped appearance. The term of the completed copies, including such has also been applied to heaps of joint- problems as MAP PROJECTIONS and research blocks in haphazard arrangements, which into all available information. Digital car- have resulted from the collapse of other tography, the use of computer-based infor- landforms or from the exhumation of iso- mation systems in the production and use lated CORESTONES. Inselberg domes may of maps, is replacing many of the more tra- degenerate into similar features following ditional methods. undercutting and collapse. cartometric testing /kar-tŏ-met-rik/A CAT See clear-air turbulence. technique enabling a cartographer to assess the accuracy value of a map provided the cataclasis /kă-tak-lă siss/ The deforma- map concerned is gridded. Accepted math- tion of rocks by the mechanical process of ematical formulas are used to test the hor- shearing and granulation. Such cataclastic izontal and vertical accuracy of the map rocks are said to have undergone disloca- against a control. The resultant answer tion metamorphism and range from gives the average deviation of the distances coarsely broken breccias to intensely de- or elevations on the map from the true po- formed mylonites. sition or elevation on the ground. catastrophism /kă-tass-trŏ-fiz-ăm/ The cartouche /kar-toosh/ A panel on a map theory, now generally thought to be false, giving its title, scale, etc. The cartographers that past geologic changes have occurred of the Dutch Renaissance and the Eliza- as a result of a number of sudden catastro- bethans specialized in highly decorative phes (compare uniformitarianism). A suc- cartouches. cession of catastrophes was also invoked to explain the extinction of organisms, linked cascade fold One of a series of minor to special creations to account for the appearance of new forms.

58 celestite catchment area The area from which a The best-known example is a green type of river and its tributaries obtain their water. CHRYSOBERYL. It may cover a few square meters or cover a large part of a continent (such as the cauldron Any collapse structure of vol- Amazon basin). Rainwater may run off the canic origin, as where a block of country surface into the river or seep into the soil rock sinks into the underlying magma. and flow to the river as GROUNDWATER. See There are no limitations as to the size of the also drainage basin. structure or the extent of subsequent erosion. catena /kă-tee-nă/ The regular repetition of a characteristic sequence of soil profiles cave A hollow that is formed in rock associated with a particular topography in by erosion, particularly by the action of the tropics. Relief and drainage are the water on LIMESTONE. Erosion starts by en- dominant controlling factors of formation. larging joints in BEDDING PLANES and at G. Milne first used the term when mapping other points of weakness. Underground the soils of East Africa in the 1930s. There streams can carve subterranean caverns, has been confusion as to whether it refers and wave action at the coast can form to soils on a uniform lithology or whether caves in cliffs. There the erosive effect may it can be used where another soil-forming be augmented by CORRASION. Water satu- factor besides relief and drainage varies rated with carbonates may drip into caves (the term toposequence is used in the latter and form STALACTITES AND STALAGMITES. instance). cavitation One of the processes of flu- cation exchange /kat-ÿ-ŏn/ (in soil sci- vial erosion that characterizes the high-ve- ence) An internal chemical phenomenon in locity parts of streams, such as waterfalls soils whereby a positively charged ion and rapids. It is relatively rare compared (cation) held on the surface of a colloid (ei- with the common processes of CORROSION ther a clay or humus particle) can be ex- and ABRASION, but very effective in terms of changed for another cation present in the erosive power where it does occur, being surrounding electrolyte. Cations include largely responsible for the rapid destruc- the soil bases, notably calcium, potassium, tion of waterfalls and rapids. Constriction sodium, and magnesium, and the acidity- of flow raises the velocity and kinetic en- inducing cations, notably hydrogen and ergy of a stream, which is compensated by aluminum. A typical example is for a cal- a decrease in the pressure of the water. This cium ion, held by its positive charge to the may lead to the formation of bubbles in the negatively charged surface of the colloid, stream, which subsequently collapse, giv- to be replaced by hydrogen from the suring off severe shock waves when velocity rounding fluid. This makes the colloid decreases as the channel widens again and more acid, the displaced calcium possibly water pressure can rise. The stress pro- being leached away. The cation exchange duced by the collapsing bubbles exerts a se- capacity of the soil is expressed in milli- vere stress on the channel walls, speeding equivalents per 100 grams of clay. This up erosion. varies with different colloids: in the simplest most highly weathered clays (kaolin) cay /kee, kay/ (key) A small sea island. It it is lowest, perhaps 3–15 m.e./100 g, is generally low-lying, possibly having a lit- reaching 80–100 in the 2:1 lattice clays tle vegetation, and usually formed in sand (e.g. montmorillonite), and then highest of or coral. The term as applied for example all, perhaps 200+ m.e./100 g, in humus col- to tiny islands or islets off the South loids. Florida coast refers to quite small coral shoals. cat’s eye Any of various semiprecious gemstones that in reflected light display a celestite /sell-ĕ-stÿt/ (celestine) A white, bright band, like the pupil of a cat’s eye. blue, or red mineral form of strontium sul-

59 celsian fate, SrSO4. It crystallizes in the or- forms. The Alpine episode of orogenic ac- thorhombic system, and generally occurs tivity extends into this era. See also geo- associated with halite and gypsum, in nod- logic timescale. ules in limestone, or with sulfur (as in Sicily). It is used as a source of strontium centigrade scale /sen-tă-grayd/ See Cel- and its compounds. See also barytes. sius scale. celsian /sell-see-ăn/ A barium-contain- central meridian The line of longitude ing type of FELDSPAR, BaAl2Si2O8. It often that forms the axis of a map projection. occurs associated with manganese ores. Usually the central meridian is in the middle of the projection, as in the Celsius scale /sell-see-ŭs/ A scale of tem- SANSON–FLAMSTEED PROJECTION. perature named for the Swedish astronomer Anders Celsius (1701–44), who was central-vent volcano A volcano that the first to divide the interval between the has one central vent, around which there is freezing point and boiling point of water a tendency for volcanic debris to accumu- into 100 parts. Although it was formerly late into a roughly symmetrical cone. known as the centigrade scale, an international agreement on units now favors the centrifugal force /sen-trif-ŭ-găl, -yŭ-găl/ term Celsius. The apparent force acting outward from the axis of rotation in a rotating system. Its cementation /see-men-tay-shŏn/ A rock- magnitude is v2/r where v is the linear ve- forming process in which fragments are locity and r is the radius of curvature of the bound together by a cement precipitated body’s path, or ω2r where ω is the angular between the grains, making a solid sedi- velocity of the body. This force is equal and mentary rock. These natural cements in- opposite to the centripetal force, which clude calcite, carbonates, iron oxides, and tends to maintain the body in its curved silica (the commonest). path. In the atmosphere, these two forces operate when air is moving in a curved Cenozoic /see-nŏ-zoh-ik, sen-ŏ-/ (Caino- path relative to the ground surface (i.e. zoic; Kainozoic) The era of geologic time around a depression). It is also a part of the following the MESOZOIC and beginning observed force of gravity, which is the re- about 65 million years ago. It includes the sultant of the true gravitational attraction PALEOGENE and NEOGENE Periods. Formerly toward the Earth’s axis and the centrifugal it was divided into the TERTIARY and QUA- force resulting from the Earth’s rotation. TERNARY Periods but Tertiary is no longer However, the centrifugal acceleration is recommended, according to the Interna- less than 1% of the acceleration of free fall. tional Commission on Stratigraphy. and proposals have been made for the Quater- centripetal acceleration /sen-trip-ĕ-t’l/ nary to be formally recognized as a sub-era The acceleration of a body traveling in a of the Cenozoic. The Cenozoic has some- curved path. It is equal and opposite to the times been incorrectly referred to as being CENTRIFUGAL FORCE per unit mass. synonymous with the Tertiary. Following the extinction of most of the reptiles, which centroclinal /sen-trŏ-klÿ-năl/ Describ- were the dominant animals of the Meso- ing a basin in which the rock strata all dip zoic, the previously insignificant mammals toward a central low point, characteristic underwent evolutionary radiation into an of cratonic areas. See also pericline. abundant, diverse, and dominant group. For this reason the Cenozoic is often Cephalopoda /sef-ă-lop-ŏ-dă/ A class of known as the age of mammals. Birds and the phylum MOLLUSCA whose members flowering plants also flourished and the in- usually have an internal or external shell vertebrate types characteristic of the Meso- divided into chambers and a foot modified zoic were replaced by essentially modern into a set of tentacles surrounding the

60 chamosite mouth. All cephalopods are marine. They chalcanthite /kal-kan-th’ÿt/ A bright are usually subdivided into three groups: blue mineral form of hydrated copper sul- the NAUTILOIDEA, the extinct AMMONOIDEA fate, CuSO4.5H2O. It crystallizes in the tri- and the Coleoidea (or Dibranchiata), clinic system and occurs in oxidized which includes the modern squid and octo- regions of copper deposits. pus and the extinct BELEMNOIDEA. Fossils of cephalopods are known from the Cam- chalcedony /kal-sed-ŏ-nee, kal-sĕ-doh- brian onward and have been used as zone nee/ A very fine-grained lustrous semi- fossils. transparent type of silica, SiO2, which may be colored by impurities (see silica miner- cerussite /seer-ŭ-sÿt/ A mineral form of als). It generally occurs as a deposit in cavities in lava and sedimentary rocks. It is lead carbonate, PbCO3. It crystallizes in the orthorhombic system, and is a sec- used as a semiprecious gemstone. ondary lead mineral, often formed by the oxidation of GALENA, and is used as a chalcocite /kal-kŏ-sÿt/ (chalcosine)A source of lead. dark gray or black lustrous mineral form of copper sulfide, Cu2S. It crystallizes in the chabazite /chab-ă-zÿt/ A colorless, orthorhombic system but is more often white, or pink hydrated calcium aluminum found as masses. It is an important source silicate, with some sodium and potassium, of copper. (Ca,Na )Al Si O .6H O. It crystallizes in 2 2 4 12 2 kal-kŏ-fÿl the hexagonal system and is a member of chalcophile / / An element that generally occurs in association with sulfur, the ZEOLITE group. especially as a sulfide. Chalcophile elements include arsenic, copper, lead, mer- chain The main equipment used in cury, silver, sulfur, and zinc. See also CHAIN SURVEYING, consisting of links of atmophile; lithophile; siderophile. thick steel wire each connected to the next by three rings. Most chains now in use chalcopyrite /kal-kŏ-pÿ-rÿt/ (copper have an overall length of 20 m, each indi- pyrites) A brass-yellow copper iron sul- vidual link being 200 mm long from its fide, CuFeS , found in hydrothermal and center to the center of the middle connect- 2 metasomatic veins and associated with the ing ring. Markers designate each whole upper portions of acid igneous intrusions. meter, while those at five-meter intervals It is the most important source of copper. have a different color. chalcosine See chalcocite. chain surveying (chaining) A surveying method used for areas containing concen- chalk A very fine-grained pure white trated detail, such as urban areas, where LIMESTONE (calcium carbonate, CaCO3) high accuracy is required. The CHAIN is run formed predominantly from COCCOLITHS, out along a convenient line, the direction of but including other invertebrate skeletal which is obtained using a prismatic com- fragments. It is the characteristic rock of pass or theodolite. Points of detail are then much of the upper CRETACEOUS system in fixed by measuring the perpendicular dis- W Europe; the White Cliffs of Dover, SE tance from the chain to the feature using a England, are a famous example of exposed tape, these measurements being known as chalk deposits. Extensive deposits also offsets. The distance along the chain and occur in the USA. the length of the offset, for every detail point, are noted on the booking sheet. As chalybite /kal-ă-bÿt/ See siderite. an alternative to offsets, a point may be fixed by measuring two lines, known as chamosite /sham-ŏ-zÿt/ A gray-green ties, from different parts of the chain, iron-containing mineral, a member of the thereby forming a triangle. CHLORITE group.

61 Chandler wobble

Chandler wobble /chan-dler, chahn-/ 2. The impact mark frequently seen on a The small measurable wobble of the Earth beach cobble (see percussion mark). with respect to its axis of rotation, discovered in 1891 and found to have a period of chelation /ki-lay-shŏn/ (in soil science) 14 months and an amplitude of 0°5′. Its The combination of sesquioxides with or- origin is unknown but it is assumed to be ganic acids. Water passing through leaf lit- connected with movement of materials inter acquires an extract from the organic side the Earth. It is named for the American material that is capable of combining with astronomer Seth Carlo Chandler (1846– the metallic cations in the soil, notably iron 1913). and aluminum, to form a chelate of organic and metal constituents. Experiments sug- channel Any waterway with free-flow- gest that leaf litter is a more powerful ing water, such as a natural stream or river, chelating agent than peat or humus, and a connection between two lakes or seas, or that the process involved is one of reduc- an artificial canal. tion and solution that renders the metal more mobile. The chelating agent in the lit- chaparral /shap-ă-al/ A type of low ter is easily extracted by water passing over dense scrub with evergreen vegetation, it, but its strength varies between different such as that in California and other parts types of litter, pine needles being among of the USA. See also maquis. the most powerful. This is generally thought to be an important process in pod- charnockite /char-nŏ-kÿt/ A rock that is zolization. mineralogically similar to leucocratic GRANULITES and contains hypersthene, cheluviation /ki-loo-vee-ay-shŏn/ The quartz, and feldspar with or without gar- combination of CHELATION and ELUVIA- net. Unlike granulites, the quartz does not TION. Water with organic extracts forms a occur in a platy flattened form. Basic vari- chelate with sesquioxides in the soil and eties rich in plagioclase feldspar may be then moves down through the profile car- termed enderbite. The formation of rying the sesquioxides in solution. It is charnockites is controversial but it is likely most effective in moving iron and alu- that they originate either by the crystalliza- minum. tion of granite magma under high-grade metamorphic conditions or by the high- chemical weathering The breakdown grade metamorphism of preexisting ig- of solid rock through a number of chemical neous rocks. reactions, which may cause the removal of cements, resulting in weakness, or the for- chart A specialized form of map used for mation of secondary minerals less resistant navigation and other specific purposes, e.g. to erosion than those of the fresh rock. The synoptic (weather) charts, star charts. The different types of chemical action that are most common chart is the hydrographic important in weathering are CARBONATION, chart. HYDRATION, HYDROLYSIS, LIMESTONE SOLU- TION, OXIDATION, and REDUCTION. chatter mark 1. A crescentic fracture seen in hard rock subjected at some time to chenier /shin-ĕ-ree/A BEACH RIDGE on a the passage of GLACIER ice. Although the mudflat, particularly in an estuary, consist- convexity of these marks can point in ei- ing of sand or shell fragments. As mudflats ther direction, a study of the fracture plane change form through water action, the from which rock material has been re- coarser material in the sediments tends to moved will indicate the actual direction of concentrate in such ridges. former ice movement. The marks are believed to be created by the impact and chernozem /cher-nŏ-zem, cher-nŏ-zyom/ rolling of boulders held loosely within the (black earth) A pedocal similar to a base of the glacier. prairie soil. It is typically found in conti-

62 chloritoid nental interiors, such as the Russian chiastolite /kÿ-ass-tŏ-lÿt/ A variety of steppes, where there is an annual precipita- ANDALUSITE in which impurities are tion of approximately 500 mm with a arranged in the form of a cross. See alu- slight rainfall maximum in summer. The minum silicates. process of calcification is dominant resulting in a soil with an A Cca C system of Chile saltpeter /chil-ee/ A mineral form horizons (a B horizon is absent from true of sodium nitrate, NaNO3, that occurs as chernozems). The dark base-rich A hori- surface deposits in Chile. It is an important zon, has mull humus resulting from the de- source of nitrates for making fertilizers and composition of the natural vegetation of explosives. grassland, incorporated to a considerable depth by rich faunal activity. This horizon chimney See smoker. may extend down to 100 cm before passing into the Cca horizon of calcium carbonate china clay See kaolin. concretions. The parent material often resembles loess. These soils with their neutral chinook /chă-nook, -nûk/ A type of föhn to slightly acid reaction and their excellent wind that occurs on the E slopes of the crumb structure are agriculturally some of Rocky Mountains. It is named for a Native the most important in the world. They fall American people that lived near the mouth of the Columbia River and origi- into the MOLLISOL order of the US SOIL TAX- nally referred to a warm moist SW wind ONOMY. that blew across this area. Later it was used for a warm dry wind on the E slopes blow- chert An extremely hard, white, gray, or ing from the Chinook region. The term is black type of CHALCEDONY that occurs as now only used for the dry wind bringing masses or layers in limestone. The nodular sudden increases of temperature and very form is known as flint. See also silica min- low humidities with rapid thaw conditions erals. when snow is on the ground. chestnut soil A pedocal typical in the chlorite /klor-ÿt, kloh-rÿt/ The group of Ukraine and the Great Plains of North minerals having a general composition America. Conditions here are more arid (Mg,Al,Fe)12 (Si,Al)8O20(OH)16. They are than in the chernozem belt and conse- structurally similar to the micas and are quently the natural vegetation consists of a composed of alternating layers of talc, tussocky grass cover. These soils are much Y6Z8O20(OH)4, and brucite, Y6(OH)12, shallower than chernozems and horizona- where Y = Mg,Al,Fe and Z = Si,Al. Chlo- tion is not so distinct. The A horizon is rites are monoclinic and typically green or chestnut brown in color and has a platy or white in color. They are common in low- prismatic rather than a crumb structure. grade metamorphic rocks, particularly This merges at about 25 cm into the B hori- those of the greenschist facies. Chlorites zon, which is lighter in color owing to the also occur as secondary minerals in ig- presence of calcium carbonate and often neous rocks owing to the hydrothermal al- gypsum. The calcium carbonate may form teration of ferromagnesian minerals such a distinct concretionary horizon (Cca) at as pyriboles. Oxidized chlorites such as only 50 cm depth. This lies above the par- chamosite, which have a high content of ent material, which is often loess. These iron, are found in argillaceous sediments as soils in the American West are subject to authigenic and detrital grains. severe wind erosion and are used mainly for grazing. They fall into the MOLLISOL chloritoid /klor-ră-toid, kloh-ră-/ A mem- order of the US SOIL TAXONOMY. ber of a group of dark green minerals with 2+ the composition (Fe ,Mg)2Al(OH)4Al3- chevron fold An accordian fold whose O2(SiO4)2, having a structure made up of 2+ limbs are of equal length. layers (Fe ,Mg)4Al2O4(OH)8 and Al6O16

63 Chondrichthyes linked by SiO4 tetrahedra. Chloritoid min- tative spatial distributions are shown by erals are found in aluminum- and iron-rich color tinting (e.g. of coalfields in an area). low-grade metamorphosed argillaceous sediments. chorographic map /kor-ŏ-graf-ik, koh- rŏ-/ A map on which regions are delin- Chondrichthyes /kon-drik-th’ÿ-eez/ Car- eated, usually large regions, e.g. countries tilaginous jawed fish, including modern or continents. Atlases and small-scale wall sharks and rays. Because of the absence of maps, particularly those showing political bone, good fossils are rare, although teeth divisions, come into this category. are often preserved. The Chondrichthyes probably evolved from the PLACODERMI choropleth map /kor-ŏ-pleth, koh-rŏ-/ A and are almost exclusively marine. They map showing quantitative spatial distribu- first appeared in the late Devonian and tions (e.g. of population) calculated from continued to be important through the average values per unit area. Mesozoic and Cenozoic to the present day. Compare Osteichthyes. chromite /kroh-mÿt/ A brownish-black mineral form of iron and chromium oxide, FeCr O , a member of the SPINEL group. It chondrite /kon-drÿt/ A stony METEORITE 2 4 that contains chondrules. Compare achon- crystallizes in the cubic system and occurs drite. in basic igneous rocks. It is the main source of chromium. chondrule /kon-drool/ A small rounded /kron/ The smallest interval of ge- body usually of olivine or enstatite found chron ologic time in the hierarchy of the in stony METEORITES. Chronomeric Standard terms used in CHRONOSTRATIGRAPHY. The equivalent chop The sea surface under the influence Stratomeric Standard term, indicating the of a fairly moderate breeze, with waves body of rock formed during this time, is the having pointed crests and steep slopes, CHRONOZONE. Chrons may be grouped to- quickly developing as the breeze mounts. gether to form an AGE. /kor-day-tă/ The phylum of Chordata chronomere /kron-ŏ-meer/ (in chrono- animals possessing, at least primitively, an stratigraphy) Any interval of geologic time. internal skeletal rod, the notochord, run- Chronomeres are not of standard uniform ning the length of the body. Members of duration. this phylum include the VERTEBRATA, which contribute significantly to the fossil record. chronostratigraphy /kron-oh-stră-tig-ră- Other subphyla consist predominantly of fee/ The branch of STRATIGRAPHY linked small soft-bodied sessile marine creatures, to the concept of time, rather than being which are rarely preserved as fossils. Some limited only to considerations of lithology of these have an active larval stage, and the and spatial distribution (see lithostratigra- evolution of vertebrates may have begun phy). There are two parallel hierarchies of with the retention of the characteristics of formal terms. The Chronomeric Standard the larvae into the adult state. terms are applied to intervals of geologic time; the Stratomeric Standard terms are C horizon The material from which soil applied to the bodies of rock laid down is formed, the lower subsoil lying between during these time intervals. The Chrono- the upper subsoil (B HORIZON) and the meric Standard hierarchy is as follows: bedrock. It contains no humus and will not EON, ERA, PERIOD, EPOCH, AGE, and CHRON; support plant life. See also horizon (def. 1). for the major divisions of geologic time, see the table at GEOLOGIC TIMESCALE. The chorochromatic map /kor-oh-krŏ-mat- Stratomeric Standard hierarchy of terms ik, koh-roh-/ A map in which nonquanti- has no equivalents to the eon and era; it

64 cirque consists of the SYSTEM, SERIES, STAGE, and semifluid when ejected from a volcano but CHRONOZONE. The individual time in- are solid by the time they fall to the tervals are not of standard duration and, ground. See also pyroclastic rock. similarly, the rock divisions are not of uniform magnitude. Some geologists regard cinder cone (scoria cone) A small vol- chronostratigraphy as essentially identical canic cone formed from accumulations of to BIOSTRATIGRAPHY whereas others insist tephra (cinders or scoriae) that have been on a formal separation of the concepts ejected under high pressure from a narrow involved, regarding biostratigraphy as sim- vent as frothy magma, which has then ply one method of calibrating chronostrati- cooled rapidly. The tephra builds up graphic scales. around the vent to form a steep conical hill with a central crater. Cinder cones may be chronotaxis /kron-ŏ-taks-iss/ The oc- a high as several hundred meters. See pyro- currence of units of rocks of equivalent age clastic rock; volcano. in separate successions. These rock units are described as chronotaxial. Chronotaxis cinnabar /sin-ă-bar/ A bright red mer- can rarely be demonstrated with certainty. cury sulfide mineral, HgS, found in veins Compare homotaxis. See also correlation. and impregnations associated with volcanic rocks. It is the main source of mer- chronozone /kron-ŏ-zohn/ The smallest cury. division of rock in the Standard Strato- meric scheme of stratigraphic classifica- CIPW classification See norm. tion (see chronostratigraphy). It indicates the body of rock that has formed during circulation index A measure of the one CHRON. Chronozones are defined at a strength of the atmospheric circulation: particular TYPE SECTION and should be used for any wind system, it commonly named after geographical localities. At the refers to the zonal component of the west- TYPE LOCALITY a chronozone may corre- erlies or the trade winds. See also general spond to a biostratigraphical ZONE but circulation of the atmosphere. other criteria are often used in calibration and in correlation with other areas. Circum-Pacific Belt A narrow belt bordering the Pacific Ocean, in which 75% chrysoberyl /kriss-ŏ-be-răl/ A usually of the present earthquakes occur. It con- yellow to green mineral form of beryllium sists of igneous, metamorphic, and tecton- aluminate, BeAl2O4. It crystallizes in the ically deformed sedimentary rocks. orthorhombic system, often as star-shaped crystals. Some types, such as alexandrite, cirque /serk/ A rounded rock basin, are used as semiprecious gemstones. See often containing a lake or CIRQUE GLACIER, also cat’s eye. enclosed by high headwalls and sidewalls, which are steep and frequently frost-shat- chrysocolla /kriss-ŏ-kol-ă/ A bright blue tered. In Scotland cirques are called cor- mineral, a hydrated silicate of copper, ries; in Wales, cwms. The floor is usually of Cu2H2Si2O5(OH)4. It occurs in thin seams smoothed striated rock and frequently has or as incrustations, usually in oxidized a deepened basin form, with an associated copper or copper sulfide deposits. convex rock lip, which may be moraine- covered. Though varying considerably in chrysotile /kriss-ŏ-tÿl/ A white, gray, or size, many cirques possess roughly the green fibrous SERPENTINE mineral, once the same proportions having a length to height chief source of commercial ASBESTOS. ratio of about 3:1. It is believed that cirques can develop cinder A dark-colored porous volcanic from any hollow in which snow can accu- fragment, generally consisting of BASALT or mulate, but in many cases they have devel- ANDESITE. Up to 30 mm across, cinders are oped from preexisting water-eroded

65 cirque glacier

cirrostratus /si-roh-stray-tŭs/ A thin headwall high-level layer cloud composed of ice crystals. It is transparent and gives the impression of a veil spreading across the sky.

ro During the day, cirrostratus clouds pro- ta ti ice duce a halo effect, which can be seen quite on lip al slip clearly around the Sun. They are usually the precursor of precipitation, heralding deepened rock basin the approach of depressions.

Cirque (cross-section) cirrus /si-rŭs/ A detached cloud in the form of a wispy streamer or filament of ice crystals. They occur at the higher levels of features. The processes responsible for the troposphere, above 6000 m, and with their creation include: FREEZE-THAW action temperatures below –25°C. When they on the headwall, which enlarges the cirque have a hooked appearance, they are known overall; abrasion by rotationally slipping as mare’s tails and signify rain. ice, which deepens the rock basin; joint- block removal, which can act over the citrine /ait-rÿn/ A yellow or yellow- whole surface area. Cirques constitute brown type of QUARTZ, used as a semi- some of the most characteristic features of precious gemstone. Dark samples are glacial erosion. sometimes called false TOPAZ. See also silica minerals. cirque glacier A small glacier found in a valley head or depression on a mountain clapotis /kla-po-tee/A STANDING WAVE slope. Being small, they can form quickly that is generally formed when the incident (within 100 years) and are usually the first sea waves meet waves being reflected from glaciers to form and the last to disappear a vertical barrier and where the depth of during a period of glaciation. The ice oc- water in front of the barrier exceeds the curs in distinct bands reflecting successive wavelength of the incident waves. An interference pattern is set up, in which there years’ accumulation of FIRN. The ice tends is no horizontal travel of the standing wave to move downward in the upper part (firn crests. The incident waves generally ap- zone), parallel with the glacier surface in proach the barrier, perhaps a seawall, the central area, and upward in the lowest breakwater, or vertical cliff-face, more or zone, i.e. a rotational movement. This is less head on, i.e. their crests are roughly because most accumulation occurs in the parallel to the barrier. Occasionally, the upper zone, beneath the headwall, whereas same phenomenon occurs when waves ap- most melting is from the lower part. This proach a very steep beach, though in this results in over-steepening of the glacier sur- case the amount of reflection is less. face, and in order to return to a more balanced shape, rotation occurs when the class A group in the taxonomic classifi- imbalance is sufficient to overcome friction cation of organisms. Several related classes between the ice and the bedrock. See also form a PHYLUM, and a class itself is com- cirque. posed of one or more ORDERS grouped together. For example, the Mammalia and cirrocumulus /si-roh-kyoo-myŭ-lŭs/A Reptilia are two classes of the phylum cloud type indicating convection in the Chordata. See taxonomy. upper levels of the troposphere. Composed of ice crystals, the cloud has a mottled ap- clast A small piece of rock that has been pearance consisting of small convective el- removed from a larger mass by some frag- ements, often regularly arranged, with mentation process. CLASTIC rocks and sedi- clear spaces between. ments are composed of clasts.

66 cliff clastic /klass-tik/ Describing a sediment 3. Montmorillonite-smectite group: these or a sedimentary rock consisting of frag- minerals are produced during the alter- ments of broken rock (clasts) that have ation of basic rocks under alkaline condi- been eroded, transported (usually by tions and are the principal components of water), and deposited. Clastic sediments bentonite and fuller’s earth. include a wide range of sediment particle 4. Vermiculite: this mineral is produced sizes from large boulders to fine-grained during the hydrothermal alteration of bi- sediments, such as silt. They are character- otite and also occurs at contacts where acid istic of coastal and shelf areas, and espe- magma has intruded basic rock. cially the littoral zone. The particle sizes may display wide and abrupt variations, clear-air turbulence (CAT) Sudden se- for rarely are littoral or shelf sediments vere turbulence occasionally encountered neatly sorted and graded, at least over large by aircraft flying at high levels of the at- distances. Detrital sediments are often de- mosphere, which is unconnected with the scribed as having a clastic structure. Clas- vertical turbulence of convection clouds. It tic rocks (fragmental rocks), consolidated appears to be the result of large vertical clastic sediments, include BRECCIA, CON- wind shears associated with high static sta- GLOMERATE, MUDSTONE, SANDSTONE, and bility. These conditions produce the effect of two unmixed layers and on the bound- SHALE. ary between them this wavelike motion de- clay 1. Particles of a size less than 1/256 velops, producing billowing. Clear-air mm in diameter. They are usually small turbulence can be detected by high-pow- ered radar. See also Kelvin–Helmholtz in- flaky CLAY MINERALS formed during the stability. weathering of other rocks. The grade of particles immediately larger than clay is cleat A joint or system of joints devel- SILT. Together they form the ARGILLACEOUS oped within a coal seam. Generally two division of clastic sediments. sets occur at right angles to each other, 2. (in soil science) In the classification used along which the coal fractures preferen- by the US Department of Agriculture, a soil tially. separate in which the mineral particles are less than 0.002 mm in diameter. cleavage 1. The tendency of a rock to break into closely spaced planar structures clay minerals Hydrous silicates, mainly or fractures as a result of deformation or of aluminum and magnesium, occurring as metamorphism. See also fan cleavage; flow platy or fibrous crystals that have a layered cleavage; fracture cleavage; slaty cleavage. structure and the ability to take up and lose 2. In minerals, the tendency to split along water. They are the main constituents of planes of weakness in the molecular frame- argillaceous rocks and are responsible for work. the plastic properties of clay. Four main groups may be considered: cleavage plane The plane of fracture or 1. Kaolinite group: kaolinite, Al(Si4O10)- a series of planar structures developed in a (OH)8, is produced during the weathering deformed or metamorphosed rock, along and hydrothermal alteration of feldspars which it splits preferentially. Cleavage and feldspathoids under acid conditions. planes also occur in mineral cleavage. The large-scale production of kaolinite by the alteration of granite gives rise to cliff A high steep rock face. Coastal cliffs KAOLIN (china clay) deposits. are attacked at their base by wave action 2. Illite group: illite and hydromicas are the and above by subaerial weathering and dominant clay minerals in shales and mud- erosional processes. The steepest cliffs are stones and are derived from the alteration those formed in resistant massive rock of micas and feldspars under alkaline con- types, such as granite, whereas softer ditions. rocks, being more susceptible to erosion by

67 climate both waves and subaerial processes, retreat 0° 5° 10° 15° 20° 1000 |||| and degrade comparatively rapidly. As cliffs retreat a WAVE-CUT PLATFORM is frequently left projecting out to sea. 2000 – Abandoned cliffs were eroded at their base by waves at a time when sea level was 3000 – some way above that of the present, as on the melting of the Pleistocene ice sheets, so 4000 – Flandrian that they now stand some distance from 5000 – the sea. – 7000 – 8000 – The synthesis of day-to-day – climate 10 000 – WEATHER variations in a locality. The ‘normal’ climatic conditions prevailing can be summarized, but since it is impossible to – include the wide range of values in a single Devensian number and climate is not an unchanging – feature, this method on its own is not a very satisfactory indicator of actual condi- – tions. – – Climate is usually taken to include the years before present – Ipswichian following weather elements: temperature, – – precipitation, humidity, sunshine, and 100 000 – Wolstonian wind velocity. Minor aspects include cloud amount, fog, snow, thunder, and gale fre- Hoxnian quencies. The major factors are summa- Anglian – rized in terms of arithmetic averages for Cromerian specific periods, usually based on at least – Beestonian 30 years of records. To indicate the range Paztonian of values within the instrumental records, – extremes are often quoted. More recently, – – Baventian greater emphasis has been made on the fac- – tors determining climate. – – Antian On a large scale, the climate is deter- 1 000 000 |||| 0° 5° 10° 15° 20° mined by latitude, by altitude, and by location relative to the continental margins and the main circulation belts of the Earth. On Changes in July mean temperature in lowland a smaller scale, more local factors can be England important, such as aspect and degree of exposure to prevailing winds. timescales. On a large scale there would be climate model See general circulation the swings between the ICE AGES and the model. warmer INTERGLACIAL periods, continuing through the whole time spectrum to indi- climatic change Climate is not static vidual years. Most of the evidence for these but constantly changes in response to vari- changes comes from the nature of sedi- ations in the factors that control it, primar- ments deposited and the fauna and flora ily the nature of the main circulation belts. they contain, because it is obviously only If records of temperature had been main- relatively recently that documentary evi- tained for the past million years, it would dence and finally meteorological instru- be evident that throughout this period ments give us more precise indicators of the there were oscillations between warmer pe- prevailing climate. The diagram illustrates riods and cooler periods on a wide range of known changes of temperature in NW Eu-

68 climatic geomorphology rope (England) in this period. However, exceeds precipitation on average through- much smaller changes became apparent, out the year; (C) humid mesothermal cli- which would not have been preserved by mates, where the coldest month has an geologic or biological evidence, so the average temperature less than 18°C but curve is not based on consistent evidence. above –3°C, with at least one month hav- The causes of climatic changes are not ing an average temperature above 10°C; well understood and likely to prove very (D) humid microthermal, where the complex. The shorter-period fluctuations warmest month is above 10°C but the will probably have a different origin from coldest below –3°C; (E) ice climates, where the factors producing an ice age, but al- the mean temperature of the warmest though mathematical models can show month is less than 10°C. These are further changes that could be produced by varying subdivided into a large number of climatic climatic controls, it is almost impossible to types based upon seasonal temperature dif- prove that any cause or combination of ferences and variations in amount and dis- causes was the real reason for past climatic tribution of precipitation. The simplicity of changes. Among probable factors influenc- the system and its relationships with vege- ing the major changes are variations in tation types have made this classification solar output, the MILANKOVITCH CYCLES, very popular although it does have limita- the movement of continental positions to tions due to its arbitrary temperature affect the oceanic circulation, mountain boundaries. building, volcanic eruptions, and the in- A more detailed classification (see crease in the carbon dioxide content of the Thornthwaite classification) system was atmosphere (see greenhouse effect). devised initially in 1933 and modified in 1948. This was based on two climatic fac- climatic classification The climate of tors, a moisture index and a thermal effi- any individual site on the Earth’s surface is ciency index, which are independent of any unique, but there are great similarities be- other geographical factor. However, the tween many records. To rationalize the values were more difficult to calculate and, large amount of data from observing although frequently quoted as an example, points, many attempts have been made to the system is rarely used in practice. Indi- classify climates into a small number of vidual aspects of the system, such as poten- categories based on specific factors of sim- tial evapotranspiration, have been more ilarity. There are two main bases for classi- fully developed. fication, empirical and genetic, the former Other empirical classifications have being based on analysis of the observations been proposed, some being based on and the latter on the atmospheric circula- human physiological responses to climate tions from which the climate results. Most in terms of wind speed, temperature, and systems have used the empirical approach, humidity combinations, others on the rela- with the earlier ones linking this with veg- tionship between the actual climate of a etation boundaries. site and a stated ideal climate. The most popular classification, the Genetic classifications are based on the Köppen climate classification, which is still atmospheric circulations that determine used in a modified form, was proposed by climate. While being meteorologically the German meteorologist Vladimir Köp- sound, in terms of the resulting tempera- pen (1846–1940) in 1918. He adopted ture and precipitation, quite varied 50°F (10°C) and 64°F (18°C) as critical weather conditions can be found within values to produce five major climatic cate- the same circulation belt and so such clas- gories incorporating precipitation regions sifications have received little popular ap- too. These are (A) tropical rainy climates, peal. with an average temperature for every month above 18°C, no cool season, and climatic geomorphology The branch abundant precipitation that exceeds evapo- of geomorphology concerning the role of ration; (B) dry climates, where evaporation climate in landscape evolution. Landscapes

69 climatic optimum are a product of processes that are condi- climatology /klÿ-mă-tol-ŏ-jee/ The study tioned by climate, geology, and base level: of CLIMATE. The emphasis on particular as- of these three, climate is said to be the most pects of climatology has produced many important, such that by its control, areas of subdivisions, such as applied, regional, differing climate tend also to have different physical, and synoptic climatology. See landforms. The concept is particularly also meteorology. popular among European geomorphologists: the French geomorphologists Jean climograph /klÿ-mŏ-graf, -grahf/ (climo- Tricart and André Cailleux elaborated it as gram) A diagram in the form of a graph an alternative to the Davisian CYCLE OF in which climatic features at any one place EROSION concept, the difference being that are plotted against each other, for example the latter states that stage of evolution is wet-bulb temperatures against relative hu- the major factor determining the appear- midities. ance of a landscape, whereas climatic geomorphology emphasizes that it is the clinometer /klÿ-nom-ĕ-ter/ An instru- processes operating, as conditioned by cli- ment that measures angles of slope or incli- mate, that dictate the form of an area. The nation, such as DIP. role of past climatic fluctuations in influencing the appearance of current land- clinopyroxene /klÿ-noh-pÿ-roks-een/A scapes is also involved. monoclinic type of PYROXENE. /klÿ-noh-zoh-ă-sÿt/ A type climatic optimum The period during clinozoisite of EPIDOTE that differs from ZOISITE in crys- which temperatures reached their highest tallizing in the monoclinic system. levels within an interglacial stage of the Pleistocene period. The climatic optimum clint See limestone pavement. of the present interglacial occurred about 5000 years ago and is known as the AT- clintonite /klin-tŏ-nÿt/ A member of the LANTIC PERIOD. MICA group of minerals that differs optically from XANTHOPHYLLITE. climatic region An area of the Earth’s surface having relatively uniform climatic closed fold A fold that has been com- properties and usually determined through pressed so much that both limbs are paral- a climatic classification system. lel. climatic zone The present distribution closure The distance between the high- of climates on the Earth’s surface exhibits a measure of zonal symmetry with the boundaries running approximately along lines of latitude. Because of this, eight prin- July cipal climatic zones have been distin- June 30°C August guished within which there is a certain 20°C September amount of homogeneity. These zones are: May 10°C the tropical rainy climate near the Equator, 0°C the two steppe and desert areas north and April October south of the Equator, the humid temperate zones, a Boreal climate in the N hemisphere, and the two polar zones. March November

February December climatograph /klÿ-mat-ŏ-graf -grahf/A January circular diagram showing the seasonal temperature variations of an area in graph- Climatograph: mean monthly maximum tem- ical form. peratures at Kew (London)

70 cloud forest est point of a structure of folded rocks or and cirrostratus clouds in the form of fila- structural traps and the lowest contour ments but without tufts or hooks; spissatus that totally encloses the structure. The clo- – denoting dense opaque cirrus cloud, sure determines the maximum area in often originating from the cumulonimbus which the structure can contain a fluid, e.g. anvil; uncinus – denoting cirrus clouds that oil. are hooked or comma-shaped and frequently parallel; stratiformis – spread out cloud A mixture of minute water into an extensive horizontal layer, applied droplets and/or ice crystals suspended in to altocumulus and stratocumulus clouds; the atmosphere and produced by conden- nebulosus – exhibiting a nebulous veil with sation or freezing of water vapor. They re- no distinct details, usually cirrostratus or main suspended within the cloud because altostratus; lenticularis – lens-shaped, with of their small size and grow only when con- sharp margins (most commonly applied to ditions are favorable for precipitation. altocumulus cloud of orographic origin); Lower clouds are usually composed almost castellanus – having a castellated or crenel- entirely of supercooled water droplets, but lated appearance connected to a common with cooler temperatures ice crystals be- cloud base (altocumulus show this charac- come more frequent until at temperatures teristic most frequently); floccus – having a below –40°C the cloud is entirely ice crys- ragged base and a small tuft or protuber- tals. Almost all clouds are found within the ance with cumuliform character above (cir- troposphere and are the result of cooling. rus, cirrocumulus, and altocumulus); This can be brought about by convection, fractus – denoting stratus or cumulus uplift over mountains, or large-scale ascent clouds with a broken and ragged appear- in depressions. See also cloud classifica- ance; humilis – denoting fair-weather cu- tion. mulus clouds, showing small vertical development and sometimes flattened; cloud base The point at which the fre- mediocris – denoting cumulus clouds of in- quency of water droplets in the atmosphere termediate vertical growth; congestus – de- becomes sufficient to reduce visibility to noting active and developing cumulus that characteristic of cloud. From the clouds, often with the appearance of a cau- ground surface this appears to be a very liflower; calvus – denoting an intermediate rapid transition but in reality it represents stage between cumulus and cumulonimbus a zone of transition. Cloud base indicates in which no cirriform part can be distin- the level at which rising air reaches satura- guished; capillatus – denoting a cumu- tion and condensation results. lonimbus with distinct cirriform appearance frequently in the form of an cloud classification Various classifica- anvil. If any further distinctive properties tions of cloud have been proposed, but the of arrangement or transparency are appar- one most commonly used is based on cloud ent then these are distinguished as vari- appearance and height. The major groups eties. In this way, three names could be are divided into ten genera following plant used to classify a particular cloud. Cirri- taxonomic classification, comprising CIR- form clouds are normally above 5000 m, RUS (Ci), CIRROSTRATUS (Cs), CIRROCUMU- alto-clouds from 2000–5000 m, and strat- LUS (Cc), ALTOSTRATUS (As), ALTOCUMULUS iform (St, Sc, Ns) at lower levels. Cumulus, (Ac), STRATUS (St), STRATOCUMULUS (Sc), cumulonimbus, and to some extent nimbo- CUMULUS (Cu), CUMULONIMBUS (Cb), and stratus develop vertically and so do not fit NIMBOSTRATUS (Ns). The determination of this height classification very well. cloud species is based on the appearance of the cloud, on its structure, and if possible cloud condensation nucleus See con- on the physical processes involved in its densation. formation. 14 species names are commonly used and can be referred to more than one cloud forest A type of forest that grows cloud genus: fibratus – applied to cirrus on tropical mountains at high altitudes.

71 cloud patterns

The trees obtain most of their moisture as ANTHRACITE. During this process the per- water that condenses out of the air as centage of carbon increases and volatiles clouds. and moisture are gradually eliminated. These are all woody or humic coals. An- cloud patterns From satellite images it other group are termed sapropelic coals, is evident that clouds exhibit a degree of and these are derived from algae, spores, large-scale organization that produces dis- and finely divided plant material. See also tinctive patterns. In the analysis of these cannel coal. images, patterns such as vortices, spirals, comma-shaped masses, wave clouds, lines, coalescence See collision–coalescence and striations may be distinguished. process. cloud seeding A method of trying to Coal Measures The uppermost of the induce clouds to give precipitation. It at- three lithological divisions of the CAR- tempts to simulate the natural precipita- BONIFEROUS System in Britain. It corre- tion mechanism either by adding dry ice sponds to the upper half of the Upper (frozen carbon dioxide), which causes Carboniferous. spontaneous freezing of water droplets and increases their rate of growth, or by adding coastal deposit All the deposits found silver iodide, which has a similar crystal at any particular coastal location are de- lattice to ice and so acts as ice nuclei to as- rived from one or more of the following sist the natural processes. four sources: (1) the cliffs along the coast in Although some success has been appar- that vicinity; (2) inland, transported by ent, it is difficult to prove that seeding can rivers or the wind; (3) offshore, moved increase rainfall because the clouds that landward by wave action; (4) farther along were seeded may have given rain naturally the coast, transported by LONGSHORE DRIFT and the observed increases are not suffi- (this material must at one time have been cient to disprove this possibility. derived from one of the other three sources). The nature of the materials and Cnidaria /nÿ-dair-ee-ăn/ A phylum of their quantities will depend upon the rock simple multicellular animals that includes types found along the coast and inland and the jellyfish and CORALS. They are aquatic, upon their relative contributions. A whole mainly marine, and may be colonial or soli- range of deposits, from massive boulders tary, attached or free-swimming. There are through shingle and sand to fine clays, can two types of individual: the polyp, which is be found in different coastal sites. cylindrical and sedentary, and the medusa, which is disk-shaped and free-living. The coastal plain A gently sloping plain Cnidaria contains three classes: the HYDRO- leading from the foot of inland upland ZOA, which have both polyps and medusae; areas down to the coast, and largely con- the SCYPHOZOA (jellyfish), which have only tinuous with the continental shelf under medusae; and the ANTHOZOA (e.g. corals), the sea. Such plains are formed by the con- which contain only polyps. The corals are tinuous processes of erosion in the inland geologically the most important cnidarians areas, transportation of material seaward, because many of them are colonial, form- and its deposition either on land, near the ing reefs, and possess hard parts, which can coast, or out to sea. Some coastal plains be preserved. have resulted from the comparatively recent emergence of parts of a flat continen- coal A carbonaceous deposit formed tal shelf due to a relative fall in sea level. from fossil plant remains. Coalification proceeds from partially decomposed veg- coastline The landward limit of the etable matter such as PEAT, through LIG- beach, the boundary between the coast and NITE, subbituminous coal, BITUMINOUS COAL, the shore, or the line that forms the bound- semibituminous coal, semianthracite, to ary between the land and the water. Per-

72 cold glacier haps the simplest application of the term is low to take the coastline as the extreme upper 1008 limit of direct wave action, which is the 1012 limit of storm wave swash during equinoc- 1016 high tial spring tides. 1020 mb 1024 COL high 1020 coastline of emergence A shoreline 1016 that reveals evidence that the land has risen 1012 with respect to the sea. Typical features in- Fig. 1: Anticyclonic col clude wave-cut platforms with overhang- ing cliffs, and raised beach terraces. 1012 1008 1004 1000 coastline of submergence A shoreline 996 that reveals evidence that the land has sunk COL 992 with respect to the sea. Typical features in- low low clude many bays and continuing erosion of 1004 1000 cliffs and headlands by wave action. 1008high Fig. 2: Cyclonic col cobble A rounded fragment of rock that is larger than a pebble but smaller than a Pressure patterns in a col boulder. The accepted size range is between 65 mm and 255 mm across. 2. A depression forming a pass over a mountain ridge or other high ground, com- coccoliths /kok-ŏ-liths/ Minute round monly formed by back-to-back cirque de- calcareous plates that formed part of the velopment or from the beheading of a protective covering of a group of unicellu- dip-slope valley in a cuesta formation by lar algae of the phylum Haptomonada (or scarp retreat. Haptophyta). They have been reported from the Upper Cambrian but were more cold front A concentrated thermal gra- common in the Mesozoic, especially the dient whose movement is such that warm Cretaceous, when they contributed largely air is replaced by cold air. Such fronts are to the formation of chalk. usually found to the rear of a depression and are associated with a sudden veering of coesite /koh-ĕ-sÿt/ A very dense form of the wind, often with a marked increase in QUARTZ, SiO2, stable at high pressures and speed, a fall of temperature, a belt of cloud, occurring in and around craters caused by and sometimes heavy precipitation. The meteorite impact. See also silica minerals. front slopes upward into the atmosphere at an angle of about 1 in 50. All these charac- cohesion (in geology) In a rock or soil, teristics refer to the ideal front, and the property by which particles or surfaces whereas most have somewhat similar adhere together. It is the part of the mate- properties, each individual will have slight rial’s shear strength that does not depend differences. See also warm front. on friction between the particles or surfaces. cold glacier (polar glacier) A glacier within which the temperature remains so col 1. A pattern of isobars resembling a low that pressure-melting does not occur. geographical col or saddle, i.e. a shallow In contrast to temperate glaciers these con- dip at a high level. A col that is elongated tain very little or no meltwater to act as a along the high-pressure axis is known as an lubricant for movement. The glacier may anticyclonic col, and if along the low-pres- remain static, being frozen to the bedrock, sure axis is called a cyclonic col. Winds are and requires great shear stress to induce light in this type of pressure system but the movement, which will be very slow. These pattern is rarely long-lasting. glaciers contain very little ENGLACIAL ma-

73 cold occlusion terial. However, cold glaciers have been the droplets merge together, they increase found with large bed loads, and because in size to form raindrops. Those smaller these can only become accumulated as a re- drops that are swept into the rear of larger sult of basal freezing and thawing (see drops may also be absorbed, the mecha- glacial plucking) it is believed that the ero- nism being known as rear capture. A rain- sion responsible may have taken place dur- drop becomes unstable once it reaches ing a period of warmer climate. around 5 mm and breaks up into smaller drops. The collision–coalescence process cold occlusion See occlusion. occurs in tropical convectional clouds, but is also applicable to clouds in the mid- cold pole That location in each hemi- latitudes. See also Bergeron–Findeisen sphere where the lowest air temperatures process. have been measured. In the N hemisphere this is the area of NE Siberia where at both collision zone The region where tectonic ° Verkhoyansk and Oymyakon, –68 C movements have caused two continents to ° (–90 F) has been recorded. The mean Jan- collide, with the closure of the ocean be- uary temperature at the former station is tween them. See also plate tectonics. –51°C (–60°F), which represents the coldest spot on average. In the S hemisphere, colluvium /kŏ-loo-vee-ŭm/ (talus) The the high-altitude research stations on the weathered debris (scree, mud, etc.) accu- Antarctic Ice Plateau have the record with ° ° mulated at the foot of a slope, which has a temperature of –89.2 C (–128.6 F) at originated from the erosion of the rockface Vostok. Both cold poles are in areas where above. radiational cooling is extreme under clear skies and low relative humidity. colony An association of animals of the same species having structural and physio- cold pool An area of cold air within the logical connections between the individu- atmosphere surrounded by air of higher als. Examples of colonial animals are the temperature at that level. Because this usu- BRYOZOA and the reef-forming corals (see ally leads to a steepening of the LAPSE RATE, Anthozoa). it may lead to instability and the development of showers. color index An index of the relative colemanite /kohl-măn-ÿt/ A white or col- proportions of light and dark minerals in orless mineral form of hydrated calcium igneous rocks. The following divisions are made according to the percentage of dark borate, Ca2B6O11.5H2O. It crystallizes in the monoclinic system and occurs as mas- minerals: sive crystals in clay nodules originating in leucocratic 0–30% Cenozoic lakes. It is an important source of mesocratic 30–60% boron. melanocratic 60–90% hypermelanic 90–100% collision–coalescence process A the- The terms felsic and mafic are synonymous ory for the process by which raindrops are with light and dark minerals respectively. believed to grow in warm clouds, based on In nearly all cases dark minerals are ferro- the mechanisms of collision, coalescence, magnesian minerals. and rear capture. Cloud droplets form by condensation of water vapor onto cloud columbite /kŏ-lum-bÿt/ A mineral that condensation nuclei (see condensation). consists of a mixed niobate and tantalate of The larger droplets, with terminal veloci- iron and manganese, (Fe,Mn)(Nb,Ta)2O6, ties increasing in proportion to their diam- in which there is more niobium than tanta- eters, begin to fall faster than the smaller lum. It is so called because niobium was droplets and, through collision with the once known as columbium, and is found in smaller droplets and coalescence in which granites and pegmatites. See also tantalite.

74 composite map columnar joint One of a series of columns of rock material having a unit hexagonal or pentagonal joints resulting in cross-sectional area must have the same columns of igneous rock, especially basalt mass. See Pratt’s hypothesis. lava. A good example is at Fingal’s Cave, Staffa, Scotland. competence 1. (in hydrology) A measure of the largest size of grain that a comagmatic /koh-mag-mat-ik/ Describ- stream can move as bed load. It is deter- ing a series of igneous rocks that are de- mined mostly by the stream velocity at the rived from the same original magma bed, which is controlled in turn by several through the various processes of differenti- factors, especially stream gradient, and the ation. See magmatic differentiation. ratio between width and depth: bed velocity is higher, with other things equal, in a comber /koh-mer/ A wave traveling in wide shallow river than a deep narrow one. deep water, which has a steep and high- With the larger grain sizes (coarse sand and breaking crest. The crests, much larger gravel) there is a sixth-power relationship than whitecaps, are blown forward under between velocity and competence. In any powerful wind action. The term has also one river, competence varies with flood been applied to certain types of spilling discharge over time; over space it varies BREAKERS. with turbulence in the form of eddies rising from obstructions on the bed, which exert comfort index A measure of climatic lift, momentarily increasing the maximum comfort experienced by humans. It is as- size of liftable debris. sumed that the ideal comfort state is be- 2. (in geology) The degree to which beds of tween 40 and 70% relative humidity and rock can be folded without flowage or air temperature between 60°F (15.5°C) change in thickness. Such strata respond to and 80°F (27°C). The index is subjective in deformation by fracture rather than flow. that individuals may prefer lower or higher temperatures depending upon their metab- compiled map A map that is produced olism, but it does give an assessment of cli- from other large-scale mapping rather than mate or weather conditions relative to an ideal state. original survey work. Compiled maps are usually reductions of other maps to which community (in ecology) Any group of selected amendments have been made be- plants and animals that occupy the same fore reduction. habitat and interact in various ways. Usu- ally the dominant feature, such as a typical complex A complicated body of rock type of plant, gives its name to the commu- that cannot be designated by any of the nity (e.g. beechwood). other formal terms used in LITHOSTRATIG- RAPHY. A complex is usually composed of a compaction The compression of a sedi- variety of rock types and it may have dis- ment with the result that it occupies less tinctive structural characteristics. The term volume (i.e., the layer becomes thinner). It may also be used informally to refer to any is generally caused by the weight of more complicated association of rocks. recent material being deposited above. It may also be caused by earth movements or, composite log The interpreted geologic on soil, by human activity. Compacted soil column encountered in a well, recon- is less porous, so that puddles and muddy structed from the evidence of geologic sam- patches may form after rainfall. pling and electrical investigations carried out in the borehole. compass traverse See traversing. composite map A map, usually com- compensation level A hypothetical piled, that brings together for ease of com- level within the Earth above which all parison data usually portrayed on two or

75 composite volcano more maps, e.g. land use and population take place until extreme supersaturation distribution. existed, but throughout the troposphere there are abundant nuclei on which con- composite volcano (stratovolcano)A densation can take place on saturation. volcano constructed of alternating layers These cloud condensation nuclei consist of of lava and pyroclastic deposits, generally tiny crystals of sea salts, industrial and nat- developed by volcanoes with fairly viscous ural smoke particles, as well as volcanic, lavas, for example, andesites. soil, or desert dusts. The condensation nuclei are around 0.1 µm to 1 µm in diame- concave slope A hill slope that is ter. On condensation, latent heat is steeper at the top than at the bottom. On a released equivalent to about 2500 joules map of the feature, contour lines are closer per gram of water, which reduces the rate together at the top of the slope. See also of cooling. convex slope. condensation level The level in the at- concentric dike (ring dike) A roughly mosphere at which CONDENSATION takes circular vertical dike developing when the place. It is normally used for calculating circular fractures produced by the upward CLOUD BASE on a thermodynamic diagram, pressure of magma in a chamber are in- by simulating the lifting of air of known truded by magma. The related structures properties of temperature and humidity. known as cone sheets are developed by a similar stress distribution. They surround a conditional instability The state of a point source (the intruding magma) and part of the atmosphere in which the ENVI- dip inward toward it, having the form of a RONMENTAL LAPSE RATE lies between the series of concentric conical sheets infilled dry adiabatic and saturated adiabatic lapse by magma. rates. This means that if a parcel of air rises from the ground it will cool at the dry adi- conchoidal /kon-koi-dăl/ Describing a abatic lapse rate (DALR) and so remain type of curving fracture marked by concen- cooler than the environment; under these tric arcuate ridges. It is characteristic of conditions the atmosphere is stable. How- minerals such as quartz and glassy and ever, if the parcel reaches saturation it will aphanitic rocks, particularly obsidian and then cool at a slower rate and could be- phonolite. come warmer than the environment on further uplift, and so become unstable. concordant coast (longitudinal coast) A coastline that parallels the hills and other conduction The mechanism by which features just inland of it. If the land sinks heat is transferred through a substance by with respect to the sea (a coastline of sub- molecular motion without overall motion mergence), the sea floods the valleys be- of the substance itself. Air is a very poor tween the hills, which become a chain of conductor of heat and this method of islands. transfer is relevant only when the air is absolutely calm. This may occur very close to concretion A roughly spherical body of the ground surface, but in most studies of material precipitated from a solution. the atmosphere, conduction can be neglected. condensation The physical process of transformation from the vapor to the liq- conduit See vent. uid state. In the atmosphere, condensation refers to the formation of liquid water from cone of depression A conical dip that water vapor as a result of the atmosphere occurs in the water table around a well as becoming saturated; for this to take place water is removed from the well. As a result, cooling must occur. If the atmosphere were the water level in the well gets lower and absolutely pure, no condensation would lower.

76 conical projection cone sheet See concentric dike. water remains in a liquid state despite supercooling. The process is most effective in confidence limit A statistical concept schists and certain limestones, notably used to assess the degree of confidence that chalk, within which very rapid shattering is can be attached to estimates of true means possible. See also frost-shattering. or standard deviations when based on only a sample of the data. For example, if the congeliturbation The movement of soil annual rainfall totals for a particular loca- materials in the PERIGLACIAL environment, tion are only known for_ 15 years, then the as a result of freezing and thawing of water mean of this sample (x) will probably dif- contained within the soil. Freezing is ac- fer from the true mean. By placing confi- companied by an expansion in the soil, but dence limits it can be stated that with a this is irregular because in a heterogeneous 95% probability the true mean for this lo- material water will not be evenly distrib- _ ± _ cation_ will be x 2y, i.e. between x – 2y uted. Similarly, thawing takes place at un- and x + 2y, where y is the standard devia- even rates, and the result is that particles tion of the data set. are transported considerable distances, even on flat ground. On slopes, material confining pressure (in geophysics) The migrates downslope under the influence of pressure exerted on a point resulting from gravity. The ice causing the disruption the weight of the overlying rock or water tends to be concentrated in the silt and column. sand fraction, coarser material being moved to the surface. confluence 1. In air flow, the rate of convergence or approach of one streamline conglomerate A RUDACEOUS sedimen- to the adjacent streamline relative to the di- tary rock in which the constituent clasts or rection of flow. It differs from CONVER- fragments are more or less well rounded. GENCE in that it does not imply an increase Compare breccia. of mass but simply the approach of streamlines of air flow. Compare diffluence. 2. The junction of two or more streams to form one. standard parallel North Pole conformable 1. Comprising an unbroken sequence of rock strata with no angu- 50° N lar discordance between them, marking a continuous period of uninterrupted deposition. See also discordant (def. 1). 50° 2. See accordant. Equator conformal projection See orthomorphic projection. congelifraction /kŏn-jel-ă-frak-shŏn/ The weathering of rocks through the freezing of Simple conical projections water within pore spaces. The accompanying expansion creates stresses, which result conical projection A MAP PROJECTION in fracture of the rock and its consequent produced by projecting the meridians and disintegration. The effectiveness of the parallels onto a cone. (See diagram above.) process varies considerably with rock type, The center of curvature of the cone is not being dependent on the size of the pores. If one of the poles, although the meridians these are too large, or open to the surface, are drawn radiating from it. The parallels the water may be forced out as tempera- are concentric circles and are their true dis- tures fall, whereas if they are too small, the tance apart. The scale is only correct along

77 coniferous forest the standard parallels; all others are too same original magma. They usually occur long. This projection is neither equal-area close together, are of about the same age, nor orthomorphic and is usually modified, and have similar chemical compositions. e.g. BONNE’S PROJECTION and the conical See also comagmatic. with two standard parallels. consequent stream A stream whose coniferous forest A forest of cone-bear- course is dictated by the slope of the land. ing GYMNOSPERM trees. The trees are See also obsequent stream; subsequent mainly evergreens (larches are an excep- stream. tion) with needle-shaped leaves, such as firs, pines, and spruces. Coniferous forests conservation (in ecology) Any mea- occur farther north than DECIDUOUS sures taken to preserve resources, such as FORESTS because the trees are better able to the soil, minerals, and living organisms withstand cold temperatures. Trees from within the environment. Methods include the forests, which are often managed, pro- preserving habitats and eliminating waste vide timber for building and wood pulp for of raw materials, and recycling those that paper making. are used. conjugate joint Any of a system of conservative plate boundary See plate joints in rocks, in which the sets of joints boundary. are of related origin. It generally consists of two sets, which intersect at 90° but share a conservative property A property of common dip or strike. air that remains unaltered when affected by specified processes. For example, when conodont /kon-ŏ-dont/ A small tooth- air is cooling at the DRY ADIABATIC LAPSE like structure up to 2 mm long and com- RATE, its potential temperature remains posed of calcium phosphate. Conodonts constant and is therefore a conservative are found in rocks of marine facies, espe- property under these conditions. cially shales, from Ordovician to Permian age and also in later strata, in which they constancy of wind An index of the de- are suspected as being DERIVED FOSSILS. Be- gree of constancy of wind direction. It is cause they are small, change in morphol- measured by determining the ratio of the ogy through time, and are widely vector mean wind to the scalar wind, and distributed, conodonts are particularly use- converting to a percentage value. It ranges ful in the correlation and identification of from 0 if all winds are equally strong and the rocks in which they occur. They have frequent in all directions to 100 if the been variously attributed to most phyla but winds are constant or unvarying in direc- their zoological affinities remain unknown tion. For the British Isles, values of between (see species); their widespread distribution 15 and 50 are typical, but in the trade wind suggests that they may have belonged to a zones it can approach 90. group of pelagic animals. constant slope The scree or talus slope, Conrad discontinuity A seismically i.e. the foot slope produced by the accumu- detectable boundary within the Earth’s lation of weathered rock debris from continental crust, separating it into a lower above. The evolution of the constant slope (basic) layer and an upper (granitic) layer. begins with a vertical rock face at the start Beneath the ocean floors the upper layer is of the cycle and at its peak it covers the free missing. It is named for the Austrian seis- face and buries it. At this stage the constant mologist Victor Conrad (1876–1962). slope, whose angle is defined by the ANGLE OF REST of the debris of which it is com- consanguinuity /kon-sang-gwă-new-ă- posed, dominates and largely determines tee/ The common features of different ig- the hillslope form. Later on, the washing of neous rocks that probably arose from the fines from the constant slope leads to its

78 continent

vores, feeding on the herbivores or other

free face carnivores.

e p free face lo contact The surface forming the bound- s t t n ary between adjacent and distinct bodies of n ta ta s s e n rock. It may be conformable within a sedi- on op o c sl c

free face rock wall mentary succession, unconformable, or Fig. 1: Fig. 2: Fig. 3: produced by intrusion or faulting. The Initiation Growth of Domination of recognition of contacts is fundamental to constant slope hill slope by LITHOSTRATIGRAPHY and much geologic in- constant slope terpretation.

free face contact metamorphism (thermal metamorphism) The changes resulting from e lop t s the intrusion of hot magma into preexist- tan ns ing cold country rock. The thermal effect co of the magma causes a recrystallization of wash slope minerals in the country rock and the Fig. 4: growth of new minerals. Contact meta- Wash of fines from constant slope evolves morphic rocks have equigranular textures, wash slope, which buries constant slope and ultimately dominates hill slope form have suffered little or no deformation, and in general do not possess any foliation. Thermal effects die out rapidly with in- Stages of slope evolution creasing distance from the contact with the igneous rock. Sedimentary rocks are converted to hornfels, quartzite, and marble. Hornfels often contain porphyroblasts or burial by a wash slope, or alluvial toe- spots of contact metamorphic minerals slope, which accumulates at the foot of the such as biotite, andalusite, cordierite, and constant slope. See also colluvium. garnet. With the exception of the expulsion of water from the country rocks, the meta- constructive plate boundary See plate morphism is largely an isochemical boundary. process. Four facies of contact metamorphism constructive wave A wave that assists are recognized: in the aggradation of a beach. For aggra- albite-epidote hornfels facies dation to occur the material moved land- hornblende hornfels facies ward must exceed that moved seaward, so pyroxene hornfels facies the SWASH must be more efficient than the sanidinite facies. BACKWASH. For a powerful swash the waves In an ideal metamorphic aureole, pyroxene must be flat, although their frequency can- hornfels rocks occur adjacent to the intru- not be too high, because the swash of a sion and pass into hornblende hornfels fa- new wave would be interfered with by the cies rocks farther from the contact. preceding wave’s backwash. The usual fre- Inclusions of country rock caught up in the quency of constructive waves is between igneous magma reach the high tempera- six and eight per minute. Compare de- tures associated with the sanidinite facies. structive wave. Such rocks may become partly fused, resulting in the formation of glass, and are consumer (in ecology) An organism known as buchites. See also metamor- that is above the first level in a FOOD CHAIN, phism; skarn. which is occupied by producers. Most PRI- MARY CONSUMERS are herbivores, feeding continent A large landmass, which rises on plants. Secondary consumers are carni- more or less abruptly from the deep ocean

79 continental borderland floor. Associated with the exposed land- broke up to form the present continents. mass are marginal areas (CONTINENTAL The presence of these supercontinents was SHELVES) that are shallowly submerged. based upon the work of the German mete- The continents occupy about 30% of the orologist and geophysicist Alfred Wegener Earth’s surface. (1880–1930) in 1910. His evidence was only qualitative but included distributions continental borderland A marine re- of rock types, flora, fauna, geologic struc- gion adjacent to, or lying in place of, a con- tures, and similarity of the shape of the tinental shelf, and displaying marked coastlines on either side of the Atlantic. topographic irregularity. A frequently Geophysicists of the time dismissed the hy- quoted example lies off the coast of South- potheses as impossible. However, in the ern California, a region of very diverse sub- late 1950s and 1960s geophysical evidence marine morphology, which includes supported the theory. submarine ranges and basins. Some of the basins are quite deep. Often the adjacent continental heat flow The flow of heat landmass displays similar topography. The reaching the surface of the Earth in a con- depths of continental borderlands are usu- tinental as opposed to oceanic region. ally far in excess of the depths normally encountered over shelf areas. continental margin (continental terrace) The combined continental shelf and slope continental climate The generalized zones, whose modes of origin are generally characteristic climate of continental interi- closely related. The depth range of the con- ors, implying a low precipitation total, a tinental margin may, therefore, be taken as large annual range of temperature with extending from the coast out to a depth of warm summers and cold winters, and low some 2000 m. Some continental margins relative humidities. are of a constructional kind, others dias- trophic. Warping, faulting, and long-con- continental crust The part of the tinued sedimentation have been shown to Earth’s crust constituting the continents, have played dominant roles in their evolu- often referred to as the SIAL. It is composed tion. Deep seismic soundings have revealed of granite rocks and reaches a thickness in that many have thick layers of sedimentary excess of 50 km under mountain regions, rocks overlying older basement rocks of a although it is generally only 33 km thick. crystalline nature. The lower continental crust was formerly believed to be of basaltic composition, sim- continental rise The submarine surface ilar to that of the oceanic crust. It is now between the abyssal floor and the base of thought that it may simply be a more dense the continental slope. Together with the phase of the granitic rocks. See also oceanic slope, the rise forms a separation zone be- crust. tween the continental shelves and the deep ocean basins. The surface of the rise is gen- continental drift The movement of the erally smooth, with gradients between continental blocks relative to one another 1:100 and 1:800. The minor irregularities across the surface of the Earth, as a result rarely amount to more than 20 m or so in of SEA-FLOOR SPREADING. The hypothesis of height or depth. The rise may be up to sev- continental drift was proposed in the early eral hundred kilometers wide, for example 1900s, but it was only with the advent of off part of the North African coast where it the PLATE TECTONICS theory that a viable is over 600 km wide. On the other hand, it mechanism was available to explain the may be virtually absent, as off the Bay of movements of the continents. It is thought Biscay. In some areas, quite large that the present continents were grouped seamounts are found on the rise. together in pre-Mesozoic times into two large supercontinents, Pangaea and Gond- continental shelf That part of a conti- wanaland. These continents subsequently nent that is shallowly submerged by the

80 Conulariidae sea. It is a gently dipping area extending The scale of the map determines the maxi- from the shoreline down to the continental mum possible number of contours per area slope at the SHELF EDGE, or to a depth of to retain clarity of detail; the terrain influ- 200 m if the slope is absent. The average ences the number of contours required to shelf width is about 70 km but shelves tend depict accurately the true nature of the to be wide off low-lying regions and nar- land (in level areas a few contours will row off mountainous regions, varying show the land characteristics whereas in from less than 1 to more than 1000 km hilly areas more contours are needed to wide. Average gradients tend to be steeper show the elevation variations within the near the coast than over the outer shelf. area); the purpose of the map also affects Globally, shelves occupy some 7.6% of the the interval: plans for builders and land ocean floor. Most shelves exhibit evidence surveyors require the maximum number of of marine erosion, especially in shallow contours per area to enable accurate mea- coastal waters. Some have carved within surements to be made. them valleys, troughs, and basins (see submarine canyon), often formed during low control point A point on a map whose sea-level stands. Many are veneered with position and usually elevation are known. loose sediments that are being moved and Control points form a network around sorted by wave and tidal action, and then which other map detail is plotted in its cor- molded into tidal banks and channels. rect position, azimuth, and elevation. continental slope A portion of the con- Conulariidae /kon-yŭ-lair-ee-I-dee/A tinental margin, dipping more steeply than family of fossil animals of uncertain zoo- the continental shelf, with a gradient of logical affinities. It has been suggested that more than one in forty, although this varies they belong to the MOLLUSCA, the ANNEL- considerably. It is bounded by the conti- IDA, or the classes Hydrozoa or Scyphozoa nental shelf on the landward margin, and the continental rise on the seaward side. Globally, the continental slope zone makes up about 8.5% of the total ocean floor, but apart from some SUBMARINE CANYONS that have been carefully explored and surveyed its precise form is not known. Where there contours is abundant sediment, especially mud, slumping and sliding is common and re- sea level sults in very irregular topography. submarine contours contour A line drawn on maps and charts joining points of the same elevation, Fig. 1: Cross section of coast and hill to show contour lines above or below sea level (see diagram). This line enables the user of the map to determine the elevation and slope of the land or sea bed (contours below sea level are called submarine contours). Measurements may be in meters, feet, or fathoms (submarine). See also contour interval; hypsomet- 150 ric tinting; supplementary contour. 100 contour interval The difference in 50 height between two consecutive index con- Fig. 2: Hill viewed as from above (as on maps) tours. The interval will depend on several factors: the scale of the map, the terrain the map portrays, and the purpose of the map. Contours

81 convection of the CNIDARIA. Some authorities place placed by less dense water. In this context, them, with a few other similar groups of the currents caused in such regions as the animals, in a separate and extinct phylum, Red Sea and the Mediterranean Sea by ex- the Conulariida. They occur in sediments cessive surface evaporation, increase in of marine facies and typically have a skele- surface salinity, and consequent sinking ton constructed from four flat triangular are claimed by some to constitute a con- plates joined at the edges to form an in- vection current. Convection currents are verted four-sided pyramid. The plates are also held, by some authorities, to occur formed of calcium phosphate and have an within the Earth’s mantle beneath the ornamentation of transverse ridges. Projec- oceans. One suggestion is that the higher tions from the free basal edges of these temperatures under the oceans at certain plates fold over to protect the terminal depths could be a consequence of convec- opening. Conulariids are found in rocks tion flow that must be operating at some dating from the Cambrian to the Triassic; depth within the mantle. This could result in size they range from about 5 cm to over in an increase in the temperature of certain 20 cm. parts of the ocean floor, such temperatures having been recorded. See also density cur- convection (in meteorology) The most rent. important form of heat transfer within the atmosphere. It takes place by the physical convective precipitation Precipitation transfer of air resulting from density varia- produced by the strong but localized verti- tions, warmer air being less dense than cool cal motion in convectional clouds found in air. Two types of atmospheric convection an unstable atmosphere. This is usually are distinguished: (1) free or natural con- produced from cumulonimbus clouds, but vection, which is essentially the transfer of cumulus clouds may also give slight rain. warm air from lower levels of the atmos- Because of the intensity of the upward phere through the mechanism of thermals movement within the clouds, convective or bubbles; (2) forced convection, which is precipitation is normally heavy but re- the vertical movement of air produced by stricted in distribution at any instant. mechanical rather than thermal forces, such as occurs through movement over up- convergence 1. A measure of the rate of lands or irregular vegetation surfaces. inflow of a fluid into a certain volume. The reverse state is DIVERGENCE. In the atmos- convection cell A continuous convec- phere, these terms are used to indicate the tion system, air that rises being replaced by horizontal components of wind velocity: air from elsewhere. This can occur on a convergence indicates retardation and di- small scale with a single thermal or convergence indicates acceleration. Surface vection cell rising above a plowed field convergence is usually accompanied by up- heated by the Sun. At the other extreme, ward air currents. In wave refraction phe- the meridional circulation within the Trop- nomena, convergence refers to the ics has been likened to a vast convection bunching together of the various wave or- cell with zones of rising air and the trade thogonals in the direction of wave ad- winds linking the vertical parts of the cell. vance. Wave energy tends to be This system is called the HADLEY CELL. concentrated in a zone of convergence. In the case of opposing currents, a conver- convection current A current within a gence is the boundary or zone at which fluid created by differences in temperature these currents meet. Usually convergence between layers causing differences in den- zones experience the sinking of some ocean sity. Convection currents are thought to water to greater depths. operate in and beneath the oceans. They 2. See convergent evolution. operate in the ocean when a portion of a water mass that is denser than the water convergence zone An area of the at- beneath it tends to sink, thereby being re- mosphere in which convergence prevails

82 coral bleaching and which is characterized by a belt of con- coombe /koom/ A small hollow in a vectional clouds, often with rain. Such steep chalk hillside. It generally results zones can be found within otherwise uni- from weathering, which opens and en- form air streams giving a short period of larges a joint in the rock. A mound of heavy rain as they pass. The best known is weathered material, COOMBE ROCK, may the INTERTROPICAL CONVERGENCE ZONE, slide down the hill and accumulate at the which is frequently seen on satellite pho- bottom of the slope. tographs and can extend for several thousand kilometers. coombe rock (head) A structureless deposit consisting of unweathered flints convergent evolution (convergence) within a matrix of chalk mud and disinte- The evolution of similar characters and grated chalk, resulting from SOLIFLUCTION structural modifications in organisms that movements during the PLEISTOCENE Epoch. are not closely related. For example, dol- The name is especially applied to the de- phins and ichthyosaurs, although only posits found in the chalk areas of S Eng- distantly related, have a very similar exter- land. nal form that is presumed to have been acquired through adaptation to similar en- coordinate A linear or angular quantity vironmental conditions. Convergent evolu- in a frame of reference such as a grid, by tion also occurred in marsupial and which positions of points on the map are placental mammalian groups, which were defined. isolated on different landmasses but occu- See chalcopyrite. pied similar habitats. Such organisms are copper pyrites known as homeomorphs. coprolite /kop-rŏ-lÿt/ The fossilized feces of an animal. Coprolites may contain other convergent plate boundary (convergent fossils, such as fragments of other animals, boundary) A region where lithospheric leaves, or pollen, that formed the diet of (or tectonic) plates are moving together the creature concerned and they thus pro- and the crust occupies a smaller area. It vide valuable information for paleoecol- may involve subduction, in which one ogy. plate dips below the other and is consumed into the mantle. Alternatively, one plate coral One of a group of animals belong- may force the crust on the other plate to ing to the cnidarian class ANTHOZOA. They pile up and form a new range of moun- secrete a hard calcareous skeleton and are tains. See also plate boundary; plate tec- important geologically in BIOSTRATIGRAPHY tonics; subduction zone. and because of their reef-building activities. See coral reef. convex slope A hill slope that is steeper at the bottom than at the top. On a map of coral bleaching A condition that occurs the feature, contour lines are closer to- in reef corals when the symbiotic unicellu- gether at the bottom of the slope. See also lar algae (zooxanthallae), which give the concave slope. coral its color, are expelled from the coral polyp hosts leaving the coral with a convolute bedding Highly disturbed, bleached white appearance. Coral bleach- folded, or crumpled laminae that are found ing is caused by stresses induced by within a single well-defined sedimentary changes in environmental conditions such layer, resulting from deformation of the as elevated sea temperatures. The coral sediment by slumping, gliding, loading, up- usually recovers under cooler conditions ward expulsion of water, or the effect of but if too many algae are lost it may die. passing eddies in the overlying water mass, Widespread bleaching events occurred on while the sediment is still in an unlithified the GREAT BARRIER REEF off Australia in state. 1998 and 2002 (when temperatures over

83 coral reef much of the reef were more than 2°C above the end of the long winter night, tempera- normal for more than two months). tures are little lower than they were at its commencement. It represents a balance be- coral reef A ridge of CORAL that forms tween surface radiational cooling and at- offshore in mainly tropical regions. For mospheric counterradiation from above coral animals to survive, the water should the intense surface inversion. Some theo- be shallow (allowing the penetration of ries suggest that advection of warmer mar- sunlight) and temperatures should be itime air may play a part in the coreless above 21°C. The reef may be partly ex- effect, but this is disputed. posed at low tide but is always covered at high tide. Coral reefs are often very steep- core sampling A method of obtaining sided. Close to the shore they are termed samples of rock, sediments, soil, snow, or FRINGING REEFS; BARRIER REEFS form farther ice by drilling a hollow tube into the ma- out to sea. See also reef. terial and extracting a reasonably undisturbed core sample. The core samples are corange line A line drawn on certain sliced into thin sections for analysis. Core hydrographic charts joining points of equal sampling can be used underwater to obtain tidal range. samples from the bed of a stream, lake, or sea. A wide range of coring devices have cordierite An orthorhombic mineral, been developed. Gravity cores drop under bluish in color and of composition their own weight and drive themselves into 2+ Al3(Mg,Fe )2Si5AlO18. The basal section the bed. Piston cores are placed on the bed of cordierite has a hexagonal outline and and forced into it to retrieve a sample. In often shows concentric twinning. The min- the case of hard-packed sand a vibro-corer eral is usually found in thermally meta- may be used, the vibrator operating after morphosed argillaceous rocks, where it is the coring apparatus has reached the bed. associated with andalusite and biotite. It also occurs in some high-grade gneisses corestone /kor-stohn/ A rounded hard and rarely in gabbro that has been contam- block of rock within a matrix of soft de- inated by argillaceous sediments. composed material, occurring in well- jointed rocks, notably granites. When the cordillera A chain of mountains, such jointed rocks are subjected to extensive as the parallel chains of the Andes in South weathering, it proceeds more rapidly in the America or the Rockies, Sierra Nevada, joint areas, where water can penetrate, re- and Coastal Ranges of North America. sulting in a series of corestones, but continued weathering will bring about their core The central sphere within the eventual disappearance. Individual core- Earth, separated into inner and outer units. stones, however, may be especially resis- The INNER CORE has the properties of a tant to weathering and these will remain as solid, whereas the OUTER CORE prevents the isolated unweathered blocks within a mass passage of S waves, suggesting that it is a of incoherent weathered rock. liquid. It is composed of a mixture of iron and nickel, together with some dissolved Coriolis effect /kô-ree-oh-lis/ A hypo- silicon and sulfur, and is separated from thetical ‘force’ that is required to relate the mantle above at a depth of 2900 km be- movement in the atmosphere to the rotat- neath the Earth’s surface by the GUTENBERG ing coordinate system of the Earth’s sur- DISCONTINUITY. face. For example, if a projectile was launched from the North Pole with a con- coreless winter (kernlose pattern)In stant southward trajectory, from space it Antarctica, the condition in which surface would appear to maintain this track. How- temperatures remain almost the same from ever, as the Earth is rotating from west to the beginning of winter to the end. Tem- east, to an observer on the ground surface peratures decrease toward winter, but by it would appear to be constantly deviating

84 corundum

path of projectile path of projectile sociation whatsoever to –1 for a perfect in- viewed from ground viewed from space verse correlation. The data must have a surface North Pole normal distribution for this coefficient, but others that do not have this constraint are available. The product moment coefficient is most frequently used for comparing rainfall values at different locations, e.g. the well-known work on Indian monsoon rainfall by the British physicist Sir Gilbert Thomas Walker (1868–1958), who was Director General of the India Meterologi- direction of Earth’s rotation cal Department, 1903–24. He found significant associations between rainfall in India Coriolis effect and sea-level pressure at various locations around the world. 2. (in stratigraphy) The process of equating parts of isolated geologic successions, usu- to the right (see diagram). In Newtonian ally in relation to time (see chronotaxis; mechanics, a body will maintain its course homotaxis). Strata may be correlated on unless acted on by other forces, so to ex- the basis of similarities in lithology or bio- plain this deflection the Coriolis ‘force’ is stratigraphically on the basis of similar introduced. Its magnitude is obtained from contained fossils. Other methods employ the equation 2ωv sin φ, where ω is the value radiometric age determinations or paleo- of the Earth’s angular velocity, v is air ve- magnetic information (see paleomagnetic locity, and φ the latitude. It therefore varies correlation). from zero at the Equator to a maximum at the poles. Without the Coriolis effect, wind corrie /kô-ree/ See cirque. flow would be at right angles to the pressure gradient, as found in equatorial areas, corrosion The process of erosion by instead of being parallel to it. It is named chemical solution. All common rock-form- for the French physicist Gustave-Gaspard ing minerals are to some extent soluble in Coriolis (1792–1843). water, even quartz, generally considered to be highly resistant, and so removal in solu- cornelian See carnelian. tion is a significant process. In humid areas, corrosion renders limestone one of corona structure The growth of a con- the most easily eroded of rocks, whereas in centric mineral zone around a core min- arid zones, where water is lacking, lime- eral. The second mineral may be a stone is highly resistant, demonstrating the REACTION RIM or the result of EPITAXIAL effectiveness of corrosion. Rivers draining GROWTH upon the primary mineral. areas of organic materials, especially bogs, swamps, and marshes, are rich in organic corrasion /kŏ-ray-zhŏn/ A type of ero- acids, and these accelerate corrosive ac- sion involving abrasion of a rock surface tion. See also abrasion; corrasion. by small fragments of rock carried along by a river or glacier. See also abrasion. corundum A mineral consisting essentially of aluminum oxide, Al2O3, but minor correlation 1. A statistical technique amounts of other ions give rise to a variety that determines the degree of association of colors. The blue of sapphire is due to the between two sets of data. The most com- presence of iron and titanium whereas the mon method in climatology is to use the variety ruby contains chromium. Corun- product moment correlation coefficient dum occurs in some silica-poor igneous whose value ranges from +1 for a perfect rocks such as nepheline-syenites. It is also direct relationship through zero for no as- found in hornfels.

85 cotidal line

/koh-tÿ-dăl/ A line drawn direction of protected boulder cotidal line ice movement rock clay on certain hydrographic charts joining points at which average high water (or low s n water) occur simultaneously. These lines o ti ia are often given as differences from the tr crag s times of high (or low) water at a ‘standard tail port’, or they may be expressed as time intervals after the time of the moon’s transit. A cotidal chart depicts both CORANGE LINES Crag and tail and cotidal lines. coulee /koo-lee/ A series of deep branch- the crag, while the protected rock behind, ing channels or gorges that are formed as which may also receive a covering of glac- MELTWATER is suddenly released from a ier-derived material, is known as the tail. GLACIAL LAKE dammed by ice at the foot of Together they form a semistreamlined a glacier, or from glacial ice that has been form. These features become visible only melted by volcanic activity. Well-known on the disappearance of the ice. coulees occur on the Columbia plateau in Washington, USA, e.g. Grand Coulee. crater A large bowl-shaped depression in the Earth’s surface, resulting from vol- coulee flow A steep-fronted and com- canic activity, out-gassing, or the impact of paratively short flow of viscous lava. It a meteorite. may be up to 100 m in thickness. crater lake A lake that forms in a vol- country rock The preexisting envelope canic crater or CALDERA. The water comes of rocks into which an igneous magma is from direct PRECIPITATION and runoff from intruded. The country rock often shows the crater sides. The water may be forcibly the marked thermal effects of CONTACT ejected if the volcano becomes active. METAMORPHISM. Xenoliths contained in the solidified magma are usually derived from craton /kray-tŏn/ Any of those parts of the country rock. the Earth’s crust that have been relatively stable (i.e. unaffected by orogenic activity) covellite /koh-vel-ÿt/ (covelline) An in- during the last 1000 million years, e.g. the digo-blue iridescent mineral form of cop- Zambia nucleus of Africa. Frequently they per sulfide, CuS. It usually occurs as represent the roots of deeply eroded an- compact or slaty masses in metasomatic cient mountain chains. zones or hydrothermal veins, associated with other copper minerals. creep The slow movement of soil and rock debris down gentle slopes under the crag A rocky outcrop on a hillside, char- influence of gravity: it is the slowest of all acterized by very steep edges. Most crags the types of MASS MOVEMENT, and it is not occur in hard-rock regions, resulting from certain whether it is a flow, which is move- erosion and weathering. ment by internal deformation and over- turning, or a slide whereby the whole crag and tail A glacial erosion feature mantle slips over the bedrock beneath. found in areas of relatively low relief, Current evidence rather supports the latter formed where a knob of particularly resis- explanation. Creep occurs in all environ- tant rock exists on the floor of a valley be- ments but is most important in the mar- neath a glacier. This knob will cause itime periglacial areas, where it is of a type deflection of the ice, protecting any less re- known as SOLIFLUCTION. It is said to occur sistant rock behind it. The knob, which is on all nonvegetated slopes of more than 5°, invariably striated and fluted on the stoss but in the areas subject to solifluction it can end (facing the oncoming ice) is known as occur on much gentler slopes.

86 Crinoidea

Evidence of creep is commonly seen was moving north, and Australia and when structures such as tombstones and Antarctica were also breaking away. North telephone poles lean upslope, as the ma- America was moving away from Eurasia, terial in which they stand has moved and by the end of the Cretaceous the At- downslope. Many processes are involved, lantic Ocean extended as far as the Arctic including freeze-thaw, wetting and drying, Ocean. Sea levels were higher than at any needle ice, and the preferential filling of other time in the Earth’s history. Creta- cavities from upslope. It is suggested that ceous rocks are widespread, the Early Cre- creep is the dominant process in shaping taceous characteristically deltaic and the convex parts of slopes. See also slope lacustrine, and the Late Cretaceous includ- convexity. ing marine deposits, such as sandstone and the characteristic chalk, a pure fine-grained crenulation cleavage See strain-slip white limestone formed largely of plank- cleavage. tonic COCCOLITHS. In the seas in which the coccoliths were deposited, bivalves, am- crepuscular ray An optical phenome- monites, belemnites, echinoids, and bony non of the lower atmosphere produced by fish also flourished. On land angiosperm the alternations of sunlight and shadow ra- plants appeared, and dinosaurs and other diating from the Sun, made visible by haze. reptiles reached their peak of development. The shadowing may be produced by gaps At the end of the period the ammonites and in low-level layer cloud, or by the irregular many other invertebrate groups and most profile of convective cloud. A similar effect of the reptiles became extinct. See K/T can sometimes be seen at sunset when the boundary event. Sun is just below the horizon and distinct rays are seen radiating from it, the dark crevasse A crack of variable width in the bands indicating parts cut off from the surface ice of a glacier, caused by shear Sun’s rays by clouds or hills. stresses set up by differential movements within the ice. These movements occur in a crest The highest point of an ANTICLINE. VALLEY GLACIER, the edges of which move A line joining the highest points of a given more slowly than the center, owing to the bed is the crest line; a plane including all frictional effect of the valley sides. In this the crest lines of successive folds is the crest case crevasses form pointing up the glacier. surface. Crevasses also occur where the ice moves over a steeper section of ground, in which Cretaceous /krĕ-tay-shŭs/ The final pe- case they are transverse to the valley. De- riod of the MESOZOIC Era. Beginning about spite the fact that their depths rarely exceed 135 million years ago and lasting about 70 30 m, crevasses are very important because million years, it followed the JURASSIC and they assist in the downward penetration of preceded the PALEOGENE (which marked both rock debris and meltwater. the start of the Cenozoic Era). The name derives from creta, the Latin word for crinanite /krin-ă-nÿt/ A type of ALKALI chalk, the characteristic rock of the period; GABBRO. extensive deposits form the white cliffs along the English Channel. The lower part Crinoidea /kri-noi-drr-ă/ The class of of the Cretaceous System consists of six the phylum ECHINODERMATA that contains stages: the Berriasian, Valanginian, Hau- the sea lilies. They are typically sessile ben- terivian, Barremian, Aptian, and Albian. thonic marine animals, consisting of a The six stages forming the Upper Creta- stem, formed of ossicles of calcite, which ceous are the Cenomanian, Turonian, Co- supports a cup (calyx) of calcite plates. On niacian, Santonian, Campanian, and the upper side of this is a centrally placed Maastrichtian. mouth surrounded by five feathery arms During the period South America began that bear pinnae. These are used in food to move westward away from Africa; India collection. Fossils are known from the

87 cristobalite

Cambrian Period onward and the class is changes. Valley cross profile can show still in existence today. Crinoids formed an such features as valley-in-valley forms, important group in the Lower Carbonifer- river terraces, extent of floodplains, sym- ous and contributed to the formation of metry or asymmetry of the valley-side bioherms and rocks. Most became extinct slopes, angle of valley-side slopes, etc., all at the end of the Paleozoic. Crinoids have of which contain clues to the geomorphic been used biostratigraphically as ZONE FOS- history of the river and the region through SILS. which it flows. See also long profile. cristobalite /kris-toh-bă-lÿt/ A white mi- crude oil Petroleum as it occurs in the crocrystalline type of silica, SiO2, which ground. It consists of a mixture of impure occurs in cavities in volcanic rocks and in hydrocarbons which, after purification, some thermally metamorphosed rocks. See can be separated to yield various fuels and silica minerals. compounds used by the petrochemical industry. critical angle The angle of incidence of a light ray which, if exceeded, will lead to crust The outermost shell of the Earth, total internal reflection. varying in thickness from 5 km under the oceans to 60 km under mountain ranges. critical distance (in seismic refraction The lower boundary is marked by the Mo- surveying) The distance between the source horovičić discontinuity. The crust is com- of seismic waves and that point in an upper posed of two units, the CONTINENTAL CRUST rock horizon at which the arrival time of a (also known by the acronym SIAL) and the direct wave is matched by the arrival time OCEANIC CRUST (SIMA). of a higher-velocity wave that has been refracted from a lower horizon. Crustacea /krus-tay-shă/ A class of invertebrate animals of the phylum ARTHRO- Cromwell Current See equatorial cur- PODA, characterized by the presence of two rent. pairs of antennae and one pair of mandibles. Crustaceans are almost exclu- cross-bedding (cross-stratification) The development of internal laminations sively aquatic and range in size from the within a stratum inclined at an oblique large lobsters to almost microscopic forms. angle to the main bedding planes, resulting Included in the six subclasses are the OS- from changes in the direction of water or TRACODA, important geologically in strati- wind currents during deposition. It is most graphic correlation, the Malacostraca commonly developed in sandstones. (crabs and lobsters), and the Cirrepedia (barnacles). Fossil crustaceans, mostly os- Crossopterygii /kro-sop-tĕ-rij-ee-ÿ/A tracods, are known from the CAMBRIAN Pe- group of bony fish (see Osteichthyes) riod onward. whose sole present-day representative is the coelacanth. The group is important for cryolite /krÿ-ŏ-lÿt/ (Greenland spar)A having given rise to the AMPHIBIA and hence mineral form of sodium aluminum fluo- to all terrestrial vertebrates. Unlike most ride, Na3AlF6. Rarely crystalline, it occurs modern fish, the Crossopterygii had fleshy as colorless or white masses. It is used as a paired fins. They were common from the flux in the electrolytic extraction of alu- Devonian to the end of the Paleozoic. minum and in making various ceramics. Compare Actinopterygii. cryoturbation /krÿ-oh-ter-bay-shŏn/ The cross profile A transverse section of a modification of the soil through the river’s channel or valley. A stream can ad- processes of CONGELITURBATION and the ef- just these parameters as well as its long fects of NEEDLE ICE in a periglacial environ- profile in response to environmental ment.

88 crystal symmetry cryptic layering See layered igneous rock. cryptocrystalline /krip-toh-kris-tă-lin, -lÿn/ Describing material that is crystalline yet so fine-grained that individual A crystals cannot be resolved even under the microscope. crystal A chemical substance with a definite geometrical form, having plane faces B at regular angles to each other, which is the expression of the regular arrangement of Crystal habit atoms or ions composing the substance. See crystal symmetry; crystal system. owing to the relative development of prism crystal axes Imaginary lines that pass and pyramid faces. through the center of a CRYSTAL. There are generally three axes, but four in the hexag- crystalline rock /kris-tă-lin, -lÿn/ Rock onal system. The angles between the axes formed by the process of crystallization, and their relative lengths characterize the i.e. metamorphic and igneous rock. structure of any crystal. See crystallographic index. crystallite /kris-tă-lÿt/ A microscopic embryonic crystal found in glassy rocks as a result of DEVITRIFICATION. Crystallites have rodlike, stellate, or feathery forms.

crystallographic index /kris-tă-lŏ-graf- ik/ A crystal face may be indexed accord- I III ing to a notation that depends upon the III intercepts made by the face with the crys- 100 100 tallographic axes. The distances along the axes from the origin to the points where the plane cuts are termed the intercepts. ABCThe ratios of the intercepts (referred to a standard parametral plane or axial ratio) Crystal form are termed the parameters of that crystal face. According to the system of Miller, the crystal form A complete set of regular reciprocals of the parameters of a crystal planes forming a crystal. The diagram il- face are called the indices. lustrates different forms found in the cubic CRYSTAL SYSTEM: A and B are simple crystal crystal symmetry The crystal faces of forms and C is a combination of the two minerals are symmetrically arranged and forms cube and octahedron. The form ex- reflect the internal regularity of the atomic hibited by a crystalline mineral may be a structure. Crystal symmetry is described by great aid to identification. reference to the following elements: 1. plane of symmetry – a plane along which a crystal habit The shape of a crystal aris- crystal may be cut into two equal halves, ing from the shape and size of faces and the one being the mirror image of the other development of different forms. In the dia- (fig. 1). 2. axis of symmetry – a line about gram the two crystals possess the same which a crystal may be rotated through combination of crystal forms but A has a 360° so that the crystal assumes a congru- prismatic habit and B a pyramidal habit ent position every 180°, 120°, 90° or 60°.

89 crystal system

hexad axis of Such axes are termed diad (2-fold), triad Different CRYSTAL FORMS can be re- rotation symmetry (3-fold), tetradplanes (4-fold), of and hexad (6-fold) ferred to the same set of crystal axes and Fig. 1 axes respectivelysymmetry (fig. 2). 3.Fig. center 2 of sym- hence belong to the same crystal system. metry – a central point about which every For example the cube, rhombdodecahe- face and edge of a crystal is matched by one dron, and octahedron are different crystal parallel to it on the opposite side of the forms of the same cubic system. crystal (fig. 3). On the basis of these symmetry ele- crystal tuff A type of tuff that contains ments, 32 different crystal classes are rec- many broken crystals. See pyroclastic rock. ognized together with 7 CRYSTAL SYSTEMS: crystal zoning Many crystals, especially cubic – 4 triad axes in igneous rocks, are zoned such that their tetragonal – 1 tetrad axis composition changes systematically and hexagonal – 1 hexad axis gradually from the core of the crystal to the Fig. 3 trigonal – 1 triad axis center of rim. The succession of zones reflects • symmetry orthorhombic – 3 diad axes changes in the composition of the melt in monoclinic – 1 diad axis which the crystal is growing or changes in triclinic – no axes. the physical conditions as crystallization Other symmetry elements may be pos- proceeds. sessed by crystalsCrystal symmetryin addition to those listed. For example, a cube has three cubic (isometric) See crystal system. tetrads, four triads, six diads, nine symmetry planes, and one center of symmetry. a3 c crystal system A category of crystal with reference to the position of the crystal faces in relation to the intercepts that the planes containing the faces make with a a1 1 three (or four) axes, which intersect at an a a origin. All crystals of the 32 different sym- 2 2 metry classes can be referred to seven crys- cubic tetragonal tal systems as follows (see diagram): 1. cubic (isometric) – 3 orthogonal tetrad c c axes of equal length, a1, a2, a3. 2. tetragonal – 3 orthogonal axes, 2 horizontal diads of equal length, and one β α vertical tetrad, a1, a2, c. a γ 3. orthorhombic – 3 orthogonal diad axes b b of unequal lengths, a, b, c. a 4. hexagonal – 4 axes, 3 horizontal diads orthorhombic triclinic of equal length at 120° apart and one c c vertical hexad at right angles, a1, a2, a3, c. β 5. trigonal – 4 axes, 3 horizontal diads of a equal length at 120° apart and one ver- 3 a2 tical triad at right angles, a1, a2, a3, c. 120° b 6. monoclinic – 3 unequal axes, one verti- a1 cal, one horizontal diad, and a third a making an oblique angle with the plane hexagonal and monoclinic containing the other two, a, b, c. trigonal 7. triclinic – 3 unequal axes, none at right angles, a, b, c. Crystal systems

90 cupola cuesta /kwess-tă/ An asymmetrical ridge is heavy and dense with considerable verti- produced by differential erosion of gently cal extent to give the appearance of a vast dipping strata. A cuesta is characterized by cauliflower, each protuberance being a its steep scarp face, well-defined crest, and tower of rapidly rising saturated air with gentle backslope, which largely conforms velocities of up to 30 meters per second. At to the dip of the strata. It is constructed of the top of the cloud there is often a more fi- a more resistant geology (e.g. sandstone or brous flattened cloud composed of ice crys- limestone) than the etched out foot (e.g. tals: this is the ANVIL. Most thunderstorms clay). are associated with this type of cloud. culmination The highest point on a cumulophyric /kjoo-myŭ-loh-fi-rik/ See structural feature. As the axis of a fold glomerophyric. often undulates up and down, a series of culminations separated by depressions can cumulostratus /kjoo-myŭ-loh-strat-ŭs/ develop. (See diagram at FOLD.) See stratocumulus. cummingtonite /kum-ing-tŏ-nÿt/ A cumulus /kjoo-myŭ-lŭs/ A type of cloud monoclinic type of AMPHIBOLE. indicating convectional activity extending above the CONDENSATION LEVEL. Cumulus cumulate rock A type of igneous rock clouds range in vertical extent from shallow fair-weather cumulus clouds indicating weak convection topped by an inversion, to the much more extensive cu- backslope mulus congestus, which almost approaches crest (mainly dipslope of resistant bed) a cumulonimbus cloud in size.

scarp resistant bed cup anemometer See anemometer. (e.g. sandstone or limestone) cupola /kjoo-pŏ-lă/ A slender upward nonresistant bed, projection from a large igneous body into typically clay the overlying country rock. See also roof pendant.

Cuesta cuprite /kjoo-prÿt/ (ruby copper) A red to black mineral form of copper oxide, that is formed largely by the accumulation Cu2O. It crystallizes in the cubic system of early-formed PRIMARY MINERALS that and occurs in oxidized copper deposits, crystallize from a magma and sink under sometimes associated with limonite. It is the influence of gravity to settle in layers on used as a source of copper. the floor of the magma chamber. Often the cumulus crystals trap small quantities of Curie point /kyoo-ree, kyoo-ree/ The liquid in the interstices and this liquid sub- temperature above which permanent mag- sequently crystallizes to produce intercum- netism disappears. Each element has its ulus minerals. Cumulate rocks may be own specific Curie point; in the case of iron found in large basic layered intrusions, it is 760°C. It is named for the French often of lopolithic form. Monomineralic physicist Pierre Curie (1859–1906). and bimineralic rocks are characteristic and cumulate dunites, peridotites, pyrox- current (in oceanography) A horizontal enites, anorthosites, and gabbros com- flow of water, the movement affecting the monly occur. whole water column or only a part or parts of it. The speed at which currents flow, and cumulonimbus /kjoo-myŭ-loh-nim-bŭs/ their direction, may vary markedly, or the The main rain cloud of convective origin. It current velocity may be remarkably con-

91 cuprite stant in a temporal sense. Often variations cuspate foreland An approximately tri- are seasonal, or they may be due to chang- angular accumulation of beach materials, ing meteorological factors. Permanent cur- usually shingle, the apex of which extends rents operate independently of the tides out to sea and produces an irregularity in and weather, although they are indirectly the coastal outline. Such forelands are cre- affected by these. These include the general ated as a result of the combination of SPITS circulatory current systems in the oceans. or BEACH RIDGES approaching each other Other currents flow on account of fresh- from opposite directions, owing to the ac- water discharge from rivers, the action of tion of two major wave sets in the area, waves and tidal motion, differences of each being more active on one side of the seawater density, and wind-drag. All or foreland. The growth of these features can certain of these currents may be superim- often be traced by the existence of many posed, one upon the other. See also density parallel shingle ridges. The maintenance of current; ocean current. a sharp projection out to sea largely depends upon the presence of a nearby is- current bedding A sedimentary struc- land or coastline, which provides shelter ture resulting from the action of either from direct frontal attack by destructive wind or water currents. It includes CROSS- waves. BEDDING and RIPPLE BEDDING. cutoff See oxbow lake. cusp A crescent-shaped mass of beach material, which may range from sand to cut-off high A warm anticyclone that quite large shingle or cobbles. The coarser has moved poleward from the main sub- material accumulates on the promontories tropical high-pressure belt, often produc- or horns between the bays and the finer ing BLOCKING in the westerlies. This system material in the bays. Cusps are regularly usually forms as a ridge in the upper west- spaced and generally display coarser ma- erly circulation and this intensifies into a terial than is found over the remainder of cellular form becoming detached from the the beach surface. Those formed purely in main flow like an oxbow lake (or cutoff) in sandy material are less common than those a river. Its influence then extends down- developed in shingle or in sand and shingle ward toward the surface. (See diagram.) mixtures. They point down the beach toward the sea and the margin between cut-off low A low-pressure system of coarser and finer material has a scalloped similar origin to the CUT-OFF HIGH but ex- shape. They vary greatly in size, ranging tending toward the Equator after develop- between several centimeters in height to ing from a trough in the upper westerly giant cusps or megacusps, such as those circulation. Cut-off lows frequently drift found on parts of the coast of West Africa. into the Mediterranean Sea area after cut- It seems that there exists a relationship be- off has taken place farther north. tween cusp spacing and swash length, and that a type of cellular water flow together cwm /kûm/ See cirque. with swash periodicity are important additional factors in their development, but cyanite See kyanite. their origin has yet to be explained satis- factorily. Waves breaking perpendicular to cycle of erosion A concept explaining the beach are thought to be the most con- the evolution of dissected land surfaces ducive to cusp formation. from uplifted areas of little relief to dissected landscapes and then to level surfaces cuspate delta /kuss-payt/A DELTA (peneplains) via a series of stages (youth, within which material is evenly deposited maturity, and old age). The process is said on either side of the river mouth. It is usu- to be cyclic because it will begin again on ally found on straight coasts where wave further uplift (see rejuvenation). The con- action is fairly strong. cept of strict erosional cycles is not now ad-

92 cycle of erosion

Waves break offshore and build up a barrier island, with a lagoon behind. As the offshore profile is deepened, the island is attacked by larger waves which push it onshore, migrating over the lagoon, until at Fig. 1 Fig. 2 maturity the barrier is coterminous with the original coast. It then continues to migrate, eroding the shore, until at theoretical high high old age a plain of marine planation is created. On a submergent coast, the initial low outline is very complex, as existing low drainage patterns are drowned. The headlands are subject to concentrated attack by Fig. 3 Fig. 4 refraction of waves, and as they are cut back spits begin to build out across the Cut-off high and low bays, eventually cutting them off and leading to their infill with waste. At submatu- rity the headlands are cut back and the hered to, because climatic changes mean bays closed; continued erosion pushes the that processes in any one area are not con- coast to maturity, at which point the shore- stant, and most landscapes have evidence line lies behind the heads of the initial of evolution under a series of cycles or drowned valleys. Thereafter the course is parts of cycles of different climatic types as as in the emergent case. follows: Glacial Arid The major process involved is the creation The key to the cycle of arid erosion is the of cirques and the retreat of headwalls, de- process of PEDIMENT formation. The Ger- stroying the preglacial landscape. Initiation man geologist Walther Penck and South involves the collection of snow fields and African geologist Lester King developed the creation of the cirques by nivation. this idea, and as a result arid lands are said Once created, headwall retreat eats into to evolve by a process of slope retreat (see the original surface, eventually leaving parallel retreat) not decline, such that the only horned peaks and arêtes. Such fea- trend is not the increase of relief to a max- tures typify the passage from youth to ma- imum in maturity and thereafter increas- turity. By full maturity the main regional ingly subdued relief but decrease of the valleys are filled with ice, only the highest area of plateau and increase of the area of peaks projecting above the ice surface as pediment. In youth the rivers incise them- NUNATAKS. The retreat of the headwall of selves, and firstly retreat produces valley- the glacial valleys extending from the side pediments; by maturity the divides cirques eventually exceeds the rate of re- have shrunk and the initial topography is treat of cirque headwalls, and the merging nearly lost; by old age retreat of slopes has of the two marks the end of maturity. The reached the point where divides are lost al- old age phase has never been observed: as together, and pediments of individual with the coastal cycle, only the phases of basins coalesce to give the ultimate youth and maturity are well defined. PEDIPLAIN. See also river. Davisian cycle (Normal cycle) Coastal Developed by the American geographer This cycle, evolved by Douglas Johnson in William Morris Davis (1850–1934) in the 1919, differs on emergent (recently up- latter part of the nineteenth century, this lifted) and submergent (recently drowned) was an attempt to devise an orderly se- coasts. On both types, the general pattern quence for the evolution of landscapes is a simplification of coastal outline. In the under a humid temperate climate. The se- emergent case, the coastline will be initially quence involved is as follows: 1. Uplift ini- straight, with a shallow offshore slope. tiates the stage of youth, when rivers begin

93 cyclogenesis to incise themselves into the landscape. cyclolysis /sÿ-klol-ă-sis/ The process of Steep V-shaped valleys are separated by decay or weakening of the cyclonic circula- broad flat interfluves. 2. Maturity of the tion around a low-pressure center. landscape, when widening of the river valleys has completely destroyed the original cyclone An area of low pressure with a landscape and valleys meet at sharp inter- series of closed isobars, usually of circular fluves. In the rivers, this stage occurs when or oval form, around its center. In the N the valleys are graded, and down-cutting is hemisphere, it is surrounded by a counter- replaced mostly by side-cutting and flood- clockwise wind circulation and in the S plain creation. 3. By old age the landscape hemisphere by a clockwise rotation, but is subdued owing to SLOPE DECLINE. The both are known as cyclonic circulation for streams meander widely on broad flood- each respective hemisphere. Compare anti- plains, losing their adaptation to structure, cyclone. a feature of maturity, due to its masking In mid- and high-latitudes, the cyclone beneath debris. Only MONADNOCKS on is usually referred to as a DEPRESSION. In rocks of great resistance in high divides rise tropical areas, it is a storm system of great above a peneplain. intensity with wind stronger than 64 knots Periglacial (120 km per hour) and is synonymous with This scheme revolves around the role of a hurricane or typhoon, although the frost shattering and solifluction. Originat- nomenclature is not fully standardized. See ing on a dissected landscape, frost shat- tropical cyclone. tering attacks the valley-side slopes, producing a frost-shattered cliff, which mi- cyclonic precipitation See frontal pre- grates into the divide. The debris from the cipitation. frost shattering is transported downslope by solifluction and meltwater to the val- cyclostrophic winds /sÿ-kloh-stroff-ik/ leys. As this process continues, the frost- A class of winds in which there is extremely shattered cliffs retreat until no solid rock strong curvature of the airflow, such as a remains. The scree slopes then dominate tropical cyclone or tornado. Under these the landscape, being progressively flat- conditions, the centripetal acceleration be- tened until by old age a landscape of faint comes the major control of the gradient relief is formed. wind. cyclogenesis /sÿ-kloh-jen-ĕ-sis/ The ini- cyclothem /sÿ-klŏ-th’em/ A series of tiation or development of a depression or sedimentary beds deposited as part of a sin- cyclone. gle cycle. The term is generally applied to Carboniferous strata, where the sequence cycloidal wave /sÿ-kloi-dăl/ (trochoidal begins with a sandstone layer, followed by wave) A wave with a flatter trough and a shale and freshwater limestone. Above the sharper crest than the typically smooth si- limestone is a clay layer, above which rests nusoidal wave. It is a steep symmetrical a coal seam. Following the coal there is a wave with a crest angle of some 120°. The return to marine conditions, with the de- wave form is that of a cycloid or trochoid, position of a marine shale, followed by a i.e. a curve that would be described by a marine limestone. Each cyclothem is sepa- point rotating within a circle that itself was rated from the next by a disconformity. being rolled along a straight path. With an The cycle represents an episode of emer- increase in wave steepness, the cycloidal gence and erosion. wave sharpens its crest and increases its asymmetry. Because the flow is of a rota- cylindrical equal-area projection See tional type, no mass transport is possible Lambert’s cylindrical equal-area projec- (see mass transport current). tion.

94 cylindrical projection cylindrical projection A MAP PROJEC- TION constructed as though a cylinder were placed around or cutting through the Earth, onto which the relevant details are projected (see diagram), meridians and parallels being drawn as straight parallel lines. There are three basic types: the sim- Equator ple cylindrical, LAMBERT’S CYLINDRICAL EQUAL-AREA PROJECTION, and the MERCA- TOR PROJECTION. In the simple cylindrical, the cylinder touches the Earth at the Equa- tor, which has the effect of making the poles the same length as the Equator and therefore causes considerable distortion in the higher latitudes. The scale along the Cylindrical projection meridians is correct, but directions are not true. It is a projection that is seldom used owing to the existence of more practical modified versions.

95 D

dacite /day-sÿt/ A dark igneous rock daughter element An element that consisting mainly of quartz and plagio- forms as a result of the radioactive decay of clase. See rhyolite. another element (e.g. radon is a daughter element of radium, which is in turn a darcy /dar-see/ A unit used to measure daughter element of thorium). the porosity of a rock (the permeability coefficient). It represents the resistance to Davisian cycle See cycle or erosion. flow through the rock of a fluid (gas or liquid). The usual practical unit is the milli- day Astronomically, a solar day is the darcy (md). period of time between successive occasions on which the Sun is in the meridian of Darcy’s law The rate of flow of a fluid any fixed place; the sidereal day is the time through a porous material (suck as a rock) between successive transits of a fixed dis- is proportional to the product of the per- tant star. As the Earth varies in its rate of meability of the material and the pressure movement during its orbit round the Sun, causing the flow. The law is named for the the solar day is not constant. In Britain the engineer and French scientist Henry Philib- climatological day extends for a 24 hour ert Gaspard Darcy (1803–58). period from 9 a.m. GMT. This is the time at which most climatological observations datolite /day-tŏ-lÿt/ A mineral form are made and so rainfall for April 21, for of hydrated calcium borosilicate, CaB- example, would in reality be the amount SiO4(OH). It crystallizes in the monoclinic that fell between 9 a.m. April 21 to 9 a.m. system as colorless or white prismatic crys- April 22. tals. It occurs in veins and in AMYGDALES. death assemblage See thanatocoenosis. datum /day-tŭm/ A fixed reference point. The vertical datum is a horizontal debris An accumulation on the surface line or a point used as the origin (zero) of soil or rock fragments. It may result from which heights and depths are mea- from glacial action (and is found when the sured. On most maps and charts it is that ice melts) or from processes (such as frost of mean sea level, e.g. in Britain the Ord- action) that cause rock to break into frag- nance Datum (O.D.) of Ordnance Survey ments. maps is mean sea level at Newlyn, Corn- wall, calculated from tidal observations be- debris load That part of a river’s total tween 1915 and 1921. Horizontal datum is load carried as solid material. It includes generally used in connection with geodetic material moving by suspension wholly information where it forms the basis for above the bed, by SALTATION, and by horizontal control. Horizontal datum is rolling and sliding along the bed. The total determined, using the reference spheroid, debris load is the sediment discharge of a from bearings (azimuths) and longitude river. See also load. and latitude readings. decalcification /dee-kal-să-fă-kay-shŏn/

96 deep weathering

(in soil science) The removal of calcium (such as earthworms) also fulfill this func- ions from a soil. The process involved is tion. carbonation, whereby rainwater percolating through humus becomes enriched with decomposition The weakening and carbon dioxide, forming an acid, which break-up of a rock mass through CHEMICAL combines with calcium carbonate in the WEATHERING. A decomposed rock will lack soil and carries it away in solution as cal- its former cohesion owing to the washing cium bicarbonate. It operates best at low away of cementing materials or the alter- temperatures, because the solubility of cal- ation of the constituent minerals. cium carbonate decreases as temperature rises. If the content of carbon dioxide in the deep (abyss) A depression in the floor of rainwater falls, then its ability to dissolve the ocean, generally at depths in excess of calcium also falls. Decalcification is said by 5000 m. some pedologists to precede LESSIVAGE and PODZOLIZATION: in a given soil it influences deep-focus earthquake An earthquake the upper profile first, and so there is a whose focus is at a depth of more than 300 point at increasing depth beyond which de- km. Most are found along the BENIOFF calcification has not proceeded, called the ZONES. acid point. deep ocean One of the fundamental di- deciduous forest Areas occupied by visions or zones of the Earth’s surface. The trees and shrubs that periodically shed total volume of deep oceans far exceeds the their leaves, usually in winter or during the total volume of land lying above sea level. dry season. Deciduous trees are also called Also, the oceans are, in general, markedly broadleaved trees. deeper than the continents are high. The Pacific Ocean is the largest ocean, occupy- decile /dess-ăl/ (in statistics) A tenth part ing, together with its adjacent seas, some of a series of values that are ranked in 50% of the total ocean area, which is 361 order of magnitude, normally quoted as million sq km. The Atlantic Ocean occu- the first or second decile, etc. If a series of pies roughly 106 million sq km and the In- values are ranked in order of magnitude dian Ocean 75 million sq km. The average they can be subdivided into any number of depth of the deep oceans is 3800 m, with a groups depending upon the amount of large percentage of the oceans varying in data. depth between 3000 and 6000 m below sea level. declination See angle of declination. deep-sea trench See trench. décollement /daykolmawn/ The plane marking the boundary between two differ- deep weathering Weathering at great ent types of deformation. The rocks above depths, occurring when suitably porous, are generally deformed, whereas those permeable, or chemically reactive rocks are below may be unaffected. It is caused by subjected to humid tropical or subtropical the upper rock series sliding over the lower weathering in areas where erosion is very during folding. slight. REGOLITHS over 100 m deep have frequently been reported and others up to decomposer (in ecology) Any organism and over 300 m are known. There is a tran- that breaks down dead plants and dead an- sition in all profiles from solid rock (at imals or their excreta. They are important depth), through partly weathered rock in the FOOD CHAIN because they set free (containing CORESTONES), to totally weath- such substances as nitrogen compounds, ered material that has lost the former struc- which pass into the soil or atmosphere. tures of the parent material. Evidence of Bacteria and fungi are the main decom- deep weathering occurs in areas that are posers, but some invertebrate animals now totally unsuitable for its development,

97 deflation in which cases weathering is assumed to recede toward the source or alternatively have occurred during former periods of the whole ice mass may become shallower, warmer climate. resulting in the landforms characteristically created by stagnant ice. During the deflation Wind removal of fine material winter an ice sheet may remain static or (silt and clay) from areas of increased even advance again slightly, causing depo- weathering. Typically, a chance depression sition of TERMINAL MORAINES or PUSH in the desert surface increases moistness at MORAINES. Successive moraines may be dis- that point, producing increased chemical tinguished in order to trace the recessional weathering. This leads to clay formation. pattern of the ice sheet. The complete dis- On drying, the clay is susceptible to wind appearance of an ice sheet may take thou- action, and when picked up can be carried sands of years, and crustal adjustments and long distances in suspension, often in the sea-level changes may continue long after- form of sandstorms. This fine material is ward. dropped elsewhere, leaving behind a concentration of coarse material as a deflation degradation 1. In general, the wearing surface. Deflation surfaces usually take the down of the land surface by processes of form of gravel-strewn areas, although this EROSION. process can also produce large depressions, 2. The lowering of stream beds over a time and sometimes OASES may result. See also period measurable in years. It is not ap- aeolian erosion. plied to the short-term cut and fill of stream beds extending over a period of a /dee-fô-rĕ-stay-shŏn/ The deforestation year or less, which is an integral part of the permanent removal of trees and shrubs, equilibrium regime of streams. Degrada- usually as a result of human activity for ob- tion can arise through a fall of sea level, taining timber or clearing land for agricul- causing REJUVENATION, or a change of flow ture or mining. The exposed soil is easily conditions giving the river increased ero- washed away by rain or blown away by the sive power, such as decreased load or in- wind, resulting in an eroded infertile land- creased runoff. Evidence of degradation scape and loss of natural habitats. See also badlands; desertification. can be found in river terraces and knickpoints. See also aggradation. deformation /dee-for-may-shŏn, deff-er-/ The alteration of rock formations that degree 1. A unit of measurement along generally results from tectonic plate move- the lines of latitude and longitude. One de- ments. The consequences include compres- gree is equal to l/360 of the Earth’s circum- sion, extension, faulting, and folding. ference on the lines of longitude. However, a degree of latitude decreases in length to- deglaciation /dee-glay-see-ay-shŏn/ The ward the poles. Each degree is subdivided shrinkage of an ice sheet or glacier through into 60 minutes, and each minute into 60 melting exposing formerly ice-covered seconds. land. The term is applied especially to the 2. A unit of temperature. See also absolute processes operating during an INTER- temperature; Celsius scale; kelvin. GLACIAL period once climatic conditions begin to improve and the ice sheets that degree-day The basic unit used for AC- may have developed in the previous CUMULATED TEMPERATURE. It can refer to ei- GLACIAL PHASE start to shrink. Although ther degrees Celsius or Fahrenheit and is considerable melting takes place at the the number of degrees above (or below) a margins, accumulation can still be main- specified datum during the day. tained in the source zones, thereby continuing the transportation of debris by delayed runoff Rainwater that flows moving ice. Most melting takes place dur- into streams (usually from springs) after ing the summer when the ice margins may having first been absorbed into the soil.

98 dendroclimatology

Delay may also be caused if rainwater is statistics and mathematics to study human temporarily locked up in snow or ice. populations geographically to determine such factors as composition, distribution, delta A large accumulation of sediment and size. deposited at the mouth of a river where it discharges into the sea (or a lake). Deltas dendrite /den-drÿt/ A fernlike branching are formed as a result of the decrease in pattern of material deposited within a rock load-carrying capacity following the decel- or mineral, often resembling a plant fossil. eration of river water on entering the com- Most dendrites are black, composed of paratively static sea (or lake). Fine clay such minerals as pyrite (iron sulfide) and material, normally carried in suspension, is pyrolusite (manganese dioxide). See moss also deposited because the very small par- agate. ticles coagulate and sink in the presence of saline water. For the sediment to accumulate the amount of material deposited by the river must exceed that removed by coastal erosional processes. Deltas grow in size because the river tends to bifurcate once a certain amount of deposition has occurred; the smaller streams (distributaries) then deposit material over a wider area. They may become abandoned later, and that part of the delta no longer receiving sediment will become eroded. See also arcuate delta; bird’s-foot delta; cuspate delta; delta deposit. Dendritic drainage delta deposit Prior to the formation of a DELTA proper, fine material is deposited on the seabed at the river mouth. These mater- dendritic drainage /den-drit-ik/ The ials (bottomset beds) are laid down hori- drainage pattern that develops where zontally and extend a considerable way out structural controls of slope, variable lithol- to sea. Above these, and making up the ogy, or fault and joint patterns are absent; major volume of the delta, are materials as a result the drainage net is entirely ran- (foreset beds) deposited with an inclination dom, with equal probability of stream flow from top to bottom of up to 35°; progres- in all directions. It characterizes areas of sive deposits extend farther out to sea and flat rocks with uniform lithology, notably thereby cause enlargement of the delta. plains, plateaus, and massive crystalline These beds are overlain by the topset beds, rocks. One example is the US Great Plains, which are again horizontal and are a sea- notably the Badlands of South Dakota. ward extension of the river’s alluvial floodplain. dendrochronology /den-droh-krŏ-nol-ŏ- jee/ (tree-ring analysis) The method of demersal /di-mer-săl/ Describing a fish using the annual growth rings of certain that lives on or near the sea floor. Flatfish, trees, in order to calculate the year in including the flounder, halibut, and sole, which they formed, for dating purposes. It are specially adapted for life on the bot- is used for dating certain fossils of wood tom, with special forms of camouflage. and wood specimens, and for the interpre- The other main types of demersal fish are tation of past environments through analy- the round fish, which include haddock, sis of the widths of the tree rings. See also cod, and hake. dendroclimatology. demography /di-mog-ră-fee/ The use of dendroclimatology The branch of

99 denitrification

DENDROCHRONOLOGY that focuses on the density log A subsurface logging tech- interpretation of former climates from nique, which records the variations with changes in width of annual growth rings of depth in the density of strata in an uncased certain tree species. By using the bristle- borehole. cone pine, a long-lived conifer from the southwestern USA, and preserved speci- denudation /dee-new-day-shŏn, den-yû-/ mens, it is possible to go back as far as ap- The weathering of rocks, the entrain- proximately 5000 BC. However, the width ment of debris, and its subsequent trans- of the tree ring does not bear a simple lin- port and deposition. Denudation is highest ear relationship with one climatic factor, in areas of high relief, heat, humidity, being related to an amalgam of many, of steep slopes, and rocks with abundant which temperature and amount of precipi- sediment yield. The highest rates of de- tation during its growing season are the nudation recorded, reaching 3000 mm/ most important. This limits the reliability 1000 years, occur in glaciated areas; of the method. lowest rates, about 1.2 mm/1000 years, occur in hot dry lowlands. Compare ero- denitrification /dee-nÿ-tră-fă-kay-shŏn/ sion. The process by which bacteria in the soil break down nitrates to produce nitrogen denudation chronology The study of gas, which passes into the atmosphere. how a landscape has evolved through time Denitrification is an important part of the to its current state, by arranging in se- NITROGEN CYCLE. quence the pieces of evidence discovered, to obtain a series of pictures of the evolu- density The mass of unit volume of a tion of the relief through a series of stages. substance at a specified temperature and For each major stage in the evolution of a pressure. In the case of the atmosphere, regional landscape the following broad density is not normally measured directly groups of features are researched. Climate, but is calculated from its relationships with geology, and base level (the independent temperature, pressure, and humidity variables) dictate the geomorphological through the GAS LAWS. For dry air at 1000 system, consisting of erosion, transport, mb and 290 K, density is about 1.2 kg/m3 and deposition. This leads to the creation (see diagram at ATMOSPHERE). of landforms, the remains of which constitute the basic research material. Partly re- density bottle See pycnometer. flecting the above factors and in turn influencing them are the flora and fauna. density current A CURRENT caused by As flora and fauna reflect so many envi- differences in density. The density of a ronmental variables, especially climate, mass of water may become different from their study in the form of pollen, insect, that of the surrounding water; if it is and mammalian remains is significant in greater, the density-driven flow is usually a environmental reconstruction. Once the BOTTOM CURRENT or underflow. On the type of environment is known, inference other hand, the density of a mass of water can often be made of the geomorphological may be intermediate between that of the processes operating. water above and beneath it, in which case Dating is done by various methods: or- the density-driven flow may develop as an ganic matter of 60 000 years old or less can interflow. A common cause of density-dri- be dated by radiocarbon methods, and for ven flow arises when relatively light fresh greater time range other methods are avail- water discharging from a river rides out able. Paleobotany has been used to estab- across and above denser saline water enter- lish pollen zones through time, with similar ing from the sea; in this case, a saline changes occurring over wide areas allow- wedge tends to drive upstream along the ing organic remains to be correlated be- bed of the river or estuary, often transport- tween regions and assigned to a relative ing appreciable amounts of fine sediment. dating system. Pedological methods can be

100 depression used to study superficial deposits of past climates still preserved. low warm front deposition The laying down of material cold front subsequent to EROSION and TRANSPORT. De- position could be described as the creative Fig. 1 part of the geomorphological system. In the fluid transport media of sea, rivers, and wind, deposition occurs when the forward movement and turbulence in low the transporting medium falls below the occlusion settling velocity of the load, i.e. when the transporting medium lacks the COMPE- TENCE or CAPACITY to carry the load any farther. Low-energy environments are warm sector therefore zones of deposition; in rivers, these occur on the inside of meander bends; cold front warm front on coasts, in bays and estuaries; in an air Fig. 2 stream, in the lee of obstructions. All these environments are characterized by lack of back-bent occlusion strong upward eddies and divergent low occlusion streamlines of flow. In the solid transport medium of ice movement, most deposition occurs where changes in the climatic regime stop the ad- warm front vance of ice and lead to its stagnation and melting, rendering it incapable of trans- warm sector porting any farther. cold front Fig. 3 depression 1. (in meteorology) A mid- Depression evolution latitude CYCLONE (low) or a weak TROPICAL CYCLONE (with wind speeds of less than Force 6 (BEAUFORT SCALE)). The term may SION), the central pressure starts rising, and also be used for smaller extratropical de- the system begins to decay. This is the gen- pressions not associated with surface eral pattern for depressions, but they vary fronts such as lee lows, thermal lows, or appreciably in their intensity, size, direc- polar lows. However, its most common tion of movement, and time of existence. usage is for the main synoptic disturbances Depressions are the main source of pre- of mid-latitudes. These are areas of low cipitation in most lowland parts of the pressure surrounded by several closed iso- mid-latitudes, the amount falling being de- bars, frequently accompanied by surface termined by distance to the depression cen- FRONTS and moving toward the northeast ter, intensity of the depression, and the in the N hemisphere and the southeast in time taken for the depression to pass. As a the S hemisphere. They appear to undergo result, the area of highest precipitation to- evolution during their movement east- tals is about 50–60° latitude where depres- ward, commencing as shallow lows with sions are most frequent and at their most widely separated fronts (see diagram) and intense stage. In the early stages of the de- only moderate winds. With time the pres- pression, the troposphere is warm and the sure at the center of the depression falls cyclonic circulation is shallow, but as it de- with a strengthening of the winds and a velops the troposphere cools and the cy- narrowing of the warm sector. Eventually clonic rotation extends to much greater the warm sector disappears from the sur- heights. The origins of the depression have face (the process being known as OCCLU- been disputed, but they appear to be due to

101 deranged drainage

DIVERGENCE in the upper atmosphere above Soils are usually very poorly developed, suitable thermal gradients near the ground. stony, or saline (see sierozem). Weathering 2. (in geology) A structurally low area in debris accumulates close to its source be- the Earth’s crust, lying between culmina- cause surface drainage cannot be main- tions along a fold axis. See fold. tained. When rain does fall it is usually in the form of sudden downpours, which can deranged drainage A type of drainage cause brief surface runoff, moving consid- pattern characteristic of recently glaciated erable amounts of debris. Mechanical areas, such as the Canadian Shield and weathering is more important than chemi- Siberian tundra, where the drainage pat- cal owing to the lack of water, and wind is tern has not yet adjusted to the structures a very active agent of erosion and trans- in the relatively recent glacially deposited portation of fine material. Vast areas of un- surface, which masks the previously ad- consolidated sand and dunes are less justed drainage pattern developed on the typical of most deserts than stony scrub- solid geology beneath. As a result, drainage lands with occasional resistant rock up- is not coordinated, and is characterized by lands. many small local drainage basins and lakes. desertification /dez-er-tă-fă-kay-shŏn/ The process by which a desert is formed by derived fossil A fossil that was origi- either changes in climate or human internally preserved in a stratum older than that vention. Natural causes also include fires in which it is ultimately found, having be- that destroy vegetation. Most often, how- come incorporated in the more recent stra- ever, the cause is overgrazing by farm animals or DEFORESTATION, resulting in tum by processes of erosion and erosion and infertility (see badlands). The deposition. If its true nature is not sus- process can sometimes be reversed, e.g. by pected, misleading conclusions regarding planting special grasses or incorporating age and stratigraphical relations of the de- into the soil water-absorbent grains of posit may be made. plastic. See also salinization. desert An area of the Earth’s surface desert pavement An area of gravelly where precipitation (usually taken as desert plain or REG, over which the abra- below 250 mm mean annual rainfall) is too sive effect of wind-blown sand has created low to compensate for evaporation a closely packed level surface. The individ- throughout the year. This reduces the types ual stones may be cemented together by of vegetation that can survive and the ab- precipitated salts drawn to the surface in sence of surface runoff produces distinctive solution by capillarity. types of landform. The atmospheric state producing deserts is either a persistent desert rose A flowerlike arrangement of high-pressure area, such as the subtropical platelike mineral crystals, often found anticyclones in the Sahara, or areas in among the sand of deserts. The most com- which natural atmospheric stability is em- mon types consist of BARYTES or GYPSUM. phasized by surface cooling due to cold water ocean currents, such as the Hum- desert soil A characteristic type of soil boldt and Benguela currents. Deserts are that has poorly developed horizons (see also found in continental interiors where profile). There is little vegetation and only mountain barriers prevent the passage of a thin organic layer, because it is too dry moisture-bearing winds; examples are the for the formation of humus. Gobi Desert and the deserts of the southwestern USA. On the margins of a desert desert varnish A thin coating, varying there are areas of climatic variability that in color from pale yellow to very dark red, experience true desert conditions in some found on the surfaces of pebbles and years but not in others. blocks in stony deserts. It is thought to be

102 devitrification caused by the deposition of iron and man- detritus 1. Rock or mineral waste pro- ganese oxides from solutions drawn to the duced by the breaking up and wearing surface by capillarity and then evaporated. away of rock surfaces; debris. Highly polished surfaces of this type can 2. Organic debris from dead or decaying occur through the extremely abrasive effect organisms, particularly the remains of of fine sand carried by strong winds. aquatic creatures that fall to the bottom of a lake or the sea. desiccation crack /dess-ă-kay-shŏn/ (mud crack; sun crack) A type of crack that de- detrivore /det-ră-vor, -vohr/ An animal velops in fine-grained deposits as a result of that feeds on organic detritus, the small shrinkage owing to the evaporation of the particles of matter formed when dead water they contain, producing polygonal plants and animals decompose. Detrivores patterns. These patterns are usually irregu- are most common at the bottom of the sea, lar and polygons can vary from a few mil- where they feed on matter that drifts down limeters up to a meter in diameter. from above. Subsequent wetting of a dried-up sediment will cause swelling and the disappearance deuteric changes /dew-te-rik/ Small- of the cracks. PATTERNED GROUND may be scale textural and mineralogical changes maintained, however, if long-lasting cracks brought about by a residual hot volatile become infilled by some extraneous ma- phase during the final stages of crystalliza- terial, such as wind-blown sand. When tion of a magma. These include the alter- preserved they act as useful indicators of ation of feldspars to albite, analcime, and zeolites and the alteration of mafic miner- paleoenvironments and orientation. als to chlorite. Deuteric changes are ex- ceedingly difficult to distinguish from desilication /dee-sil-ă-kay-shŏn/ (in soil METASOMATISM when material is intro- science) The removal of silica from a soil duced from outside. Compare hydrother- profile by intense weathering and leaching. mal process; pneumatolysis. The process is typical of tropical areas and leads to the development of latosol profiles development equation In the atmos- (see ferrallitic soil). phere there exists a high degree of compensation, so that the change in pressure destructive plate boundary See plate observed at the surface is often only a small boundary. net effect between strong DIVERGENCE or CONVERGENCE in the surface layers and the destructive wave A wave that moves reverse flow in the upper atmosphere. This more beach material seaward than land- will result in vertical motion to compensate ward, resulting in a diminution in the size for the changes. If there is divergence in the of the beach. Characteristic destructive upper levels and convergence at the sur- waves are steep, so that on breaking their face, the vertical motion will be upward BACKWASH is more active than their SWASH. with the likelihood of precipitation. They occur at high frequency, usually be- The development equation, deduced by tween 13 and 15 per minute, which means R. C. Sutcliffe, relates the difference in di- that the backwash of a preceding wave can vergence between high and low levels to interfere with the swash of the next, reduc- the thermal wind, vorticity, and the Corio- ing the potential landward movement of lis effect, the first two of which can be mea- material. Destructive waves frequently sured from charts and the last easily occur in association with local onshore calculated. If the result is positive it implies winds, which cause the setting up of a sea- cyclonic development and if negative anti- ward current on the sea floor, assisting in cyclonic development. transporting the material stirred up by the breaking waves away from the beach. devitrification /dee-vit-ră-fă-kay-shŏn/ Compare constructive wave. The slow crystallization of natural volcanic

103 Devonian glasses such as obsidian and pitchstone, Pelagic limestones rich in fossil which are metastable, often accompanied cephalopods occur in Europe and the by secondary hydration. Devitrification is Urals. Gnathostome (jawed) fish, including indicated by the presence of incipient CRYS- placoderms, were also common. By the end TALLITES and SPHERULITIC growth of quartz of the period primitive amphibians had and feldspar. The process often goes to evolved from certain crossopterygian completion and many rocks show little or fishes. There is also evidence of land plants, no evidence of a former vitreous state. such as ferns and horsetails, and of associated insects and spiders. Devonian /di-voh-nee-ăn/ The first period of the Upper PALEOZOIC. Beginning dew Water droplets deposited on the about 408 million years ago and lasting for ground after radiational cooling has re- some 45 million years, it followed the SIL- duced the temperature of the ground sur- URIAN and was succeeded by the CARBONIF- face below the dew-point temperature of EROUS. The period is named for the county the air in contact with the surface. The of Devon, England, where these rocks were source of the moisture may be either dew- first recognized as a major group. The De- fall from the atmosphere during conditions vonian System is divided into seven stages: of light wind and a downward transfer of the Lochkovian, Pragian, and Emsian form water vapor to the ground, or diffusion of the Lower Devonian, the Eifelian and water vapor from the soil and condensa- Givetian the Middle, and the Frasnian and tion onto vegetation, which is also being Famennian the Upper Devonian. These are cooled by long-wave radiation losses. The divided on the basis of fossils from rocks of latter process takes place only when the air the shallow-water marine facies, where in- near the surface is calm, but is one of the vertebrates including corals, brachiopods, most frequent sources of dew. ammonoids, and crinoids flourished. Graptolites became completely extinct and dew-point The temperature of air at the trilobites declined. which saturation will take place if the air is Outcrops of Devonian rocks occur in cooled while remaining at a constant pres- all continents with extensive deposits un- sure and moisture content. Although it can derlying areas of North America, South America, Europe, and Russia. A giant be measured directly, dew-point is nor- landmass, Gondwana, was located in the S mally determined indirectly from tables hemisphere with smaller landmasses in based on dry- and wet-bulb temperatures. equatorial areas. At the close of the Sil- urian and during the early Devonian the dew-pond A shallow artificial depres- collision of the continents of what is now sion lined with clay that collects water and North America and Europe was accompa- is used in fields without running or piped nied by extensive volcanic activity and water. The ponds are particularly common mountain uplift, especially in a belt that in- on chalk or limestone rock. It was origi- cluded New England, Nova Scotia, New- nally believed that their main source of foundland, Scotland, Scandinavia, and E water was dew because they appeared to be Greenland. Extensive continental deposits well maintained with water even during accumulated, consisting of conglomerates, dry periods. However, later investigation red silts, and sandstones. This facies is showed conclusively that natural precipita- known in Europe as the Old Red Sand- tion was most important and that dew was stone and contains remains of a large vari- of minor significance. ety of ostracoderm fish. Other Devonian sedimentary rocks in- dextral fault A transcurrent (wrench or clude the carbonate reef deposits of West- tear) fault in which the rocks on the oppo- ern Australia, Europe, and Canada. Black site side of the fault plane are offset to the shale deposits formed locally and there right. See diagram at FAULT. Compare sinis- were widespread evaporite deposits. tral fault.

104 diastem diabase /dÿ-ă-bayss/ (dolerite; microgab- agenetic changes grade into those taking bro) A rock that differs little mineralogi- place at higher temperatures within the do- cally from gabbro, of which it is the main of METAMORPHISM. medium-grained equivalent. Calcic plagioclase and augite are essential; in addition, diallage /dÿ-ă-lij/ A mineral name for diabase may contain olivine, hypersthene, diopside and augite when displaying quartz, or feldspathoids. The characteristic SCHILLER. texture is ophitic but many examples are intergranular and porphyritic. The term diamagnetic /dÿ-ă-mag-net-ik/ Describ- epidiorite is sometimes applied to altered ing a substance that has a magnetic suscep- diabase. Diabase occurs mainly as dikes, tibility of slightly less that 1. When a plugs, and sills. See also alkali basalt; diamagnetic substance is placed in a mag- basalt. In the UK the rock is known as do- netic field, its induced magnetization is di- lerite and the term diabase has been used to rected opposite to that of the applied field. refer to altered dolerite. The most diamagnetic substance is bismuth. diabatic /dÿ-ă-bat-ik/ Describing a thermodynamic process in which heat enters or diamond A crystalline form of the ele- leaves a system. There are many examples ment carbon, which occurs mainly in pipes of this in the atmosphere, such as evapora- of KIMBERLITE and in alluvial deposits. It tion, turbulent mixing, and radiation ab- crystallizes in the cubic system, forming sorption. Compare adiabatic. colorless or colored crystals (tinted by impurities), and is the hardest known min- diachronism /dÿ-ă-kroh-miz-ăm/ The eral. It has long been valued as a precious condition of a lithological unit whose base gemstone; nongem varieties are used as in- is not a time plane, i.e. whose age is differ- dustrial abrasives. ent in different successions. Diachronism occurs when the boundaries of facies move diapir /dÿ-ă-peer/ A vertical body of IG- in time. It can often be detected only if suit- NEOUS ROCK that rises into the Earth’s crust able zone fossils are available, and failure because it is less dense that the surrounding to recognize diachronism can lead to false impressions of past events and geography. rocks. SALT DOMES also rise for the same reason. See diapirism. diagenesis /dÿ-ă-jen-ĕ-sis/ A collection of processes by which loose accumulated diapirism /dÿ-ă-pi-riz-ăm/ The upward sedimentary material becomes sedimentary intrusion of a less dense rock mass through rock. Diagenetic processes are postdeposi- overlying more dense rock. It was origi- tional. In time, the pressure on a sediment nally applied to SALT DOMES, but is also an increases owing to the increasing load of important mechanism for the inplacement superposed material, and compaction re- of granitic rock types. sults, involving a reduction of pore space. Chemical reactions take place between the diaspore /dÿ-ă-spor, -spohr/ A hydrated sediment and entrapped and circulating form of alumina, AlO(OH), which occurs fluids, leading to the cementation of grains in ALUMINA and some BAUXITES. It crystal- by materials precipitated from the fluids. lizes in the orthorhombic system as gray, Calcite, silica, and hydrated iron oxides are green, or pinkish aggregates. common cementing materials. The diagenetic replacement of calcite by dolomite diastem /dÿ-ă-stem/ A depositional break may take place in calcareous marine sedi- of a very short time period, with or without ments. Such processes taking place near the erosion. The beds above and below the bed Earth’s surface at low temperatures and have the same dip and strike. The absence pressures ultimately lead to the induration of beds can be determined only by paleon- of loose aggregates and to lithification. Di- tological evidence.

105 diastrophism diastrophism /dÿass-trŏ-fiz-ăm/ Move- rocks subject to differential weathering. ment within the lithosphere, including Well-weathered rocks are obviously more folding, faulting, orogenesis, and the for- susceptible to subsequent erosion than re- mation of new ocean floor, causing large- sistant types. After a period of active ero- scale deformation of the Earth’s crust. sion, easily weathered rocks will form lowland areas, whereas resistant strata re- diatom /dÿ-ă-tom/ A microscopic ma- main as upstanding blocks. Even within a rine or freshwater alga (see algae). Diatoms single rock type certain bands may be more form an important constituent of plank- or less resistant than the main mass, result- ton, providing food for a great variety of ing in ridges and depressions respectively. aquatic animals. Their geologic impor- TORS and INSELBERGS represent good exam- tance derives from the fact that many di- ples of differential weathering and erosion atoms possess a case of silica, which may within homogeneous rocks, different joint become fossilized, and fossilized diatoms spacings accounting for the varied suscep- often form extensive deposits (see di- tibility. atomite). The earliest diatoms are found in rocks of Cretaceous age. differential shear A type of rock deformation in which movement takes place diatomite /dÿ-at-ŏ-mÿt/ A very fine- throughout the whole rock, just as in a grained siliceous rock consisting of the flow, but it occurs in distinct laminae or skeletal remains of DIATOMS. They are planes. formed under both freshwater and marine conditions. differential weathering The more in- tensive weathering of certain parts of a diatom ooze A deep-sea siliceous ooze rock mass even when the same weathering (see pelagic ooze), containing over 30% or- processes have been acting on the whole ganisms. It is a cold-water deposit, espe- mass for the same length of time. Within cially prominent in an elongated belt in the many theoretically homogeneous rocks N Pacific Ocean and flanking Antarctica. there may be variations in mineral compo- On a global scale, it occupies some 9% of sition or grain size, which can explain such the total ocean floor. Living diatoms con- differences. Joints play an important part, sist of siliceous algae belonging to the phy- especially in the case of rocks with low toplankton. They thrive in zones of porosity, as the percolation of water upwelling water, where nutrients are abun- through them causes preferential weather- dant. ing around the joints. There will be differential weathering between different rock diatreme /dÿ-ă-treem/ A volcanic vent, types subjected to the same weathering often filled by brecciated material, that has processes. been cut from the sides of the conduit by high-pressure gas charged with particles. differentiation See magmatic differenti- The best-known examples are the dia- ation; metamorphic differentiation. mond-bearing kimberlite pipes of South Africa. diffluence /diff-loo-ĕns/ The rate of separation of adjacent streamlines in the di- differential compaction The reduction rection of airflow. It is the reverse of in volume of sediments during compaction CONFLUENCE. to different degrees, depending on their porosity, grain size, and the rigidity of the diffusion The slow process by which particles that compose the rock. For exam- different fluids or fluids having different ple, shales are more compressed than sand- densities mix together as a result of molec- stones. ular movements. It obeys similar laws to thermal conduction but is too slow to be of differential erosion The erosion of importance in the atmosphere. Far more

106 dip important is mixing achieved by eddy diorite /dÿ-ŏ-rÿt/ A coarse-grained inter- transfer (EDDY DIFFUSION) in turbulent air. mediate igneous rock containing plagioclase of oligoclase-andesine composition digital cartography (digital mapping) and mafic minerals. The An50 (anorthite) See cartography. composition of plagioclase feldspar marks the division between diorite and GABBRO. In dike (dyke) A tabular body of igneous diorites, the most common mafic minerals rock that is intruded vertically and discor- are hornblende and biotite although some dantly to the structure of the rocks through rocks contain pyroxene. A little alkali which it passes. See also dilation dike; nep- feldspar may be present together with ac- tunian dike; radial dike; sill. cessory magnetite, apatite, and sphene. More acid varieties of diorite contain dike swarm A large number of dikes, quartz up to 10% and may be called often arranged in either a radial or parallel tonalites. With a further increase in quartz, pattern. tonalites pass into granodiorites. With an increase in the amount of alkali feldspar, dilatancy /dÿlay-tĕn-see/ An increase in diorites pass into syenodiorites. In the the volume of a rock deformed by pressure, USA, the term tonalite is applied to all caused by the expansion and extension of rocks containing quartz, sodic plagioclase, small cracks within it. The effect can be de- and mafic minerals and is equivalent to the tected in strained rocks just before an British tonalite (quartz-diorite) and gran- earthquake, and is the basis of one type of odiorite (in part). earthquake prediction. Both leucocratic and melanocratic diorites occur. Meladiorites containing euhedral hornblende crystals are termed dilation dike /dÿlay-shŏn/ A discordant appinites and often appear to be a peg- igneous intrusion that causes the walls on matitic facies of more normal diorite. Dior- either side of a fracture to move apart. The ites usually have equigranular textures. term is also applied to swarms of dikes that The medium-grained equivalents, microdi- have filled fractures in the Earth’s crust orites, are often porphyritic. Markfieldite when the area has been subjected to ten- is an oversaturated microdiorite consisting sional forces. of a groundmass of graphically intergrown alkali feldspar and quartz with phenocrysts dÿ-mor-fik dimorphism / / (in mineral- of plagioclase and hornblende. The vol- ogy) The existence of an element or com- canic equivalents of diorites are andesites. pound in two different crystal forms, such Dioritic rocks tend to occur in small intru- as marcasite and pyrite (both forms of iron sive masses associated with granodiorite sulfide, FeS2) or diamond and graphite and gabbro bodies and many diorites are (both forms of carbon). thought to be hybrid rocks. Compare syenite; syenodiorite. dinosaur /dÿ-nøbrevel;-sor/ A member of a group of extinct archosaur reptiles that dip 1. The angle made between the hori- were the dominant form of terrestrial life zontal plane and that of the bedding plane, from the end of the Triassic period to the measured perpendicularly to the strike, in a Cretaceous. They evolved into a great vari- stratified rock or any planar structure. See ety of both carnivorous and herbivorous also apparent dip. Compare hade. species, some of very large size. They are 2. (angle of magnetic inclination; magnetic classified in two orders: the ORNITHISCHIA dip; magnetic inclination) The angle be- and the SAURISCHIA. tween the Earth’s magnetic field at any point on the Earth’s surface and the hori- diopside /dÿop-sÿd, -sid/ A clinopyrox- zontal. It is 90° at the MAGNETIC POLES and ene with composition CaMgSi2O6. See py- 0° at the Equator. It is measured using a roxene. dip circle, an instrument in which a mag-

107 dipole field netic needle is free to rotate, in the vertical discontinuity /diss-kon-tă-new-ă-tee/ 1. plane, around a circular scale. (in meteorology) A sharp change in the value of a meteorological variable, for ex- dipole field /dÿ-pohl/ That portion of ample at a cold front, where there is usu- the Earth’s magnetic field that can best be ally a discontinuity in the temperature, described as originating from a dipole humidity, and wind velocity fields across magnet in the Earth’s interior, inclined at the frontal surface. Most meteorological 11° to the Earth’s axis of rotation. See also properties are continuous functions of nondipole field. space and time and their values can therefore be mapped in the forms of pressure dip slope The slope of the surface of the charts or isotherm maps. land that more or less mirrors the slope of 2. (in geophysics) A marked change with the rocks beneath. depth in one or more of the physical properties of the materials constituting the direct circulation (in meteorology) Cir- Earth’s interior. For example, a boundary culation in which potential energy is con- at which the velocity of earthquake waves verted into kinetic energy through the changes is a seismic discontinuity. rising and sinking of juxtaposed lighter and denser air, respectively. This occurs at discordant 1. Decribing a rock unit that scales ranging from land and sea breezes to cuts across the bedding or foliation of ad- the meridional cells of the GENERAL CIRCU- jacent rocks. Intrusive igneous rocks, such LATION OF THE ATMOSPHERE. as dikes, show discordant relationships. 2. (unconformable) Denoting a drainage direct wave A wave that travels from pattern characterized by streams cutting one point to another along the path of indiscriminately across structures. Discor- shortest distance. dance at a local level can be a product of various detailed factors, but at a regional discharge The volume of water flowing level it can be due to glacial blocking (see through a cross section of a stream. It is deranged drainage), tectonic activity (see measured by gauging mean velocity in the cross section, which is multiplied by the antecedence), or to superimposition of the cross-sectional area to give an expression drainage from a preexisting cover of rocks. of flow in m3 per second. Within a river, Compare accordant. discharge increases downstream, except in arid areas where volume is lost by evapo- discordant coastline A coastline run- ration. By far the greatest discharge of all ning at right angles to structural features the world’s rivers is that of the Amazon, shaping the landscape immediately inland with a flood-stage discharge of 180 000 m3 of it. It is most obvious where lines of hills per second, which is at least as great as the run inland, giving an undulating coastline combined discharges of its nearest rivals, with alternating cliffs and level beaches. If the Congo, Yangtze, Mississippi-Missouri, the land sinks relative to the sea, the valleys Yenisei, and Lena. between the hills become deep inlets (see fiord; ria). Such coastlines are common on disconformity /diss-kŏn-for-mă-tee/ (pa- the western side of the Atlantic Ocean. raunconformity) An unconformity that marks a considerable time gap but where disharmonic folding /diss-har-mon-il/ the beds of rock above and below the plane Folding in which differences in the COMPE- of the unconformity have a similar dip and TENCE of the various beds of rock result in strike (compare angular unconformity), variation in types of folding within the the absence of beds being detectable only fold, the less competent beds forming nu- by paleontological means. It is of greater merous folds that are smaller than those magnitude than a DIASTEM. Compare un- formed in the more competent beds enclos- conformity. ing them.

108 divergence dislocation metamorphism See meta- of a river that flows from the main course morphism. and does not rejoin it; the distributary makes its own way to the sea. Most are dispersion The separation of SEISMIC shallow and narrow. See also tributary. WAVES into groups with different frequency, resulting from variations in the ve- disturbance (in meteorology) Any locity of the waves. Such differences, more small-scale synoptic feature causing a dis- noticeable in surface waves than body turbance from the normal gradient wind waves, are caused by variations with depth flow, especially a shallow DEPRESSION (low in the density and elasticity of the rocks or cyclone). through which they travel. diurnal variation /dÿ-er-năl/ The changes disphotic zone /dis-foh-tik/ Ocean in magnitude of any climatological prop- depths at which photosynthesis is not ef- erty recorded during the solar or climato- fective because of the small amount of light logical DAY. Most elements exhibit some that penetrates to this layer, which extends variation. Temperature and relative hu- from about 80 m to 200 m (the edge of the midity have an inverse cyclic fluctuation CONTINENTAL SHELF) below sea level. See reaching peak and trough respectively also aphotic zone; euphotic zone. about 14.00 hrs. Atmospheric pressure also shows a daily variation, but it is based dissected plateau See plateau. on a 12-hourly oscillation that proceeds according to local solar time. Maxima disseminated deposits Deposits of occur about 10.00 and 22.00 and minima minerals that are formed when hydrother- at 16.00 and 04.00 hours. Because the syn- mal fluids fill small fissures and pores in a optic variations in mid-latitudes are large, rock (see hydrothermal process). They are it is only within the tropical areas that the usually found with igneous intrusions (see diurnal variation of pressure is immedi- intrusive rock), and often comprise useful ately apparent. Some proposals have been metallic minerals. made for a nocturnal maximum of precipitation but this has never been proved. dissolved load That part of a river’s load carried in chemical solution. Rainwa- divergence A measure of the rate of out- ter, being mildly acidic, can dissolve rocks, flow of a fluid from a certain volume. It is especially limestone, and then feed that dis- the opposite state from CONVERGENCE and solved content into a river via groundwater the mathematical term used to describe flow. Rivers may also be mildly acidic, es- both, convergence being negative diver- pecially if they pass through areas of bogs gence. It has important implications in the or marshes, where organic acids are pro- atmosphere as the cause of pressure duced, and they may dissolve minerals of changes and vertical motion, but is very their beds. In the Mississippi 29% of the difficult to measure directly or accurately. load by weight is carried in solution, but Values of divergence in the free atmos- this proportion varies in other streams ac- phere range up to 10–5 per second, al- cording to climate and according to what though locally, as in fronts, it can reach proportion of runoff is contributed via higher values. groundwater flow. The major ions carried In wave refraction phenomena, diver- are bicarbonate, sulfate, chloride, calcium, gence refers to the spreading out of the and sodium, although in small basins there wave orthogonals in the direction of wave is variation according to the nature of the advance. Wave energy and wave height local rocks, and in populated areas pollu- tend to decrease in areas of divergence. tion can significantly alter chemical con- In the case of ocean currents, a diver- tent. See also debris load; load. gence is the zone in which currents flow away from each other, for example at distributary /diss-trib-yŭ-tair-ee/ A branch roughly latitude 10°N where the North

109 divergent plate boundary

Equatorial Current and the countercurrent dolomitization /dol-ŏ-mÿ-tă-zay-shŏn/ associated with it separate. The process by which a calcium carbonate rock is transformed into a calcium-magne- divergent plate boundary A region in sium carbonate rock through the partial or which two of the Earth’s lithospheric (or complete replacement of calcite or arago- tectonic) plates are diverging, always oc- nite by dolomite. See carbonate minerals. curring on the ocean floor. As the plates move apart, material wells up from the dome An anticlinal fold in which the mantle beneath to form new oceanic crust. beds dip in all directions away from the Such boundaries are often associated with central point of folding. A dome may be MID-OCEAN RIDGES (the best known being merely structural or it may constitute an the MID-ATLANTIC RIDGE) and active vol- exposed landform. See also salt dome. canic action. Doppler effect A change in the mea- divide (drainage divide) The area of sured frequency of a wave due to the rela- higher ground that lies between two sepa- tive motion of the source and the recorder. rate drainage systems. The term watershed As the source moves toward the recording is also used for a divide but as watershed is station the wavelength decreases, whereas also used synonymously with drainage when the source moves away from the basin it can cause ambiguity. See also in- recorder the wavelength increases. It is terfluve. named for the Austrian physicist Christian Johann Doppler (1803–53). division 1. An informal word for any Doppler radar A type of radar that re- unit in any scheme of stratigraphical classi- lies on the Doppler effect. If the radar tar- fication, including LITHOSTRATIGRAPHY, get is approaching or moving away from BIOSTRATIGRAPHY, and CHRONOSTRATIGRA- the transmitted signal, it will affect the fre- PHY. See also stratigraphy. quency of the returning signal, being less 2. In traditional plant classifications, a tax- than that transmitted if moving away or onomic grouping corresponding to a PHY- greater if approaching. In meteorology, LUM. precipitation drops are the usual target and indicate the horizontal air motion, the fall doldrums Equatorial oceanic areas in speed of the particles, and, with certain as- which winds are light and variable and sumptions, the vertical air motion. This where navigation by sailing ships was diffi- technique is being increasingly used for in- cult. It coincides with the intertropical con- vestigations of subsynoptic levels, such as vergence zone or thermal equator with at frontal zones. Networks of radar sys- frequent thunderstorms, heavy rains, and tems, e.g. in the USA, are used to provide squalls. It is now only used as a graphic warning of severe weather and its location. term for this belt of variable winds anywhere near the Equator between the trade dormant volcano A volcano that has wind zones. erupted within recorded history but is apparently not active at present. Because no- dolerite /dol-ĕ-rÿt/ See diabase. body can predict if it will one day become active again, it is best described as an inac- doline /dol-een/ (dolina) A large SINK- tive volcano. HOLE formed by solution in KARST country. dot map (in ecology and demographics) dolomite /dol-ŏ-mÿt/ A magnesium- A map on which the distribution of a par- containing CARBONATE MINERAL of compo- ticular measurable quantity is denoted by sition CaMg(CO3)2. The term dolomite is same-sized dots (each representing a par- also used to denote a rock with a high ratio ticular number). For example, population of magnesium to calcium carbonate. densities of plants, animals, or even people

110 drainage pattern can be represented graphically in this way channels. At the edge of the basin a DIVIDE, (with each dot standing for, say, 50 trees or usually in the form of hills, mountains, or 10 elephants). a plateau, separates it from the adjacent basin. The drainage basin constitutes the double refraction See anisotropic; bire- unit of study for drainage pattern, rates of fringence. denudation, relation between precipitation and runoff, and various other geomorpho- downland An area of pasture on hilly logical factors. Little is known of the con- terrain. The term is used mainly in Aus- trols on basin size and shape, but regional tralia and New Zealand. geologic structure and tectonic history are clearly important. downs Open rolling uplands, usually on The term watershed is often used for a chalky soil. The soil is thin and there are drainage basin but as watershed is also few trees; the chief vegetation is grassland, used synonymously with divide it can often used as pasture. Plowed areas of the cause ambiguity. downs are used to grow cereal crops. The term is used mainly in S England. drainage density The ratio between the total length of stream channel in a basin, downthrow side The side of a fault that and the area that the channel occupies. has moved downward relative to the other, Closely related is stream frequency, which i.e. the side that has younger beds brought is the number of streams divided by the down against the older beds of the other total channel area. Drainage density is high side. (See diagram at FAULT.) on rocks yielding impermeable soils, e.g. shales and clays; it is also high where sedi- drag fold A small fold associated with a ment yield is high, on easily erodible rocks, larger folding structure, often found adja- e.g. clays. cent to faults where the rock strata have Other things being equal, drainage den- been bent as a result of the movement sity is governed by climate, through its con- along the fault plane. trol on runoff via rainfall and vegetation. Relief is also significant: high-relief areas drainage Any process that removes tend to have high drainage densities. rainwater from the land, whether by means Drainage density is an integral part of of streams or by artificial means (such as drainage pattern description: in particular drainpipes and channels. See also drainage it shows the degree of dissection of the basin; drainage pattern. drainage basin. drainage basin The area that supplies drainage divide See divide. water to a particular network of drainage drainage pattern The spatial arrangement of streams within a basin. It is a reflection mainly of the area’s geologic div de id structure, i.e. the nature and arrangement vi e di of faults, folds, rocks (especially lithologies), and relative relief. Past and current

divide climate and the tectonic and geomorphic history of the basin are also important. These variables control the rate and nature of stream incision and headward erosion, the basic processes by which the pattern develops. Field studies show the pattern evolves very rapidly at first, then reaches a steady state of little further change, con- Drainage basins trary to the American geographer William

111 drainage wind

Morris Davis’s original idea that the rate of are shown, as contrasted with a solid edi- development was constant through time. tion showing only the underlying rocks. See also accordant; angulate drainage; barbed drainage; dendritic drainage; de- drizzle Water droplets ranging in diam- ranged drainage; discordant (def. 2); eter from 0.2 to 0.5 µm. It forms by coa- drainage density; parallel drainage; radial lescence in stratiform cloud with only drainage; rectangular drainage; stream weak vertical motion, otherwise the order; trellis drainage. droplets would be unable to fall. It also requires a high relative humidity between drainage wind See katabatic wind. cloud base and ground surface or only a short distance between cloud and surface. dravite /drav-ÿt/ A brown variety of In both cases, evaporation of the droplets TOURMALINE that is rich in magnesium. It would occur without these constraints. generally occurs in metamorphic rocks, and is sometimes used as a semiprecious drought (drouth) A period of dryness gemstone. due to the absence of significant precipitation. It is an unsatisfactory term unless very dredging The deepening of a river, port- carefully specified because it implies some approach channel, or other such area by effect on agriculture and vegetation, but excavating loose sediment or in-situ rock this is dependent upon factors other than from the river or seabed. The dredger or rainfall alone. In purely climatic terms, it dredge is a vessel specially designed to un- can be used to indicate lack of rain. In the dertake this. Some dredgers actually dig up USA a drought is defined as a period of 21 the bed using a combination of buckets or days or more when rainfall is 30% below grabs; others suck up loose sediments me- average for the time and place. chanically using suction devices, as in a trailing suction-dredger. Dredging may be drowned coastline A strip of land performed to improve navigable depths or along the coast that has been submerged to remove obstructions such as shoals or under the sea, either because the sea level banks that are dangerous to navigation. has risen or because the land has sunk rel- Other dredgers operate farther offshore ative to the sea. Valleys are flooded, form- and recover useful minerals, such as sand, ing FIORDS or RIAS, and hills may become gravel, tin, ore, and diamonds from the islands. seabed. Dredging can have far-reaching environmental consequences, as when it in- drowned valley A river valley cut at a creases turbidity, to the detriment of fish time when sea level was at a much lower and shellfish production. level than it now is, usually during the PLEISTOCENE Epoch, and subsequently dreikanter /drÿ-kahn-ter/ A pebble with drowned as sea level rose again on melting three facets, formed by the erosive action of the large ice sheets. These former inland of windblown sand in desert regions. The valleys now exist as indentations in the wind moves the pebbles back and forth, be- coastline, their internal shapes being cause they are too heavy to be lifted en- largely hidden underwater. Differently tirely. See also ventifact. shaped coasts are produced by the flooding of geologically different areas. If the for- drift Glacial and fluvioglacial deposits. mer drainage system was perpendicular to Great thicknesses of this drift accumulated the general outline of the present coast a se- during the PLEISTOCENE Epoch, although ries of inlets separated by headlands will much has subsequently been removed by exist; if the drowned valleys lay parallel erosion. An area of glacial deposition may with the present coastline numerous elon- be referred to as one of drift topography. A gate islands, the former interfluves, will be drift map is a geologic map on which present, with narrow flooded areas be- glacial drift and other superficial deposits tween them. See also fiord; ria.

112 dune

ronmental lapse rate; saturated adiabatic profile lapse rate. dry- and wet-bulb thermometer See thermometer. direction of ice movement dry site A small hill in a wetland or marshy region, often occupied by human settlement. plan dry valley A typical subaerial river valley in all respects except that it contains no stream or river. Such valleys are mostly Drumlin found in areas of soluble limestone where the waters responsible for the cutting of the valley, at an early stage in the erosional his- drumlin /drum-lin/ A streamlined glacial tory of the area, have subsequently disap- form tending to occur in groups, all with peared underground through enlarged their long axes parallel with the direction joints and subsurface cavern systems, de- of ice movement. They may be composed veloped as a result of LIMESTONE SOLUTION. of TILL, which can vary considerably in At times of exceptional discharge the un- composition, of preexisting DRIFT, or of derground systems may be incapable of rock. Some contain stratified deposits. carrying all the water, and surface drainage Since these forms are so variable a number may return temporarily. of explanations of their origins have been Some dry valleys may have been formed suggested: there are two principal explana- in former periglacial conditions, when per- tions. Those composed of rock or preexist- mafrost rendered the ground impermeable, ing drift materials must have involved or in times of wetter climate, when the erosion, although if erosion were the only water table was higher. process, one would expect shapes similar to those of ROCHES MOUTONNÉES, which is dumortierite /dew-mor-tee-ĕ-rÿt/ A blue not the case. Drumlins composed of newly or green fibrous mineral form of hydrated deposited till must involve deposition, and aluminum borosilicate, Al8BSi3O19(OH). it has been suggested that they form around a nucleus of frozen till, or rock. De- dune A mound or hill of unconsolidated spite their variability, all drumlins are material, usually sand. Dunes are charac- linked by their common shape, and the fact teristic landforms of deserts, in an unvege- that their formation is associated with tated form, and of certain coastal and moving ice. riverside areas, where they are usually vegetated. Desert dunes are simpler in form druse /drooz/ (drusy) A cavity in a rock owing to the lack of moisture and plant or mineral vein into which large euhedral growth, the two major forms being the crystals project. A rounded cavity contain- BARCHAN (crescentic dune) and the SEIF ing crystals that project toward the center DUNE (longitudinal dune). Coastal dunes is called a geode. See also miarolitic cavity; show a development inland from the vugh. young FOREDUNE through the main MOBILE DUNE to the STABILIZED DUNE. dry adiabatic lapse rate The constant The major requirement for dune-build- rate at which air will cool upon up- ing is a sand supply; in the deserts this is lift, prior to saturation being reached. Its transported by wind from areas of erosion, value of 0.98°C/100 m depends upon the notably scarps and areas of DEFLATION, properties of the gases in the atmosphere whereas in coastal areas it is transported by and the gravitational force. See also envi- the longshore drift from a source that may

113 dune-bedding be a preexisting supply or an area of ero- and is distinguished as a dust devil only if sion creating a supply, and then left on the the surface is sufficiently sandy for the sur- foreshore ready for wind action to take it face material to be drawn into the rotating inland and build the dune. column. Less developed whirlwinds are Coastal dunes are hence due to a three- also known as dust whirls. way relationship between littoral drift of sand, blowing of sand onshore, and vege- dust storm A cloud of fast-moving tation growth trapping the sand and build- windblown dust, which occurs when the ing a dune, whereas desert dunes may be wind is strong enough to lift the dust parti- wave patterns resulting from the interac- cles from the ground. The dampness, den- tion of air flow and land surface, the dunes sity, shape, and size of the particles accumulating in areas between turbulent determines how strong the wind has to be. eddies. See also parabolic dune; star dune. Local dust storms are often associated with rain and thunder; extensive dust storms dune-bedding A type of CROSS-BEDDING usually occur in regions of low atmos- that occurs in DUNES, resulting from vari- pheric pressure. In desert areas, very small able deposition of sand by the wind. occurrences are known as DUST DEVILS. dunite /dew-nÿt/ A monomineralic ultra- dyke See dike. mafic rock consisting wholly of OLIVINE. dynamical meteorology The branch of dust bowl An area in the western Great meteorology concerned with the causes Plains region of the USA that suffered ex- and nature of motion within the atmos- tensive wind erosion (deflation) during the phere. mid 1930s, which removed the fertile topsoil. It lies mainly within W Kansas, Okla- dynamic equilibrium (in geomorphol- homa, and Texas and extends into ogy) The state of balance between erosion Colorado and New Mexico. A smaller dust and deposition to which rivers seek to ad- bowl occured in central Nebraska, North just. The emphasis on dynamic is because Dakota, and South Dakota. See also bad- rivers never actually achieve a balance of lands. no erosion or deposition, but are always actively adjusting to the constant changes dust devil A WHIRLWIND in which sand of load and discharge, etc., by some minor and dust rise into the atmosphere reaching erosion and deposition. See also equilib- an average height of around 200 m and rium profile; river. with a diameter ranging from about 3 m to over 30 m. It forms by strong convection dynamothermal metamorphism See above an intensely heated sandy surface metamorphism.

114 E

earthflow A mass movement of soil on a tity of energy liberated when the over- steep slope, well mixed with water. Such strained rocks fracture, the earthquake flows are commonest in areas with little may vary from mild quiverings to violent vegetation, where rainfall comes in sharp oscillations of the land surface. A number bursts, saturating the soil, and where the of earthquake magnitude scales have been soil has a high content of fines. The differ- developed, the RICHTER SCALE being the ence between earthflows and mudflows is best known. purely of degree: earthflows occur on gentler slopes and move at a lesser velocity earthquake focus See focus. than mudflows. The soil eventually comes to rest as a tongue at the slope foot, the earthquake intensity The degree of vi- thickness of the tongue becoming less as olence of an earthquake at a particular the velocity of flow increases. Like the point on the Earth’s surface, expressed on other mass movements, flows act on unsta- a descriptive scale. Many intensity scales ble slopes, reducing them to a more stable have been developed; in the USA the Mod- angle. ified Mercalli scale (see Mercalli scale) is currently used. It is composed of 12 levels earth hummock A rounded mound of of increasing intensity based on observa- frozen soil, up to 20 cm high, that has fine tions and descriptions of damage. material at its core and is covered with vegetation. Common in alpine and arctic re- earthquake zone See seismic zone. gions, earth hummocks create a type of PATTERNED GROUND. earth science Any one of the sciences that study the Earth. The earth sciences in- earth movement Any movement of the clude geology, geomorphology, meteorol- Earth’s crust caused by processes occurring ogy, and oceanography. beneath it. Sudden earth movements accompany earthquakes and volcanic erup- earthslide The movement of the soil tions; slow movements cause folding and mantle over a shear plane. For the process uplifting of rock strata. to operate there must be an unstable slope and a dry soil. If the soil becomes wet, it earthquake A series of shocks, subdi- may turn into an EARTHFLOW. An impor- vided into FORESHOCKS, PRINCIPAL SHOCKS, tant part may be played by water, however, and AFTERSHOCKS, which generate SEISMIC in lubricating the shear plane. See also WAVES within the Earth, as a result of the mass movement. fracturing of brittle rocks within the lithosphere. They result from the accumulation earth temperature The temperature of of forces within the rocks until they are the ground surface as determined by the strained to a point beyond which they frac- characteristic wavelengths of long-wave ture. The magnitude of an earthquake is radiation emission. the amount of energy involved and is determined from recordings of seismic waves earthworm A terrestrial annelid worm on seismographs. Depending on the quan- that is the most important of the macro-

115 easterlies fauna in the soil. Under favorable condi- motion and feeding and operated by the tions, as when the soil is moist and rich in water vascular system, an internal system lime and organic matter, there may be up of fluid-filled canals. These radii are to a million earthworms per acre. They known as ambulacra and the areas be- may pass as much as 10 tonnes per acre of tween as interambulacra. An external soil through their bodies each year. This skeletal system (test) composed of plates of material is humified and excreted in the single calcite crystals is usually present and form of worm casts, which contain more sometimes bears spines. There are five liv- humus, nitrates, and exchangeable bases ing classes: the Asteroidea (starfish), Ophi- than the surrounding soil. Besides their uroidea (brittle stars), ECHINOIDEA (sea function in the chemical and physical urchins), Holothuroidea (sea cucumbers), breakdown of plants they are also impor- and the CRINOIDEA (sea lilies). In addition, tant in improving the texture, aeration, there are a number of important extinct and drainage of the soil. classes, such as the Cystoidea. Echino- derms extend from the early Cambrian Pe- easterlies Any winds in which the zonal riod to the present day. component of air flow is from the east. They are subdivided into the tropical east- Echinoidea /ek-ă-noi-dee-ă/ The class of erlies or trade winds and the polar easter- the phylum ECHINODERMATA to which the lies. The presence of easterly winds at the sea urchins belong. They are protected by a ground surface is an essential requirement hemispherical and usually spiny test of in- to counteract the effect of the mid-latitude terlocking calcite plates, which in some westerlies on the Earth’s rate of rotation. later forms is flattened. Typically the anus is situated in the center of the upper surface easterly wave A shallow trough or dis- and the mouth is diametrically opposite. In turbance in the trade-wind flow associated later forms the mouth and anus may have with an increase of cloudiness and precipi- become displaced, the anus sometimes tation to its rear. After initial debate about opening on the oral surface. Echinoids are the status of the easterly wave, satellite marine, benthonic, and free living and are photographs have confirmed characteristic known as fossils from the Ordovician Pe- cloud patterns associated with the trough riod onward. The group was affected by although they do exhibit greater variety the widespread extinctions at the end of the than was first thought. In some instances Paleozoic Era but revived in the Mesozoic they probably act as initial disturbances for and extends to the present day. Echinoids the development of hurricanes. are used as biostratigraphic ZONE FOSSILS in the Cretaceous System. easting Any of the north-south grid lines on a map, quoted before the NORTHING eclogite /ek-lŏ-jÿt/ A coarse-grained when coordinates are being given, showing granular rock consisting essentially of distance east from the origin of the grid. bright green omphacite and deep red al- mandinepyrope garnet. Diopside, quartz, ebb tide The outgoing of the tidal and kyanite may also be present in small stream; the retreating tide, i.e. that part of amounts. Eclogite has the chemical compo- a tide cycle following the high-water stage sition of basalt and may be considered to and preceding the low-water stage. Com- be the high-pressure high-temperature pare flood tide. metamorphic equivalent. Eclogite is found as lenses and bands in regionally metamor- Echinodermata /i-kÿ-noh-der-mă-tă/ The phosed rocks of the highest grades and as phylum of marine invertebrate animals inclusions in basalt and kimberlite. that includes the starfish, sea urchins, and crinoids. The body of an echinoderm usu- ecology The scientific study of how liv- ally has five radii, along which are grouped ing organisms affect, or are affected by, hydrostatic tube feet, functioning in loco- their natural environments and by other

116 Ekman flow organisms. Both living and nonliving com- flow after the current has been disturbed ponents of the environment are considered by a bridge or shearing. See also whirlpool. (including plants, animals, soil, climate, In the atmosphere, eddies are found in a temperature, etc.). Ecology may be con- wide range of scales. Near the ground sur- cerned with individual organisms, popula- face they are important in transferring motions, or even whole communities. See also mentum, heat, and moisture. On the large autecology; ecosystem. scale, eddies in the form of depressions are a necessary mechanism for much of the economic basement Strata below which meridional transfer in the GENERAL CIRCU- there is little chance of finding economic LATION OF THE ATMOSPHERE. mineral resources, particularly oil. eddy diffusion The exchange of atmos- ecosystem An ecological unit that con- pheric matter and properties brought sists of the physical environment (both liv- about by eddies. See also diffusion. ing and nonliving) and the organisms that occupy it. It may be as small as a pond or edenite /ee-dĕ-nÿt/ A monoclinic AMPHI- tidal pool, or as large as a tropical rainfor- BOLE. est or even the whole global system. Nutri- ents and energy pass through an ecosystem edge dislocation A defect in a crystal in a certain way (see food chain). The or- lattice resulting from the insertion of an ganisms living in it may occupy various extra plane of atoms. TROPHIC LEVELS, from plants (producers) at the bottom to animals (consumers) at the effluent 1. (in geology) A stream that top. Left alone, an ecosystem achieves a flows out of a lake or other stream. balance, with all the organisms living suc- 2. In general usage, waste material dis- cessfully together. It will even usually re- charged as a liquid, e.g. from an industrial cover from a disaster such as a drought or plant, and a potential source of pollution. flood. Human intervention, however, may cause changes from which the ecosystem ejecta /i-jek-tă/ 1. Material that is thrown cannot recover, for example the large-scale out of an erupting volcano. See pyroclastic cutting down of forests or extensive pollu- rock. tion of the air, land, or oceans. See also car- 2. Material that is thrown out when a ME- bon cycle; nitrogen cycle. TEORITE impacts with the ground. It consists mainly of rock fragments, but may ecoulement /i-Kool-mĕnt/ The down- also include glassy particles that result ward sliding of large masses of rock as a re- from the melting of the meteorite or the sult of gravity. rock it hits. ectoparasite See parasite. ejecta blanket A deposit of EJECTA that surrounds an impact crater after a METE- Ectoprocta See Bryozoa. ORITE hits the ground. edaphic /i-daf-ik/ Describing those fac- ejectamenta /i-jek-tă-men-tă/ See tephra. tors of the soil (chemical, biological, and physical) that affect the growth of plants. Ekman flow /ek-măn/ (in oceanogra- Examples of such factors include moisture, phy) The movement of surface sea water in mineral content, and texture. the direction of a wind, which, as it blows, exerts a force on the sea surface in the di- eddy A rotational feature of a fluid, rection of the wind and, because of fric- which retains its identity for a limited time tional forces operating within the water while moving within the main body of the column, causes a certain thickness of water fluid and eventually amalgamating with it. to flow. The ocean’s response is highly Such eddies can frequently be seen in river complicated, especially because of Earth

117 Ekman layer rotation and the fact that water is fluid, but with height (see diagram), blowing across also because of such factors as land-ocean the isobars at low speed near the surface configuration. In the N hemisphere, aver- and parallel at higher speeds in the free at- age Ekman flow is some 45° to the right of mosphere. the wind, and the speed decreases and the direction swings increasingly to the right of elastic wave A seismic wave that is the wind direction with increasing depth. propagated through a medium by elastic Ekman flow has several far-reaching ef- deformation. fects; for instance, off the coasts of Califor- nia and Peru, Ekman flow tends to be in an elbow of capture A right-angled bend offshore direction, thereby causing zones in a river, downstream of which is the cap- of upwelling of fertile deep water near the turing river and upstream of which is the coast, which provides nutrients for fish captured river. See river capture. populations. It is named for the Swedish oceanographer Vagn Walfrid Ekman electrical logging A subsurface logging (1874–1954). technique in which an electrode or series of electrodes are lowered into an uncased Ekman layer (in oceanography) The borehole. As the sonde is raised a continu- shallow surface layer of the upper water ous record of the electrical properties of the masses of ocean circulation in which rocks through which it passes is recorded. EKMAN FLOW occurs. The thickness of the By an examination of the variations in sev- Ekman layer has been calculated theoreti- eral properties an assessment of the rock cally. The uppermost layer of the circula- types present in the borehole can be made. tion of ocean water is relatively thin, about 100 m deep. Beneath it, and extending electromagnetic radiation The form down to the ocean floor, is the deep-water of energy that travels in waves from its circulation involving deep water masses. source without the necessity of an intervening medium. The waves move with the Ekman spiral (in meteorology) The speed of light (3 × 108 m per second) in EKMAN FLOW (or effect) as it applies in the space and only slightly slower through the atmosphere. The ground surface has a fric- atmosphere. The total spectrum of this tional interaction with the atmosphere and form of radiation includes wavelengths as its effect decreases with height until, be- short as gamma rays (10–13 m) to long tween 500 and 1000 m, the GEOSTROPHIC radio waves (up to 105 m), as shown on the WIND is observed. The spiral illustrates in diagram overleaf. Within this range is vis- vector form how the wind velocity changes ible light between 0.4 and 0.7 µm, to which our eyes are sensitive. The behavior of electromagnetic radiation is explained by the following physical laws: 1. All matter with geostrophic wind at approx. 1 km a temperature above absolute zero (–273°C) emits radiation. 2. Some substances emit radiation of certain wave- height lengths only. 3. A substance that emits the maximum amount of radiation for a given temperature in all wavelengths is known as a black body (see black-body radiation). Ekman spiral The amount of radiation emitted is then proportional to the fourth power of the Wind increases in speed substance’s absolute temperature. 4. Sub- and veers in direction stances will only absorb radiation of wave- with increasing height lengths that they can also emit. 5. The hotter a substance, the shorter will be the Ekman spiral wavelengths at which most of the radiant

118 El Niño–Southern Oscillation energy is emitted. 6. The amount of radia- element A substance that cannot be bro- tion received at a point is inversely propor- ken into simpler substances by chemi- tional to the square of the distance of that cal means. All the atoms in an element have point from the radiation source. the same atomic number although, in isotopes, some of an element’s atoms have 10–15 – a different mass (because they contain – 1020 different numbers of neutrons). There 10–14 – are 92 elements that occur naturally on – 1019 Earth. 10–13 – – 1018 elevation The vertical distance of any point on the Earth’s surface above the 10–12 – gamma rays DATUM level, measured in meters or feet. – 1017 –11 10 – El Niño–Southern Oscillation /el neen- 16 – 10 yoh/ (ENSO) A periodic disturbance of –10 X-rays 10 – the ocean–atmosphere system that occurs – 1015 in the tropical Pacific at irregular intervals 10–9 – of some 3 to 8 years, and which may last – 1014 for over a year. The term El Niño (baby ultraviolet –8 10 – radiation boy) was originally applied by Peruvian – 1013 fishermen to a warm southward-flowing 10–7 – ocean current that displaces the normal visible light – 1012 north-flowing cold current in an allusion to the Christ child because it appears 10–6 – around Christmas time. It was subse- – 1011 quently used for the periodic warmings 10–5 – 10 that disrupted the fish and bird popula- infrared – 10 tions off Peru and Ecuador and has since wavelength/m 10–4 – (heat) frequency/kHz become synonymous with the larger-scale radiation 9 – 10 warm events. It now refers to the large- 10–3 – scale weather effects that are associated 8 EHF – 10 with the weakening of the trade winds and 10–2 – unusually warm surface ocean layers in the radio – 107 eastern and central Pacific Ocean. El Niño SHF frequencies 10–1 – events are connected to fluctuations in the 6 Southern Oscillation (SO), which is a see- UHF – 10 sawing in atmospheric pressure between 1 – 5 the eastern South Pacific (east of Tahiti) VHF – 10 and the Indian Ocean (west of Darwin, 10 – Australia). The coupled air–water flow is – 104 HF called the El Niño–Southern Oscillation 102 – (ENSO) and has repercussions for climate 3 MF – 10 and weather around the world. 103 – Under normal, non-El Niño, conditions 2 LF – 10 the prevailing trade winds blow westward 104 – and push large volumes of water west to- – 10 VLF ward the coast of Indonesia. The cold 105 – Humboldt current flows northward off the – 1 South American coast and then, at the equator, turns westward as the South Equatorial Current. The upwelling of cool Electromagnetic radiation water off the South American coast is nu-

119 El Niño–Southern Oscillation trient rich and supports diverse marine life El Niño event by weak high pressure and with important fisheries. When an El Niño pressures drop in the SE tropical Pacific. event occurs there is a change in the wind Drought conditions occur in Australia and pattern with a weakening of the trade southeastern Asia while in contrast the winds in the central and W Pacific. The up- warm waters bring heavy rains and flood- welling of cool water ceases, to be replaced ing to parts of the coasts of North and by warmer water that flows back from the South America, such as the normally desert west, and the nutrient supply is cut off with areas of Peru. Recent El Niños have oc- a consequent fall in marine life and adverse curred in 1982–83, 1986–87, 1991–92, effect on the commerical fisheries, such as 1994, and 1997–98. As part of this cycle the anchovy catch off Peru. The normal there is, in some years, formation of a cold low-pressure system over the W tropical region in the eastern Pacific. This is called Pacific and Indian Ocean is replaced in an La Niña (the little girl).

UNITED STATES

normal trade winds warm

PERU INDONESIA normal ocean currents

sea

AUSTRALIA P A C I F I C O C E A N

Under normal conditions the trade winds push water from east to west and warm water accumulates around Indonesia

UNITED STATES

P A C I F I C O C E A N PERU INDONESIA El Niño current

weaker trade winds increase in surface temperatures AUSTRALIA

In El Niño years, the trade winds weaken and the warm water flows back toward the coast of Latin America

120 enrichment elutriation /i-loo-tree-ay-shŏn/ The nat- emissivity /em-ă-siv-ă-tee/ The ratio of ural sorting of rock fragments into finer emission of radiation from a substance to and coarser particles. It most commonly the emission from a black body at the same occurs when the fragments are transported temperature and wavelength (see black- by water, but may also happen during py- body radiation). Values range from slightly roclastic flow, when volcanic material below 1.0 to 0.85 for most substances. flows down the side of an erupting volcano. enclave An inclusion or XENOLITH. eluviation /i-loo-vee-ay-shŏn/ The wash- enderbite /en-der-bÿt/ A rock of the ing out of fine material from a soil, espe- CHARNOCKITE group, rich in plagioclase cially from the upper part. The eluvial feldspar. horizon is the A horizon, which has less clay than the rest of the profile as a result. endomorphism /en-doh-mor-fiz-ăm/ The change in composition of igneous rock de- emerald A bright green transparent va- rived from magma as it assimilates material riety of BERYL (the color is caused by from the COUNTRY ROCK surrounding it. chromium impurities), valued as a precious See also metamorphism. gemstone. It crystallizes in the hexagonal system and occurs mainly in mica schists endoparasite See parasite. and veins of calcite. en echelon /awn esh-ĕ-lawn/ Denoting emery A black or dark gray impure form overlapping or offset geologic structures. of the mineral CORUNDUM. It occurs as fine granules, often with hematite or magnetite energy balance A concept applied to impurities. It is used as an abrasive. the static energy of the Earth–atmosphere system, rather than its dynamic energy. It emission A substance (particulate, gas, relates the net radiation flux at a surface to or aerosol) that is discharged into the at- the utilization of this available energy. This mosphere as a waste product or pollution, is consumed by the conduction of heat either from an anthropogenic source (e.g., below the ground surface, the transfer of exhaust emissions from automobiles and sensible heat to the atmosphere by turbu- emissions from industrial processes) or lence and convection, and the transfer of from a natural source (e.g., volcanic activ- latent heat by similar methods. All these ity). See also air pollution. processes are reversible. The energy balance equation is often emissions trading A mechanism that called the heat balance equation and is ex- aims to reduce pollutants and greenhouse pressed in the form R = H + LE + G where gas emissions. In emssions trading schemes R is net radiation, H is sensible heat, LE credits or permits are allocated to those the latent heat transfer, and G the compo- parties responsible for emissions (e.g. a nent transferred into the ground. company or country) by a regulatory organization. The scheme allows the party to englacial /en-glay-shăl/ Describing ma- emit in excess of its allocation by purchas- terials or meltwater contained within an ing credits or allowances from the market; ice mass. Such debris or water can reach a party that emits less than its allocation the interior ice either by movement upward can sell or trade the surplus. The concept from the bed or downward from the sur- was proposed by the KYOTO PROTOCOL, face. which sets limits on total emissions of greenhouse gases by the world’s major enhanced greenhouse effect See green- economies. The European Union Emis- house effect. sions Trading Scheme (EU ETS) came into effect in January 2005. enrichment The natural increase in the

121 ENSO proportion of one of the elements or min- level of floodplain erals that constitute a given type of rock. It may result when additional amounts of a constituent are introduced from outside sources, or when one constituent is selectively removed from the rock. It can be bedrock caused by chemical processes, as when mineral-laden water filters through a porous rock, or by mechanical processes, as when relatively light quartz is trans- stream ported away from heavier metallic minerals. Section through an entrenched meander ENSO See El Niño–Southern Oscilla- tion. to the asymmetrical cross profile of the ingrown type. However, some flood- enstatite /en-stă-tÿt/ An orthopyroxene. plain meanders do not incise themselves See pyroxene. vertically into their floodplains, but at an angle, producing an asymmetric cross pro- entisol /en-tă-sôl/ One of the twelve soil file. orders of the US SOIL TAXONOMY, approximately equivalent to the azonal category of environmental lapse rate The actual the old classification. These are recent soils rate of change in temperature with height without natural genetic horizons and they of the atmosphere, at a given time, used include lithosols (shallow stony soils), particularly when investigating the stabil- regosols (thin soils on unconsolidated ity of the lower atmosphere. The differ- drift), and alluvial soils or fluvisols (soils ences between the DRY ADIABATIC LAPSE that are constantly being added to because RATE, SATURATED ADIABATIC LAPSE RATE, and of their site on active floodplains). These environmental lapse rate are especially im- soils are not extensively used for agricul- portant in this respect when drawn on a ture but may become productive when THERMODYNAMIC DIAGRAM. The lapse rate managed properly. is steep in the lower layers during daytime heating, decreasing with height, so that by entrainment (in meteorology) The the upper atmosphere it approximates to process of mixing between the environ- the saturated adiabatic lapse rate. Inver- ment and a rising thermal in the updraft of sions are frequently present indicating a a cumulus or cumulonimbus cloud. It has warming of the air with increasing height, the effect of diluting the warmer rising air but within the troposphere they are rarely and so slightly reduces its buoyancy. This very deep. prevents the rate of cooling in the thermal from being adiabatic, but in practice the Eocene /ee-ŏ-seen/ The epoch of the PA- difference is slight. LEOGENE Period that extended from the end of the PALEOCENE, 57.8 million years ago, entrenched meander The type of IN- to the beginning of the OLIGOCENE Epoch, CISED MEANDER that results from the REJU- about 36.6 million years ago. Some classi- VENATION of floodplain meanders and fications dispense with the Paleocene, thus subsequent incision into the bedrock adding another 8.6 million years to the below. They are therefore the product of a Eocene, which then becomes the first two-stage process, unlike the one-stage epoch of the Paleogene. During this epoch origin of INGROWN MEANDERS. Entrenched mammals were abundant, perissodactyls, meanders are often distinguished from including large and small horses, and artio- ingrown meanders on the basis of their dactyls appeared and the modern carnivore symmetrical valley cross profile, contrasted families became established. The first evi-

122 epicentral angle dence of bats and whales also comes from are therefore very much more effective this epoch. than longer periods of gentle winds. Eolian transport takes place concur- eolian erosion /ee-oh-lee-ăn/ (aeolian ero- rently in three forms: suspension, SALTA- sion; wind erosion) The direct erosive ac- TION, and surface creep. A small tion of wind. This is the least important proportion of grains of less than 0.2 mm in aspect of wind action, generally of little diameter can be carried wholly in suspen- consequence in landscape formation when sion; the particle is totally buoyed up by compared with the role of wind transport the rising eddies in the air and carried and deposition. Sand blast, from the im- along parallel to the air stream. Surface pact of saltating grains, is limited to below creep accounts for about 25% of actual one meter or so from the ground surface; it sand movement, and involves the move- can undercut rocks leaving pedestals, al- ment of grains of coarse sand along the sur- though in some instances increased weath- face by the impact of the saltating grains. ering at the foot of the rock weakens it Saltation accounts for about 75% of sand beforehand. More significant than eolian flow and involves the bouncing of grains erosion by abrasion is the production of along the surface at heights of less than 1 DEFLATION hollows. These can reach 100 m, and mostly within 200 mm of the km across, and although partly due in ground surface. some cases to faulting or rock solution, they are mainly due to wind removal of eon /ee-ŏn, ee-on/ The largest interval of preweathered material down to the water geologic time in the Chronomeric Standard scheme of chronostratigraphic nomencla- table, which halts further removal and pro- ture (see chronostratigraphy), formed of duces OASES. several ERAS grouped together. eolian form (aeolian form) A landform Eötvös unit /ay-ot-vos; Hungarian ay-œt- produced by material transported by wind. vœsh/ The unit employed to express Large-scale features include DUNES, sand gravitational curvature or gradient. 1 shadows, and sand sheets, while small- Eötvös unit equals 10–6 mgal cm–1. It is scale features include sand ripples and named for the Hungarian physicist Baron ridges. Sand shadows, unlike true dunes, Roland von Eötvös (1848–1919). are deposited in the shelter of an obstacle, while sand sheets or seas are amorphous epeiric sea /e-pÿ-rik/ See inland sea. sheets with gentle swellings reaching 3–6 m. Ripples are the products of irregulari- epeirogenesis /i-pÿ-roh-jen-ĕ-sis/ Uplift ties in the surface over which the sand is or subsidence of large areas of continents passing: these initiate local concentrations or ocean basins. Compare orogenesis. of sand grain impacts on the slopes facing the wind, which become built up as ripples. ephemeral stream /i-fem-ĕ-răl/ A stream Further, each ripple acts as a take-off point that contains water only immediately after for grains in SALTATION, and since the aver- rainfall, found mainly in arid and semiarid age length of travel per jump is related to areas. For most of the year, its channel is wind speed, an even repetition of areas of dry (see gully). concentrated grain impacts occurs downwind, leading to regular ripple patterns. epicenter /ep-ă-sen-ter/ The point on the Earth’s surface situated directly above the eolian transport (aeolian transport) The focus of an earthquake. movement of sediment by wind. Below a threshold of 16 km per hour wind is inca- epicentral angle /ep-ă-sen-trăl/ The an- pable of moving sand, but thereafter sand gular distance between the epicenter of an movement rises as a cubic function of wind earthquake and a recording station, ex- speed. Short periods of high-velocity winds pressed in terms of the angle subtended at

123 epidorite the center of the Earth between verticals the soil with organic matter or the upper from these two points. eluvial horizon or both. Introduced by the US SOIL TAXONOMY, there are six common epidiorite A rock with a dioritic min- epipedons: mollic, histic, plaggen, an- eralogy derived from the low- to medium- thropic, umbric, and ochric, each being in- grade metamorphism of labradorite- dicative of a certain class of soil. For pyroxene assemblages of basic igneous example, MOLLISOLS are identified by the rocks (gabbro and dolerite), which results mollic epipedon; soils influenced by human in the formation of hornblende and a pla- use are identified by the anthropic epi- gioclase of more sodic composition. pedon (>250 ppm acid soluble salt phosphate due to farming) and the plaggen epidote A group of minerals having the epipedon (a layer >50 cm deep produced general formula X2Y3Si3O12(OH), where by manuring). X is mainly Ca but also Mn2+, Ce3+, and other rare earths, and Y = Al, Fe3+, Mn3+, epitaxial growth /ep-ă-taks-ee-ăl/ The and Fe2+. The following compositions parallel or orientated overgrowth of one occur: mineral on a crystal of another such that zoisite Ca2Al3Si3O12(OH) there is some form of continuity main- clinozoisite Ca2Al3Si3O12(OH) tained between the crystal structures of 3+ epidote Ca2Fe Al2Si3O12(OH) both minerals. Examples include the over- 3+ 3+ piemontite Ca2(Mn Fe ,Al)3- growth of augite on orthopyroxene and Si3O12(OH) idocrase on garnet. 2+ 2+ allanite (orthite) (Ca,Mn ,Ce)2(Fe , 3+ Fe Al)3Si3O12(OH) epithermal deposit /ep-ă-th’er-măl/ See The epidote minerals are monoclinic hydrothermal process. except for zoisite, which is orthorhombic. Zoisite may be gray, green, or brown in epoch An interval of geologic time in the color; clinozoisite is colorless to green; epi- Chronomeric Standard scale of chrono- dote is green to yellow-green; piemontite is stratigraphic classification (see chrono- typically red-brown; allanite is brown to stratigraphy). The equivalent Stratomeric black. A pink manganiferous variety of Standard term, indicating the body of rock zoisite is called thulite. Minerals of the epi- formed during this time, is the SERIES. Sev- dote group are found in the medium-grade eral epochs together form a PERIOD and are regionally metamorphosed rocks of the themselves formed of a number of AGES. greenschist and amphibolite facies. Epidote is produced during retrograde metamor- equal-area projection See homolo- phism and may be found on joint surfaces graphic projection. and along fractures. Plagioclase feldspar, clinopyroxenes, and hornblendes can all be Equator An imaginary line that girdles replaced by epidote minerals and basic ig- the Earth at latitude 0°. It is a GREAT CIRCLE neous rocks in particular may suffer exten- 40 076 km long. North of it is the N hemi- sive epidotization. Allanite is found as an sphere; to the south is the S hemisphere. accessory mineral in some acid igneous rocks. equatorial current Among the prominent currents that flow in the oceans are epilimnion The warmer upper layer of those in the equatorial regions. The cur- water in a lake or shallow sea. Photosyn- rents flowing at or close to the Equator in thesis may occur and green plants grow the Pacific Ocean are, in general, similar in within the epilimnion because light can pattern to those flowing in equatorial re- penetrate it. See also hypolimnion. gions of the Atlantic Ocean, there being a South Equatorial Current flowing astride epipedon /ep-ă-pee-dŏn/ A diagnostic the Equator, which is separated from the surface horizon, constituting that part of westward-flowing North Equatorial Cur-

124 equinox rent by an eastward-flowing Equatorial scribing a rock that has crystals of a uni- Countercurrent. However, this countercur- form grain size. rent is much more strongly developed in the Pacific than is its counterpart in the At- equilibrium profile (graded profile)A lantic. There are also currents that flow at long profile of a river in which there is a some depth; for instance, the Cromwell delicate balance between erosion and de- Current (also known as the Pacific Equa- position. Slope of the equilibrium profile is torial Undercurrent) flows as a narrow adjusted so that the sediment load pro- swift current in an easterly direction and at vided to the river can just be transported by a fairly shallow depth beneath the South the available discharge and channel char- Equatorial Current. The flow rate in the acteristics. Traditionally this is thought of Equatorial Countercurrent probably at- as being a concave profile, but in practice it tains some 25 million cubic meters per sec- need not be perfectly smooth. Streams with ond, with speeds of up to 2 knots. uneven long profiles have been found to be in equilibrium in the sense that in no part equatorial rainforest See tropical rain- of their profile is active erosion or deposi- forest. tion taking place. See also dynamic equilibrium; equilibrium regime. equatorial trough The belt of low pressure that occurs in equatorial areas and os- equilibrium regime The regime of a cillates in its mean location depending natural system, such as a river channel, upon the position of the overhead Sun. It coastline, slope profile, etc., when it is in a marks the convergence zone of the trade state of balance considered over time, nei- winds blowing from each hemisphere and ther eroding nor depositing. The term is is most clearly observed over the ocean most often used in connection with rivers, areas where continental modifications of where it is largely synonymous with GRADE. airflow are least. equinox /ek-wă-noks/ The period of the equatorial westerlies A zone of rather year when the Sun is overhead at local variable winds, found when the INTERTROP- solar time at the Equator. This happens ICAL CONVERGENCE ZONE extends more twice a year, about March 21 and Sept. 22, than 5° from the Equator. They probably when day and night are equal time periods represent trade-wind air that has acquired of twelve hours. Because of refraction a westerly component on crossing the through the atmosphere, actual day length Equator but their origins may vary de- is slightly longer than the true astronomi- pending upon the precise location of formation. equatorial zenithal gnomonic projec- projected view on tangent plane tion A MAP PROJECTION in which the tangent touches the surface of the Earth at the 0° Equator. All the great circles are straight lines but the angle of intersection of the pole meridians and the parallels (shown by 45° curves) becomes increasingly more acute 30° poleward, and therefore the distortion is greater. Places best represented on this pro- 0° jection are those near the Equator, e.g. Equator Africa; beyond about 30° north or south the distortion is too large for the projection to be of any use. equigranular /eek-wă-gran-yŭ-ler/De- Equatorial zenithal

125 era cal day. In NW Europe, the first and sec- erratic A rock that has been transported ond equinoxes are taken to mark the be- by ice and deposited in an area of dissimi- ginning of spring and of fall respectively. lar rock type. Volcanic and metamorphic rocks frequently make the most impressive era An interval of geologic time in the erratics because they are often distinctively Chronomeric Standard scheme of chrono- colored and their high resistance to erosion stratigraphic nomenclature (see chrono- permits their continued existence despite stratigraphy). It is formed of several considerable transportation. By linking PERIODS grouped together and a number of source outcrop with final resting place, eras may be compounded to form an EON. some insight into directions of ice move- For example, the Paleozoic Era is com- ment can be gained, although one erratic posed of six periods (the Cambrian, Or- may, in turn, be moved by several ice dovician, Silurian, Devonian, Carbonifer- masses in different directions. The exis- ous, and Permian) and lasted for some 325 tence of some erratics hundreds of meters million years, from about 570 to 245 mil- in extent indicates the phenomenal power lion years ago. available in glacial transportation. erg 1. A type of arid desert landscape eruption plume A cloud of gas, rock consisting of a very extensive sand cover, fragments, and molten magma that rises especially in the Sahara. See also ham- into the air above an erupting volcano. Its mada; reg. height depends on its density and the pres- 2. A unit of work or energy equal to the sure that forced it upwards. work done by a force of 1 dyne acting through a distance of 1 cm. 1 erg is equal escape velocity The initial velocity that to 10–7 joules. a particle, space probe, etc., on the surface of a body such as the Earth would require erosion The lowering of the land surface to overcome the gravitational influence of by agents that involve the transport of rock that body and move away from it. debris. Unlike degradation, erosion tends to be an episodic process, varying greatly escarpment See scarp. over time and space (see cycle of erosion). For erosion to occur, the eroding medium esker /esk-er/ An elongate ridge of (gravity, river flow, waves, currents, wind, rounded stratified FLUVIOGLACIAL deposits, ice, etc.) must exert a force on the land sur- consisting primarily of sands and gravels face greater than its shear strength. The with some finer and coarser materials; they elastic limit of the material then being ex- can be only tens of meters long but some ceeded, fractures form and the material extend for hundreds of kilometers. Eskers moves in the direction of the force applied form in contact with stagnant or very slow to it. The movement may be sharp and sud- moving ice from materials deposited by den, as in LANDSLIDES and ROCKFALLS, or meltwater streams, which may be under- slow and imperceptible, as with CREEP. neath, within, or above the ice mass. The Compare denudation. See also differential cross-sectional shape of an esker will de- erosion. pend largely upon the position of the melt- Soil erosion is a great problem because water stream. If it is above the ice, the esker the surface part is the most fertile and plant will have an ice core and melting will cause nutrients have to be replaced artificially. collapse; if it is in tunnels within the ice, Wind erosion of soil can be severe in semi- collapse will also occur when the walls arid regions (see dust bowl). Water erosion melt. If the tunnels are high and narrow, (sheet wash, rill action, gullying, etc.) is sharp-crested ridges are produced, whereas usually most prominent in humid lands, low wide tunnels result in flat-topped and e.g. in the Tennessee Valley. See also ab- comparatively undisturbed eskers. See also normal erosion. beaded esker.

126 Eutheria essential mineral Any of certain diag- that provides a mathematical explanation nostic minerals on the presence or absence for the distribution of conservative PLATE of which is based the classification and BOUNDARIES. Plate tectonics requires that subdivisions of igneous rocks. For exam- all conservative plate boundaries (trans- ple, the presence of calcic plagioclase and form faults) lie on small circles, the axes of pyroxene is implied in the term gabbro. which form the axis of rotation for the relative motion of the plates on each side. It essexite /ess-iks-ÿt/ A type of ALKALI also indicates that the velocity of relative GABBRO. motion across a destructive or constructive plate boundary is proportional to the an- estuarine (in ecology) Describing an en- gular distance of the particular point from vironment at a river ESTUARY, where there the axis of rotation and to the angular ve- are tidal effects and fresh and salt water locity about the axis of rotation for the mo- mix. tion of the plates. Thus velocities will vary along plate boundaries. The theorem is estuary The part of a river mouth within named for the Swiss mathematician Leon- which tides have an effect and therefore hard Euler (1707–83). where fresh and saline water are mixed. Most present-day estuaries are DROWNED eulittoral zone /yoo-lit-ŏ-răl/ See lit- VALLEYS, owing their existence to the post- toral zone. glacial rise in sea level, and for this reason they usually contain much deposited sedi- euphotic zone /yoo-foh-tik/ Ocean depths ment. Deposition will continue if the river to which abundant sunlight for photosyn- introduces more sediment than can be re- thesis penetrates, extending from the sur- moved by tidal streams and by what little face to about 80 m. See also aphotic zone; wave action is possible within the estuary. disphotic zone. Many estuaries exhibit intricate patterns of channels, largely the result of erosion by eustasy /yoo-stă-see/ Worldwide move- both incoming and outgoing tidal streams, ments of sea level. The origin of these which give flood and ebb channels respec- movements is most commonly attributed tively. See also braiding; delta. to the withdrawal and release of water due to the growth and decay of ice masses in etesian wind /i-tee-zhăn/ A wind blow- the Quaternary (GLACIO-EUSTATISM). But ing from a northerly direction in the eustatic movements can also be caused by Aegean Sea or E Mediterranean during the tectonic movements of the sea floors or summer period. Such winds are a response landmasses. It may be difficult to distin- to the shallow thermal low pressure over guish between the two, but most effects can the Sahara and the northward movement be attributed to glacio-eustasy. Since the of the Azores anticyclone to be centered last ice age there has been a gradual rise in over the W Mediterranean. sea level as conditions have become relatively warmer. This is a positive eustatic eucrite /yoo-krÿt/ A variety of coarse- change. See also base level. grained basic igneous rock containing clinopyroxenes and orthopyroxenes, eutaxitic /yoo-taks-it-il/ Denoting the olivine, and plagioclase of bytownite-anor- streaky or banded appearance common in thite composition. ignimbrites. Individual streaks or fiamme are discontinuous flattened bodies. The euhedral /yoo-hee-drăl/ Describing crys- term parataxitic is applicable when the fi- tals that are well developed and have good amme are extremely elongated. See pyro- crystal faces. Compare anhedral; subhe- clastic rock. dral. Eutheria /yoo-theer-ee-ă/ The placental Euler’s theorem /oi-lerz/ A theorem mammals. Their young are born at a late

127 eutrophic stage in development, having been nour- water molecules from the surface layers ished inside the body of the mother by and replacement by other water molecules means of a connection called the placenta. in the lower layer of the atmosphere, which Fossils suggest that the Eutheria diverged through continual motion strike the sur- from other mammalian groups in the Cre- face and become absorbed. As the mol- taceous, but their main evolutionary radia- ecules are being transferred from a lower to tion took place during the Cenozoic, after a higher energy state through evaporation, the extinction of the dinosaurs. They a supply of energy must be provided for the rapidly became the dominant land animals process to be maintained. This is a very im- and now include such diverse forms as the portant aspect of the Earth’s energy bal- flying bats and the marine whales (which ance; by this process surplus energy in are the largest living animals known). tropical areas is transferred by the flux of Compare Marsupialia. water vapor to the radiation-deficit areas nearer the poles. eutrophic /yoo-troff-ik/ (in ecology) De- If evaporation continued into still air, scribing a freshwater HABITAT in which the the overlying layer would very soon reach water is rich in plant nutrients. See eu- saturation and prevent further evapora- trophication. tion. However, air movement usually mixes lower layers by turbulence, reducing eutrophication /yoo-trŏ-fă-kay-shŏn/ the water vapor content and enabling fur- The process by which a freshwater HABITAT ther evaporation to take place. Clearly, the becomes excessively enriched with nutri- stronger the wind, the greater will be the ents. These nutrients include nitrates and rate of evaporation as long as the air is not phosphates (generally from artificial fertil- saturated. This introduces the second fac- izers washed off farmland by rainwater) tor influencing evaporation, that is the re- and possibly sewage. As a result, there is quirement for the air to be capable of rapid growth of algae (an algal bloom), absorbing moisture. An index of this is the some of which die because insufficient light saturation deficit, which is the difference can penetrate the overgrown water. The between the saturation vapor pressure and dead algae decompose, using up dissolved actual vapor pressure. If the saturation oxygen and leading in turn to the death of deficit is large, as in warm dry air, the gra- fish and other aquatic animals. dient between the moist surface and the atmosphere will be high, and so the rate of euxinic environment A marine envi- transfer will be large. With moist air, the ronment in which the bottom water is humidity gradient will be less and the rate poorly ventilated, deficient in oxygen, and of evaporation correspondingly smaller. (in extreme cases) characterized by the The measurement of evaporation is dif- presence of hydrogen sulfide, i.e. chemi- ficult because of the problem of adequately cally reducing conditions. Such conditions simulating the evaporation surface. The apply, for example, to parts of the Black standard method is to measure the loss of Sea and to certain of the Norwegian fiords, water from an evaporation pan surface, where sea-floor basins occur at different but it is accepted that this does not equate depths. One effect is that the floor of the very well with true atmospheric evapora- Black Sea is sparsely populated with living tion. Measurements of the changes in organisms. The seabed deposits in euxinic weight of soil samples are a closer ap- environments are characteristically fine- proach to reality but still differ from the grained ones containing a high percentage natural situation. of decomposable organic material. evaporimeter See lysimeter. evaporation The process by which a liquid is changed into a gas or vapor. The evaporite /i-vap-ŏ-rÿt/ A rock or deposit rate of evaporation in the atmosphere rep- formed as a precipitate from a saturated resents a net effect between the removal of solution. Evaporation from virtually closed

128 exsolution bodies of saline water is the most common of exhumation have been used to explain process involved. Evaporites are classified the creation of a number of landforms, according to their chemical composition. such as TORS and INSELBERGS. A first stage Great thicknesses of evaporite deposits consists of progressive DEEP WEATHERING occur, with minerals layered in a sequence on a rock type embodying considerable depending upon their relative solubilities. variation in its susceptibility to decomposition, which largely reflects variation in its evapotranspiration /i-vap-oh-tran-spă- jointing pattern. A highly irregular WEATH- ray-shŏn/ The combined system of vapor ERING FRONT develops and may remain for transfer by evaporation and transpiration a considerable length of time. Subsequent from the ground surface and its vegetation erosion of the REGOLITH, initiated by earth layer. Except in areas of sparse vegetation, movements or sea-level changes, will even- such as deserts, transpiration by plants is tually exhume the least weathered rock the dominant factor in the total loss of masses. water from the land surface. See hydrologic cycle. exosphere /eks-ŏ -sfeer/ The upper layer of the Earth’s atmosphere, extending up- evergreen trees Trees that do not shed wards from a height of about 600 km. It their leaves in winter or the dry season. consists mainly of hydrogen and helium in Typical evergreens include conifers and varying proportions. See also atmosphere. trees of the tropical rainforest. Compare deciduous forest. exotic A boulder or large rock body that has been transported by one of a variety of evolution The gradual change in organ- processes into an area of unrelated rock isms through time, resulting in the origin of types. Where the transporting mechanism new SPECIES. In 1859 the British naturalist is tectonic in origin the rock body is said to Charles Darwin (1809–82) suggested the be ALLOCHTHONOUS, whereas if ice is the process of NATURAL SELECTION as the means transporting mechanism such blocks are by which this could come about. The di- called ERRATICS. versity of extinct and modern organisms has resulted from their becoming adapted exposure (in meteorology) The degree to particular habitats. Evolution is gener- of openness of a site and the amount of ally a progressive change, with organisms interference to airflow by natural and arti- becoming more able to deal with ever more ficial obstacles. For meteorological instru- extreme environments. This theory of ments to give representative values, they modification and change is opposed to the must be sited correctly with a uniformity doctrine of Special Creation, which holds of exposure. The correct exposure varies that species are immutable and are re- from element to element, but is most im- placed at intervals by newly created forms. portant for rain gauges and anemometers, Much fundamental evidence for the vari- where airflow can affect the recorded val- ous pathways of evolutionary change ues. It is never measured quantitatively al- comes from paleontology. See also conver- though attempts to do so have been made. gent evolution; parallel evolution. exsolution /eks-sŏ-loo-shŏn/ (unmixing) exfoliation The splitting of rocks into a Several minerals, notably feldspars and py- series of concentric shells by a number of roxenes, are homogeneous solid solutions different weathering processes: UNLOAD- of two or more chemical end-members at ING, SPHEROIDAL WEATHERING, and FLAK- high temperatures. During cooling, the ho- ING. mogeneous mix becomes unstable and such a mineral unmixes, producing two exhumation The exposing by denuda- distinct mineral phases. For example, an tion of a surface or feature that had previ- alkali feldspar rich in potassium may un- ously been buried by deposition. Theories dergo exsolution to produce a perthite in-

129 external magnetic field tergrowth consisting of separate structural is often broadly contemporaneous with the units of sodium and potassium feldspar, in- evolutionary radiation of another unspe- dicated by the occurrence of blebs of albite cialized and often insignificant group. within an orthoclase host. extrusion The process by which magma external magnetic field That part of emerges onto the Earth’s surface through the Earth’s magnetic field that results from vents and fissures to form lava flows. The effects produced above the Earth’s surface. term is also applied to the structure or form that results from this process. See also ex- extinction The disappearance of a trusive rock. Compare intrusion. SPECIES or other group of organisms representing a particular evolutionary line. It extrusive rock A rock that forms after may occur when an organism that has be- igneous materials cool and solidify on the come highly specialized for living in a par- surface of the Earth. The rapid cooling gen- ticular environment is unable to adapt to erally results in a fine-grained type of rock. extreme changes in its HABITAT. Some ex- Compare intrusive rock. tinctions affect a wide variety of habitats, as occurred at the end of the PALEOZOIC and eye The central area of a tropical cyclone at the end of the MESOZOIC Eras. The latter (hurricane or typhoon), where wind speeds extinction affected the dinosaurs on land, are light, breaks in the main cloud sheets pterodactyls in the air, and ichthyosaurs in appear, and the driving rain stops. The di- the sea; many invertebrate groups, such as ameter of the eye varies, reaching up to 80 the ammonites, also disappeared (see K/T km in the larger storms, although its dura- boundary event). The causes of these wide- tion depends upon the speed of movement spread extinctions are not clearly under- of the storm. The weather in the eye feels stood. Major climatic changes combined particularly oppressive after the strong with unsuccessful competition with other winds preceding it, but it is soon followed groups of organisms have been suggested by even stronger winds in the opposite dias likely causes: the extinction of one group rection.

130 F

face 1. One of the surfaces of a CRYSTAL. nental facies in Britain. Correlation of one 2. The chief or most obvious surface of a facies with another may be difficult, espe- landform (e.g. the face of a CLIFF). cially if the fossils they contain are restricted to each particular facies. If this is facies /fay-shee-eez/ 1. A collection of the case they are known as facies fossils. A metamorphic rocks that have formed over unit of a particular facies may be diachro- the same range of physical conditions. Dif- nous, that is, it is present in different areas ferences in mineralogy are attributed to the at different times. variations in chemical composition of the original rock types. For example, at the facing direction The direction in which amphibolite facies, a shale may be repre- younger beds of rock lie in relation to the sented by the assemblage staurolite + gar- rocks in a FOLD. If the younger rocks lie net + biotite + plagioclase + quartz, a basalt above the fold, the facing direction is up- by hornblende + plagioclase + quartz, and ward; if they lie below, it is downward. an impure limestone by diopside + calcite + quartz. A number of metamorphic facies Fahrenheit scale /fa-rĕn-hÿt/ A scale of and subfacies are recognized in both temperature in which the freezing point of regional and contact metamorphism. water is at 32° and the boiling point at Metamorphic facies and their relative tem- 212°. The scale is named for the German peratures (T) and pressures (P) are as fol- physicist (Gabriel) Daniel Fahrenheit lows: (1686–1736). Contact metamorphic facies – low P, low to high T. fair-weather cumulus A CUMULUS cloud Zeolite facies – low T, low to intermediate with limited vertical extent. The clouds are P, gradational from DIAGENESIS. prevented from rising farther by an inver- Greenschist facies – low to intermediate P sion, usually formed by subsidence in an and T. area of high pressure. Because such areas Amphibolite facies – intermediate to high P rarely give rain, clouds formed in this way and T. are known as fair-weather cumulus. Glaucophane schist facies – low T, very high P. false bedding An obsolete term for Granulite facies – high P and T. CROSS-BEDDING. Eclogite facies – high T and very high P. See also grade (def. 1); zone (def. 1). family A group in the taxonomic classi- 2. A rock unit or group of associated rock fication of organisms. A number of related units having particular features, lithologi- families together form an ORDER, and a cal, sedimentological, and faunal, that re- family itself is composed of one or more flect some specific environmental genera (see genus). For example, the Ho- conditions. Thus, rocks deposited at the minidae, to which MAN belongs, is a family same time in different regions may be dis- of the order Primates. See taxonomy. similar because they are of different facies, as is the case, for example, with the PERMO- fan cleavage CLEAVAGE in which the TRIASSIC rocks of marine facies and conti- planes are not parallel but form a fanlike

131 fathom arrangement with either an upward or the most abundant minerals in igneous downward convergence. This arrangement rocks. Feldspars have a framework struc- often results from rotation of beds in the ture in which (Si,Al)O4 tetrahedra are limbs of a developing fold. linked together with calcium, sodium, potassium, and barium ions occupying the fathom A unit of depth equal to 1.83 m large spaces in the framework. Feldspars (6 feet). may be considered to be mixtures of the four components: fault A fracture in the Earth’s crust CaAl2Si2O8 – anorthite (An) along the plane of which there has been NaAlSi3O8 – albite (Ab) displacement of rock on one side relative to KAlSi3O8 – orthoclase (Or) the other, either in a horizontal, vertical, or BaAl2Si2O8 – celsian (Ce). oblique sense. (See illustration opposite.) Barium feldspars are rare and most feldspars are members of the ternary sys- fault basin A depression separated from tem An-Ab-Or. Members of the series be- the enclosing higher ground by faults. tween NaAlSi3O8 and CaAl2Si2O8 are plagioclase feldspars and those between fault block 1. An area of the Earth’s NaAlSi3O8 and KAlSi3O8 are alkali crust that behaves as a single unit during feldspars. faulting. Plagioclase feldspars. The replacement 2. A rock body bounded by at least two NaSi↔CaAl results in a complete grada- faults, which can be either elevated or de- tion between the two end members of the pressed relative to the surrounding region. series, calcic (anorthite 100–90% An, 0–10% Ab; bytownite 90–70% An, fault breccia The mass of broken rock 10–30% Ab; labradorite 70–50% An, fragments along a fault plane produced 30–50% Ab) and sodic (andesine 50–30% during dislocation. An, 50–70% Ab; oligoclase 30–10% An, 70–90% Ab; albite 0–10% An, 100–90% fault line The line along which a FAULT Ab). intersects the surface of the ground. Plagioclases are triclinic and exhibit multiple lamellar twinning. They are milky fault-line scarp A type of cliff, formed white or colorless. Plagioclase occurs both originally by a FAULT, whose face has re- as phenocrysts and in the groundmass of treated because of erosion. The original most basic and intermediate igneous rocks. fault line lies in front of the scarp under ac- Calcic plagioclases are characteristic of cumulated rock fragments. See also fault basic rocks and anorthosites. Sodic plagio- scarp. clases are found in intermediate rocks. Al- bite occurs in some granites and also in fault plane The surface along which a spilites. Plagioclase is also found in a wide FAULT forms. variety of metamorphic rocks. Alkali feldspars. At high temperatures the fault scarp A type of cliff formed when replacement Na↔K results in a continuous a block of rocks is forced upward by a chemical series between NaAlSi3O8 and FAULT. It is located on the FAULT LINE. See KAlSi3O8. In most volcanic rocks, the also fault-line scarp. feldspar has crystallized at a high temperature and has been quenched, thus retaining faunal province See province. its high-temperature crystal structure. Slow cooling under plutonic conditions or crys- fayalite /fay-ă-lÿt, fÿ-ah-/ A mineral of tallization at relatively lower temperatures the OLIVINE group. results in the formation of two separate feldspars, one rich in NaAlSi3O8, the other feldspar /feld-spar, fel-/ The most im- in KAlSi3O8. The homogeneous high-tem- portant type of rock-forming minerals and perature structural state of an alkali

132 P min

P max P inter Fig. 1: Thrust fault

P = principal stress

P inter

P max Fig. 2: Dextral wrench P min fault

ridge

e c spreading direction o

d sense of fault movement i

Fig. 3: Transform fault m

graben

Fig. 4: Graben

horst

Fig. 5: Horst

footwall P max hanging P inter wall P min

Fig. 6: Normal fault downthrow

Fig. 7: Reverse fault

Types of fault

133 feldspathic feldspar may become unstable at a lower stone. Kalsilite is found in potassium-rich temperature and unmixing occurs by the volcanic rocks. rearrangement of the alkali ions in the solid Leucite is a white pseudocubic mineral state. The resulting intergrowth of the two of composition KAlSi2O6 and often shows components is called perthite. complex twinning. It occurs as euhedral Sanidine is a high-temperature mono- icositetrahedral crystals in potassium-rich clinic potassium feldspar found in volcanic basic lavas. Analcime (analcite) is a white rocks, such as trachytes and rhyolites. or pink cubic mineral of composition Anorthoclase is a high-temperature NaAlSi2O6.H2O with some replacement sodium-potassium feldspar. Orthoclase K↔Na. It occurs as a late-stage mineral in (also monoclinic) and microcline (triclinic) basic and intermediate igneous rocks. are intermediate- to low-temperature The minerals of the sodalite subgroup potassium feldspars found in plutonic acid- are as follows: and alkali-rich rocks. Adularia is a very sodalite 3(NaAlSiO4).NaCl low-temperature monoclinic potassium nosean 3(NaAlSiO4).Na2SO4 feldspar found in pegmatites. Alkali haüyne 3(NaAlSiO4).CaSO4 feldspars also occur in a wide variety of These minerals are cubic, commonly blue metamorphic rocks. Sanidine and ortho- in color, and are found in alkaline igneous clase exhibit simple twinning but micro- rocks such as nepheline syenites and cline and anorthoclase have multiple twins phonolites. Lazurite is the major con- in two directions at right angles, producing stituent of lapis-lazuli, formed by the meta- a tartan effect. Alkali feldspars are color- morphism of limestone. Cancrinite is a less, white, pink, or red. hexagonal white or colorless mineral of approximate composition Na8(AlSiO4)6- feldspathic /feld-spath-ik, fel-/ Describ- (HCO3)2. It is formed as a late-stage or ing a rock or mineral aggregate that con- secondary mineral as a result of the reac- tains FELDSPAR, usually in a specified tion between nepheline and carbon diox- amount. See also feldspathoid. ide. feldspathoid /feld-spă-thoid, fel-/A fell An upland in northern England, usu- member of a group of undersaturated min- ally covered by heathland or rough pasture erals in which the alkali cations sodium for the summer grazing of sheep. and potassium are located in cavities within the framework structure of felsic /fel-sik/ Denoting light-colored (Si,Al)O4 tetrahedra. Feldspathoids appear minerals such as feldspar, feldspathoids, in place of feldspar in rocks deficient in sil- and quartz. Compare mafic. ica. They do not occur with free quartz because they react with silica to form felsite /fel-sÿt/ Any crystalline acid vol- feldspar. canic rock. Nepheline has an ideal composition NaAlSiO4 but always contains some femic /fem-ik/ Denoting the iron- and kalsilite (KAlSiO4), the amount of solid so- magnesium-rich minerals of the CIPW nor- lution increasing with increasing tempera- mative classification (see norm). Compare ture. Both nepheline and kalsilite are white salic. or colorless and hexagonal. Carnegieite is a high-temperature cubic form of nepheline fen A flat marshy low-lying region, often and kaliophilite is a rare form of kalsilite. located near the coast where rivers and Nepheline is a primary mineral in many al- lakes have silted up. Such terrain is named kaline igneous rocks and is produced dur- after the Fens in E England, where the ing the metasomatic alteration of rocks marshes have been drained and turned into through the process of FENITIZATION. fertile farmland. Nepheline also occurs as a result of the reaction between basic magma and lime- fenite /fen-ÿt/ A type of igneous rock

134 fertilizer formed by METASOMATISM through contact process. The profile is similar to a ferrallitic with a CARBONATITE intrusion. It is an al- soil, being red in color and having a similar kaline SYENITE. structure, but often clay movement results in a textural B horizon. These soils have a fenitization /fen-ă-tă-zay-shŏn/ The allow cation exchange capacity but are more kaline metasomatism of country rock, fertile than ferrallitic soils. They fall into typically gneissose, surrounding ijolite- the ULTISOL order of the US SOIL TAXONOMY. carbonatite complexes. Quartzo-feldspathic rocks may be completely made ferroaugite /fe-roh-aw-jÿt/ A mono- over to syenitic alkali feldspar-nepheline- clinic PYROXENE. aegirine assemblages, termed fenites. Feni- tization precedes or accompanies the em- ferromagnesian mineral /fe-roh-mag- placement of the carbonatite magma. nee-shăn, -zhăn/ A mineral that is rich in iron and/or magnesium and usually dark in fen soil See organic soil. color, for example olivines, pyroxenes, amphiboles, and biotite. fenster See window. ferromagnetism /fe-roh-mag-nĕ-tiz-ăm/ ferrallitic soil /fe-ră-lit-ik/ (latosol; lat- A type of magnetism exhibited by sub- eritic soil; red earth) A soil that is typi- stances (usually metals) that have a mag- cally found on the old planation surfaces of netic permeability very much larger than 1. the humid tropics. The chief soil-forming Cobalt, iron, nickel, and many of their al- process is ferrallitization where weathering loys are ferromagnetic materials, used for and leaching are intense. Rapid formation making permanent magnets. They show results in a deep profile rich in clay and hy- such behavior because, in a magnetic field, drated oxides of iron, aluminum, and man- their magnetic domains become perma- ganese. Free drainage gives a red nently aligned in one direction. coloration to the soils but they may be yellowish where drainage is poorer. Any plant ferruginous /fĕ-roo-jă-nŭs/ Describing a remains are rapidly broken down and the substance that contains iron, such as a sed- released nutrients are immediately used imentary rock (e.g. types of sandstone) again by the plants so that little organic matter accumulates. This factor plus the whose constituent grains are cemented to- kaolinitic clay results in a low cation ex- gether with iron oxide (see cementation). change capacity, which makes these soils of little agricultural value. They fall into fersiallitic soil /fer-shă-lit-ik/ (ferrisiallitic the OXISOL order of the US SOIL TAXONOMY. soil) A tropical soil formed in a less humid climate than FERRALLITIC SOILS. The ferrimagnetism /fe-ri-mag-nĕ-tiz-ăm/A profile differs in that weathering and leach- type of weak FERROMAGNETISM in which ing are not so dominant, producing a soil the magnetic domains of a substance are that is not so deep. Where the parent ma- aligned in opposite directions, although terial is particularly resistant only an A C there are slightly more of one alignment profile occurs, although sometimes an A Bt than the other. The mineral MAGNETITE is a C profile develops. The whole profile is ferrimagnetic material. richer in weatherable minerals and has a higher cation exchange capacity than fer- ferrisol /fe-ră-sôl/ A soil that has devel- rallitic soils. oped similarly to FERRALLITIC and FERSIAL- LITIC SOILS, its profile being less intensely fertilizer Any organic substance or inor- weathered and leached than ferrallitic soils ganic salt applied to the soil to improve but more so than fersiallitic soils. Unlike crop production. The nutrient elements ferrallitic soils these are found on sloping most important for plant growth are nitro- sites where surface erosion is an important gen, phosphorus, and potassium.

135 fetch fetch The distance of the stretch of open filter pressing See magmatic differentia- water over which the frictional effect of a tion. wind that is blowing in a constant direction is actually generating waves. In small sea finger lake A deep long narrow lake, areas, fetch distance is usually limited by usually occupying a valley carved by the dimensions of that sea area; in the open glacial erosion. The Finger Lakes in New ocean, however, the limit is usually im- York State are well-known examples. posed by the constantly changing meteorological situation. There is also a relation fiord /fyord/ (fjord) A long narrow sea between fetch distance, wind strength, inlet lying between steep mountain slopes, wind duration, and the maximum size of which often reach up to several hundred waves produced. For a given wind speed, meters above sea level. Fiords are glaciated the largest possible waves require a mini- valleys which, owing to a relative rise of mum fetch distance; and for a given fetch sea level after the melting of the Pleistocene distance, there is an optimum size wave ice sheets, have become flooded by the sea. that can be generated, however hard or Many fiords have very considerable long the wind blows. Very large ocean depths, some well in excess of 1000 m. Be- waves may require fetch distances of the cause of glacial action, they are character- order of 1000 km or more. ized by the existence of gouged basins separated from the open sea by sills of solid fiamme /fyam-ay/ Flattened disk-shaped rock, often capped by moraine, marking glassy bodies found in ignimbrites and im- the point where the glacier that cut the val- parting a eutaxitic structure. See pyroclas- ley lost much of its erosive power, either tic rock. through melting or slowing down. The cross section of a fiord is often U-shaped. fibrolite /fÿ-brŏ-lÿt/ A fibrous or acicular form of SILLIMANITE. See aluminum sili- fireclay A type of clay that can be baked cates. to high temperatures without crumbling or fusing. Fireclays are composed mainly of field capacity (field moisture capacity, hydrous aluminum silicate, as in illite, soil moisture storage capacity) The water kaolin, or kaolinite (see clay minerals), and that remains in the soil after it has been sat- often occur beneath beds of coal. urated and then left to drain by gravity. It is usually expressed as a percentage of the firn /fern/ (névé) A half-way stage in the oven-dried soil. transformation of fresh snow into glacier ice. Fresh snow is very loosely packed and field completion A check of a map has a density between 0.1 and 0.2. Com- under compilation carried out in the field paction and recrystallization reduce the to ensure that all the details are shown and pore space between grains, increasing the to make any corrections or amendments density. Firn exists when the density that may be necessary. Details usually af- reaches 0.5. The word means ‘of last year’, fected by the field check are names and and in this case refers to snow that has sur- civil boundaries. vived one summer. Further compaction continues until glacier ice of density 0.89 to field intensity The strength of a mag- 0.90 is formed, although this may take netic field at a particular point on the many years. Earth’s surface. firth (in Scotland) A lengthy estuary or filtering (in climatology) A method of arm of the sea. Firths bear some similarity separating components of a selected time to FIORDS and some have the closed and period from all other possible periodic deep depressions and rock sills typical of components. It is most frequently used for fiords. They developed from river or investigations into changes of climate. glacial valleys that later experienced the

136 flood postglacial transgression, becoming partly Generally there is some degree of horizon- flooded by the sea. Firths on Scotland’s E tal displacement. coast are not classed as the fiord type. The lochs and firths of the W coast of Scotland flap structure A structure resulting are fiords but have rather flatter hillside from gravity collapse, consisting of a bed slopes than, for example, the fiords of Nor- originally on the limb of an anticline that way and Alaska. has bent as it slipped downward and come to rest in an inverted position. fish One of a variety of aquatic vertebrates belonging to any of the following flash flood A flood of sudden occur- classes: AGNATHA, PLACODERMI, CHON- rence and short duration that results from DRICHTHYES, and OSTEICHTHYES. rapid runoff after heavy rainfall. Flash floods are commonest in desert and semi- fissure Any extensive break, cleft, or arid areas, where there is little or no vege- fracture in the Earth’s surface, generally tation to prevent fast runoff, although they caused by earth movements (earthquakes also occur in wetter regions, especially and faulting) or volcanic action. See also where streams occupy small drainage crevasse. basins with steep slopes. The water may flow along gullies and cause a flood many fissure eruption A volcanic eruption kilometers away from the site of the rainfall. Flash floods often occur too rapidly to through a FISSURE rather than a central be forecast and may cause considerable vent. The largest occur along MID-OCEAN damage and loss of life. RIDGES, where molten magma rises through fissures in the oceanic crust. flexure fold /flek-sher/ A type of fold best developed in rocks where there is good fjord /fyord/ See fiord. layering. As the fold develops the beds slip over one another. flaking The splitting of rocks around their margins into curved flakes, which flint A nodular variety of CHALCEDONY vary in thickness from a few millimeters to formed in chalk. See also silica minerals. a few centimeters, depending on the process involved. The most common flocculation /flok-yŭ-lay-shŏn/ (in soil process is the chemical decay of minerals, science) The process by which colloidal especially HYDRATION, causing expansion. (i.e. clay-size) particles join together form- The growth of salt crystals from solution ing groups, called floccules. Colloidal ma- beneath the surface of porous rocks can terial tends to adopt this habit in the give rise to flaking, the flakes in this case presence of neutral salts, notably salts of being frequently backed by a thin layer of calcium; in the presence of alkaline salts, salt. In both these cases there tends to be a especially sodium, the particles adopt the series of concentric layers of flakes around opposite habit and separate as independent one boulder. Flaking results in a rounding units. Calcium therefore improves the of the block, because there is a preferential structure of the soil by helping aggrega- attack on edges and corners. The process tion, and this in turn improves its agricul- can take place only above ground level, tural value; flocculation of otherwise and the final development is a flat rock sur- intractable clays is often encouraged by the face. Flaking on concave surfaces produces addition of lime to make them more pro- hollows. ductive. flame structure A sedimentary struc- flood The state of a river when the vol- ture in which an underlying bed has been ume of water flowing in it exceeds BANK- squeezed up into an overlying horizon in FULL, and water commences to spread flamelike tongues as a result of loading. away from the channel over the FLOOD-

137 flood basalt

PLAIN. Floods are produced by discharge floodplain material, but actively aggrading increases due to exceptional rainfall and rivers probably owe more to overbank de- runoff. In hydrology, much effort has been posits. devoted to the study of floods because of The existence of a floodplain does not their great geomorphic significance and mean a river is actively aggrading: mean- their cost to life and property on flood- ders migrate downstream, depositing ma- plains. Flood frequency curves have been terial on the inside of bends and eroding produced for many rivers, showing the material on the outside. Once deposited, a likely recurrence interval of floods of vary- particle will remain static in the floodplain ing magnitudes, based on the analysis of for a considerable period (about 1000 past DISCHARGE records from gauging sta- years in one stream studied) until the next tions. Geomorphologically, floods can ac- meander upstream had eroded its way complish great landscape modifications in down to the position of that grain and re- short periods, because the increased veloc- entrains it. Floodplains in streams in equi- ity of flood rivers produces an increase in librium therefore represent a temporary erosional capacity proportional to the storage for sediment on its passage through square of that velocity: thus if velocity dou- the valley. See also lateral erosion; river. bles, erosional capacity increases four times. See also geomorphic process. flood tide The incoming of the tidal stream; the rising tide, i.e. that part of a flood basalt Very fluid basaltic LAVA tide cycle following the low-water stage (usually from a FISSURE ERUPTION) that and preceding the high-water stage. Com- flows over large areas; there may be a series pare ebb tide. of such flows one after the other. The largest known area of flood basalt covers Florida Current A major surface cur- 250 000 sq km of the Deccan Traps rent on the W periphery of the Atlantic plateau on the Indian subcontinent; Ocean. It forms one of two branches of the smaller examples occur in southern Africa North Equatorial Current (see equatorial and the northwestern USA (Columbia current). At about longitude 60° W, the River Plateau). North Equatorial Current splits up into two important currents: one flows into the floodplain A relatively level area bor- Caribbean Sea but then re-enters the At- dering a river, subject to periodic flooding lantic Ocean via the Gulf of Mexico, and and made up of sediments deposited by the thereafter flows as the Florida Current. river, which bury the rock-cut valley to a The Florida Current extends from the variable depth. In the larger rivers it may Straits of Florida to roughly the latitude of contain the following features: the river Cape Hatteras. As such, it constitutes the S channel itself, OXBOW LAKES, POINT BARS, portion of the GULF STREAM system. The scars of former MEANDERS resulting from discharge rate of the Florida Current is their migration downvalley, areas of stag- probably of the order of some 26 million nant sedimenting water (sloughs), LEVÉES, cubic meters per second, but this figure backswamp deposits of fine sediments in varies significantly from one period to an- slack water away from the river, and sandy other, and almost certainly seasonally. It is flood debris. In smaller streams all these a fast-flowing current that is particularly features will not be present, but the Missis- confined to the surface waters (reaching sippi and Mekong, for example, display down to depths of between 600 and 800 them all. m), probably attaining speeds in excess of Floodplains originate from deposits 150 cm per second close to the coasts of within the river channel on the point bars Florida. An important factor in the genera- and from overbank deposits in time of tion of this current is the significant differ- flooding. It is now thought that deposits ence in water level that exists between the within the river channel are the major Gulf of Mexico coast and the Atlantic source, constituting possibly 60–80% of coast of Florida.

138 fog flowage /floh-ij/ Rock deformation re- tant landform involving the presence of sulting from the stressing of the rock be- water derived from a glacier or ice sheet. yond its limit of elasticity, but without Glacial meltwater can transport and erode fracture. material just as a normal river, but if it lies within or beneath the associated ice, it may flow cleavage A type of cleavage result- be capable of extremely efficient erosion ing from solid-state flowage in a rock, ac- due to increased pressures or velocities. companied by the regrowth of minerals. Fluvioglacial deposits can generally be dis- Traces of bedding are almost destroyed. tinguished from those of purely glacial ori- See also slaty cleavage. gin because they exhibit rounding, sorting, and stratification. fluid inclusion See inclusion. flux (flux density) A rate of flow of some flume /floom/ 1. A deep narrow gorge or quantity, usually some form of energy, ravine containing a fast-flowing stream or through a unit area. series of cascades. 2. A channel constructed in a stream to flysch /flish/ Thinly bedded marine control streamflow; some specialized sandstones, marls, shales, clays, conglom- flumes are used to measure streamflow. erates, and graywackes, which fill a trough adjacent to a rapidly rising mountain chain fluorescence /floo-ŏ-ress-ĕns/ The emis- and represent rapid erosion and deposition sion of light as a result of previously ab- prior to the main period of orogeny. The sorbing radiation. Many minerals fluoresce term was originally used by Alpine geolo- on exposure to ultraviolet light, and this gists and applies strictly to the sediments property may be used as an aid to identifi- associated with the Alpine orogeny. Com- cation. pare molasse. fluorite /floo-ŏ-rÿt/ (fluorspar) A cubic focal depth The shortest distance be- mineral form of calcium fluoride, CaF2. tween the focus of an earthquake and the The purple color of the familiar variety Earth’s surface. blue john is due to the presence of colloidal calcium; other varieties are white, green, focus The center of an EARTHQUAKE, and yellow. Fluorite is found in hydrother- where strain energy is converted into elas- mal vein deposits and as a late-stage min- tic waves. It is the source of SEISMIC WAVES eral in some alkaline and acid igneous produced during an earthquake. For con- rocks. venience it is thought of as being a point source, although strictly speaking earth- fluorspar /floo-er-spar/ See fluorite. quake waves originate from a nonfinite source, such as a fault plane. flute A sedimentary structure in the form of a hollow on the surface of a poorly foehn See föhn. consolidated bed of sediment, resulting from the scouring action of passing water. fog A cloud near the ground surface When this is infilled by an overlying bed of composed of minute water droplets in sus- silt or sand the form of this hollow is pre- pension. It is sufficiently dense for the served, and is exposed as a flute cast on the drops to produce a reduction in visibility to undersurface of the overlying bed. less than 1 km. Condensation of water to form the droplets in a fog can occur in fluvial /floo-vee-ăl/ (fluviatile) Of or re- many ways, and fogs are often classified by lating to a river. their method of formation – RADIATION FOG, ADVECTION FOG, etc. The most fre- fluvioglacial /floo-vee-ŏ-glay-shă/ (glacio- quent method is by some form of cooling, fluvial) Describing any process or resul- such as radiational cooling at night; in ad-

139 föhn vection fog the lower layers of the atmos- foliation A banded or laminated struc- phere are cooled through turbulent contact ture within a metamorphic rock, resulting with the ground. More rarely, condensa- from the metamorphic segregation of min- tion can be achieved through evaporation erals into compositionally different layers, from the surface. parallel to the schistosity. The term is also In industrial areas, the greater supply of commonly used to describe any parallel condensation nuclei and hygroscopic parti- planar element in a metamorphic rock, cles enable fogs to form even when relative such as schistosity and cleavage. humidity is less than 100%. Where smoke is abundant, this can produce a thick and food chain A series of linked feeding re- particularly unpleasant type of fog, known lationships between organisms. Each or- as SMOG (an abbreviation of smoke fog). ganism depends for its food on the one See also mist. next lowest in the hierarchy (called a TROPHIC LEVEL. The lowest level is occupied föhn (foehn) A warm dry wind that de- by PRIMARY PRODUCERS (autotrophs), such scends the leeward side of mountain ranges as plants, which provide food for the her- to give extremely rapid thaws of lying bivores (PRIMARY CONSUMERS) in the level snow in winter. Such winds occur when- above them. These, in turn, become food ever rapid descent favors adiabatic warm- for carnivores (secondary consumers). ing, whether or not there has been cooling Consumers also include decomposers (DE- TRIVORES), which break down the waste at the SATURATED ADIABATIC LAPSE RATE on products and dead bodies of all the other the upwind side of the mountains. How- organisms. A better picture of complex ever, they are of climatological significance feeding interactions between many organ- only where the descent is sufficient to pro- isms is provided by a FOOD WEB. duce a marked warming, as in the Alps, Rockies, or Andes, where airflow is at right food web A sequence of complex inter- angles to the mountain barrier and the linked FOOD CHAINS that demonstrates the maximum rate of descent achieved. feeding relationships between all the organisms in a natural COMMUNITY. fold A buckling of bedded sedimentary rocks due to deformation processes or the fool’s gold See pyrite. effect of gravity. See also orogenesis. foot wall The surface of rock beneath a fold axis An imaginary line parallel to fault plane or ore body. Compare hanging the HINGE LINE of a fold, from which the wall. rock strata dip away in different directions (downward from a crest; upward from a Foraminifera /fŏ-ran-ă-nif-ĕ-ră/ An order trough). See diagram. of planktonic organisms (see protozoa). Most species display jellylike bodies sur- fold belt See mobile belt. rounded by a shell or casing of calcium carbonate. The casing is often flask- or folding The action of forming a FOLD in sphere-shaped. Foraminiferans are most rock. See orogenesis. abundant in the surface layers of the sea, and tend to reach their maximum develop- fold limb Either of the two flanks on ei- ment near the Equator. The tests or casings ther side of the axis of a fold. The angle be- of dead foraminiferans make up a large tween the limbs is called the interlimb proportion of the organic ooze deposits, angle. for example, GLOBIGERINA OOZE, which contains some 30% foraminiferal tests. fold mountain A mountain that is formed by large-scale folding. See orogene- forecast (weather forecast) (in meteo- sis. rology) A statement about the probable

140 axial plane

plunge

Fig. 1: Symmetrical syncline Fig. 6: Asymmetric fold

Fig. 2: Symmetrical anticline Fig. 7: Recumbent fold

synclinal axis antiform open tight isoclinal Fig. 3 axial plane

upright inclined overturned Fig. 4 Fig. 8

culmination

depression

nd tre synform Fig. 5: Monocline Fig. 9

Types of fold weather events for specific areas for certain weather is expected to be warmer or time periods. Forecasts are normally given cooler, wetter or drier. at three levels of detail: (i) short-range, The methods required for working out dealing in depth with the probable events these forecasts differ for the time period infor the next 24–48 hours; (ii) medium volved. For the short-range forecast, satel- range, covering about one week, in which lite images show the major cloud belts, the expected movements of the main high radar detects the areas of precipitation, and low pressure systems are covered; (iii) and over land areas there are large num- the long-range forecast for a month or even bers of stations continuously recording the a season, in which the only details refer to weather elements. Together they provide a climatological averages and whether the fair assessment of likely events supple-

141 foredeep mented by computer calculations of pres- which the foredune is subject, and a very sure fields for 24–48 hours ahead (see open nature with low cover values and numerical weather prediction). Interpreta- large areas of bare sand. Since the foredune tions can then be made on the basis of these is produced by the coalescence of individ- forecast maps. ual grass and sand mounds, at a very early Medium-range forecasting for the USA stage it will appear broken with an uneven and Europe is largely based on experience crest line. of previous weather sequences together with a knowledge of the upper atmos- foreland /for-lănd, fohr-/ A stable area phere, particularly the Rossby waves (see of older cratonic rocks marginal to an oro- long wave) of the mid-latitude westerlies. genic belt, generally part of the continental As these to some degree control the direc- crust. When two continental plates collide tion of movement of low-pressure systems, the intervening wedge of sediments is information about the state of the upper at- squeezed between two forelands, onto mosphere aids forecasting for this which these sediments are thrust and over- timescale. However, as even quite small folded. initial disturbances can dramatically and rapidly change the weather, success is only foreset bed See delta deposit. moderate. For long-range forecasts, different tech- foreshock /for-shok, fohr-/ One of a se- niques are required. In Britain, the ANALOG ries of seismic waves recorded before the method is often used. In this method for- principal shock of an earthquake, resulting mer weather charts are compared with pre- from small slips or fractures in brittle rocks sent charts and it is assumed that as they reach their YIELD POINT, as a result similarities in the charts can be used to pre- of a buildup in stress. dict weather events. Sea-surface temperatures and their anomalies are also included foreshore The part of a beach that be- as these are important in affecting depres- comes covered and uncovered by water sion formation and energy transfer to the during the process of tidal rise and fall. It atmosphere. In the USA, long-range fore- extends from low-water spring tide level to casts are based on computer simulations of normal high-water springs level, and its the atmosphere. Details of the present state width will naturally depend upon the tidal of the atmosphere at many different levels range and gradient of the beach. are fed into powerful computer systems and then run for a time period equivalent foresight See leveling. to one month in real time to forecast pressure movements. Neither method is en- formation The fundamental unit in the tirely satisfactory and there is considerable lithostratigraphical classification of bodies debate as to whether it is theoretically pos- of rock (see lithostratigraphy; stratigra- sible to forecast movement in the atmos- phy). A formation is formally defined by a phere accurately for such long time TYPE SECTION at a type locality, on the basis periods. of readily observable lithological features that distinguish it from bodies of adjacent foredeep See trench. rock. It should preferably be marked off from these by a distinct lithological foredune /for-dewn, fohr-/ A coastal change, such as might be found at an un- dune growing in the newest line of dunes conformity, but it may have more arbitrary nearest to the sea. Some authorities put a boundaries in a gradually changing rock maximum size of about 3 m on foredunes. sequence. Thickness is not a criterion in its The usual distinguishing characteristics are definition; a single formation will often dif- dominance by the grass Agropyron fer in thickness at different points. Strati- junceum, which is capable of tolerating the graphically adjacent and related occasional drenching with salt water to formations may be associated to constitute

142 free air anomaly a GROUP. A formation may contain MEM- named for the French mathematician BERS but need not be entirely composed of Baron (Jean Baptiste) Joseph Fourier them. (1768–1830). form line A line drawn on a map to give foyaite /foy-ă-ÿt/ A type of alkali SYEN- an impression of the terrain where there is ITE. insufficient information to construct accurate contours. Form lines are usually repre- fractocumulus /frak-toh-kyoo-myŭ-lŭs/ sented by dashed lines rather than Broken or fragmented cloud rising only unbroken ones. In areas mapped by aerial slightly above the CONDENSATION LEVEL. photography there may be parts obscured Such clouds are most frequently seen on by cloud and these are often covered by early mornings when there has been suffi- form lines in the absence of alternative cient convection for the clouds to approach heighting data. the condensation level. forsterite /for-ster-ÿt/ A mineral of the fractostratus /frak-toh-stray-tŭs/ (scud) OLIVINE group. Cloud that is broken and ragged, similar in appearance to fractocumulus but found be- fosse /foss/ A long narrow depression neath altostratus or nimbostratus clouds, between the side of a glacier and the valley frequently preceding a warm front. It is the that contains it. The depression forms as result of condensation taking place in the some ice melts because of heat absorbed by moist layer beneath the main cloud base, or reflected from the walls of the valley. either by turbulent mixing or through the addition of moisture by precipitation fossil The remains of any organism that falling from higher levels. lived in the past. The term was originally applied loosely to anything ancient and fracture cleavage A series of closely dug up and as an adjective may still be used spaced fractures and parallel joints pro- in this sense (as in FOSSIL FUEL). A fossil can duced in deformed rocks that have been be the whole or any part of an animal or subjected to only minor metamorphism. plant, usually chemically altered (i.e. fossilized); it may be an impression of the fracture zone A region that lies astride a shape of an organism that has been pre- line along which faulting has occurred. It is served in some way (see cast; mold) or it often the site of earthquakes and volcanic can be simply the remains of the effects of activity, and may run across a continent or an organism in the past, such as fossil along the seabed. tracks, excrement, or burrows (see trace fossil). See also derived fossil; zone fossil. franklinite /frank-lin-ÿt/ A rare black crystalline mineral of the SPINEL group con- fossil fuel Any naturally occurring hy- sisting of oxides of iron, manganese, and 2+ 2+ 3+ 3+ drocarbon fuel such as coal, natural gas, oil zinc, (Fe ,Zn,Mn )(Fe ,Mn )2O4. It (petroleum), and peat. The fuels formed crystallizes in the cubic system and is used slowly underground by the action of pres- as a source of zinc. sure on the remains of dead plants and marine animals. Fossil fuels represent a frazil ice /fră-zil/ Small crystals of ice nonrenewable resource. that form at the edges of fast-flowing streams or in moving seawater. Fourier analysis /foo-ree-ay, -er/ (harmonic analysis) A mathematical method free air anomaly A gravity anomaly of analyzing a data series that contains pe- that takes account of altitude but does not riodic variations, such as seasonal precipi- take account of attraction effects resulting tation. These can be represented by a series from topogaphy and isostatic compensa- of sine or cosine functions. The method is tion.

143 free face free face A vertical or near vertical facet station level, although if there is an inver- of slope cut in bare rock. It is rare to find sion aloft the situation may be complex. faces actually at 90°, and various authorities have adopted different critical values freezing nucleus A tiny particle within for the minimum allowable slope; slopes of the atmosphere that, in the presence of su- above 40–45° are usually termed free faces. percooled water droplets, initiates the They occur in consolidated materials, growth of ice crystals around the nucleus. which can maintain their verticality when Water droplets in clouds do not automati- the removal of debris from the footslope cally freeze when the temperatures fall exceeds the rate of debris production from below freezing point. Being very small, the the face above. Wave erosion at the foot of droplets require some form of NUCLEUS on a sea cliff, or lateral planation by a stream which the crystalline growth can develop at the side of its valley, are situations pro- but such nuclei are not as effective or nu- ducing free faces. Owing to this removal of merous as cloud condensation nuclei (see debris from its foot, the slope becomes con- condensation) and do not produce total stantly oversteepened, and slips and slides freezing at 0°C. Instead a few ice crystals lead to stripping of material from the will appear at between –5°C and –10°C whole face equally, thereby maintaining and become more frequent with lower tem- the face by PARALLEL RETREAT. If the agency perature. The nuclei are composed of vol- removing the debris at the foot ceases to do canic dust or clay particles with a similar so, for example the accumulation of a crystalline structure to that of ice, and in beach at a cliff foot cutting off direct wave some cases fractures or splinters of ice crys- action, retreat at the foot of the free face tals may themselves act as nuclei. At tem- slows down compared with its upper part, peratures below –40°C, spontaneous and the face will tend to be gradually oblit- freezing takes place even in the absence of erated by decline to a gentle angle. See also nuclei. basal sapping. freezing point The temperature at freeze-thaw Describing weathering pro- which the liquid and solid phases of a sub- cesses involving the freezing and thawing stance are in equilibrium at standard pres- of water within preexisting rock fissures. ° When water expands on freezing, it tends sure (1013.25 mb); for water this is 0 C. to enlarge cracks; on thawing, the enlarged Because nuclei (see freezing nucleus) are cracks can contain more water than before, required for cloud droplets to freeze, water which will cause increased enlargement on can exist at temperatures far below its true renewed freezing. Eventually, particles be- freezing point. The term is therefore pri- come broken away from the main mass of marily used for ground-surface tempera- rock. Freeze-thaw processes weaken rocks ture observations and forecasts, partic- before they are subjected to other agents of ularly in connection with road surface con- weathering and erosion. A notable exam- ditions. ple is GLACIAL PLUCKING. freezing rain Rain or drizzle that falls freezing level The height above sea level onto a ground surface that is below 0°C, of the layer of the atmosphere that is at a the water freezing to give a coating of ice. temperature of 0°C. More frequently, its It can occur when a layer of rising warm height is quoted in terms of pressure level, air, such as at a warm front, is moving e.g. 850 mb. As water does not automati- slowly over a cold surface where the tem- cally freeze at 0°C (see freezing nucleus), it perature remains below freezing point. Be- would be more accurately called the melt- cause the warm air eventually reaches the ing level, representing the point at which ground surface this state is normally of solid precipitation would melt. If the short duration. ground temperature is below freezing, the freezing level would be stated as being at fresh water Water that contains less

144 frost than 2% of dissolved minerals. It is not degrees of vertical motion. In weather fore- necessarily safe to drink. casting, a front is often called a trough of low pressure. See also depression; cold friable Describing a mineral or rock that front; occluded front; polar front; warm crumbles easily (using the pressure of the front. fingers). frontal precipitation (frontal rainfall; cy- friction The mechanical force of resis- clonic precipitation) Precipitation that tance operating between two substances in originates from the movement of fronts contact. In the atmosphere, it is the resis- where there is large-scale rapid uplift and tance between the ground surface and the cooling of warm moist air along the junc- layer of air above it, although the thickness tion of cold and warm fronts in a DEPRES- of this layer varies appreciably. The term SION (low-pressure system). can also be used when referring to wind shear. frontal wave An undulation on a front The effects of friction on airflow are nu- between two air masses that travels along merous. Wind speeds are reduced near the as a wave-like perturbation. The frontal ground and there is a movement of wind wave initially has a warm front and cold across the isobars from higher to lower front but over time it starts to occlude. pressure, compared with the GEOSTROPHIC WIND when friction is excluded. The rate of frontal zone The zone of transition, in increase of speed with height away from which mixing has occurred, between two the surface is affected by the roughness of adjacent air masses. The zone may range the ground, extremes being represented by from tens to several hundred kilometers in oceans and urban areas. width. frigid zone A cold area of the Earth’s frontogenesis /frun-tŏ-jen-ĕ-sis/ The surface, either in polar regions or at high process of intensification of the thermal altitude. gradient along a frontal zone: part of the formation of a DEPRESSION. It can take fringing reef A REEF that grows directly place whenever the isotherms are suitably on bedrock at the seashore, especially in orientated during conditions of CONFLU- tropical regions. It forms a shelf whose ENCE and CONVERGENCE in the lower at- rough surface appears above the water at mosphere. low tide. See also barrier reef. frontolysis /frun-tol-ŏ-siss/ The weak- front A transition zone between air of ening of the thermal gradient at a frontal different thermal characteristics and ori- zone. This is produced under the reverse gins. On surface pressure charts, fronts are conditions of frontogenesis, i.e. with sur- depicted in two-dimensional form along face DIVERGENCE of air and often subsi- the line of maximum thermal gradient, dence. When frontolysis is taking place, where this can be determined. In cross sec- precipitation along the front stops and the tion they represent a surface of separation cloud sheet breaks up and eventually dis- between the two AIR MASSes, sloping gently appears. away from the surface warm air. Distinc- tive weather phenomena are associated frost The condition that occurs when the with fronts and hence they are very impor- temperature falls below 0°C. Subdivisions tant in short-range weather forecasting (see are made into ground frost and air frost forecast). Problems of precise definition of based on thermometer readings at grass a front occur because it represents a ther- level and screen level (1.2 m) respectively. mal gradient, the strength of which is not It is normally recognized by the icy deposit necessarily related to weather activity, so that usually forms under such temperature the same gradient may have quite different conditions, but if the air is very dry, frost

145 frost action may occur without forming icy deposits. fulls, the separating parallel depressions For weather-forecasting purposes, grades are known as swales. of severity of frost are distinguished with –0.1 to –3.5°C slight, –3.6 to –6.4°C mod- fuller’s earth A clay rich in montmoril- erate, –6.5°C to –11.5°C severe, and below lonite. See clay minerals. –11.5°C very severe. Allowances are made for wind speed and wind-chill index, which fumarole /fyoo-mă-rohl/ A vent in the influence the sensation of cold. ground in a volcanic region which emits steam and other hot gases. Temperatures ° frost action A type of MECHANICAL may be as high as 1000 C within fu- WEATHERING that results in the breakup of maroles, which occur mainly on lava flows rock through repeated freezing and thaw- and in the calderas and craters of active ing of water in its crevices. volcanoes. If sulfurous gases are also present, it is called a solfatara. frost heaving (frost heave) The swelling of the ground surface through a combina- fumigation (in meteorology) The result tion of CONGELITURBATION and the effects of the influx of heavily polluted air down to the ground surface. It most frequently of NEEDLE ICE. occurs after night-time cooling has given a frost hollow An area that has a greater temperature inversion a few hundred me- liability to frost incidence than its sur- ters above the ground surface, beneath which industrial pollution has accumu- roundings. Because cold air has a slightly lated. When convection starts after solar greater density than warm air, it tends to heating next morning, rising air motion collect in hollows or low ground and so will resume and compensating downward areas of high frost incidence are usually in flow will take the polluted air to the hollows where free drainage of air is pre- ground, giving sudden high values of pol- vented. The most famous frost hollow is lution. the Gruenloch (or Gstettneralm) sinkhole in the Austrian Alps where temperatures as ° Fungi A kingdom of heterotrophic or- low as –52 C have been recorded. ganisms, important in soil. They are less abundant in numbers than bacteria (up to frost-shattering The breaking apart of 1 000 000 per gram of soil) but greater in masses of rock by the continued enlarge- bulk, because they have a filamentous na- ment of cracks within them through ture. They require moisture and may be FREEZE-THAW action, in areas where the found in alkaline, neutral, or acid soils. temperature fluctuates for considerable pe- Thus in the latter type they number more riods around freezing point. The resultant than bacteria. Although, unlike bacteria, outcrops and debris are all highly angular they cannot oxidize ammonium com- in appearance, and this is the major char- pounds or fix atmospheric nitrogen, they acteristic of frost-shattered forms. See also are more important than bacteria in the congelifraction. formation of humus and in the promotion of soil stability. Fungi are capable of de- fugitive See volatile. composing even the most resistant plant materials such as cellulose and lignin. The fulgurite /ful-gyŭ-rÿt/ A tubular or den- mycelia of certain mushroom fungi may dritic mass of fused SILICA originating when form associations with plants called my- lightning strikes sand. corrhizas. This association is of mutual benefit to plant and fungus and where pre- full A shingle BEACH RIDGE or a BERM sent there is a marked increase in the up- composed of sand. When ridges are called take of nutrients.

146 G

gabbro /gab-roh/ A coarse-grained basic Gaia hypothesis /gÿ-ă/ An idea champi- igneous rock, the product of plutonic crys- oned in the 1970s by the British scientist tallization of basalt magma. The essential James Lovelock (1919– ) which suggests constituents are plagioclase that is more that all the living and nonliving systems on calcic than anorthite (usually labradorite) Earth form a unity that is regulated, and and clinopyroxene (usually augite or ti- kept suitable for life, by the organisms tanaugite). Many gabbros contain olivine themselves. The whole planet can therefore in addition. Accessory minerals include be regarded as a huge single organism. The magnetite, ilmenite, apatite, biotite, and hypothesis stresses the interdependence of hornblende. living things and the environment. Rocks in which orthopyroxene, usually hypersthene, predominates over clinopy- gal A unit of acceleration equal to one roxene are called norites. With an increase cm per second per second (named after in the proportion of plagioclase, gabbros Galileo). In practice this unit is too large and norites pass via leucocratic varieties and measurements of the strength of the into anorthosites; with an increase in Earth’s gravitational field at a point are olivine and pyroxene they pass via made in milligals (one thousandth of a gal); melagabbros into ultramafic rocks. With 1 milligal is equivalent to 9.8 × 10–3 N kg–1. an increase in the proportion of olivine at See gravity unit. the expense of pyroxene, gabbro passes into troctolite and allivalite. With a change gale 1. In general, any unusually strong to plagioclase of more sodic composition, wind. gabbros pass into diorites. Eucrite is a vari- 2. A wind speed averaged over a 10-minute ety of gabbro that contains olivine, two period of 34 to 47 knots. On the BEAUFORT pyroxenes, and plagioclase of bytownite– SCALE Force 8 is a gale; Force 10 a whole anorthite composition. Gabbroic rocks gale (or storm). Gales most frequently containing pyroxenes and olivines rich in occur during the passage of an intense de- iron are termed ferrogabbros. Alkali gab- pression. bros (syenogabbros) are marked by the introduction of feldspathoids or alkali galena /gă-lee-nă/ The chief lead ore feldspar. (lead sulfide, PbS), found as gray metallic Most gabbros have equigranular tex- cubes of high density and often associated tures; others are ophitic or, rarely, orbicu- with sphalerite, barytes, fluorspar, and cal- lar. Reaction rims are common features cite in hydrothermal veins. It is sometimes and rarely multiple rims involving olivine- mined for its silver content. pyroxene-hornblende-biotite may be preserved. The medium-grained equivalent of Galileo A global navigation satellite sys- gabbro is termed microgabbro or DIABASE. tem under development by the European Gabbroic rocks occur in stratiform layered Space Agency and European Union. The bodies, ophiolite complexes, and as nod- first experimental satellite (Giove-A) was ules in basalts. launched in 2005. When fully deployed, the system will consist of 30 satellites (27 gaging station See gauging station. operational) with two control centers lo-

147 Gall’s projection cated on the ground surface in Europe and into two series, pyralspite (pyrope, alman- under civilian control. It will be compatible dine, spessartite) and ugrandite (uvarovite, with the GLOBAL POSITIONING SYSTEM of the grossular, andradite). Little chemical vari- USA and the GLONASS system of Russia. ation between pyralspite and ugrandite garnets occurs. Gall’s projection /gawlz/ A cylindrical Garnets commonly develop dodecahe- MAP PROJECTION with the cylinder cutting dral and icositetrahedral forms and are a the Earth’s surface at 45° north and south variety of colors. Pyralspite garnets are (see cylindrical projection). The meridians typically pink, red, or brown. Melanite is a have true scale only at the points of inter- dark brown to black variety of andradite, section, the scale increasing poleward and rich in titanium and occurring in ijolites decreasing toward the Equator. It is neither and nepheline syenites. In general, garnets a homolographic nor orthomorphic pro- are characteristic of metamorphic rocks. jection, and is not often used. The projec- Almandine is found in medium- to high- tion was developed in 1855 by the Scottish grade regionally metamorphosed argilla- cartographer Reverend James Gall ceous sediments. Pyrope is found in (1808–94). ultrabasic rocks and eclogites, and grossular occurs in metamorphosed limestones. gamma-ray log A logging technique Many types of garnet are used as semi- run in wells that records the natural ra- precious gemstones. dioactivity of the formations through which it passes. High values are given by garnierite /gar-nee-er-ÿt/ A bright green shales, whereas sandstones give low read- amorphous form of hydrated nickel mag- ings. It can be run in cased boreholes, be- nesium silicate. It occurs as a mineral in cause it does not rely upon the electrical SERPENTINITE and is used as a source of properties of the formation. nickel. gangue /gang/ Minerals found in an ore gas cap An underground pocket of nat- deposit that have no commercial value and ural gas that occurs above a reservoir of must be removed during the refining oil. It may be tapped for use as a fuel. process. gas laws The basic physical equations gap An opening in a ridge, usually that relate the pressure (p), volume (V), formed by the action of a glacier or river. temperature (T), and density (ρ) of a per- Gaps provided routes through mountain fect gas. These include the equation of state ranges and often became the sites of human (p = RρT/M, where R is the universal gas settlement. constant and M is the molecular weight of the gas) and Boyle’s law, which states that garnet A member of a group of cubic at constant temperature, the volume of a minerals with a general formula given mass of gas is inversely proportional 2+ 3+ R3 R2 Si3O12 where the divalent metals to the pressure of the gas. This law controls are magnesium, iron, manganese, or cal- the cooling of air as it rises and expands cium and the trivalent metals are alu- into regions of lower atmospheric pressure. minum, iron, or chromium. The following six end-members are recognized: gas sand A type of sandstone or sand pyrope – Mg3Al2Si3O12 deposit that contains useful quantities of 2+ almandine – Fe3 Al2Si3O12 natural gas, which may be extracted. spessartite – Mn3Al2Si3O12 grossular – Ca3Al2Si3O12 gas thermometer See thermometer. 3+ andradite – Ca3(Fe ,Ti)2Si3O12 uvarovite – Ca3Cr2Si3O12 gastrolith /gas-trŏ-lith/ A stone or peb- Compositions corresponding to pure end- ble ingested by reptiles and birds to assist members are rare and garnets are divided in the breakdown of food in the gizzard.

148 general circulation of the atmosphere

Fossil structures associated with the re- culation of the atmosphere or oceans, or mains of extinct reptiles are thought to be both, in three dimensions. Increasingly re- gastroliths. alistic models of the general circulation are being obtained now that more is known Gastropoda /gas-trop-ŏ-dă/ The class of about the controlling factors and powerful the phylum MOLLUSCA that includes the computers with large memory stores are slugs and snails. Typically they have a shell available. Over the years the models or that is often coiled in a helical spiral; unlike simulations have become more sophisti- that of the CEPHALOPODA it is not divided cated, starting by trying to account for the into chambers. Most gastropods are ma- primary processes responsible for main- rine but some live in fresh water and others taining the large-scale features of the circu- are terrestrial. Fossils are known from the lation of the atmosphere, using the Cambrian Period onward and the class has primitive equations of motion. Sophisti- continued to flourish and diversify to the cated models now simulate physical, ther- present day. modynamic, and dynamic activity. The most complex models are coupled atmos- gauge height (gage height) See stage phere–ocean general circulation models, (def. 3). which couple an atmospheric general circulation model (AGCM) with an ocean gauging station (gaging station) (in hy- general circulation model (OGCM), and drology) A point along a stream or by a some also include factors such as the bio- lake, reservoir, or well at which instru- sphere, carbon cycle, aerosol transport, and ments are installed to monitor and collect atmospheric chemistry. These models can information, such as the height or stage of be used to provide predictions of future cli- the water above a fixed datum, stream flow mates and the rate of climate change. Gen- or discharge, water chemistry, and water eral circulation models can also be used to temperature. simulate past climatic conditions, e.g. an ice age, for paleoclimatology studies. GCM See general circulation model. general circulation of the atmos- geanticline /gee-an-ti-klÿn/ A very large phere The mechanism by which energy anticlinal structure that has developed is transferred from surplus to deficit areas within geosynclinal sediments, as a result of the world. It is constrained by a number of lateral compression. of processes that interact with the main driving force of the circulation to result in gedrite /ged-rÿt/ An orthorhombic min- the observed system of winds and distur- eral of the AMPHIBOLE group. bances. This circulation is basic to an understanding of the climates of the world gelisol One of the twelve soil orders of because it determines the direction and the US SOIL TAXONOMY (the order was not strength of winds in any latitudinal posi- in the original 10 orders but was added in tion. 1998); it comprises those soils underlain by The atmospheric circulation is very permafrost. Freezing and thawing are im- complex, but it can be simplified into a se- portant processes, accompanied by cry- ries of belts (see Fig. 1). Within the tropics oturbation (frost mixing). Gelisol soils are there are the trade winds converging to- defined as having permafrost within 1 m of ward the intertropical convergence zone the soil surface, or gelic material within 1 (ITCZ). Poleward of the subtropical anti- m and permafrost within 2 m of the soil cyclones are the main westerly winds with surface. a small area of easterlies near the poles. These mean winds are essentially zonal in general circulation model (GCM, cli- direction and do not appear to be an effec- mate model) A computer model or simu- tive system in transferring surplus radia- lation that represents the general cir- tional energy toward the temperate and

149 general circulation of the atmosphere

North Pole polar anticyclones polar easterlies mid-latitude depressions westerlies subtropical highs NE trades intertropical convergence Equator zone SE trades westerlies polar easterlies Fig. 1 South Pole

use polar front tropopa

subtropical jet

polar front jet

sinking rising cold air warm air

Arctic front

90°N trade thermal winds equator

Fig. 2 Arctic cell Ferrel cell Hadley cell

General circulation of the atmosphere

polar latitudes. However, long-term stud- tudes for only short periods of time, it is ies have shown that meridional exchange averaged out on the mean maps. takes place as shown in Fig. 2. In the trop- The driving force for the atmospheric ics, air rises at the ITCZ, releasing latent circulation is the differential pattern of net heat and gaining potential energy. This is radiation received at the Earth’s surface. If moved toward the subtropics where it de- the Earth were uniform and stationary this scends and maintains the trade winds and would give rise to a simple thermal cell be- where much moisture is evaporated from tween equatorial (surplus) and polar the sea surface to continue the convection (deficit) areas. However, the effects of the cell toward the ITCZ. This cellular circula- Earth’s rotation, the presence of mountain tion has been called the HADLEY CELL. Pole- barriers, the distribution of land and sea, ward, the mean charts show a dominance and the positions of the ocean currents all of zonal winds, but exchange here is ef- help to modify the simple thermal cell into fected by depressions, which advect warm the complex general circulation of the at- and cold air and aid their mixing. Because mosphere. See also general circulation this process takes place at different longi- model.

150 Geographical Information System genus /jen-ŭs/ (pl genera) A group in the ical indicators, and several lesser used tech- taxonomic classification of organisms that niques that help establish stratigraphy. consists of one or more related SPECIES. One Relative dating seeks to put events in order; or several genera form a FAMILY. For exam- absolute dating tries to give precise dates of ple, the genus Homo, to which man be- occurrence. In geomorphology both are longs, is placed in the family Hominidae. A used, especially in studies of the Pleistocene generic name always begins with a capital and post-Pleistocene periods. letter and is italicized. It is always included with the specific name in naming a species, geode /jee-ohd/ See druse. when it is often abbreviated to the initial letter. Thus, the species to which modern geodesy /gww-od-ĕ-see/ The science con- man belongs is Homo sapiens or H. sapi- cerned with determining the exact shape ens. A genus is defined on the basis of the and size of the Earth and the exact position species it contains, the most typical of of points on the Earth’s surface. In addition which is designated as the type species. See it investigates the Earth’s gravitational taxonomy. field, variations in tides, and the Earth’s rotation. It provides the reference surface geocentric /jee-oh-sen-trik/ At or relat- from which astronomical observations are ing to the center of the Earth. made. geochemical cycle /jee-oh-kem-ă-kăl/ geodetic surveying /jee-ŏ-det-ik/ The (rock cycle) The route followed by an el- large-scale surveying of the Earth’s surface, ement or group of elements as they circu- taking into account its curvature. Precise late through the Earth’s crustal and measurements from geodetic surveys pro- subcrustal rocks. Elements may combine vide the horizontal control points for more or separate during the cycle. Weathering of detailed surveying. igneous rocks produces particles that settle as sediments, which may become sedimen- geodimeter /jee-ŏ-dim-ĕ-ter/ A survey- tary rock or undergo metamorphism into ing instrument used for the accurate mea- metamorphic rocks. These may melt and surement of distance. The measurement is form new magma, which rises to the sur- indirect, being obtained by timing the pe- face and crystallizes as new igneous rock. riod required for a beam of light to travel from one end of a line to the other and re- geochemistry /jee-oh-kem-iss-tree/ The turn. Since the velocity of light is known, branch of geology that deals with the the distance can then be calculated. The in- chemical elements and compounds in the strument sends out a light beam, often atmosphere, water, soil, and rocks of the from a laser, which is reflected by a mirror Earth, particularly their composition and at the other end of the line to be measured. how they are formed and distributed. Suggested maximum ranges are 5 to 10 km in daylight and 15 to 25 km in darkness, geochronology /jee-oh-krŏ-nol-ŏ-jee/ depending on the light source. The geo- The science of age determination of parts dimeter is ideal for rapid baseline measure- of the Earth. It has two branches: absolute ment. (or numeric) dating and relative dating. The former is more modern, and involves Geographical Information System techniques such as carbon–14, potassium– (GIS) A computer system that includes argon, and radioactive decay methods to the hardware, software, and data for cap- put actual ages in years BP (before present) turing, storing, updating, manipulating, on organic remains, rocks, or sediments. analyzing, and displaying geographical and Relative dating includes fossil correlations, spatial data. Within the earth sciences GIS pollen analysis and correlation, matching systems have been applied, for example, to of tills from different areas on the basis of create maps displaying specific informa- weathering or geologic content, archeolog- tion, such as the geology of an area, ocean

151 geography circulation, topography, the watershed for chemical and physical interaction between a stream system, and hazard risks from natural forces (e.g. running water, waves, natural disasters for an area. ice, gravity, biological phenomena, etc.) and the Earth’s surface, which act to mod- geography The scientific study of the ify the landscape. An increasing trend in features of the Earth’s surface. Physical ge- geomorphology is toward the study of ography includes CLIMATOLOGY, GEOMOR- processes, inferring from these the land- PHOLOGY, METEOROLOGY, and PEDOLOGY; forms being created, as contrasted with human activity is covered by political or more traditional geomorphology, in which socioeconomic geography. processes were merely inferred from landforms. Processes are determined by the en- geoid /jee-oid/ The Earth pictured as a vironmental variables of climate, geology, smooth oblate spheroid, coinciding with and base level; a change in one or some of sea level and taken to continue across con- these will change the nature or rate of tinents at the same level. At all points on processes. Processes fall into the broad cat- Earth the geoid surface is at right-angles to egories of weathering, erosion. transport, the downward direction of the pull of grav- or deposition, within each of which many ity. Geodetic leveling employs the geoid as different types can be recognized. Rates of its reference surface. a process, as shown by measurements or inference, are characterized by great spatial geologic timescale (geologic column)A and temporal variation. chronological series of events in the geo- A central concept in geomorphology is logic history of the Earth, beginning in the the relative significance, in terms of work PRECAMBRIAN and extending to the present done in landscape formation, of infrequent (the HOLOCENE epoch of the NEOGENE pe- but highly destructive events such as riod). It is divided into eons, eras, periods, storms, floods, and gales and the very fre- epochs, and ages, each ascribed dates in quent but far less effective events of aver- terms of millions of years ago (see table). age conditions. In storms, whole beaches See also chronostratigraphy. can come or go overnight; rivers can change their courses and expand their geology The study of the structure and floodplains during floods; sand dunes can composition of the Earth. See geomorphol- be built and eroded in a single severe gale. ogy; geophysics; petrology. See also This is because capacity for work increases chronostratigraphy; geologic timescale. as a power function of the increase of the agency concerned; erosional capacity of geomagnetic equator /jee-oh-mag-net-ik/ rivers is proportional not to velocity, but A great circle equidistant between the geo- the square of velocity; wave energy is pro- magnetic poles. It connects all points of portional to the square of wave height; the zero geomagnetic latitude and its plane is transporting power of wind increases as at right angles to the Earth’s magnetic axis. the cube of its velocity. Major landscape features are largely shaped by the infre- geomagnetic field The Earth’s mag- quent severe events, but Leopold, Wolman, netic field, which causes a compass needle and Miller (1964) concluded that moder- to align north–south whatever its location ate events of moderate frequency are re- on the Earth’s surface. sponsible for most work done. A better understanding comes from ac- geomatics A new discipline that encom- tually measuring the processes: this shows passes geographical information systems the rates at which the same process oper- (GIS), land surveying, and map making. ates in different environments, and the The term was first used in Canada in the comparative effectiveness of different pro- mid-1980s. cesses in the same environment. It also shows the spatial and temporal distribu- geomorphic process /jee-ŏ-mor-fik/ The tion of changes, and once this is under-

152 Eon Era Period Epoch Millions of years

HOLOCENE 0.01 PLEISTOCENE NEOGENE 1.6 PLIOCENE 5.3 MIOCENE CENOZOIC 23.7 OLIGOCENE 36.6 PALEOGENE EOCENE 57.8 PALEOCENE 66.4 CRETACEOUS 144 JURASSIC MESOZOIC 208 TRIASSIC 245 PERMIAN 286 PENNSYLVANIAN PHANEROZOIC PALEOZOIC CARBON- IFEROUS 320 MISSISSIPPIAN (UPPER) 360

DEVONIAN

408

SILURIAN

438

PALEOZOIC ORDOVICIAN (LOWER) 505

CAMBRIAN

570

PROTEROZOIC PRECAMBRIAN 2500 Origin of Earths crust ARCHEAN about 4600

Geologic timescale

153 geomorphology stood, prediction to the future is easier and BIOSPHERE. It includes the Earth’s crust more reliable, and the formation of exist- (lithosphere), all bodies of water (hydros- ing landforms is more easily appreciated. phere), and the air (atmosphere). Measurements of processes are taken as they operate, by the use of rods inserted geostrophic wind /jee-oh-trop-ik/A into slopes and river banks for example, theoretical wind representing a balance of recording losses, or by the use of map evi- forces between the pressure gradient and dence recording, for example, the succes- the CORIOLIS EFFECT. This produces a wind sive growth of a spit across a river mouth, flow parallel to the isobars. Because the ef- measurements from which can show varia- fects of FRICTION are ignored, the real wind tion in rates at different periods. only approximates to the geostrophic wind above the BOUNDARY LAYER. At these levels geomorphology /jee-oh-mor-fol-ŏ-jee/ the geostrophic wind can easily be calcu- The study of the evolution of landforms, lated from the pressure gradient. excluding the major forms of the Earth’s surface, such as mountain chains and geosyncline /jee-oh-sing-klÿn/ (geotecto- ocean basins. It includes the study of the re- cline) A large linear trough developed lationships between structures and land- along the margin of a continent, within forms at the basic level, but then has two which a considerable thickness of stratified major branches: the inductive study of ex- sediments including turbidites accumu- isting landforms, from which the processes lates, with occasional extrusive volcanic of their evolution are inferred, and the de- rocks. After a long period of accumulation ductive study and measurement of actual the trough is folded and uplifted to form processes operating, inferring their ulti- mountain chains. In the light of the theory mate influence on the landscapes on which of plate tectonics and sea-floor spreading, they are acting. These two are complemen- this term has fallen into disuse. See also tary, because they approach the cause and trench. effect relationships between landforms and processes from two contrasted viewpoints. geotectocline /jee-oh-tek-toh-klÿn/ See The inductive branch is older than the geosyncline. more scientific deductive branch. jee-oh-th’er-măl geopetal cavity /jee-op-ĕ-t’l/ (spirit level geothermal / / Describ- ing heat that originates in the Earth’s inte- structure)A WAY-UP STRUCTURE produced within a closed cavity in a rock, generally rior. Evidence of geothermal activity within a fossil. On death some mud-sized includes fumaroles, geysers, hot springs, material entered the fossil and settled out and boiling mud volcanoes. with its upper surface parallel to the horizontal. Once the cavity was closed no fur- geothermal gradient The increase in ther sediment could enter. The trapped the Earth’s temperature with depth from sediment hardens and leaves a record of the its surface, expressed either in depth units past horizontal level. The empty space per degree, or more generally degrees per above the sediment is generally later in- unit depth. filled, usually by coarse calcite crystals. geyser /gÿ-zer, -ser/ A type of hot spring geophysics /jee-oh-fiz-iks/ The science that throws up jets of boiling water and that combines the principles of mathemat- steam, which are formed when groundwa- ics and physics with the use of sophisti- ter comes into contact with hot rock un- cated equipment to study the Earth and derground. High pressure increases the other planetary bodies. boiling temperature of water, but after it is forced out as steam the pressure falls and geosphere /jee-oh-sfeer/ The nonliving water boils at lower temperatures. This part of the Earth, as opposed to the living causes an intermittent cycle of events, with

154 glacial plucking eruptions taking place at intervals after pe- Bhutan, and the consequent growth of riods of apparent inactivity. many glacial lakes, nearly 50 of which are considered to be at risk of bursting through GIS See Geographical Information Sys- the ice that contains them. tem. glacial maximum The greatest extent glacial /glay-shăl/ 1. Relating to a glacier of an ice age. During the Pleistocene, at its or ice sheet. greatest extent, ice extended across the 2. (glacial period; glacial phase) The period Arctic Ocean and joined with the ice sheets during an ice age when ice sheets expand to that covered much of North America, lower latitudes. This would normally be Greenland, N Russia, and NW Europe (the associated with a global drop of tempera- Baltic, Norway, Sweden, Denmark, and ture and changes in position of the main Finland were completely covered). This ice circulation belts. See also deglaciation; is believed to have extended across the glacial maximum; interglacial; interstadial. North Sea to engulf all but the very south of the British Isles. Iceland was covered by glacial deposition The accumulation of an isolated ice cap. Antarctica was con- rocky material that has been transported cealed by ice, as today, and mountainous by a moving glacier or ice sheet. The de- areas in both hemispheres underwent posits, collectively known as DRIFT, are glaciation. At this maximum an area of usually left stranded and include drumlins, some 46 million sq km was beneath ice, erratics, eskers, and moraines. See also till. over three times that of the present day. glacial drainage system See meltwater glacial outburst flood (jökulhlaup) The channel. sudden short-duration release of water from a glacier or ice cap. The flood may glacial drift See drift. originate from the sudden drainage of an ice-dammed GLACIAL LAKE; from the sud- glacial erosion (glacial scouring) The den release of water from an englacial lake wearing away of rock by the action of a within the glacier; or from the release of moving glacier. The process is accelerated meltwater resulting from volcanic activity by the presence of meltwater streams and below a glacier or ice sheet. The erosive rock fragments beneath the ice. See also power of glacial outburst floods may form glacial plucking. deep branching channels or gorges (see coulee). Glacial outburst floods occur with glacial lake An ice-dammed body of some frequency in Iceland, where such water. A glacial lake may accumulate be- floods are named jökulhlaup (derived from tween the margin of a glacier and the val- jökull (glacier) and hlaup (flood)). In No- ley wall, or where a glacier is enclosed by vember 1996 a catastrophic flood was re- higher ground such as moraine, or where a leased following a subglacial eruption of glacier blocks a stream emerging from a the Grimsvötn volcano below the Vatna- tributary valley, or where a glacier blocks a jökul ice sheet; the flood caused severe major river valley. The sudden flood that damage with several bridges demolished results when the water breaks through the and roads washed away. ice is termed a GLACIAL OUTBURST FLOOD or jökulhlaup. Such events can be cata- glacial phase See glacial. strophic for settlements and constructions down-valley in the path of the suddenly re- glacial plucking The erosive process by leased water. In the Himalayas, rising tem- which rock material can be included within peratures (an average of 1°C from the the ice of a glacier, later becoming a tool of 1970s to 2002) have led to an acceleration glacial ABRASION. It entails the freezing of in the melting of glaciers, estimated to be the glacier ice onto bedrock and the subse- retreating at the rate of 30–40 m a year in quent plucking out of blocks on move-

155 glacial scouring ment. For this to occur FREEZE-THAW activ- shelves are, in general, very wide, averag- ity must be taking place at the ice-rock in- ing 160 km; the average shelf-edge depth is terface, thereby restricting the process to around 220 m. Shelf areas that have been TEMPERATE GLACIERS. heavily glaciated, such as that off the Nor- A prerequisite for plucking is the pres- wegian coast, often display enclosed basins ence of already loosened blocks, because and trough features. Such submarine topo- the tensile strength of consolidated rock is graphic forms are analogous to lake basins far greater than that of ice. Well-jointed found in many glaciated valleys on the con- rocks are most susceptible (an optimum tinents. Not all of the sea-floor features tes- mean joint separation of from one to seven tify to erosion; there may also be meters has been suggested). Freeze-thaw accumulations of morainic material and activity both before and after the appear- other deposits, built into or retaining a ance of the glacier will assist, while UN- constructional form. Drumlins have been LOADING joints can also form as a result of found off the coast of Maine, while con- the replacement of eroded rock by less centric stone deposits on part of the floor dense ice, providing more susceptible lay- of the North Sea are probably of morainic ers. origin. Many glacially-eroded features have become buried by shifting sediments glacial scouring See glacial erosion. and can be found only by seismic measurements. Many of the banks and channels glacial trough (U-shaped valley)A formed in glacial debris have been shaped well-developed glacial valley with a char- by a combination of wave and tidal action, acteristic U-shaped cross profile, as op- for example, in the S North Sea. posed to the V-shape of fluvially developed valleys. Such development can take place glaciation 1. (in meteorology) The by simple widening of preglacial valleys, or formation of ice crystals from supercooled by a combination of widening and deepen- water droplets in the atmosphere. It occurs ing, the latter process depending upon the in the upper layers of cumulonimbus glacier’s ability to cut into the bedrock. cloud. The typical steep sides of glacial troughs re- 2. A period of time during an ice age when flect the fact that whereas rivers occupy ice sheets grow and spread to cover exten- only the bottoms of their valleys, a glacier sive areas. tends to fill much of it, erosion taking place 3. The modification of land surfaces by wherever there is an ice-rock contact. The glaciers through the processes of glacial extent to which a valley can be deepened erosion and deposition. depends greatly upon the amount of weathering active in the valley before the glacier /glay-sher/ An accumulation of appearance of the glacier, since GLACIAL ice of limited width moving downslope, PLUCKING is the major process of rock re- under the influence of gravity, from a moval. source area. Glaciers are formed by the accumulation and compaction of snow (see glacial wastage A reduction in thick- firn). They may be contained within basins ness or areal extent of an ice mass resulting (see cirque glacier) or within preexisting from more material being lost by ablation valleys (see valley glacier). See also cold than is added by accumulation. glacier; temperate glacier. glaciated sea floor /glay-shee-ay-tid/ glacier regime The pattern of relative One of the most characteristic types of gaining or losing of ice from a glacier or ice CONTINENTAL SHELF is found off coasts that sheet. A glacier having a positive regime is have been heavily glaciated. Such shelves gaining ice and will therefore advance and tend to have highly irregular surfaces and thicken, whereas one with a negative in their deepest parts may have 200 m or regime loses ice (see ablation) and will be more of water covering them. Glaciated reduced in size.

156 glide twinning glacioeustatism /glay-shoh-yoo-stă-tiz- near drainage lines. If the gleying is sea- a˘m/ (glacioeustasy) Worldwide change sonal, the profile may have ochreous mot- in sea level due to the effect of ice. During tlings due to periodic oxidizing conditions, the PLEISTOCENE glacial periods sea level changing the iron back to its ferric state. was much lower than at present, owing to The lack of oxygen inhibits microbiologi- the presence of much water on the conti- cal breakdown of humus, which accumu- nents in the form of ice. Estimates give val- lates as acid peat. Anaerobic bacteria tend ues for this lowering of around 100 m. to produce gases toxic to large plants, such During the INTERGLACIAL periods the ice as hydrogen sulfide, ammonia, and marsh sheets melted and sea level rose again, gas (methane). Together, the effects of hy- sometimes to higher levels than those of dromorphism are sufficient to completely today. Such changes would have a world- change the direction of development of the wide effect on erosional processes, because soil profile. of the lowering of BASE LEVEL. However, these changes were not exclusively the re- gley soil /glay/ A type of intrazonal hydro- sult of the reduction in the volume of sea morphic soil that may be found in tundra, water, because deformation of the Earth’s temperate, or tropical regions where there crust at the same time also had an appre- is excessive moisture in the soil profile. The ciable effect. characteristic poor drainage may be a result of the profile characteristics, i.e. a fine- glaciofluvial See fluvioglacial. textured parent material, or the site of the soil, i.e. a low-lying area. Two main types (in geology) An amorphous (non- glass of gley can be recognized: the surface- crystalline) material that is formed by the water gley and the groundwater gley. The rapid cooling of lava or magma, such as former is found where, because of a poorly OBSIDIAN. drained subsoil, water is held within the upper part of the soil, giving waterlogged Glauber’s salt See mirabilite. conditions for part of the year. A typical profile would consist of a very dark AI glauconite /glaw-kŏ-nÿt/ A bright green horizon rich in humus. Below this there authigenic mica-like mineral with an approximate composition would be Eg and Bg horizons with typical (K,Na) (Fe,Al,Mg) - gleyed characteristics (see gleying). The C 1.2–2 4 horizon would not show any gleying and (Si7–7.6Al1–0.4O20)(OH)4.n(H2O), which is found only in marine sediments, would be similar to the parent material. particularly greensands. The groundwater gley often occurs where there are permeable sands overlying imper- glaucophane /glawk-ŏ-fayn/ A mono- vious clay. The high groundwater table clinic AMPHIBOLE common in certain kinds that is formed results in increased gleying of metamorphic rock. with depth. The surface horizons are typically aerobic, gray or brown in color, and glazed frost See black ice. rich in humus. With depth a mottled Bg and a gray/blue G horizon are common. gleying /glay-ing/ The permanent or sea- Most gley soils are under grassland vegeta- sonal presence of either perched water or tion and are used mainly for grazing. groundwater within a soil profile. This creates anaerobic conditions, leading to a glide twinning A complex form of crys- dominant process of REDUCTION of ferric tal TWINNING resulting from deformation. iron to its ferrous form, giving the soil a It results from one layer of atoms changing blue-gray color. The water may either be its position so that the lattice is reversed. perched on a heavier impermeable horizon This results in broad lamellae within the below, as in the PEATY GLEY PODZOL, or the crystal. It is commonly shown in the calcite result of a high groundwater table in areas crystals of marble.

157 Global Positioning System

Global Positioning System (GPS)A Scientists have observed that many of the satellite-based coordinate positioning tool glaciers in the Himalayas, Peru, and else- that can rapidly and accurately determine where in the world are also retreating. The the latitude, longitude, and altitude of a loss of ice masses may in itself have a criti- point on or above the Earth’s surface. It is cal effect on global warming as ice and based on a constellation of 24 satellites or- snow provides a reflective white surface biting the Earth at a very high altitude and that reflects solar radiation (see albedo). In was originally developed by the US for mil- addition the melting of permafrost in parts itary purposes but was made available of Alaska and Siberia, for example, re- for civilian use during the 1980s. It has leases methane, a greenhouse gas, into the become an important tool in cartography, atmosphere. increasing accuracy and speeding production. Among its applications in earth sci- global warming potential (GWP)An ences it has been used in studies to measure index that measures the influence that the movements of the Earth’s crustal plates, principal GREENHOUSE GASES have on the providing important evidence to support natural GREENHOUSE EFFECT in combination the theory of plate tectonics. See also with the differing length of time that the in- Galileo. dividual gases remain in the atmosphere. Carbon dioxide is used as the reference gas global warming The supposed gradual and given the value of 1; the global warm- increase in the Earth’s average air temper- ing potential of the other gases is given as a ature, generally attributed to an increasing ratio of the radiative forcing that would re- concentration of GREENHOUSE GASES sult from the emission of that gas over a (chiefly carbon dioxide, plus some other certain time period. It is calculated as the trace gases) in the atmosphere (and the ratio of the radiative forcing that would re- consequent enhanced GREENHOUSE EFFECT). sult from the emission of 1 kilogram of the It has been calculated (using computer greenhouse gas to that from the emission of modeling) that a 1.5°C rise in temperature 1 kilogram of carbon dioxide over a fixed would result in melting of ice in the polar time period (e.g. 100 years). ice caps, causing a 20-cm rise in sea levels worldwide, and widespread flooding. In globigerina ooze /glŏ-bij-ĕ-ree-/ A cal- addition changes induced in climate pat- careous deep-sea ooze (see pelagic ooze) terns may lead to shifts in vegetation zones containing over 30% organisms. It is very and extensions of desertification. Actual widespread on the deep-sea floor. The or- evidence of global warming is not yet con- ganisms largely comprise the calcareous clusive but appears to be mounting. During skeletons or tests of minute FORAMINIFERA. the 20th century the average temperatures Globally, globigerina ooze represents some on Earth rose by 0.5°C and according to 130 million sq km covering something like the Intergovernmental Panel on Climate half of the Atlantic Ocean floor, being Change (IPCC) most of the warming in the widespread in the Indian Ocean, particu- past 50 years is attributable to human ac- larly on the W side, and very conspicuous tivity. Arctic ice fluctuates with the seasons in the S Pacific Ocean. but on average has been shrinking in recent years; in 2005 the ice cover fell to 5.35 sq glomerophyric /glom-ĕ-roh-fi-rik/ (glom- km, the lowest recorded since 1978 when eroporphyritic cumulophyric) Describ- satellite records became available, and the ing a PORPHYRITIC texture indicated by the ice has also become thinner. In Antarctica occurrence of phenocrysts in aggregates or studies have shown that glaciers have been clumps set in the groundmass of an igneous retreating and the ice on the peninsula and rock. the West Antarctic ice sheet thinning, possibly caused by warming ocean tempera- GMT See Greenwich Mean Time. tures or volcanic activity, although the East Antarctic ice sheet is actually thickening. Gnathostomata /nath-oh-stoh-mă-tă/

158 grade

Vertebrates possessing jaws. The Gnatho- System but were replaced by the AM- stomata include most fish and all higher MONITES in the PERMIAN. vertebrates. Compare Agnatha. goniometer /goh-nee-om-ĕ-ter/ An in- gneiss /nÿss/ A coarse-grained metamor- strument that measures the angle between phic rock in which quartz and feldspar pre- crystal FACES. Because such angles are char- dominate over micas. The schistosity is acteristic of the crystal, they provide a use- poorly defined and segregation banding is ful means of identification. irregular and discontinuous. Coarse granular bands of quartz and feldspar alternate gorge A narrow steep-sided valley, gen- with thin often undulating schistose bands erally formed in hard rocks by the erosive in which micas and amphiboles are con- action of a flowing stream or river. A large centrated. gorge is called a CANYON. Paragneiss has an unambiguous sedimentary composition. Orthogneisses are gossan /goss-ăn/ A mass of quartz and metamorphosed igneous rocks. Rocks of hydrated iron oxides that often marks the granitic composition containing a planar outcrop of a sulfide-bearing vein, the sul- or linear orientation of minerals, imparted fides having been oxidized to soluble sul- by stress, are termed granite–gneiss. fates. Metalliferous material removed in Gneisses are formed during high-grade resolution may be redeposited at depth in a gional metamorphism. zone of secondary enrichment. See also hydrothermal process. gneiss dome See mantles gneiss dome. GPS See Global Positioning System. gnomonic projection /noh-mon-ik/A form of azimuthal map projection (see az- graben /gray-bĕn/ A generally elongated imuthal projection) in which the great cir- block of rock that has been downthrown cles of the Earth are shown as straight between two parallel faults relative to the lines. Great circle routes can thus be plot- surrounding area. It differs from a RIFT ted for navigation purposes. This projec- VALLEY in that it is a structural feature and tion is not often used because of its limited not necessarily also a topographical one. application. Compare horst. (See diagram at FAULT.) goethite /goh-th’ÿt/ A yellow-brown grade 1. The degree of metamorphic mineral, FeO.OH, formed by the oxidation change undergone by a rock. When such and hydration of iron minerals or as a di- changes can be traced from unmetamor- rect precipitate. See limonite. phosed rocks to highly metamorphosed rocks, the grade of metamorphism is said Gondwanaland /gon-dwah-nă-land/ The to increase. High-grade rocks are produced S hemisphere supercontinent, which takes under conditions of high temperatures or its name from the Gondwana system of pressures; likewise low-grade metamorphic India, thought to have existed over 200 rocks form at low temperatures and pres- million years ago. Since then it has been sures. See zone. fragmented as a result of SEA-FLOOR 2. The condition reached in the later stages SPREADING into the present continents of of a normal CYCLE OF EROSION when a river Africa, Australasia, India, South America, or slope has an EQUILIBRIUM REGIME, with a and Antartica. See also Pangaea. smooth concave long profile. Since the theory was proposed by Davis, other factors goniatite /goh-nee-ă-tÿt/ One of the ear- besides the modification of the long profile lier mollusks of the subclass AMMONOIDEA, have been taken into account, especially whose shells had simply folded angular su- adaptation to changes in the controlling ture lines. Goniatites are used as ZONE FOS- environmental factors of base level, rain- SILS in the Devonian and Carboniferous fall, and the quantity and type of the load

159 graded bedding supplied to the stream. A river that is at cording to the proportions and composi- grade is able to maintain its COMPETENCE so tions of the feldspars. that it is just able to transport its load at a Granites contain on average about 25% given discharge. It does this by adjusting its quartz and contain either a single feldspar, long profile, cross section, channel pattern, a perthitic intergrowth (hypersolvus gran- and bed roughness: any environmental ites), or two kinds, oligoclase and potas- changes will be met by changes in some or sium feldspar (subsolvus granites). All are all of these factors, so that the river uses up leucocratic rocks (see color index), the only its available energy and no more. common dark mineral being biotite. Acces- sory minerals include muscovite, zircon, graded bedding Sedimentary bedding apatite, and tourmaline. The typically in which particles show a size distribution. granular texture in which most of the crys- The coarsest material forms the base and tals are subhedral is often termed granitic. then the sequence becomes progressively Porphyritic varieties containing large finer upward. It is often present in turbid- zoned phenocrysts of white, gray, or pink ity deposits. On a larger scale such upward feldspar are common. Many granites ex- fining sequences are developed in braided hibit graphic, orbicular, and rapakivi tex- river channels, as they are infilled. See tures and contain miarolitic cavities. Some braided stream. strongly alkaline granites are chemically distinctive, having low aluminum and cal- graded profile See equilibrium profile. cium contents and characterized by the presence of the soda pyriboles, aegirine gradient wind An extension to the con- and riebeckite, and an abundance of other- cept of the GEOSTROPHIC WIND. Isobars in wise rare accessory minerals. With a de- the upper atmosphere are rarely straight, crease in the amount of quartz, granites so to account for the curved flow, follow- pass into quartz-syenites and syenites. ing Newton’s laws of motion, another Adamellites, of which Shap granite is an force must be included. This is the CEN- example, contain oligoclase and potassium TRIPETAL ACCELERATION. The equilibrium feldspars, quartz, and possibly hornblende wind for these three forces is known as the in addition to biotite. With a decrease in gradient wind. Curvature of the isobars the amount of quartz, adamellites grade can be either in the cyclonic or anticyclonic into monzonites; the term quartz-mon- sense. zonite is sometimes used as a synonym for adamellite. Granodiorites that are almost granite A member of a family of acid devoid of alkali feldspar are called trond- coarse-grained plutonic igneous rocks con- hjemites. Granitic rocks are particularly taining essential quartz, alkali, and plagio- susceptible to the pneumatolytic (see pneu- clase feldspars and small quantities of matolysis) processes of alteration, greisen- mafic minerals. The rocks of the granite- ing, tourmalinization, and kaolinization. granodiorite suite and their volcanic equiv- Microgranites are the medium-grained alents, rhyolites-dacites, are divided ac- equivalents of granites. Porphyritic vari-

GRANITE–GRANODIORITE ROCKS Dominant feldspar Coarse-plutonic Medium Fine-volcanic

alkali feldspar granite microgranite rhyolite alkali feldspar= plagioclase feldspar adamellite microadamellite rhyodacite plagioclase feldspar granodiorite microadamellite dacite

160 Graptolithina eties have been termed quartz-porphyries occur where there is differential expansion or granite-porphyries. Granophyre is a mi- of various mineral grains. The usual prod- crogranitic rock exhibiting a micrographic uct of this type of weathering is a coarse (granophyric) texture. Granodiorites are sand. volumetrically the most abundant plutonic rocks and constitute the bulk of the granule A small fragment of rock up to batholiths in orogenic regions. Granodi- 5 mm across. It is thus larger than a coarse oritic magma may be produced by anatexis grain of sand but smaller than a pebble. at deep crustal levels. When such sites of magma generation become exposed at the granulite /gran-yŭ-lÿt/ A granular tex- surface, the association of granites, tured metamorphic rock occurring in areas migmatites, and high-grade regionally of regional metamorphism of the highest metamorphosed rocks is apparent. Once grade. Granulites lack the hydrous min- generated, granitic liquids rise up in the erals, micas and amphiboles, but may crust and undergo differentiation to pro- contain pyroxene, garnet, kyanite, or silli- duce the more chemically extreme mem- manite. Characteristically the quartz and bers of the suite. At high crustal levels, feldspar crystals are flattened and their intrusive granite plutons produce marked parallel alignment gives rise to a foliation. metamorphic aureoles. Although most See also charnockite; metamorphic facies. granites crystallize from a magma, some granitic rocks may be produced by graniti- graphic intergrowth An intergrowth of zation, the pervasive metasomatism of pre- alkali feldspar and quartz commonly existing rocks. See also aplites; pegmatite. found in pegmatites and granitic rocks. The quartz is orientated along preferred di- granitization The process of metaso- rections within the feldspar and on a flat matic transformation of preexisting rocks surface resembles hieroglyphic or runic into granite due to the action of granitic writing. fluids (ichors) arising from depth. graphite (plumbago) A soft iron-gray granodiorite /gran-ŏ-dÿ-ŏ-rÿt/ A plu- massive or laminar form of pure carbon, tonic igneous rock containing plagioclase which is characteristically greasy to the feldspar of oligoclase-andesine composi- touch. Graphite is found mainly in meta- tion, subordinate potassium feldspar, bi- morphic rocks; it has many commercial otite, or hornblende. See granite. uses. granophyre /gran-ŏ-fÿr/ A microgranite Graptolithina /grap-tŏ-lith-ă-nă/A exhibiting a micrographic (granophyric) primitive group of invertebrates of the phy- texture. See granite. lum Hemichordata. Graptolites are totally extinct, fossils being confined to the Paleo- granular disintegration The separa- zoic. The body consists of a number of tion of individual grains from the main minute cups (thecae) of chitinous material, mass of a rock. It most frequently occurs in which house individual polyps. The thecae coarse-grained rocks, notably granite, and are arranged along a stem (stipe) and grap- both mechanical and chemical weathering tolites show great variation in the arrange- processes can be responsible. Mechani- ment and orientation of the thecae and the cally, the differential expansion and con- disposition of the stipes. Biseriate forms traction of different mineral grains due to have thecae on both sides of the stipe, unis- INSOLATION may cause disintegration, eriate forms along only one, and there may while crystallization of salt from solution, be a single stipe or many. within the surface pores of a rock, may Graptolites were benthonic and plank- prise grains apart. Chemically, HYDRATION tonic marine organisms and the planktonic usually involves a considerable volume forms attained a wide distribution. This, change, and granular disintegration may coupled with their great morphological

161 grassland variation and rapid evolution, makes them commonly employed in prospecting for of considerable importance in biostratigra- oil and other minerals (whose deposits phy and they are used as ZONE FOSSILS cause local anomalies in the gravitational throughout much of the Ordovician and field). Silurian Systems. Graptolites are first known from rocks of the Cambrian Sys- gravitational constant (G) The con- tem; they suffered widespread extinctions stant used in the law of universal gravita- at the end of the Silurian but some groups tion. It has the value 6.670 × 10–11 continued to the early Carboniferous. Nm2 kg–2. grassland Any region where grass is the gravitational gradient A regional shift natural vegetation, generally where there is in the measured value of gravity across an insufficient rainfall to support woodland area. or forest trees but too much for a desert to form. Tropical grasslands, also called SA- gravity See acceleration of free fall. VANNAS, occur mainly in E Africa, N Aus- tralia, and parts of N South America. gravity anomaly On the assumption of Temperate grasslands include the PAMPAS uniform density, calculations can be made of South America, PRAIRIES of central to predict the value of gravity for a partic- North America, STEPPES of central Asia and ular point on the Earth’s surface. Where Russia, and VELD of southern Africa and this value differs from that actually mea- Australia. sured an anomaly exists. A negative gravity anomaly is an area in which gravity is less grass minimum thermometer A ther- than that predicted; it is an area out of iso- mometer used to record the minimum air static balance. An area of greater gravity temperature just above (20–30 mm above) than that predicted is a positive gravity ground level, where it is often lowest dur- anomaly and usually results from a large ing the night. mass of basic rocks near the Earth’s surface. graticule /grat-ă-kyool/ The network of lines on a MAP PROJECTION representing the gravity collapse structure A tectonic lines of latitude and longitude. On some structure produced mainly as a result of maps the graticule may not be shown over downward movement under the influence the entire face of the map; in these cases the of gravity. lines shown have short graticule ticks representing lesser lines not shown. gravity fault See normal fault. gravel According to some particle size gravity fold A fold structure resulting classifications, unconsolidated material from sliding in response to gravity, as dis- (usually rock fragments rather than indi- tinct from vertical or horizontal compres- vidual mineral crystals) in the 2–60 mm sional forces operating during an orogeny. size range. It is the size of material lying immediately above sand, and below cobbles. gravity gliding The movement of a In practice, however, the term tends to be large rock mass downhill as a result of loosely used in geomorphological literature gravity. for all material of an unconsolidated nature of greater than sand size and less than gravity meter See gravimeter. boulder size (i.e. from 2–200 mm in diameter). gravity unit (gu) A unit of acceleration used with gravity measurements. One gravimeter /gră-vim-ĕ-ter/ (gravity meter) gravity unit equals 10–6 ms–2 and is often An instrument used to measure slight vari- used in place of the milligal (see gal). One ations in the Earth’s gravitational field, gravity unit is roughly equivalent to one

162 Great Barrier Reef ten millionth of the value of gravity at the grayish-brown forest soils. The humus Earth’s surface. content decreases westward from 10% to 3% owing to more podzolic conditions. gravity wave A wind-generated wave at They fall into the ALFISOL order of the US sea whose length exceeds 50 mm and SOIL TAXONOMY. whose speed of propagation is influenced mainly by gravitational forces. graywacke /gray-waks/ An ARENACEOUS sedimentary rock in which fairly angular gravity wind See katabatic wind. particles of SAND grade, mainly lithic fragments, are suspended in a matrix of much gray-brown podzolic soils Soils found finer material. See also turbidite. covering large areas of the northeastern USA to the south of the brown podzolic gray wooded soil A soil belonging to zone. They are more mildly podzolized the podzolic group, morphologically re- soils, less acid conditions resulting in more sembling a podzol but chemically resem- abundant soil fauna incorporating humus bling a gray-brown podzolic soil. It is deep into the AI horizon. An Ea horizon is typically found in the mid-continental present but it is darker (gray-brown) in states of Canada on a base-rich parent ma- color than the typical eluvial horizon of a terial under boreal forest. It has a well-de- podzol. Clay is illuviated in the B horizon, veloped Ea horizon with a platy structure which normally exhibits a blocky struc- and a Bt horizon with a blocky structure. ture. They fall into the ALFISOL order of the Leaching is less intense than in a podzol, US SOIL TAXONOMY. resulting in less movement of sesquioxides in the soil profile and a soil with a higher gray desert soil See sierozem. pH value. Gray wooded soils fall into the ALFISOL order of the US SOIL TAXONOMY. gray forest soil A type of soil found in the transition zone between forest and Great Barrier Reef A CORAL REEF lying steppe in North America and Russia. The off the NE coast of Australia, extending for typical profile consists of a thin litter below some 1900 km and varying from 30–160 which is a dark gray A horizon with a km in width. It is an example of a barrier crumb structure. Organic and mineral mat- reef found off a continental coast in con- ter have been well mixed. Below is the Ea trast to those reefs that encircle volcanic is- horizon, with its nutty structure, which is lands. It is separated from the mainland by light gray in color due to silica coating the a discontinuous channel 40–90 m deep and ped faces. Clay and humus have been re- therefore too deep for coral growth. On the moved from this horizon and deposited in seaward side of the reef, the shelf attains the illuvial Bt horizon, which has a pris- depths of at least 420 m. The coral com- matic structure. The gradation to parent posing it covers about half of the Queens- material, which is often loess, is usually land shelf region. The reef is considered characterized by calcium carbonate con- one of the best coral features in the world, cretions. Thus the soil exhibits the leaching and fears have been expressed concerning characteristics of a podzol and the calcifi- serious damage to the reef caused by cer- cation of a chernozem. From the profile tain species of starfish, especially the characteristics it has been suggested that crown-of-thorns starfish. Episodes of these soils originally developed under CORAL BLEACHING of varying intensities steppe vegetation, which was replaced by have also occurred, notably in 1998 and forest when there was a change from a 2002. Borings have indicated that recent warm dry to a cool humid climate. These coral extends down to depths of between soils may be subdivided zonally. Westward 120 and 140 m, and that sand and mud in- from E Russia to European Russia these tercalations occur within the coral. The soils change from dark gray forest soils to coral tends to be of two quite different normal gray forest soils and finally to light types, the dividing line between the two

163 great circle being the Trinity Opening. The reef may be diation but absorbs strongly in the infrared based on a step-fault platform. part of the electromagnetic spectrum and re-emits such longwave radiation in all di- great circle A circle round the Earth’s rections, thus contributing to the GREEN- surface whose plane passes through the HOUSE EFFECT. The greenhouse gases center of the Earth. Great circle routes, include carbon dioxide (the most impor- broken down into a series of RHUMB LINES, tant), methane, nitrous oxide, water vapor, are often used in navigation because they chlorofluorocarbons, and other gases. represent the shortest distance between Concerns at an international level about two points on the Earth’s surface. the impact that increased levels of greenhouse gases are having on the global cli- green flash A transient flash of green mate led to the drawing up of a treaty in light sometimes observed at sunset or sun- 1997, the KYOTO PROTOCOL, which took ef- rise. When the Sun sinks below the hori- fect in 2005. The USA, the world’s largest zon, the last rays may be seen to change emitter of greenhouse gases, withdrew from pale yellow, to orange, then to green. from the Kyoto Protocol in 2001. The sig- The rapidity of the process has given this natories of the Protocol are required to phenomenon the name of the green flash. achieve individual emission reduction tar- It requires a flat, low horizon and occurs gets for six gases or classes of gas: carbon only when the air is exceptionally clear; dioxide, methane, nitrous oxide, hydroflu- it is caused by differential refraction of orocarbons, perfluorocarbons, and sulfur the spectral colors by the atmosphere. hexafluoride. See also global warming; Normally the colors complement each global warming potential. other when the Sun is high in the sky, but as the Sun disappears at sunset, or just greenland spar See cryolite. before it appears at sunrise, the individual colors may very occasionally be identi- greensand A greenish sandstone or sand fied. deposit that consists mainly of GLAU- CONITE. greenhouse effect The process by which the Earth’s surface is warmed by greenschist /green-shist/ A type of green heat reradiated back to Earth by gases, es- metamorphic rock whose color is caused pecially carbon dioxide but also including by the presence of ACTINOLITE, CHLORITE, methane, chlorofluorocarbons, nitrous or EPIDOTE. It should not be confused with oxide, water vapor, and other gases, in the GREENSTONE. atmosphere (see greenhouse gas). The presence of the atmosphere maintains the greenstone Basic volcanic rock that has temperature of the Earth’s surface at a suffered low-grade METAMORPHISM or al- much higher level than would be expected teration. on the basis of equilibrium with solar radiation input. This is because the gases greenstone belt A group of very ancient are transparent to short-wave radiation basic and ultrabasic volcanic and sedimen- but almost opaque to long-wave radiation. tary rocks that occur on most continents. Glass has similar properties and the ef- The composition of KOMATIITE lavas within fect of the Earth’s atmosphere has been such sequences suggests that the tempera- likened to that of a greenhouse. Increases ture of the mantle was much higher at the in carbon dioxide levels resulting from the time of their formation. combustion of fossil fuels may intensify the greenhouse effect. See also global warm- Greenwich Mean Time /grin-ij/ (GMT) ing. Local time at Greenwich, London, which is located on the 0° meridian and from which greenhouse gas A gas in the atmos- the standard times of different areas of the phere that is mainly transparent to solar ra- globe are calculated. 15° longitude repre-

164 ground frost sents one hour in time from the Greenwich without the use of geographical coordi- meridian. nates, i.e. longitude and latitude. greisening /grÿ-zĕ-ning/ See pneumatol- grike /grÿk/ (gryke) See limestone pave- ysis. ment.

grit A type of coarse SANDSTONE with a large grain size, typical of the MILLSTONE 20 3 GRIT of Britain. 19 18 groin (groyne) An artificial barrier con- 17 structed on beaches and extending seaward 16 perpendicular to the coastline in order to X er stream Y trap beach materials moving along the riv 15 shoreline. Groins are usually built in 14 groups, their length, height, and separation 13 varying with type of beach material and de- 12 sired results. Although very effective at 11 building up the beach in their immediate 310 vicinity, by halting or at least slowing 41 42 43 44 45 46 47 48 49 540 550 down longshore movement of material, they may cause considerable erosion of Grid that part of the beach down-drift of the groins. grid A network of parallel lines inter- grossular /gross-yŭ-ler/ A white, yellow, secting at right angles and forming refer- or greenish member of the GARNET group ence squares over the face of a map. Grid of minerals, Ca Al (SiO ) . It occurs in lines are numbered, therefore each square 3 2 4 3 limestone, usually as a product of contact has unique easting and northing readings. metamorphism. It is used as a semiprecious Grids are used to enable identification of points on the map with respect to other gemstone. points, or distances between points. (In the grohs-vet-er-lah-gĕn diagram the junction of stream Y and river Grosswetterlagen / / X has a grid reference of 545 315). They (large-scale weather patterns) A concept are also used to determine the accuracy of devised by F. Baur to identify the major a map: by knowing the true position of a trends in atmospheric events over a partic- point in relation to the grid and the posi- ular area. It has subsequently been modi- tion as shown on a map the discrepancy be- fied and became used in Germany as a tween the two can be determined. method of assisting long-range weather forecasting. In the classification the main gridiron drainage A type of drainage anticyclone or depression center is located system characterized by many parallel and this is used to predict the likely wind streams that each have tributaries running and weather systems. at right angles to them, resulting in a rectangular network of drainage channels. See ground frost In Britain, the outcome of also trellis drainage. a fall in temperature below 0°C, measured by a thermometer at ground level on a grid zone The Earth’s surface can be ar- grass surface. In climatological statistics, bitrarily divided into grid zones, which are note is made of whether a ground frost oc- areas of different grid systems, each with curred, not its intensity. Data prior to Jan- its own origin. Grid zones enable the iden- uary 1 1961 took 30°F as the base for tification of points on the Earth’s surface ground frost.

165 ground ice ground ice Ice that is locked within the growing season The period of the year pores of the soil and rock fragments in the during which plant growth can proceed REGOLITH. It represents frozen GROUNDWA- without temperature restriction. By tem- TER. perature latitude convention, a daily mean temperature at screen level of 6°C is the groundmass The finer-grained bulk of critical lower limit; the growing season can crystalline or glassy material that com- then be determined from the monthly vari- poses an igneous rock and into which the ation of mean temperature. It is not a very coarser components (PHENOCRYSTS, XENO- precise concept because a mean tempera- ° LITHS) are set. ture of 6 C could be produced by a variety of diurnal ranges, and different plants re- ground moraine A sheet of TILL, usu- spond to different critical temperatures. ally some tens of meters thick, with a surface characterized by very low relief. groyne See groin. Ground moraine was deposited from the base of an ice sheet or glacier on the melt- grumusol /groo-mŭ-sôl/ See tropical ing of the ice, and forms a cover that tends black soil. to mask former bedrock features, because where hollows formerly existed the great- grunerite /groo-ner-ÿt/ A monoclinic est thicknesses of till were deposited. AMPHIBOLE. ground swell 1. Swell waves as they gryke /grÿk/ (grike) See limestone pave- enter shallow water, i.e. water with a depth ment. approximately half that of the wavelength. guano /gwah-noh/ Seabird excrement This causes the swell waves to decrease in found in large amounts on certain of the length and increase in height. In this sense, Pacific Islands and along part of the coast the waves are beginning to be influenced by of South America. It is of great economic the ground (seabed). importance since, being rich in nitrogenous 2. Swell waves in deep water that possess matter and phosphates, it serves as a valu- considerable height and length. able land fertilizer. groundwater Water precipitated from guide fossil A fossil of known age that, the atmosphere that has percolated into the because of its location, helps to establish ground and become trapped within pores, the age of the stratum in which it is found. cracks, and fissures. Its presence is essential for practically all weathering processes and Gulf Stream The largest and most im- for this reason more weathering takes portant permanent ocean current in the N place in well-jointed rocks than in massive hemisphere. It is most prominent in its types. In the upper layers of permeable southern portion, beyond the meeting- rock, groundwater can flow through the point of the FLORIDA CURRENT and the An- strata quite easily; however, at depth a tilles Current. In this region, it attains its level is reached below which all pore spaces greatest depth, probably some 1000 m, are water-filled (the water table). Weather- and achieves a maximum width of some ing still occurs below the water table, in the 50–70 km. Farther north, it becomes less form of reduction and hydrolysis. effective at depth, while to the south of the Newfoundland Banks it begins to split into group A division in the hierarchy of the several branches. It is not a very warm cur- lithostratigraphic classification of bodies rent, differing only little in its thermal char- of rock (see lithostratigraphy; stratigra- acteristics from the Sargasso Sea region phy). It is formed of two or more adjacent which lies close to its path. Indeed, as it and related FORMATIONS; adjacent groups spreads out northward, it tends to mix may be aggregated to form SUPERGROUPS. with colder surrounding waters and is fed

166 gymnosperms also by the rather cold LABRADOR CURRENT. gust A sudden increase of wind speed of Where strongly developed, its sharply de- short duration. Gusts are due to mechani- fined left-hand flank contrasts markedly cal interference by the ground surface, so with its more diffuse right-hand flank, that where the surface is aerodynamically where it is common for large eddies to be rough, as in cities, gustiness is increased. thrown off. The Gulf Stream brings Conversely, winds in exposed coastal sites warmer conditions to NW Europe, includ- tend to experience fewer gusts, although ing milder winters in Britain: average an- their mean wind speed can be high. nual temperatures are some 5° to 10°C above the average for the latitude. Some Gutenberg discontinuity /goo-tĕn-berg, climate models predict that the flow of the gû-/ The boundary between the Earth’s Gulf Stream will slow down if greenhouse core and mantle, at a depth of about 2900 gas emissions continue to increase with re- km, at which seismic primary (P) waves sultant GLOBAL WARMING. Studies, includ- slow down and secondary (S) waves disap- ing one that has shown a 30% reduction pear. The behavior of the waves is thought between 1957 and 2005 in the warm cur- to be caused by the change from solid rock rents that carry water north from the Gulf in the mantle to molten rock in the core. Stream, appear to support these predic- The discontinuity is named for the Ger- tions. Such a slow down may lead to cooler man–American geologist Beno Gutenberg temperatures in NW Europe. See also (1889–1960). North Atlantic Drift. guyot /gÿ-ŏt/ A relatively smooth flat- gully A narrow channel on a hillside topped SEAMOUNT or tablemount. Guyots lacking vegetation, generally formed by the are best known in the Pacific and Atlantic rapid runoff of surface water following Oceans. Their flat tops are interpreted as heavy rainfall. A gully is dry most of the being planation surfaces eroded by a com- time. See also badlands. bination of marine and subaerial processes. The tablemount is more or less conical in gully erosion A type of erosion in which shape, and the summit surface usually lies gullies develop from RILLS when sheet flow in fairly deep water. The summit depths of water becomes concentrated into dis- often range between 1000 and 2000 m; tinct channels. They originate naturally others have been sounded between 2000 near the head of the drainage pattern, but and 3500 m. Few guyots with summit they are better known as the most wide- depths less than some 1000 m have been spread and obvious signs of human in- located. Guyots may be veneered with sed- fluence on geomorphology due to the iments such as volcanic sand, gravel, and removal of vegetation cover in farming. globigerina ooze. Plowing down the slopes aids the concentration of sheet flow into gullies, which GWP See global warming potential. then erode their headwalls and advance upslope, as well as widening their valleys gymnosperms /jim-nŏ-spermz/ A group and developing tributaries. Gullies remove of primitive seed-bearing plants that differ great volumes of sediment, especially top- from the ANGIOSPERMS in having seeds that soil, and are known and feared in many are not protected by an outer covering. currently settled areas, such as central They include the conifers, cycads, and southwestern USA and southern Australia. ginkgos. Gymnosperms are known as fos- There is evidence that they were also once sils from the Carboniferous Period and widespread in Europe. It is now known flourished during the early Mesozoic. Most that the simple conservation practice of gymnosperms became extinct during the plowing parallel to contours will retard Cretaceous, presumably because of un- gully development. See also abnormal ero- successful competition with the angio- sion; arroyo. sperms.

167 gypsum gypsum A common and widespread col- spar. Gypsum is used in making plaster of orless or white mineral form of hydrated Paris and cement. calcium sulfate, CaSO4.2H2O. It crystallizes in the monoclinic system and is an gyre /jÿr/ The gross circulation of water evapotite mineral that occurs in limestones that occurs in each of the Earth’s major and shales, often associated with anhydrite ocean basins. It is caused by the convection and halite. The fine-grained massive white of warm water from the surface, the effects form is alabaster, tabular colorless crystals of prevailing winds, and the rotation of the are selenite, and fibrous crystals form satin Earth. See also ocean current.

168 H

haar /har/ A local term for advection fog applied to faults but applicable to any on the east coast of Britain. It is also called structural surface. Compare dip. sea-fret or sea-roke in NE Britain. In spring and early summer the North Sea is rela- Hadean /hay-dee-ăn/ See Precambrian. tively cold and quickly cools moist air to saturation point giving low stratus cloud. Hadley cell A cellular wind circulation Whenever winds are onshore, the low in tropical latitudes with surface winds cloud drifts inland where it is gradually dis- blowing from the subtropics at about 30° persed by heating. It is most frequent under to the Equator and winds in the upper at- anticyclonic conditions when vertical mo- mosphere blowing in the reverse direction. tion and mechanical turbulence are weak. This corresponds with the surface temperature gradient from the thermal Equator habitat The place where an organism toward the poles. This simple circulation lives, as defined by those aspects of its total was first suggested by the English meteo- environment that affect it and to which it is rologist George Hadley (1685–1768) in adapted. For example, sponges have a ben- 1735. Although it is modified by the effects thonic habitat. See also niche. of the Earth’s rotation, it is still believed to be a reasonable approximation to reality. See also general circulation of the atmos- haboob /hă-boob/ An Arabic word ap- phere. plied to any dust storm in the Sudan, occurring chiefly from May through hail Approximately spherical particles September, raised by strong winds, with- of ice, generally 5 mm or more in diameter, out reference to its origin. Many of them that fall from cumulonimbus clouds. The result from the downdrafts of cumulonim- pieces of ice can be irregular in shape and bus clouds with wind speeds reaching 90 have been known to weigh up to almost 1 knots in extreme cases, and they are often kg, although they are normally far less. followed by heavy rain or thunderstorms. They often have a concentric structure hachures /hah-shoor, hash-oor/ Short lines drawn on a map to show the relief of an area without the use of contours. They point downhill and are thicker and closer together where the gradient is steepest. Be- dump fore cartographic techniques were sufficiently advanced to enable the construction of contour lines this method was extensively used. It can be of particular effect in quarry mountainous areas. hade /hayd/ The angle measured between the vertical plane and that of the in- cline of a structural surface, generally Hachures

169 half-life with alternating layers of clear and opaque halomorphic soil /hal-ŏ-mor-fik/ A soil ice, providing some evidence of their evo- dominated by sodium salts. There are three lution within the cloud. It is believed that main types: the SOLONCHAKS or saline soils, this structure is the result of the stone being which are dominated by chlorides and sul- transported into different parts of the fates of sodium; the SOLONETZ or alkali cloud during its development. Beginning soils, dominated by carbonates of sodium; with an ice nucleus, opaque ice will form and the leached or degraded SOLOD. There around it when it is in the colder parts of are also many arid zone soils subject to the cloud. Small cloud droplets freeze some SALINIZATION. These halomorphic rapidly to produce a spherical aggregation soils are not true INTRAZONAL SOILS, but of ice with many air enclosures to give the tend to occur in certain climates, e.g. the characteristic white and opaque appear- arid to semiarid transition zone; they are ance. In the lower parts of the cloud with often related to the beds of former lakes or temperatures only slightly cooler than 0°C, seas, such as the area north of the Caspian the relatively larger droplets there tend to Sea, or to depressed areas where a water spread over the hail surface before freezing table is near enough to the surface to feed so that little air is trapped and the ice is salts upward by CAPILLARITY. In highly irri- transparent. The distribution of hail in re- gated but poorly drained areas, e.g. the San lation to the main path of the storm will Joaquin Valley of California, the water depend upon the nature of updrafts and the table may be artificially raised by irriga- cloud structure. tion, leading to salinization of formerly fer- Hailstorms occur most frequently in the tile soils through increased capillary rise of continental interiors of temperate latitudes water bringing up dissolved salts. and decrease toward the poles and Equa- tor, and over the sea. This is because con- hammada /hah-mah-dă/ (hamada)A ditions most favorable to cumulonimbus type of arid desert plain, consisting of an development are found here (intense sur- extensive almost bare rock surface, espe- face heating, sufficient concentrations of cially in the Sahara. See also erg; reg. cloud water, and strong thermal gradients). hanging valley A tributary valley in a glaciated area, the gradient of which be- half-life The time taken for one half of a comes much steeper on entering the main sample of radioactive element to decay (to valley. A large glacier will cause greater another element). It has a constant value deepening of a preexisting valley than will for any particular radioisotope, and its a smaller glacier, and therefore where glac- measurement is the basis of RADIOMETRIC iers invade a major valley and its tribu- DATING. taries the major valley will be deepened to a greater extent. On the disappearance of halide /hal-ÿd, hay-lÿd/ A compound (a the ice, the floor of the major valley will be salt) whose negatively charged element is at a lower altitude than those of the tribu- one of the halogens: fluorine, chlorine, taries, which will hang above the main val- bromine, or iodine. The halides are thus ley. Subsequent rivers will reach the main fluorides, chlorides, bromides, or iodides. valley from these hanging valleys as waterfalls. halite (rock salt) The common mineral form of common salt, sodium chloride, hanging wall The surface of rock above NaCl. It may be colorless or white, and is a fault plane or ore body. Compare foot often colored pink by impurities. It crystal- wall. lizes in the cubic system and occurs in underground evaporite deposits, derived harbor A sheltered stretch of water from ancient seawater. It is used as a where ships may anchor or tie up to buoys source of sodium compounds and chlorine. or jetties, in order to be safe from storms or See also salt dome. other adverse conditions, or to load or un-

170 heat balance load their cargoes. Some are formed by its extent with the seasons, occasionally natural features, including sheltered bays reaching the Gulf of Guinea in January but and estuaries; others are created by the rarely south of 15°N in July. construction of jetties, breakwaters, moles, and other artificial structures. Artificial harmonic analysis See Fourier analy- harbors are usually built, sometimes after sis. careful scale hydraulic model tests, so as to minimize wave action, SEICHE action, and harzburgite /harts-berg-ÿt/ An ultra- undesirable currents. Many harbors, both mafic rock consisting largely of olivine and natural and artificial, have to be dredged orthopyroxene. (see dredging) in order to maintain or to improve limiting depths. hastingsite /hayss-tingz-ÿt/ A monoclinic AMPHIBOLE. hardness See Mohs’ scale. haüyne /haw-ween/ A member of the so- hardpan A layer of hard material just dalite subgroup of FELDSPATHOIDS. below the surface of the ground, usually deposits of carbonates, hydroxides, oxides, Hawaiian See volcano. silica or, most often, clay. It may also contain some organic material. It is common in hawaiite /hă-wÿ-ee-ÿt/ A type of alkali sandstone regions and is impervious to basalt. See trachybasalt. water, thus preventing good drainage. Marshy patches and puddles may form on hazard A natural or man-made event or the surface after rain. condition that has the potential to cause harm or loss to humans and/or damage to hard water Natural water that contains the natural and built environment. dissolved ions of calcium and magnesium. These ions form a scum with soap. Water haze Atmospheric obscurity due to with temporary hardness contains dis- minute suspended solid matter, such as solved calcium hydrogencarbonate, which dust or smoke particles, in the sky. It is not is formed by the action of dissolved carbon a very precise term. See also fog; mist. dioxide on chalk or limestone. When temporarily hard water is boiled, calcium carbonate precipitates out, which forms head See coombe rock. limescale on boilers, pipework, and kettles. Water with permanent hardness generally headward erosion (headwater erosion) contains dissolved calcium sulfate or cal- The increase in length of a gully or valley, cium fluoride. These are not removed by or a stream within it, caused by erosion at boiling. the upper end (head) of the valley. harmatome /har-mă-tohm/ A white or heat The form of energy to which all gray hydrated barium aluminosilicate, other forms will eventually revert. Heat can be transferred by conduction, convec- Ba(Al2Si6O16).6H2O. It occurs in basic igneous rocks and is a member of the ZEOLITE tion, or radiation and may lead to an in- group of minerals. crease of temperature, but it can cause a change of state from liquid to vapor with- harmattan /har-mă-tan/ A dry wind out any corresponding alteration in tem- blowing from between north and east over perature. Heat is normally measured in West Africa. Because the air has had a long joules, although the former unit, the calo- trajectory across the Sahara, it is very dry, rie, is still sometimes used. cool by night but warm by day, and is laden with dust. It represents the normal heat balance The static ENERGY BAL- dry season state of the area, oscillating in ANCE between the Earth and atmosphere. It

171 heat island indicates the utilization of net radiation by result from the alteration of iron carbon- evaporation, sensible heat, and advection. ates and silicates in sedimentary rocks. heat island (urban heat island)An hemimorphite /hem-ă-mor-fÿt/ (calamine) urban area that has a higher temperature A mineral form of hydrated zinc silicate, than its rural surroundings: on calm clear Zn4Si2O7(OH)2.H2O. It crystallizes in the nights excesses of up to 10°C have been orthorhombic system and generally occurs recorded. They are the result of the differ- as fibrous crusts or masses. It is used as a ent nature of the surfaces in a city, which in source of zinc. turn affects the heat balance and airflow. See also urban climate. herbivore /her-bă-vor, -vohr/ An animal that eats green plants or algae. In ecologi- heat low See thermal low. cal terms, it is a primary CONSUMER and is generally the source of food for secondary heave 1. The horizontal displacement consumers (carnivores). See also food along a fault. chain. 2. See frost heaving. Hercynian orogeny /her-sin-ee-ăn/A hectare /hek-tair/ (ha) A metric unit of phase in the VARISCAN orogeny affecting area equal to 10 000 m2 (100 ares), and Europe and characterized by a northwest equivalent to 2.47 acres. fold trend. The term is used without time significance and covers the Carboniferous hedenbergite /hed-ĕn-berg-ÿt/ A mono- and Permian. clinic PYROXENE. hercynite /her-să-nÿt/ See spinel. helical flow (helicoidal flow) The most significant type of turbulence in streams. Superimposed on the primary downstream flow, a secondary flow moves across the surface of the stream toward the outside of the meander bends, compensated by a redeposition verse flow along the bed toward the inside erosion of the meander bends. This gives the streamflow a net corkscrew movement, concentrating erosion on the outside of the meander bends, with deposition on the inside, causing downstream propagation of the meanders. maximum current velocity as directed by helical flow helicitic structure /hel-ă-sit-ik/ Curved Fig. 1: Distribution of maximum velocity in a meandering stream due to or S-shaped trails of inclusions occurring in helical flow porphyroblasts (see porphyroblastic), especially garnets, and representing an earlier foliation. deposition hem-ă-tÿt, hee-mă- (inside of hematite / / The min- bend) eral form of ferric iron oxide (Fe2O3), an important iron ore. It occurs in two main helical current erosion types, as a massive red botryoidal or reni- (outside of bend) form ore (kidney ore) and as metallic crys- Fig. 2: Section through the outside of a tals known as specular iron ore. Hematite meander bend often occurs as a cement in sandstones producing a red coloration. Most ore deposits Helical flow

172 hoar frost heterogeneous strain /het-ĕ-rŏ-jee-nee- high-resolution satellite imagery See ŭs/ The changes in shape or dimensions satellite. occurring when a body of rock does not deform equally in all directions. A linear hill fog Persistent low cloud that covers structure would become curved and paral- high ground, associated with low atmos- lel structures would diverge after deforma- pheric pressure. It does not affect low-lying tion. regions. See also fog. heterolithic unconformity See non- hill shading (plastic shading) A means conformity. of showing relief on maps, shading the east- and south-facing slopes (the steeper heteromorphism /het-ĕ-rŏ-mor-fik/ The the slope, the darker the shading) to give existence of rocks of very similar chemical the effect of an oblique light shining from composition but with different mineral ag- the northwest over a relief model. gregates, resulting from different rates of cooling of the magmas from which they hinge fault (pivot fault) A normal fault formed. that dies out along its trend by a gradual decrease in throw. (See diagram at FAULT.) heterotrophic /het-ĕ-rŏ-troff-ik/ Describ- hinge line 1. (in folding) An imaginary ing an organism that is a CONSUMER, i.e. one that feeds on plants or other organisms line joining points of maximum curvature because it cannot synthesize its own food of the folded strata. 2. The boundary between a stable region of (unlike AUTOTROPHIC organisms). Such or- the Earth’s crust and one undergoing ganisms include some algae and BACTERIA, changes in elevation, generally as a result most protozoa, and all fungi and animals. of isostatic readjustment but also including areas of tectonic deformation. heulandite /hyoo-lănd-ÿt/ A white, gray, or brown hydrated calcium sodium alu- histosol /hiss-tŏ-sôl/ One of the twelve minum silicate, (Ca,Na )Al Si O .6H O. 2 2 7 18 2 soil orders of the US SOIL TAXONOMY, which It crystallizes in the monoclinic system and includes bog soils such as peat. Histosols occurs in basic igneous rocks. It is a mem- are characterized by accumulations of or- ber of the ZEOLITE group of minerals. ganic matter, which remains more or less undecomposed because of the waterlogged /heks-ag-ŏ-năl/ See crystal hexagonal conditions. The histic EPIPEDON (surface system. horizon) has at least 20% of its weight in organic matter, or over 30% if half the hiatus A break in a stratigraphic se- weight of the horizon is clay. Histosols are quence, either as a result of nondeposition divided into four suborders on the basis of or erosion. It represents the period of time the degree of decomposition of the organic missing between beds above and below an matter. unconformity. Although found throughout the world, histosols occupy the smallest area of all the high See anticyclone. twelve orders of the US SOIL TAXONOMY (under 1% of all soils). They tend to de- high index circulation (in meteorol- velop particularly in cooler, poorly ogy) The phase of the INDEX CYCLE when drained, more humid areas. the westerly circulation reaches its maximum intensity in specified areas. This rep- hoar frost A silvery-white deposit of ice resents a strong surface westerly flow with crystals formed on surfaces cooled below Rossby waves of long wavelength and freezing point by radiation. It may result small amplitude. Meridional exchange is at from the freezing of dew or by the direct a minimum value. sublimation of ice crystals from water

173 hodograph vapor in the atmosphere when tempera- holocrystalline /hol-ŏ-kris-tă-lin,-lÿn/ tures are less than 0°C. Describing a rock composed wholly of crystals.

360° holozoic /hol-ŏ-zoh-ik/ Describing an 340° 030° organism that feeds on complex organic 320° 050° matter (as opposed to taking in simple sol- 300° 70 uble materials). The term thus includes 60 knots most animals. 40 20 270° 090° homeomorphism /hoh-mee-ŏ-mor-fik/ The existence of crystals that have the same form and habit but different chemical com- 120° position. See crystal form; crystal habit. 230° 140° 210° 160° homeomorphy /hoh-mee-ŏ-mor-fee/ Sim- 180° ilarity in structure between organisms that are not closely related as a result of evolu- Hodograph tionary adaptation to similar habitats (see convergent evolution). The term is often restricted to similarities between more hodograph /hod-ŏ-graf, -grahf/ A dia- closely related species, in which homeo- gram used to analyze the wind field above morphy leads to confusions in taxonomy an observing station. Winds at different (see parallel evolution). pressure levels are drawn as vectors from the origin and so give an indication of the homocline /hoh-mŏ-klÿn/ A series of changes in wind speed and direction with beds of rock all of which have a similar dip height. It is then possible to make deduc- and strike, including monoclinal structures tions about the thermal patterns at these and isoclinally folded beds. The term is ap- upper levels and about the advection of plied only to small areas of the Earth’s warm or cold air. crust. hogback A steep-sided narrow ridge homogeneous strain /hom-ŏ-jee-nee-ŭs/ that stands above the surrounding plain. It The changes in shape and dimensions that generally results from the folding or fault- occur when a body of rock deforms equally ing of strata, followed by differential ero- in all directions. It results in straight-line sion that removes softer rocks from and planar structures. alongside it. homolographic projection /hom-ŏ-lŏ- Holocene /hol-ŏ-seen/ (Recent) The pre- graf-ik/A MAP PROJECTION possessing the sent epoch, covering the last 10 000 years property of equal area, i.e. the area or so of geologic time from the end of the bounded by adjoining meridians and paral- PLEISTOCENE. In Britain it follows all the lels on the map is equal in area, by ratio, to major Pleistocene glacial episodes and is that area bounded by the same meridians therefore often known as the Post-Glacial. and parallels on the ground, e.g. MOLLWEI- However, some authorities consider it to DE’S PROJECTION and BONNE’S PROJECTION. be no more than an interglacial phase of the Pleistocene. At the beginning of homotaxis /hom-ŏ-taks-iss, hoh-mŏ-/ the Holocene, Britain was isolated from The occurrence of divisions of rock occu- the rest of the continent of Europe by a pying the same position in the stratigraphic general rise in sea level. Descendants of sequence in separate successions. Such di- most fossil species of this epoch are still visions are said to be homotaxial. Al- alive today. though the divisions may be of the same

174 hot spot age, the concept does not take account of hornfels /horn-fels/ A fine- to medium- considerations of time and simply reflects grained nonfoliated rock composed of the order of deposition. Compare chrono- equidimensional mineral grains showing taxis. See also correlation (def. 2). no preferred orientation. Porphyroblasts of minerals such as biotite, andalusite, and honeycomb weathering Weathering cordierite may occur (see porphyroblastic) within jointed rocks in which joint infill- and their incipient development is often in- ings are more resistant to weathering than dicated by a spotted appearance to the the main mass of the rock, so that after ero- rock. Hornfels are the product of contact sion they project and surround recesses in metamorphism and are found in aureoles the rock. Typical infilling materials are bordering intrusive plutonic igneous rocks. iron oxides, secondary silica, calcite, and See also contact metamorphism. manganese oxides. Migration of rock cements into the joints may produce a corre- hornito /hor-nee-toh/ A small volcanic sponding decrease in the resistance of the structure resulting from the accumulation rock itself, thereby increasing the effective- of small blebs of lava thrown out around a ness of honeycomb weathering. parasitic vent; a spatter cone.

Hooke’s law A law stating that a hornstone A very fine-grained volcanic stressed body deforms to an extent that is ash. See pyroclastic rock. proportional to the force applied. Ma- terials that obey Hooke’s law are said to be horse latitudes The zone of light and elastic. The law is named for the English variable winds between 30° and 40° N and physicist Robert Hooke (1635–1703). S where the subtropical anticyclones are dominant. The name arose during the days horizon 1. (in soil science) One of a of sailing ships when unfavorable winds number of layers of soil arranged in a ver- could prolong the journey; when food be- tical sequence in the PROFILE. Each layer is came short, horses would be thrown over- reasonably uniform. Most soil-forming board or eaten. processes result in this layered arrangement but some, e.g. the action of earth- horst An elongated block of rocks worms, are destructive in this respect. bounded by faults. The sense of movement 2. (in stratigraphy) A plane within a rock of the block is upward relative to the sur- sequence that is assumed to be a time rounding rocks. Compare graben. (See dia- plane. In practice, a horizon is not a sharp gram at FAULT.) plane but a thin bed characterized by some distinctive feature of lithology or fossil hot spot An area on the Earth’s surface content. A particular horizon is sometimes where there is high thermal activity, often merely theoretical, and it may be regarded in the form of volcanoes. Hot spots are as extending through a contemporaneous thought to represent the surface manifesta- succession that carries no internal evidence tion of MANTLE PLUMES, which may origi- of its presence. nate deep withn the Earth. Most volcanic activity occurs on or near PLATE BOUND- horizontal equivalent The distance be- ARIES, e.g. the hot spot of Iceland is located tween two points on a slope projected onto on a constructive plate boundary, but there a horizontal plane, as they would be repre- are exceptions. The islands of Hawaii, for sented on a map. example, are of volcanic origin but located over 3200 km from the nearest plate horn (horn peak) See pyramidal peak. boundary. In such cases it has been hypothesized that the hot spot is virtually sta- hornblende /horn-blend/ A monoclinic tionary and successive volcanic eruptions mineral of the AMPHIBOLE group, common produce a trail of volcanoes that become in both igneous and metamorphic rocks. progressively older away from the hot spot

175 hot spring as the lithospheric plate passes above it Spoil heaps, railroad cuttings, and the (e.g. the Hawaiian Ridge–Emperor blowing out of hillsides to build towns are Seamounts chain). A hot spot below the other deliberate results. Sometimes human North American plate is located at Yellow- action can produce quite unexpected re- stone National Park in the USA. sults, especially in the coastal zone where the erection of groins, harbor arms, and hot spring See thermal spring. other artificial structures severs the littoral drift leading to the decay or growth of Hudson canyon One of numerous sub- beaches in quite unnatural situations. marine canyons that cut into or across the Large-scale recharge schemes may be used wide continental shelf lying off the E coast to overcome these problems, with convoys of the USA. It is one of the best-known sub- of trucks shuttling beach materials that can marine canyons in the world and has been no longer be moved by natural agencies. carefully surveyed several times. It cuts See also arroyo. well into the shelf opposite the Hudson River and has been traced seaward as far as Humboldt Current /hum-bohlt/ (Peru a wide submarine fan. The cross section of Current) A current of cold ocean water the canyon tends to be V-shaped, and that flows from the Antarctic northward flanking it are what appear to be small trib- along the western coast of South America. utary valleys. Core samples have yielded a It veers westward as it reaches the Equator. variety of deposits (rock, gravel, and clay), It cools winds blowing onto the coast from certain of which suggest that some sort of the Pacific Ocean, reducing rainfall on the powerful turbidity flow may have oper- western slopes of the Andes. The current is ated, perhaps when sea level was signifi- named for the German explorer and scien- cantly lower. tist Baron Alexander von Humboldt (1769–1859). See also El Niño. human influence on geomorphology In the 8000 years of the Holocene geologic humic acid /hyoo-mik/ An acid derived period during which humans have become from resynthesized organic materials that established, their activities have had con- dominate the soil humus along with the siderable geomorphological impact, princi- fulvic acids. Humic acids are polysaccha- pally in accelerating existing processes but rides and contain more carbon and nitro- to a lesser extent in creating new land- gen and less oxygen than fulvic acids. forms. The major effects are those conse- Fulvic acids tend to be more important in quent on destruction and removal of the newly formed humus but eventually they vegetation cover. Clearing the land for are overtaken by the humic acids. agriculture can cause gullying and soil erosion, leading to destruction of valuable humidity The amount of water vapor in farmland, and wind erosion of bare soil the atmosphere. It can be measured or cal- can create DUST BOWLS (see also badlands). culated in a variety of ways. The most fre- In coastal areas, human use of sand dunes quently used measure is the RELATIVE leads to vegetation destruction and cre- HUMIDITY, which is the mass of water vapor ation of BLOWOUTS. River basins can have in a given volume of air expressed as a per- higher sediment yields than those not so centage of the mass of water vapor in an much used, owing to the increased erosion equal volume of saturated air at the same from farming, etc. temperature and pressure. As its name im- Landforms created by human action in- plies, this is only a relative index and it clude lakes resulting from subsidence of varies in opposite phase to temperature, the land surface in mining areas, the Nor- even though the absolute amount of mois- folk Broads, created by peat-cutting in me- ture in the air may not change. A better sys- dieval times, and reclaimed land from the tem from the meteorological point of view sea, severely changing the coastline, as is the use of VAPOR PRESSURE, which indi- with the Zuider Zee in the Netherlands. cates the water vapor content of the at-

176 hydration mosphere irrespective of temperature. Yet The 2005 Atlantic hurricane season another measure is the SPECIFIC HUMIDITY – was the most active on record with 14 hur- the mass of water vapour per unit mass of ricanes, 7 of which became major hurri- air. The ABSOLUTE HUMIDITY is the mass of canes of category 3 or higher on the Saffir– water vapor per unit volume of air, given in Simpson Hurricane Scale. These included grams per cubic meter. hurricane Katrina, one of the most devastating hurricanes in US history, which humidity mixing ratio See mixing caused considerable flooding and damage ratio. over the New Orleans area and along the Mississippi Gulf coast, and an estimated humification /hyoo-mă-fă-kay-shŏn/ The 1200 deaths. The average number of hurri- formation of HUMUS from organic matter. canes in a season is around 6. 2. In the BEAUFORT SCALE, wind speeds humite /hyoo-mÿt/ A member of a group above 64 knots, whether or not they are of orthorhombic and monoclinic minerals linked to a tropical cyclone. Such speeds rarely occur over land. consisting of layers of Mg2SiO4, which have an olivine structure, alternating with Huygens’ principle /hÿ-gĕnz, how-/A layers of composition Mg(OH,F)2. Humite minerals are found in metamorphosed principle stating that each point of a wave limestones. front acts as a source of secondary wavelets. It was proposed in 1690 by the hummock A mound or knoll that rises Dutch physicist and astronomer Christiaan above a generally level surface, or a small Huygens (1629–95). mound of soil or turf in alpine areas (see /hÿ-ă-lin, -lÿn/ Describing a rock earth hummock). hyaline that is glassy. humus Amorphous colloidal material in hyaloclastic rock /hÿ-ă-lŏ-klass-tik/ See the soil, dark in color and composed of re- palagonite. sistant plant tissues, such as lignin, and new compounds, such as polysaccharides, hyalocrystalline /hÿ-ă-lŏ-kris-tă-lin, - synthesized by microorganisms. It is im- lÿn/ Describing a rock composed of both portant to the soil both physically and crystals and glass. chemically (its cation exchange capacity far exceeds that of the clays). See also hybridization /hÿ-bri-dă-zay-shŏn/ See moder humus; mor humus; mull humus. assimilation. hurricane 1. A TROPICAL CYCLONE with hydrate 1. A chemical compound that surface wind speeds in excess of 64 knots has water in its composition (as water of (117 km per hour) that occurs in the crystallization). Caribbean and W Atlantic, the Gulf of 2. A chemical compound produced by HY- Mexico, and in the E and central North Pa- DRATION. cific east of the dateline. They are identical to similar tropical cyclones in the W Pa- hydration The chemical addition of cific, Indian Ocean, and near Australia, water to a substance. If the substance is a which have their own local names (see cy- mineral, it usually involves a fairly consid- clone; typhoon). Hurricanes are classified erable expansion in the mineral grains. according to their strength on the SAFFIR– This may be important in producing subse- SIMPSON HURRICANE SCALE. At the center of quent mechanical weathering, in the form a hurricane is an area of light winds and of EXFOLIATION and GRANULAR DISINTEGRA- higher temperature known as the EYE, TION. Minerals affected by hydration are about which clouds and rain bands spiral prepared for further chemical weathering with the associated winds. processes, such as OXIDATION and CARBON-

177 hydraulic action

ATION, while the process is also very im- that was formed by chemical precipitation portant in the formation of CLAY MINERALS. from water, such as gypsum or halite. hydraulic action (hydraulicking) (in hydrologic cycle /hÿ-drŏ-loj-ik/ (water geomorphology) The removal of loose, in- cycle) The continuous circulation of coherent, or weathered material by flowing water between the oceans, atmosphere, water, assisting in stream-bed and bank and land. Water evaporates from the erosion. Material carried can range from oceans (and lakes and rivers) as water single minerals or aggregates of crystals to vapor in the atmosphere, where it may con- pieces of layered bed strata, e.g. slate or dense into clouds. Clouds release precipita- shale, if the materials dip downstream. See tion (rain, snow, or hail), which falls on the abrasion. land. Some evaporates, some is taken up by plants and released into the atmosphere hydraulic jump A sudden increase in when they transpire, and some runs off to mean water level in the direction of flow, form streams and rivers that flow to the which is of a rapidly varying type and leads oceans. Any that penetrates the ground to a form of stationary wave. In the sim- forms GROUNDWATER. plest terms, a surface flow at high velocity confines a deeper flow, thereby resulting in hydrology The scientific study of the the dissipation of energy through turbu- waters of the Earth on and within the lence. ground. It focuses on those aspects of the hydrologic cycle that occur close to the hydrograph A chart or graph that land surface: precipitation, evaporation, evapotranspiration, runoff, soil moisture, shows changes in a particular variable (e.g. snow and ice accumulations, and ground- discharge, stage (water level), or water water. temperature) over time for a body of water such as a stream, lake, reservoir, or bore- hydrolysis /hÿ-drol-ă-sis/ A chemical re- hole. For example, a stream discharge hy- action involving water. In geology, it is a drograph shows changes in the discharge form of chemical weathering involving a of a stream at a particular point over time; reaction between the H and OH ions a storm hydrograph shows the total dis- formed by the decomposition of water and charge at a point for a particular rain the ions of rock minerals; it occurs wher- event. ever rocks and water are in contact. Hy- drolysis is a particularly important process hÿ-drŏ-graf-ik hydrographic chart / /A in the weathering of FELDSPARS; when water map of the seabed, showing depths of comes into contact with orthoclase water, heights of underwater features, and, feldspar, potassium hydroxide and alu- sometimes, geologic information. See hy- mino-silicic acid are produced. These react drography (def. 2). with atmospheric carbon dioxide giving potassium carbonate and water, while the hydrography /hÿ-drog-ră-fee/ 1. The acid breaks down into clay minerals and study of the oceans, seas, rivers, and other easily removable soluble colloidal silica. water bodies, together with the strips of Kaolin (china clay) is produced by weath- land bordering these. It involves descrip- ering in this way. tion and measurement and the subsequent presentation of this information on hydro- hydromorphic soil /hÿ-drŏ-mor-fik/A graphic charts. type of soil found in a wide range of envi- 2. The shape of the sea floor and the de- ronments where drainage is poor, e.g. allu- posits of which it is composed, including vial flats, and therefore such soils are navigational information. classified as intrazonal. Reduction is a more dominant process than oxidation, re- hydrolith /hÿ-drŏ-lith/ A type of rock sulting in a blue/gray subsoil, with

178 hyetograph red/brown mottles. The most important 300–500°C at considerable depth and in- soil of this group is gley but also included cluding cassiterite, wolfram, and molyb- are peaty gley podzols, peaty gleys, denite veins; mesothermal deposits meadow soils, planosols, groundwater including sulfides of iron, lead, copper, and podzols, and groundwater latosols. With zinc and formed at temperatures of increasingly poor drainage these soils 200–300°C; epithermal deposits formed at grade into the organic soils. the low temperatures of 50–200°C and including stibnite, cinnabar, sulfur, silver, hydrosphere /hÿ-drŏ-sfeer/ All the wa- and gold. See also pneumatolysis. ters of the Earth, as opposed to gases (atmosphere), rocks (lithosphere) and living hydrothermal vent (smoker) An active organisms (biosphere). In all, water covers volcanic vent on the deep-ocean floor that about 74% of the Earth’s surface and has a emits hydrothermal fluids at high pressure total mass of 1021 kg. and with temperatures of around 300°C. The vents frequently contain dissolved sul- hydrostatic equation /hÿ-drŏ-stat-ik/ fides, e.g. of copper, iron, manganese, and An equation relating the variation of pres- zinc, which give the water a black color sure with height to the density of air and and these are often known as black smok- the force of gravity: dp/dz = –ρg, where ers. Vents may also be white, gray, or clear dp/dz is the rate of change of pressure with according to the minerals ejected; white height, ρ is the density of air, and g is the smokers are those releasing minerals such acceleration of free fall. It is a useful ap- as barytes and silica. Sometimes a tall ver- proximation when considering horizontal tical chimney of deposited minerals forms movement relative to the Earth and is used around the site of the vent. Hydrothermal as one of the basic equations in models of vents were first discovered in 1977 at the GENERAL CIRCULATION OF THE ATMOS- depths of 2.5 km on the Galapagos Rift PHERE. (a MID-OCEAN RIDGE) off the coast of Ecua- dor and more have since been discovered hydrostatic pressure 1. Pressure that is at other sites along mid-ocean ridges. exerted by a liquid, often water. In a body The vents support unique ecosystems: the of water, the pressure increases with depth. dissolved sulfides are oxidized by chemo- 2. (in geology) The pressure exerted on synthetic bacteria to form organic com- rock in the Earth’s crust by the rocks that pounds. Some of the animals around the overlie it. It is also termed confining pres- vents actually harbor the bacteria within sure. their body tissue in a symbiotic relationship, while others feed on the organic com- hydrothermal process /hÿ-drŏ-th’er-măl/ pounds. The abundant sea life includes Following the pegmatitic stage during the giant tube worms, giant clams, mussels, crystallization of an igneous melt is the hy- fish, and crabs. drothermal stage, when the residual fluid is a relatively low-temperature aqueous solu- Hydrozoa /hÿ-drŏ-zoh-ă/ A class of the tion. Such a fluid may effect considerable phylum CNIDARIA. As few hydrozoans pos- alteration of the crystallized portion of the sess hard parts they are not common as magma by such processes as saussuritiza- fossils, although the Paleozoic STROMATO- tion, kaolinization, and the alteration of POROIDS may belong to this class. mafic minerals to serpentine and chlorite. Many mineral deposits are formed by hyetograph /hÿ-ĕ-tŏ-graf, -grahf/ 1. A precipitation from hydrothermal solutions. chart that shows the distribution of precip- A characteristic hydrothermal deposit is itation with respect to time, either at a the sulfide-bearing vein filling rock fis- point or over an area. sures. Hydrothermal deposits may be di- 2. A type of rain gauge that records the vided into three groups: hypothermal amount and duration of rain as it falls. deposits formed at temperatures of Rain is collected in a cylinder with a float

179 hygrometer on the water surface. As the water level hypsithermal (altithermal) The North rises during precipitation, the float rises in American term for the period of time from proportion to the rate of rainfall and this is about 8000 to 3000 BP during the recorded by a pen on a rotating chart. Holocene when warmer climatic conditions prevailed. See also Atlantic period; hygrometer /hÿ-grom-ĕ-ter/ An instru- climatic optimum. ment that measures HUMIDITY (the water vapor content of the air). hypsographic curve /hip-sŏ-graf-ik/A curve showing the proportion of a land- hygroscopic nuclei /hÿ-grŏ-skop-ik/ At- scape lying at, above, or below particular mospheric particles that have an attraction elevations. The percentage of the landscape for water vapor and help in the initiation of between successive contours is calculated precipitation processes. from a relief map, and the results expressed either as simple proportions between dif- hygrothermograph An instrument con- ferent heights, or as a cumulative measure sisting of a combined hygrograph and ther- showing the percentage of area lying above mograph that records both humidity and each level, starting with 100% at sea level temperature on a single chart. and working up to nil at the absolute peaks. It has been used to look for devel- hypabyssal rock /hÿ-pă-biss-ăl/ See plu- opment of erosion surfaces, e.g. terraces tonic rock. and former peneplains, which appear on the curve as extensive areas at their partic- hypersthene /hÿ-per-sth’een/ An ortho- ular elevations. It is also used to display the rhombic PYROXENE. proportions of the Earth’s surface at particular elevations above and depressions hypervelocity impact /hÿ-per-vĕ-loss-ă- below sea level. tee/ The impact of an object (such as a METEORITE) that is moving so fast that the hypsometer /hip-som-ĕ-ter/ An instru- rocks it hits are not strong enough to with- ment for determining atmospheric pressure stand the shock waves created. The result is or altitude by measuring the boiling point generally a large crater, surrounded by IM- of water. The air pressure can be deter- PACT BRECCIA and other EJECTA. mined directly from the temperature at which boiling occurs, but altitude must hypidiomorphic /hÿ-pă-dÿ-ŏ-mor-fik/ then be obtained from the altimeter equa- Describing a rock consisting of EUHEDRAL tion. Accuracy is not very high: the boiling and ANHEDRAL crystals or in which the ma- point must be known to within 0.01 K to jority of crystals are SUBHEDRAL. Compare obtain the height to within 3 m. allotriomorphic; idiomorphic. hypsometric tinting /hip-sŏ-met-rik/ hypolimnion /hÿ-pŏ-lim-nee-ŏn/ The The differential coloring of elevation bands cool lower layer of water in a lake or shal- on a map to enable the user to determine low sea. Insufficient light penetrates for quickly the higher and lower areas of the photosynthesis to take place, so there are land portrayed. Greens usually portray no green plants and there is little dissolved lowland areas; as the land rises so the col- oxygen in the water. See also epilimnion. ors change from yellows to browns to purples. This technique is common in hypothermal deposit /hÿ-pŏ-th’er-măl/ small-scale maps, e.g. atlases and wall See hydrothermal process. maps.

180 I

Iapetus Ocean See proto-Atlantic assumed that further ice ages will be expe- Ocean. rienced in the future; as the true cause or causes of ice ages are not known it is im- ice The solid state of water. It melts at possible to predict when this will be. It is 0°C, requiring approximately 340 000 possible that changes in landmass altitude joules per kilogram (80 calories per gram) and variations in solar radiation are at least of ice (latent heat of fusion) or 2.8 × 106 partly responsible. J/kg (677 cal/g) if converted directly from From geologic records, it has been de- ice to vapor (latent heat of sublimation). duced that ice ages also existed in Permo- Pure water does not necessarily freeze to Carboniferous times about 250 million ° ice when the temperature falls below 0 C, years ago, one in late Precambrian times especially if it is in the form of small some 500 million years ago, and possibly droplets such as those found within clouds. several earlier. These former ice-age de- This has very important implications in the posits have even been found in present BERGERON–FINDEISEN THEORY of precipita- tropical latitudes, because the continents tion formation. Even when FREEZING NU- have since changed their relative positions. CLEI are present, droplets may still remain Between 1550 and 1850 temperatures in the liquid phase to temperatures of in much of the N hemisphere fell to their –30°C or less. By –40°C the water will lowest since the last ice age, and this period freeze spontaneously irrespective of the has been called the Little Ice Age. It was presence or absence of nuclei. characterized in northern regions by heavy At the ground surface, ice can take snowfalls and prolonged winters. Alpine many forms: rime, hail, black ice, and, on glaciers advanced and settlement in many a vast scale, glaciers. Because ice reflects solar radiation, and requires heat for melt- northern areas, such as Greenland, Iceland, ing, it is a very important aspect of the heat and N Norway, had to be abandoned. The balance in high latitudes. world’s climate, however, did not undergo any overall change. ice age A period in the Earth’s history when ice spread toward the Equator ac- iceberg A mass of ice in the sea that has companied by a general lowering of sur- moved from a land area to the coast, and face temperatures, especially in temperate from there into the sea, and which usually latitudes. The PLEISTOCENE epoch, ending exceeds 5 m in height above sea level. from about 10 000 years ago, experienced These ice masses, many of which result at least four major ice advances (see glacial from the calving off of fragments of glacier maximum), with the margin reaching ice, float partly above but largely beneath about 52°N over NW Europe and about the surface of the sea. They vary greatly in 45°N in NE America. With this change in shape one from the other: some are dome- location of ice surfaces the whole atmos- shaped, some are sloping or pinnacled, pheric circulation altered, the main cli- while others have a more tabular form. matic belts being compressed and pushed Glacier bergs display very irregular shapes. toward the Equator. At present we appear to be in an interglacial circulation but it is ice cap A dome-shaped permanent mass

181 icefall of ice such as might cover a highland area; sediment deposits that have been rafted by a small-scale ICE SHEET. ice, either icebergs or ice floes. They reached the ice from meltwater streams or icefall A section of a glacier at which the by being frozen into the base of ice that ice moves down a steep slope. Many trans- later became waterborne. These materials verse crevasses form across the glacier, and range from very large boulders to clays and there may be piled up SÉRACS at the foot of silts. They may be transported over consid- the icefall. erable distances to be deposited among material of an entirely different character. ice field 1. A general term for a large Icebergs and floes reach nearer to the area of ice, smaller than, and without the Equator in the N hemisphere than they do dome shape of, an ice cap. in the S hemisphere. Ice-rafted pebbles and 2. An extensive mass of sea ice. boulders seem to be capable of making pits or dents in soft sea-floor deposits, presum- ice floe An area of floating sea ice that ably as they fall to the sea floor, as was measures 20 m or more across and is es- shown by underwater photography in the sentially flat and tabular. They range in Arctic. size from small floes, 20 to 100 m across, to giant floes, over 10 km across. ice sheet An accumulation of ice hundreds of meters thick and covering vast ice fog A type of fog in which visibility is areas. Many developed during the PLEIS- less than 1 km as a result of ice crystals sus- TOCENE Epoch, and are represented by the pended in the air. Such fogs have increased present Greenland and Antarctic ice sheets. in frequency in recent years as human ac- The ice develops at high altitudes, spread- tivities extend to colder latitudes. When ing outward and submerging all the under- the air is very cold, only a little water is re- lying topography. Being unrestricted by quired before saturation takes place. If the rock walls, unlike CIRQUE and VALLEY temperature is below –40°C the ice crystals GLACIERS, the directions of ice movement will form directly on saturation. Normally within these ice sheets reflect the slopes of little evaporation takes place, but the release of water vapor from motor-vehicle the ice surface, which depend upon relative exhausts or power station cooling systems accumulation rates within the various can quickly saturate calm air, as often hap- parts of the sheet. pens in the Fairbanks area of Alaska. ice shelf Part of an ICE SHEET or ICE CAP Icelandic Low The mean low-pressure that spreads out over the sea, as around system of the North Atlantic. Low-pres- much of Antarctica. It is attached to the sure systems (cyclones or depressions) land and built up by accumulated snow form in the latitude of Newfoundland, and outward moving glaciers. move northeastward as they intensify, then gradually decay. Although their tracks ichor /ÿ-kor, -ker/ See granitization. vary quite considerably, on average they are most frequent and at their most intense Ichthyosauria /ik-th’ee-ŏ-sor-ee-ă/An in the Iceland area. As the circulation of the order of extinct reptiles that became secon- atmosphere varies with time, the precise lo- darily adapted to a marine life. In shape cation of the Icelandic Low changes. they were similar to dolphins (mammals): they had fishlike tails and the limbs were Iceland spar A transparent variety of modified as paddles. Ichthyosaurs were to- CALCITE of optical quality. See also carbon- tally aquatic and the young were probably ate minerals. born alive. They were prominent throughout the Mesozoic, especially in the Jurassic ice-rafting The transport of debris in Period, becoming extinct at the end of the sea ice. Widespread at high latitudes are Cretaceous. Compare Plesiosauria.

182 impact crater iddingsite /id-ingz-ÿt/ The red-brown al- augite. Accessory minerals include apatite, teration products of forsteritic olivine con- melanite, phlogopite, sphene, and calcite. sisting largely of iron oxides and clay Uncompahgrite is a melitite pyroxenite minerals. and jacupirangite consists largely of titanium-rich augite, magnetite, and biotite. idioblastic /id-ee-ŏ-blas-tik/ Describing a metamorphic rock consisting largely of illite /il-ÿt/ A micalike clay mineral, a hy- crystals having euhedral form. drated silicate of aluminum and potassium. It occurs in mudstone and shale. See also idiomorphic /id-ee-ŏ-mor-fik/ Describ- clay minerals. ing a rock in which the majority of crystals are anhedral. Compare allotriomorphic; illuviation /i-loo-vee-ay-shŏn/ The accu- hypidiomorphic. mulation of fine material in a soil, or a part of a soil, washed down from above. The il- idocrase /ÿ-dŏ-krayss, id-ŏ-/ (vesuvianite) luvial horizon is the B horizon: the degree A yellow, green, or brown tetragonal of development of the B is therefore a guide mineral which has an ideal formula to the time and effectiveness of operation Ca10(Mg,Fe)2Al4Si9O34(OH,F)4, found in of the illuvial processes. The materials in- thermally metamorphosed limestones. volved are sesquioxides, bases, clays, and colloidal organic matter. igneous rock /ig-nee-ŭs/ Any of the major rocks that have crystallized from a ilmenite /il-mĕ-nÿt/ A brown or black high-temperature molten silicate (or rarely mineral form of iron titanium oxide, carbonate) liquid or magma. Igneous rocks FeTiO3. It crystallizes in the trigonal sys- include PLUTONIC, hypabyssal, VOLCANIC, tem, and occurs as an accessory mineral as and PYROCLASTIC ROCKS. The composi- aggregates in igneous and metamorphic tional range of igneous rocks is described rocks, as well as in some mineral sands. It by the terms acid, intermediate, basic, and is a major source of titanium. ultrabasic. Classification may also be based upon the SILICA SATURATION princi- imbricate structure /im-bră-kit, -kayt/ A ple. See andesite; basalt; carbonatite; dior- series of thrust sheets all dipping in the ite; gabbro; granite; ijolite; lamprophyre; same direction and displaced by roughly layered igneous rock; rhyolite; syenite; the same amount. syenodiorite; trachyte. impact basin A large IMPACT CRATER ignimbrite /ig-nim-brÿt/ See pyroclastic with a diameter in excess of 200 km. It is rock. formed when a meteorite strikes the ground. There are usually terraced walls, a ijolite /ij-ŏ-lÿt/ A suite of strongly alka- sunken floor, and perhaps a hill at the cen- line undersaturated plutonic rocks often ter. The best examples can be seen on the associated with carbonatites. They are Moon and on Mercury. characterized by varying proportions of nepheline and clinopyroxene, an abun- impact breccia Fragments of rock pro- dance of accessory minerals, and the vir- duced when a meteorite hits the ground. tual absence of feldspar. Members of the Small pieces, including droplets of impact suite are classified in terms of COLOR INDEX melt, may fall back into the crater. Larger as follows: fragments are mixed with hot gases and urtite %t;30 (% dark minerals) flow out across the surface of the ground, ijolite 30–70 or are hurled into the air as EJECTA. The melteigite 70–90 preexisting rocks around the crater may pyroxenite >90 exhibit SHATTER CONES. Thus an ijolite may consist of about equal proportions of nepheline and aegirine– impact crater A crater formed when a

183 impermeable meteorite hits the ground. See impact within alkali basalt are often called nod- basin. ules. impermeable Describing a type of rock incompetent Describing beds of rock that is nonporous and therefore does not that deform plastically and flow when absorb water. Igneous rocks such as gran- stressed during folding, or folds in which ite are good examples (although joints in the thickness of the beds changes during the rock may allow water through, making folding. See also competence. it pervious). Clay is impermeable after it has become saturated with water. See also index contour A contour line that is ac- impervious. centuated by its width, in order to facilitate the reading of elevations on a map. For ex- impervious Describing a type of rock ample, where the contour interval is 10 m that, because it contains no cracks or fis- the 50 m and 100 m contours may be ac- sures, does not allow water to pass through centuated. it. Metamorphic rocks such as shale and slate are good examples. See also imperme- index cycle (in meteorology) A cyclic able. variation that the ZONAL INDEX is believed to undergo from a high-index circulation, inceptisol /in-sep-tă-sôl/ One of the with its strong zonal westerlies, to the low- index state, with much meridional transfer, twelve soil orders of the US SOIL TAXON- then back to the high-index phase, with a OMY, covering soils that are better developed than entisols but not so advanced in wavelength of about 28 days. However, further work has cast doubt on the validity formation as the alfisols, i.e. there is no of a true cycle. horizon of iron and aluminum accumulation. They are young soils in which the index fossil See zone fossil. horizons present have developed quickly. They are equivalent to the humic gley soils, index map Usually a small-scale map acid brown soils, and andosols of the old forming a base upon which certain infor- classification system. mation concerning larger scale maps, or other data, may be portrayed for easy ref- incised meander A meander of a stream erence. In series mapping the individual that is cut deeply into bedrock. This inci- sheet lines are drawn on a small-scale out- sion could arise from original development, because meanders can develop before a stream has completed downcutting (see ingrown meander). Alternatively, incision may result from REJUVENATION of typical floodplain meanders causing them to cut down and incise themselves into 1 their floodplain (see entrenched meander). 2 3 4 inclination See dip (defs. 1 and 2). inclusion The occurrence of one sub- 5 6 stance enclosed within another, e.g. a preexisting rock fragment caught up in a later rock (xenolith) or crystals of one mineral 7 8 contained in another. Small volumes of volatile constituents trapped during the growth of crystals are termed fluid inclusions. Inclusions of gabbro and peridotite Index map

184 ingrown meander line map and such information as sheet many factors: the amount and type of veg- numbers, dates of publication, and edition etation cover, the compactness of the sur- numbers are shown for each sheet. This en- face layers, the porosity of the underlying ables the user to find out immediately rock, and the amount of rainfall. The pres- which sheets cover any part of the area. ence of water already in the ground also affects the rate of infiltration. index mineral See zone. infiltration capacity The maximum Indian clinometer A surveying instru- rate at which rainfall enters soil and rock ment used in PLANE TABLING for determin- under a given set of conditions. The rate ing the heights of points fixed by changes over time; it tends to decrease intersection or radiation methods. The through time during a storm event. The height of the plane table and of the point rate is also dependant on factors such as over which it is set up must be known. The the soil texture, pore spaces, soil structure, clinometer is placed on the plane table and and the type of vegetation cover. an adjustable sight is used to view the points for which heights are required. This infrared radiation Electromagnetic ra- adjustable sight indicates a tangent value, diation with wavelengths in the approxi- positive for an elevation, negative for a de- mate range of 0.7 µm to 1 mm. It lies pression, which is multiplied by the dis- between the visible and microwave parts of tance between the points, measured from the spectrum and is divided into three spec- the table, to find the height difference. trum regions: near-infrared, middle-infrared, and far-infrared radiation. Indian summer Any period of warm settled weather in October or early No- infrared satellite imagery See satellite. vember in the N hemisphere. The term was first used in New England when under such infrastructure A structure produced weather conditions the Native Americans deep within the Earth’s crust under condi- made preparations for the coming winter tions of high temperature and pressure. It is by storing food and repairing tents. Most characterized by plastic folding, migma- years do have at least one spell that could tites, and granites. Overlying this area are be called an Indian Summer but it has no the less highly deformed rocks of the precise meaning. SUPERSTRUCTURE. induration /in-dew-ray-shŏn/ The hard- ingrown meander The type of INCISED ening of porous rocks or of soils through MEANDER that develops from origin. If a the deposition of minerals, which act as a winding course develops in a stream before cement, on or within the surface layers. it has completed downcutting, it will me- Chemical weathering usually causes the ander and cut down simultaneously, low- weakening of rocks through the removal of ering its course into bedrock. As with elements in solution. Where strong evapo- floodplain meanders, lateral erosion on the ration occurs, as for instance in the semi- outside of meander bends, complemented arid regions, these elements are brought to by deposition on the inside of bends, pro- the surface by capillarity, where they combine to cement the soil or weathered rock. There are three main types of induration rock forms. The cement in calcrete consists surface of bedrock mainly of calcite (CaCO3), that in ferricrete is of hematite (Fe2O3), while silcrete is ce- undercut slip-off slope mented by silica (SiO2). stream slope infiltration The movement of water downward into the ground. It is affected by Cross-section through an ingrown meander

185 inland basin duces an asymmetric cross profile of the inner core The innermost part of the valley and downstream migration of the Earth, believed on geophysical evidence to meanders. Compare entrenched meander. consist of solid iron and nickel and separated from the OUTER CORE at a depth of inland basin (bolson) A closed depres- 5000 km. It has a diameter of 2600 km. sion within an upland desert region toward which material is moved whenever rain Insecta /in-sek-tă/ The largest class of falls in sufficient amounts. The material is the phylum ARTHROPODA, comprising the moved both down wadis and as sheet wash only invertebrate animals adapted for on broad slopes. The finest grains and ma- flight. They are air-breathing, have three terial in solution are moved farthest, so pairs of legs in the adult, and all but the that there is a gradation from vast accumu- most primitive have one or two pairs of lations of unsorted material at the basin wings. This highly successful and diverse margin, through sand and loam, to clay group occupies an enormous range of habi- and salts in the center. Many of the mineral tats. Many fossil species are known, de- grains also exhibit salt or calcareous coat- spite the rarity of suitable environments for ings. The basins have a very flat surface preservation. Fossil insects have been when dry, although extensive cracking found in the Devonian, and some Car- may be produced by the heat. Salt lakes or boniferous dragonflies attained a wing salt marshes may form during periods of span of nearly 80 cm. rain. See also playa. Insectivora /in-sek-tiv-ŏ-ră/ The order inland sea An extensive water body of the class MAMMALIA that includes the lying in the interior of a landmass, often re- shrews, hedgehogs, and moles. Modern in- mote from the ocean. Inland seas may or sectivores are small and usually very un- may not have access to the open coast; specialized in their anatomy and feeding where they do have access it is usually re- habits. In these primitive features they restricted to a single or a few narrow sea pas- semble the first Mesozoic placental mam- sages. Inland seas display a large depth mals, which are generally regarded as early range. Those with depths generally less insectivores and the ancestors of most than 250 m are called shallow inland (or modern placental groups. epeiric) seas, e.g. the Baltic Sea. Far greater depths are encountered in the deep inland inselberg /in-sĕl-berg/ A large prominent seas, for example the Mediterranean, residual hill of hard rock, characterized by where depths range from 2000 to 5000 m. very steep slopes rising abruptly from the Very large lakes have some of the charac- surrounding surface of low relief. Insel- teristics of inland seas. All inland seas and bergs were formerly thought to be semiarid lakes experience wave action, SEICHE move- landforms, but it is now widely believed ments, and wind-induced flow. Waves on a that they are more typical of humid tropi- large lake such as Lake Kariba can be more cal climates. Some scientists believe the than 3 m high, and very much larger waves main reason for their occurrence is differ- affect some inland sea areas. The shores ential DEEP WEATHERING followed by subse- display many of the morphological forms quent EXHUMATION, which exposes the typical of sea coasts in general. Compare more massive, less weathered parts of the marginal sea. weathering profile as inselbergs. Some large forms extend to a height of more than inlier /in-lÿ-er/ An area of exposed rock 300 m and are possibly formed by succes- that is surrounded by stratigraphically sive stages of weathering, which proceed at younger strata, usually a result of folding, the same time as REGOLITH removal. As an faulting, and erosion (such as the erosion inselberg is exposed at the surface, UN- of the crest of an anticline). Compare out- LOADING joints usually develop, and move- lier. ment of rock slabs along such partings can

186 interglacial bring about the destruction of these domed instrument shelter (instrument screen, forms. See also bornhardt. thermometer screen) A structure designed to house meteorological instruments, such insequent stream /in-see-kwĕnt, in-sĕ-/ A as psychrometers, thermometers, and hy- stream that has a random course, which grothermographs. It is usually of wood, in cannot be predicted by reference to a slope, the form of a box, painted white to reflect rock type, or faults or depressions in the sunlight, and the base stands about a meter surface. As a result, it adopts a DENDRITIC above the ground surface. The shelter pro- DRAINAGE pattern. tects the instruments from direct solar radiation, wind, and precipitation; ventila- insolation /in-sŏ-lay-shŏn/ (from incom- tion to allow the free flow of air and pre- ing solar radiation) The intensity of either vent condensation is usually provided by direct or global (direct and diffuse) solar louvered sides and vents. The STEVENSON radiation on a unit area at a specified time SCREEN is the standard instrument shelter on a specified surface. Its value is depen- for thermometers for manned weather sta- dent upon the SOLAR CONSTANT, the time of tions. year, the latitude of the receiving surface, the slope and aspect, if any, of the surface, insular shelf The shelf zone surround- and the transparency of the atmosphere. ing a large island. The shelf extends from low-tide level out to the shelf-edge zone insolation weathering Weathering re- where, as with all shelves, there occurs a sulting from the expansion and contraction marked and steep descent to the deep-sea produced by temperature changes. It was floor. Insular shelves, like continental formerly believed that in arid areas practi- shelves, may be broad or narrow, but they cally all rock fragmentation was caused by tend to contrast with the zone surrounding the extreme variations in temperature and coral reefs and oceanic islands, for in these consequent differential expansion between cases the sea floor plunges steeply to the the outside and inside of blocks, which set deep-sea floor from close to the shore. The up stresses producing cracks. However, ex- shelf edge of insular shelves usually lies at perimental work has suggested that the a depth of some 200 m. process was extremely overrated, although there are many examples of fragmented intensity scale See earthquake intensity. rocks for which this process seems to be the best explanation. In addition to large-scale interception The prevention, by the cracking, GRANULAR DISINTEGRATION may vegetation cover, of a portion of precipita- be produced by the differential expansion tion or irrigation water from directly and contraction of several mineral types in reaching the ground surface. Part of this one rock. water is evaporated directly back to the atmosphere, but some will reach the ground instability The state of the atmosphere surface by running down the plant stems when thermals are able to rise freely by (STEMFLOW) and dripping from the plant their own buoyancy forces. This occurs leaves. See also throughfall. when the ENVIRONMENTAL LAPSE RATE is steeper than the DRY ADIABATIC LAPSE RATE interfluve /in-ter-floov/ The higher ground and is known as absolute instability. This between one river and the next in the same normally is found only near the ground drainage system: an upstanding area not surface. Other states of instability depend yet subject to fluvial denudation. Inter- on the relative rates of cooling of the envi- fluves become lowered and narrowed as ronment and dry and saturated thermals. denudation proceeds, until at the stage of Once condensation has taken place and the peneplanation they are almost insignifi- atmosphere remains unstable, cloud devel- cant. See also divide. opment will be extensive and heavy rainfall probable. interglacial /in-ter-glay-shăl/ A period

187 Intergovernmental Panel on Climate Change

interlocking spur One of a series of spurs on alternate sides of a river valley. young landscape They are strips of high ground that remain after a meandering river has eroded away the ground between them, and generally interfluve occur mainly in the upper and middle mature landscape reaches of the river.

intermediate contour A contour line peneplain that falls between the INDEX CONTOURS.

intermediate rock Strictly, a type of Interfluves rock containing 55–66% silica (by weight); in current usage intermediate rocks contain less than 10% free quartz, sodic plagio- of warmer climatic conditions separating clase, and/or alkali feldspar. Typical inter- GLACIALS of the PLEISTOCENE. Temperatures mediate rocks are syenite, diorite, trachyte, were probably similar to those of today, or and phonolite. See also acid rock; basic perhaps a little warmer. They are believed rock; ultrabasic rock. to have lasted longer than the glacials and very much longer than the 10 000 years or intermittent stream A stream that so since the last glacial phase. flows at only certain times of the year, as after heavy rain or when it receives water Intergovernmental Panel on Climate from springs. Such streams dry up in the Change (IPCC) An international panel dry season, usually because the water table that was established in 1988 by the United becomes lower. Nations Environment Program (UNEP) and the World Meteorological Organiza- intermontane /in-ter-mon-tayn/ 1. Lo- tion (WMO) to assess and provide infor- cated between mountains or mountain mation relevant to the understanding of ranges. climate change. The IPCC publishes peri- 2. A basin of deposition enclosed by moun- odic reports produced by scientific authors tains. and editors, including a series of major assessment reports: its First Assessment Re- intermontane plateau See plateau. port was published in 1990, the Second Assessment Report in 1995, and Third As- internal magnetic field That portion of sessment Report in 2001. the Earth’s magnetic field that originates within the Earth itself. intergranular /in-ter-gran-yŭ-ler/De- scribing a texture common in basic lavas in internal wave (boundary wave) A wave which the spaces between plagioclase crys- that occurs within a water mass, which tals are occupied by relatively small grains may be stratified or in which density may of pyroxene and olivine. Compare ophitic. vary with depth. Internal waves have quite different characteristics from those of intergrowth An interlocking growth of waves that develop at the sea’s surface. The two minerals that crystallize simultane- density variations sometimes occur ab- ously from a melt. Intergrowths may also ruptly, at sharp surfaces of discontinuity result from the EXSOLUTION of a homoge- called interfaces, and sometimes quite neous mineral to form two discrete phases, gradually. The amplitude of internal waves one included in the other. See graphic in- often far exceeds that of waves on the sea’s tergrowth; myrmekite; perthite; symplec- surface; for example, one measurement tite. recorded in the Atlantic Ocean amounted to some 60 m.

188 intraformational

International Date Line A line corre- intertidal /in-ter-tÿdăl/ Describing the sponding to the 180° meridian, deviating part of a seashore between the high- and to some extent around island groups. The low-water marks. Organisms that live date immediately to the east of the line is there have to be able to withstand periods one day earlier than to the west, because of drying out and periods of being com- 180°E is 12 hours ahead of GREENWICH pletely covered by the sea. MEAN TIME and 180°W is 12 hours behind. intertropical convergence zone (ITCZ) International Map of the World A The zone of convergence between the map series at 1:1 000 000 scale that was airstreams from the N and S hemispheres. first proposed in 1891 by the Fifth Interna- It oscillates in position with the thermal tional Geographic Congress and published equator, reaching its maximum northward by a number of countries to common inter- extent in July and maximum southward nationally agreed specifications. It was extent in January. Over the ocean areas it taken over by the United Nations in 1953 marks the meeting point of the trade but the project was officially abandoned in winds, but over the continents it becomes 1986, by which time only 800–1000 of the affected by the monsoonal circulation and originally proposed 2500 maps had been has somewhat different properties. The completed. The International Map of the ITCZ can be clearly seen over ocean areas World projection is a modified polyconic from satellite photographs as a narrow map projection having two standard paral- band of enhanced convectional activity in- lels and a scale of exactly 1:1 000 000. dicating upward motion and cloudy show- Straight lines joining points on the truly di- ery weather at the surface. Sometimes two vided top and bottom parallels of the pro- bands are found separated by about 3–5° jection represent the geographic meridians. latitude, each exhibiting the usual ITCZ Arcs of the same circle, rather than con- characteristics. Many theories have been centric circles, form the parallels. proposed to account for this feature, but as yet the real origin remains in doubt. As the intersection A type of PLANE TABLING in dual convergence zone is found only over which points of detail are fixed by setting the oceans, it is presumed that it must be up the plane table at two or more stations connected with sea-surface temperatures for each detail point and drawing lines of and upwelling. sight to it, the point of intersection of these Reference to this zone as the intertropi- lines representing the location of the point. cal front still remain, but as it has dissimilar features to temperate latitude fronts intersertal /in-ter-ser-tăl/ Describing a and is not really frontal, the use of this texture common in basic rocks in which in- term is discouraged. terstices are filled by a glassy MESOSTASIS or a fine-grained aggregate of quartz, alkali intraclast /in-tră-klast/ An ALLOCHEM feldspar, and deuteric minerals. that is a penecontemporaneously eroded fragment of calcareous material. Intra- interstadial /in-ter-stay-dee-ăl/ A warmer clasts are not derived from preexisting stage within a GLACIAL, in which the ice re- limestones. ceded, to be followed by a readvance on the return of colder conditions. Compare intraformational /in-tră-for-may-shŏ- interglacial. năl/ Existing or formed with a formation. For example, an intraformational interstice /in-ter-stiss/ The angular space conglomerate is one in which the clasts are inside a meshwork of lathlike crystals. In eroded fragments of newly consolidated basalts, interstices between feldspar crys- sediment. These clasts are then incorpo- tals may be occupied by grains of pyroxene rated immediately into the accumulating or glass. See also intergranular; intersertal. sediment.

189 intrazonal soil intrazonal soil /in-tră-zoh-năl/ A type inversion layer A layer of air in which of soil differing from surrounding ZONAL there is a temperature INVERSION, i.e. the SOILS because of the dominance of local layer is cooler than the air above it. A low- drainage or parent material over the zonal level inversion layer generally occurs in influence of climate and vegetation. These basins or valleys; when it happens over soils include hydromorphic soils (poor urban areas pollutants are trapped and it drainge), halomorphic soils (salinization), may cause SMOG. calcimorphic soils (calcareous parent material), soils dominated by human influ- invertebrates Animals that do not pos- ences, soils on organic matter, and various sess a jointed backbone formed of verte- horizons in zonal soils regarded as intra- brae. The members of all animal phyla zonal. They differ from AZONAL SOILS in except the CHORDATA are invertebrates. that they have more or less well developed Compare Vertebrata. horizons, whereas azonal soils still have little horizonation. They are often linked to inverted relief Topographic relief that zonal soils in catenary sequences, with is the opposite of the geologic structure, as poorly drained soils occurring downslope for example where anticlines form valleys from the well-drained zonal types. In the US and synclines form high ground. It is the re- SOIL TAXONOMY, these soils are linked with sult of prolonged denudation. the nearest zonal soils as aquic or halic variants, for example there is no separate involution /in-vŏ-loo-shŏn/ 1. A lobe of group of gleys. one type of material projecting upward or downward into material of another type, intrusion 1. The process by which IN- produced by the effects of CRYOTURBATION. TRUSIVE ROCKS are formed. These involutions are seen in the upper sec- 2. See intrusive rock. tions of bedded deposits and are a clear indication of former PERIGLACIAL conditions. intrusive rock (intrusion) An igneous 2. The refolding of a NAPPE structure re- rock that forms by injection of magma or sulting in very complex fold patterns. other plastic material into the Earth’s crust (the process of INTRUSION) and has solidi- ionosphere /ÿ-on-ŏ-sfeer/ The layer of fied beneath the Earth’s surface. Compare the atmosphere above the stratopause in extrusive rock. which free ions and electrons occur as a result of ionization of gas molecules by solar inversion An increase in temperature ultraviolet and X-radiation. It consists of with height in the atmosphere. This can act several layers, indicated by the letters D to as an upper limit to convection, hence in- G, which reflect electromagnetic waves versions are associated with atmospheric back to Earth and thus it is very important stability and usually dry weather. They can in radio communications. be formed by cooling beneath the inversion or warming at the inversion level and IPCC See Intergovernmental Panel on above. The former method occurs most fre- Climate Change. quently as a result of radiational cooling at night from the ground surface, an extreme iridescence /i-ră-dess-ĕns/ A series of case being over the Antarctic ice cap, and rainbow colors produced by a mineral the latter by subsidence of air under anti- caused by the interference of light within cyclonic conditions. In these circumstances its crystals or at its surface. The colors the air may be extremely dry, as occurs change as the angle of incident light above the trade wind inversion, and changes. quickly evaporates any saturated thermals reaching this level. It acts therefore as a iron bacteria Bacteria that feed on solu- very effective ceiling to convectional cloud. ble iron compounds and as a result precip-

190 isohyet itate insoluble iron(III) hydroxide, ern Cordilleras. The arcs usually include Fe(OH)3. lines of active volcanoes; moreover, they are associated with many of the world’s iron meteorite See meteorite. most severe earthquakes and with very pronounced gravity anomalies. The theory iron pan A type of HARDPAN that is rich of PLATE TECTONICS provides an explana- in iron oxide. tion for the distribution of these features. iron pyrites See pyrite. isobar /ÿ-sŏ-bar/ A line on a chart joining points of constant barometric pressure. ironstone A type of sedimentary rock At the ground surface, observing stations that is rich in iron minerals, found in beds, are often at different altitudes and correc- layers, or nodules. The iron may take vari- tions have to be made to pressure readings ous forms, including CHAMOSITE, HEMA- to provide a common level, normally sea TITE, PYRITE, and SIDERITE. level. In some countries, e.g. South Africa, much of the land surface is well above sea irradiance /i-ray-dee-ăns/ The quantity level and surface pressure readings are cor- of radiant energy received in unit time on rected to differences from the 850 mb pres- unit area of surface. It is expressed in watts sure level. The patterns produced by per square meter. joining points of equal surface pressure represent synoptic systems, such as anticy- irrigation The artificial application of clones, depressions, troughs, ridges, and water to land to provide the necessary cols, from which interpretations of the amount of water to grow or maximize weather can be made. yields of crops or other plants. isochemical /ÿ-sŏ-kem-ă-kăl/ Describ- isallobar /ÿ-sal-ŏ-bar/ A line on a chart ing metamorphic processes that involve lit- connecting places of equal pressure ten- tle or no change in chemical composition dency. Such lines are used in forecasting of a whole rock. the movements of pressure systems, particularly depressions. isoclinal folding /ÿ-sŏ-klÿ-năl/ Gener- ally upright tight folds whose axial planes isarithm /ÿ-să-rith-

191 isoline isoline /ÿ-sŏ-lÿn/ A line on a map or Earth’s surface are brought up to the sur- chart that joints points having the same face. See also Airy’s hypothesis of isostasy. value of some specified quantity, such as an ISOBAR, ISOGON, ISOHALINE, ISOHYET, isotach /ÿ-sŏ-tak/ 1. A line of constant ISOPACH, and ISOPYCNIC. wind speed. 2. A line joining points of equal distance isomeric line /ÿ-sŏ-me-rik/ A line that traveled in a period of time. joins points having the same average monthly rainfall expressed as a percentage isotherm /ÿ-sŏ-th’erm/ A line joining of the mean annual rainfall. It is helpful in points of equal temperature. Isotherm displaying similarities in rainfall regimes maps are the most common methods of by removing the effects of large totals over showing temperature changes on a climatic mountainous areas. scale. As with pressure, the effects of altitude are important and corrections to sea isometric /ÿ-sŏ-met-rik/ See crystal sys- level are normally made. tem. isothermal layer /ÿ-sŏ-th’er-măl/ Any isomorphism //ÿ-sŏ-mor-fiz-ăm// The vertical section of the atmosphere in which existence of crystals that have the same the temperature remains constant with form but different chemical compositions. height, i.e. there is a zero lapse rate, as re- Isomorphic minerals form a series of solid vealed by an upper-air sounding. Isother- solutions. See crystal form. mal layers are frequent but are usually transitory. The term formerly referred to isopach /ÿ-sŏ-pak/ (isopachyte) A line the stratosphere but it has now been super- on a geologic map joining points of equal seded as more information about the ther- thickness for a particular rock bed. mal structure of that layer became available. isopleth map /ÿ-sŏ-pleth/ A map showing quantitative spatial distributions indi- isotope /ÿ-sŏ-tohp/ One of two or more cated by lines of equal value, such as forms of the same element whose atoms isotherms. differ in their numbers of neutrons (although the numbers of protons and elec- isopycnic /ÿ-sŏ-pik-nik/ (isopycnal)A trons is the same). Isotopes therefore have line drawn on certain oceanographic and the same atomic number; they differ in meteorological charts joining points of mass but not in chemical properties. equal density. isotopic ratio /ÿ-sŏ-top-ik/ The ratio of isoseismal line /ÿ-sŏ-sÿz-măl/ A line on the amount of one isotope of an element to a map joining points on the Earth’s surface the amount of another isotope of the same of equal earthquake intensity. element, in the same sample of rock. isostasy /ÿ-sos-tă-see/ A condition of isotropic /ÿ-sŏ-trop-ik/ Having the same theoretical balance for all large portions of physical properties if measured in different the Earth’s crust, which assumes that they directions. Compare anisotropic. are floating on an underlying more dense medium. As a result of erosion or deposi- isthmus /iss-mŭs/ A narrow strip of land tion, this balance is put out of equilibrium that connects two large land masses. For and has to be compensated for by move- example, North and South America are ments of the Earth’s crust. Areas of deposi- connected by the isthmus of Panama. tion sink, whereas areas of erosion rise. In this way the roots of old mountain chains ITCZ See intertropical convergence formed many kilometers beneath the zone.

192 JK

jacupirangite /ja-kyoo-pi-răn-jÿt/A jet streams are the response to this upper nepheline-bearing basic plutonic rock. See atmospheric thermal gradient, and fre- ijolite. quently encourage further development of surface pressure features by the marked jade A green or white semiprecious convergences and divergences that are ex- stone. Jade is a tough compact variety of ei- perienced in different parts of the jet. ther jadeite or nephrite. Easterly jets have been reported between India and Africa, and low-level jets jadeite /jayd-ÿt/A PYROXENE of compo- in East Africa, but these are weaker and sition NaAlSi2O6, found in high-pressure less persistent than those of the westerly metamorphic rocks such as glaucophane zone. Above the troposphere, a POLAR schists. See also jade. NIGHT JET STREAM occurs at high latitudes in winter at about 50 mb level (40 km). jasper A red opaque variety of chal- cedonic quartz. See silica minerals. joint A surface crack in a rock, with no displacement of the pieces each side of it. jet A type of hard COAL that can be See jointing. carved and polished for decoration. jointing The system of cracks in a mass jet stream A narrow band of winds at of rock constituting, in many rock types, high altitudes, with speeds above 60 knots the only means by which water can infil- and having strong vertical and lateral trate the rock mass. Consequently, within shear. Two main regions of jet streams are an area of a single rock type in which there known in the troposphere, one in the sub- are variations in the extent of jointing, tropical westerly circulation (the subtropi- well-jointed areas will become weathered cal jet stream), which is fairly constant in more quickly and to a deeper level than the location for any given season and so ap- more massive parts of the outcrop, which pears clearly on mean charts, and a second may even remain as solid rock. associated with the polar front (the polar- front jet stream). This is highly variable in jökulhlaup See glacial outburst flood. its location, being dependent on the transitory thermal gradients of mid-latitude de- joule /jool/ (J) The unit of work or en- pressions. Because surface depressions ergy in the Système International (SI), form where divergence in the upper atmos- equal to the amount of work done by a phere is strong, they are frequently initi- force of one newton in moving an object ated downstream of a trough in the upper one meter in the direction of the force. It is westerly circulation, with anticyclones equivalent to 1 watt second or 0.239 calo- forming in areas of upper air convergence ries. It is named for the British physicist upstream of the trough. This development James Prescott Joule (1818–89). takes place in zones of large surface temperature gradient, which in turn causes junction The plane of contact between marked temperature contrasts in the upper two adjacent and distinct bodies of rock. atmosphere as depressions intensify. The Such a junction may be conformable, un-

193 jungle conformable, or produced by intrusion or sulfate and potassium chloride, faulting. MgSO4.KCl.3H2O. It crystallizes in the monoclinic system and occurs as under- jungle An imprecise term for a tropical ground granular masses, often associated forest or mangrove region that has dense with rock salt deposits. It is used for mak- undergrowth. See tropical rainforest. ing other compounds of magnesium and potassium. Jurassic /jucirc;-rass-ik/ The period of the MESOZOIC Era that followed the TRIAS- Kainozoic /kÿ-nŏ-zoh-ik/ See Cenozoic. SIC and preceded the CRETACEOUS. It lasted for about 65 million years, from 208 to kaliophilite /kal-ee-off-ă-lÿt/ A rare type 144 million years ago. It was named for the of KALSILITE. See feldspathoid. Jura Mountains in Europe, where rocks of this age were first studied. The Jurassic Sys- kalsilite /kal-să-lÿt/ A potassium alumi- tem is subdivided into eleven stages. The nosilicate that occurs in nepheline. See Lower Jurassic consists of the Hettangian, feldspathoid. Sinemurian, Pliensbachian, and Toarcian Stages; the Middle Jurassic consists of the kame /kaym/ An isolated hummock of Aalenian, Bajocian, Bathonian, and stratified FLUVIOGLACIAL sediments, mainly Callovian Stages; the Oxfordian, Kim- sands and gravels, associated with slow- meridgian, and Tithonian Stages form the moving or stagnant ice. Kames may be Upper Jurassic. Continental rifting of the formed in a variety of ways. Debris-laden supercontinent Pangaea began toward the meltwater streams may emerge from the end of the Triassic; by the Middle Jurassic snout of a decaying glacier, depositing North America was moving away from their load into an ice-dammed lake. On Eurasia and Africa and the Atlantic Ocean melting, that part of the accumulation for- was opening. Following a marine trans- merly adjacent to the snout may form a gression, the characteristic rocks of the steep ice-contact slope, although a certain Jurassic System are clays and limestones, amount of slumping will occur. Alterna- some of them oolitic. There were coral tively, material may be deposited in melt- reefs, and other important invertebrates in- water pools on the surface of a stagnant cluded brachiopods, bivalves such as oys- glacier. The accumulated sediments will be ters, ammonites (on which much of the lowered to ground level on melting, form- system is zoned), and irregular echinoids. ing a kame. On land, dinosaurs continued to flourish and diversify and other reptiles included kame terrace An accumulation of FLU- aerial pterosaurs and marine forms. Fossils VIOGLACIAL sediments deposited by melt- of both Archaeopteryx, the earliest known water flowing between a decaying glacier bird, and the first mammals come from and its valley sides. As with KAMES, the ice Jurassic rocks. A marine regression toward contact slope suffers collapse on melting of the end of the period resulted in nonmarine the ice. Kame terraces can be continuous or deposits. discontinuous, deposition occurring only where meltwater channels could widen, juvenile water Water that exists as de- thereby reducing velocities of flow. These posits within the Earth’s magma and first features are flat-topped and steep-sided, appears at the surface during volcanic ac- sloping gently down-valley. Long contin- tivity. ued existence of these terraces after deglaciation depends upon the extent of kaersutite /kair-soo-tÿt/ A monoclinic subsequent fluvial and slope processes. AMPHIBOLE. kaolin /kay-ŏ-lin/ (china clay) A soft kainite /kay-nÿt, kÿ-/ A mineral consist- white clay used in medicine, as a filler for ing of a hydrated double salt of magnesium paper, and in the manufacture of ceramics.

194 kelvin

It consists of deposits of minerals of the ground also cools. The density of air in- KAOLINITE group that are produced by the creases when the temperature falls, and if weathering and hydrothermal alteration of the ground surface is sloping, these heavier feldspars in granitic rocks. See also clay layers will give rise to a downslope gravita- minerals. tional flow. Where concentrated in a valley, night-time MOUNTAIN WINDS can be kaolinite /kay-ŏ-lin-ÿt/ A white or gray quite strong, but the most extreme form of CLAY MINERAL consisting of hydrated alu- katabatic wind is off the Greenland and minum silicate, Al2Si2O5(OH)4. It crystal- Antarctic ice caps. A mean hourly speed of lizes in the triclinic system and is derived by 156 km per hour was reported at Cape alteration of alkali feldspar, feldspathoids, Denison (67.9°S, 142.7°E) under such con- and other silicates. It is the main compo- ditions. See also anabatic wind. nent of KAOLIN (china clay). katafront /kat-ă-frunt/ Any frontal sur- kaolinization See pneumatolysis. face at which the warm air is descending relative to the cold. As a result of this sub- karren /ka-rĕn/ Relatively small irregu- sidence, weather activity at a katafront is lar hollows and elongated grooves cut on normally weak with only a belt of strati- limestone surfaces as a result of solution. form cloud indicating its presence. Some At the most basic level they may be divided fronts vary in character along their length into rundkarren, having rounded separa- and may take the form of an ANAFRONT tions between grooves, and rillerkarren, near the depression center and a katafront with sharp separations. It is believed that farther away. whereas the latter are the result of running water, the former may be at least partly katophorite /kat-off-ŏ-rÿt/ A mono- created beneath a cover of snow, soil, or clinic AMPHIBOLE. peat. The term is also used for the whole dissected surface and is the German equiv- kelp Certain species of seaweed of con- alent of LIMESTONE PAVEMENT, the French siderable length and thickness. These sea- equivalent being lapiés. weeds, which are usually brownish in color, thrive below low-tide level, and may karst Denoting any area within which be luxuriant enough to form kelp forests. limestone solution landforms such as SINK- Of particular importance are the large lam- HOLES, UVALAS, and LIMESTONE CAVERNS have become well developed. The term inarians, which anchor themselves to rocky stems from the regional name for the mas- or stony bottoms and, depending on their sive limestone area on the Dalmatian coast rigidity, either stand fairly erect or move of the Adriatic Sea, where practically all with the tidal currents. They influence the the landforms have been produced by solu- movement of bottom materials, and may, if tion and almost all the drainage is under- uprooted by strong tidal flow or wave ac- ground. Prerequisites for a fully developed tion, carry their stony attachments shore- karst landscape include a considerable ward or alongshore. They are also effective thickness of strong soluble well-bedded at times in damping wave action. Kelp and well-jointed limestone, moderately forests are important as fish feeding heavy rainfall, and sufficient altitude to grounds. allow an extensive flow of underground water. kelvin (K) The unit of thermodynamic temperature in the Système International katabatic wind /kat-ă-bat-ik/ (drainage (SI), equal to 1/273.16 of the temperature wind; gravity wind) A downslope flow of of the triple point of water. 1 kelvin is cool air that develops on calm clear nights equal to 1°C and a temperature expressed when the ground surface cools by radia- in kelvins is the same as the ABSOLUTE TEM- tional losses and the air in contact with the PERATURE. It is named for the British theo-

195 Kelvin–Helmholtz instability retical and experimental physicist Baron keratophyre /ke-ră-tŏ-fÿr/ A fine-grained William Thompson Kelvin (1824–1907). igneous rock associated with basaltic lava. See spilite. Kelvin–Helmholtz instability The development of waves at the surface separat- Kerguelen-Gaussberg Ridge /ker-gĕ-lĕn ing two layers of the atmosphere having gowss-berg/ A large ocean feature ex- different temperatures and wind speeds. tending southward from the East African These waves sometimes increase in ampli- coast, reaching almost to Antarctica. The tude and become unstable, rolling into vor- ridge branches out, one branch reaching tices, which break and give CLEAR AIR westward to the MID-ATLANTIC RIDGE, and TURBULENCE. Very occasionally moisture another trending eastward, and then cir- conditions may be such that clouds form cumscribing the Australian continent and on the waves and make the vortices visible. extending toward the East Pacific Rise. It is named for Baron William Thompson Surveys conducted off the continent of Kelvin (see kelvin) and the German scien- Antarctica show that the continental slope tist and philosopher Hermann von Helm- south of the Kerguelen-Gaussberg Ridge is holtz (1821–94). in places made up of several tilted plainlike features with gentle undulations. Rocks Kelvin wave A tidal phenomenon in dredged from the Kerguelen-Gaussberg which a progressive wave traveling along a Ridge yield samples of basalt and conti- tidal channel is affected by the Earth’s ro- nental-type rock. tation to the extent that the tidal range (in /ker-nÿt/ A soft white or color- the N hemisphere) is increased along the kernite less mineral form of hydrated sodium bo- right-hand side of the channel and de- rate, Na B O .4H O. It crystallizes in the creased on the left-hand side (with the ob- 2 4 7 2 monoclinic system and is an important server facing the direction in which the source of boron compounds, including progressive wave is traveling). Usually, the BORAX. energy of the progressive wave is much di- minished, so preventing a true am- kernlose winter /kern-lohs/ See coreless phidromic tidal system from operating (see winter. amphidromic system). The Kelvin wave phenomenon can be observed in the Eng- kettle hole A depression in the ground lish Channel, where the tidal range along surface in a glaciated area. As a mass of ice the English coast is small compared with stagnates, material is deposited from above that along the French coast. During any the ice surface, and FLUVIOGLACIAL deposits tidal state, the sea surface slopes from one also accumulate. In this way masses of ice side of the Channel to the other (see can become buried beneath DRIFT. Once kelvin). buried the ice takes a considerable time to melt, and when this finally happens, col- kelyphitic rim /kel-ă-fit-ik/ See reaction lapse occurs and a depression is formed, rim. which may subsequently become filled by a lake or by sediments. Not all kettle holes kentallenite /ken-tal-ĕ-nÿt/ A syenogab- are associated with stagnant ice, because bro containing olivine and augite together small lakes and streams may freeze while with plagioclase and orthoclase feldspars sediments are accumulating, but those that and large poikilitic crystals of biotite. See are will be found in association with KAMES alkali gabbro. and ESKERS. kenyte /ken-yÿt, keen-/A TRACHYTE con- key See cay. taining large rhombic orthoclase phenocrysts. Medium-grained varieties may be khamsin /kam-sin, kam-seen/ A hot dry termed rhomb-porphyry. southerly wind blowing across Egypt from

196 K/T boundary event the Sahara. It precedes depressions passing eastward along the North African coast and hence is most common between April and June. According to Arab tradition it blows for 50 days.

se level kidney ore A form of the iron-contain- former ba knickpoint ing mineral HEMATITE, which occurs in el lev base masses whose shape resembles solidified current bubbles or kidneys.

Knickpoint kimberlite /kim-ber-lÿt/ (blue ground)A brecciated carbonate-rich phlogopite-bearing PERIDOTITE found in pipes and dia- tremes piercing very old metamorphic knickpoint (rejuvenation head) A break rocks. Many kimberlites contain DIA- of slope in the long profile of a stream, re- MONDS. sulting from a fall of base level rejuvenat- ing the seaward portion of the stream. This kinetic energy The energy of a body de- REJUVENATION leads to renewed downcut- rived from its movement. Its magnitude is ting, and the creation of a new lower long expressed as ½ mv2 where m is the mass of profile; the point where this profile meets the body, and v is its velocity. The atmos- the established profile is the knickpoint. Knickpoints are often marked by wa- phere possesses large amounts of kinetic terfalls if the break of slope is steep, or energy by virtue of its motion, but this is rapids if less steep. Once created, the constantly being lost through surface fric- knickpoint tends to migrate upstream and tion and turbulence. It is replenished by be lowered in height; the rate at which it conversion of potential energy derived advances and the degree to which it loses originally from solar radiation and upward its height depends on the lithology in which motion of air. it is cut. In unconsolidated material it will rapidly disappear. The increased turbu- kingdom The largest group in the taxo- lence associated with break of slope accel- nomic classification of living organisms. erates erosion and rapidly lowers its Five kingdoms are usually recognized: An- height. imalia includes all animals; Plantae includes all plants; Fungi includes all fungi; knoll A small rounded hill. See also Protista (or Protoctista) includes algae, hummock. protozoa, and slime molds; and Monera (Bacteria or Prokaryotae) comprises the knot A unit of speed, being one NAUTI- bacteria. Kingdoms are divided into phyla. CAL MILE per hour. 1 knot is equal to 0.515 See also taxonomy. meters per second. kink plane A deformation structure in komatiite /kŏ-mat-ee-ÿt/ A series of an- which the orientation of a foliation is cient extrusive igneous rocks that contain changed as a result of slippage or gliding. some of the oldest PERIDOTITE lavas. They are characterized by large branching crys- klippe /klip, klip-ĕ/ Originally, any rock tals, indicative of rapid cooling of the body isolated by erosion. The term is now magma from which they derived. restricted to the outlier or erosional remnant of a NAPPE. Köppen climate classification See climatic classification. knee fold A GRAVITY FOLD, generally zigzag in shape. (See diagram at FOLD.) K/T boundary event (Cretaceous–Ter-

197 Kuroshio Current tiary boundary event) The time at the beneath the Kuroshio Current is only end of the Cretaceous period, when (ac- partly saline and cold, being of sub-Antarc- cording to fossil evidence) large-scale ani- tic origin. The Kuroshio Current and the mal extinctions took place. For example, FLORIDA CURRENT have roughly the same dinosaurs, ammonites, and belemnites flow rates. ceased to exist. The cause is thought to be a general and rapid rise in world tempera- kyanite (cyanite) A light blue, white, or tures. Because of high concentrations of gray aluminum silicate mineral, Al2SiO5. It the element iridium in rocks of the time, crystallizes in the triclinic system and oc- some geologists think that the climatic curs in pressure-metamorphosed rocks, change resulted from a major meteorite im- such as amphibolite and some mica schists. pact which filled the Earth’s atmosphere See also aluminum silicates. with debris, cutting off most of the sunlight (iridium is relatively abundant in mete- Kyoto Protocol An international agree- oroids). ment that aims to curb the air pollution that contributes to GLOBAL WARMING. Its Kuroshio Current /koo-rosh-ee-oh/An measures include targets to reduce world- ocean current constituting one of the im- wide emissions of GREENHOUSE GASES by in- portant western BOUNDARY CURRENTS in the dustrialized countries. It was originally world’s ocean circulation system. It devel- formulated in 1997 at the Third Confer- ops following a gradual deflection of part ence of Parties to the United Nations of the North Equatorial Current (see equa- Framework Convention on Climate torial current) in the Pacific Ocean, being Change (UNFCCC) in Kyoto, Japan. The at first rather diffuse, but in more northern agreement commits the signatory industri- tracts becoming concentrated between the alized nations to specified reductions in shelf region off the Ryukyu Islands and a emissions of six greenhouse gases or classes submarine ridge. At about 35°N, the cur- of gas: carbon dioxide, methane, nitrous rent swings away from the Japanese main- oxide, hydrofluorocarbons, perfluorocar- land and flows for some 8000 km before bons, and sulfur hexafluoride. The Proto- again splitting up into branches. In con- col also introduced the concept of trast to the water beneath the GULF STREAM, EMISSIONS TRADING. Following ratification water that is relatively warm and fairly the Protocol came into force on February saline because of its continual replen- 16, 2005. The USA indicated its decision ishment of very saline water from the not to ratify the Protocol in 2001. See also European Mediterranean Sea, the water Montreal Protocol.

198 L

Laborde projection /la-bord/A MAP injected it has sufficient pressure to arch PROJECTION resembling Mercator’s trans- up the overlying strata. Therefore although verse, the only difference being that it has a flat base, its upper surface is con- Laborde uses a spheroid and not a sphere. vex. It is generally restricted to areas of east–west extent, for which it is orthomor- lacuna /lă-kyoo-nă/ The time interval phic. The projection is named for the missing between beds above and below an French artillery officer and general Jean unconformity, including both the nonde- Laborde in 1926. positional and erosional components.

Labrador Current One of several lacustrine /lă-kuss-trin/ Describing some- major surface currents in the N Atlantic thing that lives in, is related to, or is pro- Ocean, bringing cold water southward duced by a lake. from the Arctic Ocean region. The Labrador Current flows in on the W flank lagoon A stretch of water that is more of Greenland, and is matched on the E or less enclosed and often shallow, pro- flank by the East Greenland Current. tected from the open sea by a spit, These influxes of cold water meet, in the tombolo, bay-mouth bar, or other kind of surface layers, with the relatively warm barrier. These barriers are not complete, and saline GULF STREAM and NORTH AT- for lagoons invariably possess a free con- LANTIC DRIFT water. Both cold currents nection with the open sea, however narrow carry icebergs southward, and these tend this may be. The barrier provides protec- to become concentrated in the ‘Gateway of tion from waves, although wave overwash Icebergs’ east of the Grand Banks. This as- is common to many lagoonal areas. Tidal sembly of bergs is most common between current action is usually minimal within mid-March and mid-July. Another clima- the lagoon itself. The water may be fresh, tological feature of the region is the high saline, or brackish. Moreover, the salinity frequency of fogs, especially off the coast level in a lagoon may vary with time. The of Newfoundland. Winter cooling of the deposits of lagoons are characteristically combined waters from the North Atlantic fine-grained, with a significant proportion Drift, the Labrador Current, and the East of organic matter. Greenland Current may increase their density sufficiently to cause the surface wa- lahar /lah-har/ A mudflow composed ters to sink, perhaps to depths in excess of mainly of volcanic debris. If the unstable 1000 m. debris accumulating on the sides of a volcano is lubricated by heavy rain, it will labradorite /lab-ră-dor-ÿt/ A variety of flow under gravity. The flow that covered plagioclase FELDSPAR. Herculanium in AD 79 may have been of this type. laccolith /lak-ŏ-lith/ A concordant intrusive igneous body with a dikelike feeder, lake A body of water completely sur- usually forming small lenslike features less rounded by land. It lies in a depression in than 5 km in diameter. As the magma is the Earth’s surface or in an artificially cre-

199 Lambert’s cylindrical equal-area projection ated location behind a dam. Most lakes water move by sliding over each other, contain fresh water, although some very with no mixing. Energy is transmitted from large ones (such as the Caspian Sea and one layer to the next by the viscosity be- Dead Sea) have saline water. A very small tween the layers, while at the boundary lake is generally referred to as a pond or (ground surface, stream bed, stream wall, pool. Some lakes have streams or rivers etc.) movement is nil. Laminar flow is rare flowing into them, and some dry up at cer- in natural streams or air flows, except at tain times of the year. In ecological terms, boundary contacts. a lake can be considered as a complete ECOSYSTEM. lamprophyre /lam-prŏ-fÿr/ One of a group of medium-grained basic alkali-rich Lambert’s cylindrical equal-area pro- igneous rocks occurring as dikes and sills. jection A MAP PROJECTION drawn so that These rocks are characterized by the pres- the scale along the Equator is correct, but ence of mafic phenocrysts, biotite, am- all the other parallels are exaggerated in phibole, or augite, and a groundmass length. The scale on the meridians de- containing orthoclase or sodic plagioclase creases away from the Equator, causing the feldspar. parallels to become closer poleward. The Minette and vogesite are varieties con- projection is equal-area because the ratio taining orthoclase feldspar and in which between the meridians and parallels re- biotite and hornblende respectively is the mains constant. It is seldom used outside predominant mafic mineral. Analogous the equatorial zones because of the degree rocks containing plagioclase are called ker- of distortion of shape north and south of santite and spessartite. Camptonite con- about 40°. The projection is named for the tains plagioclase and a brown amphibole German mathematician, physicist, as- called barkevikite. Monchiquite is a tronomer, and philosopher Johann Hein- feldspar-free lamprophyre containing phe- rich Lambert (1728–77). nocrysts of biotite, augite, amphibole, and olivine in an analcime-rich matrix. Lambert’s zenithal projection An Lamprophyres are melanocratic hy- equal-area MAP PROJECTION used mainly in pabyssal equivalents of diorites and syen- polar areas, where the pole is the center of ites and are associated with such plutonic the projection and the meridians are intrusions. straight lines radiating from it, making the bearings true. The parallels are concentric land breeze The weaker nocturnal circles. Lambert also constructed an equivalent of the SEA BREEZE. It results from oblique (again equal-area, showing coun- the unequal cooling of the different sur- tries within 40° with good shape) and an faces at night, but because the temperature equatorial projection (used for countries difference is small, wind speeds are low. It astride the Equator, equal-area). is most evident when the pressure gradient is weak and skies are clear, so allowing ra- Lamellibranchia /lă-mel-ă-brank-ee-ă/ diational cooling over the land to be at a See Bivalvia. maximum. The surface wind is part of a larger con- lamina /lam-ă-nă/ A thin layer of sedi- vection cell, which maintains air continu- mentary rock, up to 1 mm thick, that ity. Above the land breeze is a weak airflow differs in some respect from the layers on from sea to land. each side of it. It may, for example, have a particular color, particle size, or composi- land bridge A strip of land that joins tion. two larger landmasses and may provide a route for the migration or spread of plants laminar flow The nature of movement and animals. See also isthmus. of a fluid over a smooth surface at very low velocities, whereby parallel ‘slabs’ of air or landform The various surface features

200 lateral erosion of the Earth that contribute to the land- lapilli /lă-pil-ÿ/ Round fragments of solid scape, such as plains, mountains, and val- lava thrown out by an erupting volcano; leys. small volcanic bombs. They are a form of EJECTA up to 65 mm across. See also pyro- Landsat Any of the unmanned orbiting clastic rock. earth resources satellites, launched by NASA from July 1972. They use remote- lapis-lazuli /lap-is-laz-yŭ-lÿ, -lee/ The sensing devices, such as infrared cameras semiprecious variety of LAZURITE. See to provide information and images of the feldspathoid. Earth’s surface, including land use, vegetation, geology, and pollution. lapse rate The change of temperature in the atmosphere with height. Distinctions landscape The geomorphological and are made between the decrease of tempera- artificial features that make up the Earth’s ture in the free atmosphere, the ENVIRON- scenery. It thus includes the natural LAND- MENTAL LAPSE RATE, and the decrease of FORM as well as buildings, civil engineering temperature under adiabatic conditions in works, and agricultural modifications. thermals, the DRY ADIABATIC LAPSE RATE and SATURATED ADIABATIC LAPSE RATE. landslide The sudden downslope movement of rock and debris due to failure larvikite /lar-vik-ÿt/ (laurvikite) A dark along a shear plane. The movement is blue iridescent type of SYENITE, used as an ornamental stone. started when the stress in the FREE FACE exceeds the resisting power of the potential The energy required or re- shear plane: this loss of resistance may be latent heat leased on a change of phase. In the atmos- due to increased weight of the face, e.g. if it phere, water can exist as a solid, a liquid, becomes wetted; it may be due to under- or a vapor, representing increasing energy cutting of support at the foot of the slope states respectively. Thus freezing and con- by some process of BASAL SAPPING; or it may densation release latent heat, melting and be due to a sudden triggering by an earth- evaporation require heat. Latent heat is ex- quake. tremely important in atmospheric pro- As a result, a convex scar is left upslope, cesses, enabling energy to be transferred with a mass of jumbled debris at the base in within the general circulation, from ground a tonguelike form. Areas of rapid down- to atmosphere, and as a reservoir of heat in cutting by rivers, or with variable litholo- the atmosphere. gies, or swelling clays that expand on Specific latent heat is the latent heat re- wetting are susceptible; so too are undercut quired or released per unit mass. sea cliffs and river banks, and areas of ice formation. Like all MASS MOVEMENTS, the lateral accretion Material that accumu- effect is to render an unstable slope more lates at the side of a channel, such as that stable. deposited on the inner sides of bends on a meandering river. landslide surge See tsunami. lateral erosion River erosion in a hori- langley A unit of energy equal to 1 calo- zontal direction. Within the meandering rie per square centimeter. It is mainly used reach of a river, erosion on the outside of for radiation, but as the calorie has been bends leads to downstream migration and largely superseded by the SI unit the JOULE, constant paring away of the bluffs at the the langley is decreasing in use. It is named side of the valley; this effectively erodes the for the American aeronautics pioneer valley width laterally. In floods, the ele- Samuel Pierpoint Langley (1834–1906). vated water level fills the channel and raises the water table, saturating the banks. lapiés /lap-ee-ay/ See karren. When the floods subside, the support on

201 lateral fault the weakened (wetted) banks is removed lateritic soil /lat-ĕ-rit-ik/ 1. A soil devel- and mass slumping occurs, hence widening oped on part of a relic LATERITE, or with an the channel laterally. By these processes, indurated layer within its profile. some FLOODPLAINS are eroded rather than 2. A red tropical soil developing into lat- deposited; they consist of a rock-cut bench erite. It undergoes leaching of silica and with a thin veneer of alluvium, unless they bases, the concentration of aluminum and form because of a base-level rise, when a iron oxides increasing, as happened in the deep purely aggradational floodplain will formation of the relic laterites. As leaching result. of silica proceeds, the silica:sesquioxide ratio falls; in 1927 Martin and Doyne cal- lateral fault See strike-slip fault. culated that while laterites had a ratio of %t;1.33, lateritic soils had a ratio of lateral moraine A ridge of material 1.33–2.0, showing that they were follow- along the side of a glacier (or along the side ing the same development trend but had of a valley formerly occupied by a glacier), not yet been desilicified as much as true la- primarily composed of angular and un- terite. However, relic laterite probably de- sorted material that has fallen from over- veloped by much larger-scale processes and hanging rock outcrops as a result of the term lateritic soil has now been re- FERRALLITIC SOIL FREEZE-THAW weathering, although it is be- placed by (latosol) or, in lieved that a certain proportion of the ma- Australia, red loam. terial is derived from within the glacier itself. The ridge may consist extensively of laterization /lat-ĕ-ră-zay-shŏn/ The con- ice, because once a layer of waste about 0.5 centration of iron and aluminum oxides in the upper layers of the soil and the removal m thick has developed, the ice beneath is of silica and bases, either in the formation protected from melting and hence it stands of relic LATERITES or in current soil-forming above the surface level of the rest of the processes in the tropics. In the case of relic glacier. See also moraine. laterites, laterization is thought to be a result of a fluctuating water table. Opinions laterite /lat-ĕ-rÿt/ A layer of deposits vary: the rising and falling groundwater formed from the weathering of rocks in (resulting from seasonal dryness and wet- humid tropical conditions and consisting ness) either brings up iron and aluminum mostly of iron and aluminum oxides. It from the pallid zone and concentrates it at hardens on exposure to the atmosphere a point of maximum upward extension, or and is therefore used for building. It is not it causes iron and aluminum being washed a soil, because it is produced by processes down in percolating rainwater to be pre- operating on a far larger scale than typical cipitated on meeting the alkaline ground- pedologic processes. It can occur in two water, or it causes a combination of the forms, either buried and soft or as a super- two. It is a large-scale process, operating ficial capping (cuirass) of hardened red ma- over geologic timescales. In the case of cur- terial on uplifted areas that have had their rent tropical soil formation laterization de- overburden stripped off. Most laterites denotes the process of individualization of veloped in the Cenozoic Era, and are hence iron and aluminum oxides, and to a lesser relic. In profile they display a strong hard- extent oxides of titanium, chromium, and ened reddish surface horizon, dominated nickel, by intense weathering and leaching, by ferric oxide, which breaks up with removing bases and silica. This process depth into a mottled horizon, which in turn now tends to be called ferrallitization, passes down to a depleted pallid zone, leading to the development of FERRALLITIC which may be 50 m thick and reach to the SOIL (latosol). bedrock. In a pure laterite, the red horizon will be composed of 90–100% iron, alu- laterolog /lat-ĕ-ră-log/ A subsurface log- minum, and titanium oxides. See also lat- ging technique, which measures variations erization; limonite. in the conductivity of strata.

202 layer cloud latite /lat-ÿt/ An extrusive igneous rock cumulation of single extrusions, termed consisting of alkali feldspar and plagio- lava flows, produces a volcano or lava pile. clase. See syenodiorite. There are numerous textural terms used to describe lavas, such as aphyric, porphyritic, vesicular, glassy, holocrystalline, etc. See also aa; block lava; columnar joint; North Pole N pahoehoe; pillow lava. See lava flow. 60°N lava dome (volcanic dome) A mass of solidified lava, built up from one or more lava flows, that forms a rounded steep- Equator sided mound. It forms from lava that is too viscous to flow far and instead piles up around the volcanic vent from which it is S slowly extruded. Most lava domes are formed from dacite or rhyolite lavas. A Latitude succession of lava domes have formed in the crater of Mount St Helens, Washing- latitude The angular distance of a point ton, USA, following the major eruption of on the Earth’s surface north or south of the 1980. Equator, measured as the angle subtended at the center of the Earth by an arc along a lava flow A stream of molten LAVA meridian between the point and the Equa- flowing from a fissure or vent of an active tor. The Equator has no angle whereas the volcano, and the solidified rock it produces poles are at 90°. Latitude is measured in when it cools. An acidic siliceous lava, with degrees, minutes, and seconds. Parallels of a high viscosity, tends to move slowly in latitude form approximate concentric cir- short thick flows whereas basic basaltic cles, with the poles as the center (see dia- lava, with a low viscosity, flows quickly in gram). See also longitude. long thin streams. latosol /lat-ŏ-sôl/ See ferrallitic soil. lava plateau A large area of elevated land gradually built up by a series of LAVA laumontite /law-mŏ-nÿt/ A hydrated sil- FLOWS from a volcanic FISSURE. See also icate of calcium and aluminum, flood basalt. Ca(Al2Si4O12).4H2O. It crystallizes in the monoclinic system, and occurs in veins in lava tube (lava tunnel) A long hollow schists and cavities in igneous rocks. It is a below the surface of a solidified LAVA member of the ZEOLITE group of minerals. FLOW, created when lava that was still molten continued to flow after the surface Laurasia /law-ray-zhă, -ahă/ The N had become solid. Such hollows occur hemisphere supercontinent believed to most frequently in basaltic lava, which has have been present before 200 million years a low viscosity when molten. ago. This supercontinent subsequently fragmented to form the present Greenland, Eu- lawsonite /law-sŏn-ÿt/ A white mineral rope, Asia (excluding India), and North of composition CaAl2(OH)2(Si2O7)H2O, America. See also Pangaea. found in low-temperature regionally metamorphosed rocks, particularly glauco- laurvikite See larvikite. phane schists. lava A molten or partly molten mass of layer cloud A cloud having little vertical MAGMA extruded from a volcano or vol- development. Such clouds are formed canic fissure and the rock that forms as a under conditions of zero or weak uplift; result of cooling and solidification. The ac- precipitation may be extensive from nim-

203 layered igneous rock bostratus clouds but because uplift is lechatelierite /lesh-ă-tel-yĕ-rÿt/ A rare weak, intensity is low. Clouds of this type silica glass. See silica minerals. are cirrostratus, altostratus, stratus, stratocumulus, and nimbostratus. lee The side of a hill or other prominence that is sheltered from the prevailing wind. layered igneous rock Many large intrusions of gabbroic igneous rocks are lee waves Stationary waves of air gener- stratified, containing layers in which differ- ated, under certain conditions of atmos- ent minerals are concentrated, which sug- pheric stability, on the leeward side of hills gests that the dominant differentiation or mountain ranges. Their wavelength is process (see magmatic differentiation; usually between 3 and 30 km. Lee waves metamorphic differentiation) was the grav- are frequently seen when the crests of the wave reach saturation point to give a lens- itational settling of early-formed crystals. shaped cloud outlining the upward mo- Often mineralogically similar layers are re- tion. This cloud is constantly forming on peated in a rhythmic fashion. Systematic the upwind side and being dissipated in the changes in mineral composition from the area of descent while appearing to remain bottom to the top of a layered sequence are stationary; it is classified as altocumulus termed cryptic layering. Typically, olivines lenticularis. Lee waves occur when a stable and pyroxenes become progressively richer layer of air is sandwiched between layers of in iron (relative to magnesium) and plagio- lower stability above and below, with a clase trends to more sodic compositions, strong and steady wind flow. changes which are compatible with crystallization from residual liquids that are lepidolite /lep-ă-dŏ-lÿt/ A pink, purple, differentiating toward more acid composi- or gray lithium-bearing mineral, a hy- tions. drated silicate of aluminum, lithium, and potassium, sometimes also containing fluo- lazurite /laz-yŭ-rÿt/ A deep blue to violet rine and rubidium. It crystallizes in the complex sodalite mineral, (Na,Ca)8- monoclinic system as platy aggregates oc- (Al,Si)12O24(S,SO4), the chief component curring in pegmatites, and is a member of of lapis-lazuli. It crystallizes in the mono- the MICA group of minerals. It is a principal clinic system, and occurs in pegmatites and source of lithium. metamorphic rocks. See also feldspathoid. lessivage /less-ee-vahzh/ The mechanical leaching The action of water moving movement of clay down a soil profile under down through a soil profile carrying soil the influence of water and its redeposition materials with it in suspension or solution. lower down. It occurs in soils subject to It includes the mechanical action of water, LEACHING but where acidity is neutral or slight, so that the clay is not broken down e.g. LESSIVAGE; its chemical action, as in the into its constituents but moves as a whole. weathering processes of HYDRATION, HY- It does nevertheless operate better in less DROLYSIS, and CARBONATION; and its partly alkaline conditions, and DECALCIFICATION biochemical action, as when it contains generally precedes lessivage. leaf leachates and moves material by Diagnostic signs include an increase in CHELUVIATION. Leaching removes bases, the heaviness of texture with depth, and clays, organic matter, and the various micromorphological examination can sesquioxides; in its varying forms it is re- show clay skins on ped surfaces and along sponsible for the processes of lessivage, channels. Clay skins are evidence of rede- PODZOLIZATION and LATERIZATION. Soils position of clay; increased heaviness alone subject to leaching become increasingly is only suggestive, as Bt horizons (textu- acid with strongly developed eluvial and, if rally heavier B horizons) can be due to a the leached material is redeposited, illuvial lighter horizon above or increased weath- horizons. ering in the B due to increased moisture.

204 lightning leucite /loo-sÿt/ A white or gray potas- strument must be adjusted until the line of sium aluminum silicate, KAlSi2O6, which sight is perfectly horizontal, by centering a occurs in basic lavas and other igneous bubble that moves as the basal footscrews rocks. It crystallizes in the tetragonal sys- are turned. Once leveled it can be used to tem and is used in making fertilizers. See sight onto the staff. The telescope incorpo- also feldspathoid. rates a diaphragm with engraved lines, the reading at the central line being that used leucitite /loo-să-tÿt/ A volcanic rock that for heighting calculations. is composed of LEUCITE and a pyroxene. See nephelinite. leveling A method of surveying used for obtaining heights of points in relation to leucocratic /loo-kŏ-krat/ See color index. that of a point of known height. A LEVEL is set up between a point of known height levée /lev-ee/ A berm or bank elevated and one for which a height is required; it is above the general level of the floodplain, then adjusted to give a horizontal line of bordering many streams in their alluvial sight. A sight, known as a backsight, is floodplain sections. Usually made of coarser made onto a LEVELING STAFF at the known material than the rest of the floodplain, point and the staff reading noted. A read- they originate in times of flooding when ing is then obtained by making a foresight the overbank flow decreases in speed and onto the staff at the other point. The dif- volume away from the stream, depositing ference between the two staff readings the coarse materials first as it rapidly loses gives the difference in height between the COMPETENCE. If the river lacks a coarse two points. The process continues by mov- fraction, alluvium being mostly silt and ing the level between the second point, now clay size, levées may be absent. of known height, and another unknown one, and so on. A leveling traverse (see tra- leveche /lĕ-vesh/ A hot dry wind that versing) returns to the first known point, as blows from the south in southern Spain. It a check on accuracy. generally precedes a depression moving along the Mediterranean Sea. See also leveling staff A staff used in surveying khamsin; sirocco. in conjunction with a LEVEL for determining heights. Most staffs consist of three sec- levéed channel A submarine channel in tions, which are either telescopic or simply the deep ocean flanked by marked levées, clip together, to reach a height of 4 m at not unlike some of the levée features that their full extent. They embody a small bub- border many river channels on land. Such ble to enable the user to hold them verti- submarine channels usually lie at the lower cally. The staff has heights marked clearly ends of SUBMARINE CANYONS, such as the La in black or red on white or yellow, which Jolla Canyon off the coast of California. can be read to the nearest millimeter. How- Here, at a depth of some 600 m, the true ever, when viewed through the telescope of canyon terminates in a channel flanked by a level, the figures appear upside-down, natural levées. At its seaward extremity, and care must be taken in reading them. the channel merges into a submarine fan. Sand layers have been found in the fan and lherzolite /ler-zŏ-lÿt/ An ultramafic rock on the levées on each side of the channel. consisting largely of olivine together with Channels with levées are also strongly de- orthopyroxenes and clinopyroxenes. veloped across many abyssal fans. life assemblage See biocoenosis. level A surveying instrument used in conjunction with a LEVELING STAFF to de- lightning The visible discharge of a termine heights. A level is basically a tele- thunderstorm and the natural mechanism scope fitted to an adjustable base, which for neutralizing the high electrical fields clamps onto a tripod. Before use the in- that build up. Air is a good insulator but as

205 lignite charge production increases the potential tated, or metasomatic. Organic limestones gradient in a thunder cloud, and water are formed from the calcareous skeletons droplets decrease the insulational proper- of living organisms, and include in situ BIO- ties of air, eventually the insulation is over- HERMS and accumulates such as CHALK. come and the lightning flash results. It Among the various precipitated limestones consists of a line of highly ionized air mol- are those that are part of an EVAPORITE se- ecules along which electricity can flow for quence, and OOLITES. Clastic limestones are a very short time. those derived from preexisting calcareous High-speed film recordings of lightning rocks. Metasomatic limestones are other discharges have clarified the development limestones that have undergone diagenetic of a flash. A cloud-to-ground discharge is change. The most common change in- started by a streamer, which develops volves the alteration of calcite to dolomite. downward from the cloud in a series of Descriptive classifications employing the steps. It is of low luminosity. When this relative proportions of ALLOCHEMS, MI- streamer has approached to within about 5 CRITE, and SPARITE are increasingly used. to 30 m of the ground, a highly luminous return stroke comes up to meet it and trav- limestone cavern A subterranean cave els up the ionized channel to the cloud. developed in limestone as a result of the en- This is the main visible flash, which travels largement of joints and fissures by solu- at speeds of about 0.1 times that of light, tion, larger caverns occurring in less passing a current of 10 000 amperes but resistant well-jointed limestone. Most cave only lasting for about 100 microseconds. systems comprise both large open caves Several more streamers and return strokes and small restricted passages, all of which may develop. See also thunder. continue to grow in size as long as unsaturated water flows through them. Some lignite /lig-nÿt/ (brown coal) Low-grade caves are considerably extended horizon- brown coal with a high moisture content. tally, which may reflect a position corresponding to the present or a former water limb See fold limb. table. The largest caves are found in hard well-jointed limestones of low porosity; limburgite /lim-berg-ÿt/ An undersatu- soft limestones, such as chalk, are unsuit- rated basic igneous rock occurring as flows able for major cave development. and dikes and consisting of phenocrysts of augite and olivine embedded in a matrix of limestone pavement A flat exposure of brown basaltic glass. bare limestone resembling a rough pavement and consisting of flat or ridged irreg- lime An imprecise term with several ularly sized and shaped blocks (clints), meanings. To a chemist, lime (also called separated by clefts (grikes or grykes) quicklime) is calcium oxide, CaO, made by formed by the widening of joints by solu- heating LIMESTONE. Slaked lime (made by tion. Grikes can measure up to 0.5 m treating lime with water) is calcium hy- across and frequently 2–3 m deep. droxide, Ca(OH)2. To an oceanographer, the term lime is generally applied to CAL- limestone solution A weathering pro- CIUM CARBONATE, CaCO3, especially when cess that occurs in limestone. Limestone is it is dissolved in seawater or laid down in the only common rock type in which SOLU- the shells of mollusks and other aquatic an- TION is important in the development of its imals. See also carbonate minerals. characteristic landforms. Because it is composed of calcium carbonate, solution is limestone A rock formed from carbon- easily achieved by very weak carbonic acid ate minerals, principally CALCITE but in- (formed from water and dissolved carbon cluding others such as DOLOMITE. dioxide from the air and soil). Many lime- Limestones can be classified broadly on a stones possess a well-developed system of genetic basis as clastic, organic, precipi- joints and bedding planes; the vast major-

206 lithostratigraphy ity of the water flowing through the rock lithification /lith-ă-fă-kay-shŏn/ The pro- passes along these fissures, in which solu- cess by which sedimentary deposits de- tion takes place and they therefore become velop into sedimentary rocks. enlarged, forming better watercourses. In theory more limestone is dissolved as the lithographic limestone /lith-ŏ-graf-ik/ temperature or acidity of the solution in- A dense fine-textured type of limestone, so creases, but it is probably more effective to called because it was formerly used for lith- have a rapid turbulent flow of ample sup- ography. It is a kind of MICRITE. plies of water over a large surface area of rock. lithophile /lith-ŏ-fÿl/ An element that occurs mainly as silicates and is therefore limnology /lim-nol-ŏ-jee/ The scientific most concentrated in the Earth’s crust. study of lakes and other bodies of fresh Lithophile elements include lithium, oxy- water. It considers all the living and non- gen, silicon, and sodium. See also at- living factors that affect them. mophile; chalcophile; siderophile. limonite /lÿ-mŏ-nÿt/ The yellow to lithosol /lith-ŏ-sôl/ AZONAL SOIL with an brown amorphous and cryptocrystalline AC or (A)C profile (i.e. no illuvial B hori- oxidation and hydration products of iron. zon) developed on fresh and imperfectly Goethite and hematite are important con- weathered rock or rock debris. These soils stituents together with colloidal silica and often form on steep slopes where runoff clay minerals. Limonite is found as an al- erodes developing soil and little water per- teration product in all kinds of rocks, as a colates down through the profile to pro- precipitate in bog iron ore, and as a major mote LEACHING and soil development. The constituent of gossan and laterite. surface horizon is typically dark due to organic matter accumulation, but may be lineament A large-scale linear topo- coarse-textured and light colored if slope graphic feature that is structurally con- wash is intense. On eroded land, subject to trolled. Lineaments often appear as long poor agricultural practices, these soils can linear features on aerial photographs and be artificial. A special type is the ranker de- frequently result from faults, where ero- veloped on acidic rocks. These soils are sion has worked selectively on the softer part of the ENTISOL order of the US SOIL material of the fault zone. TAXONOMY. line scale A divided scale given on a map lithosphere /lith-ŏ-sfeer/ That part of to enable distances to be directly measured the Earth including the CRUST and the and read off in terms of distances on the upper MANTLE above the ASTHENOSPHERE. ground. lithospheric plate /lith-ŏ-sfe-rik, -sfeer-ik/ linkage analysis A statistical procedure (tectonic plate) Any one of a series of used to group individuals into clusters. The large blocks of continental or oceanic ma- measures of similarity used are coefficients terial of which the Earth’s crust is com- of association, coefficients of correlation, posed. These plates, whose base is marked or distance measures, depending upon the by the asthenosphere, move across the sur- type of data and the nature of the problem face of the Earth as a result of SEA-FLOOR being analyzed. SPREADING. The six major lithospheric plates are named Eurasian, Indian, Pacific, litchfieldite /lich-feeld-ÿt/ A type of al- American, African, and Antarctic. See also kaline SYENITE containing albite, nepheline, continental crust; oceanic crust; plate tec- and potassium feldspar. tonics. lithic /lith-ik/ Describing anything com- lithostratigraphy /lith-oh-stră-tig-ră-fee/ posed of or related to stone. The branch of STRATIGRAPHY concerned

207 lit-par-lit gneiss

LOAD Major division Subdivision Nature of load (1) dissolved load chemical content, dissolved in water (2) solid debris load (a) suspended load finest solid particles, supported by water (b) saltation load sand/fine gravel, bouncing along bed (c) bed load coarsest material, rolling and sliding on bed solely with lithological features and with strictly the combination of the intertidal naming and elucidating the spatial rela- and SURF ZONES, i.e. the region lying be- tions of rock units. It is the most purely de- tween highest high water and the outer scriptive of the systems of stratigraphic edge of the surf zone. The width of the lit- classification, incorporating the least toral zone varies according to the prevail- amount of inference, and does not take ac- ing wave conditions and the slope of the count of the evolution of organisms as sea floor. The eulittoral zone extends be- shown by contained fossils (see also bios- tween high water and the limit of attached tratigraphy) or of time (see also chronos- plants, which is usually at a depth between tratigraphy). Hence lithostratigraphic units 40 and 60 m. In global terms the littoral are often diachronous (see diachronism). zone occupies an area of about 150 000 sq The hierarchy of lithostratigraphic divi- km. sions consists of SUPERGROUP, GROUP, FOR- MATION, MEMBER, and BED (see also llanos /lah-nohz, lyah-nos/A SAVANNA complex). Formally designated lithostrati- type of grassland found in northern South graphic units (formations) are given geo- America, particularly in the Orinoco River graphical names with capitalized initial basin. letters. load (stream load) The material carried lit-par-lit gneiss /lee-par-lee/ A coarsely along by a river (stream). Material is incor- banded or foliated gneiss consisting of al- porated into a river by its erosive action on ternating layers of quartz-feldspar and the valley bed and walls. It can also be sup- mafic minerals. The felsic material, which plied by groundwater flow, or direct from may have originated by partial melting, has valley sides by wash, CREEP, or LANDSLIDES. intruded along the schistosity planes of the Once in the river, load can be split up as in metamorphic rock. See also migmatite. the table. The predominance of the different types of load varies between rivers and litter The accumulation of leaves and in the same river over time, e.g. DISSOLVED twigs on the surface of the soil. It is the raw LOAD is important in rivers receiving a material for the formation of the soil steady groundwater flow and little over- HUMUS; beneath the fresh litter there is a land runoff; in floods, transporting ability fermenting layer and below this the humus. is increased owing to velocity increases, and debris that at low flow moves as BED Little Ice Age See ice age. LOAD may start to saltate (see saltation), and saltating load may be carried in sus- littoral drift /lit-ŏ-răl/ See longshore pension (see suspended load (def. 1)). Gen- drift. erally bed load is the least important part: in the Mississippi, only 8% of load moves littoral zone The region immediately as bed load, compared with about 65% in flanking the coast on its seaward side, suspension and saltation.

208 long profile loam A type of soil with characteristics N 0° between those of sand and clay. It is usually rich in humus. It drains reasonably 60°E well and yet retains sufficient moisture to promote good plant growth. North Pole loch The Scottish name for a lake. Com- Eq r pare lough. uato S lode A deposit of minerals occupying a seam or vein. Lodes probably formed when Longitude hot gases or liquids were forced through existing rocks before cooling and solidifying. longitude The angular distance of a point on the Earth’s surface east or west of lodestone MAGNETITE that is strongly a central meridian (see diagram), measured magnetic. by the angle between the plane of the meridian through the point and that of the lodgment till Unstratified glacial ma- central meridian. In 1884 it was interna- terial deposited directly from beneath a tionally agreed that the meridian from glacier or ice sheet. Such deposits tend to which the readings would be taken would be somewhat compacted due to the weight be the one passing through Greenwich, of ice at one time above them. See till. London, now considered as 0 degrees east, Compare ablation till. west. In passing through 15 degrees of longitude at the Equator there is a local time loess /loh-iss/ A fine-grained, unconsoli- difference of one hour. See also latitude. dated yellowish sedimentary deposit originating as windblown soil. It is very soft and longitudinal dune See seif dune. easily eroded to form gullies. It is porous but can form extremely fertile soil. The longitudinal wave See primary wave. thickest deposits occur in northern China; it is also found widely in the USA, central long profile A section drawn from Europe, central Asia, and Argentina. source to mouth along a stream or river, showing the changes in gradient through- Lomonosov Ridge /lom-ŏ-noss-ŏf/A out its length. The ideal form is an expo- vast submarine ridge that effectively di- nential concave curve, with far greater vides the Arctic Ocean into two separate gradients at the head than at the mouth. basins. The ridge was first discovered by This profile results from the fact that dis- Russian scientists while they were conduct- charge increases downstream and hence ing their oceanographic surveys from field velocity increases, so less gradient is stations set up on drifting pack ice. The needed to transport the load supplied. Si- ridge runs directly underneath the North multaneously the size of debris decreases Pole, stretching from Ellesmere Island, downstream, most probably owing to north of Canada, across to the Novo abrasion of the debris, but possibly owing Sibirsk Islands off the Siberian mainland. to sorting, and so there is a further reduc- Such data as has been collected suggests tion in energy needed, allowing a further that the ridge may be a faulted feature reduction in gradient. rather than volcanic. Soundings across the Surveyed profiles in fact rarely appear ridge have revealed what appears to be a as the ideal profile; rather they are a series truncated upper surface lying at a depth of of partial curves, due to past base-level some 1400 m. The ridge is named for the changes, with further irregularities due to Russian writer and chemist Mikhail Vasile- lakes, pools, bars, hard rock bands, etc. In- vich Lomonosov (1711–65). sufficient time to grade a smooth profile is

209 longshore bar

source beach and the net effect of swash and backwash is a small longshore movement. Longshore currents aid this process. In most areas one set of waves predominates, mouth resulting in an undirectional longshore

base level movement, but some coasts may be influ- height above enced by two sets in opposition, causing distance from source the material to move to and fro along the same stretch of beach. Ideal long profile long wave (Rossby wave) The smooth broad wave-shaped contour patterns on a pressure chart of the upper atmosphere. a general cause of irregularity, as well as They have wavelengths up to about 2000 the fact that if a river flows over different km and there are usually either four or five lithologies, the size of debris supplied will such waves around the westerly wind belt vary: the coarser the debris the steeper the of the N hemisphere. Both heat and mo- slope needed, so gradients over sandstone mentum are transferred poleward by the are steeper than those on shale and lime- long waves and they also have a significant stone. See also cross profile; river. effect on CONVERGENCE and DIVERGENCE at the surface. Long waves are also named (ridge; full) A sand BAR longshore bar Rossby waves for the Swedish–American lying roughly parallel to the coastline, in meteorologist Carl-Gustav Arvid Rossby the intertidal zone or in the surf zone. (1898–1957). longshore current The current that is lopolith /lop-ŏ-lith/ A large concordant generated over the whole width of the SURF igneous intrusion, which is saucer-shaped ZONE at those times when waves approach in cross section. The type example is the and break at an angle to the shore. Long- Duluth gabbro, Canada, which has a vol- shore currents tend to flow parallel to the 3 shore, and probably attain their maximum ume of approximately 200 000 km . rate when the wave crests near breakpoint are angled approximately 30° to the shore- lough /lok/ The Irish name for a lake. line. Field measurements have revealed Compare loch. that such wave-induced currents are roughly of uniform speed across the whole Love wave (L wave; Q-wave) A surface width of the breaker zone, but that they wave produced by an earthquake in which rapidly weaken seaward of the surf zone. the particle motion is in a horizontal direc- Tidal and other currents may be superim- tion, perpendicular to the direction of posed upon the longshore currents, tending wave propagation. It causes multiple re- either to strengthen or to nullify them. flections between the surface and the top of the layer of rock below. It is named for the longshore drift (littoral drift) The British mathematician Augustus Edward movement of beach materials, sand or Hough Love (1863–1940) who discovered shingle, along the shore parallel with the it. See also primary wave; Rayleigh wave; coastline. This process is basically due to secondary wave; seismic wave. the effects of waves breaking on the beach obliquely. Despite WAVE REFRACTION, low See depression (def. 1). waves infrequently break perfectly perpendicularly to a beach and their SWASH tends low index The state of the westerly cir- to move material landward diagonally. culation when flow is weak and there is ap- However, the returning water, in the form preciable meridional transfer, as in a of the BACKWASH, moves directly down the BLOCKING situation.

210 lysimeter low-velocity zone See asthenosphere. tary rock formed of material that was once mud. Examples include MUDSTONE, SHALE, loxodrome /loks-ŏ-drohm/ See rhumb and SILTSTONE. line. luxullianite /luks-ul-ee-ă-nÿt/A TOUR- luster The appearance of the light re- MALINE-QUARTZ-FELDSPAR rock produced flected from the surface of a mineral, which by the boron pneumatolysis of granite. is a qualitative characteristic often providing an aid to identification. The kinds of L wave See Love wave. luster are: 1. metallic – displayed by galena and lysimeter (evaporimeter) An instrument pyrites. for measuring the amount or rate of down- 2. vitreous – displayed by glass and quartz. 3. resinous – displayed by amber. ward water percolation and water storage 4. silky – displayed by gypsum. in a vegetated block of soil that is enclosed 5. pearly – displayed by talc and muscovite. (in a tank, column, or plot) and open to the 6. adamantine – displayed by diamond. sky. From the measurements an estimation of the rate of EVAPOTRANSPIRATION can be lutite /loo-tÿt/ Any fine-grained sedimen- obtained.

211 M

maar /mar/ A shallow volcanic crater proximations of the compositions of the not associated with a single vent or cone. magmas from which they originated. Maars are thought to be formed by many small explosive eruptions, sometimes at the magma chamber A reservoir within the top of a DIATREME. Many fill with water Earth’s lithosphere in which molten rock and form lakes. accumulates before being emptied at the Earth’s surface through a volcano. Such a macroclimate /mak-roh-klÿ-mit/ The chamber exists beneath most volcanoes, general climate of a large area. Compare and is replenished from below after being microclimate. emptied during a volcanic eruption. macrometeorology /mak-roh-mee-tee-ŏ- magmatic differentiation /mag-mat-ik/ rol-ŏ-jee/ The study of the large-scale at- A number of processes causing the gradual mospheric processes responsible for the evolution of the composition of an igneous differentiation of the macroclimates on the melt and the separation of the MAGMA into Earth’s surface, e.g. the general circulation two or more fractions of contrasting com- of the atmosphere. Compare mesometeo- positions. Several mechanisms are recog- rology; micrometeorology. nized: 1. Fractional crystallization. One of the main processes producing composi- tional variation in igneous rocks is the maculose /mak-yŭ-lohs/ Having a knot- gravity settling of early-formed minerals. ted or spotted appearance, usually describ- Such crystals having densities higher than ing a metamorphic rock. The irregularities the enclosing magma sink to the bottom of are thought to result from the growth of the magma chamber. In some cases, light new minerals during heating. minerals such as leucite float upward. The composition of the residual liquid differs maf-ik mafic / / Describing dark-colored from that of the original melt because of minerals. Mafic minerals are approxi- the removal of the solid crystalline ma- mately equivalent to FERROMAGNESIAN MIN- terial. 2. Filter pressing. During the later ERALS. Compare felsic. stages of crystallization, residual liquid may be squeezed out from a crystal mush magma /mag-mă/ A hot silicate liquid and rise upward in the crust. 3. Flow dif- beneath the Earth’s surface containing sus- ferentiation. The movement of a magmatic pended crystals and dissolved gases. IG- liquid within a dike or lava flow often con- NEOUS ROCKS are formed from the centrates phenocrysts toward the center of crystallization and solidification of magma the body. 4. Liquid immiscibility. During and are said to be magmatic. A lava repre- cooling, two liquid fractions of a magma sents magma extruded at the Earth’s sur- may become immiscible and separate. face whereas magma solidifying at depth Sulfides separate from a silicate magma gives rise to HYPABYSSAL and PLUTONIC during the early stages of crystallization. ROCKS. During the solidification of magma, The role of liquid immiscibility in igneous the volatile constituents escape and thus petrogenesis is limited, but this process compositions of igneous rocks are only ap- may account for the segregation of carbon-

212 magnetic stripes ate liquids from mafic alkaline magma. 5. magnetic interval The variable period Gaseous transfer. The upward streaming of time between MAGNETIC REVERSALS in the of gas bubbles and dissolved materials re- Earth’s polarity. The paleomagnetic scale sults in the concentration of volatiles near must be calibrated by some independent to the top of a magma body. method of assessing time, such as radiometric dating. The rocks formed during a magnesite /mag-nĕ-sÿt/ A white, yellow, magnetic interval are known as a MAG- or gray major mineral form of magnesium NETIC DIVISION, which can be used in the carbonate, MgCO3. It crystallizes in the correlation of stratigraphic successions. trigonal system and occurs in masses in See also paleomagnetic correlation. sedimentary rocks. It is used as a refractory material and as a source of magnesium. See magnetic iron ore See magnetic. also carbonate minerals. magnetic meridian An imaginary line magnetic anomaly A departure from on the surface of the Earth joining the mag- the predicted value of the Earth’s magnetic field at a point on the Earth’s surface. netic poles. It indicates the direction of the horizontal component of the Earth’s mag- magnetic declination See angle of dec- netic field. lination. magnetic pole Either two of the points magnetic dip See dip (def. 2). on the Earth’s surface at which the lines of magnetic force are vertical. These points magnetic division A stratigraphical are slowly changing with time and do not unit (see stratigraphy) consisting of the coincide with the geographical poles. rocks formed during one MAGNETIC INTER- VAL (see also paleomagnetic correlation). magnetic reversal (polarity reversal) The establishment of magnetic divisions Changes in the direction of the Earth’s constitutes an important method in the magnetic field’s induction by 180° (i.e., the CALIBRATION and CORRELATION of strati- North Pole became the South Pole, and graphic sequences, especially in the Neo- vice versa) that have been revealed by paleogene Period. magnetic studies (see paleomagnetic correlation). Based upon these reversals, magnetic elements The seven parame- which have been recorded in rocks forming ters of the geomagnetic field at a particular at that time, a polarity/time scale has been locality on the Earth’s surface. They in- established for the last 4.5 million years of clude a value for the field with respect to the Earth’s history. geographic north (X), east (Y), the vertical (Z), the horizontal (B), the total induction magnetic storm A more or less violent (F), its ANGLE OF DECLINATION (D), and in- disturbance of the Earth’s magnetic field, clination (I) (see dip (def. 2)). probably caused by the arrival of charged magnetic equator An imaginary line particles ejected from the Sun in solar round the Earth joining all points where flares. the ANGLE OF DECLINATION is zero. magnetic stripes The roughly parallel magnetic field A field of force that ex- bands of ocean floor with alternating di- ists as a result of a circuit carrying an elec- rections of induction: a normally magne- tric current, or the presence of a permanent tized stripe will be enclosed by two magnet. reversely magnetized stripes. These patterns can be picked up by shipborne instru- magnetic inclination See dip (def. 2). ments and used to correlate areas of ocean floor offset along TRANSFORM FAULTS.

213 magnetite magnetite /mag-nĕ-tÿt/ (magnetic iron covering of hair or fur. They are skeletally ore) A shiny black magnetic mineral, an distinguished from reptiles by having only oxide of iron, Fe3O4, sometimes containing one bone in each lower jaw, which also dif- some magnesium. It crystallizes in the fers in its articulation with the cranium. cubic system and occurs as granular or Mammals evolved from mammal-like compact masses in various igneous and reptiles in the late Triassic; the earliest metamorphic rocks. It is a member of the forms were small flesh-eating active crea- SPINEL group and an important source of tures similar to modern shrews. Fossils of iron. these primitive Mesozoic mammals are rare and consist mainly of teeth. Mammals magnetometer /mag-nĕ-tom-ĕ-ter/An existed throughout the dominance of the instrument used for measuring the strength dinosaurs but did not achieve supremacy of the Earth’s magnetic field. on land until the Cenozoic, after the extinction of most reptilian groups, when magnetopause /mag-nee-tŏ-pawz/ The their main radiation took place. The two region where the Earth’s MAGNETOSPHERE main mammal groups, the Marsupialia ends and interplanetary space begins. and Eutheria, separated from the ancestral stock by the late Cretaceous. They both magnetosphere /mag-nee-tŏ-sfeer/ The give birth to living young (as opposed to area around the Earth in which the Earth’s laying eggs). The most primitive group is magnetic field is present. The field is con- the Monotremata, which comprises egg- fined to this area as a result of the interac- laying mammals whose modern represen- tion between the Earth’s magnetic field and tatives are the duck-billed platypus and the solar wind. The magnetosphere ex- echidnas (spiny anteaters). tends out away from the Earth’s surface for a much greater distance on the side away man Modern man, Homo sapiens, be- from the Sun. longs to the family Hominidae of the mammalian order PRIMATES. Primates that are magnitude See earthquake. sometimes classified as hominids are known from the Miocene and perhaps malachite /mal-ă-kÿt/ A blue-green min- even earlier. In the Pliocene and early Pleis- eral form of hydrated copper carbonate, tocene of Africa the remains of a variety of Cu2CO3(OH)2. It crystallizes in the mono- manlike creatures have been found; they clinic system and occurs in the oxidation are usually placed in two genera, Australo- zone of copper deposits. It is used for mak- pithecus and Homo. Most authorities being ornaments and as a semiprecious gem- lieve that Homo evolved from one of the stone, and is a minor source of copper. species of Australopithecus. Homo erectus, a hominid of robust construction, was pre- malignite /mal-ig-nÿt/ An alkaline sent in Africa, Asia, and possibly Europe nepheline-containing igneous rock, a type during the mid-Pleistocene. In the later of SYENITE. Pleistocene two species of Homo are recognized: modern man, H. sapiens, and mallee /mal-ee/ A type of scrubland that Neanderthal man, H. neanderthalensis. occurs in semiarid regions of Australia. Primitive stone tools have been found from The vegetation consist mostly of low- rocks dated as Pliocene in age. growing kinds of eucalyptus. manganese nodule A deep-sea mineral Mammalia /mă-may-lee-ă/ A class of concretion. Somewhat similar in form to warm-blooded vertebrates that nourish PHOSPHORITE nodules, manganese nodules their young on milk secreted by mammary (one of the AUTHIGENIC minerals) are the glands in the female. Mammals have a rel- only potential deep-sea mineral deposit atively large brain, morphologically differ- that could be economically recovered from entiated teeth, and (in most) a body the deep ocean floor utilizing present-day

214 map series technology. They seem to be largely con- has been brought to the surface by a vol- fined to deep-sea clays, but have also been canic explosion. found in association with oozes. The nodules are surprisingly heavy, averaging some mantle plume An upwelling of buoyant 24% manganese, 14% iron, 1% nickel, hot mantle material that is hypothesized to 0.5% cobalt, and 0.5% copper. The vari- originate deep within the Earth, possibly at ous minerals frequently form roughly con- the core–mantle boundary. Mantle rock centric layerings on stones and other hard begins to melt, thereby becoming less objects, such as sharks’ teeth, present on dense, and as a result this material rises to the sea floor. These layerings build up the base of the Earth’s lithosphere where it gradually on account of the slow precipita- spreads out. In so doing it causes the over- tion from sea water of manganese oxides lying crust to be domed up. The surface ex- and other mineral salts. Surveys have re- pression of a mantle plume is known as a vealed that the nodules are widely distrib- HOT SPOT. uted on the deep ocean floor and are sometimes surprisingly abundant. For ex- map A graphic representation on a plane ample, in certain parts of the Pacific surface of the Earth’s surface or part of it, Ocean, concentrations of 20 000 tonnes showing its geographical features. These per sq km have been located. are positioned according to pre-established geodetic control, grids, projections, and manganite /mang-gă-nÿt/ A black or scales. gray semiopaque lustrous mineral form of hydrated manganese oxide, MnO(OH). It map projection Any method of repre- crystallizes in the monoclinic system and senting the curved surface of the Earth on occurs in hydrothermal veins, usually asso- a flat plane surface such as a piece of paper. ciated with BARYTES or CALCITE, and in A grid printed on the map represents lines deep-sea MANGANESE NODULES. It is a minor of latitude and longitude, which are drawn source of manganese. according to different mathematical formulas, depending on the area to be mantle That part of the Earth’s interior mapped. Some projections give the prop- beneath the base of the CRUST and above erty of equal area (homolographic), others the Earth’s OUTER CORE, roughly between true bearings (azimuthal), and others 30–2900 km beneath the Earth’s surface. It achieve a compromise. is thought to consist of ferromagnesian silicate minerals such as olivine and pyrox- map scale The ratio of the distance mea- ene. Essentially it behaves as a solid, sured on the map to the distance on the although within the asthenosphere it may ground between the same points. On a map be partly molten. with a scale of 1:50 000, for example, one unit on the map is equal to 50 000 of the mantled gneiss dome (gneiss dome)A same unit (e.g. centimeters) on the ground. structure present in Precambrian shield Most maps now are metric and therefore areas, consisting of a central core of gran- have ratios such as 1:50 000, 1:100 000. ite rock surrounded by regionally meta- The larger the ratio, the smaller the scale of morphosed rocks, mainly gneisses, that has the map. been exposed by erosion and covered by a The scale used is largely determined by series of younger sediments. The area was the purpose of the map: for index maps then subjected to orogenesis, during which and wall charts of the world a small-scale the granitic core was remobilized and rose map would normally be used; for town up, causing the overlying strata to be plans the larger the scale, the more accu- arched upward. rate and detailed the plan can be. mantle nodule A small fragment or map series A collection of maps, usually block of rock from the Earth’s MANTLE that at the same scale, using the same specifica-

215 maquis tions and generally identified by a series 200 m. The shallow marginal seas occupy number. Map series normally cover indi- the continental shelf areas and are thus vidual countries but can cover the entire generally less than 200 m deep. There is an Earth’s surface, e.g. the International Map obvious contrast between the open ocean of the World 1:1 000 000. and those sea areas juxtaposed between mainland and groups of offshore islands. maquis /mah-kee/ A type of scrubland Compare inland sea. that occurs in the Mediterranean districts of France. The vegetation consists mainly marigram /ma-ră-gram/ The graphic of deep-rooted low-growing evergreen record obtained from the use of an instru- bushes and trees, whose leaves are able to ment that monitors the rise and fall of survive the hot dry summers. water level throughout a tide cycle. From this, an average water-level curve, known marble A rock composed largely of cal- as a tide curve, can be plotted. Such curves cite or dolomite, produced by the regional or records enable oceanographers to iden- or contact metamorphism of limestones. tify the types of tidal motion affecting par- During the metamorphism of a pure lime- ticular areas. stone, calcite recrystallizes to produce coarser interlocking grains, often with a marine borer An organism that is capa- preferred orientation imparting a weak ble of performing marine erosion, usually schistosity. A variety of minerals may form by a boring process. Such erosion may be when impure limestones are metamor- limited to fairly local areas but is signifi- phosed, including quartz, diopside, cant not only because of the way in which forsterite, grossular, tremolite, micas, and rocks become riddled with holes but also epidote. because many boulders, smaller stones, shells, and flakes of rock may be affected in marcasite /mar-kă-sÿt/ A pale bronze- this way. Particularly susceptible are soft yellow iron sulfide mineral, FeS2, polymor- limestones and sandstones. Borers abound phous with pyrite but having ortho- in the sea and are particularly active in the rhombic symmetry. Marcasite is found shallow littoral zone. Boring algae are ef- mainly as a replacement mineral in sedi- fective, and certain sponges attack rocks; mentary rocks and as radiating nodules, es- some worms, chiefly the annelids, together pecially in chalk. It is used as a with a variety of mollusks, excavate pits semiprecious gemstone. and tubes in rocks. Crustaceans and fish such as the eel often enlarge the cavities mare’s tails See cirrus. they tend to occupy. Serious damage may even be caused to timber and concrete margarite /mar-gă-rÿt/ A pearly pink structures, for example groins. mineral form of calcium aluminum silicate, (Ca,Al)Si2O10(OH)2. It crystallizes in the marine ecosystem The littoral and sub- monoclinic system and is a member of the littoral zones are characterized by an enor- MICA group of minerals. mous diversity of plant and animal life which, taken with their physical environ- margin See plate boundary. ment, constitute the marine ecosystem. The system is highly dynamic. Most sublittoral marginal sea (adjacent sea) A sea area plant life occurs as plankton but there are that is significantly open to the adjacent also a number of bottom-living seaweeds. ocean. Marginal seas may be of the deep The range of species of sublittoral animals type or the shallow (shelf sea) type. The is enormous. The food chain of sublittoral deep marginal seas usually lie between the life begins with plant plankton (phyto- continental blocks and offshore submarine plankton), which consume nutrient salts ridges, which may or may not have islands, present in the sea water and also carbon and often display depths well in excess of dioxide. Animal plankton (zooplankton)

216 mass movement feed actively on the phytoplankton and are marl An ARGILLACEOUS sedimentary rock themselves eaten by the smaller species of in the form of MUDSTONE that has a high fish. The small fish are consumed by larger proportion of calcareous material in its fish, and so on. Decomposition of dead composition. fauna and flora refurbishes the supply of nutrients. Thus, the individual organisms marsh An area of soft wet land that has in the marine ecosystem are interdepen- poor drainage and frequently becomes wa- dent. The marine ecosystem is highly sus- terlogged. The dominant vegetation con- ceptible to damage from marine pollution sists of grasses, reeds, and sedges. Marshes and other forms of human intervention. occur in depressions left by retreating glaciers and in estuaries, especially where there marine geology The scientific study of is a DELTA. On flat low-lying land they gen- the floor of the ocean and its various phys- erally overlie impermeable rock where the ical features, rocks, and sediments. water table is near the surface. If an estuarine marsh is periodically covered by the maritime air mass A large mass of air tide, it can become a salt marsh. The drain- that has its source region over the ocean, or ing of marshes to create arable land com- is modified after passage over an expanse pletely changes the ECOSYSTEM. See also of ocean. Such air masses have significant bog; swamp. effects on weather, although the effects of, say, a tropical air mass and a polar air mass Marsupialia /mar-soo-pee-ay-lee-ă/ will be very different. Mammals whose offspring are born at an immature stage and continue their development within a special pouch (mar- maritime climate The generalized char- supium) on the belly of the mother. This acteristic climate of the temperate parts of and other (skeletal) differences distinguish the Earth’s surface that are affected by the them from the placental mammals (see Eu- sea. The sea acts as a vast storehouse of the theria). Fossils show that the marsupials Sun’s energy and changes its temperature diverged from the placentals in the Creta- only slightly from winter to summer. ceous, when they appear to have been Hence areas experiencing a maritime cli- much more abundant than they are today. mate have low annual ranges of tempera- They are known from the Tertiary of Eu- ture (cool summers and mild winters) and rope and America, but as the placentals be- an even distribution of precipitation came the more successful group, the throughout the year with no dry season. marsupials gradually became confined to Some coastal areas may not experience a isolated areas such as Australasia and maritime climate if the prevailing winds South America. In these regions marsupial are offshore. species evolved to resemble placental species elsewhere (see parallel evolution). marker bed (marker horizon) A bed of rock having some distinctive characteristic massif /ma-seef/ An area of high ground of lithology or contained fossils that per- or mountain landscape with no level areas mits its easy recognition wherever it oc- and features that distinguish it from the curs. Such a bed can often be inferred to surrounding terrain. The constituent rocks represent a very short period of time or, in are generally more rigid and older than the case of beds resulting from certain those around the massif. kinds of volcanic eruption, to be virtually instantaneous in its formation. Marker mass movement (mass wasting) The beds thus have great use in STRATIGRAPHY downslope movement of rock and soil ma- and in the CORRELATION of strata. terial on unstable slopes, that is slopes whose angle is greater than the natural markfieldite /mark-feeld-ÿt/ A por- ANGLE OF REST of the constituent material phyritic microdiorite. See diorite. and whose SHEAR strength is not capable of

217 mass transport current

old l l profile a f r e o

n e a d l i p l new profile s r a e h s

debris pile

Fig. 1: Slope angle becomes Fig. 2: Shear plane develops Fig. 3: Slide/fall lowers too steep to be stable slope angle

Mass movement maintaining this angle. Gravity, together mass wasting See mass movement. with other agents such as wind and water, may act to produce a mass movement of matrix /may-triks/ The fine-grained some kind, whereby slope angle becomes component of a rock into which the reduced to a more stable value. Mass coarser components are set. In sedimentary movements constitute flows, slides, and rocks, the matrix is often the material that falls of rock material from a slope; they cements the larger grains or pebbles to- may be sudden and short-lived, as in the gether. In igneous rocks the GROUNDMASS is case of ROCKFALLS, or very slow and long- equivalent to the matrix. lived, as in the case of CREEP. Slides and falls act along a shear plane within the maximum temperature The highest rock. Flows, on the other hand, involve in- value recorded by a thermometer in a spec- ternal deformation and movement by over- ified time. The normal period between ob- turning of the material. Commonly mass servations is the day, but the mean monthly movements are compound: flows on gen- maximum and mean annual maximum are tler slopes often start on steeper slopes often extracted from the daily data. The above as a slide or fall. See also basal sap- maximum temperature is recorded by a ping; earthflow; earthslide; landslide. mercury thermometer, which has a constriction near its bulb. As the temperature mass transport current The slow drift rises, the force of expansion is strong and of water beneath waves, in the direction of pushes the thread of mercury up the col- wave propagation, that arises from the na- umn. If the temperature then falls, the con- ture of ORBITAL MOTION beneath waves. Be- traction force is weaker and the cause, in the case of deepwater waves, the constriction prevents the mercury return- orbits of water particles are almost circular ing to the bulb. It remains at the position of but not quite closed 1oops, each particle of the highest temperature. On a world scale, water in motion not only orbits but also the highest maximum air temperature moves slowly forward in the direction of recorded, 57.8°C (136°F), was reported wave propagation. This asymmetry or or- from Azizia, Libya, on September 13, bital motion in deep water becomes even 1922. more pronounced in shallow water, for here the orbits become increasingly ellipti- meadow soil See gley soil. cal with decreasing depth. Theoretically a cork placed on the sea surface in deep mean The arithmetic mean or average is water should both bob up and down and a statistical method of representing the shift slowly in the direction of wave travel. magnitude of a data set. It is obtained by

218 mechanical weathering summing the individual values and divid- centrating erosion on the outside of bends ing by the number of individuals. It gives with deposition on the inside, and there- meaningful results only when the data have fore continuing meander development a normal distribution. For example, the downstream. Individual meanders will mean rainfall in desert areas is often calcu- tend to migrate downstream by this pro- lated on the basis of many years without cess of erosion of the outside of meander rain interspersed with exceptionally heavy bends, and deposition on the insides. If the falls and may indicate an annual total that erosion and deposition exactly comple- has never been experienced. ment each other, the meander train retains its form but migrates down valley. How- ever, erosion rates typically exceed deposition rates in some meanders, leading to maximum erosion modifications. The distribution of velocity of stream deposition flow in a meandering reach is not even: be- as point bar cause of the pattern of helical flow, veloc- deposition ity is highest on the outside of meander as point bar bends, least on the inside. There is therefore active erosion in the former areas, de- streamline position in the latter. As the outsides erode migration of of maximum flow and move downstream, so the insides of meander the bends sediment and move downstream as well. Thus the whole reach migrates Meander down valley. See also entrenched meander; incised meander; ingrown meander. meander /mee-an-der/ A sinuous curve or loop in a river channel. The American mean sea level The level of the surface geologist William Morris Davis (1850– of the sea averaged over all tide states, 1934) stated that meanders originated being actually determined from the average when a river used up excess energy by side- of hourly heights. The periods over which cutting when it could no longer cut down, but since Davis other causes have been put readings are recorded vary; computations forward: 1. Local erosion and deposition have been based on a month, a year, and patterns will lead to the development of even a 19-year period. All levels, including one meander, which will then influence the those of the sea surface, coastal land eleva- stream-flow pattern, propagating the sys- tions, and the levels of coastal structures tem of bends down river. The initiation of are referred to a chosen datum plane local erosion and depositions is due to a known as the tidal datum. See also sea chance abrasion in the channel, e.g. a fallen level. tree or stream-bed shoals. 2. Long profiles of rivers characteristically develop POOLS mechanical weathering (physical weath- AND RIFFLES. Shallowness over the riffles ering) The breakdown of solid rock ei- leads to increased velocity and bed rough- ther through the development of cracks ness, and hence greater energy loss over the produced by strain as a result of internal or riffles than the pools. Since rivers try to dis- external stresses, or by the movement away tribute energy loss as evenly as possible from each other of the individual grains throughout their lengths, it may be that within a rock. The cause of stresses within meander development in the pools would rocks are many and mechanical weathering be one way of equalizing energy loss. Oth- can operate at a number of different scales. ers have said the riffles are the site of initi- The major types of mechanical weathering ation. 3. Irrespective of the cause, the are: FREEZE-THAW weathering, EXFOLIA- divergence of primary downstream flow TION, GRANULAR DISINTEGRATION, UNLOAD- will set up a secondary HELICAL FLOW con- ING phenomena, WETTING-AND-DRYING

219 medial moraine

WEATHERING, FLAKING, and INSOLATION of the slumping of large masses of unstable WEATHERING. rock debris, associated with destructive PLATE BOUNDARIES. medial moraine A ridge or spread of angular material extending lengthwise melanite /mel-ă-nÿt/ A black or dark along the center of a glacier. Moraines may brown member of the GARNET group of be formed as a result of the joining of two minerals, containing significant amounts LATERAL MORAINES as two glaciers con- of titanium. verge, or of material originating at a spur between two confluent glaciers. Beaded melanocratic /mel-ă-noh-krat-ik/ See forms occur, reflecting a greater availabil- color index. ity of rock waste in summer periods. On the melting of the glacier the medial melilite /mel-ă-lÿt/ A member of a group moraine may be left as a ridge running of minerals with composition (Ca,Na)2- 2+ down the middle of the valley. See also (Mg,Fe ,Al,Si)3O7 found mostly in ther- moraine. mally metamorphosed limestones and some strongly undersaturated basic ig- median ridge See mid-ocean ridge. neous rocks such as NEPHELINITES. median valley (median rift; axial rift melilitite /me-lil-ă-tÿt/ A member of the zone) A deep cleft that follows the axis of NEPHELINITE group of minerals. a MID-OCEAN RIDGE, marking the site of a constructive PLATE BOUNDARY. melteigite /mel-tee-ă-jÿt/ See ijolite.

Mediterranean climate In Köppen’s meltwater Melted ice from a glacier, CLIMATIC CLASSIFICATION the distinctive cli- which may travel over the surface of the ice mate of hot summers and mild wet winters (supraglacial), within it (englacial), or be- of the Mediterranean basin, which he des- neath it (subglacial), carrying material with ignated a Cs climate. Similar climatic it and achieving a certain amount of sort- regimes are found in California, Chile, S ing and rounding of rock fragments in the Africa, and W and S Australia. process. At ice margins meltwater is responsible for the formation of distinctive meerschaum /meer-shăm, -shawm/ (sepio- landforms, such as KAMES and ESKERS, lite) A mineral form of hydrated magne- while the presence of meltwater at the ice- sium silicate, Mg2Si3O6(OH)4. It is yel- rock interface is essential for successful lowish, pink, or white and claylike, and is glacial erosion in the form of GLACIAL used as a building stone and for making to- PLUCKING. See also meltwater channel. bacco pipes. meltwater channel (glacial drainage megabreccia /meg-ă-brech-ee-ă, -bresh-/ channel) A channel cut by glacial melt- A type of very coarse BRECCIA, with the water in either solid rock or drift deposits, largest fragments more than 1 km across, the most favorable locations being beneath often the result of rockfalls or landslides. the ice, where water restricted to tunnels Compare microbreccia. can move at high velocity, carrying an ero- sively powerful load, and marginal to the megaripple /meg-ă-rip-ăl/ See sand wave. ice, where large volumes of water derived from the ice surface can accumulate and melange /may-lahnzh/ A sedimentary flow. After the disappearance of the ice, rock consisting of a jumbled mass of vari- these channels bear little similarity to nor- ous rock fragments including schists, lime- mal fluvially-developed ones. They may stones, cherts, quartzites, graywackes, and start abruptly, frequently in unlikely loca- a wide range of other rock types. They are tions, crossing present-day drainage di- thought to have been deposited as a result vides; they may be unusually straight and

220 meridian steep-sided and may contain only a small Francis Richter in 1956, is the one cur- stream compared with the channel size, or rently used in the USA. It ranges from I sometimes no stream at all. (not felt except by a very few) up to XII (damage total; objects thrown into the air). member A division in the lithostrati- Compare Richter scale. graphic classification of bodies of rock (see lithostratigraphy; stratigraphy). It is part Mercator projection /mer-kay-ter/An of a FORMATION, distinguished by some orthomorphic cylindrical MAP PROJECTION particular lithological characteristic. that is most commonly used for navigation charts. Although there is great exaggera- Mercalli scale /mer-kah-lee/ A description of area in the higher latitudes, the tive scale devised in 1902 by the Italian vol- straight-line bearings are correct. It is canologist Giuseppe Mercalli (1850–1914) therefore used for portraying information for measuring the intensity of earthquakes of a directional nature, e.g. ocean currents. in terms of the damage they cause. The Similar to the Mercator is the TRANSVERSE scale underwent several modifications in MERCATOR PROJECTION. The projection is Europe and in 1931 was adapted and pub- named for the Dutch cartographer and lished in the USA by the American seismol- seismologist Gerardus Mercator (1512– ogists H. O . Wood and Frank Neumann as 94). See also orthomorphic projection. the Modified Mercalli scale (MM scale). This scale, following further modification meridian /mĕ-rid-ee-ăn/ A line of longi- by the American seismologist Charles tude passing through any given point and

poles cannot be shown 80°

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80° 180° 150° 120° 90° 60° 30° 0° 30° 60° 90° 120° 150° 180°

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Mercator map projection

221 meridional circulation the North and South Poles. Meridians are vective, depending upon their mode of ori- semicircles of equal length and cut the gin. A denser network of observing sta- Equator and all other parallels at right an- tions is required to obtain adequate data gles. The Greenwich meridian is taken to for this scale of investigation than would be the prime meridian from which all oth- be needed in macrometeorology. ers are measured. Distance between two adjacent meridians decreases with distance mesopause /mes-ŏ-pawz, mee-sŏ-/ The from the Equator. See also magnetic merid- upper limit of the mesosphere at a height of ian. about 80 km. It represents a temperature minimum (–80°C) between the mesosphere meridional circulation /mĕ-rid-ee-ŏ-năl/ and the ionosphere. Any large-scale atmospheric movement from south to north or vice versa. Compare mesosphere /mes-ŏ-sfeer, mee-sŏ-/ 1. zonal circulation. The layer of the upper atmosphere between 50 km and 80 km above the Earth’s sur- mesa /may-să/ An isolated flat-topped face. Through the layer temperatures de- hill with steep sides most frequently found crease from values of about 0°C at the in old-established landscapes based on hor- stratopause, its lower boundary, to about izontally bedded strata. This often exhibits –80°C at the mesopause, its upper bound- alternate layers of comparatively hard and ary. It is not thought to affect weather con- soft rocks, which on dissection produce ditions in the troposphere. steep and flat slopes respectively. Mesas 2. That part of the Earth’s interior lying be- often have almost vertical upper slopes, de- neath the ASTHENOSPHERE. veloped on the resistant cap rock, followed by much flatter slopes extending down to a mesostasis /mes-ŏ-stay-siss, -stass-iss, general plain. The cap rock may be a lava mee-sŏ-/ Glassy or late-stage crystalliza- flow or the same effect can be produced tion minerals, such as quartz, alkali where a resistant soil horizon has been cre- feldspar, zeolites, or calcite, which infill IN- ated. TERSTICES in igneous rocks. meseta /may-set-ă/ The high plateau re- mesothermal deposit /mes-ŏ-th’er-măl, gion in central Spain that tilts to the west. mee-sŏ-/ See hydrothermal process. mesocratic /mes-ŏ-krat-ik, mee-sŏ-/ See Mesozoic /mes-ŏ-zoh-ik, mee-sŏ-/ The color index. era of PHANEROZOIC time that followed the PALEOZOIC and preceded the CENOZOIC, mesolite /mes-ŏ-lÿt, mee-sŏ/ A white fi- comprising three periods: the TRIASSIC, brous calcium-containing ZEOLITE mineral, JURASSIC, and CRETACEOUS. It lasted about whose composition is intermediate be- 179 million years, from 245 million years tween those of natrolite and scolecite. ago to the beginning of the Paleocene, 66.4 million years ago. It is often known as the mesometeorology /mes-ŏ-mee-tee-ŏ-rol- age of reptiles, for it was during this time ŏ-jee, mee-sŏ-/ The study of the atmos- that the Reptilia underwent their major pheric processes that give rise to weather evolutionary radiation, producing such di- phenomena having scales that are interme- verse forms as the dinosaurs, pterosaurs, diate in size between those of MACROMETE- and ichthyosaurs. Most reptiles became ex- OROLOGY and MICROMETEOROLOGY. The tinct before the end of the era, and the actual size of systems in mesometeorology mammals, birds, and flowering plants first is a matter of some dispute but it is some- appeared. Invertebrate groups important where from approximately 10 to 150 km. during the Mesozoic include the Am- Mesosystems can be subdivided into those monoidea and Belemnoidea. The Alpine topographically induced, free-atmosphere episode of orogenic activity began at the convective, and free-atmosphere noncon- end of this era. See also geologic timescale.

222 meteorite metabasite /mĕ-tab-ă-sÿt/ Any meta- with regional metamorphism are those morphosed basic rock. incurred at depth within the Earth’s crust. 3. Dislocation metamorphism, developed metamorphic differentiation /met- as a result of the mechanical deformation ă-mor-fik/ In medium- to high-grade or CATACLASIS of preexisting rocks and lo- metamorphic rocks some constituents, es- calized along shear belts, fault planes, and pecially quartz and feldspar, migrate by a thrust planes. diffusion process into preferred zones, Prograde metamorphic assemblages are leaving behind micas and amphiboles. Lay- formed in response to an increase in inten- ers of contrasting mineralogical and chem- sity or grade of metamorphism. Retro- ical compositions result and constitute a grade mineral assemblages are formed in FOLIATION parallel to the SCHISTOSITY. metamorphic rocks that have become subjected to a lower grade as the intensity of metamorphic facies A method of clas- the metamorphism wanes. See also facies sifying METAMORPHIC ROCKS in terms of the (def. 1); zone (def. 1). chemical composition of their constituent minerals, which is thought to depend on metasomatism /met-ă-soh-mă-tiz-ăm/ A the temperatures and pressures at which metamorphic process by which mineralog- they were formed. ical and chemical changes occur in rocks as a result of interaction with migrating fluids metamorphic rock One of the three introduced from an external source. Meta- main types of rock, formed from one of the somatic changes should be distinguished other types (sedimentary or igneous) by from those produced by PNEUMATOLYSIS. METAMORPHISM. Well-known metamorphic See fenitization; granitization. rocks include amphibolite, gneiss, hornfels, phyllite, marble, migmatite, quartzite, Metazoa /met-ă-zoh-ă/ A group of ani- schist, shale, and slate. mals whose bodies are formed of many cells. The Metazoa includes all animals ex- metamorphism /met-ă-mor-fiz-ăm/ The cept the sponges (PORIFERA). process by which mineralogical and chemical changes take place in the solid state in meteoric dust Extraterrestrial fine dust preexisting sedimentary or igneous rocks that reaches the Earth’s surface through as a response to the imposition of new entering the Earth’s gravitational field. It physical or chemical conditions. Metamor- contributes to a small degree to marine sed- phic changes take place at temperatures iments. Some deep-sea cores have been ranging from 100°C to those in excess of found to contain meteoric dust, often in the 600°C, when rocks begin to melt. The min- form of magnetic spherules, perhaps only eral assemblages produced when rocks 0.1 to 0.5 mm in diameter. Meteoric dust have attained chemical equilibrium are de- may be transported in the atmosphere for pendent upon the chemical compositions considerable distances across the oceans, of the original rocks and the temperature and, if present in significant amounts, may and pressure conditions prevailing, pro- later be identifiable in certain seabed sam- vided that the metamorphism was iso- ples. One estimate suggests that some five chemical and involved no METASOMATISM. million tonnes of meteoric dust fall onto Different types of metamorphism are the Earth’s surface annually. Meteoric dust recognized: 1. CONTACT METAMORPHISM, is probably a small though important part i.e. the thermal metamorphism developed of some deep-sea RED CLAY deposits, and in rocks that are intruded by hot magma. meteoric spherules have been found to be 2. Regional or dynamothermal metamor- plentiful in some slowly deposited deep-sea phism, developed over large areas that lutites. have suffered intense deformation and the regional emplacement of granitic bodies. meteorite A solid body that has fallen to The temperatures and pressures associated the Earth’s surface from an extraterrestrial

223 meteorology source. Meteorites are composed of a tween sheets of (Si,Al)O4 tetrahedra and nickel-iron alloy (typically 90% iron and hydroxyl ions. The general formula is 10% nickel) and silicate minerals, mainly olivine and orthopyroxene. Three main X2Y4–6,Z8O20(OH,F)4, types are distinguished: 1. Stony meteorites containing mainly silicate minerals plus where X = K,Na,Ca; Y = Al,Mg,Fe,Li, and some nickel-iron alloy. They are divided Z = Si,Al. The main mica minerals are: into chondrites and achondrites depending muscovite K2Al4(Si6Al2O20)(OH,F)4 upon the presence or absence of small paragonite Na2Al4(Si6Al2O20)(OH)4 rounded bodies consisting of olivine or or- margarite Ca2Al4(Si4Al4O20)(OH)4 thopyroxene, termed chondrules. 2. Stony- phlogopite 2+ iron meteorites contain about equal K2(Mg,Fe )6(Si6Al2O20)(OH,F)4 proportions of silicate minerals and nickel- biotite 2+ 3+ iron alloy. 3. Iron meteorites consist K2( Mg,Fe )6–4(Fe ,Al,Ti)0–2- largely of nickel-iron with only accessory (Si6–5 Al2–3O20)(OH,F)4 silicates. zinnwaldite 2+ Certain nonvolcanic glasses, termed tek- K2(Fe ,Li,Al)6(Si6–7Al2–1O20)(OH,F)4 tites, are also thought to be of extraterres- lepidolite trial origin. These are small bodies K2(Li,Al)5–6(Si6–7Al2–1O20)(OH,F)4 composed of acid glass and containing Clintonite and xanthophyllite are related lechatelierite. No tektite fall has been wit- to phlogopite by the substitution of cal- nessed and their origin remains controver- cium for potassium. sial. See also SNC meteorite. All micas have a perfect basal cleavage, which reflects their layered structure. meteorology The science of the atmos- ‘Books’ of mica, found in pegmatites, are phere. It is concerned with the physics, so called because they have the appearance chemistry, and movement of the atmos- of piles of pages. The cleavage flakes of phere and its interactions with the ground micas are flexible and elastic. Muscovite surface. Some meteorological interest and paragonite are colorless whereas bi- has also developed in studying the atmos- otite and phlogopite are usually dark pheres of other planets by applying the shades of red, brown, and black. Mus- principles obtained from the Earth’s at- covite and biotite are common in schists, mosphere to different environmental con- gneisses, and granitic rocks. Biotite is also ditions. Because the main emphasis of found in basic and intermediate igneous meteorology has been on explaining and rocks. Phlogopite occurs in some peri- forecasting surface weather, meteorology dotites and metamorphosed limestones. is primarily concerned with the tropos- The lithium micas are found almost exclu- phere and stratosphere. Higher levels of sively in pegmatites. the atmosphere, where different techniques of data collections are needed, tend to be micrite /mÿ-krÿt/ Chemically precipi- regarded as part of geophysics. See also tated CALCITE having microscopic grains of climatology; macrometeorology; meso- less than 0.01 mm in diameter, present in meteorology; micrometeorology; synoptic some LIMESTONES; the terminology used in meteorology. the petrographic description and classification of limestones is based on the presence miarolitic cavity /mÿ-ă-rŏ-lit-ik/ An ir- of micrite, SPARITE and ALLOCHEMS. regular cavity within a plutonic rock into which large euhedral crystals project. microbiome /mÿ-kroh-bÿ-ohm/A COM- Compare druse; vugh. MUNITY of living organisms that occupies a comparatively small area. Examples in- mica A mineral that has a layered struc- clude a hedgerow, pond, or wood. See also ture in which cations are sandwiched be- biome; habitat.

224 mid-ocean ridge microbreccia /mÿ-kroh-brech-ee-ă, -bresh-/ micropaleontology /mÿ-kroh-pay-lee- A type of rock consisting of very small an- on-tol-ŏ-jee, -pal-ee-/ See microfossils. gular fragments in a finer matrix. Typical particle size is less than 0.2 mm. Compare microseism /mÿ-krŏ-sÿ-zăm/ A small ir- megabreccia. regular earth tremor and quavering, which makes up the background pattern recorded microclimate /mÿ-kroh-klÿ-mit/ The on seismometers. Such tremors can result climate within a few meters of the ground from natural sources such as waves break- surface, resulting from the interaction being on a beach, or from the passing of tween soil, atmosphere, and vegetation. heavy trucks. microcline /mÿ-kroh-klÿn/ A variety of Mid-Atlantic Ridge A very large relief feature forming part of the MID-OCEAN alkali FELDSPAR. RIDGE system, which trends north–south through the Atlantic Ocean, connected by microfossil /mÿ-kroh-foss-ăl/ A fossil so Kerguelen Island and St Paul Island with small that a microscope is required for its the Indian Ocean Ridge. The relief of the study. Microfossils include bacteria, di- elevation above the adjacent ocean-basin atoms, protozoa, some Crustacea, the lar- floor is 1 to 3 km, and the width in most vae of certain animals, and the isolated places is more than 1000 km. It is essen- skeletal parts or fragments of organisms. tially a wide fractured arch taking up the Apart from their taxonomic and evolution- center third of the Atlantic Ocean. Along ary interest, microfossils are important in the crest region are the rift mountains and the correlation of rocks of which only high fractured plateaus, jointly making up small samples are available, such as the a strip 80 to 300 km wide. Some of the cores from boreholes. The study of micro- higher parts of the ridge rise above sea level fossils is known as micropaleontology. to form islands. microgabbro /mÿ-kroh-gab-roh/ See di- mid-latitude cyclone See depression. abase. mid-ocean ridge (median ridge) A mas- microlite /mÿ-krŏ-lÿt/ A small incipient sive system of submarine elevations that crystal that is found in glassy rocks. Unlike extends, in global terms, over a distance of CRYSTALLITES, microlites are sufficiently about 60 000 km, and traverses all of the large for their nature to be determined major oceans. Its general form is that of a under the microscope. broad arch that stands, on average, 1 to 3 km above the adjacent deep-sea floor, and micrometeorology /mÿ-kroh-mee-tee-ŏ- which has experienced fracturing along its rol-ŏ-jee/ The study of small-scale atmos- crest region. Along much of its length, the ridge is more than 1000 km wide. Parts of pheric processes, usually operating near the ridge crest region are gashed by a steep- the ground surface. Experiments and re- sided rift feature (see median valley), search in this subject require large numbers whereas other fracture zones (some of of accurate instruments, which result in them known as TRANSFORM FAULTS) tend to most studies being limited to small areas be perpendicular to the main axis of the and short duration under ideal conditions. ridge. The rift is coincident with a belt of The enormous variety of surface condi- shallow earthquake epicenters. The floor tions results in a complex interaction be- beneath the ridge is generally hot and the tween the atmosphere and the ground to basaltic rocks of the ridge-crest and ridge- give multitudinous microclimates; in fact flank region appear to be quite young in each instrumentation site could be classed geologic terms. as unique. Compare macrometeorology; In terms of the PLATE TECTONICS theory, mesometeorology. such mid-ocean ridges mark the sites of

225 migmatite constructive PLATE BOUNDARIES where the present it is nearest in the N hemisphere lithospheric plates are gradually moving winter (January 7) and farthest in N hemi- apart. It is at these boundaries that basaltic sphere summer (July 7). The effects of these magma from the Earth’s mantle reaches the radiation changes vary from latitude to lat- surface to emerge as lava and be added to itude. The variations likely to be produced the oceanic crust in the form of pillow were calculated by the Serbian mathemati- lavas and dikes. Because these eruptions cian Milutin Milankovitch (1879–1958) are relatively frequent by geologic scales in 1940. It is believed that these cyclic the area does not have time to adjust im- changes may have great importance for the mediately to regain isostatic balance but commencement of ice ages, but the associ- sinks as the sea floor spreads away from ation is not as close as might be expected. the ridge. See also sea-floor spreading. millibar /mil-ă-bar/ (mb) One thou- migmatite /mig-mă-tÿt/ A coarse hetero- sandth of a bar, a cgs unit of pressure. Be- geneous gneissose rock consisting of bands 6 and patches of quartz-feldspar granitic ma- cause 1 bar is equal to a pressure of 10 terial and mafic material consisting mainly dynes per sq cm, one millibar is equal to 2 2 of biotite or hornblende. Migmatites have 1000 dynes/cm or 100 newtons/m (pas- the appearance of mixed rocks, comprising cals). The millibar is still in use in meteo- portions that resemble granitic igneous rology. material and mafic portions that are metamorphic and possess a SCHISTOSITY. They milligal /mil-ă-gal/ See gal. are associated with metamorphic rocks that have formed at the highest grades. Millstone Grit The middle of the three Under such conditions, partial melting lithological divisions of the CARBONIFEROUS takes place producing mobile granitic ma- System in Britain. It corresponds to the terial, which invades and reacts with the lower half of the Upper Carboniferous (or metamorphic rocks. Namurian) Stage.

Milankovitch cycles /mi-lank-ŏ-vich/ A mineral Any naturally occurring sub- theory of cyclical changes in the Earth’s stance having a definite chemical composi- orbit in correlation with long-term climate tion. When a mineral of commercial value change. The Earth does not revolve in a cir- is present in some rock body in economi- cular orbit round the Sun at a constant ve- cally sufficient quantity to merit extrac- locity, but over a period of many years tion, the body constitutes a mineral changes take place affecting the amount deposit. and distribution of solar radiation received by the Earth. First, the tilt of the Earth’s mineralization /min-ĕ-ră-lă-zay-shŏn/ axis of rotation relative to the plane of its The process by which minerals are formed orbit varies between 21.8° and 24.4° over within a rock, resulting in a vein (lode) or a period of 40 000 years. This affects the other deposit. There are various mecha- seasonality or thermal range between summer and winter. Secondly, the ellipticity or nisms by which this may take place, in- eccentricity of the Earth’s orbit varies over cluding replacement (of one mineral by a period of about 100 000 years. This re- another), impregnation (of rock by min- sults in a greater seasonal range of radia- eral-laden gases or liquids), and the filling tion receipt. When the orbit is at its most of preexisting fissures. eccentric, there will be a 30% difference between aphelion and perihelion, com- mineralogy /min-ĕ-ral-ŏ-jee/ The scien- pared with 7% at present and none when tific study of minerals. It includes their the orbit is circular. Finally, the season classification and deals with their composi- when the Earth is nearest the Sun changes tion, formation, and physical and chemical over a period of about 21 000 years. At properties.

226 mist minette /mă-net/ A type of LAMPRO- ality, refractions from the clear sky. A su- PHYRE consisting of biotite and orthoclase. perior mirage may occur above a flat sur- It occurs in DIKES and other intrusions. face of much lower temperature than the air above, as over ice or a cold sea in sum- minimum temperature The lowest mer. Light in this case is bent downward temperature recorded during a given pe- from the object toward the viewer. riod either in a Stevenson screen (minimum air temperature) or on the ground (grass misfit stream A stream that is appar- minimum temperature). Both temperatures ently too small for its valley. It is charac- are normally measured by an alcohol ther- terized by meander bends in its channel of mometer in which the meniscus drags a smaller amplitude and greater intricacy metal index to the lowest temperature then than the meanders in the valley itself. The leaves it as the column rises when tempera- length of the valley meanders is ten times tures increase. Global minimum tempera- that of the current meanders. There is an tures are found on the Antarctic ice caps. established relation between meander length and discharge in streams, and on Miocene /mÿ-ŏ-seen/ An epoch of the that basis it has been calculated that the NEOGENE Period extending from the end of streams that eroded the valley meanders the OLIGOCENE about 23.7 million years must have had a bankfull discharge ago, for 18.4 million years to the beginning 80–100 times the current streams. This of the PLIOCENE. Grasses evolved and probably occurred at the end of the ice age, spread and, by their effects on the environ- when the glaciers thawed; the periglacial ment, may have contributed to changes in climate of that period would have reduced the mammals, which became more modern vegetation and soils to a minimum, allow- in appearance. Miocene mammals in- ing all the meltwater to flow off the land to cluded pigs, deer, horses, rhinoceroses, ele- the rivers, with none lost by infiltration phants, monkeys, apes, and possibly into the soil. The resulting torrents cut the hominids. Alpine orogenic movements current valley. took place during this epoch. Misfits could also evolve through the capture of a stream’s headwaters, reducing miogeosyncline /mÿ-ŏ-jee-oh-sing-klÿn/ its volume, and leaving it a misfit in its val- A geosyncline in which there are no vol- ley. See river capture. canic products or processes associated with its sedimentation. mispickel /miss-pik-ĕl/ An old name for ARSENOPYRITE. mirabilite /mă-rab-ă-lÿt/ (Glauber’s salt) A yellow mineral form of hydrated Mississippian /miss-ă-sip-ee-ăn/A pe- sodium sulfate, Na2SO4.10H2O. It crystal- riod within the American classification of lizes in the monoclinic system, and occurs geologic time that extends from the end of in salt lakes and around hot springs. It is the DEVONIAN, about 360 million years used as a source of sodium sulfate. ago, for about 40 million years to the beginning of the PENNSYLVANIAN. The Missis- mirage An optical phenomenon result- sippian System corresponds approximately ing from the refraction of light through lay- to the Lower CARBONIFEROUS elsewhere. It ers of air having very large temperature was named for the Mississippi River Valley gradients. The rapid change of temperature where the rocks associated with this period vertically produces varying refractive in- are well exposed. dices of the air and so light appears to travel in curved paths. The most common mist The state of atmospheric obscurity, is the inferior mirage above a heated composed of suspended microscopic water ground surface. Light is strongly refracted droplets and with an atmospheric relative upward near the surface so that the appar- humidity of at least 95%, in which visibil- ent pools of water on the ground are, in re- ity exceeds 1 km. Visibility is reduced to a

227 mistral lesser extent than FOG. The term is also mock sun (parhelion) An optical phe- used for dense but shallow condensation nomenon seen in the sky as a result of the phenomena at ground level when the refraction of sunlight by hexagonal ice ground surface may be totally obscured, crystals with vertical axes. This is most fre- but hedges or animals can be seen quite quent with cirrostratus clouds. Usually two clearly. Compare haze. mock suns are seen, equal distances on either side of the Sun, and appear as brighter mistral /mee-strahl/ A strong cold dry areas of the cirrostratus clouds. northerly wind that is funneled down the Rhône Valley in S France and then blows mode The percentage of each of the across the delta into the Mediterranean component minerals contained in an ig- Sea. Its frequency and strength have neces- neous or metamorphic rock. Compare sitated agricultural adaptations to prevent norm. crop damage. These take the form of hedges or screens oriented east–west to model A theoretical representation of a protect plants. The winds are linked to in- process, system, or object developed to tensification of depressions in the Gulf of show the essential features in a simplified Genoa. comprehensible form, or to enable predictions to be made. See also general circula- mixed tide Any tide that is not clearly of tion model. the diurnal or semidiurnal type. Such tides might be referred to as intermediate tides. moder humus HUMUS intermediate be- The tidal wave is characterized by a signif- tween MULL and MOR HUMUS. icant inequality in either the low-water or the high-water levels and a double low Modified Mercalli scale See Mercalli water and a double high water occur dur- scale. ing each tidal day. mofette /moh-fet/ A type of SOLFATARA mixing ratio The ratio of the mass of an that also produces large amounts of carbon atmospheric gas to the mass of air with dioxide. See also fumarole. which the gas is mixed. It is most frequently used for water vapor, the humidity mogote /moh-goh-tay/ A large limestone mixing ratio, and is expressed in g kg–1. hill that rises on a tropical or subtropical KARST landscape. mobile belt (orogenic belt; fold belt)A long linear area of the Earth’s crust under- Moho See Mohorovičić discontinuity. going intense deformation, often accompanied by seismic and volcanic activity. These Mohole project /moh-hohl/ An aban- structures originate as deep trenches, doned project originally proposed in order which fill up with a thick wedge of sedi- to obtain samples of the rocks of the upper ment. As two continents later collide this mantle. It was hoped to drill down through wedge between them is compressed and de- the Earth’s crust to the Mohorovičić dis- formed to form a mountain chain. continuity. A series of drillings were started in 1961 but the project was aban- mobile dune A coastal sand dune at the doned in 1966. stage when its vegetation (mainly marram grass) is open, leaving sometimes 50% or Mohorovičić discontinuity /moh-hoh- more bare sand subject to wind movement roh-vee-chich/ (Moho) The seismic and hence mobile. Their appearance is of a boundary that marks the junction between confused mass of sandy depressions and the base of the Earth’s crust and the top of grassy knolls superimposed on a profile the mantle. This boundary varies in depth that shows a gentle windward slope and a below the Earth’s surface from 5 to 10 km steep lee slope. beneath the ocean and between 55 and 70

228 monadnock km beneath the continents. The boundary BROWN EARTH, BROWN CALCAREOUS SOIL, is marked by an increase in the primary (P) and associated types such as humic gley, wave velocity from 6.9 to 8.2 km per sec- and solonetzic soil. ond. It is named for the Croatian geologist Andrija Mohorovičić (1857–1936). Mollusca /mŏ-lus-kă/ The phylum of invertebrate animals that includes snails, Mohs’ scale /mohz/ A scale of hardness mussels, and octopuses. Mollusks are un- devised to aid the identification of miner- segmented creatures having a head, a vis- als. Ten reference minerals were chosen, ceral region, a muscular ventral organ for the softest, talc, being assigned a hardness locomotion known as the foot, and a man- of 1 and the hardest, diamond, 10. Each tle, which generally secretes a hard calcare- mineral on the scale can be scratched by ous shell. The geologically important those of higher numbers: 1. talc. 2. gyp- classes of this phylum are the GASTROPODA, sum. 3. calcite. 4. fluorite. 5. apatite. 6. or- the BIVALVIA, and the CEPHALOPODA, which thoclase. 7. quartz. 8. topaz. 9. corundum. includes important extinct forms such as 10. diamond. the AMMONOIDEA and BELEMNOIDEA. An- Other common substances are useful in cient mollusks seem to have been marine testing hardness: finger nail (2.5), copper animals inhabiting shallow waters and coin (3), glass (5.5), and a penknife blade their shells are known as fossils from the (6.5). The hardness of many minerals Cambrian Period onward. Some, such as varies with direction. For example, the the Bivalvia in the Carboniferous, have hardness of kyanite varies between 5.5 and been used in stratigraphic CORRELATION. 7. The scale is named for the German min- eralogist Friedrich Mohs (1773–1839). Mollweide’s projection /mawl-vÿ-dĕz/ A MAP PROJECTION showing the entire molasse /mŏ-lass/ A term originally used Earth’s surface on an ellipsoidal base. The by Alpine geologists for a particular se- major axis (Equator) is twice the length of quence of Miocene sediments. The term the minor axis (central meridian). The geo- has since been used to denote sequences of graphic parallels are straight lines, as is the sediments deposited under continental or central meridian; the other meridians are freshwater conditions after a period of curved, the curvature increasing toward mountain-building. Compare flysch. the outside limits of the projection. The distortion of shapes and directions is there- mold A fossil in which the original skele- fore considerable, although the projection tal parts have been dissolved away, leaving is homolographic and consequently used a space that preserves their shape. In some for distributions. This projection is some- cases a mold may preserve features of only times interrupted (see diagram overleaf): the outer or inner surfaces of such struc- each continental area has its own central tures as shells. These are known respec- meridian (C.M.), with different ones north tively as external and internal molds. The and south of the Equator; the interruptions infilling of a mold with secondary material are in the oceans. The projection is named produces a CAST. for the German astronomer and mathematician Karl Brandan Mollweide (1774– mollisol /mol-ă-sôl/ One of the twelve 1825). soil orders of the US SOIL TAXONOMY. Mol- lisols are soft calcimorphic soils, developed molybdenite /mŏ-lib-dĕ-nÿt/ A silver- in subhumid to semiarid conditions, char- gray molybdenum sulfide mineral, MoS2, acterized by a dark strongly structured sur- found as an accessory in acid igneous rocks face horizon, often almost 1 m deep and and in hydrothermal veins. more than 50% saturated with bases (mainly calcium), which is termed a mollic monadnock /mŏ-nad-nok/ Any hill epipedon. It includes the CHERNOZEM, standing up above the surface of a flat or CHESTNUT SOIL, PRAIRIE SOIL, RENDZINA, gently undulating plain. These hills may be

229 monazite

pole North American C.M. pole Eurasian C.M. 75° 60° 45° 30° 15° 0° 0° 150° 120° 90° 30° 0° 30° 60° 90° 120° 150° 15° 30° 45° 60°

75° pole pole pole South American C.M. South African C.M. Australian C.M.

Mollweide’s interrupted projection composed of rocks that are more resistant monomineralic /mon-ŏ-min-ĕ-ral-ik/ De- to weathering than those around them; al- scribing a rock consisting almost wholly of ternatively they may be located in former one mineral, e.g. dunite, anorthosite. drainage divide positions. In the latter case they will have been subjected to active flu- monsoon A large-scale reversal of vial erosion for a shorter period of time winds in the tropics (from the Arabic word for season) due to differential heating of than the rest of the area and it is for this continent and ocean, but because the circu- reason that they remain as upstanding lation can be affected by many other fac- masses. tors, the monsoon varies in character from one area to another. It is best developed in monazite /mon-ă-zÿt/ A rare-earth min- India, SE Asia, and China but N Australia eral of composition (Ce,La,Y,Th)PO4 and E and W Africa all show similar wind found as an accessory in acid igneous rocks reversals. The summer season is normally and as a detrital mineral in certain beach the rainy period of the monsoon with sands. southwesterly winds in the N hemisphere, the dry season having winds from the N monchiquite /mon-chă-kÿt/ An alkaline sector. type of LAMPROPHYRE. Monsoon Current A seasonal current monocline /mon-ŏ-klÿn/ A bending of that occurs in the N part of the Indian rock strata produced in sedimentary se- Ocean where the surface currents reflect closely the seasonal changes in the wind di- quences that have deformed under condi- rection as the monsoon season develops. tions favoring the development of a normal The North Equatorial Current is clearly fault. Two areas of horizontally bedded apparent during February and March, at sediments are left at different elevations the time when the northwest monsoon is but still connected by a steeply inclined se- blowing. An Equatorial Countercurrent is ries of the same beds. See diagram at FOLD. also strongly developed, with its axis aligned roughly along latitude 7°S. How- monoclinic /mon-ŏ-klin-ik/ See crystal ever, when the southwest monsoon com- system. mences in August–September, the North

230 mountain wind

Equatorial Current is replaced by the Mon- posited by former glaciers and ice sheets. soon Current and flows eastward. These This material may be transported on the currents are generally located within the surface of the ice (supraglacial), within the uppermost layers of water. See also equa- ice (englacial), or beneath the ice (sub- torial current. glacial). See ground moraine; lateral moraine; medial moraine; push moraine; montane /mon-tayn/ Describing a moun- terminal moraine. tain or mountainous area, or something associated with it, such as a montane tropical mor humus Acid HUMUS, which forms forest. in wet lime-deficient environments, e.g. podzolic conditions. Most of the organic monticellite /mon-tă-sell-ÿt/ A calcium- breakdown is achieved by the fungi in the bearing variety of OLIVINE. soil but it is a slow process with the result that the L, F, and H layers (see profile) are montmorillonite /mont-mŏ-ril-ŏ-nÿt/A clearly differentiated. Compare mull type of CLAY MINERAL consisting of a hy- humus. drated aluminum silicate with the unusual property of swelling when it absorbs morphogenetic zone /mor-fŏ-jĕ-net- water. It also undergoes ion exchange of its ik/ (in climatic geomorphology) An area calcium, potassium, and sodium ions. It is of the Earth in which climate and relief are the main component of bentonite and distinctive, through the control of climate fuller’s earth. on geomorphic process and the control of process on landform. These zones run Montreal Protocol (Montreal Protocol roughly parallel to major climates, which on Substances that Deplete the Ozone conform to latitude. Early attempts to di- Layer) An international agreement de- vide the Earth into such zones used very signed to regulate ozone-depleting sub- simple climatic data (mean annual rainfall stances and protect the atmospheric ozone and mean annual temperature) but modern layer (see ozone). The treaty was drawn up attempts use much more sophisticated pa- in Montreal in 1987 by the United Nations rameters. The French geographers Jean Environment Program (UNEP), and came Tricart and André Cailleux identified four into force in 1989 when ratified by most of major world zones, running from a cold the industrialized nations. It sought a 50% zone in the north, through a forested zone reduction in the use of chlorofluorocar- and subarid zone to the equatorial tropical bons (CFCs) by 1992, relative to 1986 lev- zone; within each, climates and landforms els. Subsequent amendments (London, are in sympathy through the link of pro- 1990; Copenhagen, 1992; Vienna, 1995; cess. Montreal, 1997; and Beijing, 1999) have introduced other types of control measures morphological species See species. and added new controlled substances to the original list. moss agate A type of CHALCEDONY that has fernlike inclusions of black dendritic monzonite /mon-zŏ-nÿt/ See syenodior- PYROLUSITE (manganese dioxide). ite. moss animal See Bryozoa. moonstone A bluish or silvery iridescent type of orthoclase (an alkali mother-of-pearl See nacre. FELDSPAR), used as a semiprecious gemstone. See also sunstone. mountain wind A form of KATABATIC WIND in mountainous areas where, after moraine /mŏ-rayn/ A depositional fea- night-time radiational cooling, strong ture composed of glacial TILL, which may breezes are concentrated in the valleys as be in association with active ice or de- the cold air has subsided downslope.

231 mud mud Sediment having a preponderance ture in metamorphic and sedimentary of particles measuring less than 0.06 mm in rocks that resembles a rod or column. diameter. Under natural conditions, muds generally form very plastic masses com- mullite /mul-ÿt/ A colorless mineral form prising sediment particles mixed with of ALUMINUM SILICATE, approximate for- water. The term applies to both silty and mula Al6Si2O13. It crystallizes in the ortho- claylike particles. Mud may be deposited, rhombic system and is used as a refractory. according to prevailing conditions, in both shallow water and deep-sea areas. Shel- multiple intrusion An intrusion of ig- tered environments are particularly suscep- neous rock (see intrusive rock) consisting tible to mud deposition, as for example in of several masses of similar composition. sheltered lagoons and estuaries, and along Most DIKES are multiple intrusions. low-energy beaches and tidal flats. Once partly consolidated into mud banks, the multiple reflection A seismic wave that material tends to be remarkably cohesive has been reflected more than once and as a and therefore resistant to scour; many mud result may be recorded more than once on banks are eroded primarily through side- a seismic profile. scour because of the gradual migration of tidal channels. multispectral satellite imagery See satellite. mud crack See desiccation crack. muscovite /mus-kŏ-vÿt/ One of the mudflat A low area of fine silt that lies major types of MICA, K2Al4(Si6Al2O20)- along the shore of an estuary or on the lee (OH,F) . It crystallizes in the monoclinic side of an island. It supports no vegetation 4 system as colorless, green, or pale brown and is generally covered and uncovered by plates and occurs in various igneous rocks, the tide. Various types of arthropods, mol- mainly pegmatites. It is used as an electri- lusks, and worms live in the mud, and as a cal insulator and lubricant. result mudflats are important feeding grounds for several species of wading muskeg /mus-keg/ A type of BOG that oc- birds. curs in TUNDRA areas. It forms when the mudflow See earthflow. surface permafrost melts in summer, and generally supports lichens and mosses, mudrock See mudstone. such as sphagnum moss. The best-known muskegs are found in N Canada. mudstone (mudrock) An ARGILLACEOUS sedimentary rock that is less fissile along mylonite /mÿ-lŏ-nÿt/ A rock formed bedding planes than SHALE. during dislocation metamorphism by extreme granulation and shearing. Mylonites mud volcano See sand volcano. are banded or streaky rocks and may contain augen of undestroyed crystals or rock mugearite /moo-jee-ă-rÿt/ An alkaline fragments in a fine-grained matrix. They basalt. See trachybasalt. are commonly found at thrust planes along which extensive movements have taken mull humus Alkaline HUMUS, which place. See cataclasis; pseudotachylite. commonly forms in a mild, moist, and base-rich environment, e.g. chernozem myrmekite /mer-mĕ-kÿt/ Wormlike in- conditions. It is rapidly incorporated into tergrowths of quartz in plagioclase the soil by the abundant soil fauna, partic- feldspars produced as a result of the re- ularly earthworms. Compare mor humus. placement of potassium feldspar by sodium feldspar, during which excess silica mullion A tectonically produced struc- is liberated.

232 N

nacre /nay-ker/ (mother-of-pearl) A lus- natural gas A mixture of flammable hy- trous iridescent coating on the inside of the drocarbon gases – mainly ethane and shells of various mollusks, such as oysters. methane – that occurs in underground de- It consists mainly of calcium carbonate, posits (often below the sea bed), either CaCO3, and is the substance that PEARLS alone or in association with oil. It is are made of. thought to have formed from the remains of microscopic organisms that became NAO See North Atlantic Oscillation. buried and compressed in underground sediments. It is an important FOSSIL FUEL. nappe A large tectonic feature that owes its origin to a combination of folding and natural selection The process proposed thrusting. Nappes are basically large hori- by the English naturalist Charles Robert zontal recumbent folds that have traveled Darwin (1809–82) in 1859 to explain the for many tens of kilometers along thrust EVOLUTION of organisms from one species planes. They are well developed in the to another. It attempts to account for the Alps, where they were first described. origin and diversity of all organisms. Plant and animal species produce relatively large native Denoting the state of occurrence numbers of offspring that differ in various of certain elements or minerals when they ways. If an organism possesses certain vari- are uncombined with other elements or ations that make it better suited to its envi- minerals. ronment, this organism is more likely to survive and reproduce. If these variations natrolite /nat-rŏ-lÿt/ A colorless, white, are inheritable the likelihood of its off- or yellow mineral form of hydrated sodium spring also possessing them is increased. In aluminum silicate, Na2(Al2Si3O10).2H2O. this way, and over many generations, radi- It crystallizes in the orthorhombic system cal changes may arise. as needlelike crystals, usually in cavities in basaltic rocks. It is a member of the ZEO- nautical mile A distance equal to 1 LITE group of minerals. minute of longitude along a GREAT CIRCLE, equivalent to about 1853 m. A speed of 1 natural arch An archway on an ex- nautical mile an hour is a KNOT. posed headland that results from erosion by wave action. Generally, a cave in chalk Nautiloidea /naw-yă-loi-dee-ă/ An al- or limestone rock becomes enlarged so that most extinct subclass of mollusks of the it cuts through to the other side of the class CEPHALOPODA. The external shell of headland. the nautiloids is similar to that of the related AMMONOIDEA, except that in fossil natural bridge A bridge of rock formed forms it is straight or only slightly coiled in limestone by erosion. It generally results and in all types the suture lines are simple when the roofs of two underground cav- curves, showing none of the complex con- erns situated close to each other collapse, volutions characteristic of the Am- leaving a bridge of rock between the result- monoidea. The oldest fossils come from ing holes in the ground. upper Cambrian rocks and their shells

233 neap tide sometimes reached a length of up to 4.5 m. needle ice Needles of ice, often several These straight-shelled forms were extinct centimeters long, growing beneath the sur- by the end of the Triassic Period. The face layer of debris and able to lift frost- group as a whole declined from the Devon- shattered rock fragments and soil particles ian Period onward; very few genera existed perpendicularly from the ground surface. during the Mesozoic and only one, Nau- On melting of the needles (often called pip- tilus, has persisted to the present day. krakes), the fragments are deposited a little way downslope. Material can progress neap tide A TIDE of relatively small considerable distances and, as the process range that occurs near the time of quadra- tends to be more effective in coarser mature of the Moon (i.e. near the time of the terial, it also achieves a certain amount of Moon’s quarters). The tidal range falls sorting. Needle ice occurs in periglacial en- below the average range by about 10 to vironments, most effectively where there is 30%. Because of this low range, the tidal diurnal freezing and thawing. current speeds during a neap tide are significantly less than those attained during a negative gravity anomaly See gravity SPRING TIDE. anomaly. nearshore circulation system The sys- negative movement of sea level A rise tem of currents, notably those induced di- of the land relative to the sea due either to rectly or indirectly through wave activity, an actual rise of the land through tectonic that operate within and closely adjacent to movements, or a fall in sea level through the SURF zone. These currents include the the locking up of water in the form of ice, shoreward-directed MASS TRANSPORT CUR- as happened in the Pleistocene Period. RENTS beneath waves, the wave-induced Whatever the cause, more land is created, LONGSHORE CURRENTS, the powerful sea- and the relative relief of the land, from sea ward-flowing RIP CURRENTS, and other cur- level to mountain tops, is increased. The rents associated with the expanding major result is the extension of river sys- rip-head zones. Superimposed on these tems out across former sea-bed areas and currents there is invariably a reversing tidal their REJUVENATION. Raised beaches, ma- current, and, on those occasions when rine beaches, knickpoints, river terraces, local winds are blowing, wind-induced and incised meanders are all evidence of currents. In the vicinity of rivers and estu- past negative movements. See also positive aries, there may also be density-induced movement of sea level. currents. The relative importance of the various types of current varies both spa- nekton /nek-tonn/ (nektonic or- tially and temporally. ganisms) Pelagic organisms that swim and move actively, such as fish. Compare nebular hypothesis An idea proposed plankton. by the Marquis de Laplace (1749–1827) in 1796 in which he suggested that the ma- nematath /nem-ă-tath/ A series of volca- terial now forming the Sun and planets noes that form as one of the Earth’s LITHOS- originated as a disk-shaped nebula or gas PHERIC PLATES (tectonic plates) moves cloud, which contracted into discrete bod- slowly across a thermal center (see hot ies. spot). The farther a volcano is from the thermal center, the older it is. The youngest neck (plug) An erosional remnant repre- volcanoes are generally still active, whereas senting the former conduit that fed a vol- the older ones may be dormant. cano. This conduit was filled with lava and pyroclastic material that was more resis- Neogene /nee-ŏ-jeen/ The latter period tant to erosion than that of the enclosing of the CENOZOIC Era, comprising the volcanic cone, which has since been eroded MIOCENE, PLIOCENE, Pleistocene, and away. Holocene Epochs. It began at the end of the

234 névé

OLIGOCENE Period, 23.7 million years ago, phyres. These volcanics are restricted to and extends to the present. Historically, stable continental areas and occur in asso- the QUATERNARY has been widely consid- ciation with ijolites and carbonatites. ered to be a period, equated by some authorities with the Pleistocene and Holocene nephrite /nef-rÿt/ A tremolitic AMPHI- Epochs, but it had never been ratified as a BOLE. See also jade. chronostratigraphic division within the international geologic time scale. It has since neptunian dike A roughly vertical body been suggested that the Quaternary should of sediment, usually sandstone, that has in- be considered as a sub-era. filled a fissure in a preexisting rock body exposed at the Earth’s surface or on the sea nephanalysis /nef-ă-nal-ă-siss/ The analysis of cloud patterns on weather satellite floor. photographs. The photographs cover large areas of the Earth’s atmosphere and sur- neritic /ni-rit-ik/ Describing the shallow- face and new techniques were required to water marine environment lying between describe and analyze the larger features of low-tide level and a depth of some 200 m. cloud patterns compared with the appar- The environment rarely covers a distance ently random distribution seen by the sur- from the shore in excess of several hundred face observer. On a nephanalysis the kilometers; on a global scale, it covers following information can be portrayed: roughly 30 million sq km or about 10% of cloud elements, cloud masses, cloud pat- the total ocean area. The environment is terns, cloud systems, cloud bands, cloud characterized by very diverse sediments (of types (stratiform, cumuliform, cirriform, which fine to coarse terrestrially-derived cumulonimbus, or stratocumuliform), per- sediments mixed with what are mostly cal- centage cloud cover, cloud boundaries, and careous organic remains tend to predomi- synoptic interpretations of these features. nate) and BENTHOS, and a wide array of nepheline /nef-ĕ-lin/ A major member of environmental conditions, as for example the FELDSPATHOID group of minerals. in the degree of light penetration. The whole of the sea floor in the neritic zone nephelinite /nef-ĕ-lin-ÿt/ A strongly un- probably experiences some measure of dersaturated basic to ultrabasic feldspar- wave disturbance and almost certainly free volcanic rock, having a plutonic tidal current action. equivalent within the IJOLITE-MELTEIGITE series. Nephelinites consist of NEPHELINE net radiation The difference in value which may occur both as euhedral phe- between all downward incoming short- nocrysts and as anhedral groundmass wave radiation and upward outgoing long- grains, together with a variety of mafic wave radiation fluxes at the Earth– minerals, typically titanaugite, biotite, atmosphere interface. olivine, and magnetite. Assemblages of LEUCITE plus mafic min- neutron logging A subsurface logging erals constitute the leucitites. Ugandite is technique used in uncased boreholes to an augite-rich olivine-leucitite. Melilitites record the presence of fluids. When used in are composed of melilite plus mafics and often contain small amounts of nepheline conjunction with a gamma-ray log it can be and calcite in addition. Alnoite is a dike used to calculate the porosities of the for- rock largely composed of melilite and bi- mations encountered. It measures the in- otite together with pyroxene, calcite, and tensity of radiation in the borehole after it olivine. Mafurite is an olivine-kalsilite has been exposed to a radioactive source. rock. Nephelinites and leucitites are composi- névé /nay-vay/ 1. See firn. tionally related to phonolites and leucito- 2. The area in which firn exists; firn field.

235 New American Comprehensive Soil Classification

New American Comprehensive Soil ther be oxidized, by Nitrobacter, to nitrate, Classification See soil taxonomy. which can be used by plants. DENITRIFICA- TION, a conversion of nitrates back to ni- New Red Sandstone A succession of trogen gas, most of which is unusable, may red sandstones forming the continental fa- occur when soils become poorly aerated cies of the PERMIAN and TRIASSIC Systems in and acid. See nitrogen cycle. Britain. nitrogen /nÿ-trŏ-gĕn/ The most abun- niccolite /nik-ŏ-lÿt/ (nickeline) A cop- dant gas in the atmosphere and a critical per-colored mineral form of nickel ar- constituent in the soil, which can only be senide, NiAs. It crystallizes in the used directly by a few specialized bacteria. hexagonal system, and occurs associated To be of widespread value it has to be con- with ores of copper and silver. It is used as verted into the nitrate form (see nitrifica- a source of nickel. tion). In nature, nitrogen is involved in cyclic changes termed the NITROGEN CYCLE. niche /neesh/ (in ecology) The place or status of an organism in its COMMUNITY or nitrogen cycle The series of chemical ECOSYSTEM. The niche can be defined in reactions by means of which nitrogen cir- terms of its moisture and temperature and, culates through the global ECOSYSTEM. Ni- from the organism’s point of view, the trates and other inorganic nitrogen availability of food and the presence of compounds in the soil are taken up by competitors, and how well adapted it is to plants and turned into proteins and other live there. No two species can occupy ex- organic nitrogen compounds. These are actly the same niche, or they would be in eaten by herbivores, which are in turn direct competition all the time. eaten by carnivores. The excreta and dead bodies of all these animals returns nitrogen nickeline /nik-ĕ-lin/ See niccolite. to the soil, where decomposers (such as fungi and bacteria) reconvert it to an inor- nimbostratus /nim-boh-strat-ŭs/ The ganic form. Some bacteria that live in the main rain-bearing cloud of temperate lati- soil cycle nitrogen between the atmosphere tudes. It consists of a gray cloud layer of and plants. See nitrification; nitrogen fixa- appreciable depth beneath which are fre- tion. quently low ragged clouds, called FRACTO- STRATUS clouds or scud. They are most nitrogen fixation The absorption of at- often associated with low-pressure areas mospheric nitrogen by some heterotrophic and precipitation may be prolonged, al- bacteria. There are two groups: the non- though it is not usually heavy. symbiotic, e.g. Azotobacter, which make the nitrogen part of their tissue so that niter /nÿ-ter/ (saltpeter) A colorless or when they die it is made available by nitri- white mineral form of potassium nitrate, fication; and the symbiotic bacteria, e.g. KNO3. It is found as a surface deposit in Rhizobium, which form nodules on the caves and in dry regions. It has many uses: roots of leguminous plants and pass ni- in curing meat; in making glass, pyrotech- trogenous compounds on to their host. nics, and explosives; and as a fertilizer. See also Chile saltpeter. nivation /nÿ-vay-shŏn/ (snow patch erosion) Any of a number of processes asso- nitrification /nÿ-tră-fă-kay-shŏn/ The ciated with a patchy snow cover over conversion of nitrogen by bacteria to a unconsolidated rocks, such as glacial deform usable by higher plants. When ma- posits. The most important erosional pro- terial decays, ammonia is often liberated cess is FREEZE-THAW, the role of the snow and this is toxic to many plants. Some bac- being to supply meltwater to assist in the teria, namely Nitrosomonas, can oxidize process. The thickness of snow is an im- ammonia to nitrite and in turn it may fur- portant factor, because if it is too thick it

236 normal will tend to protect surface materials from nonconformity (heterolithic unconfor- the effects of atmospheric freeze-thaw cy- mity) An unconformable contact be- cles. The most active nivation takes place tween overlying younger sedimentary around the edges of snow patches. The rocks, and underlying older igneous or presence of PERMAFROST greatly increases metamorphic rocks. efficiency since it stops the meltwater soak- ing away. In order to achieve any erosion, nondipole field /non-dÿ-pohl/ That weathered material must be removed, and small part of the Earth’s magnetic field that consequently the process is most efficient is superimposed upon the main DIPOLE on slopes, where SOLIFLUCTION and melt- FIELD. This small additional component is water runoff are the principal transporta- responsible for the irregularities of the tion agents. The characteristic form Earth’s magnetic field. It is irregular in its produced by nivation is the NIVATION HOL- distribution and is different in each hemi- LOW. sphere; as a result the Earth’s magnetic dip poles are not antipodal, the north magnetic ° ° nivation hollow A hemispherical hol- dip pole being at 75 N, 101 W, while the ° ° low of variable size produced by NIVATION south magnetic dip pole is at 67 S, 143 E. processes. These hollows can be described as transverse, longitudinal, or circular, de- nonsequence /non-see-kwĕns/ A short pending on the shape of the associated break in sedimentation, causing a type of snow patch and its relationship with the unconformity detectable only by paleontol- slope. The downslope part of the hollow is ogical techniques. generally flat or gently inclined, while the nordmarkite /nord-mark-ÿt/ A member upslope part is steeper. It is considered that of the SYENITE group of minerals. this backslope recedes gradually upslope, increasing in height, and that in this way norite /nor-ÿt/ A coarse-grained basic ig- the hollows may develop into cirques, as neous rock consisting mainly of orthopy- more and more snow can be accommo- roxene and plagioclase. See gabbro. dated, and FIRN begins to form. norm A chemical analysis of an igneous nok-tă-loo-sĕnt noctilucent clouds / / or metamorphic rock expressed in terms of Clouds observed in the high atmosphere standard minerals (the normative miner- when it is almost dark at ground level. als), the proportions of which form a basis Sometimes they show brilliant colors, al- for comparison and classification. The though they more usually have a bluish- norm is distinct from the MODE, which is white or yellow appearance. Almost all the actual mineral composition of a rock. A other clouds are limited to the troposphere, quantitative classification for igneous but the height of noctilucent clouds has rocks was devised in 1903, based upon a been shown to be about 80 km, sometimes normative calculation by the American reaching speeds of 300 knots. Their true petrologists W. Cross, J. P. Iddings, L. V. nature and origins are not completely un- Pirsson, and H. S. Washington, and termed derstood, but they are believed to consist of the CIPW classification. ice particles, saturation being reached through orographic wave development res- normal 1. (in climatology) Denoting the onated from the surface. average value of any climatological element over a specified time period. The nodule 1. A rounded concretion in sedi- usual time period is 30 years, which is be- mentary rocks. lieved to be sufficiently long to average out 2. See inclusion. year-to-year variations but not long 3. A small lump containing nitrogen-fixing enough to contain trends in the value of the bacteria on the roots of certain plants, par- elements, such as temperature or precipita- ticularly legumes. See nitrogen fixation. tion.

237 normal fault

2. (in statistics) Denoting a distribution of takes depressions into NW Europe bring- data that corresponds to an accepted fre- ing mild wet conditions, while the Mediter- quency distribution about a population ranean has dry conditions. In a low (or mean. negative) NAO index phase there is a reduced pressure gradient between a weak normal fault (gravity fault) A type of Azores High and Icelandic Low resulting in fault in which the predominant displace- an east–west orientation in which depres- ment is downward in the same direction as sions take a southerly route across the Iber- the inclination of the fault plane. This re- ian peninsula bringing wet conditions to sults from a stress configuration in which the Mediterranean; N Europe experiences the principal maximum stress is vertical cold dry conditions. The NAO changes be- with the other two principal stresses (min- tween high and low index phases at ran- imum and intermediate) being horizontal. dom, but usually shows either a See fault. predominantly positive or negative average for the year. It is closely related to the ARC- North Atlantic Drift The portion of TIC OSCILLATION. the major surface current in the North At- lantic that is an extension of the GULF North Equatorial Current See equato- STREAM current. The flow of the North At- rial current. lantic Drift is generally in a northeasterly direction. It tends to be more diffuse than northern lights See aurora. the Gulf Stream, being characterized by several shifting bands of water. Each band northing /nor-thing, -thing/ Any of the displays large meander flows that appear, east–west grid lines on a map, quoted after disappear, and reappear in a constantly al- the EASTING when coordinates are being tering pattern of flow patterns. Large ed- given, showing distance north from the ori- dies are often thrown off the main stream. gin of the grid. In its more easterly portion, the North At- lantic Drift bifurcates, one stream moving nosean /noh-see-ăn/ A member of the north-northeastward toward the Polar sodalite group of FELDSPATHOIDS. Seas, the other stream (the Canary Cur- rent) moving east-southeastward. Being a nucleation /new-klee-ay-shŏn/ 1. (accre- relatively warm current, especially on action) A process by which it has been sug- count of being partly replenished at depth gested that continents grow, younger fold by fairly warm water from the Mediter- belts being added to their margins. This is ranean Sea, it has an important ameliorat- consistent with the present theory of plate ing influence on the climate of northwest tectonics because these fold belts originate Europe’s ocean flank. in the deep trenches bordering continents that are overriding the adjacent ocean North Atlantic Oscillation (NAO)A floor. large-scale pattern of fluctuations in the 2. The first stage in precipitation and the pressure systems over the North Atlantic growth of crystals in a melt. that is most marked in winter. It has significant influence on the weather across the nucleus (in meteorology) Any minute Atlantic and surrounding continents, espe- solid particle suspended in the atmosphere. cially of Europe, but also affects the E coast Nuclei have been classified on the basis of of the USA. During what is known as the size into Aitken nuclei (smaller than 0.2 high (or positive) NAO index phase there µm), large nuclei with radii between 0.2 is a large difference in pressure between the and 1 µm, and giant nuclei with radii low-pressure system around Greenland greater than 1 µm. The Aitken nuclei are and Iceland (the Icelandic Low) and the most abundant in the atmosphere, reach- high-pressure over the Azores (the Azores ing about 10 000 per cm3, but there are far High). The strong northerly orientation more in industrial areas than over the

238 nutrient cycle oceans. The nuclei are named for the Scot- forecast as accurately and quickly as possi- tish physicist and meteorologist John ble. Aitken (1839–1919). Identification of the nuclei is very difficult, even with electron nummulites /num-yŭ-lÿt/ Benthonic ma- microscopes, unless they possess a distinct rine protozoa forming a family of the crystalline form. FORAMINIFERA. Their skeleton, up to 80 Some nuclei have an attraction for mm in diameter in some species, consists of water and are known as hygroscopic; oth- a smooth flattened disk-shaped spiral di- ers are nonhygroscopic. This is very impor- vided into many separate chambers. Num- tant in enabling nuclei to act as favorable mulites are known from the Cretaceous points for CONDENSATION as part of the Period but were most abundant in the precipitation process. There are undoubt- Eocene and Oligocene Epochs; they are edly sufficient such nuclei in the atmos- used locally in stratigraphic correlation. phere because condensation always takes place as soon as saturation is reached. nunatak /noo-nă-tak/ A rock peak stick- Compare freezing nucleus. ing out above the surface of an ice sheet. In many instances these peaks were formerly nuée ardente /new-ay ar-dahnt/ An in- covered with ice and only subsequent re- candescent cloud of gas and volcanic frag- duction in the extent of ice cover has ments emitted during certain types of brought about their emergence. volcanic eruptions. See pyroclastic rock. nutation /new-tay-shŏn/ The small os- numerical weather prediction (NWP; cillation superimposed upon the preces- numerical forecasting) The production sional motion of the Earth. This results of weather forecasts through the use of from the gravitational attraction on the complex numerical models. Numerical Earth’s equatorial bulge from the Sun and weather forecasting was first propounded Moon. This attraction varies continuously in the early 20th century by the Norwegian as the Sun and Moon change their posi- meteorologist Vilhelm Bjerknes (1862– tions with respect to the Earth. 1951) and his associates, and developed by the British mathematician Lewis Fry nutrient cycle (in ecology) The transfer Richardson (1881–1953). With the advent of nutrients from one stage in an ECOSYS- of computers, especially from the 1950s, TEM to another stage. For example, within computer models were developed that a forest leaves that fall to the ground rot could be applied to the production of and are broken down by bacteria to pro- weather forecasts. Modern numerical vide nutrients in the soil, which are needed weather prediction features feeding obser- for the growth of plants. Green plants use vations of the current state of the atmos- the energy of sunlight (in PHOTOSYNTHESIS) phere (obtained from sources that include to make more tissue, which provides food networks of land stations, buoys, ships, for herbivores (primary consumers). The aircraft, and satellites) into powerful com- herbivores, in turn, provide food for carni- puters and using sophisticated computer vores (secondary consumers). See also bio- models in order to calculate the weather geochemical cycle.

239 O

oasis A place in a dry landscape (such as cooled rhyolitic lava (see rhyolite). It is a desert) that has water and so can support sometimes used as a semiprecious gem- vegetation. The water may come from a stone. river that flows across the desert on its way to the sea, or there may be a well for rais- occlusion (occluded front) A front that ing groundwater to the surface. Natural develops during the later states of the evo- oases have long been sites of human settle- lution of a DEPRESSION when the air of the ment. warm sector is no longer at the ground surface. As the cold front normally travels obduction In plate tectonics, the pro- more quickly than the warm front, it cess by which OCEANIC CRUST is forced over slowly reduces the area of the warm sector the edge of and incorporated into CONTI- until it merges with the preceding front to NENTAL CRUST at a destructive PLATE complete the occlusion process. The occlu- BOUNDARY. See also subduction zone. sion is therefore a compound zone with warm and cold front characteristics. If the objective analysis (in meteorology) A method of interpreting a two-dimensional air behind the original cold front was scalar field of some meteorological ele- colder than that ahead of the original ment, usually pressure, so that the results warm front it is known as a cold occlusion, from a particular data set are independent and if warmer, as a warm occlusion. The of human subjectivity. The usual way is by frontal structure of the occluded system is fitting a two-dimensional surface to the shown in the diagram. In reality individual data by means of a least-squares method, occlusions are more complex. Sometimes, which is a statistical process in which the as the depression moves eastward along sum of the squares of the difference be- the occlusion, the line of airmass contrast tween actual and predicted values is re- to the north of the depression swings coun- duced to a minimum. terclockwise giving a further period of obsequent stream A stream that flows in the opposite direction to a CONSEQUENT STREAM, often against the direction of dip. In scarpland areas, the obsequent streams subsequent develop after the consequent and subse- stream quent pattern has dissected the strata to dip slope produce the characteristic sequence of limestone and sandstone scarps, with clay consequent stream obsequent vales; obsequent streams will flow down stream clay vale the scarp faces, tributary to the subsequent master streams. scarp obsidian /ob-sid-ee-ăn/ A black, brown, or red lustrous type of volcanic glass with Obsequent, subsequent, and consequent a conchoidal fracture, derived from rapidly streams

240 oceanic crust

generally quite flat but also has volcanic features that include SEAMOUNTS (either oc- warm air aloft curring individually or in large groups), and its linear-type archipelagos. It also features the deep-sea TRENCHES, the deepest cool air parts of the ocean. cold air ocean current A distinct flow of water Fig. 1: Warm occlusion in the ocean. Almost all the water in the ocean is in a constant state of movement, true stagnation occurring only at the bot- warm air aloft tom of certain deep basins hemmed in by rock sills. Generally, currents in the oceans occur at fairly low speeds, especially within cool air the deep water masses, but there are also cold air well-defined currents (such as part of the Gulf Stream) that may flow as rapidly and strongly as some of the largest rivers on Fig. 2: Cold occlusion land. Neglecting those currents that flow within the surf zone, ocean currents result Occlusions from three particular sets of circumstances: wind stresses acting on the sea surface, tidal motion, and differences in the density rain. This is known as a back-bent occlu- of seawater. The density differences arise sion. because of differential heating and cooling, differential salinity (perhaps because of ocean The water mass (excluding lakes evaporation, ice melting, river discharge, and seas completely surrounded by land) and so on), and changes in turbidity levels. that covers part of the surface of the Earth. The wind-induced ocean currents may be The water mass as a whole constitutes the directly or indirectly driven. HYDROSPHERE. The interface between the hydrosphere and the atmosphere is thus ocean floor The whole of the seabed the sea surface, while the interface between seaward of lowest low-water mark, includ- the hydrosphere and the lithosphere is the ing the continental shelf, continental slope, sea bottom. The DEEP OCEAN lies beyond and deep-sea floor. It is characterized by the shelf edge or shelf break. diverse relief forms, some of them of immense size. Significant tracts of the deep- ocean basin floor That part of the sea floor are remarkably flat and smooth. ocean floor that extends from the CONTI- In spite of considerable depths of water in NENTAL MARGIN on the continental flank to the ABYSSAL ZONE, most of the ocean floor the MID-OCEAN RIDGE on the oceanic flank; experiences some current action. In global together these constitute the three major terms, the ocean floor is vast: some 360 morphological divisions of the ocean floor. million sq km out of a total of some 510 The ocean basin floor has been further sub- million sq km for the Earth’s surface as a divided into a number of physiographic whole. Ocean floor occupies about 61% of provinces, two of which (the elevated the N hemisphere and 81% of the S hemi- oceanic rises and the very deep abyssal sphere. In general, the ocean floor arches floor zones), are of particular importance. upward rather than downward. Truly con- The ocean basin floor occupies very exten- cave basins are rare. sive areas: approximately a third of the At- lantic Ocean floor, a third of the Indian oceanic crust That part of the Earth Ocean floor, and three quarters of the Pa- above the Mohorovičić discontinuity form- cific Ocean floor. The ocean basin floor is ing the floor of the ocean basins. It is usu-

241 oceanicity ally up to 15 km thick and consists of three from submarine earthquakes or explosive layers: layer one composed of young and submarine volcanic eruptions. Like land- poorly consolidated sediments, layer two slide surges and very big storm waves, they composed of basalt pillow lavas and dol- may cause severe damage and loss of life erite dikes, and layer three composed of along coasts. Oceans also experience tidal dolerite dikes and gabbro. The ocean crust waves: these waves operate as standing os- is generated at the constructive plate cillations that are modified by the Earth’s boundaries and destroyed at the destruc- rotation and move around nodal centers tive plate boundaries. See also continental known as amphidromic points (see am- crust. phidromic system). Within the water masses of the oceans, there occur inter- oceanicity /oh-shă-niss-ă-tee/ The de- facial waves known as INTERNAL WAVES. gree of oceanic influence on the climate of See also wave. an area. This is indicated by low annual and diurnal ranges of temperature, and ocellar /oh-sell-er/ Describing a texture abundant precipitation throughout the of some volcanic rocks in which PHE- year. NOCRYSTS are enveloped by small radially or tangentially arranged crystals of another oceanite /oh-shă-nÿt/ A basaltic rock mineral. containing a high proportion of olivine phenocrysts. Compare ankaramite. ocelli /oh-sell-ee/ Small spherical or dropletlike bodies found in certain alkaline oceanography /oh-shă-nog-ră-fee/ The igneous rocks. Ocelli contain mineral as- study of the oceans, with particular refer- semblages supposed to have crystallized ence to their overall form, the nature of the from a liquid that was immiscible with the sea floor and associated sediments, the host magma. characteristics of the ocean waters, and the types of fauna and flora living within the ocher /oh-ker/ A yellow, brown, or red oceans. This field has widened its scope to type of mineral iron oxide – or clay embrace aspects of human intervention in strongly colored by iron oxide – which is the ocean environment since the oceans powdered and used as a pigment. Red have assumed such importance economi- ocher usually contains HEMATITE, whereas cally and strategically. It is a multidiscipli- the yellow and brown forms are generally nary subject, embracing physics, based on LIMONITE. mathematics, biology, and several others. offlap /off-lap/ The disposition of sedi- ocean trench See trench. ments laid down as a sea regresses, the progressively younger sediments being ocean wave A wave affecting ocean wa- deposited seaward of the shoreline that ters. Ocean waves of some kind are operat- marked the maximum extent of the former ing all the time, some affecting the surface marine transgression. In this way, instead of the sea, others the internal water masses. of older rocks being covered by younger Some waves are generated by wind blow- deposits, they are exposed. The beds that ing over the sea’s surface, often traveling mark the maximum extent of the former far beyond the generation area as SWELL. transgression have the largest areal devel- Large waves with long periods may spread opment. See also overlap. from storm centers and travel thousands of kilometers before affecting distant coasts. offset See chaining. Some waves or wave energy may be reflected, or generated, in a seaward direc- offshore bar A bar that is partly or tion, as with SURF BEATS or the occasional completely submerged beyond the SURF catastrophic landslide surges (see tsunami). ZONE. Considerable littoral transport of tsunami waves are relatively rare and result sediment, especially sand, may occur along

242 olivine the crest of nearshore bars and offshore Cover of 8 oktas corresponds to a com- bars that lie in fairly shallow water because pletely overcast sky. of wave and tidal action. See also barrier beach. Old Red Sandstone A succession of conglomerates, red shales, and sandstones offshore zone The zone extending from forming the continental facies of the DE- the outer limit of the SURF ZONE to the VONIAN System in NW Europe. These rocks depth at which waves cease to influence the contain abundant remains of ostracoderm seabed. But because experimental evidence and gnathostome (jawed) fish. suggests that such wave influence occurs almost everywhere over continental shelves Oligocene /ol-ă-goh-seen/ The epoch of (with the exception, possibly, of shelves the PALEOGENE Period that followed the that have been abnormally deepened), the EOCENE and preceded the MIOCENE. It offshore zone effectively extends out to the began about 36.6 million years ago and SHELF-EDGE zone. lasted for some 13 million years. Many mammalian groups common in the Eocene ogive /oh-jÿv, oh-jÿv/ 1. (in statistics) A became extinct but others continued to cumulative frequency curve. It constitutes flourish, including camels, rhinoceroses, one of the most effective ways of repre- and the first pigs. The main phase of Alpine senting a frequency series, because uneven folding began at the end of the Oligocene. class intervals do not distort the curve and ol-ă-goh-klayss interpolation is easy. Calculation and plot- oligoclase / / A sodic plagioclase FELDSPAR. ting are also easy. The accumulated values for each class interval are determined as a oligotrophic /ol-ă-goh-troff-ik/ Describ- percentage of the grand total, or simply ing a lake or other body of water that is de- plotted as frequency against the element ficient in plant nutrients and so has very concerned. clear water (because of the scarcity of 2. (in glaciology) A broad-banded surface plankton). The lowest cold levels of water pattern on a glacier. In general, ogives are rich in dissolved oxygen, but the de- curve down-glacier as a result of relatively posits on the bottom contain little organic faster ice movement at the center of the material. See also eutrophic. glacier. A number of different types of ogive have been recognized: the banding in olivenite /o-liv-ĕ-nÿt, ol-ă-vĕ-/ A rare some cases is of alternating white ice, green mineral form of hydrated basic cop- which contains many air bubbles, and dark per arsenate, Cu2(AsO4)(OH). It crystal- ice, which contains few; others may relate lizes in the orthorhombic system and to longitudinal pressures in ice flow. occurs in deposits of copper. oil sand Any porous rock that is im- olivine /ol-ă-veen, ol-ă-veen/ A member pregnated with liquid hydrocarbons or of a group of orthorhombic rock-forming contains deposits of oil (petroleum). Com- minerals consisting of (SiO4) tetrahedra pare gas sand; tar sand. linked by divalent metal cations. The general formula is R2SiO4, where R = Mg, oil shale A type of fine-grained shale Fe2+, Mn, or Ca (in part). A complete gra- that can be strongly heated to produce oil dation in chemical and physical properties (petroleum). The resulting shale oil is rich exists between the two end-members, in unsaturated hydrocarbons. forsterite (Mg2SiO4) and fayalite (Fe2SiO4). oil trap See trap. Olivines are green, brown-green, and yellow-green and show little or no cleavage okta (in meteorology) A measure of but possess a chonchoidal fracture. Peridot cloud cover equal to one-eighth cover. is a pale green semiprecious gemstone vari-

243 omnivore ety. Magnesium-rich olivine is common in ooze See pelagic ooze. basic and ultrabasic rocks whereas fayalite occurs in acid igneous rocks. Forsterite and opal A cryptocrystalline or colloidal va- monticellite (CaMgSiO4) are found in riety of hydrous silica, SiO2.nH2O (see sil- metamorphosed limestones. ica minerals). It usually contains between 3 and 9% water, and occurs in various col- omnivore /om-nă-vor, -vohr/ An animal ors, from transparent to multicolored; the that feeds on both plants and other ani- reddish type is called fire opal. Found in ig- mals. Typical omnivores include cock- neous rocks and near hot springs, it is roaches, bears, chimpanzees, hogs, and highly valued as a gemstone. human beings. See also Carnivora; herbi- open fold A fold in which the interlimb vore. angle is greater than 70°. See diagram at FOLD. omphacite /om-fă-sÿt/ A member of the PYROXENE group of minerals. ophiolite /off-ee-ŏ-lÿt, oh-fee-/ The sequential association of ultramafic rocks, oncolite /onk-ŏ-lÿt/ A rounded, chalky gabbros, dolerite dikes, pillow lavas, and body that originated as algae. Most oncol- cherts occurring within eugeosynclinal enites are up to 2 cm across, although some vironments. Ophiolites are thought to rep- are five times that size. They are thought to resent slices of the basaltic ocean crust that have been formed when sediments became have been tectonically emplaced onto con- trapped round an algal growth that was tinental margins. See also spilite; Stein- then rolled around by currents in the shal- mann trinity. low sea. ophitic /oh-fit-ik/ Describing a texture onion weathering (onion-skin weather- of common occurrence in basalts and doing) Small-scale expansive EXFOLIATION lerites in which large plates of clinopyrox- in which the splitting away of layers of ene completely envelop earlier-formed rock resembles that of concentric onion euhedral plagioclase plates. The term sub- skins. ophitic may be used when the pyroxenes only partly enclose the plagioclases. onlap /on-lap/ See overlap. Ophitic texture is a specific example of POIKILITIC texture. onyx A type of CHALCEDONY that has alternating straight bands of various colors orbicular /or-bik-yŭ-ler/ Describing cer- (typically whitish with black or brown). It tain plutonic rocks that contain large ovoid is used for making ornaments and cameos. bodies made up of concentric shells of alternately light and dark minerals. The OR- See also banded agate. BICULES are the result of rhythmic crystallization around XENOLITHS, which oolite /oh-ŏ-lÿt/ An allochemical LIME- have acted as nuclei. Compare rapakivi STONE that is formed predominantly from structure. ooliths. These are more or less spherical bodies formed by the precipitation of orbicule /or-bă-kyool/ A large ovoid or carbonate in concentric layers around a spherical rocky mass consisting of concen- nucleus. They are usually small, up to tric shells of minerals. Most orbicules are about 2 mm in diameter. Ooliths larger up to 15 cm in diameter, although some than this are often known as PISOLITHS. Oo- may be as large as 3 m across. See orbicu- lites are common in the JURASSIC system of lar. Britain. Some noncalcareous oolites are known, such as those formed from iron orbital motion Waves on the sea’s sur- minerals. face experience orbital motion of the water

244 organic horizons particles beneath each wave; concurrently, cian the Llanvirn and Llandeilo series, and the wave form is propagated along the the Late Ordovician the Caradoc and sea’s surface, in the direction of wave ad- Ashgill series. Fossil evidence suggests that vance, at speeds that are dependent on the the landmasses were in very different loca- length of the waves. Beneath the waves, the tions to those of today. During the period orbital motion of water particles takes the major landmass Gondwanaland ex- place at much lower speeds. The orbits of tended from the South Pole north to the the water particles are nearly circular and tropics; the landmasses of Laurentia and have their greatest diameter just below the Baltica began to move toward each other sea’s surface, but the diameters decrease narrowing the Iapetus Ocean between rapidly with increasing depth. The orbital them. There is evidence that tectonic and paths are referred to as open, because the volcanic activity was extensive and intense, circular orbits are not completely closed with the beginning of mountain building loops; with each wave period, a water par- (the Caledonian orogeny) along the sub- ticle moves slightly in the direction of wave duction zone of the E margin of what is propagation. When the waves enter shal- now North America. Sedimentary rocks of low water, orbital motion becomes dis- the period are deep-water graptolitic shales torted, the orbital paths becoming and mudstones and calcareous sandstones, increasingly elliptic as waves run into in- mudstones, and limestones. Common fos- creasingly shallow water. Beneath a given sils include brachiopods (articulate forms shallow water wave, the long diameter of were dominant), trilobites, and gas- the ellipses decreases with increasing tropods. The first corals, echinoids, depth, while at the bed the water particles crinoids, and Bryozoa appeared and the no longer orbit but merely oscillate to and straight-shelled nautiloids reached their fro along a straight line, although again peak. The first fossils of ostracoderm ver- with a slight asymmetry shoreward. See tebrates are found in Ordovician rocks. also mass transport current. ore A naturally occurring usually rocky order A group in the taxonomic classifi- material from which a useful product (such cation of organisms. Several related orders as a metal or one of its minerals) can be ex- constitute a CLASS, and an order itself is tracted. The commercial worth of a metal- composed of one or more FAMILIES. For ex- lic ore depends on the value of the metal ample, the Primates and Carnivora are two and the percentage of it in the ore, bearing orders of the class Mammalia. See taxon- in mind how difficult it is to extract the ore omy. and, subsequently, the metal itself. See also mineral. Ordnance Datum See datum. organic acid (in soil science) Simple Ordovician /or-dŏ-vish-ăn/ The period products such as oxalic acid, (COOH)2. in the PALEOZOIC Era that followed the 2H2O, acetic acid, CH3COOH, and lactic CAMBRIAN and preceded the SILURIAN. It acid, CH3CH(OH)COOH that are re- began about 505 million years ago and leased when organic matter is attacked by lasted for about 67 million years. The the soil microorganisms. These acids name was proposed for rocks in the Arenig greatly increase the power of leaching in Mountains of N Wales, part of an area in- soils. See podzolization. habited by a Celtic tribe, the Ordovices. Sedimentary rocks containing fossils of the organic horizons The L, F, and H lay- Ordovician occur in all present-day conti- ers in a soil PROFILE, which lie above the nents; the graptolites, an abundant and mineral soil. However, humified organic rapidly evolving group, are used as the base matter is also usually found intermixed for the Ordovician geologic timescale. The with the mineral matter in the A horizon Early Ordovician consists of the Tremar- and may also be present lower down the doc and Arenig series, the Middle Ordovi- soil profile in the B horizon.

245 organic matter organic matter (in soil science) Ma- rocks by their removal of nutrients. Larger terial that forms as a result of the incorpo- plants can affect the surface atmosphere ration of plant and animal remains in the through shading while even in areas of only soil and their breakdown by the microor- shallow soils, weathering has been seen ganisms present. Some of this material is surrounding roots at great depths, owing broken down completely into soluble sub- to their localized ability to alter the con- stances but the more resistant material stituent minerals chemically. tends to remain, and with the new resynthesized compounds this forms the soil Ornithischia /or-nă-th’iss-kee-ă/ An order HUMUS. Organic matter rarely forms more of dinosaurs characterized by a pelvis sim- than 5% by weight of the soil yet it is ilar to that of birds, although they were not highly significant in promoting a produc- bird ancestors nor very closely related to tive soil because it improves the structure them. They were all herbivorous and had and provides a source of important mineral correspondingly modified dentitions; some nutrients. ornithischians had a horny beak. For avoiding saurischian predators some devel- organic reef See reef. oped rapid bipedal locomotion, while the more slow and bulky forms evolved horns organic rock Any sedimentary rock and other types of armor. Ornithischians that consists essentially of the remains of became extinct at the end of the Cretaceous plants or animals. Period (see K/T boundary event). Compare Saurischia. organic soil (bog soil) A soil with an accumulation of peat more than about 70 cm orocline /ô-rŏ-klÿn/ An orogenic belt in depth. Such soils form in water-satu- that has a marked change in trend. This is rated environments where the decomposi- thought to have resulted from horizontal tion is slow. In upland regions with high forces operating within the Earth’s crust. rainfall, rapid leaching of bases results in the formation of an acid peat called blan- orogenesis /ô-rŏ-jen-ĕ-sis/ The process ket bog. Similar acid soils, called raised by which mountains are formed, i.e. the de- bogs, may occur in lowland sites where formational processes such as thrusting, rainfall is sufficient. The most characteris- folding, and faulting, which result from the tic lowland organic soil, however, is fen collision of two continental plates. The soil (or peat), which is neutral to alkaline in sediments between the continents are then reaction as a result of saturation by base- strongly deformed, causing them to spill rich waters. When this black peat is out onto the older stable continents and be drained, rich agricultural soils are formed. compressed into long linear mountain chains. There is frequently associated ig- organic weathering The breakdown neous activity. Compare epeirogenesis. and decomposition of rocks caused by plants and animals. Both mechanical and orogenic belt /ô-rŏ-jen-ik/ See mobile chemical breakdown are involved. Bur- belt. rowing animals and plant roots can physically separate rock particles and can mix or orogeny /ô-roj-ĕ-nee/ A mountain- transfer material elsewhere. Through the building period. Major orogenic phases respiration of vegetation, the carbon diox- since the Precambrian include the Caledon- ide content of the soil may be substantially ian, Variscan, and Alpine orogenies. increased, having a marked effect on weathering, while soil bacteria can also be orographic cloud /ô-rŏ-graf-ik/ A cloud very important, especially in chemically re- that owes its formation to rising air result- ducing conditions. Lichens can exist on ing from airflow over mountains. Because bare rock surfaces and are frequently re- air is forced to rise over high ground there sponsible for the initial decomposition of will always be some upward component in

246 oscillatory current the wind field. If the air is already moist, or traced shoreward into decreasing depths. vertical motion extensive, the condensa- For this reason, where orthogonals bunch tion level may be reached and orographic together at the coast, or conversely when cloud formed. If there are stable layers they tend to spread out, one can delimit within the airstream, LEE-WAVE clouds may zones of wave energy concentration (zones be seen. These are the most easily recogniz- of CONVERGENCE), and zones of energy dis- able type of orographic clouds. sipation (zones of DIVERGENCE) respectively. orographic precipitation Rainfall and other forms of precipitation resulting from orthomagmatic /or-th’oh-mag-mat-ik/ the vertical motion of an airstream, caused Describing the stage in the crystallization by the presence of uplands. However, it is of a magma during which the early-formed fairly rare for orographic precipitation to primary minerals having high crystalliza- fall without either convective or cyclonic tion temperatures are formed. The or- processes also acting. The orographic com- thomagmatic stage is followed by periods ponent is normally weak and merely acts as of pegmatitic, pneumatolytic, and hy- an enhancement to the other two mecha- drothermal crystallization. nisms. orthomorphic projection /or-th’ŏ-mor- orpiment /or-pă-mĕnt/ A yellow to or- fik/ (conformal projection) Any MAP PRO- ange mineral form of arsenic sulfide, As2S3. JECTION on which the shape of an area of It crystallizes in the monoclinic system, and the map is the same as the corresponding occurs in association with CINNABAR and area of the Earth’s surface. REALGAR, often in lodes containing gold, lead, or sulfur. It may also be found in de- orthopyroxene /or-th’oh-pÿ-roks-een/ posits around hot springs. Despite its One of the PYROXENE group of minerals. highly toxic nature, it has long been used as a pigment; it is also used as a source of ar- orthoquartzite /or-th’oh-kwort-sÿt/ See senic. quartzarenite. orthite /or-th’ÿt/ See allanite. orthorhombic /or-th’ŏ-rom-bik/ See crystal system. orthoclase /or-th’ŏ-klayss/ A potassic alkali FELDSPAR. oscillation (in meteorology) Any cyclic variation of a climatic element above and orthoferrosilite /or-th’oh-fĕ-ross-ă-lÿt/ below its mean value or position. Its most One of the PYROXENE group of minerals. frequent use is as the Southern Oscillation (see El Niño) where an oscillation with a orthogeosyncline /or-th’oh-jee-oh-sing- period of 2.33 years was found in the sea- klÿn/A GEOSYNCLINE that is associated sonal distribution of pressure and to a with both continental and oceanic environ- lesser extent temperature and rainfall over ments, and contains both volcanic and the oceanic areas of tropical latitudes. See nonvolcanic areas in association with the also North Atlantic Oscillation. accumulating sediments. oscillatory current The type of current orthogonal /or-th’og-ŏ-năl/ (wave ray) A that waves induce at the seabed arising line that is perpendicular to a series of from the ORBITAL MOTION of the water par- wave crests. This is usually done on the ticles beneath waves. The larger the waves, basis of a wave refraction diagram, which the deeper such currents operate. As shows a series of wave crests in the vicinity waves, especially long waves, enter shallow of the coast or a harbor. In theory, the water, the orbital motion of the water par- wave energy between any two orthogonals ticles tends to be elliptic, while at the remains roughly the same as they are seabed the particles oscillate to and fro

247 oscillatory wave about a mean position along a straight line. placed as the animal increases in size. They This gives rise to an oscillatory current at are usually very small, less than 1 mm in the bed. Beneath the wave crest there oc- length, but some species grow up to 20 mm curs a short sharp acceleration shoreward, across. Varied ornamentation of the shells while beneath the flattish trough of the is of use in taxonomy. Ostracods range wave there is a more protracted seaward from the Cambrian Period to the Holocene movement of water. Such an oscillating and because of their small size, distinctive type of flow beneath waves often has a pro- forms, and widespread distribution, they found influence on the movement of bed are valuable in the calibration and correla- materials (although far less an influence on tion of the rocks in which they occur, espe- materials moving continually in suspen- cially in the cores from boreholes. sion), and may cause pebbles to move rapidly shoreward. Ostracodermi /oss-tră-koh-der-mÿ/ Fos- sil jawless fish (see Agnatha), having an oscillatory wave A wave, often of the outer protective covering of bony plates progressive oscillatory type, in which only and scales. Usually not more than 30 cm the waveform on the sea’s surface advances long, they were common inhabitants of Sil- in the direction of wave propagation, while urian and Devonian ponds and rivers and a the water particles beneath the wave orbit few fossil fragments are known from the in almost closed loops. Whereas the wave- Ordovician. This group probably gave rise form may be propagated across the sea’s to the jawed fishes. surface quite rapidly, at a speed that is largely dependent on the length of the outburst flood See glacial outburst wave, the gradual shift of the water parti- flood. cles in the direction of wave travel may be quite slow. outcrop The area on the Earth’s surface where a particular rock type or body is pre- oscula See Porifera. sent. This includes both exposed areas and others covered by drift. Osteichthyes /oss-tee-ik-th’ee-eez/ The class constituting the bony fish. It includes outer core The layer of the Earth be- the ACTINOPTERYGII (teleosts and more primitive ray-finned fish) as well as the tween 2900 km and 5000 km beneath its surface. It is bounded at 2900 km by a seis- fleshy-finned CROSSOPTERYGII. Some early groups had a body armor of closely fitting mic discontinuity (see Gutenberg disconti- scales of complex construction; these were nuity) that separates it from the overlying reduced and simplified in later forms. The mantle. On geophysical evidence it is class is characterized by the development thought to have a composition of iron alloy of some kind of lung, which may be con- and nickel and to behave essentially as a verted into a swim bladder in advanced liquid, unlike the INNER CORE, which is forms for maintaining hydrostatic equilib- solid. rium. Fossil fish first appeared in the late Silurian, since when the class has contin- outgassing The action of heat in remov- ued to expand in diversity and abundance. ing occluded gases from rocks. The out- Compare Chondrichthyes. gassing of water vapor and other gases from molten rocks in the primeval Earth is Ostracoda /oss-tră-koh-dă/ A group of believed to be the source of the atmos- aquatic arthropod animals that belong to phere. the class CRUSTACEA. The body is completely enclosed within two calcareous outlier /owt-lÿ-er/ An area of exposed valves forming a carapace. Unlike the younger sediments completely surrounded shells of the BIVALVIA, these have no growth by older rocks, usually as a result of fold- lines; they are periodically completely re- ing, faulting, and erosion. Compare inlier.

248 oxidation outwash plain (sandur) A widespread tant in PEDIMENT evolution. Most authori- area of fluvioglacial deposits produced by ties attribute to it a major capacity to shape deposition from meltwater streams emerg- the landscape. ing from the margins of an ice mass. The material decreases in size with distance overlap (onlap) The disposition of beds from the ice and is frequently well bedded. lying above an unconformity, where they Some unstratified till may be found among were deposited by a transgressing sea. Each the gravels, indicating a period of re- successively younger bed extends farther advance of the ice margin. These outwash onto the previous land surface than its pre- plains may exhibit irregular surfaces, re- decessor. See also offlap. sulting from the presence of kettle holes, and are frequently found in association oversaturated rock An igneous rock with other fluvioglacial landforms, such as that contains free quartz. Examples include KAMES, KAME TERRACES, and ESKERS. granites and rhyolites. See silica saturation. overflow channel A valley carved out overstep The disposition of strata asso- by water that overflows from a lake. Often ciated with an unconformity where meltwater from a glacier or ice cap over- younger transgressive marine beds en- flows from the lakes that form when the ice croach onto progressively older beds of the melts. Some overflow channels become underlying sequence. deep gorges. overturned fold See overfold. overfold (overturned fold) A type of FOLD in which very strong compression has oxbow lake (cutoff) A crescent-shaped caused the rock strata to be pushed right lake occupying the abandoned channel over. (oxbow) that was formerly part of a stream meander. Oxbow lakes are typical features overland flow (wash; rain wash; sheet of meandering streams, and result from the erosion) The movement of water as a stream’s ability to erode laterally being shallow unchanneled sheet over the soil concentrated on the outsides of meander surface. Overland flow, which is interme- bends. Once created the oxbow rapidly diate between flow of water through the silts up. (See diagram overleaf.) soil and surface channeled flow, occurs An alternative origin of cutoffs has been once the infiltration capacity, or ability to suggested by laboratory experiments, hold and store water, of the soil is ex- which showed that cutoffs develop when ceeded. Water then collects in depressions stream gradient falls below a critical limit, on the surface and begins to flow as a because the stream needs a minimum gra- broad shallow layer, often giving a glisten- dient to flow, and meanders effectively de- ing sheen to the surface. The flow is not crease gradient by increasing length over a even but turbulent, moving as a series of given fall. If the meanders develop to such waves, although LAMINAR FLOW in sheets 8 an extent that they rob the stream of its cm deep has been recorded. Speeds and minimum necessary gradient, cutting off depth of flow are limited but soil particles that meander will restore the gradient can be entrained and moved. The thickness needed. See also meander. of the sheet determines the size of material moved. Overland flow is most devastating oxidation (in geology) The chemical on bare soils of limited permeability, espe- weathering process involving the reaction cially on long steep slopes. The depth of between rocks and atmospheric oxygen, flow increases downslope from nil at the the oxygen usually being dissolved in hill crest until at the downslope end the water. The products of oxidation are ox- sheet becomes channeled into rills and ides and hydroxides, iron being the mineral ceases to exist. It tends to erode a hydraulic most frequently affected and its oxidation concave depression, and as such is impor- products give many weathered rocks their

249 oxide

erosion ered and leached, generally lacking in fertility, and in the case of the relic laterites may well include deposits of a more geo- deposition deposition logic than pedological origin.

oxygen One of the main gases of the at- erosion mosphere (about 20% by volume). From Fig. 1: Pattern of erosion and deposition the meteorological point of view, oxygen is in a meandering reach most important as an absorber of radiation in the ultraviolet wavelengths (0.13–0.17 µm). It dissociates at high levels of the atmosphere into monatomic oxygen (O), which combines with oxygen molecules (O2) to give OZONE (O3).

oyster reef A REEF largely formed from the consolidation of oyster shells. The Fig. 2: As a result of the pattern of erosion maximum development of oysters tends to and deposition, meander migrates as shown be in the brackish water areas skirting coasts, as within certain estuaries, or in bays with fairly narrow inlets. A reason- oxbow able supply of fresh water from rivers, to reduce salinity levels, is also an important factor. Oysters do also occur in the open sea, although in lesser numbers. The reefs they give rise to are common to many la- Fig. 3: In a storm, the narrow neck is goonal areas in the humid zone. The reefs breached leaving the oxbow may attain a significant thickness, for instance, commonly 10 m or more along Oxbow lake formation parts of the coast of Texas.

ozone Triatomic oxygen (O3), which reddish or yellowish color. A certain forms in the upper atmosphere especially amount of oxidation may also result from at about 20–25 km level (the ozone layer or bacteriological activity. Compare reduc- ozonosphere). It results from the dissociation. tion of molecular oxygen into single atoms of oxygen, which then combine with other oxide A chemical compound of oxygen oxygen molecules to give ozone. Below this and one or more other elements. In miner- level of formation, it gradually disperses alogy, the non-oxygen component is a toward the ground surface, but being a metal or metals; metal oxides make up a chemically reactive gas it is quickly re- large percentage of all minerals. duced to oxygen. Ozone absorbs strongly in the 0.23–0.32 µm waveband, preventing oxisol /oks-ă-sôl/ One of the twelve soil potentially damaging ultraviolet radiation orders of the US SOIL TAXONOMY character- from reaching the ground surface. It also ized by the presence of free iron and alu- absorbs at other wavelengths, but these are minum oxides (Fe2O3 and Al2O3) and of less importance. possibly other oxides of titanium and The distribution of ozone would be ex- chromium, as well as a high proportion of pected to correspond to its rate of produc- simple clays (especially kaolin) and some tion under conditions of prolonged quartz. This includes the currently devel- exposure to ultraviolet radiation. How- oping ferrallitic soils and the relic laterites ever, it seems to reflect vertical and hori- of tropical areas. Oxisols are highly weath- zontal movements in the stratosphere

250 ozone rather better. High latitudes have the max- As a result, more damaging ultraviolet ra- imum levels of ozone in spring with a min- diation can reach the Earth’s surface, pos- imum in the fall. sibly also leading to climate change. In Since the 1980s scientists have detected 1987 the signing of the Montreal Protocol ozone holes, particularly near the South established the first global agreement to re- Pole, in which significant depletions of strict CFCS. stratospheric ozone occur at certain times Ozone also occurs within the lower tro- of the year. They are thought to be caused posphere as a major constituent of photo- mainly by chlorofluorocarbons (CFCs), chemical SMOG where it is a pollutant used as aerosol propellents, which enter potentially harmful to plants and to human the atmosphere and break down the ozone. lung tissue.

251 PQ

Pacific ring of fire The circum-Pacific it followed the CRETACEOUS Period and was system of earthquakes, volcanoes, and succeeded by the EOCENE Epoch. Occasion- ocean trenches that surrounds the Pacific ally the time represented by this epoch is basin’s oceanic plates where they are sub- included within the Eocene. Paleocene ducting below other plates. It extends rocks are of both marine and nonmarine around the Philippines, Japan, and the W facies. Following the extinction of the large coast of North and South America. See also reptiles at the end of the Cretaceous (see plate tectonics. K/T boundary event), mammals became abundant and diversified into a variety of packing The arrangement in space of primitive and now mainly extinct groups. the atoms or ions in a CRYSTAL or the grains Insectivores were common and by the end in a rock. The individual particles can be of the epoch primates and rodents had thought of as tiny spheres, and they can be evolved. packed in various ways. The closest is hexagonal close packing, in which in any paleoclimatology /pay-lee-oh-klÿ-mă-tol- plane each sphere is centered on a corner of ŏ-jee, pal-ee-/ The study of the climates a regular hexagon. In cubic packing they of earlier geologic periods. The evidence are located at the corners of a square. In a for the former climate is obtained from rock, the packing of the grains determines sediments and fossils. Complications arise its porosity. from the fact that the continents have been changing their latitudinal positions pahoehoe /pah-hoh-ee-hoh-ee/ A lava through time, so the time period and loca- having a surface that resembles coils of tion of deposition have to be determined twisted rope. Compare aa; block lava. before conclusions can be deduced. palagonite /pă-lag-ŏ-nÿt/ Hydrated bas- paleoecology /pay-lee-oh-ee-kol-ŏ-jee, altic glass, usually yellow or orange, pal-ee-/ The study of the ecology of fossil formed when basalts are erupted beneath organisms, i.e. the relationships of organ- water or ice. Palagonite breccias or tuffs isms in past geologic ages with their non- produced by the fragmentation of the living environment and with other animals glassy material are termed hyaloclastic and plants. Paleoecology involves the in- rocks. vestigation of the rocks in which the organisms are found to determine paleobotany /pay-lee-oh-bot-ă-nee, pal- paleoenvironmental information and de- ee-/ The branch of PALEONTOLOGY that tails of TAPHONOMY. TRACE FOSSILS often deals with the nature and evolution of provide information regarding the behav- plants through geologic time, as shown by ior of an animal. fossil remains. Paleogene /pay-lee-ŏ-jeen, pal-ee/ The Paleocene /pay-lee-ŏ-seen, pal-ee-/ The lower period of the CENOZOIC Era, com- first epoch of geologic time in the PALEO- prising the PALEOCENE, EOCENE, and GENE Period. Beginning about 66 million OLIGOCENE Epochs. It lasted for about 42.5 years ago and lasting for 8.6 million years, million years from approximately 66.5 to

252 pallasite

24 million years ago, when it was followed paleontology /pay-lee-on-tol-ŏ-jee, pal- by the MIOCENE Epoch of the NEOGENE. ee-/ The study of all aspects of ancient organisms, including their TAXONOMY, paleomagnetic correlation /pay-lee-oh- anatomy, ECOLOGY, and EVOLUTION. The mag-net-ik, pal-ee-/ The correlation of evidence for this comes from FOSSILS pre- the residual magnetism in rocks. The direc- served in rocks, which are either the re- tion of polarization of the magnetic field of mains of the organisms themselves or the Earth changes periodically (see mag- TRACE FOSSILS. Much paleontological work netic reversal). It is possible to measure the is applied to the elucidation of the strati- residual magnetization of suitable rocks graphical relationships between bodies of and to discover the direction of polarity at rock by means of BIOSTRATIGRAPHY. The the time of their formation. By this means branch of the subject that deals with microscopic organisms is known as micropa- a sequence can be established showing nor- leontology; fossils of such organisms are mal and reversed periods in the Earth’s MICROFOSSILS. magnetic field. It may be calibrated by some means, such as radiometric dating Paleozoic /pay-lee-ŏ-zoh-ik, pal-ee-/ The (see calibration), to provide a valuable tool first era into which PHANEROZOIC time is CORRELATION for the of stratigraphic se- divided. It followed the PRECAMBRIAN and quences of rock. See also magnetic divi- consists of the following periods: the CAM- sion; magnetic interval. BRIAN, ORDOVICIAN, SILURIAN, DEVONIAN, CARBONIFEROUS, and PERMIAN. Systems of paleomagnetism The study of the RE- the first three periods constitute the Lower MANENT MAGNETISM preserved in rocks and Paleozoic, and the second three the Upper changes in the Earth’s magnetic field Paleozoic. It lasted for about 325 million through geological time. During the time years from 570 to 245 million years ago, of their formation those rocks in which when it was followed by the MESOZOIC Era. magnetic iron-bearing minerals are present The fauna of the Paleozoic is characterized acquire a permanent record of the Earth’s by an abundance of invertebrate types, magnetic field at that time; such rocks in- many of which, such as the rugose corals clude mafic igneous rocks and lake and (see Anthozoa), trilobites, graptolites, and marine sediments. During the cooling pro- productid brachiopods, became extinct at cess of mafic lava small crystals of mag- the end of the era. In the course of the Pa- netite form and while the lava is still fluid, leozoic fish evolved and the first amphib- are aligned with the Earth’s magnetic field. ians appeared, flourished, and declined. Once cooled and solidified the alignment Two episodes of major orogenic activity of the crystals is frozen in place. The align- occurred, giving rise to mountain ranges ment also occurs when magnetite particles and associated tectonic activity. The CALE- DONIAN OROGENY was the earliest, occur- settle in the sediments on the floor of a ring at the end of the Silurian and the body of calm water, e.g. a lake. The record beginning of the Devonian; the VARISCAN of reversals in the Earth’s magnetic field is orogeny took place toward the end of the particularly evident on the ocean floor era. See also geologic timescale. where there is little erosion to disturb the sediments. The magnetic reversals re- paleozoology /pay-lee-oh-zoh-ol-ŏ-jee, corded in the eruptions of basaltic lava pal-ee-/ The branch of PALEONTOLOGY along mid-ocean ridges have provided evi- concerned with the nature and evolution of dence for SEA-FLOOR SPREADING. Paleomag- animals through geologic time by the in- netism has also supported plate tectonics vestigation of their fossil remains. theory and has been used to locate the positions of continents in relation to each pallasite /pal-ă-sÿt/ A stony-iron METE- other in the past. See also magnetic rever- ORITE that has grains of OLIVINE in a nickel- sal. iron matrix.

253 paludal paludal /pă-loo-dăl, pal-yŭ-/ Describing desert areas. The coastal blowout dune is a something that lives in, is related to, or is type of parabolic dune that occurs adjacent produced by a marsh. to a beach (see blowout). palustrine /pă-lus-trin/ Describing plants paragonite /pa-ră-gŏ-nÿt/ One of the that grow in or deposits that accumulate in major types of MICA, a hydrated sodium a marsh. aluminum silicate, Na2Al4(Si6Al2O20)- (OH)4. It forms colorless, yellowish, or palygorskite /pal-ă-gorsk-ÿt/ Any of a green crystals and occurs mainly in various number of claylike minerals, hydrated metamorphic rocks. magnesium aluminum silicates, (Mg,Al)2- Si4O10(OH).4H2O. They form thin fibrous parallel A line of latitude, running par- crystals that resemble cardboard and occur allel to the Equator round the Earth in desert soils. through any given point. The plane of each parallel is at right angles to the Earth’s palynology /pal-ă-nol-ŏ-jee/ The study axis. Parallels range in length from a point of fossil spores, especially POLLEN. Because at each pole to the circumference of the spores are usually adapted to resist de- Earth at the Equator, which is the only par- struction and to be dispersed over large dis- allel that is a GREAT CIRCLE. Parallels are tances, they are valuable for use in the uniformly spaced along the meridians. CORRELATION of the rocks in which they occur. They are also important environ- parallel drainage A type of drainage mental indicators and have been used in pattern that develops where there is strong monitoring climatic change during the structural control in one direction, streams Quaternary Period. Palynology forms part being strictly guided in that direction. A of micropaleontology (see paleontology). uniform regional slope, rigid alternation of hard and soft lithologies, or parallel folds pampas A region of temperate grassland will produce parallel streams; recently in South America, found in Argentina and glaciated areas may also have parallel Uruguay near the Plate River estuary. Used drainage nets if the surface is covered in originally as pasture for cattle and sheep, today much of it has been plowed for fluted ground moraine or drumlin fields. growing crops such as alfalfa, corn, and wheat. parallel evolution The CONVERGENT EVOLUTION of closely related and anatomi- Pangaea /pan-jee-ă/ A hypothetical su- cally similar organisms, which undergo percontinent that is thought to have ex- nearly identical evolutionary changes isted until near the close of the Triassic through time. Period. This single continent was composed of all the present-day continents, parallel retreat A form of SLOPE EVOLU- which have since been derived from it as a TION in which the original shape and angle result of repeated episodes of SEA-FLOOR of the slope is maintained as it erodes SPREADING. This supercontinent is thought (compare slope decline). Hence it remains to have fragmented initially into two seg- parallel in new positions to its previous po- ments, LAURASIA to the north and GOND- sitions. Walter Penck and later Lester King WANALAND to the south. used this principle as the basis for their scheme of the CYCLE OF EROSION. As a parabolic dune A U-shaped type of method of slope evolution it is strongly re- DUNE in which the curve of the crest bows lated to geomorphic processes, in that a outward in the downwind direction, and slope will remain parallel only if the debris elongated arms extend upwind (opposite being eroded in its retreat is removed at an to barchan dunes in which the arms extend equal rate from its base. If the debris accu- downwind). They are common in coastal mulates, the slope declines.

254 particle size

The idea was first formulated in rela- paratectonics /pa-ră-tek-tonn-iks/ The tion to the tropical scheme, with the origi- tectonics of stable areas, such as old fold nal work being in Africa; since then, G. H. belts and cratons. Dury claims to have identified it as a viable process in the humid temperate lands, and paraunconformity /pa-ră-un-kŏn-form- it can be seen in areas of BASAL SAPPING, e.g. ă-tee/ See disconformity. active sea cliffs. S. A. Schumm has suggested it operates preferentially on certain parautochthonous /pa-raw-tok-th’ŏ-nŭs/ lithologies through their control on pro- Describing rock bodies that are midway cess: on sandstones, dominated by over- between ALLOCHTHONOUS and AUTOCH- land flow, parallel retreat operates, THONOUS, i.e. that have been displaced only whereas on clays, dominated by creep, the short distances. slope declines. It seems that a number of situations can favor parallel retreat, not parent material The material from merely certain climates; lithology and which the soil forms, which may not be the processes, in various permutations, are same as the underlying bedrock. Before the also vital. importance of climate was recognized, parent material was thought to be the domi- paramagnetic /pa-ră-mag-net-ik/De- nant soil-forming factor. Certain features scribing a material that has a magnetic per- of the parent material, such as texture, meability slightly greater than 1. In a mineral composition, and porosity, have a considerable influence on the soil-forming magnetic field, a paramagnetic mineral is processes, e.g. a limestone soil rich in cal- magnetized in proportion to the field cium will delay the process of acidification strength. Platinum is a paramagnetic in a humid climate. Parent material is likely metal. to have a greater influence in temperate regions where soils have been forming for a parameterization /pă-ram-ĕ-tĕ-ri-zay- relatively short time, e.g. in Scotland shŏn/ (in numerical weather prediction) brown earths have been recorded on basic The inclusion of the effects of meteorolog- igneous rocks and podzols on acid igneous ical systems that are smaller than the grid rocks. See also soil formation. on which the model is based. It involves ap- proximating the statistical effects of these parhelion /par-hee-lee-ŏn, -hel-yŏn/ See small-scale processes in terms of the large- mock sun. scale factors. partial melting The geologic process in parasite An organism that lives on or in- which a rock is subjected to extremely high side another organism (the host) on which temperature and pressure so that it partly it is totally dependent for food and energy. melts and the liquid flows away. The liquid This may or may not harm the host. Exter- then solidifies to form of a different type of nal parasites are called ectoparasites; inter- rock. nal ones are endoparasites. partial pressure In a mixture of gases, parasitic cone A small cone on the such as the atmosphere, the partial pres- flank of a major volcanic cone. These cones sure of one constituent is the pressure it develop frequently and indicate that the would exert on its own while occupying magma trying to reach the surface has the same volume as the whole mixture. The found an easier route than via the main sum of all the individual partial pressures vent. They are usually several tens of me- of the components is equal to the pressure ters beneath the main vent. of the whole mixture. parataxitic /pa-ră-taks-it-ik/ See eu- particle size The effective diameter of a taxitic. particle. Particle size analysis is the tech-

255 passive continental margin

weathering. Patinas are distinctive in color CLASSIFICATION OF PARTICLE SIZES and physical properties, their thicknesses often reflecting age. Class Size range (mm) patterned ground A distinctive geo- boulder > 200 morphological feature mostly found in cobble 60–200 periglacial areas. Where the ground surface gravel 2–60 is flat, STONE POLYGONS or CIRCLES may de- coarse sand 0.6–2 velop, either in groups or in isolation, medium sand 0.2–0.6 which may grade into STONE STRIPES even fine sand 0.06–0.2 on very gentle slopes. In nonperiglacial cli- silt 0.002–0.06 matic areas the most likely cause of pat- clay < 0.002 terning is cracking due to heat shrinkage (see desiccation crack). nique of determining the proportion of the pays A low plateau of land in northern different particle sizes present (each parti- France. cle size is termed a separate) in a sample. There are several classifications in use; peacock ore See bornite. the main classes in the British standard classification are shown in the table. There pearl An accumulation of white or are three main techniques of analyzing par- cream-colored, usually spherical, NACRE in- ticle size: by sieving in a stack of sieves of side the shell of a mollusk, such as a clam decreasing mesh size, by allowing particles or oyster. It forms when layers of nacre to settle in a tube of water, the rate of set- gradually build up on a foreign body, such tling being proportional to size, and by as a grain of sand. Pearls are valued as pre- micrometric methods. It is a technique cious gemstones; most prized are black commonly used in soil science and geo- pearls. morphology, e.g. to describe texture classes in soil, or to infer the origin of a peat A mass of fibrous plant debris, sedimentary deposit in geomorphology. which is only partly decomposed, and is See also phi scale; Udden–Wentworth often dark brown in color. This is a result scale. of wet anaerobic conditions, which retard microorganism activity. Certain soils such passive continental margin (passive as peaty gleys and peaty gleyed podzols margin) A margin of a continent that have peat as one of their main horizons; does not coincide with a plate boundary other soils may have a thicker accumula- (or margin) and where there is minimal tection of peat. (See organic soil.) tonic activity. Examples include the At- lantic margin of North and South America. peaty gley podzol A hydromorphic Compare active continental margin. variant of PODZOL, characterized by a blue- gray waterlogged area above the imperme- passive glacier A GLACIER that accumu- able iron pan, capped by a thick peat lates little material and loses little material; horizon. Above and below the gleyed area technically, its rates of alimentation and the color is orange-brown because of the ablation are about equal. This occurs when free drainage and oxidation, as contrasted there is not much winter snow and a cool with the reducing conditions in the gleyed summer with little melting of the ice. Such area. Such soils are most probably polyge- glaciers move very slowly, causing little netic, being originally BROWN EARTHS that erosion and transporting few materials. became degraded to podzols with forest clearance, and subsequently became gleyed patina /pat-ă-nă/ A film or skin on the when the iron pan developed to the point surface of a boulder, produced by chemical where it became impermeable. Once gley-

256 pediplain

gles of between 9° and 0.5° or less, being peat bounded on their upslope ends by a sharp break of slope, the PIEDMONT ANGLE, at the point where they meet rock residuals. Such A horizon features are said to be typical of arid and mineral and organic horizon–ungleyed semiarid regions, although some would attribute this type of surface with a much gleyed B horizon– wider occurrence. iron pan impermeable Pediments have a concave long profile, with the angle of slope decreasing away p o aining B horizon s us st from the piedmont angle. They are cut in sible hum iron pan bedrock, often with a capping of gravel or alluvium, and display great regularity of form over wide areas. Pediments have been ungleyed B– free drainage identified at various scales, ranging from valley-side types to the continental-scale forms that the South African geologist Lester King identified in Africa. C horizon parent material Pediments evolve by PARALLEL RETREAT of the upstanding residuals, leaving a broad area of low relief as they retreat. King and Walther Penck, who see this as Peaty gley podzol the usual tendency for landscape evolution, therefore see pediments as the end prod- ing had set in, microbiological activity was ucts of denudation. The processes respon- inhibited and so litter was not broken sible are not certain, but much discussion down but accumulated as peat. It is char- has centered on erosion by running water. acteristic of many gently sloping uplands. Because the pediments have a concave profile, analogies have been drawn between pebble A rounded rock fragment from 5 their evolution and that of the long profiles mm to 65 mm across. It is thus larger than of rivers: both are hydraulic curves. The a granule but smaller than a cobble. form of this water erosion is disputed: some authorities consider that the scarp re- ped An aggregate of soil particles, form- treats by headward erosion of streams ing a crumb of soil. emerging from the upland, leaving the low- level surface as it retreats; others consider pedalfer /pĕ-dal-fer/ A leached soil in that pediments are due to lateral planation which iron and aluminum increase relative of streams, while a major school of thought to silica during pedogenesis. Pedalfers are has stressed the importance of sheet wash, soils of humid climates, including PODZOLS, which has been recorded up to a meter BROWN EARTHS, LATERITES, and PRAIRIE deep on pediments. King states that pedi- SOILS. See also soil formation. ments begin on weak rocks and then extend into harder rocks as the scarp retreats pedestal rock A rock that has been by stream erosion at the base. Several dif- shaped like a mushroom by the erosive ac- ferent processes may be responsible, with tion of windblown sand (corrasion). In different emphases in different areas. desert regions, the corrasive action of sand is strongest about a meter above the pediplain /ped-ă-playn/ A broad land- ground and most pedestal rocks have scape of low relief broken by isolated resid- ‘stems’ of this height. ual uplands, which meet the pediplain in a sharp PIEDMONT ANGLE. It is formed pediment A surface of low relief that through the coalescence of a number of slopes away from residual uplands at an- PEDIMENTS. According to the South African

257 pedocal geologist Lester King and German geolo- largest crystals occur in pegmatites, occa- gist Walther Penck, a pediplain is the logi- sionally measuring meters in length, and cal final product of subaerial denudation in because of the concentration of rare ele- an area of PARALLEL RETREAT (compared ments, pegmatites may be of economic im- with a PENEPLAIN in areas of SLOPE DE- portance. Coarse-grained facies of igneous CLINE). Pediments tend to be smooth with rocks other than granite may be termed gentle concave profiles, broken by few pegmatitic. Pegmatites represent a stage in stream lines. It is difficult to be categorical the crystallization of a magma during as to whether a surface of low relief consti- which a residual silicate melt and an aque- tutes a true pediment, owing to the fact ous gas phase coexist over the temperature that landscapes have not evolved under range 500–700°C. The growth of large constant climates and hence a single cycle, crystals may be ascribed to conditions of but are polycyclic in origin (see polycyclic slow cooling (and therefore slow crystal- landscape). lization), rapid diffusion, and low viscosity due to the high concentration of volatiles. pedocal /ped-ŏ-kăl/ Unleached soil in See also aplites. which lime accumulates during pedogenesis, e.g. CHERNOZEM, CHESTNUT SOIL, and pelagic /pĕ-laj-ik/ 1. Denoting the envi- SIEROZEM. ronment of the open ocean. A classification of ocean sediments, based on distance from pedogenesis /ped-ŏ-jen-ĕ-sis/ See soil the coast, might include beach, shelf, formation. hemipelagic, and pelagic sediments. The latter have been precipitated from the open pedology /pĕ-dol-ŏ-jee/ The scientific ocean or have accumulated from the or- study of soil, including its classification, ganic remains of planktonic marine ani- origins, and composition. See soil; soil for- mals. The main types of pelagic sediment mation. are the PELAGIC OOZES and RED CLAY. The hemipelagic-abyssal environment is taken pedon /ped-on, pee-don/ A three-dimen- to lie within several hundred kilometers of sional body of soil, including the full the coast, at depths of over 1000 m. The SOLUM and the PARENT MATERIAL, with a pelagic-abyssal environment also occurs in circular or hexagonal cross section, rang- depths greater than 1000 m but is farther ing in size from 1 to 10 square meters. In- from the coast. Similar sediments occur in troduced by the US SOIL TAXONOMY method the hemipelagic environment but in this of classification, it replaces the earlier use case they are often mixed with land-de- of the two-dimensional profile as the low- rived material. est unit of classification. It includes all the 2. Denoting communities of marine organ- properties formerly noted from the profile, isms (free-swimming nekton and floating plus notes on interfingering of horizons. plankton) that survive within the ocean Similar contiguous pedons are grouped waters quite independent of shore or into polypedons, which can then be amal- seabed environments. Compare benthos. gamated into soil series, and hence mapping of soil is dovetailed into classification. pelagic ooze Organic deposits made up of shells and other hard parts of marine or- pegmatite /peg-mă-tÿt/ A very coarse- ganisms, which cover large tracts of the grained igneous rock occurring as segrega- deep-sea floor. The nature of the skeletons tions, veins, and dikes within and of the various organisms determines the emanating from granite bodies. Pegmatites kind of deposit ultimately formed, the de- consist largely of alkali feldspar and posits being named after the most promi- quartz, often graphically intergrown, and nent organisms present. They include the contain abundant accessory minerals such calcareous GLOBIGERINA and PTEROPOD as micas, tourmaline, topaz, spodumene, OOZES, and the siliceous RADIOLARIAN and beryl, cassiterite, and wolframite. The DIATOM OOZES. Nearly all the oozes are soft

258 pentlandite and are easily disturbed by crawling or There are no large peneplains on the burrowing organisms and deep-sea cur- Earth, because no landscape has evolved rents, a fact confirmed by deep-sea pho- under just one cycle. All have polycyclic tographs. The radiolarian and pteropod origins, interrupted by base-level move- oozes occupy very limited areas of deep-sea ments and process variations. The pene- floor; diatom ooze, a cold-water deposit, plain concept is developed from the theory occupies some 9% of the total ocean floor, that slopes evolve by decline, such that as while globigerina ooze, the most wide- the cycle nears its end the remaining inter- spread of the oozes, globally occupies some fluves gradually become inconspicuous as 130 million sq km. weathering and rill wash continue to erode them, with the slope foot zones in the val- Pelean See volcano. leys being areas of little activity, mostly ac- cretional in nature. Relative relief Pelecypoda /pel-ĕ-sip-ŏ-dă/ See Bivalvia. diminishes, therefore leaving an almost flat plain. This contrasts with the ideas of the Pele’s hair, Pele’s tears /pel-ee/ See py- South African geologist Lester King and roclastic rock. German geologist Walther Penck, who stated that slopes evolve by PARALLEL RE- pelite /pel-ÿt/ A metamorphosed ARGILLA- TREAT, leaving broad PEDIPLAINS and small CEOUS rock. Compare psammite; psephitic upstanding residuals as the final products. rock. Penman formula A method of estimat- pellet An ALLOCHEM that is well rounded ing POTENTIAL EVAPOTRANSPIRATION. Evap- but has no internal structure. Pellets may oration loss is expressed in terms of the be produced by algae or they may be the solar radiation, mean air temperature, feces of invertebrates. mean atmospheric relative humidity, and mean wind speed. It was devised in the pendant See roof pendant. 1940s by the British physicist Dr Howard Latimer Penman (1909–1984) and the for- penecontemporaneous /pen-ĕ-kŏn-tem- mula and subsequent modifications have pŏ-ray-nee-ŭs/ Describing geologic events been applied in climatology and in agricul- that appear to have occurred at almost the tural and soil studies. same time. For example, the deposition and erosion of a bed of sediment are de- Pennsylvanian A period within the scribed as penecontemporaneous if the bed American classification of geologic time underwent erosion during, or just after, its that followed the MISSISSIPPIAN, and pre- deposition. ceded the Permian. It began about 320 million years ago and lasted for about 34 peneplain /pen-ĕ-playn/ An extensive million years. The Pennsylvanian System area of low relief dominated by broad corresponds approximately to the Upper floodplains and gentle interfluves, with iso- CARBONIFEROUS elsewhere. It was named lated residuals (monadnocks) left upstand- for the State of Pennsylvania, where rocks ing in areas of resistant rocks. from this period are widespread and have According to the American geologist produced much coal. William Morris Davis’s original CYCLE OF EROSION scheme, the peneplain is the end pentad /pen-tad/ A period of five succes- product of subaerial denudation. His sive days. It is often used in climatological scheme being formulated mostly in the work because the period divides exactly humid temperate climate of New England, into the number of days in a year, leap he regarded this as the normal cycle, with years excepted. the PEDIMENT and INSELBERG landscapes of arid and semiarid areas being deviations pentlandite /pent-lănd-ÿt/ A yellow or due to climatic controls. orange mineral, an iron-nickel sulfide,

259 percentile

(FeNi)9S8. It crystallizes in the cubic sys- dome they dip away from the center tem, and occurs as masses, generally asso- (quaquaversal). ciated with PYRRHOTITE. It is used as a source of nickel. peridot /pe-ră-dot/ An olive-green gem variety of OLIVINE. percentile The percentage division of a ranked data series. For example, the upper peridotite /pe-ră-dot-ÿt/ An ultramafic five percentile indicates the value that is ex- rock consisting wholly or largely of olivine, ceeded by 5% of the data, and the lower together with other ferromagnesian miner- five percentile is the value above which als, and devoid of feldspar. 95% of the values occur. periglacial /pe-ră-glay-shăl/ Describing percolation /per-kŏ-lay-shŏn/ The pro- the climate, physical processes (see altipla- cess by which water moves downward, nation; congelifraction; congeliturbation; under the influence of gravity, through freeze-thaw; frost-shattering; needle ice; pores in soil or cracks and joints in rock. solifluction), and resultant landforms (see The water may carry dissolved chemicals, involution; patterned ground; permafrost; or may dissolve minerals from the soil (see pingo; stone polygons or circles; stone leaching). stripes) characteristic of an area bordering an ice sheet or glacier. Although ice sheets percoline /per-kŏ-lin/ A line just below and glaciers do not occur in the periglacial the surface that denotes the extent of water zone, it is still extremely cold, temperatures seepage through soil. It generally follows remaining below freezing throughout the the slope of the surface, but may be af- winter and rising slightly above only dur- fected by plant roots and the activities of ing the summer. Periglacial zones are re- burrowing animals. stricted to high latitudes today but were far more widespread during the PLEISTOCENE percussion mark A circular chip out of glacial periods. a pebble, caused by a sharp blow against another pebble when they were moving perihelion /pe-ră-hee-lee-ŏn/ The point rapidly, probably transported by water. on the Earth’s orbit that is nearest the Sun. It is also applied to the nearest point to the perennial stream A stream or river that focus of any orbiting body. Compare aphe- flows permanently throughout the year. lion. Compare intermittent stream. period An interval of geologic time in perfect elasticity The property of a ma- the Chronomeric Standard scale of chrono- terial that can regain its original form after stratigraphic classification (see chrono- an applied force has been removed. stratigraphy). The equivalent Stratomeric Standard term, indicating the body of rock perfect plasticity The property of a ma- formed during this time, is the SYSTEM. Pe- terial that retains a new form after an ap- riod and system are usually known by the plied force is removed. same name; for example, rocks constituting the Jurassic System were formed during periclase /pe-ră-klayss/ A pale-colored the Jurassic Period. Periods are formed of a cubic magnesium oxide mineral of compo- number of EPOCHS grouped together and sition MgO, found in metamorphosed are themselves compounded to form ERAS. dolomites and limestones. Thus the Triassic, Jurassic, and Cretaceous Periods together constitute the Mesozoic pericline /pe-ră-klÿn/ A fold structure, Era. such as a basin or dome, in which the beds dip around a central point. In a basin they periodicity /peer-ee-ŏ-diss-ă-tee/ An os- dip toward the center (CENTROCLINAL); in a cillation in a time series of a climatic ele-

260 Permo-Triassic ment that is found to recur at approxi- pore spaces within the material are con- mately equal time intervals. There have nected. been numerous searches for periodicities in climatic data because they would be of Permian /per-mee-ăn/ The final period great assistance in long-range forecasting. of the PALEOZOIC Era. Beginning about 286 Unfortunately, although periodicities can million years ago and lasting some 40 mil- be found for short time periods, they are lion years, it followed the CARBONIFEROUS rarely persistent. and was succeeded by the TRIASSIC (which marks the start of the Mesozoic Era). It is Perissodactyla /pĕ-riss-ŏ-dak-tă-lă/ The named for the Perm region of the Russian order consisting of hoofed mammals hav- Urals. The lower part of the Permian Sys- ing an odd number of toes, such as the tem, the Cisuralian Epoch, consists of four horse and rhinoceros. Horses evolved from ages: the Asselian, Sakmarian, Artinskian, hyracotherium, a small dog-sized five-toed and Kungurian. The next epoch, the animal of the Eocene, by increasing in size Guadalupian, consists of the Roadian, and, as an adaptation to fast locomotion of Wordian, and Capitanian Ages. This was the plains, by gradually losing all but one followed by the Lopingian Epoch compris- toe on each foot. They acquired high- ing the Wuchiapingian and Changhsingian crowned teeth with a complicated enamel Ages. pattern, well suited for chewing a diet of Large-scale movements of the crustal coarse grass. Horses originated in North plates continued into the Permian; the con- America and spread to Europe, Asia, and tinent of Laurasia in the N hemisphere be- Africa. Other, now extinct, types of peris- came linked to Gondwanaland in the S sodactyl lived in the early Tertiary, some hemisphere and, as a result, by the middle reaching a large size, but the order has not of the period the vast supercontinent, Pan- achieved the success of the Artiodactyla. gaea, was created. The continuation of the Allegheny orogeny in E North America perlitic /pĕ-rit-ik/ Describing glassy saw the culmination of the formation of rocks that contain concentric onion-like the Appalachian fold belt; in Europe and cracks caused by contraction during cool- Asia the corresponding Variscan orogeny ing. of the Hercynian Mountains was also di- minishing in intensity by the end of the pe- permafrost /per-mă-frôst/ Permanently riod. Characteristic rocks of the Permian frozen soil occurring in PERIGLACIAL areas, are red continental sediments and evapor- where the winter temperatures rarely ex- ites. They are widespread in North Amer- ceed freezing point and the soil is frozen to ica with great thicknesses of deposits considerable depths. In summer only the occurring in Texas, New Mexico, Nevada, top meter or so of soil thaws, that beneath and Utah. remaining frozen and acting as an imper- The period saw the extinction of many meable barrier to percolating water. This animal groups, including the tabulate and top layer, which is subject to freezing and rugose corals (see Anthozoa), productid thawing, is known as the active layer and brachiopods, trilobites, and blastoids, and when melting occurs it rapidly becomes a great reduction in others. Reptiles be- saturated and hence mobile (see muskeg; came abundant and pteridophyte plants solifluction). Pedogenic and cryoturbation were superseded by gymnosperms. See also processes are confined to this layer. Permo-Triassic. permanent hardness See hard water. Permo-Triassic /per-moh-trÿ-ass-ik/ The rocks of the PERMIAN and TRIASSIC Periods permeability /per-mee-ă-bil-ă-tee/ The in Britain, when these are considered as a ability of a rock, sediment, or soil to allow single unit. Because continental conditions pore fluids and gases to pass through it. prevailed for much of the Permian and Tri- This is governed by the extent to which the assic Periods in Britain recognition of any

261 perovskite boundary between the two systems is diffi- high viscosity, and occurs seeping out of cult. the ground or in underground deposits (from which it is extracted by drilling). The perovskite /pĕ-rof-skÿt/ A black or crude extract is distilled to produce various brown cubic mineral form of calcium ti- fuels and starting materials for the petro- tanate, CaTiO3, but usually with some chemical industry. substitution of rare earth elements for calcium and niobium for titanium. It is found petrology /pi-trol-ŏ-jee/ The scientific as an accessory mineral in some highly un- study of rocks, which deals with their clas- dersaturated alkaline igneous rocks and in sification, origins, distribution, and com- thermally metamorphosed limestones. position. Petrology involves mineralogy, chemistry, petrography, and petrogenesis. persistence The continuance of any synoptic event or climatic element beyond its pH A measure of the acidity or alkalinity expected duration; the term is frequently of a solution. It is the negative logarithm of used for sea-surface temperatures or pres- the hydrogen ion concentration. A neutral sure anomalies. In a time series of climatic solution has a pH of 7; a pH of more than data, persistence also indicates a nonran- 7 is acidic, less than 7 is alkaline. dom tendency for relatively high or low values to occur in succession. phacolith /fak-ŏ-lith/ A minor concordant igneous intrusion, usually lensoid (the perthite /perth-ÿt/ An intergrowth of or- concave surface facing downward) and thoclase and albite feldspar in which the al- present in folded strata. bite occurs as patches or lenticles in the orthoclase host. Perthite may be produced phanerocrystalline /fan-ĕ-roh-kris-tă-lin, by the reaction of potassium feldspar with -lÿn/ (phaneritic) Describing an igneous sodic fluids or by the unmixing of an orig- rock in which the constituent crystals can inally homogeneous alkali feldspar. See an- be seen with the naked eye. Compare tiperthite; exsolution. aphanitic. perthosite /per-thŏ-sÿt/ A member of Phanerozoic /fan-ĕ-rŏ-zoh-ik/ The geo- the SYENITE group of minerals. logic time that has elapsed since the end of the PRECAMBRIAN, comprising the PALEO- Peru Current See Humboldt Current. ZOIC, MESOZOIC, and CENOZOIC Eras. It has lasted for some 570 million years, from the pervious Describing a rock through CAMBRIAN Period to the present. The name, which water can pass along cracks and fis- meaning visible life, refers to the fact that sures. The water can in this way percolate clearly recognizable fossils are found in as far as the water table and form part of rocks laid down during these periods. In the supply of groundwater. Precambrian rocks fossils are extremely rare and often of obscure biological affini- petrogenesis /pet-rŏ-jen-ĕ-sis/ The ori- ties. gin or mode of formation of rocks. phase boundary (boundary line) (in petrography /pi-trog-ră-fee/ The study geophysics) The line along which two con- of the mineralogical and textural relation- stituents meet where only two are present. ships within rocks revealed by observation In a more complex system it is the line of thin sections and hand specimens. along which any two liquid phases meet. petroleum /pĕ-troh-lee-ŭm/ A naturally phenakite /fen-ă-kÿt/ (phenacite)A occurring complex mixture of flammable white or colorless quartzlike mineral form hydrocarbons, known also as oil or crude of beryllium silicate, Be2SiO4. It crystallizes oil. It is a green to black liquid, often of in the hexagonal system, and occurs in

262 photocontour map veins in granite. When cut it has a brilliant feldspar, nepheline, and aegirine–augite. luster and it is used as a semiprecious gem- The typically aphanitic groundmass has a stone. flinty appearance and the rocks ring when struck with a hammer and fracture sub- phenoclast /fen-ŏ-klast/ A large frag- conchoidally. Trachytes containing acces- ment or clast set in a finer-grained matrix sory nepheline up to 10% are termed of a sedimentary rock. phonolitic trachytes. In many phonolites, analcite, sodalite, or leucite accompany phenocryst /fen-ŏ-krist/ A large gener- nepheline. With an increase in the content ally euhedral crystal contained in many ig- of leucite and the disappearance of neous rocks and set in a fine-grained nepheline, leucite-phonolites pass into matrix or groundmass. Such rocks are said leucitophyres. to be porphyritic. The phenocrysts are the Tinguaite is an obsolete term for first crystals to form as a melt cools and are phonolite when it occurs as a medium- thus minerals having the highest crystal- grained dike rock. lization temperatures. Phonolites often occur with strongly undersaturated basic lavas, nephelinites, phillipsite /fil-ips-ÿt/ A fibrous mineral and in association with nepheline-syenites form of a hydrated aluminosilicate of cal- in oceanic islands and continental rift envi- cium, sodium, and potassium, (Ca,Na,K)3- ronments. (Al3Si5O16).6H2O. It crystallizes in the orthorhombic system, and is one of the ZE- phosphate /fos-fayt/ A salt of phos- 3– OLITE group of minerals. phoric acid, containing the ion PO4 . Many minerals are composed of phos- phi scale A logarithmic scale of particle phates, although few are common, except or grain size in sediments or sedimentary for APATITE. rocks. The scale is a logarithmic-based modification of the UDDEN–WENTWORTH phosphatic nodule /fos-fat-ik/A SCALE. The conversion formula is used: rounded mass that occurs in sedimentary φ = –log2d rocks that have been laid down on the sea where φ is the size expressed in phi units floor. Up to 30 cm across, they consist of and d is the grain diameter (expressed in pieces of corals, mica flakes, sand grains, millimeters). The scale was devised by the and shell fragments. American geologist William Christian Krumbein. See also particle size. phosphorite /fos-fŏ-rÿt/ (phosphate rock) A calcic phosphate deposit that oc- phlogopite /flog-ŏ-pÿt/ A brownish curs as a marine deposit in roughly nodu- mineral form of potassium magnesium lar form and (according to sampling and aluminum silicate, with some iron, K2- photographs) sometimes in quite large con- (Mg,Fe)6(Si6Al2O20(OH,F)4. It crystallizes centrations. It is mainly used for the manu- in the monoclinic system, and occurs facture of fertilizers. It is found on some mainly in ultrabasic igneous rocks and shelf areas, and over parts of the continen- metamorphosed limestones. It is a member tal slope. It is currently dredged at a num- of the MICA group of minerals. ber of offshore sites, for example, off the E coast of the USA. phonolite /foh-nŏ-lÿt/ A strongly undersaturated lava, the volcanic equivalent of photochemical smog See smog. nepheline-syenite. Phonolites are leucocratic rocks containing a high proportion photocontour map /foh-toh-kon-toor/ of sanidine or anorthoclase feldspar and A topographic map produced by aerial nepheline. Mafic minerals include soda photography. All the detail normally ap- pyriboles and biotite. Most phonolites are parent on topographic maps is shown, the porphyritic and possess phenocrysts of information being extracted from the pho-

263 photogrammetry tographs using stereoscopic equipment. phreatic activity /free-at-ik/ The violent This technique has been extensively used reaction that results from hot lava coming for 1:50 000 series mapping of level under- into contact with cold water. The surface developed areas of the world, e.g. part of of the lava chills and forms a glassy skin. Arabia and Botswana, especially if a fairly The pressure of gasses such as water vapor rapid result is required. present within the lava then cause this surface to fracture. Small fragments are hurled photogrammetry /foh-tŏ-gram-ĕ-tree/ into the air, the reaction continuing until a The technique of producing maps and tuff ring has built up, which separates the charts using stereoscopic equipment to ob- lava and water. tain reliable measurements from aerial photography. See photomap. phreatic eruption A volcanic eruption caused by escaping steam generated when photomap /foh-toh-map/ A map pre- a lava flow comes into contact with pared by adding grid information, names, groundwater. boundaries, and other map data to a re- production of photographs or photomo- phreatic water GROUNDWATER, espe- saics. It is a quick method of producing cially that occurring below the water table, maps, and is often used in areas of little re- i.e. in the ZONE OF SATURATION (phreatic lief, e.g. deserts, or for towns. Contours zone) where all fissures and voids are filled can be added to these maps for use in areas with groundwater. Phreatic water may of relief difference, although this greatly supply springs and wells. Compare vadose water. lengthens the time taken to produce the map. phyllite /fil-ÿt/ A metamorphosed rock resembling slate but of a coarser grain size. photoperiodism /foh-toh-peer-ee-ŏ-diz- The cleavage or schistosity surfaces have a ăm/ The response of plants to variations lustrous sheen caused by muscovite and in day length. Some plants flower only chlorite. Phyllites may possess incipient when the day length (photoperiod) exceeds banding due to the segregation of quartz a specific time; others flower only in pho- and feldspar into layers parallel to the toperiods of less than ten hours. This is cleavage. The characteristics of phyllites obviously important in the natural dis- are essentially intermediate between those tribution of plants. of slates and schists. photosynthesis /foh-tŏ-sin-th’ĕ-sis/ The phylum /fÿ-lŭm/ (pl. phyla) A major chemical processes by which green plants, group in the taxonomic classification of or- algae, and other chlorophyll-containing organisms. Animal phyla include the Mol- ganisms use the energy of sunlight to make lusca, Cnidaria, and Chordata. Each complex organic compounds from carbon phylum is composed of one or more dioxide (from the atmosphere) and water. CLASSES. In traditional plant classification The main reaction is catalyzed by chloro- systems phyla are known as divisions. See phyll, and oxygen is released into the at- taxonomy. mosphere as a by-product. It is a vital process because nearly all plants and ani- physical geography The branch of ge- mals rely either directly or indirectly on it ography that includes certain aspects of for their existence. geomorphology, hydrology, climatology, meteorology, oceanography, and pedol- phototroph /foh-tŏ-troff/ A living or- ogy. ganism that obtains energy from sunlight, such as all green plants. See photosynthe- physical weathering See mechanical sis. weathering.

264 pingo phytoplankton /fÿ-toh-plank-tŏn/ See is almost definitely produced by different plankton. mechanisms in different areas. See also pediment. picotite /pic-ŏ-tÿt/ A dark brown variety of SPINEL that is rich in chromium. piedmont glacier A large fan-like lobe of ice spreading out over lower relief picrite /pik-rÿt/ An ultramafic rock con- formed when a valley glacier extends be- sisting of FERROMAGNESIAN MINERALS and yond the valley walls that have contained accessory calcic plagioclase feldspar. and confined it. It is associated with the terminus or snout of the glacier. The piedmont /peed-mont/ A gentle slope Malaspina Glacier in SE Alaska, consid- leading from the foot of a mountain range ered the classic example of a piedmont down to comparatively flat land. In arid glacier, is formed from the merger of sev- and semiarid areas, where these features eral valley glaciers and is also known as a most frequently occur, they consist of an compound glacier. eroded upper segment (known as a PEDI- MENT), which makes an abrupt angle with piemontite /peed-mont-ÿt/ A member of the mountain front, followed by an accu- the EPIDOTE group of minerals. mulation form or BAJADA, consisting of transported debris from the mountains. pigeonite /pij-ŏn-ÿt/ A calcium-poor This merges into a flat INLAND BASIN or monoclinic PYROXENE. playa. pillow lava A lava extruded under piedmont angle The sharp inflexion be- water and having the appearance of pil- tween lowlands and uplands in pediment lows piled one upon another. The outer and inselberg landscapes. Characteristi- skin of the lava is chilled on extrusion and cally, the scarp of the upland slopes at a bubble of lava grows, flattening under its 25–35°, passing to the slope of the pedi- own weight and producing the characteris- ment at 9° or less over a very short dis- tic pillow shape. tance, leaving an abrupt angle. This angle can be very sharply defined, but usually it pilotaxitic /pil-ŏ-taks-it-ik/ Describing a is a concavity, with the break of slope close-packed felted arrangement of acicu- spread over a considerable horizontal dis- lar MICROLITES. In many trachytes and an- tance. desites, feldspar laths exhibit parallelism Its origin is closely linked with the due to flow and are deflected around phe- processes of PARALLEL RETREAT of the up- nocrysts in the direction of flow, this tex- land and beveling of the pediment. Some ture being called trachytic. geologists have found that the angle is maintained by intense weathering in the pinch and swell A deformation feature scarp foot zone, caused by runoff from the developed when competent rocks are pediment and through flow emerging at squeezed. The more competent beds ex- the angle; in other cases it coincides with a tend and thin toward their margins and tectonic or geologic boundary. Others con- eventually break. The less competent beds sider it to be a product of contrasted deform plastically to fill any space. See processes, e.g. unconfined wash on the boudinage. residual with channeled water on the pediment (Bryan), or turbulent wash on the pingo /ping-goh/ A dome-shaped hill residual with laminar wash on the pedi- found in PERMAFROST areas. Pingos vary in ment (King); still others have explained it size but seldom exceed 60 m in height and by slope process, the increased volume of 300 m in diameter. The top may be broken, water in the scarp foot area undercutting so that it resembles a crater, and often con- the slope of the residual, leading to mass tains a lake. Internally they frequently con- movements which keep the angle sharp. It sist of outward dipping beds of stratified

265 pipkrake sand or silt. Many have a visible or sup- members, albite (NaAlSi3O8) and anorthite posed core of ice. They are believed to have (CaAl2Si2O8). See feldspar. been formed as a result of bulging, produced by subsurface pressure build-up plain An extensive region of low-lying within isolated groundwaters before the land, which is generally flat or gently un- complete extension of permafrost over the dulating. Most plains are formed by depo- area. sition of eroded sediments; others are created by the wearing away of higher land pipkrake /pip-krayk/ See needle ice. (DENUDATION), forming a peneplain. pisolith /pis-ŏ-lith/ A pea-sized accretion planation surface /plă-nay-shŏn/ A sur- that occurs in some sedimentary rocks face of low relief, the end product of a cycle (such as pisolite). It consists of concentric of subaerial or marine erosion. Character- layers of calcium carbonate, possibly re- istically such surfaces bevel indiscrimi- sulting from the biochemical encrustation nately across structures; they may be of algae. See also oolite. PEDIPLAINS, PENEPLAINS, or surfaces of marine erosion. In order to develop, a plana- pitch The direction of dip of the axis of tion surface needs a long period of a fold, measured by the angle between the base-level and climatic stability: such con- axis and the horizontal on the axial plane. ditions have not existed since the end of the Neogene Period, because of the oscillations pich-blend pitchblende / / A massive in the base level and climate of the Pleis- form of URANINITE. tocene and the post-Pleistocene. Those planation surfaces that do exist therefore are pitchstone See rhyolite. relic and much dissected. They have been reconstructed by analysis of relief, which pivot fault See hinge fault. shows in general concordance of summit placentals /plă-sen-tălz/ Placental mam- levels at the level of former planation sur- mals (see Eutheria). faces. placer A surface deposit of sand or plane tabling A rapid surveying method gravel that contains significant quantities for fixing detail in fairly open areas. The of valuable minerals, such as chromite, di- plane table is a flat board with an attached amonds, gold, platinum, or tin. Small sheet of paper onto which a baseline is amounts of the minerals can be removed by drawn to scale between two known points. panning; large-scale extraction usually in- The whole stands on a tripod, which is set volves dredging and concentration of the up over one end of the baseline, with the mineral by various processes. drawn line aligned with the actual line. Points of detail are then sighted using an Placodermi /plak-ŏ-der-mÿ/ A class of ALIDADE and rays to them drawn directly extinct jawed fish comprising a number of onto the paper. Once enough points have primitive but varied groups common in the been covered, the plane table is set up at Devonian. They had paired fins and, in the the other end of the baseline, aligned, and arthrodires and antiarchs, an armor of pro- rays drawn to the same detail points. Their tective bony plates. Some placoderms were actual positions on the map occur at the of great size, reaching a length of up to points of ray intersection on the paper. 10 m. More accurate fixing can be achieved by drawing rays from three known points. plagioclase feldspar /play-jee-ŏ-klayss/ Any member of a series of minerals with planetary boundary-layer See bound- compositions varying between two end- ary layer.

266 plastic relief map planetary wind Any of the world’s plant cover The vegetation of a particu- major winds, affecting large areas of the lar area. The geomorphological signifi- globe. They include the TRADE WINDS of the cance of the plant cover is as a balance tropics, the south-westerlies of northern between the soil and weathered rock be- temperate regions, and the north-westerlies neath and the processes acting on the sur- or ROARING FORTIES in the S hemisphere. face from above. Its loss greatly accelerates rates of erosion, e.g. in areas of cultivation, planimetric map /plan-ă-met-rik/A overgrazing, human trampling, or destruc- map on which, unlike topographic maps, tion of vegetation by burning or pollution. no vertical information such as contouring The beneficial effects of vegetation include is shown. the binding action of the roots, the divert- ing action of its mass on water flowing on plankton /plank-tŏn/ Aquatic and usu- the surface, shelter from wind action, and ally microscopic organisms that float and the increase in organic matter on the soil drift passively in the sea and other bodies surface, which improves aggregation and of water (compare nekton). Plankton con- hence the ability of the soil to hold water, sists mainly of animal larvae, PROTOZOA, thereby reducing runoff. Vegetation can and algae, such as DIATOMS. Phytoplankton hold soil on a slope that would otherwise refers to plant and algal forms, on which be too steep for stability and hence be all other marine organisms depend, di- eroded; it also intercepts rainfall, prevent- rectly or indirectly, for their survival; zoo- ing RAINSPLASH erosion. In ecological plankton refers to the immense variety of terms, plant cover provides HABITATS for animal forms. The distribution of both various organisms. phytoplankton and zooplankton throughout the oceans is very patchy; plankton- plastic deformation of ice A process rich areas form highly fertile zones in the operating in glacier flow. Glaciers do not oceans, for example where UPWELLING cur- move down their valleys as rigid masses rents occur. simply sliding across the bedrock. Ice is a Plankton is of considerable geologic im- crystalline solid and as such changes shape portance as it contributes to rock-building; (or deforms) at temperatures near its melt- for example, much of the chalk (see Creta- ing point. Movements between adjacent ceous) consists of COCCOLITHS, the skeletal remains of past planktonic organisms. Fos- crystals are limited but individual crystals sil plankton is studied in micropaleontol- can deform internally, although little ogy (see microfossil), which has important change in shape appears to occur over a pe- stratigraphical applications. riod of time: this indicates that recrystallization accompanies the deformation. planosol /plan-ŏ-sôl/ An INTRAZONAL Individual crystal deformation is achieved SOIL developed in flat areas, principally by relative movements of layers, one above under humid continental warm summer another, parallel to the basal plane. Field climates. Planosols are characterized by a evidence has shown that ice is molded as it moves across its bed, but that resultant fea- whitened A2 horizon passing abruptly to a B horizon of high clay content, either due tures are maintained within the ice beyond to in-situ weathering or LESSIVAGE from the the location of molding. This suggests that A horizon. Periodic waterlogging results the ice acts plastically under pressure, but from the poor drainage due to the high clay that it remains rigid on removal of the pres- content in the B horizon and lack of relief. sure. They intergrade with a number of soils, including GRAY-BROWN PODZOLIC SOILS, plastic relief map A three-dimensional PRAIRIE SOILS, and surface-water GLEY SOILS. map produced by printing a topographic In the US SOIL TAXONOMY they are split be- map on plastic and then molding the plas- tween a number of orders, including mol- tic to fit the relief. It is generally used only lisols, alfisols, and ultisols. for demonstration purposes because there

267 plastic shading are still certain inaccuracies involved in terial and sediment deposition along their this method. margins. These sediments are compressed and folded when two continental regions plastic shading See hill shading. collide by orogenic processes, which results in the formation of fold mountain chains. plate See lithospheric plate. These have a central nucleus of older rocks, frequently cratons. plateau A fairly flat elevated area of land. A dissected plateau is broken up by playa /play-yă/ A flat-bottomed en- river valleys or canyons, an intermontane closed basin in a desert, sometimes occu- plateau is surrounded by mountains, and pied by an ephemeral playa lake. There some plateaus may incorporate the moun- may be deposits of EVAPORITES just below tains themselves. the surface, and similar deposits on the surface when any lake dries up. See also inland plateau basalt See flood basalt. basin. plate boundary (plate margin) The Pleistocene /plÿ-stŏ-seen/ The earlier LITHOSPHERIC PLATE edge of a (tectonic epoch of the QUATERNARY, extending from plate). There are three types of boundary: the end of the PLIOCENE about 1.6 million 1. Constructive, where new sea floor is years ago, until the beginning of the added to the plates on each side of the HOLOCENE Epoch. During this period, often boundary, e.g. at the Mid-Atlantic referred to as the ICE AGE, the world expe- Ridge. rienced great fluctuations in temperature, 2. Destructive, where sea floor is destroyed resulting in cold periods (GLACIALS), sepa- through subduction. This is usually rated by warmer periods (INTERGLACIALS). marked by a deep ocean TRENCH and re- The Earth’s climate cooled by about sults from one plate overriding another. 5–10°C during the glacials. In North See also subduction zone. America ice sheets developed in Canada 3. Conservative, where plates slip passively and advanced S into the N USA at least past each other without destroying or four times before retreating. In the Alps, adding sea floor. This occurs along four main glacial episodes can be recog- TRANSFORM FAULTS. nized: the Gunz, Mindel, Riss, and Würm. plate tectonics A theory arising from a series of ideas developed in the early 1960s pleochroism /plee-ŏ-kroh-iz-ăm/ A prop- proposing that the surface of the Earth is erty of some crystals that display different composed of a number of relatively thin colors when viewed from different direc- plates of rigid material. These LITHOS- tions under transmitted plane-polarized PHERIC (or tectonic) PLATES extend down to light. It is caused by selective absorption of the low-velocity zone of the upper mantle. some wavelengths along the different crys- They are all in motion relative to one an- tal axes. other and it is through these movements and the consequent collisions between the Plesiosauria /plee-see-ŏ-sor-ee-ă/ A group plates that the present distribution of al- of Mesozoic reptiles that were secondarily most all volcanic, seismic, and orogenic ac- adapted to an aquatic life but, unlike the tivity is controlled. The plates are bounded ICHTHYOSAURIA, were not fishlike. They by PLATE BOUNDARIES. The plates them- had a rigid flattened trunk and well-devel- selves are composed of either oceanic or oped powerful paddles for propulsion. A continental crustal types, or a combination long neck provided the necessary rapid of both. Only the oceanic parts of the flexibility for catching the fish on which plates grow or are destroyed; the conti- they fed. Some plesiosaurs attained a nents ride along on these plates and grow length of 17 m. The group became extinct only slowly by the addition of volcanic ma- by the beginning of the Cenozoic.

268 podzol

Plinian See volcano. ceding or succeeding periods, usually on a geologic timescale. Many of the semidesert Pliocene /plÿ-ŏ-seen/ The second epoch areas of the tropics experienced pluvials of the PALEOGENE Period, preceded by the during full glaciation in polar regions. This MIOCENE and followed by the PLEISTOCENE was a response to the southward move- Epoch. It began about 5.3 million years ment of the circulation belts and depres- ago and lasted some 3.7 million years. sion tracks. Evidence for these wetter Mammals similar to modern forms existed, periods comes from greater water erosion, and there were definite hominids, includ- increased plant growth, and animal re- ing species of Australopithecus and Homo mains found in areas in which they would (see man). not be found under present conditions. However, this has been disputed because plucking See glacial plucking. lower temperatures could have reduced evaporation, making the same rainfall plug 1. Any roughly cylindrical vertical amounts more effective. body of intrusive igneous rock, usually relatively small. pneumatolysis /new-mă-tol-ă-sis/ A pro- 2. See neck. cess occurring during the final stages of the crystallization of acid igneous rocks when plumbago /plum-bay-goh/ See graphite. residual borofluoric gas escapes along joints and fissures and brings about miner- plunge The tilt of the axis of a fold from alogical changes in the crystalline parent the horizontal, the value of plunge being rock (compare metasomatism). Three the angle between the axis and the hori- kinds of pneumatolysis are recognized: 1. zontal lying in the same vertical plane. See Greisening. Alteration at the margins of diagram at FOLD. granites to assemblages of muscovite, quartz, and topaz (greisen) is the result of plunging breaker See breaker. the action of fluorine-bearing vapors. Original feldspars are pseudomorphed by pluton /ploo-tonn/ A deep-seated major aggregates of mica, often varieties rich in intrusive body of coarse-grained igneous lithium such as zinnwaldite. The end prod- rock, generally of granitic composition. uct of the process is a quartz-topaz rock known as topazfels. 2. Tourmalinization. plutonic rock /ploo-tonn-ik/ An igneous The boron pneumatolysis of granitic rocks rock that crystallizes at depth and cools results in the growth of TOURMALINE at the slowly, resulting in a coarse grain size. expense of feldspar and biotite. The rock, Granite is an example. Igneous rocks of the luxullianite, contains radiating aggregates same composition may show striking tex- of black tourmaline but some feldspar sur- tural differences depending on their mode vives. The final stage is a quartz-tourma- of occurrence. Those that are extruded on line assemblage known as schorl rock. 3. the Earth’s surface as lava flows and cool Kaolinization. Feldspars in granitic rocks rapidly, producing a fine grain size, are may be altered to aggregates of kaolinite called VOLCANIC ROCKS. Hypabyssal rocks and sericite under the action of high-tem- are those that have crystallized at levels in- perature aqueous fluids. The end product termediate between those of plutonic and of this pneumatolytic/hydrothermal pro- volcanic conditions. They are medium- cess is KAOLIN. grained and occur mostly as sills and dikes. Gabbro and dolerite are, respectively, the podzol /pod-zol/ (podsol) A soil charac- plutonic and hypabyssal equivalents of terized by an ashen-colored acid eluviated basalt. A horizon and a B horizon illuviated with iron or humus, possibly in the form of a pluvial period /ploo-vee-ăl/ A period of compact pan. The A horizon is depleted of time experiencing greater rainfall than pre- bases and sesquioxides by LEACHING

269 podzolic soil processes, especially CHELUVIATION, while can reduce the sesquioxides themselves, the B horizon is divided into an upper part and do not rely on the anaerobic reduction (Bh) dominated by the redeposited humus of iron from its insoluble ferric to its solu- and a lower part (Bs or Bfe) dominated by ble ferrous state. Podzolization is a zonal the redeposited iron, and to a lesser extent, process in areas of natural LEACHING and aluminum. If both the Bh and Bs horizons coniferous vegetation, which together are present, the soil is an iron-humus pod- favor intense cheluviation, the chelates of zol; if the Bh only is present, a humus pod- pine litter being among the most powerful. zol; if Bs only, an iron podzol. Intrazonally, it is the dominant process on Podzols are zonally developed soils in coarse-textured parent materials, or where the TAIGA zones of Russia and North Amer- forest clearance has increased leaching. ica, where they form a mosaic with regosols, gleys, and peats, and develop from poikilitic /poi-kă-lit-ik/ Describing ig- initiation directly on the parent material. neous rocks in which large crystals of a Elsewhere podzol is intrazonal, e.g. on mineral completely enclose crystals of ear- coarse deposits such as sands and gravels lier-formed minerals. Poikiloblastic de- or human-induced by forest clearance. As notes a similar texture in metamorphic the profile matures, the Bh-s horizons may rocks. See also ophitic. become impermeable and lead to GLEYING producing hydromorphic variants such as poikiloblastic /poi-kă-lŏ-blas-tik/ See PEATY GLEY PODZOLS. poikilitic. podzolic soil /pod-zol-ik/ (podsolic soil) point bar A depositional feature that A soil with certain features of morphology develops on the inside of meanders, com- and genesis akin to true podzols. There is a plementing the erosion that occurs on the large number of varieties: in North Amer- outside of meander bends. Point bars ex- ica there are gray-brown podzolics in a belt tend downstream from the point of maxi- below the true podzols, which develop mum curvature of the meander bend, slowly by increased leaching, destruction leaving a trough between themselves and of the Bt horizon, and graying of the A2 the bank, which eventually becomes filled horizon to brown podzolics. In the south, up by the sedimentation of fine material. in the subtropical areas, are red-yellow See meander. podzolics, which have free aluminum and ferric oxides like other tropical soils but Poisson’s ratio /pwah-sawnz/ The ratio also clay skins in the B horizon. between the fractional longitudinal strain and the fractional lateral strain in a de- podzolization /pod-zŏ-lă-zay-shŏn/ (pod- formed material. It is equal to the ratio of solization) The movement of clays and change in diameter divided by change in sesquioxides down the soil profile and length. It is named for the French mathe- their deposition in the B horizon. The dom- matician and physicist Siméon Denis Pois- inant process is CHELUVIATION by organic son (1781–1840). acids, but opinions vary as to the other processes involved. Some pedologists be- polar Describing the climate that is lieve that podzolization operates best in characteristic of the regions within the Arc- acidic and anaerobic conditions, so that tic and Antarctic Circles (around the North the cheluviation is preceded by DECALCIFI- and South Poles). They have permanently CATION, LESSIVAGE, and clay destruction by low temperatures, a short growing season, weathering. Others have suggested that and no trees. acidic conditions are not necessary, and decalcification is therefore not a necessary polar air depression See polar low. preliminary; it has also been argued that anaerobic conditions (i.e. waterlogging) polar easterlies The easterly wind belt are not necessary because the organic acids between the weak polar anticyclone and

270 polar zenithal gnomonic projection the westerly depression tracks. Because of strong near-gale or gale force winds and the variability of the intensity and tracks of heavy showers of precipitation, much as the mid-latitude depressions and the weak- snow, but is short-lived and rapidly dissi- ness of the polar anticyclone, the polar pates over land. Intense polar lows are easterlies are rarely strong or persistent. sometimes known as Arctic hurricanes and the cloud pattern in satellite images has a polar front The major frontal system, similar appearance to that of a tropical cy- situated in the N Pacific and N Atlantic, clone. separating tropical and polar air masses. Depressions are often initiated on this pro- polar night jet stream A very steep nounced thermal gradient and the Norwe- thermal gradient in winter giving corre- gian meteorologist Vilhelm Frimann Koren spondingly very strong westerly winds, sit- Bjerknes (1862–1951) based his theory of uated around the stratospheric cold pole at frontal evolution on it. In summer the ther- levels of 20–30 km. During summer the mal contrast across the front is less and its stratosphere at this level warms up because position is much more variable than in of absorption of ultraviolet radiation, and winter. a reversal of the wind system takes place to give stratospheric easterlies. polar-front jet stream See jet stream. polar wandering curve A theoretical polar glacier See cold glacier. line produced for a particular point on the Earth’s surface by joining successive paleo- polar high (polar anticyclone) An ex- magnetic pole positions through time. tensive area of semi-permanent high pressure in the polar latitudes. The anticyclone over Antarctica (the Antarctic high) is projected view semi-permanent whereas that over the Arc- on tangent plane tic (the Arctic high) is more seasonal. The 0° 30° 30° polar high is a source of very cold and dry air. 60° 60° polarity reversal See magnetic reversal. 90° 90° 60° polarization colors See birefringence. 45° polarized light In ordinary light the 15° electromagnetic vibrations take place in all Equator directions in a plane at right angles to the direction of propagation of the ray of light. When vibrations are confined to a single direction in this plane, the light is said to be plane-polarized. Certain crystals constrain ordinary light to vibrate in only two direc- Polar zenithal tions at right angles to one another; i.e. they polarize it. See anisotropic; birefrin- polar zenithal gnomonic projection A gence. MAP PROJECTION in which the center of the projection is the pole, from which the polar low (polar air depression) A small straight lines representing the great circles shadow depression or cyclone that forms radiate (see diagram). The parallels are over the ocean, poleward of the polar front portrayed as concentric circles. It is com- (or other major baroclinic zone), and ex- monly used to show the polar areas, the extends horizontally for several hundred aggeration becoming too great in areas kilometers. It brings severe weather with farther away from the center.

271 polder polder A low-lying tract of land that has to the present-day vegetation in different been reclaimed from a body of water such conditions allows deduction, by analogy, as sea or lake and is enclosed by embank- of the environment that the vegetation ments or dikes. Polders are frequently lived in. This method has shown well-de- below the level of the sea or lake from fined sequences of changes for each of the which they have been reclaimed. They are interglacials and the postglacial period, especially common in the Netherlands on which allowed relative correlation between the North Sea coast but also occur else- different sites, and hence dating. where. pollution dome An accumulation of pole of rotation (Euler pole) A pole polluted air in a low dome-shaped layer about which a lithospheric plate rotates over a large urban or industrial area. The during sea-floor spreading and continental pollutants are trapped in the warm air drift. below a temperature inversion (see inversion layer) when conditions are calm or polje /pol-yay/ The largest type of solu- winds are light and there is persistent high tion depression found in limestone areas, pressure. See also heat island. extending up to 250 sq km. The floors support a certain amount of surface drainage, pollution plume 1. A trail of polluted are alluvium-covered, and often uneven, air that extends downstream from a pollu- reflecting the collapse of former cave sys- tion source, such as a large urban or indus- tems. Poljes are frequently surrounded by trial area. steep marginal walls, up to 100 m in 2. In a stream or in groundwater, an area height, while considerable numbers show of pollutants that moves along the flow signs of being tectonically bounded basins. from the point of origin. pollen Microscopic spores of the higher polyconic map projection /pol-ee-kon- plants (gymnosperms and angiosperms). ik/ A modified CONICAL PROJECTION in Because pollen is very resistant to destruc- which each parallel is treated as a standard tion and may be carried for large distances parallel on a normal conical projection. All by the wind, fossilized pollen provides a the parallels and the central meridian are valuable means of correlating the rocks in truly divided, the meridians being con- which it occurs. Pollen may also be used as structed by joining the divisions on the par- a sensitive environmental indicator and has allels. This projection is neither an proved particularly useful in monitoring ORTHOMORPHIC PROJECTION nor a HO- climatic change during the Quaternary. MOLOGRAPHIC PROJECTION and is therefore The study of pollen and other spores is restricted to showing small areas. See also known as PALYNOLOGY. map projection. pollen analysis A technique of both rel- polycyclic landscape /pol-ee-sÿ-klik/A ative dating and environmental reconstruc- landscape that has evolved under a number tion, consisting of the identification and of geomorphic cycles (see cycle of erosion) counting of pollen types preserved in peats or part cycles of different types. The inde- and lake beds. It is based on the assump- pendent variables of geomorphology are tion that the vegetation in an environment climate, geology, and base level: a change is in equilibrium with it, and therefore re- in any one starts a new cycle. Regional flects its conditions; dispersal of pollen studies of landscape evolution emphasize leads to some of it being preserved in re- that most if not all landscapes have experi- ducing environments, such as peat beds enced changes in these factors and have and lake bottoms. Subsequent careful close hence evolved under several different cy- sampling and analysis allows the structure cles. This realization has led to the aban- of the vegetation at each particular horizon donment of the youthful, mature, and in the deposit to be worked out. Reference senile labels for landscapes, because they

272 porphyroblastic assume that each landscape is a product of pools and riffles /pol-ee-fayz/ The alter- only the current cycle, variation being due nating gravel bars (riffles) and pools that to the STAGE of that cycle. Instead each occur along a stream: these features are landscape can be seen as composed of ele- regularly spaced, usually such that the dis- ments from several different cycles, only tance between successive pools is 5–7 times some of which are related to current condi- the channel width. In meandering streams, tions. lengths of meanders are about twice the distance between pools, and there may be a polygenetic /pol-ee-gĕ-net-ik/ (in geo- relationship between the pool and riffle morphology) Describing a soil or land- formations and MEANDER creation. scape that has evolved under a number of differing conditions that occurred in suc- Porifera /pŏ-riff-ĕ-ră/ The sponges: a cessive phases of its history. Each of these phylum of simple sessile aquatic (mostly conditions has played a part in influencing marine) multicellular animals with a its current appearance. With landscapes, a saclike body, often containing small cal- change in base level, climate, or geology careous or siliceous skeletal elements will institute a new cycle and hence super- known as spicules. Water is drawn into the imposition of varying conditions in its evo- body through small holes and expelled lution; with soil, a change in climate, from an opening, the oscula, during which vegetation, topography, or drainage will food particles are filtered off. The spicules institute a new set of soil-forming condi- may become fossilized. Fossil sponges are tions and hence change the direction of the useful environmental indicators and have soil’s evolution. Some authorities consider contributed to the growth of bioherms and this term to be synonymous with POLY- formation of rocks. They are known to CYCLIC when applied to landscapes; others have extended from the Cambrian Period consider that polygenetic should be used to to the present day, and structures found in describe only the influence of minor Precambrian rocks have been attributed to changes in conditions, leaving polycyclic this group. for the major changes. porosity The extent to which a body of polymorphic transition /pol-ee-mor-fik/ soil, rock, or sediment is permeated with A change in minerals that involves a cavities between grains, usually expressed change in their atomic structure but not as a percentage of the volume. These pores their chemical composition, e.g. aragonite are filled by air and water (air is mainly in (CaCO3) to calcite (also CaCO3). the larger macropores and water in the mi- cropores), which impede air movement polymorphism /pol-ee-mor-fiz-ăm/ The and allow water to move by CAPILLARITY ability of some substances to exist in two only. In sands and compact soils, where the or more structurally distinct forms, each particles lie close together, porosity is low having its own characteristic properties but (25–30% in compacted soils); conversely, having identical chemical compositions. where organic matter content is high, pro- Calcium carbonate, CaCO3 occurs as arag- moting good aggregation, porosity is high, onite and calcite. Andalusite, kyanite, and reaching possibly 60%. Cropping, which sillimanite are polymorphs of the alu- removes organic matter, eventually lowers minum silicate, Al2SiO5. Graphite and dia- porosity, and hence aeration. mond are mineral polymorphs of the element carbon. porphyritic /por-fă-rit-ik/ Describing an igneous rock that possesses large crystals polypedon See pedon. called PHENOCRYSTS set in a finer-grained groundmass. polyphase deformation Folding and faulting occurring in several periods during porphyroblastic /por-fă-roh-blas-tik/ De- a single orogeny. scribing a metamorphosed rock containing

273 porphyry large crystals set in a fine-grained matrix. of deformation (usually metamorphic min- The large crystals are called porphyrob- erals). lasts by analogy with the corresponding igneous term, PORPHYRITIC. potassium feldspar A type of alkali FELDSPAR that contains potassium alumi- porphyry /por-fă-ree/ A medium- or nosilicate, KAlSi3O8. The two principal fine-grained igneous rock containing nu- crystal forms are the monoclinic ORTHO- merous PHENOCRYSTS. The porphyritic CLASE and triclinic MICROCLINE. Adularia mineral may be indicated by the prefix of a and sandine are also alkaline feldspars. mineral name, e.g. quartz-porphyry. A rhomb-porphyry is so called on account of potential energy The energy possessed the distinctive shape of the feldspar phe- by a body as a result of its position. It is nocrysts. measured by the amount of work required to move that body from a position of zero positive area A large area of the Earth’s potential energy, usually sea level, to its crust that has remained above sea level for new position. Stationary air at high levels a long period of geologic time. of the troposphere therefore has a high potential energy. positive gravity anomaly See gravity anomaly. potential evapotranspiration (PE) The evaporation from an extended surface of a positive movement of sea level A fall short green crop, actively growing, com- of the land relative to the sea, usually due pletely shading the ground, of uniform to sea-level rise, as has happened since the height, and possessing an adequate supply of soil moisture. It is essentially a function end of the last ice age. The results are basi- of climate rather than the nature of the veg- cally twofold: flooding, and deposition of etation, and as such was used by Thornth- material creating broad low plains. waite as a method of climatic classification Flooding produces estuaries or rias (see Thornthwaite classification). It can be from inundated river valleys; estuaries estimated or measured more easily and ac- characterize lower-lying areas (e.g. the curately than ACTUAL EVAPOTRANSPIRATION Chesapeake and Delaware bays on the At- and so has achieved more universal accept- lantic Coast of the USA), rias the more ability in studies of evaporation. rugged areas (e.g. in SW Britain, SW Ire- land). If the valleys inundated are glacial in potential temperature The tempera- origin, fiords result (e.g. in Norway). Low- ture of a parcel of air if it is uplifted or sub- lying parts of the land may be flooded sides at the DRY ADIABATIC LAPSE RATE to a wholesale, creating new seas and severing standard pressure, usually 1000 mb. Use is islands from the mainland (the British Isles also made of the wet-bulb potential tem- were severed from the rest of Europe by the perature by changing the temperature of postglacial sea-level rise). the parcel of air at the SATURATED ADIA- The effect on rivers is that CAPACITY and BATIC LAPSE RATE from the wet-bulb tem- COMPETENCE are reduced, leading to perature of the original level to 1000 mb. AGGRADATION in the lower reaches, which This is useful because the wet-bulb poten- fills up the newly created estuaries and cre- tial temperature is a conservative property ates broad alluvial floodplains. The lengths in such processes as evaporation and con- of drainage systems are reduced, leaving densation, and both dry and saturated adi- buried portions beneath the elevated sea abatic temperature changes. level. See also negative movement of sea level. pothole /pot-hohl/ 1. A circular hole worn in rock in an eddy of a stream or postkinematic /pohst-kin-ĕ-mat-ik/De- river, often at the foot of a waterfall, by scribing minerals developed after a period moving pebbles and gravel.

274 precipitable water

2. See sinkhole. most of geologic time. Precambrian rocks contain rare FOSSIL evidence and, be- prairie A region of flat treeless temper- cause of their extreme age, have often been ate grassland of Canada and the USA, ex- subjected to a great deal of subsequent al- tending from Alberta to Texas. The teration. The original stratigraphical rela- prairies have summer rain and fairly dry tionships are therefore frequently obscure. winters. Once home to large herds of The SHIELD areas represent the largest areas bison, cattle, and sheep, much of the east- of exposed Precambrian rocks. ern part has been plowed for growing ce- The Precambrian is often now divided real crops, particularly wheat. Repeated into three eons. The earliest time span is plowing in the drier western prairies com- the Hadean (‘beneath the Earth’), from bined with drought conditions resulted in which time virtually no rocks or fossils re- erosion and the formation of a DUST BOWL main. During the Archean eon (about 3.8 during the 1930s. to 2.5 billion years ago) evidence for the prairie soil (brunizem) Dark soil of sub- earliest known life forms (early bacteria) humid grasslands in which the profile has appears in rocks (see stromatolite). Rocks been decarbonated but leaching has not of the Proterozoic eon contain fossil evi- produced any movement of sesquioxides. dence of the first multicelled organisms, in- The carbonate appears as flecks in the C cluding jellyfish and soft marine worms. horizon. These soils develop under tall Calibration and correlation of isolated suc- grass vegetation and have a brown or gray- cessions is achieved mainly by radiometric brown A horizon grading to a yellow- dating. brown B horizon, with blocky structures and a light parent material at around one precession /pree-sesh-ŏn/ A form of mometer. Their exact status is not certain. tion of a rotating body that results when a They may be degraded CHERNOZEMS or couple, having its axis at right angles to the more related to the podzolic group, with axis of rotation, is applied to the body so which they intergrade at their boundary, or that it then turns about a third mutually their distinctive profile may be caused by a perpendicular axis. As a result of the grav- water table standing in the profile produc- itational attraction of the Sun and Moon, ing GLEYING. In the US SOIL TAXONOMY they the Earth’s axis precesses and traces out a fall in the order of MOLLISOLS. conical figure in space.

Pratt’s hypothesis A proposed mecha- precipitable water /pri-sip-ă-tă-băl/ The nism of hydrostatic support for the Earth’s amount of water that could be obtained if crust (named for the British physicist John all the vapor in a standard column of air Henry Pratt (1809–71). This hypothesis re- was condensed onto a horizontal surface of lies upon crustal density being greater unit area. It is a useful index of the mois- under mountain chains than under oceans. ture content of air above a specified point, If this were true at a datum level, called the although precipitation processes are never COMPENSATION LEVEL by Pratt, rock columns of equal diameter would have so efficient that all the water would be pre- equal mass. cipitated and it neglects the effects of vapor advection. It is calculated by the formula: p1 Precambrian /pree-kam-bree-ăn/ The Mw = (1/g)∫ rdp geologic time prior to the Cambrian Period p2 or the rocks stratigraphically below the where Mw is the precipitable water, g is the Cambrian System. Because the age of the acceleration of free fall, p1 and p2 are the Earth’s crust has been estimated at about pressures (mb) at the top and bottom of the 4600 million years, and the beginning of layer, r is the mixing ratio, and dp is the the Cambrian is dated around 570 million depth of the individual layer of mixing years ago, the Precambrian represents ratio.

275 precipitation precipitation /pri-sip-ă-tay-shŏn/ The predator An animal that kills other ani- deposition of water in solid or liquid form mals (the prey) for food. It is a secondary from the atmosphere. It therefore covers a (and sometimes tertiary) CONSUMER in a wide range of particles including rain, driz- FOOD CHAIN. zle, sleet, snow, hail, and dew. Precipita- tion is initiated within clouds by the prehnite /pray-nÿt, pren-ÿt/ An ortho- Bergeron–Findeisen process (see Berg- rhombic mineral of composition Ca2A12- eron–Findeisen theory), by the collision– Si3O10(OH)2 found chiefly in basic lavas as coalescence process, or by a combination a secondary mineral associated with ZEO- of both operating together. For precipita- LITES. It is also found with pumpellyite in tion to reach the ground a number of other low-grade regionally metamorphosed conditions must be satisfied. The droplet rocks. or ice crystal produced by one of the above mechanisms must be sufficiently heavy to pre-kinematic /pree=kin-ĕ-mat-ik/De- overcome upward vertical motion and scribing minerals that are formed before large enough to withstand evaporation be- deformation. neath the cloud base. Thus, some clouds can be seen to be giving precipitation that pressure (in meteorology) The weight of does not reach the ground. air vertically above a unit area centered on a point. It is measured by balancing the precipitation effectiveness A measure force exerted by the atmosphere with a of the usefulness of the annual rainfall total dense liquid, usually mercury. As a result for agricultural purposes or for hydrology. of this, atmospheric pressure was formerly For example, 600 mm of rain in temperate quoted as a number of millimeters of mer- latitudes with low evaporation maintains cury representing the length of the mercury humid vegetation and surplus runoff for column required to balance air pressure. It river flow. In the tropics, such an annual is now measured in MILLIBARS. Sea-level total would give semidesert. Precipitation values range from extremes of about 890 effectiveness was used by Thornthwaite as mb in hurricanes to 1060 mb in strong an- a basis for climatic classification (see Thornthwaite classification). However, the ticyclones. sparsity of data of both measured and cal- pressure gradient force The force that culated POTENTIAL EVAPOTRANSPIRATION, from which precipitation effectiveness is acts on an air molecule by virtue of the spa- obtained, has prevented its use on a global tial variations in pressure at any horizontal scale. level. It is the primary motivating force of air movement in the atmosphere. precipitation variability The annual variation in rainfall totals, although pressure release See unloading. shorter time periods can be used. It is assessed by many statistical methods but the pressure system A pattern of isobars most common is the coefficient of varia- that exhibits coherent weather characteris- tion, which is the standard deviation of the tics. It normally denotes low-pressure areas annual totals divided by the mean annual or anticyclones. value, multiplied by 100 to be expressed as a percentage. In Britain, values range from pressure tendency The local rate of below 10% in the NW to 20% in S Eng- change of surface pressure. It is recorded at land, but these are low compared with most observing stations with a 10 point many other parts of the world. Malden Is- code to distinguish various trends of the land in the central Pacific is believed to pressure trace in the preceding time period, have the highest rainfall variability with usually 3 hours. It is very useful for indi- annual rainfall totals varying between 30 cating future movements of pressure sys- mm and 1422 mm in a thirty-year period. tems.

276 Proboscidea prevailing wind The most frequently large brain, and an unspecialized dentition. occurring wind at any site. In most parts of Fossil primates are known from the begin- the globe this will correspond with the cli- ning of the TERTIARY onward; they proba- matic zone. Occasionally local factors, bly evolved from the INSECTIVORA. such as wind funneling in a valley, become Monkeys appeared in the Oligocene and dominant. Also for shorter time periods, the remains of humanoid forms have been the prevailing wind may be different from found in Pliocene strata (see man). its annual value, as in monsoon areas. prime meridian See meridian. primary consumer (in ecology) An organism in the second level of a FOOD CHAIN, primitive equations The physical equa- usually a HERBIVORE. It provides food for tions governing momentum in the atmos- secondary consumers (carnivores). phere. They have been used most extensively in models simulating the gen- primary magma A magma whose com- eral circulation of the atmosphere. position has remained unchanged since it was first formed. principal shock The suite of waves produced by the main movement in an earth- primary mineral A mineral that was quake. See also aftershocks; foreshock; formed at the same time as the rock seismic wave. containing it and which has remained unchanged in composition. Compare sec- prismatic Describing the habit of some ondary mineral. crystals that are elongated and show well- developed prism faces. See crystal habit. primary producer (in ecology) An organism at the lowest level of a FOOD CHAIN. prismatic compass A magnetic com- It ‘feeds’ on inorganic substances. Green pass within which the needle, or the zero of plants and photosynthetic algae are exam- a 0° to 360° scale, always points toward ples; they produce carbohydrates from car- magnetic north. Also incorporated is a bon dioxide and water using the energy of sighting arrangement, consisting of a verti- sunlight in PHOTOSYNTHESIS. They can then cal hairline at the far side and a prism with become the food for consumers in the next a sight at the observer’s side, which allows level of the food chain. the object and the graduated scale to be viewed simultaneously. Readings taken primary wave (P wave) A type of SEIS- with such a compass are true bearings from MIC WAVE, the first to be recorded on a magnetic north. seismogram of an earthquake. These longitudinal waves travel by a series of com- Proboscidea /proh-bŏ-sid-ee-ă/ The order pressions and rarefactions (pushes and of mammals that includes the modern ele- pulls) in the direction of propagation, i.e. phants and the extinct mammoths. Fossil each particle vibrates backward and for- proboscideans are known from the ward along the direction in which the wave Oligocene. They were small creatures with is traveling. P waves can pass through long jaws, both upper and lower bearing solids, liquids, and gases. See also sec- tusks, and short trunks. Evolutionary mod- ondary wave. ifications associated with the subsequent increase in size have occurred: solid limbs Primates /prÿ-may-teez/ The order of and flat feet support the great weight; the mammals that includes man, apes, mon- trunk has evolved into an efficient food- keys, and lemurs. They are characterized gathering organ; the teeth have lengthened, by adaptations for an arboreal habitat, and the jaws have shortened, in modern such as flexible limbs and hands, anteriorly forms accommodating only one large directed eyes providing stereoscopic vision, grinding tooth at a time, which is replaced often a long tail for balance, a relatively through life. Formerly a widespread and

277 producer diverse group, the Proboscidea are today crest and the lowest point of the wave confined to Africa and S Asia. trough, although the direction of the current as between crest and trough is re- producer See primary producer. versed. profile 1. (soil profile) The arrangement projection (in cartography) See map of soil HORIZONS found between the projection. ground surface and the parent material. The normal depth of a profile in temperate promontory /prom-ŏn-tŏ-ree/ A small latitudes is about a meter. Pedologists, no- peninsula or headland that projects into tably Dokuchaev (see soil formation), have the sea. Wave action can cause erosion on assigned letters to certain horizons. The both sides, which are likely to have cliffs surface horizon is known by the letter A and possibly caves. and is usually rich in organic matter and plant nutrients. Because material is often Proterozoic /prot-ĕ-rŏ-zoh-ik/ See Pre- leached from this horizon down the profile cambrian. to the subsoil B horizon, it is known as the eluvial horizon (see eluviation). The B hori- proto-Atlantic Ocean (Iapetus Ocean) zon, known as the illuvial horizon (see illu- An ocean that existed between the late Pre- viation), contains less organic matter and cambrian and early Devonian times. more closely resembles the parent material Within this ocean, which reached its maxi- owing to the lesser effect of the soil-form- mum extent during the Ordovician, a series ing processes. This horizon usually merges of sediments were deposited on an ocean into the C horizon, which is the weathered floor of pillow lavas. When the two conti- parent material. Well-developed soils con- nents on each side of the ocean closed, sist of more than these three horizons and these sediments were subjected to orogenic subdivision occurs. Characteristics of cer- processes that resulted in the formation of tain horizons are symbolized by a suffix or the Caledonian mountains of Norway and prefix, e.g. Bh = humus-rich B horizon; Ap Scotland, and the Appalachian mountains = plowed A horizon. Superficial organic of North America. Evidence for the closure horizons are denoted by the letters L (lit- of this ocean can be found in Anglesey, ter), F (fermenting), and H (humus). Wales, where typical rocks associated with Groups of soils having a similar suite of a destructive PLATE BOUNDARY are present horizons are linked in broad soil zones. in fairly close association with pillow 2. See river. lavas. progressive wave A wave that can be protozoa /proh-tŏ-zoh-ă/ Small, usually recognized by the progressive movement of microscopic, unicellular organisms, for- the waveform at the sea surface. For exam- merly classified as animals but now usually ple, such a wave would be propagated included with certain other organisms in along a channel of infinite length, having the kingdom Protista (or Protoctista). Pro- been generated at one or other end of it by tozoa of the orders FORAMINIFERA and RA- a wave paddle. In progressive waves, the DIOLARIA possess skeletons, either secreted speed of propagation of the waveform will or of agglutinated material, which may be largely depend upon the depth of water, fossilized. They are present in large num- one example being a tide wave that may be bers in marine plankton, and some very long in relation to the water depth. As Foraminifera are benthonic. Fossils are with wind-generated waves, the speed of known from the Cambrian Period onward, wave propagation far exceeds the speed at perhaps even the Precambrian; they are im- which the water particles themselves ad- portant in micropaleontology, and in vance. The wave-induced currents associ- stratigraphic correlation, especially of ated with progressive waves are at their rocks from borehole cores. Under certain maximum at the highest point of the wave conditions their remains accumulate in suf-

278 pteropod ooze ficient numbers to contribute to rock for- (c) Infiltration by a different mineral or mation. substance of a cavity or mold previously occupied by a soluble crystal, e.g. clay provenance The source area from pseudomorphs after halite. which the particles composing sediments 2. A fossil in which the original skeletal are derived. substance of the organism has been replaced by a secondary material in the province (faunal province) A large re- course of fossilization, which preserves its gion characterized by a particular assem- shape. See also cast; mold. blage of animal species, which differs from contemporaneous assemblages in similar pseudotachylite /soo-doh-tak-ă-lÿt/A environments elsewhere. The differences glassy material produced by the fusion of between faunal provinces have arisen be- crushed rock by frictional heating, and cause the communities have been geo- found as narrow veinlets and streaks in graphically isolated from each other over mylonite zones. long periods of time and have evolved in different ways. The detection of ancient psychrometer An instrument that mea- faunal provinces is valuable in reconstruct- sures the RELATIVE HUMIDITY of the atmos- ing paleogeography. phere. It is a type of HYGROMETER and consists, in its basic form, of a wet-bulb psammite /sam-ÿt/ A metamorphosed and a dry-bulb thermometer that are mounted side-by-side. ARENACEOUS rock (see sandstone). Com- pare pelite; psephitic rock. pteridophytes /te-rŏ-dŏ-fÿts/ Plants that have a vascular system, leaves, stems, and psephitic rock /sĕ-fit-ik/ A metamor- roots but reproduce by spores (rather than phosed RUDACEOUS rock. Compare pelite; seeds). They are predominantly terrestrial psammite. and include the ferns and horsetails. Pteri- dophytes were especially abundant in the pseudo-bedding See unloading. late Paleozoic but declined during the Mesozoic. These plants formed much of pseudo-karren /soo-doh-ka-rĕn/ KAR- the coal-measure forests in the Carbonifer- REN (grooves) found on the surfaces of ous Period, when many grew to the size of rock types not normally associated with modern trees. the process of solution. pterodactyl /te-rŏ-dak-tăl/ A colloquial /soo-doh-morf/ 1. A min- pseudomorph term for one of the PTEROSAURIA, taken eral that has assumed the external form of from the name of one of the genera of this another earlier mineral. Pseudomorphs order (Pterodactylus). may be produced as a result of: (a) Replacement or alteration of one min- pteropod ooze /te-rŏ-pod/ A calcareous eral by another. In certain cases the deep-sea ooze deposit (see pelagic ooze) pseudomorph may be a polymorph of the containing more than 30% organisms. It original mineral, e.g. calcite after aragonite comprises the shells of pelagic mollusks (see polymorphism). Other common exam- and may also include the dead bodies of ples of pseudomorphism include gypsum tiny swimming snails and marine butter- after anhydrite, iddingsite after olivine, flies, especially in the vicinity of the Equa- and kaolinite after feldspar. tor. It is limited to a depth range of 1500 to (b) Encrustation or investment when a 3000 m, i.e. it lies in generally shallower mineral is deposited as a crust on crystals water than GLOBIGERINA OOZE, being par- of another, e.g. quartz on fluorite. Some- ticularly common in the vicinity of coral is- times the first mineral is removed, leaving a lands and on submarine elevations that lie negative pseudomorph. well offshore. Viewed on a global scale, it

279 Pterosauria is very limited in extent, occupying a fairly below freezing point at night. In the Boli- small north-south strip in the center of the vian and Peruvian puna there are some rich S Atlantic and several deep-water sites off deposits of minerals. Brazil and in the N Atlantic. Collectively, these areas amount to only 1% of the total push moraine A ridge of material accu- ocean floor. mulated by the bulldozing action of an advancing glacier or ice sheet. These mor- Pterosauria /te-rŏ-sor-ee-ă/ An order of aines tend to be more convex in profile extinct flying reptiles (subclass AR- than TERMINAL MORAINES and can easily be CHOSAURIA), prominent in the Mesozoic. differentiated by considering the internal The body was small in relation to the size structures, which show signs of faulting of the wings, which were formed of a thin and thrusting. If really distinct faults occur membrane of skin supported by an elonga- in the material, then it was almost certainly tion of the fourth finger. This arrangement frozen when incorporated into the push differs from that found in the birds (see moraine. See also moraine. Aves). The early Jurassic species were suited to flapping flight, but by the late puy /pwee/ A plug of volcanic rock, Cretaceous Period pterosaurs were charac- sometimes left standing when the sur- teristically adapted for gliding, and were rounding rock has been eroded away. among the most efficient gliders known. They ranged from sparrow-sized creatures P wave See primary wave. to the enormous Pteranodon, which had a wing span of up to 9 m. Some had teeth; pycnometer /pic-nom-ĕ-ter/ (density bot- others were toothless. tle) An instrument for measuring specific gravity. It consists of a small glass bottle of ptygmatic structure /tig-mat-ik/ Any known volume which is filled and weighed one of a series of highly contorted parallel (to find the specific gravity of the liquid). folds, as commonly shown by veins of PEG- Alternatively, it is filled with a dense liquid MATITE in MIGMATITE or other high-grade and a weighed sample of mineral grains in- metamorphic rock. troduced, which displaces some liquid. The specific gravity of the mineral can then be puddingstone A type of CONGLOMER- found. ATE consisting of small rock fragments or pebbles in a sandy matrix, so called be- pyralspite /pÿ-răl-spÿt/ A chemical se- cause of its supposed resemblance to a fruit ries of GARNET minerals. pudding. pyramidal peak (horn; horn peak)An pulaskite /pûl-ăss-kÿt/ A member of the individual steep-sided mountain peak SYENITE group of minerals. formed in a similar way to an ARÊTE but in this case involving three or more converg- pumice /pum-is/ Highly vesicular, usu- ing CIRQUE headwalls, thereby isolating a ally acid, volcanic rock. See pyroclastic single residual rock mass rather than a lin- rock. ear divide. pumpellyite /pum-pel-ee-ÿt/ A hydrous pyranometer /pÿ-ră-nom-ĕ-ter/ Any in- calcium-bearing mineral found in low- strument for measuring scattered and grade regionally metamorphosed rocks. global radiation on a horizontal surface. Instruments that measure global (direct puna /poo-nă/ A region of high, flat, and and diffuse) radiation are also known as bleak land in the South American Andes, solarimeters. up to 4000 m above sea level. Because of the altitude, the air is thin and tempera- pyrargyrite /pÿ-rar-gă-rÿt/ A dark red tures are low, even in summer; they fall to black mineral form of silver antimony

280 pyroclastic rock sulfide, Ag3SbS3. It crystallizes in the trigo- confined under pressure. This gas may be nal system and commonly occurs in associ- derived from the magma or from the trans- ation with other silver-bearing minerals. It formation to steam of water from the sea is an important source of silver. or a crater lake coming into contact with magma. Phreatic eruptions are caused by pyrheliometer A radiation instrument steam produced when lava flows come into for measuring direct solar radiation only. contact with groundwater. Diffuse radiation is excluded by having a The main kinds of pyroclastic materials 5° aperture continuously facing the Sun. It and rocks are as follows: requires sophisticated equipment to main- Bombs and blocks. Bombs are large frag- tain the aperture in this position and is ments with a rounded to subangular shape therefore used only at a few special sites. and erupted in a plastic condition. During flight, aerodynamically modified shapes pyriboles /pÿ-rib-ŏ-leez/ A group of are produced, the most common being a minerals consisting of PYROXENES and AM- spindle form. Blocks are large angular frag- PHIBOLES. ments ejected in a solid condition. Accu- mulations of blocks and bombs, known as pyrite /pÿ-rÿt/ (iron pyrites; fool’s agglomerate, occur near volcanic vents. gold) A pale brass-yellow mineral form Showers of hot blebs of magma that flatten of iron sulfide, FeS2, commonly occurring and weld on impact with the ground are as cubes and octahedra. It is the most wide- called spatter or agglutinate. spread and abundant sulfide mineral, Lapilli. Round to angular fragments of di- found as an accessory in igneous rocks, in ameter between 64 and 2 mm. Most of the hydrothermal and replacement deposits, in irregular vesicular fragments known as cin- contact metamorphic rocks, and in sedi- ders or scoriae are of lapilli size. A special ments laid down under reducing condi- form of lapilli,known as Pele’s tears tions. It is used as a source of sulfur. The (named for the Hawaiian goddess of fire, mineral’s common name of fool’s gold de- lightning, and volcanoes), are droplets of rives from its bright golden color when first lava that solidified in the air to pear-shaped exposed to air. pieces of glass. The droplets trail behind them threads of liquid that solidify to glass pyroclast /pÿ-rŏ-klast/ Any material, filaments, known as Pele’s hair. from fine dust and ash to large blocks of Ash. Tephra fragments less than 2 mm in rock, ejected from the vent of an erupting diameter. Consolidated ash is known as volcano. tuff. The most common variety, vitric ash, is formed by the disruption of magma by pyroclastic flow /pÿ-rŏ-klass-tik/A expanding gas. The gas causes a frothing of mixture of hot gases and PYROCLASTS that the magma, which becomes torn apart. moves under gravity along the surface Some highly vesicular masses survive as from the vent of an erupting volcano. Such pumice. Most is completely disintegrated flows can move quickly and be extremely to shards. These small glass septa, which dangerous, destroying anything in their separated individual bubbles, have charac- path. See also pyroclastic rock. teristically curved and forked shapes. Ash may be carried a great distance from the pyroclastic rock A rock formed by the volcanic source by the prevailing winds accumulation of fragmental materials and is deposited in layers, often graded, thrown out by volcanic explosions (liter- draping over the underlying topography. ally, fire-broken). Such material is known Consolidated ash with a high proportion as TEPHRA or ejectamenta and may be ex- of larger fragments is sometimes called pelled as solid fragments or in the molten lapilli tuff. Crystal tuff contains a high pro- state, chilling in the air and producing vit- portion of broken crystals, which represent reous material. All volcanic eruptions are the PHENOCRYSTS present in the magma be- the result of the release of gas that has been fore eruption.

281 pyrolusite

Ignimbrites (welded tuffs). Fragmental monoclinic and have a continuous chain flows having some of the characteristics of structure of SiO4 tetrahedra linked by shar- lava and some of air-fall pyroclastic ma- ing two of the four corners. Cations link terial. Ignimbrites typically have a streaky the chains laterally. The general formula of or banded appearance known as eutaxitic pyroxenes is X1–pY1+pZ2O6, where X = structure. The glossy streaks, usually Ca,Na; Y = Mg,Fe2+,Mn,Li,Al,Fe3+,Ti, and darker in color than the surrounding ma- Z = Si,Al. In the orthopyroxenes (ortho- trix, are laterally discontinuous, unlike rhombic pyroxenes), p = 1 and the content flow banding. Individual streaks called fi- of trivalent ions is small. In clinopyroxenes amme are arcuate or elliptical in plan and (monoclinic pyroxenes), p varies from 0 as are thought to represent pumice fragments in diopside (CaMgSi2O6) to 1 as in spo- or lava blebs flattened by the weight of the dumene (LiAlSi2O6). ignimbrite and welded together as the flow Orthorhombic pyroxenes deflates. The shards that compose the ma- The orthopyroxenes have a composition 2+ trix are also flattened and drawn out. (Mg,Fe )2Si2O6 and form a series from The flows producing these deposits are enstatite (MgSiO3) to orthoferrosilite called ash flows and the rocks produced (FeSiO3) that is produced by the replace- are ignimbrites, ash-flow tuffs, or welded ment Mg↔Fe2+. Intermediate composi- tuffs. Not all ash flows become welded and tions in the series include bronzite and in many flows welded eutaxitic material hypersthene. Orthopyroxenes are found in passes up into unwelded material. Ash basic and ultrabasic igneous rocks and flows are mostly of acid composition and high-grade metamorphic rocks. are the result of deposition from NUÉES AR- Monoclinic pyroxenes DENTES, incandescent clouds of gas and ash Most clinopyroxenes can be considered to produced by the explosive vesiculation of be members of the four-component system, magma. CaMgSi2O6–CaFeSi2O6–MgSiO3–FeSiO3. Within the system, three series are recog- pyrolusite /pÿ-rol-yŭ-sÿt/ A soft pow- nized: dery or fibrous black mineral form of man- 1. diopside–salite–hedenbergite ganese dioxide, MnO2. It crystallizes in the Ca(Mg,Fe)Si2O6 tetragonal system, and occurs in associa- 2. augite–ferroaugite 2+ 3+ tion with other manganese minerals. It is (Ca,Na,Mg,Fe ,MnFe Al,Ti)2- used a source of manganese, and as a de- (Si,Al)2O6 2+ 2+ colorizer and oxidizing agent. 3. pigeonite (Mg,Fe ,Ca)(Mg,Fe )Si2O6. The pigeonite series are calcium-poor pyrope /pÿ-rohp/ A deep yellow-red to clinopyroxenes. The diopside series have black member of the GARNET group of min- higher Ca/Mg + Fe ratios than the augites. erals, Mg3Al2Si3O12. It crystallizes in the Clinopyroxenes rich in titanium are called cubic system, and occurs in kimberlite and titanaugites. Pigeonite is found in basic other ultrabasic igneous rocks. Transpar- lavas but under the slow cooling condi- ent examples are valued as semiprecious tions associated with plutonic crystalliza- stones. tion, pigeonite expels calcium-rich ions as lamellae of augite and inverts to an or- pyrophyllite /pÿ-roff-ă-lÿt/ A soft cream- thorhombic structure. Diopside and heden- colored, gray, or green mineral form of hy- bergite are found in calcium-rich and drated aluminum silicate, AlSi2O5(OH), iron-rich thermally metamorphosed sedi- which resembles talc. It crystallizes in the ments respectively. Hedenbergite also oc- monoclinic system, and occurs in meta- curs in intermediate and acid igneous morphic rocks such as schist as thin flakes. rocks. Pyroxenes of the augite-ferroaugite series are found in basic igneous rocks. pyroxene /pÿ-roks-een/ A member of a Omphacite, a high-pressure pyroxene re- group of ferromagnesian rock-forming sembling augite but with some CaMg re- minerals. Pyroxenes are orthorhombic or placed by NaAl, is found in ECLOGITES.

282 Quaternary

Sodic pyroxenes may be considered cedony, jasper, and opal. Many of these are to be members of the system CaMgSi2O6– used as gemstones. Quartz is a component 3+ CaFeSi2O6–NaFe Si2O6, in which the re- of granite and other acid igneous rocks; it placements Ca(Mg,Fe2+)↔ NaFe3+ occur. also occurs in metamorphic rocks The sodic end-member is aegirine (or ac- (quartzite) and some sedimentary rocks 3+ mite), NaFe Si2O6, and as the name (sandstone). It has many uses, mainly in the implies, aegirine-augite, (Na,Ca)(Fe3+, manufacture of abrasives and glass. 2+ Fe ,Mg)Si2O6 is of intermediate composition. Sodic pyroxenes are found in inter- quartzarenite /kwort-sa-rĕ-nÿt/ (ortho- mediate and acid alkaline igneous rocks quartzite) An ARENACEOUS rock having a and glaucophane schists. composition including more than 95% Other pyroxenes include spodumene QUARTZ. (LiAlSi2O6), found in lithium-rich pegmatites, and jadeite (NaAlSi2O6), found in quartz diorite A coarse-grained igneous high-pressure metamorphic rocks. Com- rock consisting mainly of PLAGIOCLASE pare amphibole. FELDSPAR, sometimes also with some OR- THOCLASE. The minerals biotite and horn- pyroxenite /pÿ-roks-ĕn-ÿt/ An ultra- blende occur as the dark constituents, with mafic rock consisting wholly of PYROX- up to 30% quartz as the light component. ENES. Pyroxenites may be monomineralic or bimineralic. quartzite /kwort-sÿt/ A tough massive rock consisting almost wholly of QUARTZ pyrrhotite /pi-rŏ-tÿt/ A yellow-brown and usually having a pale color. A quartzite magnetic mineral form of iron sulfide with is the product of the metamorphism of a a variable amount of sulfur, average for- pure sandstone, during which process the mula Fe7S8. It crystallizes in the hexagonal quartz grains recrystallize and become in- system, and often occurs associated with terlocking. See also psammite. nickel sulfide, when it is a major source of nickel. quasi-geostrophic motion Atmospheric motion that approximates to the GEO- quaquaversal /kwah-kwă-ver-săl/ See STROPHIC WIND. It is one of the assumptions pericline. used in the preparation of numerical weather forecasts. Because it is not quite quartile /kwor-tÿl. -tăl/ If a data series is geostrophic flow, it cannot be used for de- arranged in order of magnitude, a quartile riving other quantities based on the indicates a quarter of the values. It is more geostrophic wind. frequently quoted as upper and lower quartile, the former separating the highest quar- Quaternary /kwă-ter-nă-ree/ A subdivi- ter of the values and the latter the lowest sion of the CENOZOIC Era, formed of two quarter. It can therefore be used as a mea- epochs, the PLEISTOCENE and HOLOCENE. sure of the dispersion or range of the data. The Quaternary has been considered to be a period (comprised of the Pleistocene and quartz One of the most important rock- Holocene Epochs) within the Cenozoic forming minerals, consisting of silicon Era, following the TERTIARY Period but al- dioxide, SiO2 (see silica minerals). It crys- though the term has been widely used in- tallizes in the trigonal system, and occurs formally it has never been formally as sand and as deposits of highly transpar- recognized. Recent proposals are for the ent colorless crystals (rock crystal), some- Quaternary to be formally recognized as a times tinted by impurities. These give rise subera of the Cenozoic, and for it to span to the varieties known as rose quartz and the past 2.6 million years. During the Qua- smoky quartz, as well as to amethyst and ternary parts of Europe were subjected to citrine. Quartz is also the basis of cryp- four major advances of the ice sheets, tocrystalline minerals such as agate, chal- which were separated by warmer inter-

283 quicksand glacial episodes, and most British Pleis- nary is the time when humans became the tocene rocks consist of glacial and associ- dominant terrestrial species. ated fluvial deposits. Fossil animals and plants were essentially modern. The land quicksand An area of mud and sand containing a large amount of water and fauna alternated between forms adapted to thus almost liquid in composition. It can cold conditions, such as the mammoth and occur on the shore or near a river, where woolly rhinoceros, and species now re- the water table is near the surface and stricted to the tropics. Pollen and the re- drainage is poor. mains of beetles have proved valuable in monitoring climatic change. The Quater- Q-wave See Love wave.

284 R

radar (radio detection and ranging) A cator), radar can measure the distribution method of active REMOTE SENSING in which and intensity of surface precipitation and electromagnetic energy in the range of mi- distinguish particularly intense storms such crowave wavelengths is used to detect ob- as tornadoes, hurricanes, thunderstorms, jects by their ability to reflect the beams. and hailstorms. Scanning about the vertical Radar systems emit pulses of microwave (Range-Height Indicator) enables the radiation and record the returned mi- height and structure of precipitating clouds crowave echo. The resulting image is pro- to be obtained as well as the altitude of the duced from analysis of the time and melting level, if present. Operation over a strength of the returned pulse. Radar uses period of time clarifies the evolution of wavelengths that are able to penetrate many of these atmospheric systems and it is clouds so can provide an image of the at the subsynoptic scale that most advances Earth’s surface in areas normally obscured in radar meteorology have occurred. by cloud cover. Side-looking airborne radar (SLAR) is an airborne instrument radial dike A dike that radiates from a that emits beams of microwaves to the central-vent volcano. This radial fracture ground at an angle perpendicular to the pattern is the result of the volcano’s super- aircraft’s flight path to produce a three- structure swelling prior to an eruption as dimensional image of the land surface. The pressure builds up. ground surface is illuminated at an oblique angle that enhances features such as folds radial drainage A drainage pattern that and faults. Synthetic aperture radar (SAR) develops when structural control is in the carried on aircraft or satellites has wide- form of a volcanic dome or cone, or some spread applications, e.g. in land-use and other sort of dome. Slope will cause vegetation studies, studies of the cryo- streams to radiate out from their common sphere, geology, oceanography, and hy- center at the crest of the dome like the drology. The technique of radar inter- spokes of a wheel, e.g. in the English Lake ferometry using SAR (InSAR) has particu- District. lar potential for the solid earth sciences as it enables the measurement of motions of radiation 1. (in meteorology) The portion the ground surface with considerable preci- of the ELECTROMAGNETIC RADIATION spec- sion. Areas of potential use include the trum that is emitted by the Earth and the study of crustal deformation, earthquakes, Sun. In terms of wavelength, this is radia- and volcanoes. See also radar meteorology. tion between 0.1 and 70 µm,encompassing part of the ultraviolet, all the visible, and radar meteorology The investigation part of the infrared spectrum. This type of or determination of aspects and properties wave differs from most other waveforms in of the atmosphere by radar. Using suitable that it does not require an intervening wavelengths, radar offers great possibilities medium for its propagation; it can travel of investigating the areal distribution of through a vacuum. many meteorological elements, which oth- The behavior of electromagnetic radia- erwise would be unobserved. By scanning tion is described by certain physical laws. in the horizontal field (Plan Position Indi- All substances above the absolute zero of

285 radiation balance temperature (–273°C) emit radiation in higher temperatures than would otherwise amounts and wavelengths dependent on be expected. their temperature. Some bodies emit and 2. A type of PLANE TABLING in which points absorb radiation in certain wavelengths of detail are fixed by ascertaining direction only; this is particularly true of gases (see using an ALIDADE and distance by measur- atmospheric window). The ideal radiating ing on the ground with a tape. The method body is a black body (see black-body radi- may be used over short distances, dispens- ation), which emits the maximum amount ing with the necessity to set up the plane of radiation for its temperature, the table at two stations, as in INTERSECTION amount being proportional to the fourth methods. power of its temperature on the Kelvin scale. Absorption and emission take place radiation balance The net effect of the at the same wavelengths for any substance. difference between incoming and outgoing The hotter a body, the shorter will be the radiation at any point. These two factors wavelengths at which the maximum rarely balance. Normally there is a surplus amount of emission will take place. These of radiation on the ground surface during laws determine the way in which bodies the day, which helps to warm the surface react to radiation and enable the utilization and atmosphere, and a deficit at night of solar energy by the Earth and atmos- when cooling takes place. The atmosphere phere to be explained. The electromagnetic has a negative radiation balance at all radiation emitted from the Sun is termed times. Taking the Earth and atmosphere solar radiation (or solar energy). It repre- together, the areas equatorward of 38° sents nearly all the energy available to Earth. have a radiation surplus and poleward The Sun has an emission temperature of there is a radiation deficit. 6000 K, giving an energy maximum in the visible light wavelength. This reaches the radiation fog Nocturnal cooling result- top of our atmosphere at the rate of about ing from terrestrial radiation losses can 2 calories per sq cm per minute or 1.35 lead to the air near the ground surface kW m–2. On its passage through the at- reaching the saturation point so that con- mosphere, some of this short-wave radia- densation takes place, causing radiation tion is reflected back to space by clouds fog. It is most likely to occur during long, and dust, some is scattered by gas mol- clear, and calm nights with a moist atmos- ecules and dust particles to give diffuse ra- phere. In industrial areas, the abundance of diation, and some (about 18%) is absorbed condensation and hygroscopic nuclei facil- by water vapor, carbon dioxide, and dust. itates fog formation and it may even occur The remainder reaches the ground surface with relative humidities less than 100%. where some is reflected and the rest is ab- However, urban warmth frequently coun- sorbed. On a global average, this amounts teracts this factor and so city centers may to approximately 47% of the radiation be almost clear of fog while it is still quite reaching the top of the atmosphere. dense in the cooler suburbs. The Earth’s surface has a mean emission temperature of about 290 K as a result radioactive decay The spontaneous of solar heating. Thus terrestrial radiation change of one atomic nucleus into another is in the longer wavelengths with a maxi- through the emission of a photon (gamma mum about 10 µm. The gases of the at- ray) or a particle (alpha ray or beta ray), or mosphere have a very different response to by electron capture. It is the basis of RA- this radiation and much of it is absorbed DIOMETRIC DATING. See also half-life; iso- (see absorption). Certain wavelengths are tope. unaffected, especially those between 8 and 12 µm (radiation windows). The atmos- radiocarbon A radioactive ISOTOPE of phere warms as a result of this absorption carbon, commonly one of mass 14. Its de- and atmospheric counterradiation is re- tection is the basis of radiocarbon dating of turned to the surface and helps to maintain organic remains. See also half-life.

286 rainfall radiocarbon dating (carbon-14 dating) and daughter isotopes of a radioactive el- A method of RADIOMETRIC DATING in which ement. Because these decay reactions the age of an organic material is deter- progress at a known rate, the age of the mined by measuring the proportion of ra- rock is calculated from the ratio present. diocarbon (carbon-14) within the carbon content of the material. Carbon-14 is pro- radiosonde /ray-dee-oh-sond/ A package duced in the upper atmosphere from the of small electronic instruments for measur- action of cosmic waves on nitrogen-14. ing pressure, winds, temperature, and hu- Living organisms continuously absorb car- midity in the upper atmosphere, together bon-14 in the form of carbon dioxide from with a small radio transmitter that is at- the atmosphere, or through the food chain, tached to a helium-filled meteorological during their lives, but on death no more is balloon, which lifts the radiosonde to alti- taken in and the amount present decreases tudes of about 30 000 m. Observations of through radioactive decay at the known pressure, temperature, and humidity are half-life rate. Radioactive carbon-14 has a transmitted to a ground station. The bal- 5730-year half-life and can be used for dat- loon is tracked by radar and from its posi- ing materials up to about 70 000 years. tion wind velocities and direction can be The method is used for dating archaeolog- calculated. ical artifacts and for materials from the Holocene and Pleistocene. It was pioneered rain PRECIPITATION in the form of liquid by the American physical chemist Willard water drops ranging in size from about 0.5 F Libby (1908–80) in the 1950s. mm to 5 mm in diameter.

Radiolaria /ray-dee-oh-lair-ee-ă/ An order rainbow An optical phenomenon con- of PROTOZOA whose members have an in- sisting of an arc of light across the sky bro- ternal skeleton composed of silica. They ken up into the spectral colors. It results are one of the constituents of marine from the refraction and internal reflection plankton. In certain conditions their re- of sunlight through falling water drops. mains accumulate to form RADIOLARIAN The intensity of the rainbow depends upon OOZE, and analogous ancient radiolarian the size of the drops, larger sizes producing cherts are known. Fossils from rocks as old brighter colors. Some of the light may be as the Precambrian have been attributed to reflected twice within the raindrop to pro- this group. duce a double rainbow effect. radiolarian ooze A siliceous ooze (see rain day In Britain, a climatological DAY pelagic ooze) deposit containing more than from 9 am GMT within which 0.2 mm or 30% organisms. It is reddish or brownish more of rainfall is recorded. See also wet in color and results from the deposition of day. minute siliceous skeletal remains of radio- larians. The deposits are very limited in ex- raindrop size spectrum The range of tent when viewed on a global scale, being raindrop sizes within or beneath a precipi- almost confined to a narrow tract of deep- tating cloud. The range of sizes varies with sea floor in the Pacific Ocean, located just the rate of rainfall. At low rates (0.1 north of the Equator. mm hr–1) most drops will have a diameter of less than 0.75 mm and there will be very radiolarite /ray-dee-oh-lair-ÿt/ A hard few above 1.25 mm. With heavier rain, the fine-grained siliceous rock formed predom- upper range of drop sizes increases and the inantly from the skeletal material of RADIO- density of drops rises markedly. LARIA. rainfall The water equivalent of all forms radiometric dating /ray-dee-oh-met-rik/ of precipitation from the atmosphere, re- A method of dating rocks by determining ceived in a rain gauge. This includes dew, the relative proportions present of parent hoar frost, and rime on the collecting area

287 rainforest of the gauge, because on melting it is indis- rain shadow An area on the leeward tinguishable from the rest of the catch. It is side of a range of hills where rainfall totals now recorded in millimeters in most coun- are less than would be expected from their tries of the world. The value quoted repre- latitudinal position. It is the result of a dry- sents the amount of water that has fallen ing-out of the winds through precipitation on a horizontal surface assuming no on the upwind side of the hills. runoff, percolation, or evaporation. Rainfall has been classified into three rainsplash The impact of raindrops on types depending on its mode of formation: the soil. It influences slope evolution in two convective, frontal (or cyclonic), or oro- ways: firstly, as the drop hits the ground graphic rainfall. and rebounds, it brings up with it particles of soil, which tend to fall downslope; sec- rainforest A type of dense forest that ondly, this process will, if continued, com- grows in regions that have heavy rainfall pact the soil surface, often breaking up soil all the year round. Most rainforests occur structures, and reduce the infiltration ca- in the tropics, although there are some in pacity (ability to hold and store water) of warm temperate regions. They are among the soil. This promotes surface flow, either OVERLAND FLOW RILLS the most complex ECOSYSTEMS in the as an or as , which world, with a great diversity of plants and then erode the surface. animals. Their plants are also an important rain wash See overland flow. source of atmospheric oxygen (from PHO- TOSYNTHESIS). See also deforestation. raised beach An inland terrace of deposits that marks the location of a former rain gauge An instrument for measuring shoreline, but is now well above the pre- rainfall. In its simplest form it is essentially sent sea level. There may be cliffs inland of a funnel and a collecting vessel placed ver- the raised beach. Most of these features re- tically into the ground. Different types of sult from uplift of the land following glacia- rain gauge are used throughout the world tion, although some were produced by the but most are set about 30 cm above the movement of lithospheric (tectonic) plates. ground surface. This is to avoid splashing during heavy rain, to retain snow, and to raised bog See organic soil. reduce the effects of wind eddying around the gauge. It is widely accepted that a rain range 1. A line of mountains that forms gauge does not catch the precise amount of a continuous barrier, perhaps with some rain falling on a surface, largely because of gaps or passes. the wind eddying effect, but as long as all 2. An open area of pasture, as in the west- sites are standardized there is comparabil- ern USA. ity between records. The rainfall is meas- 3. The difference between the highest and ured by emptying the contents of the lowest values of a variable, such as temper- collecting vessel into a special cylindrical atures or the heights of the tides. flask, which is graduated in relation to the diameter of the gauge. The rainfall can be rapakivi structure /rap-ă-kee-vee/An read off directly from the water level in the ORBICULAR structure exhibited by some flask. Recording versions, e.g. the tipping- granites in which large oval pink ortho- bucket rain gauge, can be linked to auto- clase crystals are mantled by white oligo- matic weather stations. More sophisticated clase. versions also record rainfall intensity. rapids A stretch of fast-flowing turbu- rain pit A sedimentary structure formed lent broken water at a break in the long in subaerially exposed soft fine-grained profile of a river that is not a vertical drop. sediments as a result of the impact of rain- Rapids often mark KNICKPOINTS, some- drops upon the sediment’s surface. times in the form of the remains of an

288 reaction series eroded waterfall. If the fall in base level containing the direction of wave propaga- causing rejuvenation results in a retreat of tion. See also Love wave. the shoreline some distance offshore, because the coastal area is gently sloping, the reaction rim A CORONA STRUCTURE con- knickpoint may be spread over a consider- sisting of a concentric shell (or shells) of a able horizontal distance and rapids will be mineral often arranged in a radial or fi- created rather than a waterfall. Rapids can brous manner around a primary mineral. also occur where a river crosses a hard rock Such rims, in igneous rocks, may be the re- band, not yet graded to fit a smooth long sult of a reaction between an early-formed profile. Rapids and waterfalls tend to be mafic mineral and the liquid with which it fairly quickly eliminated owing to the in- is no longer in equilibrium. Thus a later creased turbulence and erosion they them- member of the REACTION SERIES is pro- selves create. duced, e.g. olivine may be mantled by pyroxene or amphibole. Kelyphitic rims are rarefaction wave After the impact of a secondary coronas formed by late-stage or METEORITE with the ground, a wave of de- metasomatic fluids reacting with primary compression that follows the initial com- crystals, e.g. iddingsite rimming olivine. pressional shock wave through the rocks at the impact site. It is the rarefaction wave reaction series During magmatic crys- that causes the ejection of debris from the tallization, in order to maintain equilibrium between crystals and liquid, minerals crater (see ejecta). interact chemically with the liquid. The reaction may be progressive so that a contin- raster In geographical information sys- uous series of homogeneous solid solutions tems (GIS), a method of representing spa- is produced. In the plagioclase feldspars, tial data as a matrix of cells (or pixels). the first-formed crystals are richest in cal- Geographic space is divided into cells in a grid to which properties or attributes can be assigned. Remote-sensing satellites capture data in this form and it is transmitted discontinuous continuous to ground stations where it can be analyzed reaction series reaction series and distributed. (iron:magnesium ratio increasing) ravine A small narrow deep-sided valley olivine anorthite carved by the erosive action of a river or produced by faulting. bytownite orthopyroxene raw sienna See sienna. labradorite raw soil Soil with an (A)C profile, produced by incipient soil-forming processes clinopyroxene on fresh rock. andesine raw umber See umber. hornblende oligoclase Rayleigh wave /ray-lee/ One of two types of surface wave produced either nat- biotite albite urally by earthquakes or volcanic eruptions, or artifically by an explosion (named for the British physicist John William quartz Strutt, 3rd Baron Rayleigh (1842–1919)). alkali feldspar In Rayleigh waves the motion of surface particles is elliptical in the vertical plane Reaction series

289 realgar cium and as the temperature falls these the same as or different from the original react with the liquid to become progres- (primary) grains, and they may be signifi- sively more sodic. The reaction series in- cantly larger. volving the ferromagnesian minerals is discontinuous, each reaction taking place only over discrete temperature intervals and corresponding to the transformation of one mineral to another of different crystal structure. At high temperatures, magnesium-rich olivine crystals are the first to appear in a basic magma but as cooling proceeds, olivine is no longer stable and magnesium-rich orthopyroxene crystallizes. Likewise orthopyroxene is stable over a limited temperature range and reacts to produce clinopyroxene. Ferromag- nesian minerals have both high- and Rectangular drainage low-temperature variants, those rich in magnesium preceding those rich in iron. rectangular drainage A drainage pat- The common rock-forming minerals have tern that is characterized by streams flow- been arranged by the Canadian geologist ing in two directions at right angles to each Norman Levi Bowen (1887–1956) in the other. Neither direction is subordinate, order in which the reactions take place and both being equally developed. The most constitute two series. usual control is right-angled faulting in an The early high-temperature members of area of little other structural guidance. both series generally crystallize together; hence gabbros contain magnesium-rich rectilinear slope /rek-tă-lin-ee-er/A olivine and pyroxenes together with calcic slope or individual facet of a slope that is plagioclases. The low-temperature miner- straight, i.e. has a single angle of slope. als, alkali feldspar, mica, and quartz, are associated in granitic rocks. When the re- recumbent fold A fold type in which action between crystals and the liquid is the axial plane is almost horizontal. See di- unable to go to completion because of too agram at FOLD. rapid cooling, early-formed members of the reaction series persist as relics in the recurrence interval (return period) The final rock and zoned crystals (see crystal average interval of time between occur- zoning) and REACTION RIMS are often ob- rences of a particular event. In hydrology it served. is used for events such as floods or particular amounts or intensity of rainfall (e.g. a realgar /ree-al-ger/ A rare orange-red flood of a certain magnitude or above may mineral form of arsenic sulfide, As2S2. It have a recurrence interval of 100 years); crystallizes in the monoclinic system as such information is used in flood forecast- compact aggregates in hydrothermal veins ing. It is also used in geology for the aver- and hot springs; it may be found in associ- age time interval between earthquakes of a ation with ORPIMENT. It is used as a pig- given magnitude in a particular area. ment and as a source of arsenic. red beds Layers of sedimentary rocks Recent See Holocene. consisting mainly of sandstone, siltstone, and shale colored red by iron oxide recrystallization /ree-kris-tă-li-zay-shŏn/ (HEMATITE). An example is OLD RED SAND- The formation of new mineral grains in a STONE. rock while it is still in the solid state. The composition of the new material may be red clay (brown clay) A pelagic deposit

290 regelation that covers large parts of the abyssal floor, mon in the tropics. Other organic reefs in- actually about a quarter of the Atlantic and clude OYSTER REEFS and serpulid reefs Indian Oceans respectively, and something (which result from the cementing action approaching a half of the Pacific Ocean, or brought about by certain types of marine collectively some 28% (approximately 130 worms). The presence of a freshwater out- million sq km) of the total ocean floor. The flow from reefs locally inhibits organic deposit is characterized by a low silica and growth, causing gaps through the reefs. carbonate content. The clay particles may be mixed in places with manganese nod- reentrant A small valley or area of low- ules, whale bones, sharks’ teeth, pumice, land that stretches into higher ground. It and other assorted materials. generally results from erosion by water, but may be caused by FREEZE-THAW weath- red earth See ferrallitic soil. ering. red rain Rain colored red by dust parti- reflection profile A seismic profile pro- cles, usually fine sand. A phenomenon of duced and recorded by equipment designed mid-latitudes, an example is the red rain to reflect energy from layered rock bodies. that sometimes falls on S Europe containing dust picked up in the Sahara. reflux The process by means of which dense concentrated salt solutions move reduction /ri-duk-shŏn/ A chemical pro- downward through the EVAPORITE on the cess that occurs in rocks and in the gleyed floor of a PLAYA or other basin. It is proba- and leached parts of soil, whereby oxygen bly one of the mechanisms that leads to the is removed from the weathering material. increasing magnesium content in some sed- The continuous presence of water renders imentary rocks. oxygen scarce, leading to its displacement, for example from ferric iron oxide (Fe2O3) refolded fold A fold that has been sub- to give ferrous iron oxide (FeO), in which jected to more than one period of folding. state it is rendered more soluble and hence The folds produced in the first period of potentially more mobile. The characteristic folding have further fold trends over- red and yellow colors of OXIDATION are ab- printed on them by subsequent periods of sent, being replaced by greens and grays. folding. Bacteria are important in this process. reforestation The planned replanting of reef A ridge, island chain, or area of trees in an area where they have been cut rocks that projects above the surface of the down or destroyed (see deforestation). It is sea for all or part of the tide or it may be particularly important as a method of pre- permanently submerged. Reefs constitute venting soil erosion and the formation of distinct navigational hazards. A reef may infertile land. be formed of solid rock or pebbles, but the term is more commonly applied to organic refractory inclusion /ri-frak-tŏ-ree/A reefs (BIOHERMS). They are sometimes at- type of irregular mineral grain found, tached to the coast and act as groins by col- along with carbonaceous chondrules, in lecting debris against them (see fringing some chondrite METEORITES. Up to 2 mm reef), sometimes parallel to the coast and across, the inclusions consist of high tem- separated from it by a lagoon too deep for perature-resistant oxides of aluminum, cal- coral growth (see barrier reef), sometimes cium, magnesium, and titanium. in tabular sheetlike masses (apron reefs), and sometimes in circular form (see atoll). reg A type of arid desert plain consisting Coral, a hard calcareous material, dead or of gravel, especially in the Sahara. See also alive, may be predominant in all of these erg; hammada. (corals are at present the commonest reef- building organisms). CORAL REEFS are com- regelation /ree-jĕ-lay-shŏn/ A process of

291 regeneration thawing and refreezing within a GLACIER, regression The retreat of the sea from a which contributes toward the down-valley land area. movement of ice. It is believed that pressure within the ice of a glacier causes the rejuvenation The revitalizing of streams melting of some ice crystals. The resultant by an increase in their erosive capacities re- meltwater will move to locations at which sulting from a fall in BASE LEVEL. This pressure is less, i.e. normally downslope, causes oversteepening of the lower reaches, and then refreeze. The process has been and increased erosion cuts out a new long classed as the primary cause of ice motion profile which will intersect the original but many authorities on the subject con- long profile at a KNICKPOINT. Tributary valleys meeting the main stream seaward of sider it to be only a secondary factor aris- the knickpoint will also be rejuvenated and ing from flow. begin to cut down; if they do not keep pace with the incision of the master stream, regeneration (in ecology) The natural there will be a marked break of slope at the regrowth of plants that follows destruction junction, leaving them ‘perched’. Below the (such as of grassland after fires or of trees knickpoint, the former valley floor will be after forest fires or felling). It is an impor- left hanging above the stream as it cuts tant factor in the long-term stability of down to a new level, becoming a RIVER TER- many ECOSYSTEMS. RACE. The knickpoint will advance upstream at a rate dependent on the lithology regime The total economy or habit of a of the bed material and the nature of the natural system, e.g. fluvial regime, estuar- flow over it; if it meets a hard rock band it ine regime, coastal regime. See also equilib- may be halted, and so the effects of the re- rium regime. juvenation will not be felt upstream. Valley-side slopes will also be rejuve- regional metamorphism See metamor- nated, because the incision of streams will phism. oversteepen their lower portions, and temporarily create a valley-side facet adjacent regolith /reg-ŏ-lith/ (waste mantle)Un- to the stream. consolidated weathered material between the ground surface and the bedrock, which rejuvenation head See knickpoint. may reach thicknesses of 60 m in the tropics. It is formed either in situ, by the weath- relative dating The ordering of rocks or ering of the underlying bedrock, or it is fossils in terms of the GEOLOGIC TIMESCALE, transported into the area by water, wind, without taking into account their absolute or ice. The upper biochemically weathered ages. portion of the regolith is the soil. relative humidity The most frequently used index of atmospheric HUMIDITY. It is regosol /reg-oV-sôl/ A thin azonal soil the actual vapor pressure of the air ex- that may develop where one soil-forming pressed as a percentage of the saturation factor delays the soil-forming processes. vapor pressure at the same temperature. It Such soils are young in development and can be measured directly by changes in lack a B horizon, resulting in an AC profile length of a hair or a skin in a hygrometer, with the A horizon, because of organic or indirectly using wet- and dry-bulb tem- matter, being darker than the C. Regosols peratures. Because the value of relative hu- are typically developed in unconsolidated midity changes inversely with temperature, deposits such as loess, recent till, or sand even for the same moisture content of the dunes, and the parent material dominates atmosphere, it is not a very precise unit. the characteristics of the profile. They are classified in the ENTISOL order of the US relative plate motion The motion of SOIL TAXONOMY. one LITHOSPHERIC PLATE in relation to an-

292 representative fraction other. Compare absolute plate motion. See coloring, or shading to indicate the differ- also plate tectonics. ent heights of various features. See contour; hachures. relative relief The degree of dissection of a landscape, shown by the difference in remnant magnetism The magnetism height between the tops of drainage divides ‘frozen’ into a mineral or rock as it cools and the bottoms of adjacent valley floors. through its Curie point. Cartographically it is approached by calculating the difference between the highest remote sensing The process of obtain- and lowest points in each grid square of the ing information about the surface of the relief map, and then expressing the results Earth and the atmosphere by using meth- as a series of isolines joining areas of equal ods that involve no physical contact. It relative relief. According to the normal includes the use of orbiting satellites, CYCLE OF EROSION, relative relief increases spacecraft, and aircraft, which transmit the to a maximum in maturity and thereafter information down to a ground station. The decreases. One geomorphologist has based most commonly employed techniques are a scheme for landscape attractiveness on aerial photography, infrared photography, relative relief, high relative relief producing and radar, and usually computers are em- high scenic appeal. See also relief. ployed in interpreting the data. See also Landsat; satellite. relative vorticity See vorticity. rendzina /rend-zee-nă/ Soil developed relic sediment (in oceanography) A ma- on soft calcareous parent material with an rine deposit, usually present on shelf areas, AC profile (no illuvial B horizon). The A is that is incompatible with contemporary usually dark brown or black, rich in car- marine environments. For example, much bonates and humus, with a good crumb or relic sediment on the continental shelf sur- granular structure, and a pH value above rounding Britain was initially deposited neutral. There are no free sesquioxides as there under glacial or interglacial condi- in the related TERRA ROSSA soils, which also tions. Much of the material lying on the develop on calcareous material. It may be shelf off S California is relic, especially the sands that contain extinct shallow-water that the rendzinas are less mature than the foraminifera, and which currently lie on terra rossa soils, or that the terra rossas de- outer parts of the shelf. Some sands off velop on hard limestone, whereas rendzi- Long Beach, California, have been found nas develop on soft. Rendzinas occur in to include Pleistocene fauna. Many relic humid to semiarid climates, under grass or shelf sediments are now undergoing sort- grass-and-tree vegetation, and are perhaps ing and transport, under the action of best known as the soils of chalk downland. waves and tidal flow, gradually shifting to more compatible sedimentary environ- reniform /ren-ă-form/ Denoting the mas- ments. Modern sediments are those being sive kidney-shaped form in which some derived under contemporaneous condi- minerals occur. Reniform hematite is tions. called kidney iron ore. Compare botryoidal. relief The variation in elevation or physical outline of a landscape, shown on maps replacement (in geology) The natural by the use of contours, spot heights, hyp- substitution of one mineral for another, sometric tinting, and hill-shading. Relief is generally by the action of gases or solu- also used synonymously with RELATIVE RE- tions. See mineralization; reflux. LIEF. Positive relief indicates land rising above the general level, i.e. hills. representative fraction (RF) The ratio between distance on a map and distance on relief map A map that uses contours, the ground, expressed as a fraction.

293 Reptilia

Reptilia /rep-til-ee-ă/ A class of cold- resorption The remelting of crystals due blooded terrestrial vertebrates whose to reaction with the magma from which young complete their development within they originally crystallized. Early-formed a tough-shelled egg; they are therefore pro- phenocrysts frequently fail to maintain tected from desiccation and totally inde- equilibrium with the melt and are partly re- pendent of an aquatic environment. sorbed. Markedly anhedral crystal forms Reptiles evolved from the AMPHIBIA in the are produced and a REACTION RIM of a new Carboniferous Period and their main radi- mineral may result. ation began at the end of the Permian. Dur- ing the Jurassic and Cretaceous a great resultant wind The vector average of variety of reptiles came to dominate life on all wind velocities and directions for a land. These ranged from the giant DI- given level at a given place over a given pe- NOSAURS to the flying PTEROSAURIA and the riod of time. A wind velocity may be di- secondarily aquatic ICHTHYOSAURIA and vided into zonal (east) and meridional PLESIOSAURIA. All these groups became ex- (north) components. To determine the re- tinct at the beginning of the Cenozoic, sultant wind of a series of wind velocities, when they were replaced by the mammals the individual components of the actual (see K/T boundary event). Modern reptiles wind are summed, squared, added to- include crocodiles, lizards, and snakes. gether, and the square root taken. That is: √ Σ 2 Σ 2 resultant wind = ( VN) + ( VE) reservoir (in geology) A rock that has a The emergence of an under- high porosity and good permeability and is resurgence ground stream, usually where it encounters able to store and transmit fluids or gases. impermeable rock after having flowed through permeable strata. residual clay A type of clay that is formed where it lies by the weathering of reticulated Describing something, such rock. It may result from the removal of as a lode or vein, that has a netlike appear- nonclay minerals from the rock, or from ance or structure. chemical changes to FELDSPAR. return period See recurrence interval. residual deposit Rock fragments that are left behind after weathering and ero- reverse fault A type of fault in which sion of the preexisting rock. SCREE, for ex- the movement along the inclined fault ample, consists of such rock fragments left plane has been up-dip. This results from a on a hillside. Rockfalls or transport by ice principal stress configuration in which the or water may move some residual deposits maximum principal stress is vertical, from their original site. See also residual whereas the intermediate and minimum clay. principal stress directions are horizontal. (See diagram at FAULT.) residual hill An isolated small hill, all that is left of a larger mass of high ground Reynolds number /ren-ŏldz/ (Re) The that has suffered erosion or land move- extent to which viscosity modifies a fluid ment. See monadnock; puy. flow pattern depends upon the speed of flow (u), the width of an obstacle placed resistance thermometer See thermom- across the flow or of the flow passage (D), eter. and a value for the kinematic viscosity (ν). The relationship, uD/ν gives the Reynolds resistant rock Any rock that has re- number (named for the Irish engineer and sisted weathering and erosion better than physicist Osborne Reynolds (1842–1912). surrounding rocks because it is relatively It is an important number in laboratory harder. Such rocks may form higher simulations of prototype flow conditions, ground (see residual hill). because ideally the Reynolds number for

294 ria scale hydraulic model flow should be the the groundmass. Phenocrysts of sodic pla- same as for the prototype, i.e. to maintain gioclase and quartz are also common, the scale relationships, the small model size latter often markedly anhedral owing to re- has to be compensated by increasing the sorption. The dominant mafic mineral is flow rates in the model. biotite, although hornblende and clinopyroxene also occur. RF See representative fraction.. In dacites, sodic plagioclase is dominant over sanidine and, together with quartz, rheidity /ree-id-ă-tee/ Deformation of a occurs as phenocrysts. With a decrease in substance by plastic flow as a result of an the amount of quartz, rhyodacites and applied stress over a long period. dacites pass into LATITES and ANDESITES but the relative proportions of minerals are dif- rheomorphism /ree-ŏ-mor-fiz-ăm/ The ficult to assess in fine-grained volcanic process in which a rock changes composi- rocks and classification is often based upon tion while it flows. It may or may not result chemical analyses. Rhyolitic and andesitic from intense heat or pressure. lavas constitute the calc-alkaline volcanic suite, which is characteristic of island arcs rhodochrosite A pink, gray, or brown and orogenic regions. mineral form of manganese carbonate, Strongly alkaline rhyolites, rich in MnCO3. It crystallizes in the hexagonal sodium and poor in aluminum, are found system, and occurs in masses in hydrother- in continental areas, particularly those sub- mal veins. It is used as a source of man- jected to rift movements, and are the vol- ganese and as a semiprecious gemstone. canic equivalents of alkali granites. These lavas contain anorthoclase feldspar and rhodonite /roh-dŏ-nÿt/ A pink or soda PYRIBOLES. An arbitrary division at brownish translucent mineral form of 12.5% femic constituents separates pantel- manganese calcium silicate, (Mn,Ca)SiO3. lerites from the more leucocratic comen- It crystallizes in the triclinic system, and oc- dites. With a decrease in the amount of curs in metamorphic rocks as granular quartz, alkali rhyolites pass via quartz-tra- masses containing veins of black man- chytes into TRACHYTES. ganese dioxide. It is sometimes used for Many lavas of rhyolitic composition making ornaments. occur in the glassy state, as OBSIDIAN and pitchstone. Obsidian is a typically black rhombochasm /rom-bŏ-klaz-ăm/ A par- natural glass with a conchoidal fracture allel-sided break in the Earth’s continental but pitchstone has a dull appearance. crust, the break being filled by oceanic Whereas obsidian contains very little crust. The fracture is thought to be the re- water, pitchstone may contain up to 10% sult of SEA-FLOOR SPREADING. and owes its appearance to secondary hydration and devitrification. Most volcanic rhumb line /rum/ (loxodrome) A line of glasses contain crystallites; many contain constant bearing, used in navigation, cut- spherulites and exhibit perlitic cracking. ting all meridians at the same angle. Com- Rhyolitic lavas are often flow-banded pare great circle. and it is sometimes difficult to distinguish such rocks from ignimbrites with para- rhyodacite /rÿ-od-ă-sÿt/ See rhyolite. taxitic structure. See pyroclastic rock. rhyolite /rÿ-ŏ-lÿt/ A fine-grained volcanic rhythmite /rith-mÿt/ A sedimentary de- rock. Rhyolites, rhyodacites, and dacites posit that has alternate layers of coarse- are the volcanic equivalents of GRANITES, grained light and fine-textured dark adamellites, and granodiorites respectively. material. Most were formed on the beds of Rhyolites usually contain sanidine, the coldwater lakes. See varve. high-temperature form of potassium feldspar, both as phenocrysts and microlites in ria /ree-ă/A DROWNED VALLEY eroded by

295 richterite subaerial (nonglacial) processes at a time favoring tension in the Earth’s crust, com- when the sea level was lower than it now is. pression, or the cracking of a crustal dome Like river valleys of the present, rias have a along the crest. There is often associated V-shaped cross section, which deepens sea- volcanic activity along the sides of the val- ward and narrows inland, possibly bifur- ley floor. Famous rift valleys include the cating. Former MONADNOCKS may remain East African system, extending more than as small islands. Rias are produced wher- 4000 km from Syria to the Zambezi, and ever a dissected area of hills and valleys, the Rhine rift valley between Mainz and perpendicular to the coastline, has been Basle. See also graben. submerged as a result of a postglacial rise in sea level. Erosion of the projecting inter- rigidity modulus The ratio of stress to fluves eventually reduces the indentation of strain when the stress is a shear. This is cal- such coasts. culated by dividing the tangential force per unit area by the angular deformation. richterite /rik-ter-ÿt/ A monoclinic AM- PHIBOLE. rill A minute ephemeral channel at the head of drainage systems, forming at the Richter scale /rik-ter/ A logarithmic point where unconfined sheet wash be- scale that was devised by the American comes concentrated into definite channels. seismologist Charles Francis Richter (1900– This usually occurs on the lower concave 85) in 1935 for comparing the magnitude portions of slopes, because there the vol- of EARTHQUAKES. Although originally de- ume of sheet flow has built up to a suffi- vised for Californian earthquakes, it has cient amount for channeling to occur. Rills since been adopted for worldwide use. It is are said to be responsible for shaping the based on the amplitude of seismic waves concavity of the basal part of the slope. recorded by seismographs at a number of Downslope they run into more permanent stations, with adjustments made for dis- gullies, which constitute the smallest tribu- tances between the seismographs and the taries of drainage systems. Rills carry earthquake epicenter. The scale has no water only in storms. See also gully ero- upper limit but earthquakes of magnitude sion; overland flow. 9 and above are rare and extremely destructive. rillerkarren See karren. ridge See mid-ocean ridge. rill mark A small channel found on beaches, aligned more or less up and down ridge of high pressure A long narrow the beach. The channels develop because of region of high atmospheric pressure that the seaward flow of water that has perco- extends from an ANTICYCLONE. It is associ- lated into the upper parts of the foreshore ated with short-lived anticyclonic weather, during the period around mid- to high- with generally short dry sunny spells. It is water or just after, or during the action of sometimes known as a wedge. STORM waves with powerful swash action between low- to mid-water. As the tide re- riebeckite /ree-bek-ÿt/ A monoclinic treats, or as the waves subside, the infil- AMPHIBOLE. trated water may flow out on to the beach face and cut the rills. Once developed, the rift valley (taphrogeosyncline) A struc- rills tend to channel this flow seaward and tural and topographical feature formed possibly enlarge themselves. The seawater within the Earth’s crust consisting of a may be reinforced by freshwater seepages steep-sided flat-bottomed valley, the rocks from landward, for instance where springs of the valley floor having subsided between issue from a rock platform beneath the two parallel faults or two parallel series of beach. step faults. There are conflicting views as to the origin of rift valleys, the main ones rime A deposit of white ice crystals

296 river capture formed by the freezing of supercooled rection. They can be used, therefore, to in- water droplets onto surfaces below 0°C. It dicate the existence of and something of grows on the windward side of the surface, the nature of wind and water currents. Rip- especially on sharp edges, by impaction as ples tend to be larger in more exposed the droplets drift past in the wind. areas and in deeper water offshore. Very large ripples are usually termed megarip- ring complex A circular igneous intru- ples or SAND WAVES. Dune areas above sive body of rock including both concentric high-water mark may display numerous dikes and cone sheets. ripple features. ring dike See concentric dike. rise A broad elevated area of the sea floor, similar to a MID-OCEAN RIDGE but rip A turbulent area of water where tidal lacking a median rift valley. currents meet head-on, where waves meet a current flowing in the opposite direction, river A channeled flow of water running where a tidal current suddenly enters shal- downhill under the force of gravity. Most low water, or in a RIP CURRENT. rivers flow into the sea, although some enter lakes and a few flow into desert rip current A strong well-defined cur- areas, where they dry up. The course of a rent that flows away from the shore. Such river from start to finish is called its profile. currents are wave-induced, providing the The source is usually on high ground, at a mechanism whereby water piled up at the spring or marsh, or where the water table coast because of mass transport, and possi- reaches the surface. It then generally flows bly wind, is returned seaward. The current through mountains, typically through speeds are usually fairly high. They are rocky areas as a turbulent flow along nar- usually manifested by a fairly narrow zone row, steep-sided valleys. There may be wa- of agitated water and by an accentuated terfalls or cascades. The river then amount of sandy material in suspension. broadens along its middle course through They consist of feeder currents that flow an upland area. As the valley widens, me- inshore of the breakers and more or less anders may form; there may also be bluffs. parallel to the shore as longshore currents, Finally the lower course of the river crosses the neck, where the current is concentrated a wide floodplain, where large meanders into a RIP and flows seaward through the may become cut off as oxbow lakes. The surf zone, and the expanding rip-head widest part of a large river as it enters the where the rip loses impetus within or be- sea is an estuary, which may include a yond the surf zone. They account for many delta. drownings. river bar A build-up of waterborne silt ripple bedding Small-scale cross-bed- (alluvium) in the channel, along the banks ding resulting from the rapid deposition of or at the mouth of a river. It is generally sand in the form of ripples. exposed at low tide. See also braided stream. ripple mark The most common minor beach morphological form, consisting of river basin The area from which water fairly regular and generally small ridges is collected by a river. See catchment area; formed in sediment (usually sand, but pos- drainage basin. sibly shell and fine gravel) on a river bed, in the inter-tidal zone, or on the seabed sea- river capture The diversion of the headward of low-water mark. Fossil ripples or streams of a river into an adjacent river ripples formed by the wind may occur with more powerful headward erosion. It above high-water mark. Ripples are caused cuts back its valley into that of the weaker by water or wind flow, and are aligned river, thus enlarging its drainage basin at more or less perpendicularly to the flow di- the expense of the other. Rivers whose

297 river cliff headstreams have been captured often flow direction of ice movement as MISFIT RIVERS in valleys that are too large d, the oo to have been eroded by the present capac- sm ed irregular d, iat ity of the river. The ELBOW OF CAPTURE is he str freeze-thaw lis nd the bend at which the captured head- po a surface streams are diverted. river cliff See bluff. Roche moutonée river profile A section of a river along its length from source to mouth. See river. valley face has a broken shattered appearance as a result of freeze-thaw processes, river regime The variations in the vol- which are assisted by increased pressure- ume of water in a river that take place with melting induced by the presence of the rock the changing seasons, which can be plotted mass. as a graph. In temperate regions, the great- The term is extremely old and is widely est flow is usually in the winter (corre- used for similar landforms of greatly con- sponding to the greatest precipitation). In trasting size. As a more suitable alterna- monsoon and savanna regions, it is usually tive, the term stoss and lee topography has in the summer (the rainy season); a river been suggested. may even dry up in the winter dry season. rock An assemblage of MINERALS, gener- river terrace (stream terrace) A bench- ally cemented or consolidated together. It like landform bordering many rivers, ele- may contain only one type of mineral, or vated above the current streams and many. The surface of the Earth is made up ending in scarps suspended on the valley of rocks. They are classified into three side. These terraces constitute all that re- major groups: IGNEOUS ROCK, SEDIMENTARY mains today of past abandoned valley ROCK, and METAMORPHIC ROCK. floors formed when the river was flowing at a higher level than today. Most com- rock crystal A colorless transparent va- monly they are paired on each side of the riety of QUARTZ. See silica minerals. valley, the product of REJUVENATION of the river or climatic changes (via their influ- See geochemical cycle. ence on stream discharge, base level, and rock cycle sediment load). They consist of a rock-cut bench, with or without a thin veneer of al- rockfall A type of landslide involving luvium. purely dry materials. Basal sapping or some other type of trigger mechanism will Roaring Forties The region between institute an instantaneous downslope 40°S and 50°S where the westerlies blow movement, leaving a pock-marked slope at with great regularity across the open the source and a confused pile of talus at ocean. The frequent depressions and the foot. See also mass movement. strong winds produce a stormy climate unfavorable to shipping. rock flour Fine powdery rock produced by the abrading action of a glacier, which roche moutonnée /rosh-moo-tonn-ay/ grinds particles off the rock it flows past. A glacial erosion form found in areas of Most of the rock flour is transported by low relief and consisting of a resistant rock subglacial streams, which have a milky or mass, which has been eroded to comprise a inky appearance as a result. gently sloping up-valley face and a steep down-valley face. The up-valley face is rock pedestal See pedestal rock. smoothed, polished, and striated, the effects of glacial abrasion, while the down- rock salt See halite.

298 rutile roller A large wave rolling in onto an ex- of CORUNDUM (the color is caused by posed coast, often following a storm, in- chromium impurities). It crystallizes in the creasing its height before breaking in a trigonal system, and has been synthesized. rather destructive manner. Such waves make up a series of long-crested forms. ruby copper See cuprite. They characterize such coasts as the South Atlantic, the South Indian Ocean, and the rudaceous /roo-day-shŭs/ Describing a West Indian islands. CLASTIC sedimentary deposit or rock in which the constituent fragments are of rel- roof pendant A small downward pro- atively large size. It is formed of GRAVEL, jection of country rock into an underlying i.e. the clasts are greater than 2 mm in di- igneous body. See also cupola. ameter. ropy lava Volcanic lava that has solidi- rudite /roo-dÿt/ A type of sedimentary fied in long strands. A thin crust forms on rock that consists of coarse grains (more the molten lava while it is still moving; the than 2 mm across). The category includes liquid center continues to move and form BRECCIA and CONGLOMERATE. the linear structure. See also pahoehoe. rundkarren See karren. rose quartz A pink QUARTZ sometimes used as a semiprecious gemstone. runnel A depression extending along a beach perpendicular to the direction of Rossby wave /ross-bee/ See long wave. wave approach. Runnels separate and lie parallel with BEACH RIDGES. rotational slip The semicircular movement of a mass of loose material, such as runoff That part of total precipitation ice, rock, or soil, down a concave slope. left to flow into rivers after evaporation and transpiration by plants have taken rotation of the Earth The turning of place. It has several components: rain the Earth on its axis, which it completes falling into the channel of the river, surface every 24 hours. As different parts of the runoff, rainfall that soaks into the soil Earth face the Sun, day and night occur, moving laterally toward the river to reach whose lengths vary because of the 2.5° tilt it as ‘interflow’, and water that percolates of the Earth’s axis. At the EQUINOXES, there through the soil to the water table, feeding are 12 hours daylight and 12 hours dark- steadily all the year round to the stream as ness everywhere. groundwater. Over the world as a whole, around one fifth of rainfall becomes roughness length A measure used in runoff, but regional variations are very micrometeorological studies as an indica- great, dependent on climate, the nature of tor of degree of roughness of a surface to vegetation and soil, whether the rain comes airflow. It is determined by extrapolating in severe storms or widespread gentle the observed relationship between wind showers, etc. speed and height to the point where the wind speed becomes zero. Its value is about rutile /roo-teel, -tÿl/ A brown, violet, or one tenth that of the true height of the black tetragonal mineral. It is the com- roughness elements, i.e. for a lawn of thick- monest polymorph of titanium dioxide, ness 1 cm it is about 0.1 cm. TiO2, and is widespread as an accessory mineral in igneous and metamorphic rubellite /roo-bell-ÿt/ A pink or red form rocks. It also occurs in high-temperature of TOURMALINE, used as a semiprecious veins with quartz and apatite and as an al- gemstone. teration product of sphene and ilmenite. It is an important source of titanium. See also ruby A deep red transparent gem variety anatase; brookite.

299 S

sabkha /sab-kă/ A salt flat that has a sur- and Senegal in the west to Ethiopia and face encrusted with HALITE; there are also Sudan in the east. Overgrazing has re- calcium and magnesium salts in the de- moved much of the vegetation, with conse- posit. It runs along the coast just inland quent soil erosion. DESERTIFICATION (from and is periodically flooded by the sea. the Sahara) is occurring in the north. saccharoidal /sak-ă-roi-dăl/ Describing salic /sal-ik/ Denoting the silicon- and a fine- to medium-grained granular rock aluminum-rich minerals of the CIPW nor- resembling sugar. mative classification (see norm). Compare femic. saddle A broad shallow depresion in a mountain ridge forming a pass. salina /să-lÿ-nă/ A saltpan, named after those that occur in semiarid areas of Spain. saddle reef A deposit of minerals that See playa. occurs at the top of an anticline fold. In some places in Australia and Canada, sad- saline /say-lÿn/ Describing any solution dle reefs in folded slate beds contain gold. that contains a salt, particularly common salt, sodium chloride, NaCl. The salinity of saeter /see-ter/ An area of pasture in the seawater, for example, is 35 000 parts per Norwegian mountains, above the treeline, million (of which 85% is sodium chloride). used for the summer grazing of cattle. salinity The extent to which water con- Saffir–Simpson Hurricane Scale A tains dissolved salts. Together with tem- scale of one to five based on the intensity of perature and oxygen content, it is one of a hurricane at a particular time that can be the fundamental properties of sea water. A used to estimate the potential damage and wide range of salinity occurs in the various flooding if the hurricane makes landfall. seas of the world: normal sea water has a All winds in the scale use the US one- salinity of approximately 34.33 grams per minute average. During 2005, the most ac- thousand grams (3.433%), although it tive Atlantic hurricane season on record, may be as high as 4.0% (in the Red Sea and three hurricanes (Katrina, Rita, and Persian Gulf) or less than 3.3% (at high lat- Wilma) reached category five intensity. itudes). The differences arise largely as a re- Category five hurricanes to have made sult of variations in the rate of evaporation. landfall in the USA are the Labor Day Hur- Increased salinity raises the relative den- ricane of 1935, Camille, in 1969, and An- sity, lowers the freezing point, and greatly drew, in 1992. The scale was developed in influences marine ecosystems to the extent the early 1970s by the Americans Herbert that fauna and flora have to adapt to saline Saffir (1917–), a consulting engineer, and conditions. A knowledge of salinity is also Robert Simpson (1912–), then director of important in understanding the circulation the US National Hurricane Center. of ocean water.

Sahel /să-hel/ A large region to the south salinization /sal-ă-nă-zay-shŏn/ The ac- of the Sahara, extending from Mauritania cumulation in water or soil of salts of mag-

300 sand nesium, potassium, and sodium. It often layer of anhydrite or gypsum (calcium sul- occurs in arid and semiarid regions, where fate) or calcite (calcium carbonate), possi- the rate of evaporation is greater than the bly containing some sulfur. The rocks rate of precipitation. Plants cannot grow in below the dome are deformed, and may the salty soil and in this way salinization hold deposits of oil and natural gas. contributes to DESERTIFICATION. salt-earth podzol See solod. salite /say-lÿt, sal-ÿt/ A monoclinic PY- ROXENE. salt flat A level salt surface constituting the bed of a former salt lake. salt 1. Chemically, a compound formed when an acid is neutralized by a base. Most salt lake A highly saline lake formed in salts are crystalline ionic compounds, and an INLAND BASIN. most minerals are composed of salts. 2. See halite. salt marsh See marsh.

saltpeter /sawlt-pee-ter/ See niter. direction of paths of wind/water flow individual grains San Andreas fault /san-an-dray-ăs/A 1125-km transcurrent fault that runs through California. It is on the boundary between two lithospheric plates (the Pacific ground surface/river bed and the N American plates) that intermittently move laterally, producing friction Saltation that causes earthquakes. /sal-tay-shŏn/ A mechanism of saltation sand Particles of mineral or rock with a sediment transport, whereby individual size range of 0.06–2.00 mm in diameter grains move by bouncing off the surface of (i.e. between silt and gravel): the land or bed. It is the most common 0.6–2.0 mm coarse sand. form of transport by wind. It is much less 0.2–0.6 mm medium sand. significant in rivers because of the viscosity 0.06–0.2 mm fine sand. of water as compared with air; the ‘sticki- Rocks formed from sediment in this range ness’ of water slows down the rise of the grain and cushions its fall, so the height of are known as sandstones. saltation is but a few grain diameters as According to the British soldier, ex- compared with several thousand grain di- plorer, and scientist Ralph A. Bagnold ameters in air. The initial rise of the grain (1896–1990) in 1941, sand is that material is near vertical, followed by a gentle down- lying above a lower limit of material capa- ward flight in the direction of wind or ble of carriage by suspension (mostly dust) stream flow, terminated by impact again and below an upper limit of material inca- with the surface at an angle of 10–16°. In pable of movement either by direct wind air, this impact may cause the grain itself to force or by the impact of falling grains. rebound, or dislodge another grain on the Windblown sands are said to be mostly 0.3 surface, which will then saltate. In water, to 0.15 mm in diameter. the lifting force is eddy turbulence, which Most sands are predominantly made of temporarily overcomes the weight of the QUARTZ, other material being too easily grain, imparting an upward movement. eroded to survive for long. The quartz is derived from the weathering of quartz- salt dome A circular domelike structure bearing rocks, subsequently reduced in size formed by the upward movement of a col- by water or ice abrasion. Desert sand umn of salt (HALITE), generally below strata grains are more rounded than fluvial or of sedimentary rock. It may be capped by a marine types.

301 sand bank sand bank A bank of sand and other sand stream A broad tract of moving sediments that occurs in estuaries and sediment (largely of sand grade) seaward along the open coast where the tidal of low-water mark, possibly tens of kilo- streams tend to flow in a rectilinear fashion meters in width. Sand streams tend to ex- and where transportable sediments are tend over considerable distances, roughly fairly abundant. The form of the bank and parallel to the coast. The streams are evi- the pattern of a group of banks often re- dent from various field data, including flect the nature of tidal flow. The sur- morphological forms such as SAND RIB- rounding or included channels may be of BONS, SAND BANKS, and SAND WAVES, infor- the ebb or the flood type, depending on the mation relating to sediment grain-size relative dominance of one or other tidal distribution, and a knowledge of waves stream. The banks tend either to be linear and tidal streams. Sand streams may occur or parabolic in shape. anywhere on shelf areas, but are especially developed near to coasts when the tidal sand dune See dune. currents flow in rectilinear fashion. One such stream moves northward out of the sand ribbon A submarine morphologi- southern bight of the North Sea, toward cal form of linear shape that, like a sand the Netherlands and Germany. wave, occurs where tidal streams are suitable for its formation and where trans- sandur See outwash plain. portable sediment is sufficiently abundant. The ribbons occur within certain estuaries sand volcano (mud volcano) A sedimentary structure produced in poorly con- and along parts of the open coast, seaward solidated sands and clays that have a high of low-water mark. They are often only a water content. As a result of shaking, a few centimeters thick, but may be up to lower layer of sediment will be injected up 100 m wide and extend for several kilome- and through the overlying bed. When this ters. The general trend of sand ribbons off process is completed the lower bed builds the S coasts of Britain is parallel to both the up as a small cone above the upper bed. coast and the directions of the strongest Where the process does not reach comple- tidal streams. Where surface streams are tion or the overlying bed is very thick, less than 1 knot, the boundaries of the rib- FLAME STRUCTURES are produced. bons tend to be somewhat obscure; in contrast, where surface tidal streams attain sand wave (megaripple) A morphologi- more than 1 knot, the boundaries become cal form that resembles a desert dune and more strongly defined. Such morphological has been called an underwater dune. The information is clear from high-precision sediment is usually of sand grade. The echo sounding. structures are created by tidal flow, and, if large, may have many smaller sand waves sand shadow See eolian form. superimposed upon them. They often occur on the surface of SAND RIBBONS. They sand sheet See eolian form. tend to be orientated perpendicular to the main tidal flow direction. Most display a sandstone A lithified ARENACEOUS de- measure of asymmetry, which enables the posit. It is a CLASTIC sedimentary rock com- net direction of sediment transport to be posed of grains between 0.0625 mm and 2 determined, the steepest face indicating the mm in diameter. The particles may be direction of travel. A small difference, per- bound by a secondary cement such as CAL- haps only 0.1 knot, in the flood and ebb CITE or various iron minerals, or be welded peak velocities, is sufficient to cause asym- together by pressure. A great variety of metry of form and to make the sand waves sandstones are known, and they can be migrate in one or other direction. The classified by both mineralogical and tex- crests of sand waves may be straight or sin- tural characteristics. uous; their height, as in the case of the

302 satellite southern North Sea, may be as much as 15 made up mainly of the remains of algae, m. In contrast, off Florida, many of the with some mineral fragments. Sapropels sand waves are less than 1 m in height. form in anaerobic conditions at the bot- Some sand waves are formed in the rela- toms of lakes or shallow seas, and when tively deep water near the shelf edge, for compacted change into shale containing example off SW England, where their coal, bitumen, or oil (see oil shale). heights vary between 8 and 12 m, and their average length is some 850 m. saprotroph /sap-rŏ-troff/ (saprobe; sapro- phyte) An organism that feeds on dead sanidine /san-ă-deen/ A potassic alkali or decaying organic matter (plant or ani- FELDSPAR. mal). The best-known saprotrophs are fungi. Sanson–Flamsteed projection /san-sŏn flam-steed/ A modified CONICAL PROJEC- SAR See radar. TION and a special case of Bonne’s projection. The projection is named for the sardonyx /sar-don-iks/ A type of CHAL- French cartographer Nicolas Sanson CEDONY that has alternate parallel bands of (1600–67) and the English astronomer reddish-brown and white. It is used as a John Flamsteed (1646–1719). Both the semiprecious gemstone. central meridian and the Equator (standard parallel) are straight lines that are satellite (artificial satellite) A man- truly divided and at right angles to each made spacecaft that orbits the Earth, other. All other parallels are horizontal moon, or another planet. Since 1957, nu- straight lines, equally divided and equal merous satellites have been placed in Earth distances apart. This is a sinusoidal map orbit. Some of these have had specific me- projection because the meridians are sine teorological purposes and tremendous curves drawn through predetermined points amounts of new information have ap- on the parallels. Although the shapes are peared as a result of these instruments. distorted away from the center of the pro- Continuous records are now received of jection, it is an equal-area projection and cloud patterns, and infrared photographs therefore commonly used in atlases for and experiments have shown that it is pos- areas astride the Equator, e.g. Africa. sible to obtain vertical temperature profiles. Geostationary satellites, which are saponite /sap-ŏ-nÿt/ A magnesium-rich orbiting at the same rate as the Earth and claylike mineral related to montmoril- are therefore providing a continuous lonite. See clay minerals. record of the same portion of the atmosphere and surface, can determine the wind sapphire A colored transparent variety field at cloud level from the varying cloud of CORUNDUM (the color is caused by traces patterns, in addition to the other facilities of impurities such as oxides of chromium, offered by the normal satellite. The move- cobalt, and titanium), valued as a precious ments of tropical cyclones (hurricanes, ty- gemstone. It may be deep blue (the most phoons) can be tracked from their prized), pale blue, green, or yellow. It crys- formation enabling advisories and warn- tallizes in the trigonal system, and has been ings to be issued to lands in their predicted synthesized. path. See Landsat. Developments in satellite imagery and saprolite /sap-rŏ-lÿt/ The part of the data have contributed greatly to the obser- weathering profile that, although weath- vation of features on, above, and below the ered, remains in situ and retains such orig- Earth’s surface and the information thus inal structures of the parent rock as obtained can be incorporated into GEO- jointing, banding, and veining. GRAPHICAL INFORMATION SYSTEMS (GIS). Originally restricted to simple photo- sapropel /sap-rŏ-pel/ A loose deposit graphs, satellite imagery can now provide

303 satin spar widely diverse imaging at a variety of resolutions. High-resolution satellite imagery, 40 in which images with 1-meter resolutions are now available in the public domain, has widespread applications. For example, in cartography it is used in updating and cre- 30 ating maps and navigation charts; other uses include locating potential sites in mineral exploration, showing areas of deforestation, and locating pollution and its 20 sources. Multispectral satellite imagery can be used to reveal subtle topographic and

vegetation features. Infrared satellite im- vapor pressure/mb agery, which depicts the intensity of in- 10 frared radiation emitted from Earth, can be applied in the study of volcanoes. Satellite supercooled imagery has been used to study variations water in ocean characteristics over wide areas; ice

| | | | | || 0 for example, data from AVHRR instru- –20 –10 0 10 20 30 ments (high-resolution visible and infrared temperature/°C light radiometers) on such satellites as the NOAA Polar Orbiting Environmental Saturation vapor pressure curve Satellite series enable sea surface temperature and cloud patterns to be mapped. In 1999 the first of a series of TERRA satel- saturation 1. The condition in which a lites to monitor effects of human activity sample of moist air is in equilibrium with on the global environment was launched; an open water surface at the same temper- measurements recorded are to include that ature and pressure. In this state there is a of pollution in the lower atmosphere. See balance between the number of molecules also Galileo; Global Positioning System. leaving the water surface and the number returning from air to water. The point of satin spar A type of GYPSUM (calcium saturation can be reached by adding mois- sulfate) that has translucent white fibrous ture to a system or cooling it. The capacity crystals. The same name is also sometimes of air to hold water is largely determined given to a type of calcite (calcium carbon- by temperature as shown in the diagram of ate) of similar appearance. the saturation vapor pressure curve. 2. See silica saturation. saturated adiabatic lapse rate The rate of cooling of a parcel of saturated air Saurischia /sor-iss-kee-ă/ An order of as it rises. Because the amount of latent dinosaurs distinguished from the ornithis- heat released depends upon air tempera- chians by having a pelvis constructed on ture, the lapse rate is not a constant. At the typical reptilian plan. These dinosaurs high temperatures the rate is slow because were primarily carnivorous and some grew much latent heat is released on condensa- to a very large size: Tyrannosaurus, nearly tion, whereas when it is very cold little heat 7 m high, was the largest carnivorous land is released and its value approaches the DRY animal known and the herbivorous species ADIABATIC LAPSE RATE. See also environ- were even larger, Diplodocus, for example, mental lapse rate. reaching a length of nearly 28 m. Like other dinosaurs the group became extinct saturated rock An igneous rock that at the end of the Cretaceous period. See contains neither quartz nor feldspathoids. K/T boundary event. Compare Ornithis- See silica saturation. chia.

304 schlieren saussuritization /saw-sŭ-ră-tă-zay-shŏn/ water vapor molecules, which break up or The alteration of plagioclase to a fine- scatter its unidirectional beam to give dif- grained assemblage of albite, epidote, fuse radiation. This scattering is selective in calcite, and sericite during low-grade meta- that blue wavelengths are scattered more morphism and hydrothermal processes. easily than the longer red wavelengths and so diffuse radiation from the sky appears savanna A region of tropical grassland, blue. The reduction of blue light from the occurring mostly in a band between the direct beam results in the Sun’s disk ap- tropical forests and the deserts (between 5° pearing red or yellow. and 20° north and south of the Equator). Larger dust particles scatter light with- There are few trees, except for solitary out any wavelength dependence. Hazy drought-resistant acacias, baobabs, and conditions with high dust concentrations eucalyptuses. See also savanna climate. therefore give a whitish hue to the sky. savanna climate A tropical climate scheelite /shay-lÿt, shee-/ A white, yel- with a seasonal rainfall regime. It has sim- low, green, or brown mineral form of cal- ilar properties to the monsoon climate, the cium tungstate, CaWO4. It crystallizes in rain being associated with the INTERTROPI- the tetragonal system, and occurs in quartz CAL CONVERGENCE ZONE, although the sea- veins and as deposits in contact metamor- sonal reversal of winds is not necessarily as phic rocks. It is used as a source of tung- well developed. The name is derived from sten. savanna vegetation characterized by tall grasses. schiller /shil-er/ A striking play of colors exhibited by some minerals, particularly scablands A type of highly eroded ter- feldspars and pyroxenes, in hand speci- rain that was formed by sudden extensive men. This is due to very small inclusions of flooding after the melting of glaciers and iron ore, orientated along certain cleavages ice sheets. There is little or no soil on the or partings, which reflect the light when bare rock surface, and so very few plants the crystal is rotated. See diallage. grow. A well-known example occurs in the northwestern USA. schist /shist/ A strongly foliated coarse- grained rock in which mica minerals are scapolite /skap-ŏ-lÿt/ A member of abundant and their subparallel orientation a range of minerals that vary in composi- produces a marked SCHISTOSITY. The schis- tion between Na4(Al3Si9O24)Cl and tosity is accentuated by the segregation of Ca4(Al6Si6O24)CO3, mainly by the substi- minerals into thin layers alternately rich in tution NaSi↔CaAl. They are found in a micaceous minerals and quartz/feldspar. wide variety of metamorphic rocks. Schists in general are the product of regional metamorphism. scar A short steep slope of generally bare craggy rock, usually in a limestone region. schistosity /shis-toss-ă-tee/ A variety of SLATY CLEAVAGE that is well developed in scarp (escarpment) A steep clifflike SCHISTS and to a lesser extent in GNEISSES. slope often of considerable size, which rises Platy minerals such as micas, amphiboles, above the surrounding land surface. Such and chlorites recrystallize or crystallize structures result from faulting (see fault perpendicular to the direction of maxi- scarp; fault-line scarp) or the differential mum stress. All traces of the original bed- erosion of gently inclined strata (see ding are destroyed and the rock readily cuesta). splits along schistosity planes. See also foliation. scattering In the atmosphere, a process in which the electromagnetic radiation schlieren /shleer-ĕn/ Streaked-out patches from the Sun is affected by air, dust, and found in igneous rocks, representing soft-

305 schorl ened xenolithic material drawn out along scrub A type of vegetation consisting of the direction of flow. low shrubs and small trees that grows in regions that have unreliable rainfall, such as schorl /shorl/ A black variety of TOUR- the margins of deserts. Examples include MALINE. the chaco of Paraguay, CHAPARRAL of North America, the MALLEE of Australia, scirocco /să-rok-oh/ See sirocco. and the MAQUIS of the Mediterranean area. scolecite /skol-ĕ-sÿt, skoh-lĕ-/ A white scud See fractostratus. mineral form of hydrated calcium aluminum silicate, Ca(Al2Si3O10).3H2O. It Scyphozoa /sÿ-fo-zoh-ă/ The class of the crystallizes in the monoclinic system as phylum CNIDARIA that includes the jelly- groups of radiating crystals. It is a member fish. Because they possess no hard parts of the ZEOLITE group of minerals. they are rare as fossils, although remains attributable to them have been found in scoria cone See cinder cone. rocks as old as the Precambrian. scoriae /skor-ee-ee, skoh-ree-ee/ See py- sea breeze A local wind resulting from roclastic rock. differential surface heating between land and sea. It blows only during the day when Scotch mist A mixture of thick cloud land areas heat up relative to the sea to and heavy drizzle, which derives its name produce a weak thermal pressure gradient from frequent occurrences in the hillier from sea to land. Initially the wind blows parts of Scotland. It is formed in maritime down the pressure gradient force at right tropical air when uplift over the hills pro- angles to the coast, but as the system con- duces large amounts of condensation and tinues the Coriolis effect comes into play, cloud droplets, some of which are large causing a veering of the wind until eventu- enough to give drizzle. It can occur any- ally the breeze may be almost parallel to where that the conditions of weak uplift of the coast. It can extend up to 80 km or almost saturated but stable air are fulfilled. even more inland and is separated from the warmer air by a sea-breeze front, rather scour Localized erosion, for example like a cold front. Above the surface sea tidal scour in estuaries, removing sediment breeze is a returning circulation from land periodically and depositing it again at an- to sea maintaining continuity. See also land other stage; in rivers periodic bed scour oc- breeze. curs during periods of high flow, compensated at low flow by infill of de- sea-floor spreading A concept first for- pressions, a process sometimes called mulated in the 1960s by the American geo- scour-and-fill. In connection with glacial physicist Harry H. Hess (1906–69), by erosion, scour refers to the etching and pol- which a satisfactory mechanism for CONTI- ishing of solid rocks by rock material in- NENTAL DRIFT was found. This theory corporated in the ice. showed that the ocean floor is one of the youngest and most active parts of the scree An accumulation of loose rock Earth’s surface. Magma rising from the fragments on a hillside or at the base of a Earth’s mantle reaches the surface along weathered cliff. Scree that slips downhill the MID-OCEAN RIDGES. The magma cools (when it is wet) becomes slightly sorted, and becomes part of the Earth’s crust. Re- with the larger pieces at the bottom. peated rifting of the area accompanied by the addition of further magma causes the screw dislocation A type of defect in a older material to be displaced sideways. crystal lattice that results from the twisting Because the Earth is not expanding in di- of one part of the lattice relative to the ameter, the generation of new sea floor is other. compensated by its destruction and reab-

306 sea valley sorption into the mantle at destructive steep and they display relatively small cir- plate boundaries. As the oceans spread the cular or elliptic summit areas. On a global continents are moved across the surface of scale, seamounts are very numerous and the Earth as part of a large LITHOSPHERIC widespread, being especially numerous in PLATE. The ocean continues to expand until the Pacific Ocean. Their summits lie in the movement of other plates causes its rate depths of between 200 and 2500 m, al- of destruction to exceed its rate of con- though the majority of them lie between struction. At that time the ocean begins to 1000 and 2000 m. Many stand on the CON- close. Examples of spreading oceans are TINENTAL RISE. Although some seamounts the Red Sea and Atlantic Ocean; an exam- are found clustered in roughly linear ple of a closing ocean is the Mediterranean groups, many others rise from the sea floor Sea. Spreading rates are quite low, being in quite independently of their immediate the order of 4 cm per year but varying from neighbors, the floor between them often place to place along a mid-ocean ridge, and being relatively flat. Three or more from ocean to ocean. See also plate tecton- seamounts in a line comprise a seamount ics. chain, three or more not in a line make up a seamount group, and three or more sited sea fog Any fog over the sea, whatever on a rise or ridge comprise a seamount its origin. It is most frequent over cold sea range. See also guyot. surfaces where contact cooling can lead to saturation of the overlying moist air. An sea roke See haar. exception to this is ARCTIC SEA SMOKE, which is an evaporational fog. season A subdivision of the year consisting of a period of supposedly uniform or sea-fret See haar. similar climatic characteristics. The length and properties of the seasons vary across sea level The average level of the sea the globe. In tropical areas, temperatures surface at a given time and in relation to a are fairly uniform throughout the year and chosen datum (see mean sea level). Because distinction is made between the wet and of the numerous irregularities and slopes dry seasons only. In temperate latitudes, present on the sea surface, an accurate the seasons are based on the equinoxes and measure of sea level is rather difficult. solstices, but for climatological purposes Apart from day-to-day variations, sea level the divisions are on a monthly basis: (in the has changed with time, sometimes showing N hemisphere) spring – March, April, May; a net upward trend, at other times showing summer – June, July, August; fall – Sep- the reverse. This has happened for many tember, October, November; winter – De- different reasons; among these are isostatic cember, January, February. These periods readjustment, geosynclinal sinking, local do have some average significance, but in- tectonic instability, geodetic changes, and dividual days may have weather conditions glacioeustatism. Hence, superimposed on appropriate to any season. In polar re- the short-term fluctuations of sea level due gions, the changeover from winter to sum- to tides, winds, etc., are longer-term fluctu- mer and vice versa is very sudden and there ations. Global sea level is closely related to is thus a two-season year as in the tropics. climate change; it has been estimated that sea levels are rising by about 1–2 mm per sea surge A large movement of water in year as a result of the reduction in ice the oceans, which generally causes large sheets and the thermal expansion of the waves and high tides. Sea surges may be oceans. See also base level. caused by strong winds associated with deep DEPRESSIONS, and can cause extensive seamount A submerged and isolated el- flooding of low-lying areas, especially if evation on the deep-sea floor. Seamounts they travel up an estuary. stand over 1000 m above the surrounding sea floor. Their slopes are comparatively sea valley An elongated depression cut

307 sea wave in the sea floor and having a valley-like in a rock as a result of the chemical alter- form. Such features display a variety of ation or breakdown of the original min- forms: SUBMARINE CANYONS are deeply in- eral. Compare primary mineral. cised and possess steep walls, whereas many others have a broad cross-sectional secondary reflection A signal received valley form, with relatively gently sloping at a receiver such as a geophone (hy- sides that contrast markedly with the steep drophone) from a particular reflecting rock canyon walls. Some valleys have a trough- horizon after it has been reversed in its di- like form, like that off the Mississippi Delta rection of propagation at least twice by or that off the Ganges Delta. Their floors other reflecting horizons, and therefore ar- are broad and flat. Some resemble land val- rives considerably later than a primary re- leys by having levée features along one or flection from that horizon which has had a both flanks (see levéed channel). Some are direct path of propagation. merely short gullies, sometimes arranged in a subparallel series. Eroded submarine val- secondary vegetation Plants that grow leys have been grouped into shelf channels in an area that has been cleared of its pri- and submarine canyons. The former group mary vegetation, for example following a may be further subdivided into glacial fire or human activity. troughs, tidal scour channels, and drowned river valleys. secondary wave (S wave; shear wave) A type of SEISMIC WAVE that travels through sea wave A wave in the generating area, the solid body of the Earth. It is propagated i.e. the area in which waves are being ac- by the oscillation of the particles at right tively formed. As the waves travel beyond angles to its direction of propagation. This this area they become longer-crested and type of wave cannot be transmitted by liq- more regular, being known as SWELL. Be- uids. See also primary wave. cause there may be present at any one time waves of quite different lengths (perhaps secular trend A climatic variation that ranging from a few centimeters to several can be observed during the period of in- hundred meters) and different heights, su- strumental records and indicates a persis- perimposed one upon the other, the sea’s tent tendency for the mean value to surface may present a very confused pic- increase or decrease. It is most easily deter- ture. The size of sea waves depends largely mined by smoothing the data series by on three variables: the speed of the wind, RUNNING MEANS. wind duration, and available fetch. See also wave. secular variation Changes in the properties of the Earth’s magnetic field over a secondary depression A DEPRESSION long period of time. that forms along a front within the general circulation of a primary depression. Ini- sediment Particulate material that has tially it is less intense than the parent low, been deposited in a fluid medium. The fluid but on many occasions it intensifies and concerned is mostly water, but eolian sedi- eventually incorporates the original de- ments are not uncommon. pression. sedimentary rock A rock formed by secondary enrichment The concentra- lithification (consolidation and compres- tion of valuable metals in the lower part of sion) of sediments laid down by wind or a vein as a result of the weathering of the water. The sediments may consist of fine upper part. Dissolved metals seep down- particles of preexisting rock, which may be ward and react with the lower-grade com- igneous, sedimentary, or metamorphic, or pounds, enriching them with metal content. be derived from plant or animal remains. The resulting rock strata may subsequently secondary mineral A mineral present be uplifted, tilted, folded or faulted. Sedi-

308 seismic array mentary rocks include CLASTIC rocks 50 m have led to deposition rates of up to formed of land sediments, including clay, 1 m per century. Synthetic seaweed, which silt, sand, and gravel; consolidated types is made of a special plastic material, has are sandstone and shale. Organic sedimen- been used in experiments to test its effec- tary rocks, usually lacking land sediment, tiveness in building up beaches. Spartina include LIMESTONE and DIATOMITE (com- traps marsh muds, marram grass traps posed of the fragmentary remains of sea dune sands, and mangroves collect estuar- creatures), COAL (derived from plants), and ine and tidal lagoon muds. coralline (derived from coral). Chemical sedimentary rocks are those formed by seiche /saysh/ A fluctuation in water chemical processes, and include the EVAP- level sometimes occurring in the open sea, ORITES and some ironstones. in semienclosed bays, harbors, lakes, and other water bodies. The period of oscilla- sedimentary structure A structure tion generally ranges from a few minutes to formed within sedimentary rocks during several hours. Small seiches may be a per- their deposition (primary structure) or manent characteristic of many harbor after deposition (secondary structure). basins. The period of oscillation is deter- Such structures include CROSS-BEDDING, mined by the resonance characteristics of FLUTE casts, BIOTURBATION and RAIN PITS. the water body in question, usually being a function of the actual dimensions and sedimentation /sed-ă-men-tay-shŏn/ The shape of the water body. The more severe process of sediment deposition. Sedimenta- seiches often result from rapid changes in tion of any particular particle size takes wind fields and barometric pressure. Har- place when the velocity of flow in the bor basin seiches may result from wave transporting medium falls below its termi- penetration. nal, or settling, velocity. seif dune /sÿf/ (longitudinal dune)A sedimentology /sed-ă-men-tol-ŏ-jee/ The sand dune occurring as a long chain run- study of sediments and sedimentary rocks. ning parallel to the prevailing wind direction. Such dunes tend to form in areas sediment transport The movement of mineral grains by wind, water, or ice. The where the wind regime is two-directional, force of gravity also moves sediments in with a gentle prevailing wind that supplies rockfalls and on scree slopes. See eolian the sand and short-term cross winds, often transport; saltation; suspended load. of greater strength, that help build the dune, producing curved faces. They can sediment trap A natural or artificial de- originate ab initio, or from BARCHANS when vice that serves to trap part of the sedi- the wind regime changes. Their character- ments moving across or above the sea istic form is of lines of ridges, running par- floor. Artificial traps have been success- allel over long distances, rising to summits fully used for experimental purposes in cer- periodically. The dunes grow downwind of tain limited situations, for example in some the prevailing wind, with little lateral Alpine lakes, and in scale hydraulic model movement. Individual chains reach 100 km experiments. In nature, certain types of in length, but several distinct chains can seaweed, mangrove, spartina, marram continue one line for much greater dis- grass, and so on perform similar functions. tances. Heights up to 210 m have been Soft algae, which in temperate seas may at- recorded, while the distance between suc- tain lengths of up to 10 m, not only break cessive lines varies from a maximum of 500 the force of waves but may reduce the or- m to a minimum of 20 m. bital velocity beneath waves to the extent that sediments being carried in suspension seismic array /sÿz-mik/ A series of seis- may settle out on the seabed. Off S France, mometers laid out in an L- or T-shaped poseidonia beds in depths of between 5 and pattern.

309 seismic discontinuity seismic discontinuity A boundary be- malayan chain. There are larger areas that tween rocks at which seismic waves sud- experience virtually no seismicity. denly change speed, because of the different densities of the rocks on each side seismogram /sÿz-mŏ-gram/ A record of of the boundary. Major examples include the frequency and magnitude of the oscilla- the GUTENBERG DISCONTINUITY and the MO- tions produced during an earthquake, HOROVIčIć DISCONTINUITY. recorded on a time-correlating device. seismic event A short-lived event that seismograph /sÿz-mŏ-graf, -grahf/An acts as a source of energy for the formation instrument that detects and records the of SEISMIC WAVES, e.g. an earthquake or ex- seismic waves generated by earthquakes or plosion. other tectonic movements and explosions. The epicenter of the earthquake can be cal- seismic noise See microseism. culated from the recordings of a number of seismograph stations. seismic reflection The technique of determining the structure of a rock body by seismology /sÿz-mol-ŏ-jee/ The scien- measuring the time taken for a pulse from tific study of earthquakes, including their a source at the surface to travel to that origins and manifestations. One branch of body, be reflected from a seismic disconti- the science concentrates on seeking meth- nuity within that rock body, and return to ods for predicting earthquakes. the surface. seismometer /sÿz-mom-ĕ-ter/ An elec- seismic refraction The technique of de- tronic or mechanical device that detects, termining the velocity and attitude of a amplifies, filters, and records the motions subsurface rock body by measuring the of the Earth in a particular direction. A shortest-time travel path from a source to a seismic set is a series of three seismometers set of receivers distributed around the rock orientated at right angles to each other. body. /sell-ĕ-nÿt/ A colorless transpar- seismic shooting A technique used in selenite geologic surveys in which explosives are ent mineral form of calcium sulfate, detonated underground and the resulting CaSO4, a type of GYPSUM. It crystallizes in seismic waves detected at various locations the monoclinic system, as separate large (using seismometers). crystals or in crystalline masses. seismic wave A wave generated by an selva /sel-vă/ A type of dense tropical explosion or earthquake within the Earth rainforest, as in the Amazon basin and or on its surface. There are four main types parts of central Africa. See tropical rainfor- of seismic wave: PRIMARY WAVE, SEC- est. ONDARY WAVE, RAYLEIGH WAVE, and LOVE WAVE. sepiolite /see-pee-ŏ-lÿt/ See meerschaum. seismic zone (earthquake zone) A nar- septarium /sep-tair-ee-ŭm/ A large row well-defined belt in which the majority rounded nodule of clay ironstone or lime- of earthquakes occur. These zones usually stone, with a characteristic network of ra- coincide with the junctions between lithos- diating cracks that contain calcite or other pheric plates, especially in association with minerals. island arcs, mid-oceanic ridges, major fracture zones, and young orogenic belts. séracs /say-raks/ An extremely irregular There are two main continental seismic ice surface, usually at the foot of an ICEFALL zones, one bordering the Pacific and the on a glacier. It is formed when the slowly other corresponding to the Alpine–Hi- flowing ice decreases speed, and crevassed

310 shell ice piles up into impenetrable pillars and basis of all official meteorological read- pinnacles. ings. sericite /se-ră-sÿt/ Secondary muscovite shadow zone An area on the Earth’s occurring as fine-grained flaky aggregates surface whose EPICENTRAL ANGLE is greater formed from the alteration of FELDSPARS. than 103° and less than 143° from an earthquake focus. Within this area seismic series A division of rock in the waves are received only after they have Stratomeric Standard scheme of strati- been reflected at the Earth’s surface. graphic nomenclature (see chronostratigraphy). It indicates the body of rock that has shale A well-laminated ARGILLACEOUS formed during one EPOCH. A series consists sedimentary rock that is fissile, and splits of several STAGES grouped together and sev- easily along bedding planes (compare mud- eral series may be combined to form a SYS- stone). The fissility is related to the dispo- TEM. In the past the term has been applied sition of clay minerals within the rock. loosely to bodies of rock characterized on the basis of gross lithology and contained shallow inland sea See inland sea. fossils, but a continuation of this informal practice is not generally recommended. shard See pyroclastic rock. serozem /se-rŏ-zem/ See sierozem. shatter cone A striped conical feature in rock formed by compressive shock waves serpentine One of a group of minerals that follow the impact of a meteorite. The having the general composition Mg3- cones, which may be up to several meters Si2O5(OH)4 and a layered structure. They across, are aligned with their tops oriented are monoclinic, typically green or white, toward the center of impact. They are and occur in two main forms: CHRYSOTILE found in most sedimentary rocks and in and ANTIGORITE. Chrysotile is fibrous and granite. is used in the manufacture of asbestos, whereas antigorite has a platy habit. Ser- shear (shearing stress) A force tending to pentines are found in altered basic and deform a rock mass through the movement ultrabasic rocks where they have formed of one part of it relative to another, as for from the breakdown of olivines and pyrox- example at a fault or thrust plane. See also enes. differential shear. serpentinite /ser-pĕn-tee-nÿt/ A rock con- shear joint A shear fracture, constitut- sisting largely of SERPENTINE, formed by the ing a site of possible future shearing. hydrothermal alteration of ultramafic rocks such as dunite and peridotite in shear wave See secondary wave. which olivine and pyroxene have been converted to serpentine. sheet erosion See overland flow. sessile benthos See benthos. sheeting See unloading.

Seventh Approximation See Soil Tax- shelf edge (shelf break) The break of onomy. slope between the CONTINENTAL SHELF and the CONTINENTAL SLOPE, where a marked shade temperature (in meteorology) slope increase occurs. The shelf-edge depth The temperature of the air measured in the varies considerably from place to place, shade, usually by a thermometer in an in- ranging between about 100 and 350 m, but strument shelter. In this way, direct sun- the norm is taken as roughly 200 m. light, breezes, and variations in shade do not affect the measurement, which is the shell The hard casing of certain marine

311 shield organisms. On the death of the organisms which is shallow enough to permit active the shells are deposited on beaches or on movement of sea-floor sediments under the the sea floor. Many remain virtually intact influence of waves. Some authorities use for long periods, especially in environ- the term to mean that part of the beach ments devoid of marked wave and tidal ac- that is periodically covered then uncovered tion. Others rapidly become pulverized by the tide, i.e. the FORESHORE zone. into minute particles or broken into several fragments. In tropical areas, beaches often shoreline The line that is produced by consist of shell sands, perhaps mixed with the intersection of a sea or lake surface Bryozoa, Foraminifera, and echinoid frag- with the sloping beach-face or shoreface. ments. Outside tropical areas, shell With regard to US National Geodetic Sur- beaches are much less common. They are vey marine charts and surveys, the shore- usually of rather coarse material and there- line approximates to the average line of fore tend to be steep. Shelly material may high tides. However, the term is often used figure prominently in reef-building, for in- loosely to indicate coastline. stance in the case of OYSTER REEFS. shore platform See wave-cut platform. shield A very large rigid area of the Earth’s crust made up of Precambrian shoshonite /shoh-shoh-nÿt/ An extru- rocks, which have been unaffected by later sive igneous rock with a groundmass of orogenic episodes. Shields represent areas potassium-rich ORTHOCLASE and augite, of the Earth’s earliest formed continental olivine, and sometimes biotite mica. crust. Examples include the Baltic Shield and the CANADIAN SHIELD. sial /sÿ-al/ The Earth’s CONTINENTAL CRUST, which is composed of granitic rock shield volcano A VOLCANO with gently types that are rich in silica (Si) and alu- sloping sides and a wide base. It is built up minum (Al). by successive flows of highly liquid basaltic lava, usually issuing from many fissures. siallitic soil /sÿ-ă-lit-ik/ A soil with a See also flood basalt. medium to high silica to aluminum ratio. Such soils are typical of the temperate shingle Pebbles that make up a beach on zone, because tropical weathering removes a seashore. They have been rounded by silica and lowers the ratio, producing a soil rolling back and forth up the sloping beach in which the aluminum content is high. with the tides (attrition). See also gravel. Siberian anticyclone (Siberian high)A shock wave A wave of compression cre- large area of high pressure that dominates ated when the speed of something exceeds the mean pressure field over Siberia, Rus- the speed of sound in the medium in which sia, in winter. It is emphasized by extensive it is traveling. Within a rock, for example, radiational cooling of the Eurasian land- a shock wave (as from a meteorite impact) mass. may deform, melt, or even vaporize the material, perhaps altering its composition side-looking airborne radar See radar. at the same time (termed shock metamorphism). siderite /sid-ĕ-rÿt/ (chalybite) A yellow to brownish-black mineral form of iron shonkinite /shonkă-nÿt/ A type of alka- carbonate, FeCO3. It crystallizes in the line SYENITE containing orthoclase feldspar hexagonal system, and occurs mainly as and various mafic minerals. veins in sedimentary rocks. It is used as a source of iron. See also carbonate minerals. shoreface The narrow zone that lies to seaward of low-water mark and which is, siderophile /sid-ĕ-rŏ-fÿl/ An element therefore, always covered by water but that has an affinity for iron, such as cobalt

312 silica minerals or nickel. Siderophile elements are found in silica minerals Quartz, tridymite, and metallic meteorites and are probably con- cristobalite are the three commonly occur- centrated in the Earth’s core. See also at- ring polymorphs of SILICA (SiO2). mophile; chalcophile; lithophile. Tridymite and cristobalite are stable at intermediate and high temperatures, respec- sienna (raw sienna) /see-en-ă/ A yellow- tively, but neither form persists at high brown earthy pigment containing iron pressures and both are absent from plu- oxide. When it is roasted, its color changes tonic rocks. The polymorphs have distinct to reddish brown (called burnt sienna). crystal structures built from SiO4 tetrahedra but the pattern of linking is different in sierozem /sye-rŏ-zem/ (serozem; gray each case. In addition, each polymorph has desert soil) A soil occurring in semidesert a low- and high-temperature modification, areas in more arid zones than the CHESTNUT the transitions taking place rapidly at the SOILS, and characterized by low organic particular inversion temperature involved. matter contents, a lack of leaching, and ac- Changes from one polymorphic form to cumulations of lime at the top of the C another are very sluggish and tridymite horizon or lower B, typically within 30 cm and cristobalite both exist metastably at of the surface, and possibly reaching up to ordinary temperatures. Coesite and stish- the surface. Vegetation cover is scanty: ovite are high-pressure high-density forms short grass and scattered brush. These soils of silica found near meteorite impact are included in the aridisol order of the US craters. Chalcedony is the cryptocrystalline form of silica containing minute crystals of SOIL TAXONOMY. quartz with submicroscopic pores. Opal is a hydrous cryptocrystalline or amorphous significant wave A wave that has the form of silica. Lechatelierite is a rare average period and height of the highest metastable silica glass. 33.3% of the waves in the WAVE SPECTRUM. Quartz is stable over a wide range of Calculation of significant wave character- physical conditions and because silica is istics requires careful wave measurement at the most abundant oxide in the Earth’s sea and subsequent analysis of this infor- crust, it is a very common mineral. The dif- mation, or the use of a wave analyzer in a ferent colored varieties are: colorless – rock scale model that involves wave action. crystal, yellow – citrine, gray-brown to black – smoky quartz, pink – rose quartz, silcrete /sil-kreet/ A surface deposit, that and violet – amethyst. Quartz is trigonal occurs in semiarid regions, consisting of and the absence of cleavage is characteris- gravel and sand cemented together by tic. On Mohs’ scale it is the standard min- chert, opal, and quartz. eral of hardness 7. Quartz is an essential constituent of silica-rich igneous rocks, silica /sil-ă-kă/ Silicon dioxide, SiO2. It such as granites, rhyolites, and pegmatites, is the chief mineral in chert, diatomite, and is found in oversaturated, intermedi- and sand, and in crystalline form it makes ate, and basic rocks. Quartz often occurs up all the various varieties of QUARTZ intergrown with alkali feldspar in graphic and other SILICA MINERALS. Opal consists of granite. Vein-quartz, precipitated from hy- hydrated noncrystalline (amorphous) sil- drothermal solutions, is often associated ica. with ore minerals. Because of its hardness and resistance to chemical weathering, silica concentration For an igneous quartz is the most abundant detrital min- rock, the percentage of SILICA it contains. It eral and is concentrated to give rise to is a common way of classifying such rocks. sands and gravels of various types, which, Those with more than 66% silica are on lithification, constitute the arenaceous termed acid, 55–66% silica are intermedi- rocks. Authigenic quartz is often deposited ate, 45–55% are basic, and less than 45% around detrital grains, cementing them to- are ultrabasic. gether. During metamorphism, the quartz

313 silica saturation of sediments and igneous rocks recrystal- morphite, melilite), cyclosilicates (beryl, lizes to a coarser grain size. Quartz is also axinite), inosilicates (amphiboles and py- produced by the released silica during the roxenes), phyllosilicates (micas and clay metamorphic breakdown of preexisting minerals), and tektosilicates (quartz, zeo- minerals. At high grades, segregation and lites, feldspars). mobilization take place giving rise to quartz veins and pegmatites. silicification /să-liss-ă-fă-kay-shŏn/ The Tridymite and cristobalite are typically process by which SILICA (usually in the found in cavities in acid volcanic rocks, form of chalcedony or quartz) enters pores such as rhyolites and andesites, and may be or replaces other minerals in a rock. produced as a result of the passage of hot gases. sill A near-horizontal tabular intrusive Chalcedony occurs in numerous vari- body of igneous rock, usually dolerite, of eties. Carnelian is red. The banded variety, roughly uniform thickness but thin relative agate, is formed by intermittent deposition to its area. It is concordant with the planar in cavities. Jasper is a red-brown color. structures of the rock types into which it Chert and flint are opaque, dull-colored, or intrudes. See also dike. black varieties, the former being the massive or stratified form and the latter being sillimanite /sil-ă-mă-nÿt/ A pale green, found as nodules in chalk. Chalcedony is gray, or brown mineral form of aluminum found in hydrothermal veins, amygdales, silicate, Al2SiO5. It crystallizes in the or- and sediments. thorhombic system, generally as fibrous Opal occurs in many color varieties, the aggregates (fibrolite) in metamorphic precious variety being iridescent. It is de- rocks. It is used as a refractory. See also posited at low temperatures from silica- aluminum silicates. bearing waters and is found around geysers and hot springs. Diatomite is a rock made silt A fine-grained deposit that ranges in up almost entirely of the accumulated opa- sediment particle size from 0.06 mm to line skeletons of diatoms. Radiolarian and 0.002 mm, and is therefore significantly diatomaceous deposits may recrystallize to finer than sand. Like clays, silts may in- form chert. clude clay minerals, and also hydroxides and oxides of iron, silica dioxide, and nu- silica saturation The extent to which a merous other fine material particles. Clay rock contains SILICA, this measure consti- and silt together form the ARGILLACEOUS di- tuting a convenient method of classifying vision of sediments. Silts result not only igneous rocks into three groups: 1. oversat- from decomposition of certain in-situ urated – rocks containing free silica as rocks and rock particles but also from quartz. 2. saturated – rocks in which all the gradual abrasion processes (impact, grind- silica is combined and in sufficient quantity ing, and rubbing). Fine silty material tends to exclude feldspathoids. 3. undersaturated to collect in sheltered quiescent marine en- – rocks containing feldspathoids. vironments and can be a serious threat to navigation. It is sometimes difficult to silicates /sil-ă-kayts, -kits/ A group of dredge, often returns rapidly from spoil- minerals constituting about one third of all grounds to the dredged areas, and can in- minerals and approximately 90% of the undate seaweed beds where fish and other Earth’s crust, in which feldspar (60%) and marine organisms normally abound. quartz (12%) are the most abundant. The basic structural unit common to all sili- siltstone A fine-grained layered sedi- cates is the SiO4 tetrahedron. Silicates are mentary rock derived from SILT. It resem- classified on a structural basis according to bles shale but contains less clay. See also how the tetrahedra are linked together. mudstone. The six classes of silicates are as follows: nesosilicates (olivine), sorosilicates (hemi- Silurian /sa-loo-ree-ăn, sÿ-/ The last pe-

314 SI units riod in the Lower PALEOZOIC beginning lower regions of the Earth’s oceanic crust about 438 million years ago, at the end of that consists of sima. the ORDOVICIAN Period, and lasting about 30 million years, until the start of the DE- singularity /sing-gyŭ-la-ră-tee/ The an- VONIAN. The Silurian System is divided into nual recurrence of particular types of four series: the Llandovery, Wenlock, Lud- weather at certain times of year as illus- low, and Pridoli, but there are many alter- trated by the BUCHAN SPELLS. Unfortunately natives throughout the world. It was their reliability is low except in a very gen- named by the British geologist Sir Roderick eral sense. Impey Murchison (1792–1871) for the Celtic tribe, the Silures, that inhabited sinistral fault A fault at which move- much of SE and central Wales where Sil- ment is to the left relative to the side of the urian rocks are well developed. Rocks of fault plane on which the observer is stand- the Silurian occur throughout the world; in ing. Compare dextral fault. North America they are exposed in the Ap- palachians and across the Mid-West, no- sinkhole (sink; pothole) A saucer-like tably around the Great Lakes. The Niagara hollow, typical of limestone areas, varying Falls are on an escarpment of Niagaran from 1 to 1000 m in diameter and from a limestones from the Silurian. Important few centimeters to 300 m in depth, pro- earth movements were taking place within duced by solution (see limestone solution) the period: the later stages of the Taconian or by rock collapse. The ratio of diameter orogeny of North America and the Cale- to depth is usually about 3:1. They are usu- donian orogeny in NW Europe. The Iape- ally formed on flat or gently sloping sur- tus Ocean, which separated Scotland on faces, because here water can accumulate the North American continent from the and stand, thereby causing solution. Sink- rest of the British isles on the Europe and holes may also be simply enlarged vertical Scandinavian continent, was continuing to joints, but whatever their shapes they often close. During the Upper Silurian extensive act as channels down which surface water evaporite deposits were laid down with the seeps to underground drainage systems. widespread regression of the seas. See also swallow hole. Similarity in stratigraphy between the Ordovician and Silurian is reflected in the sinusoidal projection /sÿ-nă-soi-dăl/ See fauna, which seems to be a continuation of Sanson–Flamsteed projection. the Ordovician fauna. Trilobites and graptolites became less common; the latter were sirocco /să-rok-oh/ (scirocco) A wind of almost extinct by the end of the period. similar origin to the Egyptian KHAMSIN oc- Brachiopods were very varied and crinoids curring farther west between Algeria and and corals increased in importance. Ostra- Libya. It is a hot dry southerly wind on the coderm fish became more abundant and North African coast, but on crossing the the first gnathostomes (jawed vertebrates) Mediterranean it can pick up much mois- appeared. The first evidence of land plants ture and bring extensive low stratus cloud also comes from Silurian rocks. to the Italian coast. sima /sÿ-mă/ The Earth’s OCEANIC CRUST, SI units (Système International d’Unités) which is composed of basaltic rock types A system of units used, by international that are rich in both silica (Si) and magne- agreement, for all scientific purposes. It sium (Mg). The term was formerly used for is based on the meter-kilogram-second that portion of the continental crust below (MKS) system and replaces both the cen- the CONRAD DISCONTINUITY but this was in- timeter-gram-second (cgs) and Imperial accurate and led to confusion with the (fps – foot, pound, second) systems. It con- oceanic crust. sists of seven base units and two dimen- sionless units (see Table 1). Measurements simatic crust /sÿ-mat-ik/ The denser of all other physical quantities are made in

315 skarn derived units, which consist of combina- sleet Precipitation comprising ice pellets tions of two or more base units. Eighteen with a diameter of 5 mm or less formed of these derived units have special names when raindrops or melting snowflakes (see Table 2). Base units and derived units freeze or refreeze. In Britain the term is with special names have agreed symbols, used for melting snow or a mixture of rain which are used without a period. and snow. Decimal multiples of both base and derived units are expressed using a set of slickensides 1. Small parallel grooves or standard prefixes with standard abbrevia- striations formed on the surface of fault or tions (see Table 3). Where possible a prefix joint surfaces as a result of the movement representing 10 raised to a power that is a of rocks against each other. Small platy multiple of three should be used (e.g. mm is minerals are developed on them, from preferred to cm). which the relative sense of movement of the rocks can be determined. The surface of skarn A calc-silicate mineral assemblage the slickenside is smooth in the direction of produced during the CONTACT METAMOR- movement, but rough in the opposite di- PHISM and METASOMATISM of highly cal- rection. careous rocks. 2. (in soils) The polished and smoothed surfaces formed in subsurface soils by two sky-view factor The proportion of the masses sliding past each other. Slickensides sky that is visible to a person. It is an im- are common in swelling clays where there portant parameter in urban climatology. are large changes in water content. slack tide (slack water) The period or state of the tide in which there is negligible horizontal flow of water or when the tidal inside outside currents are at virtually zero velocity. This period occurs, in the case of a rectilinear slip-off slope undercut tidal stream, when the currents are revers- (deposition of slope ing their direction. Some authorities con- point bar) (erosion) sider the period covers that time when flood and ebb velocities are less than 0.1 Section through a meander bend knot. slip-off slope The gentle slope on the in- SLAR See radar. side of a MEANDER bend opposite the undercut slope that occurs on the outside of a slate A fine-grained rock in which the meander bend owing to erosion. The depo- parallel orientation of platy crystals of mica sition of POINT BARS and creation of slip-off and chlorite results in the production of a slopes is mostly attributable to the HELICAL perfect slaty cleavage. The individual con- FLOW in meandering reaches, which constituent minerals cannot be resolved by the stantly transfers sediment from eroding naked eye but some slates contain porphy- areas on the outside of bends to the de- roblasts or are spotted, the spots represent- positing areas on the inside. ing embryonic porphyroblasts. Slates are formed during the low-grade regional meta- slip sheet A gravity collapse structure morphism of mudstones, siltstones, and developed in the beds on the limb of an another fine-grained argillaceous sediments. ticline. If the beds at the base of the limb fracture, the overlying beds will slip down- slaty cleavage A parallel alignment of ward under the influence of gravity and platy minerals in fine-grained rocks, at then spill out onto the adjacent strata. right angles to the direction of compression. Slaty cleavage is usually also a form slope An inclined surface whose length of AXIAL PLANE CLEAVAGE. is determined by the horizontal distance

316 TABLE 1: BASE AND SUPPLEMENTARY SI UNITS

Physical quantity Name of SI unit Symbol for SI unit length meter m mass kilogram(me) kg time second s electric current ampere A thermodynamic temperature kelvin K luminous intensity candela cd amount of substance mole mol *plane angle radian rad *solid angle steradian sr

*supplementary units

TABLE 2: DERIVED SI UNITS WITH SPECIAL NAMES

Physical quantity Name of SI unit Symbol for SI unit frequency hertz Hz energy joule J force newton N power watt W pressure pascal Pa electric charge coulomb C electric potential difference volt V electric resistance ohm Ω electric conductance siemens S electric capacitance farad F magnetic flux weber Wb inductance henry H magnetic flux density tesla T luminous flux lumen lm illuminance (illumination) lux lx absorbed dose gray Gy activity becquerel Bq dose equivalent sievert Sv

TABLE 3: DECIMAL MULTIPLES AND SUBMULTIPLES USED WITH SI UNITS

Submultiple Prefix Symbol Multiple Prefix Symbol 10–1 deci- d 101 deca- da 10–2 centi- c 102 hecto- h 10–3 milli- m 103 kilo- k 10–6 micro- µ 106 mega- M 10–9 nano- n 109 giga- G 10–12 pico- p 1012 tera- T 10–15 femto- f 1015 peta- P 10–18 atto- a 1018 exa- E 10–21 zepto- z 1021 zetta- Z 10–24 yocto- y 1024 yotta- Y

317 slope concavity from crest to foot and whose angle is de- upslope, the volume passing a given point termined by the inclination of the slope increases downslope, so an increasingly from the horizontal. Usually a slope is said steep slope is needed to transmit the debris, to have one angle in broad terms, but most hence a convex slope. Today, this idea is slopes are composed of a number of differ- still widely accepted, even though creep is ent parts, called facets if straight and el- now thought to operate as a slide rather ements if rounded, each having its own than a flow, so that the thickness of the de- slope angle. See also slope concavity; slope bris does not increase downslope. The Ger- convexity; slope decline; slope evolution. man geologist Walther Penck said convex slopes were a product of uplift acting faster slope concavity The bottom parts of than downcutting; Nevin M. Fenneman hill slopes are typically concave, except in (1908) thought it might be due to the ac- certain areas where this basal concavity tion of wash; S. Schumm found that creep has been buried by aggradation. Analogies does produce a convex profile, but because have been drawn between the profile of a it operates only on clay lithologies, convex concave slope and the long profile of a profiles tend to form only in claylike ma- river; this similarity of form has also pro- terials. It may also be that because the voked claims of a similarity of origin, the upper part of a hill is attacked by weather- concavity being essentially a product of ing on all sides, the tendency is to weather running water action, especially rill and out a convex slope. In reality a number of gully action. This type of slope form is pro- factors may be responsible. nounced on lower slopes because there is a greater catchment area for running water slope correction The reduction of dis- than on upper slopes, there is increased tances measured on a slope in the field to weathering of debris downslope, produc- their correct horizontal value for presenta- ing smaller particles that can move on a tion on a map. The corrected figure is ob- more gentle slope, and there is increasing tained by multiplying the measured length impermeability as debris becomes finer by the cosine of the slope angle. downslope, leading to increased running water action. For running water action, an slope decline The original CYCLE OF impermeable surface is a prerequisite and EROSION, as formulated by the American consequently concave slopes are well de- geographer William Morris Davis, was veloped on the finer rocks, e.g. clays, based on a progressive lowering of slope shales, and argillites, and less well devel- angles (slope decline) as contrasted to the oped on the permeable rocks, e.g. sand- later schemes of the German geologist stone, chalk, and limestone. Plentiful Walther Penck and the South African geol- rainfall is also important and, other things ogist Lester King, emphasizing PARALLEL being equal, the concave parts of slope pro- RETREAT of slopes. Slopes will decline when files tend to become more pronounced more material is eroded upslope than is re- with increasing age, as weathering of de- moved from the foot of the slope, leading bris advances, making it finer and less per- to a differential rate of evolution, the upper meable, and hence more able to support part retreating faster and hence lowering running water. See also slope convexity; the overall slope. For material to accumu- slope evolution. late at the foot of a slope there must be no effective BASAL SAPPING, e.g. in a sea cliff cut slope convexity The upper parts of hill off from the sea by the growth of a shingle slopes are characteristically convex in bank or sand spit, protecting it from wave shape, and a good deal of discussion on action, it has been seen that parallel retreat slope evolution is directed at suggesting gives way to slope decline. Others have causes for this shape. Early workers in- found that lithologies of certain types favor cluded the American geomorphologist G. slope decline, e.g. S. Schumm found decline K. Gilbert, who said convexity was a prod- was a feature of slope evolution in clays. uct of CREEP; as the soil mantle moves from Originally, this type of slope evolution was

318 smoky quartz seen as characteristic of the normal cycle in through time, there is no reason why cur- humid temperate areas, i.e. as a climati- rent slope forms should have any strong re- cally controlled process. Today it is known lation to the current processes operating on that slopes in the same climate can undergo them. very different evolutions and slopes in different climates take the same course. slump A single mass movement of rock Whether or not a slope undergoes decline debris and soil downhill. It most often oc- depends on process and lithology as well as curs when water-holding permeable rocks climate. overlie impervious ones (such as shale). Rocks may shear away, leaving a scarred slope evolution Studies of slope evolu- rockface. Slumping also occurs underwater tion are concerned with the relation beat the edges of continental shelves. tween slope form and the processes operating on that slope, the lithology of the slurry A liquid mixture of mud and rock, and the stage of evolution. It is inti- water, which readily slips downhill. It most mately linked to studies of the general ten- commonly occurs in clays and shales. dencies of landscape form, i.e. whether they tend to retreat parallel to themselves smectite /smek-tÿt/ A member of one of (see parallel retreat), leaving INSELBERGS the major groups of CLAY MINERALS. and PEDIMENTS, or whether they tend to decline (see slope decline), ultimately produc- smithsonite /smith-sŏn-ÿt/ A white, yel- ing a PENEPLAIN. low, pink, pale green, or blue mineral form Many different techniques and ideas of zinc carbonate, ZnCO3, known also as have resulted from these studies. Some ge- calamine in the UK. Its color is caused by ologists have studied the current evolution impurities. It crystallizes in the trigonal of small-scale slopes of different rock types system, and occurs in beds and veins in cal- to see the influence of lithology; others careous rocks, often associated with HEMI- have studied the role of climate, through its MORPHITE. control of geomorphic process, by comparing slopes in different latitudes, or on the smog The polluted air that occurs above shady and sunny sides of valleys; others urban and industrial areas. The term is de- have related slope form to tectonic action, rived from the words smoke and fog. The or studied the role of base-level changes in burning of fossil fuels, especially coal, re- producing polycyclic slopes. Early workers leases sulfur compounds and smoke parti- inferred a series of sequential stages cles into the atmosphere; sulfurous smog through which any slope might evolve with forms if these particles are unable to be dis- time, whereas some modern workers have persed because of a persistent INVERSION. successfully identified actual time series in The concentration of toxic chemicals may the form, for example, of a growing spit at build up to harmful levels. Photochemical a cliff foot progressively cutting that cliff smog, a hazy brown blanket of smog, is off from wave action and hence inducing a created when sunlight reacts with pollu- change of process. Finally, some have em- tants in the air. It is most prevalent in ployed a statistical approach, measuring urban areas with high concentrations of the angles of slopes and finding those val- motor vehicles; heat from burning fossil ues around which angles tend to bunch fuels causes nitrogen and oxygen to chem- (characteristic angles). Despite all these ef- ically combine to form nitrous oxide and forts, slope evolution is still a highly com- hydrocarbon vapors. Under sunlight these plex topic, although many general models undergo photochemical reactions, one of have been produced. In any particular the substances formed being OZONE. slope, its shape is determined by the balance of losses of rock by erosion and gains smoker See hydrothermal vent. by deposition, but because the geomorphic processes operating may have changed smoky quartz A yellow to dark grayish-

319 SNC meteorite brown type of QUARTZ, which owes its altitude is determined by mean summer color to the presence of impurities. It is temperatures, but local climate conditions sometimes used as a semiprecious gem- such as aspect or shelter from snow-bear- stone. ing winds can produce variations.

SNC meteorite Any of a few small snow patch erosion See nivation. stony METEORITES (named after Shergotty, Nakhala, and Chassigny, the three main soapstone (steatite) A rock composed ones) that have significantly different com- largely of TALC. positions and ages from usual meteorites. Some planetologists believe that they orig- soda lake An inland area of water con- inated from mantle material on Mars up to taining high concentrations of dissolved 1.3 billion years ago. sodium salts, particularly the carbonate, chloride, and sulfate. The highly alkaline Snell’s law A law stating that the angle waters do not support any life. of refraction R of a light ray is related to the angle of incidence I by the equation: sin sodalite /soh-dă-lÿt/ A major member of I/sin R = µ, where µ is a constant called the the FELDSPATHOID group of minerals. refractive index of the material. The law is named for the Dutch mathematician and soil The natural accumulation of uncon- physicist Willebrord van Roijen Snell solidated mineral particles (derived from (1591–1626). weathered rocks) and organic matter (humus) that covers much of the Earth’s snout The down-valley margin of a glac- surface and forms the supporting medium ier. for plant growth. The exact classification of soils is diffi- snow Solid precipitation composed of cult because soil occurs as a graded contin- individual ice crystals or aggregations of uum, breaks in this continuum being ice crystals forming snowflakes. The for- chosen subjectively for classification. In the mer occurs when temperatures are well past classification was based on the PRO- below freezing and the moisture content of FILE, but in the US SOIL TAXONOMY classifi- the atmosphere is not high. As tempera- cation the unit is the PEDON and rather than tures approach 0°C, snowflakes are found being based purely on factors of soil for- and these increasingly aggregate into larger mation (as formerly) it is concerned with flakes at about freezing point. Snow is very morphology, HORIZON arrangement, and difficult to measure accurately. It tends to other properties. A soil phase is the lowest block rain gauges or be blown out. It is unit in the hierarchy of soil classification, normally recorded by inserting a graduated and it reflects the erosional state of the soil, ruler into a flat surface of undrifted snow. e.g. its depth, stoniness, and degree of ero- The water equivalent of 30 cm of snow is sion. A soil association is a collection of approximately equal to 25 mm of rainfall. soils grouped on the basis of geographic proximity even though they may differ snowfield A large accumulation of greatly in profile characteristics (this snow in a mountainous region. If it is thick grouping is usually used in the generaliza- enough, downward pressure gradually tion of a detailed map in order to reduce it converts it to ice, which may form glaciers. to a smaller scale). A soil series is a collection of soils similar in all respects except snow line The altitude of the lower limit for the texture of the surface horizon and of permanent snow. In tropical areas the its erosional state. Series are the basic soil snow line is about 5000 to 7000 m, but mapping units. A soil type is a subdivision gradually decreases poleward, until it of a soil series on the basis of the texture of reaches sea level in the Arctic Seas, Green- the A horizon, these textural variations land, and Antarctica. In general terms, its usually being related to slight changes of

320 soil slope. A soil group is a group of soils shar- of varying degrees of these three variables. ing a similar type and sequence of hori- color is an important factor in the recogni- zons, occurring over wide areas with tion and description of soils, particularly as similar temperature and moisture regimes it often reflects formation, e.g. blue-gray (differences in surface horizon due to agri- colors typify gleying, red the concentration culture are discounted). A soil order is the of ferric oxide, and gray or grayish brown highest level of generalization in soil classi- often shows the accumulation of calcium fication (the US Soil Taxonomy has 12 or- carbonate. ders) and is based on the type and Soil moisture is fundamental to plant development of horizons present. growth and whether it becomes available Soil structure denotes the mode of binding together of individual soil particles into secondary units (aggregates or peds), mostly by organic matter. There are seven TABLE 1: SOIL STRUCTURE TYPES main structure types (see table) and the grade of structure can be classified into Type Characteristics structureless, weak, moderate, or strong, platy thin leafy flaky depending on the observable degree of ag- layers gregation. Structure is important to soil prismatic prismlike units with fertility, because it influences POROSITY flat tops; mainly (and hence air and water supply), bulk den- subsoil sity, heat transfer, etc. Cultivation tends to columnar prismatic units break down structures by removing or- with rounded tops ganic matter, the major binding agent. blocky rectangular blocks; Soil texture denotes the proportion of common in humid the various PARTICLE SIZES in a soil. There zones are four main texture classes (see table) and subangular blocky blocky with many intermediate grades. Texture can be rounded edges determined accurately by laboratory analy- granular rounded and sis and allocated to a specific class by a tex- porous; mainly ture triangle, or more approximately in the surface soil field by rubbing a wet sample between the crumb soft, rounded, and fingers and estimating the proportion of very porous; sand (gritty feel), silt (silky feel), and clay mainly surface soil (plastic). Loams are generally the best agricultural soils because they contain a good TABLE 2: SOIL TEXTURE CLASSES mixture of all particle sizes. Soil consistency denotes the degree of Name Composition cohesion and adhesion in a soil, hence its sand > 85% sand ability to resist deformation or rupture. It percentage of silt is described by the following terms: wet, plus 1.5 times the sticky, plastic, nonplastic, moist, loose, fri- percentage of clay able, firm, dry, and hard, with a number of is not more than intermediate grades. 15% Soil color as defined by the Munsell loam 7–27% clay Color System is composed of three vari- 28–50% silt ables: hue, value, and chroma. Hue is de- < 52% sand termined by the wavelength of the light silt 80% or more silt reflected from it, chroma is the purity or < 12% clay strength of the color, and value is the rela- clay < 40% clay with tive lightness or intensity of the color. < 45% sand and Using a color code, a soil sample is allo- < 40% silt cated to a particular class that is composed

321 soil creep to plants depends to a large extent on the background of the others where these are size and distribution of the soil pores (see constant. porosity) within which the water is held. In Climatic factor. This is thought to be the general the finer the texture of the soil the most important factor in soil formation. It greater its capacity to store water. Water is is very complex because it can considerably held by the soil with varying degrees of influence the effect of the other factors. Pa- tenacity; the less the moisture content the rameters of fundamental importance are more firmly it is held. rainfall (regime and intensity), temperature Each soil horizon is determined by the (maximum and minimum, soil tempera- original parent material and subsequent tures, and duration of sunshine), and evap- additions, losses, and transformations of otranspiration. The effects of these single materials within it. Especially important in components can be measured, e.g. the the processes accomplishing horizon differ- depth to which calcium carbonate is entiation are those that transfer material leached can be broadly correlated with the from one horizon to another, because these amount of rainfall. The combined influ- differentiate the eluvial A horizon from the ence of temperature and precipitation is illuvial B. See also abnormal erosion; well illustrated in Russia, where the se- cheluviation; decalcification; eluviation; quence of soil zones changes from north to erosion; gleying; humification; illuviation; south in relation to an increase in tempera- laterization; leaching; lessivage; podzoliza- ture and a decrease in precipitation. This tion; soil formation. relationship between soils and climatic zones forms the basis of the zonal classifi- soil creep The slow movement of soil cation of soils. particles downhill. It occurs particularly in Organic factor. Organisms are responsible wet areas and is so slow that it cannot nor- for the breakdown of plant tissue resulting mally be seen. Nevertheless, in time large in the accumulation of organic matter, the quantities of rock fragments and soil slip to recycling of nutrients, structural stability, the bottom of the slope. and profile mixing in the soil. Vegetation itself, by binding the soil together, prevents soil erosion The removal of soil by the erosion. Humans through their farming action of wind or water. It occurs most practices such as cropping, burning, and ir- often when vegetation has been removed, rigation are playing an increasing role in usually by fire or human activity. See bad- soil formation. Organisms are often de- lands; Dust Bowl; erosion. pendent on climate but they may act as independent variables. For example, certain soil formation (pedogenesis) The major soil types are associated with partic- process by which soil forms. The early Rus- ular vegetation types and when the vegeta- sians, particularly Vasily Vasilievich tion type changes in the same climatic Dokuchaev (1840–1903), stated that soil region a change in soil type may result. forms as a result of five factors: local cli- Time factor. The time a soil takes to de- mate, PARENT MATERIAL, plant and animal velop is measured from when fresh rock is organisms, relief and elevation, and the age first exposed at the surface, e.g. when the of the land surface. In 1941, the Swiss- Pleistocene glaciation left the British Isles American pedologist Hans Jenny (1899– some 10 000 years ago. Soil develops grad- 1992) tried to formulate the Russian work ually (the rate is partly dependent on par- in a mathematical equation, s = f(cl, o, r, ent rocks, e.g. soils on till will develop in p, t ....), where s = soil, cl = climate, o = orless than 1000 years whereas those on re- ganisms (including man), r = relief, p = par- sistant quartzite will take much longer), ent material, and t = time. However, all the probably through an (A)C, AC, A(B)C, factors are interrelated and the equation ABC horizon sequence, and eventually a would be very difficult to solve in the field, stage is reached in which the soil is in equi- but in certain cases it is possible to evaluate librium with the environment and can be the effect of one of the factors against the called mature. However, because the sys-

322 sole mark tem is dynamic and any environmental gap between mapping and broad classifica- change will affect the soil, it is better to use tion, as so typically happened with many absolute or relative datings than terms older systems. To some extent, the Soil such as young, mature, and old. Taxonomy is now being used at a world Topographic factor. Topography affects level, and the FAO has published a world soil moisture. Drier shallower soils occur soil map based on it. See alfisol; andisol; at the top of a slope, where drainage is aridisol; entisol; gelisol; histosol; incepti- more rapid. Soils at the slope foot tend to sol; mollisol; oxisol; spodosol; ultisol; ver- be deeper because of an increase in subsur- tisol. face weathering due to the presence of more moisture and the accumulation of soil temperature The temperature material eroded upslope. As a result of as- recorded at any level within the soil (stan- pect, soils on south-facing slopes (in the N dard depths being 30 cm and 100 cm) with hemisphere) tend to be drier than those fac- a soil thermometer, which consists of a ing north. Because vegetation type is mercury thermometer that has a horizontal closely related to soil moisture, through section as usual, but is then bent through vegetation topography also has an indirect 90° to a predetermined length where the influence on soil development. See also bulb is located. The diurnal temperature catena. wave decreases very rapidly with depth and below 10 m there is usually little sign soil horizon See horizon (def. 1). of an annual wave. soil moisture storage capacity See soil texture The arrangement and sizes field capacity. of the particles in SOIL. Soils based on clay, for example, have a fine texture but rela- soil profile See profile (def. 1). tively poor drainage, whereas soils containing sand have a coarse texture and Soil Taxonomy The US soil classifica- good drainage. A mixture of the two types tion system, published by the US Depart- is called LOAM. ment of Agriculture (USDA). The system was originally published in 1960 as the solar constant The intensity of RADIA- Seventh Approximation (the name coming TION received from the Sun on a unit area from the fact that it represented the seventh of a horizontal surface at right angles to the attempt by its authors to find an ‘ideal’ solar beam above the atmosphere, at the classification) and subsequently revised in Earth’s mean distance from the Sun. Its 1975. It originally comprised 10 orders but value is 1.35 kW m–2. There has been con- subsequent revisions since 1975 have seen siderable debate as to whether this value is the addition of 2 new orders: the andisols a constant or not. Before the use of satel- and, most recently, the gelisols. Unilke ear- lites it had to be measured from high-alti- lier classifications based on pedogenesis, tude observatories where atmospheric the emphasis is placed on properties of interference could be reduced but not elim- soils and diagnostic horizons that affect pe- inated. It now appears that the value is al- dogenesis or result from it. As a result the most constant except in the shortest part of twelve orders of soils have some unusual the spectrum where the amount of energy groupings; for example, there is no sepa- received is small. rate group for gleys, and podzols occur in more than one order. Although the nomen- solarimeter See pyranometer. clature is somewhat obtuse, and laboratory analysis to measure certain properties is solar radiation See radiation. necessary before definite classification, there is the great advantage that the major sole mark A sedimentary structure levels are as tightly defined as the field formed at the base of sandstone and silt- mapping units, so that there is no awkward stone beds that overlie softer sediments.

323 solfatara solfatara /sohl-fah-tah-rah/ See fuma- form a textural Bt horizon (a process not role. present in a true brown earth). This is clear from a study of the structure of the soil as solid geology The geology of rocks un- the Bt horizon has a prismatic or blocky derlying glacial and other superficial de- structure whereas the surface horizon often posits. See also drift. has a weak crumb/blocky structure. These soils are common in the humid climate of solifluction /sol-ă-fluk-shŏn/ A process NW Europe on freely drained fine-textured of mass movement on slopes, usually re- parent materials. They fall into the alfisol stricted in its application to areas of order of the US SOIL TAXONOMY. periglaciation (see periglacial). The major process of solifluction concerns the pres- solod /sol-od/ (salt-earth podzol) A soil ence of PERMAFROST. In spring, melting resulting from the leaching of a SOLONETZ takes place in the top layer of the soil, but soil, forming a white-gray A horizon from the water produced cannot drain away be- the upper B horizon of the solonetz. This cause beneath this active layer the ground bleached A horizon, as in true PODZOLS, is remains frozen and hence impermeable. deficient in iron and aluminum, has a Consequently, the upper soil becomes sandy texture and an acid reaction. The B highly saturated, and its cohesion may be horizon is gleyed, with mottlings and a reduced to such an extent that flowage can columnar and prismatic structure devel- take place and material be moved down- oped in a compact clay. These soils repre- slope, often creating characteristic terrace sent the leached end of the development forms and burying former vegetation continuum from SOLONCHAK through layers, although the presence of well- SOLONETZ to solod in which salts are pro- developed vegetation usually hinders the gressively removed through leaching, ei- process. ther naturally, sometimes owing to a Solifluction sometimes also embraces falling base level, or artificially through the effects of NEEDLE ICE and CONGELITUR- leaching by irrigation water. BATION. solonchak /so-lon-chak/ (white alkali soil) solitary wave A water wave consisting Saline soil in which sodium chloride and of only one elevation which, once formed, sodium sulfate form more than 0.3% of the may travel practically unchanged as a sin- total soil. During drought the salts dry out gle hump for a considerable distance. The at the surface giving a white color. Saline actual water particles move forward a groundwater exists within two meters of short distance as the wave passes by, i.e. the surface, and this supplies the salts for the passage of a solitary wave causes a net upward rise by CAPILLARITY to the surface shift of the water particles beneath the soli- soil; it also imparts gley features to the soil, tary wave in the direction of wave propa- usually just the lower part of the profile but gation. The solitary wave is neither possibly the whole profile if the water table preceded nor followed by other waves, unis near the surface. These soils are charac- less these are generated quite indepen- terized by a pH value of more than 8 (more dently. TSUNAMI waves, which may be of than 15% of exchangeable cations are very limited height at sea, possess some of sodium), a flocculated structure, no illuvial the characteristics of solitary waves. B horizon, and one or two salt layers; they are friable and soft in a moist state, crusty sol lessivé /sohl-less-ee-vay/ A soil that when dry. They are fairly common in arid, corresponds to the gray-brown podzolic semiarid, and subhumid regions, where soil of North America. The profile is char- they may occur naturally in depressions or acterized by a thin surface accumulation of as a result of secondary salinization of the moder humus. Below this the A and Eb zonal soils, or they may be artificial due to horizons have been depleted of clay, which excessive irrigation or irrigation with has been eluviated down the profile to water dominated by sodium salts. In the US

324 sorting

SOIL TAXONOMY they are classified in the times causing mechanical breakdown of ARIDISOL order. rocks (see granular disintegration; flaking). The efficiency of the solution process de- solonetz (black alkali soil) Soil domi- pends largely on the volume of water flow- nated by sodium carbonate. Solonetz de- ing, but the presence of weak acids within velops from SOLONCHAK by the leaching of the water may cause increased solution. some of the sodium, due to increased rain- See also limestone solution. fall, improved drainage, or irrigation, which deflocculates the soil mass. As a resolution breccia A BRECCIA in which the sult the structure is single-grain when wet, original rock has been fragmented by the but on drying the illuvial horizon develops removal of soluble material. a massive columnar structure. It is this structure difference that originally divided sonic log A subsurface logging tech- the solonetz from the solonchak. Whereas nique run in a borehole, which measures the solonchak has an AC profile, increased the time taken for a compressional sound leaching in the solonetz produces a dark wave to travel through 30 cm of the adja- gray or brown B1 horizon enriched with cent formation. In this way a continuous colloids, and lighter gray B2. The C horizon record of the properties of the rocks is enriched with carbonates and salts, while against depth is recorded. at the surface alkaline solutions of humic material accumulate in wet periods which sonoprobe /sonn-ŏ-prohb/ A continu- on drying produce black crusts. They may ously recording acoustic reflection instru- be partly zonal soils, being concentrated at ment, which has made an important climatic transition zones at 15° and 40°N, contribution to the study of the continental but traditionally they are considered as in- terrace and some deeper areas. Its use pro- trazonal. In the US SOIL TAXONOMY they vides data on the approximate thickness, are classified partly as aridisols, partly as distribution, and character of marine sedi- mollisols. ments and the surface morphology and internal structure of exposed or buried rock solstice /sol-stiss/ The time of maximum areas. Various sound sources are employed or minimum declination of the Sun. The to give varying degrees of penetration of summer solstice in the N hemisphere oc- the seabed. There are many applications of curs on or about June 22 when the Sun is the sonoprobe technique to offshore min- overhead at the Tropic of Cancer, its far- eral exploration and engineering surveys, thest north. The winter solstice is about for example in the laying of submarine December 22 when the sun reaches pipes and cables and in the siting of oil rigs. 23°30′S, the Tropic of Capricorn. At the summer solstice is the year’s longest day sorting The process by which materials and at the winter solstice, the shortest. forming a particular sediment are graded according to size by natural processes. If solum /soh-lŭm/ The surface soil and the the size range is confined within narrow subsoil, excluding the parent material. limits, so that most of the material is silt size (as in eolian brickearth) or fine sand solution (in geology) A basic form of size (as in many beach sands), the deposit is chemical weathering in which solid ma- well sorted. If the deposit has many size terials are dissolved by water. Rates of so- ranges (as in a slump deposit, for example) lution vary considerably from one it is poorly sorted. substance to another; silica is fairly insolu- Sorting is best in sediments subject to a ble and its removal rate increases with tem- long distance of transport, with repeated perature, whereas the most soluble rock reworking, as happens to beach sands minerals are calcium, sodium, magnesium, being washed along a coast by wave action. and potassium. Halite and gypsum are very In slump deposits, transport is sharp and soluble, subsequent recrystallization some- short with no time for any sorting.

325 soufrière

The basis of sorting concerns the set- spar A general term for any lustrous tling velocity of a particle; each size and translucent or transparent mineral that density of grain has a minimum level of en- cleaves easily. Examples include Iceland ergy necessary to overcome its weight and spar and various other forms of CALCITE. keep it in suspension. If the energy environment falls below that critical level, the sparite /spar-ÿt/ Coarse CALCITE of a grain settles out. Hence on a coast, where grain size in excess of 0.01 mm in diameter, wave energy shows a gradient from high on which may infill cavities within and headlands to low in bays, there is a corre- around ALLOCHEMS in LIMESTONE. The ter- sponding sorting of sediment, the coarsest minology used in the petrographic descrip- fractions in the high-energy areas, the finer tion and classification of limestones is fractions in the low-energy areas. based on the presence of sparite, MICRITE and allochems. soufrière /soo-free-air/ A volcano that emits sulfurous gases; a large solfatara. See spatter cone A volcanic structure con- fumarole. sisting of a steep-sided mound of lava that has issued from a central vent or along a sound 1. A waterway somewhat wider fissure. The lava takes the form of lumps and cinders, adhering together, that have than a STRAIT linking two bodies of water, or a passage between an island and the come from sprays of frothing molten mainland. magma. 2. A relatively long sea arm or inlet. species The basic group in the taxonomic classification of organisms, which is sounding 1. A measure of the depth of usually defined as a collection of similar in- water. Before the advent of modern echo- dividuals that are capable of interbreeding sounding techniques, a sounding line with to produce fertile offspring. In classifying a lead weight on the end was used, but this fossil species this criterion is impossible to often gave very inaccurate readings be- satisfy, and these species are usually de- cause of the effect on the sounding line of fined on the basis of structural similarities wave and current action. Almost all sound- (which in practice are also used in classify- ings are now obtained using echo ing modern species). This type of species, sounders. These devices are usually set to defined solely on morphological character- record at speeds of some 1600 m/s. Mod- istics, is known as a morphological species. ern sounding techniques largely stem from Difficulties in defining fossil species arise the use of acoustic equipment in submarine because of the absence of soft parts, whose warfare during World War II. structural characteristics provide an im- 2. (upper-air observation) (in meteorology) portant means of classifying modern A vertical profile of the chemical and/or species. In addition the boundaries be- physical properties of the atmosphere (e.g. tween the successive fossil species of a con- relative humidity, dry-bulb temperature, tinuously evolving line must be arbitrary, wind direction, and wind speed). Observa- because the species tend to grade into each tions may be obtained directly, such as other. Some fossil species do not show ob- from a radiosonde, or indirectly through vious affinities with any organisms alive satellite imaging. today. Examples of such species are those that produced CONODONTS: different skel- South Equatorial Current See equato- etal elements belonging to this group are rial current. given different specific names, although in life they may have formed part of a single southern lights See aurora. individual. A species is defined with reference to a southern oscillation See El Niño; oscil- TYPE SPECIMEN, which is selected as a stan- lation. dard and formally described. The name of

326 spilite a species is always italicized and preceded a sphere, used as a reference surface for ge- by the name of the GENUS to which it be- odetic surveys. Depending on the area of longs. For example, the species that in- the world in which the survey is being car- cludes modern man is Homo sapiens, or H. ried out, different spheroids are used as a sapiens. A species name is often followed reference datum, e.g. in the British Isles, the by the name (in abbreviated form) of the Airey spheroid (named for the British person who first formally described the mathematician George Biddell Airey species. (1801–92)) is used, whereas in much of Africa, Clarke’s spheroid (1880), intro- specific latent heat See latent heat. duced by the British geodesist Alexander Ross Clarke (1828–1914), is used. specular iron ore /spek-yŭ-ler/ See hematite. spheroidal weathering /sfeer-oi-dăl/A form of EXFOLIATION that takes place on speleology /spee-lee-ol-ŏ-jee/ The scien- boulders below ground level. Initially an- tific study of caves and caverns, including gular joint blocks are gradually rounded their exploration, geology, and mineral- owing to preferential weathering on the ogy. corners, a resultant block consisting of a solid rounded core surrounded by spher- speleothem /spee-lee-ŏ-th’em/ A min- oidal shells. The term is sometimes re- eral formation in a cave that results from stricted to exfoliation involving no change the action of water. Most consist of CAL- in volume (any weathering products are re- CITE (calcium carbonate), and the most typ- moved in groundwater), in contrast to ical are STALACTITES AND STALAGMITES. FLAKING. The layering within spheroidal blocks is due to the chemical migration of spessartite /spess-er-tÿt/ (spessartine) An elements within the rock, creating zones of orange to deep red member of the GARNET enrichment and depletion. Any totally group of minerals, Mn3Al2Si3O12. It occurs rounded blocks found above ground level in igneous and thermally metamorphosed have probably been exhumed (see exhuma- rocks. It is used as a semiprecious gem- tion). stone. spherulitic /sfe-rû-lit-ik/ Describing a sphalerite /sfal-ĕ-rÿt/ (zinc blende, blende, texture commonly found in glassy or cryp- black jack) A yellow to red-brown lus- tocrystalline acid rocks in which acicular trous mineral form of zinc sulfide, ZnS, crystals of quartz and alkali feldspar are ra- sometimes containing some cadmium and dially arranged to form a sphere. iron. It crystallizes in the cubic system, and Spherulitic growth is the result of DE- occurs in hydrothermal veins and metaso- VITRIFICATION of glassy material or rapid matic replacements. It is the chief source of crystallization of viscous magma. See vari- zinc. olitic. sphene /sfeen/ (titanite) A monoclinic spicule See Porifera. mineral of composition CaTiSiO5 but with some replacement of O by OH and F. spilite /spil-ÿt/ Any of several basaltic Sphene is a common ACCESSORY MINERAL in lavas containing albitic plagioclase and in igneous rocks. It is also found in metamor- which the primary mafic minerals have phosed calc-silicate rocks. been altered to chlorite and epidote. Al- tered acid rocks associated with spilites are sphenochasm /sfee-nŏ-klaz-ăm/ A tri- called KERATOPHYRES and may contain phe- angular block of oceanic crust between nocrysts of albite and hornblende in a two continental areas. groundmass of albite, chlorite, epidote, and quartz. Spilitic rocks occur as PILLOW spheroid /sfeer-oid/ A figure resembling LAVAS interbedded with marine sediments.

327 spilling breaker

They appear to be basalts and rhyolites of spit An elongated accumulation of sand the ocean floor, which have suffered post- or shingle (or both) attached to the coast at magmatic soda metasomatism due to inter- one end and extending out to sea. Spits action with sea water. See also ophiolite; occur most frequently where the coastline Steinmann trinity. changes direction abruptly. The longshore movement of beach material is an essential spilling breaker See breaker. factor in the formation of spits. The material is moved along one straight section of spinel /spi-nel/ One of a group of oxide the coastline and, rather than rounding a minerals with cubic symmetry, often oc- sharp headland, accumulates seaward in curring as octahedral crystals. The general the original direction of movement. Spits 2+ 3+ 2+ formula is R R2 O4 where R = Mg, Fe, sometimes form across estuary mouths, or Zn, Mn, or Ni and R3+ = Al, Fe, or Cr. two may approach each other from two Most spinels fall into one of three series de- facing headlands (a double spit). A com- pendent upon the nature of the trivalent mon feature is a seaward end that curves cation: spinel (Al), magnetite (Fe3+), and toward the land. This may be produced by chromite (Cr). Ulvöspinel also has a spinel wave refraction or two different sets of structure. waves may be in action, one causing accu- There is a continuous chemical series mulation and the other the rounding of the 2+ end. between hercynite (Fe Al2O4) and true spinel (MgAl O ) because of the replace- 2 4 spoh-dŏ-sôl ment Fe2+↔Mg. Minerals of the spinel spodosol / / One of the SOIL TAXON- series are a wide variety of colors includ- twelve soil orders of the US OMY including ashen or podzolized soils, ing red, brown, blue, green, and black. characterized by a spodic B horizon con- Magnetite (Fe2+Fe 3+O ) is black and 2 4 taining humus and silicate clays with alu- forms a continuous series with ulvöspinel minum or iron that has been moved down (Fe 2+TiO ). Chromite is Fe2+Cr O but 2 4 2 4 the profile from the bleached A horizon with some replacement Mg↔Fe2+. There above. This includes the true PODZOLS, and is a complete chemical series between the PODZOLIC SOILS and groundwater pod- chromite and hercynite due to the replace- ↔ zols. Spodosols develop mostly in humid ment Cr Al. The brown variety, picotite, cool temperate climates or on acidic parent is a chromium-rich hercynite. materials. Spinels occur in high-temperature metamorphosed rocks, especially limestones. spodumene /spoj-û-meen/ A lithium- Hercynite is found in metamorphosed bearing clinopyroxene. See pyroxene. pelites and in basic and ultrabasic rocks. Magnetite is ubiquitous in metamorphic sponges See Porifera. and igneous rocks and is an important iron ore. Chromite is the chief chromium ore spontaneous potential log (SP log)A and, like magnetite, often occurs in subsurface logging technique that continu- monomineralic bands in layered basic ig- ously records the spontaneous potential neous bodies. present at different depths. This results from electrochemical reactions between SP interval (in seismology) The time the drilling mud and formation fluids, and that elapses between the arrivals of the first small electromotive forces resulting from secondary (S) and primary (P) waves fol- the penetration of drilling fluids into the lowing an earthquake. It provides a means formation. of calculating the distance to the earthquake’s focus. See also seismic wave. spot height A height above the vertical datum, represented on a map by a dot lo- spirit level structure See geopetal cav- cator with the elevation printed beside the ity. dot. Unlike bench marks or trigonometri-

328 stage cal points, spot heights are not identifiable stability The state of the atmosphere on the ground. where vertical air movement is limited owing to the slow rate of cooling of the EN- spreading rate The rate at which new VIRONMENTAL LAPSE RATE. If the environ- ocean floor is added to the oceanic plates at mental lapse rate is less than the SATURATED constructive PLATE BOUNDARIES. It is there- ADIABATIC LAPSE RATE then the state is fore directly related to the rate at which known as absolute stability, because no ris- new ocean floor is produced. ing thermal can cool at a slower rate and so will quickly become colder than the envi- spring A place where water flows out of ronment and sink back. These conditions the ground because the water table inter- are typical of summer anticyclones. sects the surface. Thus the water flows out from above an impervious rock, such as stabilized dune A coastal dune in which clay, shale, or slate. the cutting off of direct wind action by the growth of another dune line in front, and spring equinox See equinox. the development of a virtually continuous vegetation cover, has ended sand move- spring line The line where a water table ment by the wind. These dunes generally intersects the surface of the ground, behave a very subdued topography, unlike a tween layers of pervious and impervious MOBILE DUNE, and a species-rich vegetation. rocks, and there is a spring or springs. See spring. stable zone A part of the Earth’s crust that is not undergoing mountain building (orogeny) or deformation through crustal spring tide A TIDE that has a relatively movements. Such zones are generally near large range, rises and falls to the greatest the centers of continents, away from plate extent from mean tide level, and occurs at boundaries. or near the times of full and new moon. A period of spring tides allows waves to stack A small pillarlike island just off- break farther up or down the beach, and at shore, generally at the end of a headland. It those times when strong onshore winds co- has been detached from the headland by incide with the time of high water, this may the erosive action of waves, which may lead to flooding of the hand behind the have first formed a cave that was gradually beach. A wider tract of beach is intermit- enlarged. See natural arch; natural bridge. tently uncovered compared with a period of NEAP TIDES. Also, tidal currents tend to stage 1. The point to which a landscape be stronger during spring tides than during has evolved in a CYCLE OF EROSION, which neap tides. determines its appearance. Landscapes used to be labeled youthful, mature, or se- spur An area of high ground that pro- nile, but this labeling is now being dropped jects into lower ground. A spur in the because most landscapes are polycyclic (see upper parts of a river valley may form IN- polycyclic landscape), so one cannot at- TERLOCKING SPURS, but farther down the tribute their appearance purely to the stage valley they are likely to be eroded. of evolution in the current cycle, and because evidence is increasingly being pro- squall A sudden increase of wind speed duced that landforms do not adjust lasting for several minutes before dying steadily with time through equally long away quickly. To be classed as a squall, the stages: they evolve very rapidly at first and wind should rise by at least 16 knots (8 then achieve a steady state of little further ms–1) and reach at least 22 knots (11 change. ms–1). A squall is often, although not nec- 2. A division of rock in the Standard essarily, accompanied by heavy rain or a Stratomeric scheme of stratigraphic classi- thunderstorm. fication (see chronostratigraphy). It indi-

329 stagnant ice cates the body of rock that has formed dur- with a lapse rate of 6.5°C/km up to 11 km ing one AGE. A number of stages grouped at the tropopause. together constitute a SERIES and the stages themselves are formed of several CHRONO- standard parallel The line of latitude ZONES. Stages are often spoken of in exclu- selected in the construction of a map pro- sively biostratigraphic terms but they may jection that is projected at its true length. be calibrated and defined by methods not involving fossils. They are usually named standing wave (stationary wave) A type after geographical localities and have the of water wave in which there is a surface ending -ian, as in the Oxfordian, a stage of oscillation that does not travel along the the Jurassic System. sea’s surface. The surface oscillates up and 3. (gauge height) The height of a water sur- down between fixed points (nodes). A crest face in a river or stream above an estab- at one moment becomes a trough at the lished fixed datum level. next, and so on. Water particles have maximum horizontal travel at the nodes and stagnant ice An ice mass that no longer maximum vertical travel at the antinodes receives an adequate supply of ice in the ac- (or loops). The height of a standing wave is cumulation zone to maintain movement. approximately twice that of the initial inci- The ice melts downward from the surface, dent waves. Such a situation is best ob- meltwater frequently forming lakes mar- served when waves approach a vertical ginal to the ice, which may subsequently be barrier such as a cliff or seawall and are re- evidenced by deltaically-bedded terrace deflected so as to meet the incoming waves. posits. Debris-covered ice will melt more The depth of water must be sufficient to slowly than that exposed, and in this way prevent the incoming waves from break- large masses can be isolated, resulting in ing. Standing waves may also be produced KETTLE HOLE formation at a later stage. The at sea, for instance when similar PROGRES- large amounts of meltwater present are re- SIVE WAVE trains meet from opposite direc- sponsible for the characteristic ice-stagna- tions. See also clapotis. tion features, namely KAMES and ESKERS. star dune A complex DUNE that has a stalactites and stalagmites Two types star shape when seen from above, and has of SPELEOTHEMS that occur in limestone slip faces on three or more arms that radi- caverns. Stalactites are tapering pendants ate from a central point. Star dunes grow projecting downward from the rocks of vertically and are the largest type of desert limestone caverns. They are formed from dune; they are also believed to be the old- drips of water containing dissolved cal- est. They occur in areas with multidirec- cium carbonate, which on evaporation de- tional wind regimes. Star dunes occur in posit a small trace of solid material. the Grand Erg Oriental in the central Sa- Stalactites take many thousands of years to hara and in the Badain Jaran Desert, form and are usually associated with sta- China. lagmites, which develop on the cave floor in a similar way. Stalagmites tend to be staurolite /stor-ŏ-lÿt/ A mineral with an 2+ much broader at their bases than stalac- approximate composition Fe Al4Si2- tites, and in time they extend upward. O10(OH)2. It shows orthorhombic symme- Eventually stalagmite and stalactite may try and is brown to yellow. It has a join to form a complete pillar. structure of alternating layers of kyanite 2+ and a composition Fe (OH)2 but with standard atmosphere The idealized at- some replacement Mg↔Fe2+. Staurolite is mospheric structure in terms of tempera- found in medium-grade regionally meta- ture and pressure for all heights. It is used morphosed argillaceous rocks, often in as- for the calibration of altimeters, etc. Sur- sociation with kyanite and almandine. face characteristics are taken as 15°C and 1013.25 mb for temperature and pressure steatite /stee-ă-tÿt/ See soapstone.

330 stone stripes steering The control of the direction and stilbite /stil-bÿt/ A white, gray, or red- speed of movement of pressure systems by brown mineral form of a hydrated some atmospheric factor. Thermal steer- aluminosilicate of calcium, sodium, and ing, or movement in the direction of the potassium, (Ca,Na2K2)(Al2Si7O18).7H2O. thermal wind, works quite well for nonin- It crystallizes in the monoclinic system as tensifying depressions, but in developing sheaflike aggregates in hydrothermal veins lows and anticyclones it is less reliable. in igneous rocks. It is a member of the ZE- OLITE group of minerals. Steinmann trinity /stÿn-măn/ The common association within geosynclinal sedi- stishovite /stish-ŏ-vÿt/ A very dense ments of SPILITES, CHERT, and SERPENTINITES product of QUARTZ, SiO2, produced at ex- (and other ultramafic rocks). The trinity is tremely high pressures, possibly following named for the German geologist Gustav a meteorite impact. See also silica minerals. Steinmann. See also ophiolite. stock A discordant igneous intrusion, stemflow The portion of precipitation usually of coarse-grained granitic rock, re- or irrigation water that, having been inter- sembling a batholith but having a surface cepted by a plant, flows down the stem to exposure of less than 50 sq km. Compare reach the ground surface. See also intercep- boss. tion; throughfall. stone polygons or circles A form of steppe An area of flat open temperate periglacial PATTERNED GROUND found in grassland in an arid region that will not flat areas. The polygons or circles consist support trees, particularly that of south- of fine-grained material, which is usually eastern Europe and Asia. If the land is saturated with water and frequently plowed, it provides fertile soil for growing domed upward, surrounded by slightly cereal crops. In this respect, it is similar to raised walls of coarser stones, which pro- PRAIRIE. ject into the soil for several centimeters. The diameter of these features, which may Stevenson screen An instrument shel- occur either in large groups or in total iso- ter: the standard housing for meteorologial lation, varies considerably: the larger instruments, named for the Scottish engi- forms are several meters across and are neer Thomas Stevenson (1818–87). It con- found in areas of long-continued freezing sists of a wooden box on legs, the base of and extensive summer thaw, whereas the box being 1 m above the ground, in smaller examples reflect more frequent which thermometers are placed. Ventila- freeze-thaw cycles. Their method of formation is provided by louvered sides, which in tion is still not totally clear, but is probably normal conditions allow adequate air concerned with pressures set up by re- movement but prevent solar radiation peated freezing and thawing, which move reaching the thermometer bulbs. Access is the coarser fragments both upward and provided by (in the N hemisphere) the outward. north-facing side of the screen being hinged; this again prevents sunlight affect- stone stripes A type of periglacial PAT- ing the temperature records when records TERNED GROUND that forms in a similar are being taken. way to STONE POLYGONS OR CIRCLES when the processes responsible are acting under stibnite /stib-nÿt/ A lead-gray lustrous the influence of gravity on a slope. The re- mineral form of antimony sulfide, Sb2S3. sult is a series of alternating coarse and fine Crystallizing in the orthorhombic system, parallel downslope stripes of stone. The it occurs in hydrothermal veins and re- coarse lines can vary from a few centime- placement deposits, often with lead, mer- ters to about two meters wide, while the cury, and silver. It is the principal source of finer stripes between them are often at least antimony. twice this width. Stone polygons developed

331 stony-iron meteorite on flat ground may become elongated to- ier is coming. It is exposed to the abrasive ward a slope and then grade into stripes as action of the moving glacier, and as a result the slope angle increases. usually has gentle slopes and rounded features. stony-iron meteorite See meteorite. strain The deformation of a body of stony meteorite See meteorite. rock as a result of stress. It may be a change in shape, volume, or both. See heteroge- storm 1. In general, a period of strong neous strain; homogeneous strain. winds, often accompanied by heavy rain. Major storms are called CYCLONES, HURRI- strain hardening The increasing CANES, or TYPHOONS, depending on where strength of a rock as it deforms plastically. they occur. 2. In the BEAUFORT SCALE, force 11 with strain rate The rate at which a material wind speeds of 56–63 knots. deforms. This is usually expressed as a percentage by which the original length is al- storm beach An accumulation of coarse tered per second. beach material found at the very highest levels of a beach profile, far above the high- strain-slip cleavage (crenulation cleav- est points reached by high spring tides. age) A type of cleavage superimposed These boulders and shingle blocks attain upon slaty cleavage. It is typified by tabu- such a height as a result of the extreme lar bodies of rock between regularly spaced power of very infrequent storm waves. Al- cleavage planes. though their overall effect on a beach is destructive, the SWASH of these waves can strait A comparatively narrow water- throw some material to great heights and way that links two sea areas or other large rapid percolation of water prevents its sub- bodies of water. Some authorities, how- sequent removal by the BACKWASH. ever, restrict the term to gaps in isthmian links or island chains. storm hydrograph See hydrograph. stratification /strat-ă-fă-kay-shŏn/ The storm surge The rapid rising of water condition shown by sedimentary rocks of level during a storm as a result of wind being disposed in horizontal layers or BEDS, stresses acting on the sea’s surface, or a known as strata. The term is also occasion- PROGRESSIVE WAVE resulting from certain ally applied to igneous bodies that display meteorological circumstances. If this situa- analogous parallel textures. tion occurs at the time of high SPRING TIDES, the tidal levels may significantly exceed stratigraphic succession /strat-ă-graf-ik/ predicted values and serious flooding and The series of the Earth’s rock strata damage may result. This occurred with dis- arranged in sequence from the earliest to astrous results at Galveston, Texas, in the latest in age. See also geologic 1850. In August 2005 Hurricane Katrina timescale. produced a storm surge of up to 9 m which, in combination with the wave ac- stratigraphic trap See trap. tion and extremely high winds, caused severe flooding and damage in the affected stratigraphy /stră-tig-ră-fee/ The branch parts of the Gulf Coast of Louisiana and of geology concerned with the descrip- Mississippi; the failure of levées and flood- tion and classification of bodies of rock walls left parts of New Orleans under 6 m and their CORRELATION with one another. of water. Various aspects of this are dealt with in LITHOSTRATIGRAPHY, BIOSTRATIGRAPHY, and stoss /shtohss, stoss/ The side of a hill stratomeric and chronomeric CHRONO- that faces the direction from which a glac- STRATIGRAPHY. This separation of the vari-

332 stream order ous concepts involved in stratigraphy is stream A body of water on the Earth’s necessary if different levels of inference are surface flowing within a natural channel. to be distinguished. The term is applied to all such flows, ranging from the smallest creek to large rivers. stratocumulus /stray-toh-kyoo-myŭ-lŭs/ A layer cloud within which weak convec- stream capacity See capacity (def. 1). tion is taking place. It often forms extensive sheets, which have dark areas stream discharge hydrograph See hy- separated either by small areas of clear sky drograph. or by thinner cloud. It occurs most frequently in winter anticyclones or over cool stream frequency See drainage density. ocean areas where anticyclones are dominant. streamline A line that is parallel to the instantaneous direction of the wind field at stratomere /strat-ŏ-meer/ (in stratigra- all points along it, showing overall air phy) Any segment of a sequence of rock. movement. The term is used particularly in Stratomeres are not of standard uniform tropical areas where isobars are of little use size nor need they have been formed during in indicating the wind field because of the equal intervals of geologic time. breakdown of the geostrophic relationship. stratopause /strat-ŏ-pawz/ The bound- stream load See load. ary between the stratosphere and mesosphere. It is found at a height of about 50 km and represents the temperature maxi- ° 1 mum of about 0 C resulting from energy 1 1 1 release in the ozone layer. 2 1 2 2 2 2 stratosphere /strat-ŏ-sfeer/ That layer 2 1 of the atmosphere above the troposphere 3 extending to a height of about 50 km. The lower stratosphere is almost isothermal but 3 temperatures increase gradually to their 1 secondary maximum of about 0°C at the 2 stratopause. Winds in the stratosphere can 3 2 1 be quite strong, especially the POLAR NIGHT 2 JET STREAM. 4 1 stratovolcano /stray-toh-vol-kay-noh/ See composite volcano. Stream order in a hypothetical drainage pattern stratum /stray-tŭm, strat-ŭm/ See bed. stratus /stray-tŭs, strat-ŭs/ A layer cloud stream order The classification of a with a fairly uniform base found at lower stream in an integrated drainage pattern levels in the atmosphere or actually at the that is broken down into stream segments ground surface in the form of hill fog. It and the segments then ranked in a hierar- may give drizzle or Scotch mist. chy according to their allotted order. At the head of the basin, the first small streak A colored mark left when a min- tributaries are first order; two or more eral is drawn across a piece of unglazed first-order tributaries unite to give a sec- porcelain. It is characteristic of the mineral ond-order tributary; two or more second- and commonly used as an aid to identifica- order tributaries unite to produce a tion. third-order tributary, etc., up to the major

333 stream terrace stream of the basin, whose order defines green bacteria (cyanobacteria). Fossilized the order of the basin. Quantitative com- stromatolites dating back to Precambrian parisons can then be made between basins, (Archean) times have been found. and studies made of the relationships between orders. Studies of this kind have pro- stromatoporoid /strom-ă-toh-por-oid/ duced apparent laws of drainage basin An extinct organism that built laminated development. See also bifurcation ratio. reef structures in the Paleozoic. They were particularly common during the Silurian stream terrace See river terrace. Period. Their taxonomic affinities are obscure but they may belong to the Hydro- stress An applied force per unit area, zoa, a class of the phylum CNIDARIA. They causing rocks to undergo STRAIN. Stress can and the tabulate corals (see Anthozoa), take the form of tension, compression, or which were also important reef-builders in shear. this era, seem to have preferred different conditions, for they are rarely found in as- striation /strÿ-ay-shŏn/ A line that has sociation. been cut or scratched into a rock surface over which a glacier has passed. Such lines Strombolian See volcano. are usually short, but sometimes exceed one meter, and are best seen on gently slop- strontianite /stron-shee-ă-nÿt/ A coloring faces of fine-grained hard rock, up and less, white, yellow, or green mineral form over which the ice moved. Ice itself cannot of strontium carbonate, SrCO3. It crystal- cause such abrasions; the projections of lizes in the orthorhombic system and oc- rock debris, held rigidly in the basal ice, act curs in nodules and veins in limestone as the cutting tools. Each striation can in- rocks. It is used as a source of strontium dicate two diametrically opposed direc- and its compounds. tions of ice movement, and they have often been used to discover movements of for- structural geology The branch of geol- mer ice masses. ogy that deals with the structural features of rocks, their analysis, and their descrip- strike The direction along a rock stra- tion, including the forces that create the tum at right angles to the true dip. features. strike line (structure contour) A line structural high A geologic structure of joining points of equal elevation above or positive relief, such as a dome or anticline. below a selected stratigraphic datum. structural low A geologic structure of strike–slip fault (tear fault, transcur- negative relief, such as a syncline or basin. rent fault, wrench fault) A vertical or near- vertical fault in which the displacement is structural trap See trap. horizontal and motion is parallel to the strike of the fault. The movement along the structure contour See strike line. fault is described as left lateral (or sinistral) if the block on the side of the fault opposite subaerial erosion /sub-air-ee-ăl/ ERO- to an observer moves to the left, and right SION that takes place at the surface of the lateral (or dextral) if the block moves to the Earth. right. The San Andreas Fault zone is a well- known example of a particular type of subcrop /sub-krop/ The disposition of a strike–slip fault, the TRANSFORM FAULT. bed of rock beneath the surface and its contact with the undersurface of younger beds. stromatolite /strŏ-mat-ŏ-lÿt/ A lami- The term is used by the petroleum industry nated concentric structure formed of in the reconstruction of the paleogeogra- calcium carbonate and produced by blue- phy of an area.

334 submarine fan subcrustal convection /sub-krus-tăl/ after the wave has broken movement is sea- Gross movement of semifluid magma in ward. Thus sand accumulates from two di- the Earth’s mantle resulting from convec- rections, although an optimum size is tion, which in turn causes the movement of reached eventually, beyond which no fur- the crustal plates. See plate tectonics. ther increase in height is possible, owing to the effect of the bar on waves passing it. subduction zone /sub-duk-shŏn/ (destruc- The outermost bar, in deepest water, is in- tive plate boundary) An area on the frequently fully developed and is formed Earth’s surface where two lithospheric by the rare strong storm waves; it is unaf- plates are converging and one, containing fected by the smaller storm waves respon- oceanic lithospheric material, is being sible for the bars nearer the shore. The overriden by the other, which may be innermost bar tends to move its position as oceanic or continental. The overridden wave heights vary; movement is seaward plate is forced down to descend into the as- with increasing wave height and landward thenosphere in a process known as subduc- with decreasing height. tion. Where the lithospheric plate descends, it forms a deep-sea TRENCH, a submarine canyon A deep steep-sided long linear depression that is the deepest valley or trench cut into the continental feature of the Earth’s surface, and its posi- shelf or continental or insular slope. Many tion is marked by a series of earthquake are deeply incised into the solid rock of the foci. With descent, the lithospheric ma- sea floor, with V-shaped cross profiles, and terial is subjected to greater temperatures some form deep winding gorges. Well- and pressures, as a result of which some known submarine canyons include those starts to melt to form magna, which then off the mouth of the Congo River, the rises upward through the overlying crust to Hudson River, and Cape Breton in S produce a linear chain of volcanoes paral- France. Canyons may have tributary inci- lel to the trench. If the oceanic lithosphere sions and and these sometimes display a subducts below oceanic lithospheric ma- dendritic pattern; others occur singly or as terial the volcanoes form an island volcanic part of a system of subparallel incisions. arc. See also plate tectonics. These features are thought to have originated through erosion by submarine TUR- subglacial /sub-glay-shăl/ Describing BIDITY CURRENTS. See also sea valley. the region underneath a glacier or ice sheet. submarine contour See contour. subhedral /sub-hee-drăl/ Describing crystals with partially developed crystal submarine eruption A volcanic erup- forms. Compare anhedral; euhedral. tion from a vent in the oceanic crust on the sea floor. Most of these eruptions take sublimation /sub-lă-may-shŏn/ The place on SEAMOUNTS or along MID-OCEAN process by which ice is converted to vapor RIDGES. and water vapor into ice with no intervening liquid state. Chemically, the term is submarine fan A fan-shaped mass of used only for the first process. sediment normally found at the lower end of a SUBMARINE CANYON. Much of this ma- submarine bar An offshore accumula- terial may have been deposited under the tion of sand, more or less parallel with the action of submarine slides and turbidity coastline, that never becomes exposed by flows. For example, a very large fan exists the tide. Submarine bars occur in areas in at the lower end of the Congo Canyon, which the tidal range is extremely small where slides or turbidity flows occur about and usually form in groups of two or three. 50 times annually. The Mississippi Delta is They are formed at the point at which steep flanked on its western side by a trough val- storm waves break. Seaward of this point ley that leads, at a depth of some 1800 m, material is moved toward the land, then into a large submarine fan. In both cases,

335 submarine valley very substantial amounts of muddy sedi- subtropical high (subtropical anticy- ments are carried to the coast by the rivers. clone) One of the anticyclones in the belt of semipermanent high pressure in the sub- submarine valley See sea valley. tropics. submergence The covering of land by subtropical jet stream See jet stream. the sea, either because the land has sunk or sea level has risen. See also cycle of erosion; succession The vertical sequence of fiord; ria. rock units in a particular place. A number of local successions may be combined to subsequent stream A stream that has produce an idealized succession demon- developed on a weak substructure, such as strating the sequential order of strata over a clay vale, or in regional joint or fault pat- a larger region. terns, at right angles to streams consequent to the dip of the slope. Subsequent streams suevite /swee-vÿt/ A type of BRECCIA con- owe their development to the accelerated sisting of angular fragments of rock in a rate of headward erosion they are able to glass matrix. perform in these weaker areas, rapidly extending themselves and often coming to in- sugar loaf An INSELBERG, especially in tegrate and dominate the drainage pattern. the S coastal area of Brazil. See also consequent stream; obsequent stream. sulfur A native element (S) of characteristic yellow color found around volcanic vents and hot springs. subsidence The widespread downward movement of air associated with surface sulfur bacteria Bacteria that release ele- DIVERGENCE. The rate of descent is usually mental sulfur as a result of their metabo- fairly slow, of the order of a few meters per lism. They may, for example, oxidize hour, but this results in adiabatic warming hydrogen sulfide (to obtain energy) or re- and a low relative humidity. Near the lease sulfur through a type of PHOTOSYN- ground, mixing takes place so that extreme THESIS. See also iron bacteria. values of relative humidity are unusual. The weather associated with large-scale sulfurous smog See smog. subsidence is always dry although cloud layers may result from mixing with moister sulfur oxidation A process in soil sci- air near the ground. ence analogous to nitrification, in this case the bacterium Thiobacillus converting in- subsoil See profile. organic sulfur compounds into the sulfate form. This may occur solely by chemical subsolidus /sub-sol-ă-dŭs/ A mixture of means but most appears to be biochemical compounds, such as minerals, below their in nature. melting points in which chemical reactions may still take place (in the solid state). summer solstice See solstice. subsurface eluviation The process by sun crack See desiccation crack. which water falling on the soil and sinking in eventually moves downslope as a sub- sunshine Direct radiation received from surface flow; as it does so it dissolves or the Sun. It is one of the climatological el- transports soil and rock material with it. ements recorded at most observing sta- This process has tended in the past to be tions. overlooked in studies of slope evolution. However, evidence has shown that in sunshine recorder Sunshine is fre- places it can be very effective. quently measured by the CAMPBELL–STOKES

336 supersaturation

RECORDER, which consists of a glass sphere Often various relict weathering horizons that focuses the Sun’s rays onto a card, are included in this term, but this is not burning a trace on the card whenever strictly correct because these materials are bright sunshine occurs. The length of the related to the rocks beneath. trace indicates sunshine duration. Al- lowance is made for the seasonal variation supergroup The largest division in the in declination of the Sun. As it records only hierarchy of the lithostratigraphical classi- bright sunshine many hours are not fication of bodies of rock (see lithostratig- recorded, especially in early morning or raphy; stratigraphy). A supergroup is late evening, or in conditions of severe formed of two or more associated and ad- haze. No distinction is made between the jacent GROUPS. different heating capacities of winter and summer sunshine. The Campbell–Stokes superimposition The most common recorder is named for an employee of the process by which a drainage pattern be- British Board of Health, Reverend J. F. comes discordant. Drainage originates on a Campbell, who originally designed it. It land surface and develops a conformable was subsequently improved upon by the pattern with the structures in that surface; British mathematician and physicist Sir as dissection proceeds this surface is George Gabriel Stokes (1819–1903). In the eroded away, and the drainage is let down USA the MARVIN SUNSHINE RECORDER has onto the newly exposed rocks below. been used as the standard instrument of the These have different structures and litholo- National Weather Service; the time scale is gies from the original surface, and so the supplied by a chronograph. It is named for drainage pattern no longer conforms to the the American meteorologist Charles Fred- structure over which it flows. Hence it has erick Marvin (1858–1943). A number of been superimposed from a former cover automatic electronic sunshine recorders onto the one it currently occupies. Once su- have been developed for use in automatic perimposition has occurred, the drainage weather stations. may adjust to the structures of its new surface, and mask its true origin. On Dart- sunstone A translucent type of oligo- moor, England, the E–W and N–S clase (see feldspar) that appears to glow rectangular pattern superimposed from the with a reddish light, caused by tiny parallel former Eocene and Cretaceous cover has inclusions of HEMATITE. See also moon- been supplemented by a NW–SE and stone. NE–SW pattern following the joint pattern of the granite. Current drainage is there- supercooling The cooling of a liquid to fore a mixture of accordant and discordant a temperature below its normal freezing streams. point. This is a very common phenomenon in the atmosphere because ice does not superposition (principle of superposi- form in cloud droplets at 0°C because of tion) The principle that if one set of the sparsity of suitable CONDENSATION nu- strata occurs on top of another in a succes- clei. Supercooling is essential to the BERG- sion the upper unit was formed later (ex- ERON–FINDEISEN THEORY of precipitation cept in the case of strata that have been formation. overturned tectonically). This principle provides the basis of understanding the se- superficial deposits Material of Holo- quence of events in time as shown by a par- cene and Pleistocene age that lies on top of ticular succession of rocks. It was first the solid geology, being extraneous ma- proposed by the Danish scientist Nicolaus terial transported and deposited by various Steno (1638–87) in 1669. geomorphological processes. They include glacial drifts, terrace gravels, alluvium of supersaturation The state of air that rivers, raised beach deposits, windblown contains more water vapor than the sand and loess, solifluction deposits, etc. amount required to saturate it. However,

337 superstructure the abundance of condensation nuclei usually prevent this situation arising in the atmosphere. 200 superstructure The higher levels within an orogenic belt, which have deformed in a 275 brittle manner as a result of their near-sur- 250 face position. These levels have suffered 225 only minor metamorphism. Compare in- 200 frastructure. 100 supervolcano An exceptionally large 100 ft contour interval with supplementary volcano that begins as a boiling reservoir of contours to show a small hill magma risen from the mantle to within the Earth’s crust, building in pressure until it finally erupts in a massive and devastating interval of 100 ft (40 m) may not show up explosion. Unlike the majority of volca- the hills. (See diagram.) noes, which are cone-shaped, supervolca- noes may be immense CALDERAS and can be supratenuous folding /soo-pră-ten-yoo- hard to detect. For example, the magma- ŭs/ A form of folding that results from filled caldera (about 70 km by 30 km) of differential compaction. Sediments are Yellowstone National Park was only de- bent around more competent materials tected in the 1960s through infrared SATEL- such as concretions or coral reefs. LITE imagery. Yellowstone has been on a regular eruption cycle of approximately surf Broken water resulting from turbu- 600 000 years, the last eruption being lent wave activity between the outer limit some 640 000 years ago. Calculations have of the SURF ZONE and the swash-backwash been made that indicate that during the zone on the beach. 20th century parts of the caldera rose by over 70 cm, raising concerns that the over- surface boundary layer See boundary due eruption may be imminent. layer. During the largest volcanic eruptions forces are great enough to eject volcanic surface runoff The part of rainfall that dust, debris, and gases, including sulfur runs off the surface of the land and does dioxide, into the upper atmosphere. The not filter into the ground or evaporate into sulfur dioxide combines with water to the atmosphere. form droplets of sulfuric acid, which form a reflective barrier to incoming solar radia- Water that stays at or tion with the effect of reducing tempera- surface water tures. The last supervolcano to erupt was near the surface of the land, as opposed to Toba, in Sumatra, 74 000 years ago. The GROUNDWATER. volcanic winter that ensued created a global catastrophe and is believed by some surf beat A type of long wave activity geneticists to have pushed human life to the that is evident at the coast by irregular os- brink of extinction. cillations of water level in the surf zone. These oscillations have periods of several supplementary contour A contour line minutes. A possible cause is the arrival of that may be shown (often as a dashed line) groups of higher-than-average waves, on maps of areas of detailed or irregular re- which have the effect of piling up water in lief patterns in addition to the standard in- the nearshore zone and causing a reflection terval for the area. For example, in an area of long-wave energy. The presence of surf where the land is generally level but there beat has been demonstrated by precision are several isolated small hills, a contour wave recorders set up in the offshore zone.

338 swash surf zone (breaker zone) The strip of material is sufficiently fine-grained.) Ex- water along the shore in which breaking periment has shown that at the break-point waves are actively dissipating their energy. of waves and in the swash zone, sandy ma- They do this by a process that involves sur- terial held in suspension is fairly evenly dis- face turbulence at a time when the waves tributed from the sea surface down to its or swell have become unstable and break. bed. Suspended load transport is of great Usually this is because of their encounter- importance in the dispersal of pollutants in ing very shallow water, but it may also be the sea. due to their meeting with opposing cur- 2. The fine sediment carried in suspension rents or wind. This complete breakdown of by a body of flowing air. waves or swell must not be confused with breaking seas or so-called white horses, in suspension A type of transport in which which there is only partial collapse, at sea, fine particles of sediment are carried in of wave crests (see breaker). The surf zone water. The particles remain in suspension, is characteristically wide off gently sloping held up by eddies caused by turbulence. coasts and relatively narrow off steeply The material being carried in this way is shelving coasts. The width of the surf zone called the SUSPENDED LOAD. It is the sus- fluctuates with constantly changing wave pended particles that make some rivers conditions; also, it is usually relatively look milky or muddy. wide in the case of sand beaches and narrower in the case of shingle or boulder swale /swayl/ See full. beaches. surge See sea surge. surface stream surging glacier A glacier that has a brief period of comparatively rapid flow. It occurs when accumulated ice in an ice reser- stream disappears underground voir reaches a critical amount and at swallow hole, usually suddenly surges downward at up to a hun- conical and occurring where dred times the normal speed. stream meets permeable rock surveying The construction of maps and plans by accurately recording the rela- stream reappears usually tive positions and heights of features on the at the base of permeable Earth’s surface, and plotting them to some layer of rock and continues as surface stream suitable scale. This is achieved by measuring distances, directions, and heights, the major surveying methods being: TRIANGU- LATION, TRAVERSING, CHAINING, PLANE Swallow hole TABLING, TACHEOMETRY, and LEVELING. See also geodetic surveying. swallow hole A SINKHOLE that is usually the site at which a stream disappears un- suspended load 1. The sediment car- derground. See diagram. ried in SUSPENSION within a body of water, beneath waves or in current flow, as op- swamp An area of soft wet land that has posed to the sediment that moves wholly or poor drainage and is generally water- intermittently in contact with the bottom logged. The dominant vegetation consists (bed load and saltating load). Shoreward of trees, such as eucalyptus, mangroves, of wave break-point, a great deal of fine maples, palms, and willows, depending on material may be carried in suspension be- the climate. See also bog; marsh. neath sea waves. (This also occurs in deeper water offshore provided that the swash The movement of a fairly thin

339 S wave layer of turbulent water up a beach, fol- their volcanic equivalents are divided as lowing the breaking of a wave. This water shown in the table. can move material, the extent of the move- Syenites range from oversaturated ment depending on the nature of the waves quartz-bearing types through saturated and of the beach. Most landward move- types containing neither quartz nor ment is achieved by flat waves, within feldspathoid to undersaturated strongly al- which the oscillation of water particles kaline varieties containing little or no tends to be elliptical. The oscillation in feldspar. Division may also be made ac- steeper waves is more circular, and the cording to the composition of the feldspar breaking water strikes the beach material into potassic (orthoclase, microcline), from directly above, giving equal opportu- sodipotassic (perthites), and sodic (albite, nity for up and down beach movements. oligoclase) types. The mafic minerals are Some of the swash water infiltrates into usually hornblende and biotite but the the beach material and the rest returns to more alkaline varieties contain sodic pyri- the surf zone as BACKWASH. The edge of boles. Common accessory minerals include swash action is frequently obvious from magnetite, apatite, and sphene. such signs as the wetted area of beach and Nordmarkite is an oversaturated the line of seaweed, driftwood, mica flakes, sodipotassic syenite containing microp- etc., left at the upper limit of swash flow. erthite, quartz, aegirine, and riebeckite. Swash action on a falling tide may lead to With an increase in the amount of quartz the formation of swash channels, as water to over 10%, nordmarkite passes into al- tends to become channeled on its return kali granite. Perthosite and pulaskite are flow down the beach. sodipotassic leucocratic syenites. In the former, the feldspar is perthite and in the S wave See secondary wave. latter, antiperthite. Larvikite is a distinctive variety containing dark blue feldspars swell 1. Waves that, having developed displaying strong SCHILLER. The feldspars under wind stress in the generating area, are antiperthitic oligoclase and orthoclase have sufficient energy to travel beyond the and, in addition, the rock contains aggre- wind field or to survive after the wind has gates of mafic minerals, titanaugite, dropped. They may travel into an area in olivine, and biotite. Shonkinite is a satu- which the winds are far weaker or absent rated potassic melasyenite containing or- or they may ultimately travel into another thoclase feldspar and mafic minerals. wind field. Such swell waves are usually Nepheline is the commonest feldspathoid fairly regular in form, with somewhat rounded crests. Their energy may enable them to travel considerable distances. In general, swell decreases in height and in- INTERMEDIATE PLUTONIC ROCKS creases in length as it travels away from a WITH VOLCANIC EQUIVALENTS wind field. Swells from several directions may be superimposed and swell may also Plutonic Volcanic carry relatively short and sharp-crested (sodic plagioclase) (andesine/oligoclase) waves, which are generated by local winds. 2. A dome or anticline, especially one that syenite trachyte, phonolite rises from the sea floor without reaching (dominant alkali feldspar/feldspathoid) the surface. syenodiorite trachyandesite syenite /sÿ-ĕ-nÿt/ A coarse-grained inter- (monzonite) (latite) mediate igneous rock in which the propor- (alkali feldspar = plagioclase feldspar) tions of alkali feldspar or feldspathoids are dominant over sodic plagioclase; in fact diorite andesite many syenites contain no plagioclase. The (dominant plagioclase feldspar) intermediate plutonic igneous rocks and

340 syngenetic present in undersaturated syenites but and the volcanic equivalents are the tra- other varieties contain sodalite, analcime, chyandesites or latites. The alkali feldspar and leucite. Some of the many different in trachyandesites is usually sanidine, oc- kinds of alkali syenite are: curring as microlites in a pilotaxitic or tra- Borolanite, containing orthoclase and chytic groundmass set with plagioclase nepheline-orthoclase intergrowths termed phenocrysts. Trachyandesites occur in as- pseudo-leucite, thought to have been sociation with andesites in calc-alkaline formed from the breakdown of leucite; ma- orogenic volcanic suites. Compare diorite; lignite, containing potassic feldspar and syenite. nepheline; foyaite, containing a perthitic feldspar and nepheline; litchfieldite, con- syenogabbro /sÿ-en-oh-gab-roh/ See al- taining potassic feldspar, albite, and kali gabbro. nepheline. Syenites containing nepheline and albite are called monmouthite or mar- symbiosis /sim-bÿ-oh-sis, sim-bee-/ The iupolite; in the former nepheline predomi- close association that exists between two nates, in the latter, albite. The feldspar-free organisms of different species in which types constitute the IJOLITE series. each partner benefits from the relationship. Microsyenites are the medium-grained A common example of symbiosis is the re- equivalents and the plutonic rock names lationship between flowering plants and apply with the prefix micro-. Most are por- some insects, in which the insects feed on phyritic and contain phenocrysts of ortho- the nectar of the plant, pick up grains of clase. The volcanic equivalents of syenites pollen, and later transfer them to another and alkali syenites are trachytes and flower (thus pollinating it). See also para- phonolites. site. Syenitic rocks are found in intrusive complexes often associated with continen- symmetry See crystal symmetry. tal rifting, in eroded volcanic islands of alkaline affinity, and within differentiated symplectite /sim-plek-tÿt/ An inter- dikes and sills. See also diorite; syenodior- growth of two minerals, one mineral being ite. riddled with complex wormlike inclusions syenodiorite /sÿ-en-oh-dÿ-ŏ-rÿt/ (mon- of the other, as in MYRMEKITE. zonite) A coarse-grained acid and intermediate rock that lies chemically and syncline /sing-klÿn/ A basin-shaped fold mineralogically between syenites and dior- in which the beds dip toward each other. ites. Syenodiorites are characterized by ap- See diagram at FOLD. proximately equal amounts of alkali feldspar and plagioclase of oligoclase-an- synclinorium /sing-klă-nor-ee-ŭm, -noh- desine composition. The mafic minerals ree-/ A syncline of regional extent, which are usually either hornblende or biotite, or is composed of a series of minor folds. both, but in some varieties augite occurs, often rimmed by hornblende. Syenodorites synecology /sin-ee-kol-ŏ-jee/ The scien- containing accessory quartz are called tific study of interactions within ecological quartz-monzonites and with an increase in COMMUNITIES, and between communities the amount of quartz pass into adamellites. and their environment. With a change to plagioclase of more calcic composition and an increase in the amount synform /sin-form/ A synclinal structure of mafic minerals, syenodiorites pass into composed of sediments whose precise syenogabbros. stratigraphic relationships are not known. Syenodiorites usually occur in small See diagram at FOLD. volumes associated with granite masses. The medium-grained varieties are termed syngenetic /sin-jĕ-net-iki/ Describing micromonzonites or microsyenodiorites mineral deposits that formed at the same

341 synkinematic time and by the same process as the rock in area at a particular moment in time, which which they occur. is shown on a SYNOPTIC CHART. This is built up from surface weather observations of synkinematic /sing-kin=ĕ-mat-ik/De- such elements as wind velocity, air temper- scribing minerals that are formed during a ature, cloud amount, dew-point tempera- period of deformation. ture, and pressure tendency. Isobars are constructed from pressure observations synoptic chart /si-nop-tik/ A chart or and fronts inserted where appropriate. It is map that summarizes the weather condi- most directly concerned with day-to-day tions over an extensive area at a given mo- weather forecasting. ment in time. It includes isobars to show atmospheric pressures, temperature meas- syntexis /sin-teks-iss/ See assimilation. urements, wind speeds and directions, cloud cover, etc. synthetic aperture radar See radar. synoptic index A method of assessing system A division of rock in the Stan- the sequences of pressure systems in a quantitative manner. Instead of isolated dard Stratomeric scale of stratigraphic wind readings the overall direction of sys- classification (see chronostratigraphy). It tem movement is taken into account, then indicates the body of rock that has formed aggregated to give 10-day or monthly to- during one PERIOD. System and period gen- tals. erally have the same name; for example, the Triassic System is the body of rock synoptic meteorology The section of formed during the Triassic Period. A sys- meteorology concerned with the descrip- tem, which does not have a standard uni- tion and analysis of synoptic weather in- form thickness, is formed of a number of formation, i.e. the weather over a wide SERIES grouped together.

342 T

tabular Describing a crystal or rock for- to Siberia. It has cool summers and very mation that is thin, wide, and long – cold winters, with frozen subsoil for much shaped like a table. of the year. Fallen needles from the trees create PODZOL soil. tacheometry /tak-ee-om-ĕ-tree/ (tachyme- try) A surveying method for finding the talc /tal’k/ A white or pale green mineral location and height of points using a with a mica-like layered structure and theodolite (set up at a point of known composition Mg3(Si4O10)(OH)2. Talc is height and position) and a leveling staff greasy to the touch and very soft, having a (placed at the unknown points). The direc- hardness of only 1 on the Mohs’ scale. It is tions of the unknown points are obtained formed during the hydrothermal alteration by measuring horizontal angles from a of ultrabasic and basic rocks and during known direction, while the distance from low-grade thermal metamorphism of the theodolite position is calculated using a siliceous dolomites. Rocks known as soap- formula involving the difference in staff stone or steatite consist almost wholly of readings for two cross-hairs on the dia- talc. phragm of the theodolite, and the angle of elevation or depression, derived from the talus /tal-ŭs/ See colluvium. vertical circle reading. Another formula, using the staff reading of the central hair tangential folding A type of folding and the angle of elevation, permits the produced in homogeneous rocks, analo- height of the point to be calculated. The gous to the bending of a block of rubber height of the theodolite must always be with circles drawn on it; the circles become taken into account whenever this method distorted but there is no sliding movement is being used. See also traversing. of any kind present. tachylite /tak-ă-lÿt/ A dark basic glass tantalite /tan-tă-lÿt/ A black mineral formed by the rapid chilling of BASALT and found particularly at the margins of dikes. form of mixed tantalates and niobates of iron and manganese, (Fe,Mn)(Ta,Nb)2O6. taconite /tak-ŏ-nÿt/ A fine-grained iron- It crystallizes in the orthorhombic system, rich laminar sedimentary rock, commonly and occurs in alluvial deposits, pegmatites, having layers of chert. The iron may be in and granites. It is the chief source of tanta- the form of carbonate, oxide, silicate, or lum. sulfide. It is used as a low-grade ore of iron. taphonomy /ta-fon-ŏ-mee/ The study of taenite /tee-nÿt/ An alloy of iron and the processes affecting an organism from nickel (with 27–65% nickel) that occurs in its death to its possible fossilization. iron METEORITES. Taphonomy elucidates the many differences that exist between a fossil assem- taiga /tÿ-gă/ (Boreal forest) A large re- blage and the community (or communities) gion of coniferous forests that occupies N of living plants or animals from which it latitudes from Canada, across Scandinavia, came (see thanatocoenosis). It is a neces-

343 taphrogeosyncline sary preliminary to investigations in PALEO- tectonics The study of the Earth’s defor- ECOLOGY. See also actuopaleontology. mational movements and their effect on sedimentation and geomorphology. taphrogeosyncline /taf-roh-jee-oh-sing- klÿn/ See rift valley. tektite /tek-tÿt/ See meteorite. tarn A small lake, usually occupying a Teleostei /tel-ee-oss-tee-ÿ, tee-lee-/ The CIRQUE. group of bony fish dominant since the Cre- taceous, occurring in both freshwater and tar pit A deposit of natural BITUMEN in a marine environments. The body covering is depression at the surface of the land. Ani- usually reduced to thin scales of bone. mals sometimes become trapped in such Teleosts show great morphological varia- pits, which are a good source of recent fos- tion and many fossil species are known. sils. telluric current /tĕ-loor-ik/ A naturally tar sand A type of sedimentary rock that occurring electric current at or near the contains commercially useful amounts of Earth’s surface. ASPHALT. tellurometer /tel-yŭ-rom-ĕ-ter/ A sur- taxonomy /taks-on-ŏ-mee/ 1. The study veying instrument for precise distance and practice of naming organisms and clas- measurement. Like the GEODIMETER it is in- sifying them into a hierarchy of groups direct in its measurements. The time re- based on the similarities between them. It is quired for radio microwaves to travel from implied that this also represents their evo- one point to another is determined and cor- lutionary relationships. Thus the members rections applied for the meteorological of a lower group show more similarities conditions to convert this time to that in a and are therefore more closely related than vacuum. The distance is then calculated those of a higher group. The group at the from the known velocity of the waves, and base of the hierarchy is the SPECIES. The further corrections applied for slope and groups above the level of species, in as- altitude. The best results are obtainable cending order, are GENUS, FAMILY, ORDER, along a line with few obstacles, in moder- CLASS, PHYLUM, and KINGDOM. Any of these ate sunshine, with a light breeze and low groups may be subdivided into smaller relative humidity. ones; for example a class may contain several subclasses. temperate Describing a moderate cli- 2. See Soil Taxonomy. mate, such as that of the mid-latitudes north and south of the Equator, character- tear fault See strike–slip fault. ized by warm summers and cool winters. It extends from the Tropics of Cancer and tectogene /tek-tŏ-jeen/ A buckling of the Capricorn to the Arctic and Antarctic Cir- Earth’s crust that results in the down-buck- cles, and thus lies between the tropics and ling of granitic rocks deep into the Earth’s the frigid zone. crust to form the roots of mountains. It also results in the upward-buckling of the temperate glacier A glacier containing shallower levels in the crust to form oro- considerable amounts of water above, genic mountain belts. within, and beneath the glacier ice. Water at the ice-rock boundary promotes easy tectonic breccia /tek-tonn-ik/ A type of sliding and consequently such glaciers flow BRECCIA that has been formed through faster than other types. This movement movements of the Earth’s crustal plates produces erosion of the abrasive type, (see plate tectonics). while the presence of water allows freezing and thawing at the ice margins, an impor- tectonic plate See lithospheric plate. tant factor in GLACIAL PLUCKING.

344 terra rossa temperate grassland A type of GRASS- cluding bombs, ash, cinders, pumice, and LAND that occurs in the world’s TEMPERATE lapilli. See pyroclastic rock. regions. It includes the PRAIRIES of North America and the STEPPES of Russia. They tephrite /teff-rÿt/ An olivine-free type of have warm wet summers and cold dry ALKALI BASALT. winters, making them ideal pasture and, when plowed, good land for growing cereal terminal moraine An accumulation of crops. TILL material that develops against the front of a glacier. Such moraines are best temperature An index of the heat con- formed when the front is slowly advancing tent of a substance, which determines the or is stationary, because in these circum- flow of heat between one substance and stances the front will be high and fairly another. Because the specific heat capacity steep. Such forms will finally be deposited of different substances varies appreciably, on the retreat of the glacier, thereby mark- temperature cannot be equated directly ing its farthest limit from the source. See with heat. A variety of scales are in use. In also moraine. most parts of the world the Celsius (centigrade) scale is used. For scientific work, the termite Ants and termites are abundant in tropical soils, their activity being con- ABSOLUTE TEMPERATURE was formerly used but temperature is now measured in spicuous in the form of termite mounds, which generally are about one meter high. KELVINS. See also maximum temperature; Their main benefit appears to be in im- minimum temperature; soil temperature. proving the structure of the soil. temperature inversion See inversion terra fusca /te-ra foo-ska/ A soil that, layer. like TERRA ROSSA, develops on limestone in a warm subcontinental climate. Terra temporary hardness See hard water. fusca are brown clay loam soils, neutral or slightly alkaline in reaction, occurring tension crack (gash; fissure; or fracture) under cork oak or maquis vegetation. Be- A crack that develops in competent beds as neath the brown A horizon is a relict red B they are deformed. The rock is put under horizon, suggesting that they may be de- tension and fractures. This usually happens graded terra rossas. at several sites and sets of tension cracks develop en echelon. These fractures are terrain correction See topography cor- usually infilled by secondary minerals such rection. as calcite. terra rossa /ross-a/ Red soil that devel- tension joint A joint produced as a re- ops on hard limestone in warm subconti- sult of tension, usually developed parallel nental climates. These soils typify the karst to the fold axis of an anticline. areas adjacent to the Mediterranean, and also occur in S Australia, Texas, central tephigram A type of THERMODYNAMIC Spain, and Israel. They typically have a DIAGRAM used in meteorology that shows clay-loam texture, with some free sesqui- the changing properties of the atmosphere oxides, a variable depth, and low base sta- with height. The properties displayed on the tus; they pass directly and abruptly to the diagram are: isotherms, i.e. lines of con- parent material below, with per-haps a stant temperature; dry adiabats; isobars, humus surface horizon. Their origin is un- i.e. lines of constant pressure; saturated certain: they were formerly thought to rep- adiabats; and saturation mixing ratio lines. resent a weathering residue. They may develop from TERRA FUSCA on deforesta- tephra /teff-ră/ (ejectamenta) Fragmen- tion, because they occur only under gar- tal materials thrown out by a volcano, in- rigue vegetation. They are related in some

345 terra roxa way to RENDZINAS, which seem to occur tica. Episodes of orogenic movements ex- more typically on soft limestone (e.g. tended through the period with the forma- chalk); it may also be that the terra rossa tion of major mountain belts, including the soils are more mature, because their lower formation of the Alps and Himalayas (at- base status and content of free sesqui- tributed to the movement north of India oxides shows more intense weathering. and its collision with Asia), the Rockies, and the Andes of South America. Extensive terra roxa /roks-a/ A Brazilian red earth volcanic activity took place along the west (see ferrallitic soil) with a high percentage coast of North and South America. of titanium and manganese oxides and a The Tertiary saw the evolution and low aluminum oxide:iron oxide ratio, de- gradual increase in abundance of modern veloped on a parent material of basic ig- invertebrates and mammals, with a corre- neous rocks. Compare terra rossa. sponding reduction of primitive groups. The modern angiosperms became the dom- terrigenous deposit /tĕ-rij-ĕ-nŭs/ A sed- inant plants. iment from land washed into the sea as mud, where it slumps down the continental teschenite /tesh-ĕ-nÿt/ A nepheline-free shelf under gravity or the affect of currents. type of ALKALI GABBRO. The deposits are named after their colors, and include blue mud, gray mud, green Tethys Sea /teth-is/ (Tethyan Ocean)A mud, and red mud. The color, in turn, de- large ocean that existed before the north- pends on the composition. ward movement of Africa relative to Eu- rope, of which the Mediterranean Sea is the Tertiary /ter-shee-air-ee, -shă-ree/ A sub- surviving remnant. Within this ocean, division of the CENOZOIC Era that followed which existed for a long period of time, the the CRETACEOUS nearly 65 million years sediments of the Alps and Himalayan ago, and lasted some 63 million years until mountains were deposited. As the ocean the beginning of the QUATERNARY. The Ter- closed as a result of SEA-FLOOR SPREADING tiary was formerly regarded as an era, but this wedge of sediments was compressed because of its short length was subse- into the present-day mountain chains and quently classified as a period. The divisions associated orogenic features. of the Tertiary, which previously formed periods, are now regarded as epochs. They tetragonal /te-trag-ŏ-năl/ See crystal are the PALEOCENE, EOCENE, OLIGOCENE, system. MIOCENE, and PLIOCENE. Although this definition of the Tertiary as a period has been tetrahedrite /tet-ră-hee-drÿt/ A gray sul- widely used, it has not been give formal fide mineral that contains copper and anti- status. The International Commission on mony, sometimes also with iron and Stratigraphy now recognizes the Cenozoic arsenic, typical composition (Cu,Fe)12- as comprising the PALEOGENE and NEOGENE Sb4S13. It crystallizes in the terahedral sys- Periods. Some authorities have suggested tem, and occurs in veins associated with that the Tertiary should be regarded as a copper, silver, and zinc. It is used as a subera. source of copper. During the Tertiary the spatial distribution of the continents was similar to that of texture The small-scale structures rec- today. There was intense volcanic activity ognized in hand specimens and THIN SEC- in NW Scotland, N Ireland, the Faeroe Is- TIONS of rocks, originating from the lands, and Greenland associated with the geometrical relationships of the minerals rifting and separation of Eurasia and that constitute the rocks. Some of the tex- North America between Scandinavia and tural terms in common use are: Greenland. Massive eruptions of basaltic 1. Terms describing grain size: coarse-,. lava occurred on the Deccan trap. Australia medium-, and fine-grained, APHANITIC, and South America separated from Antarc- HYALINE.

346 thermal wind

2. Terms describing the degree of crys- fication) for use in his climate classification tallinity: HOLOCRYSTALLINE, HYALOCRYS- scheme. It is a measure of the amount of TALLINE, HYALINE. heat given to a specific area expressed in 3. Terms describing grain shape: EUHE- terms of the potential evapotranspiration DRAL, SUBHEDRAL, ANHEDRAL, IDIOMOR- that would result. PHIC, ALLOTRIOMORPHIC, HYPIDIO- MORPHIC. thermal equator The line of highest 4. Terms describing the relationships be- mean surface air temperature. Because tween grains: EQUIGRANULAR, INTER- land areas tend to absorb heat more than GRANULAR, OPHITIC, POIKILITIC, PORPHY- oceanic areas, the thermal equator has a RITIC, APHYRIC, poikiloblastic, PILO- mean position in the N hemisphere. In July, TAXITIC, SPHERULITIC, interstitial. it reaches about 20°N over the continents and in December is located at about the ac- thanatocoenosis /th’an-ă-toh-see-noh-sis/ tual Equator except in E South America (death assemblage) An assemblage of where it does extend farther south over fossils composed of the remains of animals Amazonia. that have been accumulated by various agencies after their death and thus may not thermal low (thermal depression, heat have lived together in life. They may have low) An area of low pressure produced become affected by scavengers and by me- by intense solar heating of the ground sur- chanical breakage in the process. Most fos- face. The heating reduces the density of the sil occurrences are thanatocoenoses. air, which often results in rising air and a Compare biocoenosis. fall in surface pressure. The best examples of thermal lows are the monsoon lows, theodolite /th’ee-od-ŏ-lÿt/ An accurate such as those over the Thar Desert. These surveying instrument used for the measure- do not give rise to rain because they are ment of horizontal and vertical angles, capped by strong anticyclones, which pre- consisting of a telescope that can be rotated vent all thermals reaching condensation in both the horizontal and vertical planes. level. theralite /th’e-ră-lÿt/ An analcime-free thermal metamorphism See contact type of ALKALI GABBRO. metamorphism. thermal /th’er-măl/ A volume of air that thermal spring (hot spring) A spring possesses buoyancy relative to its cooler that produces hot water from under- surroundings. Thermals tend to arise as a ground, heated by geothermal energy. If it result of strong insolational heating at the ejects steam as well as boiling water, it is ground or a steep environmental lapse rate. termed a GEYSER. See also smoker. Some will reach the condensation level producing cumulus clouds, others may lose thermal wind A theoretical wind used their buoyancy earlier. The thermal rises to indicate the horizontal temperature field by slowly mixing with its cooler surroundings at its upward margin and by entrainment in its wake. The mixing with environmental air means that strictly the 12 knots 315° thermal is not cooling adiabatically, but in thermal wind practical terms the difference is rarely sig- 21 knots 255° nificant. vector of wind at base of layer thermal depression See thermal low. vector of upper wind 28 knots 280° thermal efficiency A concept devised by Thornthwaite (see Thornthwaite classi- Thermal wind

347 thermocline in a layer of air. Actual winds change in the which cause differences in density. Ocean vertical as a result of variations in the ther- water circulation tends to be controlled by mal field at higher levels. The thermal wind two principal factors: the wind stresses is a measure of the difference between the that affect the sea’s surface and the distrib- wind at the top and at the base of the layer. ution of density within the water masses. It is obtained by subtracting the geo- These two factors are closely related and strophic wind vector at the base of the influence one another. Thermohaline con- layer from that at the top. See diagram. vection, largely due to the cooling of surface waters at high latitudes, gives rise to thermocline /th’er-nŏ-klÿn/ A subsur- pronounced currents operating in the face layer of water within the ocean in oceans. which there is a marked decrease in water temperature as the depth increases. The thermometer An instrument for meas- water layer above the thermocline is essen- uring temperature by recording a change in tially homogeneous or isothermal: the dif- a heat-dependent property of a substance. ferences in temperature and salinity in the The property chosen will depend on such vertical column are minimal. This upper- factors as the accuracy required and the most layer seldom exceeds 100 m in thick- range of temperatures to be measured. The ness. Hence, the thermocline is located at most common method relies on noting the the bottom of the surface water masses. length of a column of mercury (or alcohol The maximum vertical temperature gradi- where low temperatures are involved) en- ent is generally found at depths between closed in a sealed glass capillary rising from 100 and 200 m. Some distance beneath the a small bulb. The length of the column thermocline, in depths greater than 200 to varies as the mercury expands and con- 300 m, the water masses are surprisingly tracts with changing temperature. Other constant in their characteristics, the tem- properties used include electrical resistance perature gradient rapidly decreasing to- (resistance thermometer), the variation in ward the ocean floor. the pressure of a gas kept at constant volume (gas thermometer), and the magnitude thermodynamic diagram /th’er-moh-dÿ- nam-ik/ A diagram on which the proper- of the EMF produced by a bimetallic junc- ties of a parcel of air may be indicated in tion (thermocouple). terms of its pressure, temperature, and hu- A dry- and wet-bulb thermometer con- midity. Adiabatic lapse rates are also fre- sists of two thermometers side by side, one quently included on the diagram, from of which has its bulb enclosed in a wet which it is possible to make assessments of muslin bag. The difference in readings be- atmospheric stability and cloud base. tween the two thermometers enables the relative humidity of the atmosphere to be thermodynamics The branch of calculated from standard tables (the wet- physics concerned with the study of the bulb thermometer will have a lower tem- movements or relations of heat. perature due to the cooling effect as the liquid evaporates). thermograph /th’er-mŏ-graf, -grahf/A self-recording thermometer, which consists thermophile /th’er-mŏ-fÿl/ A plant that of a temperature-sensing mechanism and a can survive in very hot climates. system of time portrayal. A continuous record of temperature can be obtained thermoremanent magnetization /th’er- from it. moh-rem-ă-nĕnt/ (TRM) The permanent magnetism possessed by igneous rocks thermohaline circulation /th’er-moh- after they have solidified from the molten hal-ÿn, -hay-lÿn/ The circulation of sea state. It is produced in the rocks by the water within the oceans that arises from Earth’s magnetic field, and the direction of differences of salinity and temperature, the magnetism in ancient rocks provides

348 thrust evidence of the history of the Earth’s geol- which it can be identified and its character- ogy. istics in terms of rainfall, temperature, moisture index, and concentration of ther- thermosphere /th’er-mŏ-sfeer/ The layer mal efficiency understood. Because of its of the Earth’s atmosphere above the complexity and difficulties, it has not be- mesosphere, in which the temperature in- come widely used since its final version creases with height. The pressure ranges appeared. It is named for the American –8 from about 0.01 mb to 10 mb. The ther- climatologist and geographer C. Warren mosphere falls within the IONOSPHERE. Thornthwaite (1899–1963). thickness (in meteorology) The vertical threshold The lip of a CIRQUE, which thickness of air between any two specified may act as a dam, holding back water after pressure levels. The most common layer is the ice has melted and creating a tarn. the 1000 mb–500 mb, but others could be used. The thickness of the layer is directly threshold wind speed The lowest wind proportional to its mean temperature, warm temperatures being associated with a speed that permits air to pick up grains of thick atmosphere. rock dust, sand, or soil. See aeolian transport. thin section A thin slice of rock, cut and ground to a uniform standard thickness throughfall The portion of precipita- (usually 0.33 mm) and mounted on a glass tion or irrigation water that falls through a slide for petrographical study under a covering of vegetation, without being in- microscope. tercepted, to reach the ground surface. See also interception; stemflow. tholeiite /th’oh-lee-ÿt/ One of the three main types of BASALT. throughflow The rainwater that flows down a hillside through the soil. It occurs thomsonite /tom-sŏn-ÿt/ A white min- when more rain falls on the surface of the eral form of hydrated sodium calcium alu- ground than can be absorbed quickly minum silicate, NaCa2(Al5Si5O20).6H2O. downward by the soil. It crystallizes in the orthorhombic system, and occurs in crevices in basic igneous throw The vertical change in level of a rocks and in amygdales within lava. It is a previously continuous bed of rock as a re- member of the ZEOLITE group of minerals. sult of faulting. thorn forest An area of thorny SCRUB thrust A low-angle reverse fault (see dia- that occurs in regions that have prolonged gram at FAULT) that extends over a large dry weather. The plants generally have distance. In thrusting large rock bodies can long roots (to reach underground water), be displaced over each other, with the over- thick bark (to prevent water loss), and riding block moving upward as it passes sharp thorns (to discourage browsing ani- over the lower block. It results from a prin- mals). cipal stress configuration in which the Thornthwaite classification A system maximum and intermediate stresses are of classification based on climatic effi- horizontal, while the minimum stress is ciency, or the capacity of a climate to sup- vertical. The Moine Thrust of NW Scot- port the growth of plant communities. It is land, which is 150 km long, carries Pre- based on available moisture, the annual cambrian rocks far over Cambrian and variation of temperature, and the degree of other Precambrian rocks. These large dis- association between the temperature and placements are often facilitated by gravity precipitation regimes. The climate of an and the presence of fluids in the pores of area is described by a four-digit label from the rocks undergoing deformation.

349 thulite thulite /thoo-lÿt/ A pink manganese-con- lar time, higher than at a neighbouring taining type of zoisite. See epidote. point. A tidal current will be produced causing water to move from the high to the thunder The noise heard accompanying low area. Although moving with some ve- a LIGHTNING FLASH resulting from the sud- locity, tidal currents are not noted as ero- den heating and expansion of air as the sive agents, but may be important in the flash passes through the ionized atmos- transport of material thrown into suspen- phere. Because the speed of sound is far less sion by waves. The most notable tidal cur- than that of light, thunder is always heard rents operate within straits where high tide after the flash. An approximate measure of at each end occurs at different times. distance from the storm is 1 mile for every 5 seconds between flash and thunder. tidal flat A wide flat area of barren or marshy land that is covered and uncovered thunderstorm A storm that is produced at high and low tides. It is made up of un- from CUMULONIMBUS cloud and is accom- consolidated sediments. See marsh; mud- panied by thunder and LIGHTNING, and flat. usually strong winds and rain. It involves large convection currents, which can result tidal hypothesis A theory of the origin in very heavy rain, often causing flooding. of the Solar System, which suggested that a If the convection currents carry drops of star approached close to the Sun, and that water high enough, they freeze and as a re- the tidal attraction coupled with the Sun’s sult hail may accompany the rain. unstable nature caused some of the Sun’s mass to be torn off, later to cool as the tidal current 1. The periodic horizontal planets. The theory has fallen into disfa- flow that occurs in response to the rise and vor. See nebular hypothesis. fall of the tide. Such a current, like the vertical oscillation of the tide, arises from the tidal limit The highest point in an estu- gravitational attraction between the Earth, ary or river inlet that is reached by sea the Sun, and the Moon. Near the coast, water at high tide. tidal currents tend to be rectilinear and reversing. Farther offshore, they tend to flow tidal prism The quantity of sea water in a more rotary manner. Tidal currents are that floods into or ebbs out of a harbor, es- responsible for the transport across shelf tuary, or other sea inlet as a result of tidal areas of large quantities of sediment, partly flow (not taking into account the freshwa- along the sea floor, partly in suspension ter discharge from streams, rivers, or above it. Because they are reversing cur- canals). It is usually measured in cubic me- rents near the coast, the residual flow in ters. The stability of tidal inlets is largely one or other direction often determines the determined by the relationships that exist net direction of sediment transport. Tidal between the tidal prism and littoral drift: currents, which in limited situations attain the ebb flow, reinforced by freshwater dis- speeds even in excess of 9 or 10 knots, tend charge, tends to evacuate sediments from to flow most rapidly at the sea surface, but inlets whereas the flood flow, reinforced by rather more slowly near the seabed. Bot- the littoral drift, tends to infill inlets with tom friction accounts for this difference. sediments. Dredging is often necessary to Tidal currents create several distinctive maintain the desired balance because most morphological features, including tidal inlets tend toward progressive siltation. banks and channels, and depressions eroded out of soft rocks. tidal range The difference in height be- 2. The movement of water between two tween the water level at high and low tide. places differentially affected by the rise and This range is never a fixed value because fall of the tides. Two tidal regions may op- wherever tides occur there is always varia- erate in close proximity, such that the tion between great ranges, associated with water level at one point is, at any particu- the especially high and particularly low

350 till fabric tides of spring type, and small ranges asso- Equator, and on the mountainsides at alti- ciated with the neap tides. Whenever a sin- tudes of up to about 1000 m. The temper- gle value for tidal range is quoted for a ature is constant and rainfall heavy place, it is a mean value. A major geomor- throughout the year – conditions that favor phological importance of the tidal range is the growth of tropical rainforest. that it controls how much of a beach profile will come under the influence of wave tierra fria /free-ah/ The cold regions of action, and therefore the nature of the Central and South America. They occur coastal features. higher in the Andes, at up to 3000 m near the Equator. Rainfall is spread throughout tidal stream A movement of water into the year, and the natural vegetation is and out of bays, estuaries, and other re- coniferous forest on the lower slopes with stricted coastal openings, associated with scrub at higher altitudes. the rise and fall of the tides. These streams may move comparatively rapidly and can tierra templada /tem-plah-dah/ The cause considerable erosion both in flood TEMPERATE regions of Central and South and ebb. In the fine sediments of estuaries America. They occur at altitudes of be- different streams are utilized by the rising tween 1000 and 2000 m in the equatorial and falling tide, and the results of their ero- Andes. Natural forest covers much of the sion are complex patterns of interdigitating land, although some areas have been channels, termed flood and ebb channels cleared (and the mountain slopes terraced) according to their mode of formation. See for agriculture. also tidal current. tiger’s-eye A yellow-brown type of tidal wave See tsunami. QUARTZ, colored by iron oxide impurities. Fibrous inclusions of crocidolite give the tide The regular rising and falling of mineral its shiny banded appearance, mak- water level resulting from the gravitational ing it popular as a semiprecious gemstone. attraction that exists between the Earth, the Sun, and the Moon. Depending on their tight fold A type of fold with parallel or relative positions, either SPRING TIDES or slightly diverging limbs, with an interlimb NEAP TIDES result. Tidal predictions can be angle of less than 30°. See diagram at FOLD. made, although other influences such as local meteorological conditions make very till (boulder clay) The unstratified ma- accurate predictions difficult. With the rise terial deposited by glaciers and ice sheets. and fall of the tide, the horizontal move- Characteristically unsorted, it contains an- ment of water gives rise to TIDAL CURRENTS. gular material ranging from clay-sized par- The response of the oceans to tide-produc- ticles to huge boulders. These may exhibit ing forces is based on a series of amphi- STRIATIONS and can be derived from a wide dromic points (see amphidromic system), variety of rock types. See also ablation till; around which the tide oscillates in the lodgment till. manner of a PROGRESSIVE WAVE. Diurnal tides are those with one low water and one till fabric The general pattern of orien- high water during the tidal day; semidiur- tation of elongate stones within TILL ma- nal tides have two low waters and two high terial. Analysis of the positioning of waters during the tidal day. pebbles has shown that in many cases a large proportion are located with their tie See chaining. long axes parallel with the direction of former ice flow, and with a low angle of dip tierra caliente /tye-rah kal-yen-tay/ The downstream. Frequently another minor hottest regions of Central and South Amer- concentration exists, consisting of stones ica. They occur on the coastal plains, on whose long axes are perpendicular to the the lower slopes of the Andes near the direction of movement. It is believed that

351 timberline the former were moved by sliding and the Mediterranean containing many examples. latter by rotation about their long axes. Where the coastal outline is suitable, nor- Tills of different ages have been distin- mal longshore movements of beach ma- guished on the evidence of their different terial may produce a spit, which eventually stone orientations. extends to reach an island. timberline (treeline) The highest alti- tonalite /toh-nă-lÿt/ A quartz-diorite. tude on a mountainside or the highest lati- See diorite. tude at which trees will grow. Trees do not grow beyond this line because it is too cold, topaz /toh-paz/ A variably colored or- there is insufficient soil, or there is not thorhombic mineral of composition enough rainfall. See also snow line. Al2(SiO4)(OH,F)2 found in acid igneous rocks, such as granites and pegmatites. time-distance curve (T.D. curve)A Some types are used as semiprecious gem- graph that shows the relation between the stones. arrival times of various seismic waves and their distances to the epicenter of an earth- topoclimatology /top-ŏ-klÿ-mă-tol-ŏ- quake. Such curves provide seismologists jee/ The study of the interaction between with various kinds of data, such as infor- topography and climate at the local scale. mation about discontinuities in the rocks Stress is placed on the varying slope condi- through which the waves pass. tions (as they affect radiation receipt), the local water balance, raindrop trajectories, time series A set of data arranged in the geostrophic wind flow, and gravita- order of occurrence with equal time inter- tional winds. These modifications of the vals between each value. Such series are heat and water balances produce a distinc- common in climatology because standard tive topoclimate, which may be widely dif- records are taken at equal intervals and ex- ferent from the macroclimate as recorded tend over many years. A time series can be in an instrument shelter on a horizontal for individual values or for averaged values surface. such as annual mean temperature. Time series can be analyzed by a variety of statisti- topography /tŏ-pog-ră-fee/ The relief, cal techniques such as spectrum or drainage, roads, vegetation, and cultural harmonic analysis. features of the Earth’s surface. tinguaite /ting-gwă-lÿt/ See phonolite. topography correction (terrain correction) The correction of gravity measure- titanaugite /tÿ-tăn-aw-jÿt/ A titanium- ments for the effects of local topography, rich variety of AUGITE. See pyroxene. because hills and valleys cause variations in the strength of the Earth’s gravitational titanite /tÿ-tăn-ÿt/ See sphene. field. tombolo /tom-bŏ-loh/ A form of coastal toposequence See catena. SPIT, composed of sand or shingle, that links an offshore island to the mainland. topset bed See delta deposit. Two tombolos (double tombolos) may link one island to different parts of the coast, topsoil The dark fertile soil at the sur- enclosing a lagoon between them. The face, the A HORIZON. usual existence of comparatively shallow water in the straits between island and tor A small hill projecting abruptly from coast assists in the reduction of wave en- a gently undulating land surface. Tors usu- ergy in this area, thereby promoting depo- ally consist of stacks of joint blocks, which sition of material. Tideless areas seem remain in situ and attached to bedrock at especially suited to tombolo formation, the depth, together with many collapsed

352 trachybasalt blocks. The most favored explanation of symmetry and are variably colored al- tor formation postulates two cycles: the though black varieties are common. Tour- first involves initial differential DEEP maline is found in veins and pegmatites WEATHERING, the least jointed rocks being associated with granites. the least weathered; the subsequent period The pneumatolytic alterations of of EXHUMATION exposes the former undu- granitic minerals caused by the introduc- lating WEATHERING FRONT and leaves the tion of boron is called tourmalinization. unweathered blocks as tors. Many tors dis- The rock luxullianite is a tourmalinized play UNLOADING sheets, which must have granite in which the biotite and much of developed as exhumation proceeded. the alkali feldspar has been replaced by radiating aggregates of acicular tourmaline. torbanite /tor-bă-nÿt/ A type of dark Quartz is the only mineral to survive tour- brown OIL SHALE that contains 70–80% of malinization and the end product is a tour- carbonaceous material. maline-quartz assemblage known as shorl rock. See pneumatolysis. tornado (twister) A violently rotating column of air characterized by a funnel- tourmalinization /toor-mă-lin-ă-zay-shŏn/ shaped cloud, which may reach to the See pneumatolysis; tourmaline. ground surface, accompanied by a roaring noise. On a local scale the tornado re- trace element (in geology) An element presents the greatest wind intensity of all that occurs in a rock or mineral in very surface weather conditions. Most ane- small quantities (much less than 1%), and mometers and even barographs are dam- is not regarded as an essential part of its aged if a tornado passes overhead, but falls composition. of up to 200 mb and wind speeds of up to 500 km per hour are believed to be experi- trace fossil A fossilized remnant of the enced. The tornado moves across country effects of an organism in the past, rather with the wind flow and direction of the than the remains of the organism itself. wind at a higher level, leaving a swath of Trace fossils may be the fossilized feces of destruction wherever the funnel cloud has an organism or structures produced by it reached the ground. The precise origins are that have been preserved in the sediment, not fully understood, but in their severest such as tracks and burrows. ACTUOPALEON- form the following environmental condi- TOLOGY is used in relating these fossils to tions are required: warm moist air at low the species that produced them. Trace fos- levels, an INVERSION at about 2000 m, a sils are classified in a taxonomic system tongue of dry air between 850 and 700 mb, analogous to that applied to the organisms and a triggering mechanism to initiate the themselves, characteristic trace fossils individual storm. These factors occur most being described as ichnospecies. They are frequently in the mid-West of the USA, often used in elucidating the paleoecology where tornadoes are a great problem. of a species. However, tornadoes of lower intensity occur in many countries. Tornadoes and trachyandesite /tray-kee-an-dĕ-zÿt, trak- other severe weather are tracked in the ee-/ A type of SYENODIORITE. USA by a system of radar stations that use Doppler radar. trachybasalt /tray-kee-bă-sawlt, -bass- awlt, trak-ee-/ The fine-grained equiva- torrid zone The tropical areas between lent of syenogabbro (see alkali gabbro), 23.5°N and 23.5°S. displaying mineralogical and chemical features intermediate between TRACHYTES and tourmaline /toor-mă-lin, -leen/ The tour- ALKALI BASALTS. Hawaiite and mugearite maline minerals all have the general for- are members of the alkali basalt volcanic 2+ mula NaR3 Al6B3Si6O27(OH,F)4 where R suite containing andesine and oligoclase = Fe2+, Mg, or (Al + Li). They have trigonal feldspar, respectively, together with

353 trachyte basaltic mafic minerals. These lavas are strong low-level inversion, preventing the distinguished from andesites largely on vertical development of cloud. These areas, chemical grounds and field associations. such as the Canary Islands or the Galapa- Benmoreite is chemically intermediate be- gos Islands, tend to have low rainfall. tween mugearite and soda trachyte. Crossing the oceans increases the depth of the moist layer near the surface, the inver- trachyte /tray-kÿt, trak-ÿt/ The volcanic sion becomes less intense, and disturbances equivalent of syenite, ranging from over- more frequent, so that on the western sides saturated to undersaturated in composi- of the oceans the trade winds provide an tion. Some varieties contain sanidine and adequate rainfall in most areas. oligoclase feldspars, others contain a single anorthoclase feldspar. Feldspars occur trajectory /tră-jek-tŏ-ree/ (in meteorol- both as phenocrysts and as close-packed ogy) A line drawn to indicate the actual microlites in the groundmass. The micro- movement of a particle of air over a certain lites usually have a subparallel flow orien- time interval. Ideally the path should be tation and swirl around the phenocrysts, three-dimensional, but as little is normally imparting a characteristic trachytic tex- known in detail about the vertical compo- ture. Mafic minerals include biotite, nent, the horizontal projection of the path clinopyroxene, and hornblende. In the is portrayed. The term is also used for the sodic varieties, soda pyriboles and fayalite downward motion of a precipitation parti- are common. Trachytes may contain up to cle as far as it can be assessed from upper 10% accessory quartz. Oversaturated air information. sodic varieties are termed pantelleritic trachytes which, with an increase in the transcurrent fault See strike–slip fault. amount of quartz, pass into pantellerites (alkali rhyolites). Varieties containing ac- transform fault A fault present within cessory nepheline are termed phonolitic the Earth’s oceanic crust (see diagram at trachytes. When the nepheline content FAULT). Such faults have strike-slip dis- reaches 10%, phonolitic trachytes pass placement and are orientated at right an- into phonolites. Kenytes are a variety of gles to the MID-OCEAN RIDGES (constructive sodic phonolitic trachyte containing dis- plate boundaries), which they offset for tinctive rhombic phenocrysts of anortho- several tens or even hundreds of kilome- clase. ters. These faults lie parallel to small cir- Trachytes are the intermediate mem- cles, the axis of which is the axis of bers of the alkaline basalt volcanic suite rotation for the relative motion of the and are found mainly in ocean islands and plates on each side. Transform faults form areas of continental rifting and vulcanism. conservative plate boundaries, one of the three types of PLATE BOUNDARY between traction load See bed load. lithospheric plates (the other two being constructive and destructive plate bound- trade winds The predominantly east- aries). Along conservative plate boundaries erly winds that blow steadily over the the lithospheric plates are sliding past each ocean areas and less steadily over conti- other. nental interiors in the tropics, converging toward the Equator. They play a very im- transgression Flooding of land area by portant role in the GENERAL CIRCULATION a positive movement of BASE LEVEL, result- OF THE ATMOSPHERE by evaporating mois- ing in onshore migration of the high-water ture in large quantities from the tropical mark. A transgression results in the short- seas, which helps maintain the global heat ening of rivers by the drowning of former balance by removing surplus heat from valleys, leading to the creation of fiords, these areas. On the eastern side of the trop- rias, and estuaries: these features are rela- ical oceans, the trade winds are very steady tively shortlived because rapid aggradation in direction and speed and have a very soon fills them up with silt. Past transgres-

354 traversing sions at a particular point can be identified for the Dutch cartographer and seismolo- by studying the deposits left. Fossil remains gist Gerardus Mercator (1512–94). may also show changes from freshwater to marine types. In the postglacial period transverse valley A valley that cuts there have been two major transgressions, across the prevailing geologic structure of the first one in the Boreal period the land, or at right angles to the general (7600–5500 BC) being the most important, alignment of the underlying rock strata. which have elevated sea level to its current extent after the low stand of the last glacial trap (oil trap) A geologic structure in period. Marine transgression can be local which hydrocarbons (oil and/or natural or worldwide. gas) become trapped and accumulate. Hy- drocarbons migrate away from their transpiration The process of water loss source rock under pressure and accumulate from the surface of a plant. Moisture is where an effective barrier to their further drawn up from the soil by the plant roots migration exists. Traps can occur in a vari- and passes up through the stem to the ety of ways. Since hydrocarbons, both oil leaves where it evaporates as water vapor and gas, are less dense than water, they from stomata (pores) on the surface of tend to accumulate in the highest points of leaves. The movement of water up through the trap, displacing most of the water pre- the plant is known as the transpiration viously present in the rock pores. The bar- stream. The process cools leaf surfaces, riers to further migration can result from which in turn cools surrounding air. tectonic activity, as in the case of fault, unconformity, and anticlinal traps, and from lateral changes in the properties of the rock transport (in geology) The link in the body, as in the case of facies change traps geomorphological system between sites of and reef traps. When more than one type of erosion and sites of deposition. The media trap exists in the same place it is termed a of running water, wind, and currents trans- composite trap. A distinction is made be- port material in varying proportions by tween a structural trap, formed by tectonic bed creep, saltation, and suspension. In activity (such as folding and faulting), and rivers, bed creep is dominant; in air, salta- a stratigraphic trap, where, for example, a tion is most important. Solution can also layer of fine sediments may act as an im- be important in rivers. Waves transport by permeable cap above coarser sedimentary beach drift of pebbles and sand in the surf layers. See also salt dome. See illustration zone, and the setting up of LONGSHORE overleaf. CURRENTS. Ice transports as a solid medium, by shunting material in front, in- traversing /tră-ver-sing/ A surveying corporating material in its base, and carry- method for finding position by measure- ing slope-eroded material on its top. ment of direction and distance, using either Gravity is inherent in most transport; on a prismatic compass and tape (compass slopes it is directly responsible for down- traverse), or a theodolite and leveling staff slope movements and it provides energy (tacheometric traverse). If the starting and for flowing water. finishing points are of known position, whether a single or two distinct stations, transverse Mercator projection A then the traverse is closed. If only one sta- MAP PROJECTION constructed in the same tion, at one end of a traverse line, is of way as the MERCATOR PROJECTION but hav- known position the traverse is open or un- ing the cylinder tangential to a meridian closed. The former type is better because it rather than to the Equator. It is used allows easier correction if errors are made. mainly for small areas with a north–south Unlike a compass traverse, a tacheometric orientation, and for all British Ordnance traverse will not only fix position but also Survey maps. Rhumb lines on this projec- height. To obtain heights for points fixed tion are curved. The projection is named by compass and tape, a leveling traverse

355 travertine

must be run, starting and finishing at a point of known height. See tacheometry.

travertine /trav-er-tin, -teen/ Deposits of calcium carbonate formed by precipitation from hot springs.

treeline See timberline. Fig. 1: Unconformity trap tree-ring analysis See dendrochronology.

d i r e c t i o Fig. 2: Facies change n o f s l es o gi p olo e lith of ike str

Trellis drainage

trellis drainage A drainage pattern that develops where two sets of structural con- Fig. 3: Fault trap trols of a different type occur at right angles. If a steep regional slope is crossed by varying hard and soft lithologies, short stream segments will follow the slopes, while the soft lithologies will rapidly develop long subsequent streams that will come to dominate the pattern. One example is in the Jura Mountains of France; another is the Ridge and Valley province in the Appalachian mountains of the eastern Fig. 4: Anticlinal trap USA. A similar pattern can also arise if glaciation of an area of former consequent streams proceeds at right angles to the initial drainage. Subsequents will develop following the grain of the glacial advance, as expressed in fluted ground moraine or drumlins, and come to dominate the system.

tremolite /trem-ŏ-lÿt/ A monoclinic Fig. 5: Salt dome trap mineral of the AMPHIBOLE group.

Types of trap trench (deep-sea trench, foredeep, ocean

356 Triassic

orogenic belt with island arc earthquake activity trench

lithosphere

asthenosphere X earthquakes X X (Benioff zone)

Trench trench) A long narrow deep furrow in are the basis of all surveying because they the Earth’s crust, generally developed at provide the locations of the few initial con- the margin of a continent where an oceanic trol points, from which subsequent detail plate is being subducted (see subduction mapping may take place using other meth- zone), and often bordered by an ISLAND ods. ARC. Trenches can also develop where one oceanic plate overrides another. They Triassic /trÿ-ass-ik/ (Trias) The first pe- mark the site of a destructive PLATE BOUND- riod of the MESOZOIC Era, beginning about ARY and the surface expression of a BENIOFF 246 million years ago, at the end of the PER- ZONE. Most trenches have steep V-shaped MIAN, and lasting for some 40 million years walls and are often terraced, with a flat until the beginning of the JURASSIC. The Tri- floor that is formed by a sediment infill. assic System has three main divisions. The They are the deepest parts of the oceans: Lower Triassic consists of the Gries- the greatest known depth (10 911 m) was bachian, Nammalian, and Spathian Stages; recorded in the Mariana Trench, and simi- the Middle Triassic comprises the Anisian lar depths occur in the Tonga and Kuril and Ladinian Stages; and the Karnian, No- Trenches. rian, and Rhaetian Stages form the Upper Triassic. The name reflects the threefold di- trend The general alignment, direction, vision of these rocks in Germany: Bunter, a or bearing of a geological feature such as a lower unit of continental red-colored sedi- fold. See diagram at FOLD. ments; Muschelkalk, a middle unit of marine limestone, sandstone, and shale; and triangulation /trÿ-ang-gyŭ-lay-shŏn/ The Keuper, an upper unit of continental rocks. accurate location of a number of points by A wide variety of sedimentary rocks is evi- dividing the area containing them into a se- dent from the Triassic, with some igneous. ries of triangles for which the values of in- Toward the end of the Triassic the super- ternal angles and the lengths of sides are continent of Pangaea, which extended ascertained. Only one side of one triangle from pole to pole, started to show signs of need be measured physically, because once breaking up with rifting along the Tethys all the angles are known, the side lengths seaway. can be computed using trigonometry. After Triassic faunas, following the Late Pa- a BASELINE has been measured very accu- leozoic extinctions, show a typically Meso- rately, the angles are measured using a zoic character. Modern corals appeared, THEODOLITE, usually a one-second instru- ammonites developed, and bivalves re- ment, which is set up in turn at all the sta- placed the declining brachiopods. The evo- tions to be fixed. Triangulation schemes lution of the Reptilia produced such

357 tributary diverse types as the dinosaurs and the ma- trochoidal wave /troh-koi-dăl/ See cy- rine ichthyosaurs and plesiosaurs. cloidal wave. tributary /trib-yŭ-tair-ee/ A secondary troctolite /trok-tŏ-lÿt/A GABBRO consist- stream or river that flows into a larger ing of olivine and plagioclase feldspar of river. There may be many tributaries, all of labradorite composition. which collect rainwater and groundwater from the main river’s catchment area. trona /troh-nă/ A white, gray, or yellow- Rivers that flow through semiarid or arid ish mineral mixture of hydrated sodium areas tend to have few tributaries. See also carbonate and sodium hydrogen carbon- distributary. ate, Na2CO3.NaHCO3.2H2O. It crystallizes in the monoclinic system, and occurs triclinic /trÿ-klin-ik/ See crystal system. as tabular crystals or evaporite deposits. It is used as a source of sodium compounds. tridymite /trid-ă-mÿt/ A white crystalline high-temperature derivative of trondhjemite /tron-yem-ÿt/ A coarse- QUARTZ, SiO2. It crystallizes in the or- grained igneous rock consisting mainly of thorhombic system, and occurs in cavities quartz and plagioclase, with some biotite; in acid volcanic rocks. See silica minerals. it lacks feldspar. See granite. trigonal /trig-ŏ-năl/ See crystal system. trophic level /troh-fik/ The position an organism occupies in a FOOD CHAIN. The Trilobita /trÿ-lŏ-bÿ-tă/ An extinct class main levels are PRIMARY PRODUCERS (such of the phylum ARTHROPODA whose mem- as plants, at the lowest level), PRIMARY bers are common and widely distributed as CONSUMERS (such as herbivores, at the next fossils. Trilobites had an oval flattened seg- level), and secondary consumers (such as mented body divided longitudinally into carnivores, usually at the highest level). three lobes and transversely into three regions: an anterior cephalon, a thorax, and tropical black soil A black heavy-tex- a posterior pygidium. They possessed a tured soil characteristic of tropical lowland large number of appendages, including one areas, either plainlands or the downslope pair of antennae. Most trilobites were ends of CATENAS, which are subject to sea- about 50 mm in length although a few sonal drying and cracking due to the reached over 500 mm. The first fossils are shrinking and swelling of the dominant found in lower Cambrian rocks; the group montmorillonitic clay during dry and wet declined from the Ordovician Period on- periods respectively, caused by the ability ward and by the end of the Paleozoic it was of the clay to take up water in its lattice. extinct. The trilobites formed a wide- During the wet swelling periods, soil is spread, diverse, and rapidly evolving group pushed up to form swells (gilgai) and in the and their fossils are of great value in the dry phases cracks develop, which infill calibration and correlation of Lower Pal- themselves from the surface, thereby main- eozoic rocks. taining a kind of cycle, which makes the soils self-mulching or inverting. There are triple junction A point at which three possibly 40 or so different names for these lithospheric plate boundaries meet. soils, including vertisol, mbuga, tir, black cotton soil, grumusol, morgatitic, black tripoli /trip-ŏ-lee/ A pale-colored porous earth, black turf, and regur, each of which sedimentary rock containing silica. It is denotes a regional variant differing in some powdered and used as an abrasive for pol- small way. As a group, they are similar to ishing. CHERNOZEMS, but have less organic matter and differences in the surface horizons, due TRM See thermoremanent magnetiza- mainly to the higher temperature and tion. greater evaporation of their environment.

358 tropical rainforest

Tropical black soils have indistinct hori- tropical easterlies See trade winds. zons due to their self-mulching nature, often with calcareous or gypseous concre- tropical grassland A type of GRASSLAND tions; silt and clay content is high (possibly that occurs in the world’s tropical regions. 85%+), pH is neutral or alkaline, and par- These areas have seasonal rainfall followed ent materials typically basic, e.g. marine al- by prolonged droughts preventing the luvium, limestone, or basalt. growth of trees. See also savanna. tropical cyclone 1. A low-pressure sys- tropical podzol (groundwater podzol; tem of tropical latitudes. It is a general geant podzol) A soil developed in low- term used to describe a cyclonic storm of lying tropical areas, often adjacent to river- any intensity, distinctions of wind strength ine, marine, or deltaic areas, subject to a being made by other terms such as tropical fluctuating water table. They are charac- depression, tropical storm, and hurricane terized by their striking colors and overde- (or typhoon). The diagram shows the dis- veloped A2 horizon, which can be two tribution of tropical cyclones, which are meters deep. They develop by the leaching clearly restricted to certain areas. They de- of sesquioxides and organic material to the velop only in oceanic areas where the sea B horizon, with precipitation taking place temperature is above 27°C and at least 5° at the level of the water table. of latitude away from the Equator. It appears that the warm sea surface must pro- tropical rainforest (equatorial rainfor- vide energy through evaporation, 27°C est) Lush evergreen forest that grows in representing its lower limit, and as the the tropics, generally in regions between 5° CORIOLIS EFFECT is zero at the Equator, the north and south of the Equator. There are storm must be at least 5° away before rota- constant high temperatures and it rains tional components of airflow can develop. throughout the year. Such forests support a The precise origins of tropical cyclones are very large number of different species of debatable, but some form of initial distur- animals and plants. The soil, however, is bance in the airflow seems to be a prereq- shallow and poor, and easily eroded away uisite. if trees are felled. Even if the soil is used for 2. See hurricane. agriculture after the trees have gone, the

areas with sea-surface temperature of the warmest month above 27°C

Distribution of tropical cyclones

359 Tropic of Cancer soil nutrients are consumed after only a pattern of low pressure having much few seasons and the ground becomes no greater length than width and a concave longer suitable for crops. curvature of the isobars toward the main depression center. All fronts occupy Tropic of Cancer The line of latitude troughs of low pressure, but not all troughs 23°30′N. At the summer SOLSTICE in the N are frontal. They are normally associated hemisphere, the Sun is directly overhead at with precipitation and cloud. the Tropic of Cancer, the most northerly latitude at which this occurs. trowal A trough of warm air aloft. This indicates the presence of a layer of warmer Tropic of Capricorn The line of lati- air in the upper atmosphere, which is not in tude 23°30′S. At the summer SOLSTICE in contact with the ground surface. This type the S hemisphere, the Sun is directly over- of situation occurs in an occlusion. head at the Tropic of Capricorn, the most southerly latitude at which this occurs. truncated spur A SPUR that formerly protruded into a valley but has been partly tropopause /trop-ŏ-pawz/ The bound- or completely separated from the high ary between the troposphere and the ground by the action of a moving glacier. stratosphere. It represents the point at which temperatures stop falling before the tsunami /tsoo-nah-mee/ A seismic sea isothermal conditions of the lower stratos- wave generated in the ocean by submarine phere. On occasion, there may be more earthquakes, explosive volcanic eruptions, than one tropopause. The height of the or mass slides underwater. Although fairly tropopause varies, being higher above rare, such waves can have catastrophic warm air than cold. Consequently its mean effects when they do occur. Perhaps of lim- height is greatest over tropical latitudes at ited height (about a meter) in the open above 16 km but only about 8 km over ocean, they may travel hundreds or even polar latitudes. It also changes because of thousands of kilometers, and they attain different weather conditions. The tropo- considerable heights (up to tens of meters) pause is not a continuous surface between in shallow coastal water and at breakpoint. the tropics and poles, but is broken at the Their length at sea may be 150 km or latitude of the subtropical jet stream and is more, and they usually travel at high suddenly lower on the poleward side. speeds, probably between 500 and 1000 These breaks enable mixing to take place km per hour. They have caused immense between tropospheric and stratospheric havoc and destruction and heavy loss of air, which would otherwise not occur be- life, especially along low-lying coasts. The cause the isothermal layer of the stratos- catastrophic Asian tsunami of December phere acts as a stable inversion. 26, 2004, which resulted in over 283,000 deaths and considerable damage, was tropophyte /trop-ŏ-fÿt/ A plant that can caused by a massive earthquake of magni- survive in both dry and wet climates, such tude 9 that had its epicenter 30 km below as deciduous trees. the seafloor, 250 km south-southeast of Banda Aceh, Indonesia. It occurred as a troposphere /trop-ŏ-sfeer/ The lowest 1200-km stretch of the Indian lithospheric layer of the atmosphere, where almost all (tectonic) plate was thrust some 20 m weather phenomena develop. It takes its under the Burma plate, the thrust-faulting name from the Greek word tropos mean- releasing elastic strains that had accumu- ing a turn: it is the atmospheric layer where lated in the SUBDUCTION ZONE, and raising turning and convective mixing is domi- the seafloor by several meters. The conse- nant. quent tsunami waves caused damage along the coastlines of 13 countries including In- trough /troff/ A pressure system on a donesia (with waves up to 10 m high on the weather chart distinguished by an isobaric coastline of Sumatra), Sri Lanka, India,

360 turbidity current

Thailand, Myanmar, Malaysia, The Mal- turbidite /ter-bă-dÿt/ A sediment de- dives, and the East African countries of So- posited in water by TURBIDITY CURRENTS. malia and Kenya. Because the speed of a turbidity current is A landslide-generated tsunami, some- proportional to the square root of effective times known as a landslide surge is a large density, its lower part will be more heavily and often destructive wave caused by the charged with sediment and will move falling of a mass of rock, or perhaps parts faster than the less heavily charged upper of a glacier, from cliffs or hill slopes bor- part. For this reason the particle size of the dering a body of water. Such events have sediment transported decreases from the been witnessed on the open coast and in base of the flow upward, a consequence lakes. Because of the generally unpre- being that a form of sediment grading, or dictable nature of rockfalls and rockslides, graded bedding, occurs in the deposit ulti- detailed studies of landslide surges have mately laid down. A typical sequence in rarely been possible. One rockfall in Lituya such a deposit is for coarse material at the Bay, Alaska, which resulted from a 1958 base to give way to finer material above, earthquake, caused the fiord water to and possibly be covered in turn by lutite. If swamp some 500 m up the mountainside a series of turbidity flows occur in the same on the opposite shore of the fiord. A 15-m area, a rhythmic pattern of sedimentation high wave was generated along the fiord, arises, with superimposed layer upon layer traveling at an estimated 160 km per hour. of graded turbidite material. Many sandy Somewhat similar occurrences have been turbidites contain a significant amount of recorded in Japan, Norway, and elsewhere, silt and clay. but they are comparatively rare. Some scientists have predicted that turbidity /ter-bid-ă-tee/ 1. (in meteorol- there is potential for a landslide to be gen- ogy) The property of the clear atmosphere erated on the island of La Palma in the Ca- that leads to a reduction of solar radiation, nary Islands through the collapse of part of usually due to particles of dust and smoke, a volcano during a future eruption. This which lead to attenuation rather than mo- may generate a tsunami capable of crossing lecular scattering. It is therefore a measure the Atlantic Ocean to bring damage to the of atmospheric pollution either natural or Caribbean and eastern seaboard of the USA. resulting from human activities. Tsunamis are sometimes incorrectly 2. (in oceanography) The stirring up of sed- called tidal waves. iment by water. tufa /tew-fă/ Deposits of calcium carbon- turbidity current A flow of dense sedi- ate formed by precipitation from water ment and water. Such currents develop and including stalagmites, stalactites, flow- with the stirring of sediment, resulting in stone, and travertine. the formation of a layer that is denser than the surrounding water. Such a situation oc- tuff A type of PYROCLASTIC ROCK made curs if strong wave action, a submarine up principally of small fragments of con- slide, or an earthquake disturbance affects solidated volcanic ash. the sea floor in an area in which sediments, especially the finer-grained ones, are rea- tundra /tun-dră/ A flat or undulating re- sonably abundant. In the case of a flat gion located north of the TAIGA in the sub- seabed and in the absence of currents, the arctic lowlands of North America, Europe, suspended material would tend to settle and Asia. Summers are cool although they out again, but where the seabed slopes (e.g. may be sunny; for more than six months of on CONTINENTAL SLOPES) the suspended the year in winter temperatures never rise material usually begins to flow, most above freezing and there is a thick layer of rapidly when the slope is steep, and thereby PERMAFROST. No trees grow on the tundra initiates a turbidity current. Turbidity cur- and the main vegetation is grasses, lichens, rents can be very dense and may possibly and mosses. attain speeds of 80 km per hour or more.

361 turbulence

They have been powerful enough to break after the Sun has set below the horizon in submarine cables and to erode channels in the evening or before it rises in the morn- the deep-sea floor. They lead to sediment ing. It can last for up to an hour in Arctic deposits in characteristic morphological and Antarctic regions, but is over in a few forms. minutes near the Equator. turbulence Irregular movements of a twinning The growth of two or more fluid, such as air in the lower atmosphere crystals so that they join together, either in- or water in a rapidly flowing river. See tur- tergrown or in contact, with different ori- bulent flow. entations of the crystal axes. It can occur with several minerals, such as calcite, fluo- turbulent flow The usual type of move- rite, gypsum, feldspar, staurolite, and ru- ment characteristic of air and water in na- tile. ture, in which the net forward movement of the fluid has superimposed upon it a twister See tornado. chaotic pattern of secondary eddies, carrying molecules from one layer in the flow to type locality 1. (in paleontology) The either higher or lower layers. This constant place from which a fossil that is the TYPE exchange of molecules speeds up slower SPECIMEN of a species comes. layers and slows down faster ones; the ed- 2. (in stratigraphy) The site of the outcrop dies are principally responsible for creating of rock used to define a particular strati- hydrodynamic lift, so important in sedi- graphical division. See formation; lithos- ment transport. LAMINAR FLOW becomes tratigraphy; stratigraphy; type section. turbulent when the viscosity of the water falls as temperature increases, releasing the type section The particular outcrop of liquid from the confines of flowing purely rock that has been selected as the standard as laminar layers. Alternatively, increased section for the definition of the limits of a velocity, increased bed roughness, or a de- lithostratigraphical unit because it clearly crease in water depth can all promote tur- demonstrates the characteristic lithological bulent flow. In waterfalls and rapids the features of the unit. The place in which it greatly increased velocity and lowering of the water surface promotes a special type occurs is known as the TYPE LOCALITY. See of turbulence, ‘shooting’ flow, which accel- also formation; lithostratigraphy; stratig- erates erosion and leads to rapid lowering raphy. of the falls or regrading of the rapids. The scale of turbulence in the atmos- type species The species regarded as phere is wide, ranging from small eddies typifying the GENUS to which it belongs. rising above a strongly heated ground sur- The type species is often the species whose face to mid-latitude depressions, which characteristics were used in defining the represent large-scale turbulence in the genus. westerlies. It is important in the atmosphere as an effective mechanism of disper- type specimen A specimen of an organ- sal. ism that was orginally selected and preserved to define the morphological turquoise A complex mineral form of characteristics of a SPECIES. It may not nec- hydrated, copper aluminum phosphate, essarily be typical of the species as a whole. CuAl6(PO4)4(OH)8.5H2O, valued for its light blue or green color. It crystallizes in typhoon A TROPICAL CYCLONE or HURRI- the triclinic system, and occurs in nodules, CANE with winds above 64 knots (117 km small masses, or thin veins in various rocks. per hour or Force 12 on the BEAUFORT It is used as a semiprecious gemstone. SCALE). The term is used for such storms in the W and N Pacific Ocean and comes twilight The period of weak daylight from a Chinese word meaning great wind.

362 U

Udden–Wentworth scale (Wentworth sisting largely of ferromagnesian minerals) scale) A scale of particle or grain size in are ultrabasic but some pyroxenites may sediments or sedimentary rocks. The grain- have more than 45% silica and are basic size classification is: boulder, over 256 rather than ultrabasic in composition. Cur- mm; cobble, 64–256 mm; pebble, 4–64 rent usage has tended to equate the two, mm; granule, 2–4 mm; sand (with subdivi- but strictly, ultrabasic and ultramafic are sions very fine, fine, medium, coarse, and chemical and mineralogical terms respec- very coarse), 0.06–2.0 mm; silt (with sub- tively. See acid rock; ultramafic rock. divisions very fine, fine, medium, and coarse), 4–62.5 µm; clay, 0.06–4.0 µm. It is ultramafic rock /ul-tră-maf-sik/ An ig- named for the American geologists Johan neous rock consisting largely of FERROMAG- August Udden (1859–1932) who proposed NESIAN MINERALS. Such rocks may be the original scale in 1898, and C. K. Went- divided into three groups: 1. Peridotites, in worth, who produced a modified and ex- which olivine is dominant and feldspar ab- tended version of the scale in 1922. See sent (dunite, harzburgite, lherzolite, also particle size; phi scale. wehrlite). 2. Pyroxenites, consisting wholly of orthopyroxenes and clinopyroxenes /yoo-gran-dÿt/ A chemical se- ugrandite (bronzitite, websterite). 3. Picrites, which ries of GARNET minerals. contain accessory feldspar (oceanite, ankaramite). ulexite /yoo-leks-ÿt/ A white mineral With an increase in the amount of pla- form of a hydrated sodium calcium borate, gioclase, picrites grade into gabbros. Most NaCaB5O9.8H2O. It crystallizes in the triclinic system as rounded masses of silky ultramafic rocks are plutonic and occur in hairlike fibers, and occurs as evaporite de- layered intrusions, ophiolite complexes, posits in arid regions and lake basins. It is zoned ultrabasic bodies, and as nodules in used as a source of boron. basaltic lavas. ultisol /ul-tă-sôl/ One of the twelve soil ulvöspinel /ûl-vŏ-spi-nel/ See spinel. orders from the US SOIL TAXONOMY, denoting highly weathered soil formed in sub- umber (raw umber) A greenish-brown tropical to tropical climates, which has a earthy pigment that contains oxides of iron surface horizon containing residual iron and manganese, together with silica and oxides and an illuvial horizon beneath rich lime. It can be roasted to give dark brown in clay. Base status and fertility is low. It is burnt umber. See also sienna. so named because it contains soils at the ultimate stage of weathering and includes the unaka /yû-nay-kă/ An area of high red-yellow podzolics, red-brown lateritics, ground that rises above a PENEPLAIN (which and associated hydromorphic variants. has been eroded away around it), larger than a MONADNOCK. ultrabasic rock /ul-tră-bay-sik/ An igneous rock containing less than 45% silica uncompahgrite /un-kŏm-pah-grÿt/ See (by weight). Most ultramafic rocks (con- ijolite.

363 unconformable unconformable /un-kŏn-for-mă-băl/ See ee-ă-niz-ăm/ (actualism) The theory, discordant (def. 2). now generally accepted, that all geologic changes have occurred by the gradual ef- unconformity / un-kŏn-for-mă-tee/A fect of processes that have been operating surface representing a period of nondeposi- over a long period of time and are still tion or erosion separating rocks of differ- going on today. Compare catastrophism. ent ages. Some unconformites show a marked angularity, the beds above and units For general scientific purposes, SI below the unconformity surface having dif- UNITS are now widely used throughout the ferent dips and strikes, whereas other un- world, However, in some disciplines older conformities can be detected only by systems of units still persist. For example, paleontological means. Compare discon- in meteorology and climatology many el- formity. ements are still measured in Imperial units and knots continue to be used for wind ve- underground drainage In limestone locity and ocean currents in most coun- areas solution can take place to such an ex- tries. Although the SI unit of pressure is the tent that joints, bedding planes, and other pascal, it has been agreed that the millibar fissures become progressively enlarged, will continue to be used in meteorology. forming hollows (SINKHOLES), UVALAS, or POLJES) on the surface, and cavern systems unloading (sheeting; pseudo-bedding; underground. Linked cave systems can pressure release) A weathering process lead the water of a stream from a sinkhole resulting in the division of rock masses into at one point to a spring some considerable sheets separated by fractures. These sheets distance away, thereby eliminating surface tend to increase in frequency and decrease drainage for that stretch. in thickness toward the top of an exposure; they are concentric, and essentially follow underground stream A stream that the topographic outline of the ground sur- flows underground for all or part of its face. The usual explanation is that the frac- course, most commonly in limestone re- tures are due to the expansion of the rock gions. The erosive action of such streams on reduction of the confining pressure, can carve various formations under- achieved by the removal of overlying ma- ground. See underground drainage. terials by erosion. These features occur most frequently in granites. undersaturated rock A type of igneous rock that is deficient in silica and therefore unmixing See exsolution. contains FELDSPATHOIDS. Undersaturated rocks include ijolites, nephelinites, and al- upper-air observation See sounding kali basalts. See silica saturation. (def. 2). underthrust A low-angled fault result- upslope fog A form of ADVECTION FOG ing in the movement of the foot wall rela- resulting from the cooling of moist air to tive to the hanging wall. saturation point as it is forced to rise up the windward slope of uplands. It has the ap- undertow A seaward flow of water be- pearance of stratus cloud from below. neath, or in the vicinity of, breaking waves. Some authorities claim that the backwash upthrown Describing the side of a of waves constitutes a periodic sheet-flow FAULT that appears to have moved upward seaward, describing this as an undertow. relative to the other side (by an amount The term has also been used (erroneously) called the upthrow). See also heave. to mean the RIP CURRENT flows that occur within fairly confined zones. upwarp The uplifting or uparching of a large area of the Earth’s crust, usually in re- uniformitarianism /yoo-nă-for-mă-tair- sponse to isostacy. The area of Scandinavia

364 uvarovite has gradually been rising after having been tion of the macroclimate produced by ex- depressed by the weight of the ice during tensive urban areas. The marked difference the last ice age. in the nature of the surface in rural and urban areas alters the local heat balance to upwelling An ascending water current, such an extent that, even in an area of uni- not necessarily from the bottom of the sea, form topography, the city evolves its own which transports colder water up into the climate. This has become the subject of surface layers of the sea. Such currents much research in recent years and to dis- occur particularly in tropical and subtrop- tinguish it from the macroclimate deter- ical seas, and in the water flanking the mined from Stevenson screens at standard Antarctic, especially in those regions where exposures, it is called the urban climate. surface currents diverge (DIVERGENCE See also heat island. zones). Upwellings also occur in some inland or marginal sea areas, including the urban heat island See heat island. Caspian Sea and Black Sea. They may stem from strong offshore winds that tend to Urstromtäler /oor-strohm-tah-ler/ In N drag the surface water layer seaward so to Europe, wide trenches or valleys eroded by be replaced by colder water from beneath. meltwater from the ice sheet as it retreated Upwellings occur off the coasts of Califor- northward at the end of the last ICE AGE. nia, Peru, and Ghana. They tend to operate seasonally and to form cold zones within urtite /oor-tÿt/ See ijolite. otherwise warm sea areas. They are of primary importance in bringing to the surface waters that are rich in nutrients; these in U-shaped valley See glacial trough. turn may support thriving fisheries. Shifts in the zones of upwellings have proved dis- uvala /yoo-vah-lă/ A large depression astrous to the fishing industry. found in limestone areas, resulting from solution (see limestone solution). They uralitization /yû-ral-ă-tă-zay-shŏn/ The may be several kilometers in diameter, and late-stage or metasomatic alterations (see frequently result from the coalescence of a metasomatism) of primary igneous pyrox- number of adjacent DOLINES. In such a case enes to fibrous pale green amphiboles. the uvala may have a scalloped margin. See also polje. uraninite /yû-ran-ă-nÿt/ A black radioactive uranium oxide mineral, UO2, found uvarovite /oo-vah-rŏ-fÿt/ A rare bright as an accessory in acid igneous rocks and in green member of the GARNET group of min- hydrothermal veins. erals, Ca3Cr2Si3O12. It occurs in chromite- rich serpentinites. It is used as a urban climate The distinctive modifica- semiprecious gemstone.

365 V

vadose water /vay-dohs/ Groundwater tend down-valley to comparatively low al- above the water table in permeable rock, titudes. e.g. limestone. Underground streams in this zone flow with free air surfaces. Com- vanadinite /vă-nay-dă-nÿt/ A brilliant pare phreatic water. orange, red, or brown mineral, a chloride and vanadate of lead, Pb5(VO4)3Cl. It vagrant benthos See benthos. forms hexagonal crystals or fibrous masses in deposits of lead ores, and is used as a valley A long depression, lower than the source of vanadium. surrounding terrain, generally formed by the erosive action of a glacier or river. Its vapor pressure The pressure exerted by width and depth may be as little as a few the molecules of a liquid or solid that es- meters or many kilometers. The cross-sec- cape from the surface. In meteorology it is tional shape depends on the prevailing that part of the total atmospheric pressure rock type and its origin (see glacial trough; exerted by any water vapor that is present. V-shaped valley). Young valleys, in moun- Vapor pressure is measured indirectly from tainous areas, tend to be steep and narrow, dry- and wet-bulb temperatures using ta- whereas in lower regions the valleys are bles or a humidity slide rule to obtain the generally broader and less steep. See also precise value. river. variolitic /va-ree-ŏ-lit-ik/ Denoting a valley breeze The daytime equivalent of texture occurring in basaltic glasses similar the MOUNTAIN WIND. It is formed by greater to SPHERULITIC texture in acid glassy rocks. solar heating on the inclined mountain Variolites usually consist of radial or slopes relative to the air at the same level over the valley. Because wind speeds tend sheaflike aggregates of plagioclase crystals. to be higher during the day as a result of general turbulence and convection, the val- Variscan /va-ră-skan/ The period of ley breeze is more easily masked than the mountain-building in late Paleozoic time mountain wind. that affected Europe and includes both the Armorican and Hercynian phases. valley glacier An accumulation of ice moving down a preexisting valley and re- variscite /va-ră-sÿt/ A greenish mineral stricted in width by the valley walls. Such form of hydrated aluminum phosphate, glaciers may develop as a result of the AlPO4.2H2O, which occurs as nodular merging of a number of CIRQUE GLACIERS in masses. valley-head locations at times of deterio- rating climate, in which case they are varve /varv/ A thin bed of sediment in known as the Alpine type, or they may be glacial lakes that represents a seasonal in- developed at the very edge of an ICE CAP or crement. Spring and summer glacial melt- ICE SHEET, when they are known as the out- ing produces a sudden influx of coarse let type. If the supply of ice to the source sediment upon which finer materials settle areas is sufficient, valley glaciers can ex- during the remainder of the year. These an-

366 vesuvianite nual rhythms can be used in dating events sess an internal skeleton of cartilage or within the PLEISTOCENE. bone, including a jointed backbone formed of vertebrae. Vertebrates are active animals vauclusian /vaw-kloo-zhee-ăn/ A spring and consequently are usually bilaterally that flows from underground in a lime- symmetrical. The sense organs and feeding stone region. apparatus are grouped anteriorly, where they and the brain are protected by a skull. veering In meteorology, a clockwise Vertebrates have a dorsal hollow nerve change in wind direction, such as from cord and, in most higher groups, two pairs southwesterly to westerly. See also back- of limbs. Fossil evidence indicates that ver- ing. tebrates appeared in the Ordovician (see Ostracodermi), since when they have vegas In Spain, the local name for irri- evolved to occupy a great variety of envi- gated lowland areas used for agriculture. ronmental niches. vein A thin sheetlike deposit of minerals vertical corrasion /kŏ-ray-zhŏn/ Abra- in a crack, fault, or joint in rock. Most sive erosion (corrasion) on a rocky river minerals in veins were deposited after vol- bed that acts downward and forms POT- canic activity by hot fluids (gases or liq- HOLES, producing a gradual lowering of the uids) that solidify when they cool. bed. veld /velt, felt/ (veldt) A large area of vertisol /ver-tă-sôl/ One of the twelve grassland that occurs in southern Africa, soil orders of the US SOIL TAXONOMY, de- where it is too dry for trees to grow. Some noting mixed and inverting soils of the has been plowed to grow cereal crops, such TROPICAL BLACK SOIL group. Vertisols are as corn and wheat. split into uderts, which crack open for less than three months in a year, and usterts, vent (conduit) The subterranean passage which open and close more than once a from the underlying magma chamber year. through which volcanic products, i.e. lava, ashes, and vapor, are discharged at the vesicle /vess-ă-kăl/ A small rounded cav- Earth’s surface. Where a volcano has only ity within lavas formed by bubbles of gas one such hole, debris accumulates around coming out of solution during the solidifi- it as a roughly symmetrical cone: this is cation of the lava. The concentration of termed a CENTRAL-VENT VOLCANO. Where vesicles at the top surfaces of lava flows there is more than one eruptive site, usually serve as a way-up criterion. Elongate hol- along a large fracture in the crust, it is low tubes formed by the escape of steam termed a FISSURE ERUPTION or volcano. through lavas flowing over wet ground are called pipe vesicles. Highly vesicular lava ventifact /ven-tă-fakt/ A pebble or stone fragments are termed pumice. See also worn and polished by the action of wind amygdale; pyroclastic rock. and sand, and found in the gravel-strewn parts of the deserts. If a ventifact becomes vesiculation /vĕ-sik-yŭ-ler/ The forma- worn in such a way that a roughly triangu- tion of small cavities in molten igneous lar cross section evolves, it is known as a rock as it cools. The gases, previously in so- DREIKANTER (German = three sides). lution, form bubbles as the pressure decreases when the magma rises nearer the vermiculite /ver-mik-yŭ-lÿt/ See clay surface. If a great deal of gas is released, an minerals. explosive volcanic eruption may occur.

Vertebrata The subphylum of the Chor- Vesuvian See volcano. data to which FISH, AMPHIBIA, REPTILIA, AVES (birds), and MAMMALIA belong. They pos- vesuvianite /vĕ-soo-vee-ă-nÿt/ See idocrase.

367 viscous flow viscous flow Flowage that occurred in Water and gases are also released at minor once molten rocks, for example lavas, or vents, geysers, hot springs, fumaroles, and imperfect fluids, such as asphalt. This re- solfataras. sults in highly complex fold patterns, each fold being distinct from and related to its volcanic ash Fragments of PYROCLASTIC neighbors. ROCK less than 2 mm cross. viscous lava Slow-flowing acidic LAVA, volcanic bomb See pyroclastic rock. which tends to build up steep volcanic cones. See also lava flow. volcanic breccia A type of BRECCIA consisting of angular fragments of volcanic visibility (in meteorology) The greatest rock more than 65 mm across. distance that the eye can see. If there is a marked variation with direction then the volcanic dome See lava dome. lowest visibility is recorded, because visibility is mostly used for aviation purposes volcanicity /vol-kă-niss-ă-tee/ (vulcani- and therefore the minimum value is impor- city) The various processes involved with tant. Reports at night are based on dis- the formation of magma and how it moves tances to unfocused lights of moderate or beneath and through the Earth’s crust. known intensity. volcanic neck (volcanic plug) A cylin- vitrophyre /vit-rŏ-fÿr/A PORPHYRITIC drical column of solidified magma that oc- lava in which the groundmass is wholly cupied the core of an inactive volcano. It is glassy. exposed by erosion of the less resistant surrounding volcanic rock formations. See vogesite /voh-jĕ-sÿt/ A type of LAMPRO- neck. PHYRE that contains biotite and hornblende, and in which ORTHOCLASE volcanic rock Any igneous rock that is predominates. formed on the surface of the Earth as a result of volcanic action. The chief basic vol- volatile (fugitive) A substance, normally canic rock is BASALT; its acidic equivalent is gaseous, that is dissolved under pressure in RHYOLITE. Because of their rapid cooling, a magma. The main volatile constituents of such rocks consist of very small micro- magmas are water and carbon dioxide, to- scopic crystals; even more rapid cooling (as gether with smaller quantities of chlorine, with a submarine volcano) results in the hydrochloric acid, fluorine, hydrofluoric formation of glassy OBSIDIAN. acid, hydrogen sulfide, sulfur, and boron compounds. volcanism /vol-kă-niz-ăm/ (vulcanism) The solubility of volatile substances in- The various processes involved in the ejec- creases with pressure so that during the as- tion of molten rock (magma, which be- cent of magma and the attendant lowering comes lava) from a volcano or hot water of pressure, the magma is unable to retain and steam from a FUMAROLE or GEYSER. these constituents in solution. Volatiles concentrate at the top of the magma cham- volcano A fissure or vent on the Earth’s ber in a residual fluid from which peg- surface connected by a conduit to the matites, hydrothermal veins, and mineral Earth’s interior, from which lava, gas, and deposits may originate. During escape, pyroclastic material are erupted. Each vol- volatiles effect mineralogical changes on canic eruption has its own characteristics, the early-formed crystals (PNEUMATOLYSIS). but they can be classified generally as: 1. Volcanic eruptions are accompanied by Hawaiian Fairly quiet eruptions in which the release of large quantities of steam and fluid lava is erupted from fissures or pits, carbon dioxide. The explosive release of with gas being liberated freely. When the these gases produces pyroclastic rocks. eruption is accompanied by spurting gases,

368 vulcanology incandescent spray is thrown into the air. direction as the Earth, and negative if anti- 2. Strombolian More violent than the cyclonic. Absolute vorticity is the relative Hawaiian type, the eruption taking place vorticity plus the component of the Earth’s more spasmodically as trapped gas escapes rotation about its axis; this is zero at the from a more viscous lava confined in a Equator and a maximum at the poles. crater. Eruptions may be every few minutes. During violent activity bombs are V-shaped valley Traditionally, fluvial ejected, which may be accompanied by valleys of youthful and mature rivers are lava flows. 3. Vulcanian Characterized by said to have a V cross profile, with steep viscous lava whose surface rapidly solidi- walls and narrow bottoms, while by senil- fies. Beneath this crust gas accumulates ity, the broadening floodplain produces and builds up pressure until the crust shat- more of a U-shape. Irrespective of age, ters. This results in large quantities of py- most valleys display a V-shape near their roclastic deposits ranging from large source, with some trend toward a broaden- bombs to fine ash. 4. Vesuvian Similar to 2 ing bottom near their mouth. Streams with and 3 but with gas-charged lava being shot sandy and gravelly banks tend to have a V- violently up into the air, emptying the lava shape as the material slumps on erosion of column to a considerable depth. 5. Plinian the foot slope to its angle of rest, and rain- A culmination of type 4 in which a violent fall landing on the valley sinks in and does blast of gas rises to a height of several kilo- not run off and sculpture the gentle side meters. The gas and vapor on reaching this walls; conversely, valleys in cohesive silty height spreads into a large cloud. The fall- or claylike material tend to maintain their out of ash is low, being confined to ma- walls as the river cuts sideways, leaving a terial removed from the conduit. 6. Pelean flat bottom, with steep side walls and Very violent eruptions accompanied by hence a U-shape. Climate can also be im- nuées ardentes and hot avalanches of inco- portant: shady (north- and west-facing in herent constantly expanding self-explosive the N hemisphere) valley sides have a dif- lava, lubricated by hot gases and vapors. ferent microclimate from sunny (south- Such eruptions are characteristic of highly and east-facing in the N hemisphere) walls viscous lava, whose gas content cannot and this can influence the rate and type of readily escape. processes acting, hence lowering the two See also supervolcano. side walls at different rates to produce an irregular valley shape. If precipitation is volcanology /vol-kă-nol-ŏ-jee/ (vulcanol- high, the valley walls become clothed in ogy) The scientific study of volcanoes, dense vegetation, which slows down ero- including their origins, action, structure, sion, maintaining a V-shape, whereas in and classification. Volcanologists are also arid zones where there is no vegetation U- conducting research into the prediction of shapes often result from increased erosion volcanic eruptions. of valley walls. vorticity /vor-tiss-ă-tee/ The three-di- vugh /vug, vûg/ (vug) A cavity within a mensional rotation of a fluid about an axis, volcanic rock lined with deuteric or SEC- measured as a vector of twice the local rate ONDARY MINERALS. Compare amygdale; of rotation of an individual fluid element. miarolitic cavity. As rotation occurs in most aspects of atmospheric motion, vorticity is very impor- Vulcanian See volcano. tant. Even if air is still relative to the Earth, it will still be rotating in space because of vulcanicity See volcanicity. the Earth’s rotation. To distinguish these two types, relative vorticity is taken as ro- vulcanism See volcanism. tation relative to the Earth’s surface; it is positive if cyclonic, i.e. rotating in the same vulcanology See volcanology.

369 W

wacke /wak-ĕ/ A type of young SAND- warm front A strong thermal gradient STONE with poorly sorted grains in a matrix in the atmosphere in which, from its direc- of clay and fine silt. tion of movement, warm air is replacing cold air. The slope of a warm frontal sur- wackestone /wak-ĕ-stohn/ A type of face is about 1 in 150, and so weather phe- limestone consisting of grains of calcium nomena associated with it precede the carbonate in a matrix of fine-grained lime surface front by many kilometers. The first mud. indication is the spreading of cirrus clouds across the sky, followed by cirrostratus, al- wadi /wod-ee, wah-dee/ A normally dry tostratus, then nimbostratus as the rain valley in a desert or semidesert environ- starts falling. Temperatures rise, relative ment. In such locations rain falls very in- humidity increases, and the wind veers as frequently but when it does so it is often in the front passes. Warm fronts are a major the form of violent downpours of limited contributor to annual rainfall totals in duration. As normal fluvial activity does coastal locations of temperate latitudes. not exist, large amounts of weathering de- Compare cold front. bris accumulate in situ. Channel flow following a desert cloudburst can have a warm occlusion See occlusion. considerable erosional effect, cutting these warm rain Rain falling from clouds that narrow valleys with nearly vertical walls are entirely warmer than freezing and into the weathered rocks. Wadis may be re- which therefore must have developed by licts from times of wetter climate. the COALESCENCE process alone. wake The trail of eddies or vorticities warm sector The region of warmer air that develop on the leeward side of an ob- existing in a mid-latitude DEPRESSION be- struction to wind flow. Large vertical ve- tween warm and cold fronts. It eventually locities may be induced in essentially disappears from the surface during the evo- horizontal flow by this method, as on the lution of the depression as the cold front lee side of hills, buildings, or wind breaks. catches up the warm front (see occlusion). Weather in the warm sector is usually no- waning slope The gentle concave foots- ticeably milder than the preceding and fol- lope lying beneath the FREE FACE. The wan- lowing air streams. Precipitation is very ing slope is equivalent to the pediment, by variable, varying from none if there is a which name it is more commonly referred strong ridge of high pressure to heavy, es- to. It comprises the gentle slope of the val- pecially over mountain areas, if the warm ley floor, terminating in a drainage line or sector is potentially or conditionally unsta- floodplain. Waning slopes are said to be- ble. come more widespread at the latter part of the cycle of erosion, as the dominant wash See overland flow. process becomes retreat of hillsides (back- cutting), rather than downcutting of val- washout A filled-in river channel that leys. See also waxing slope. cuts into preexisting sediments. It is usually

370 wave developed on deltas and other areas of below the falls also obscures the break and slow-flowing rivers. The channels are usu- tends to restore a smooth long profile. ally infilled with sands, which often contain fragments of the eroded bed. They are water gap A pass through a ridge of very common in the Coal Measures and mountains through which a stream or river deltaic Jurassic beds. The sand body infill- flows. ing the channel is sometimes known as a horse. watershed 1. See divide. 2. See drainage basin. wash slope (alluvial toe-slope) That part of a hill slope lying at the foot of the waterspout The oceanic equivalent of a CONSTANT SLOPE, formed by the washing TORNADO. It occurs over seas or large lakes out of fines from the scree in the constant when a funnel-shaped cloud descends from slope. This fine material, often lying in the a cumulonimbus cloud to produce violent river valley, has a lesser angle of rest than agitation of the sea surface and a very local the coarser constant slope debris, and so intense rotation of wind. Waterspouts are there is a gentle break of slope between the never as severe or long-lasting as the vari- two components. ety of tornado found in the mid-West of the USA. waste mantle See regolith. water table See groundwater. water balance (in meteorology) The movement of water between and within water vapor The distribution of water the atmosphere and ground surface on a vapor in the atmosphere varies greatly, global scale. It involves the balance be- with largest amounts in the humid tropics tween precipitation, evaporation, advec- and least in polar regions and to a lesser ex- tion of moisture in the air, ocean-current tent the desert areas. As well as being nec- circulations, and river runoff on land. essary for precipitation, water vapor has There is a long-term balance, so that no another important meteorological role in area of the Earth is continuously losing or absorbing long-wave or terrestrial radia- gaining moisture. The values of the com- tion. With carbon dioxide, it acts as a very ponents of the water balance are often effective mechanism for preventing the loss known only very approximately, especially of radiation that would otherwise occur. over the ocean areas. The mean temperature of the Earth is therefore partly a response to the presence water cycle See hydrologic cycle. of water vapor; without it, it would be appreciably lower. Water vapor also waterfall A vertical fall of water at a represents the highest energy state of steep break in the long profile of a stream. water and so it can act as an energy store. This may be the product of base-level fall When condensation to liquid or sublima- producing a large KNICKPOINT, local dis- tion to solid takes place, energy is released, placement due to an earthquake, or irregu- which may give rise to an increase of tem- larities in the long profile produced by hard perature. bands of rock through which the river has been unable to grade a smooth profile water wave See wave def. 2. owing to lack of erosive power or insufficient time. In areas of horizontal strata, wave 1. A periodic disturbance or varia- breaks through individual rock beds will tion in a medium (solid, liquid, or gas) or often result in a steep fall. Once created the in space that transfers energy from one falls immediately begin to retreat upstream place to another. and become less pronounced owing to low- 2. (water wave) An undulation or defor- ering of their height through the concen- mation of a water surface. The period of tration of erosion on the lip. Deposition water waves ranges from a second or so to

371 wave base several hours. The size of waves ranges water or sharp promontory. Diffraction from tiny capillary waves (only a few cen- usually leads to a marked reduction in timeters long) to the large storm waves, wave height. The various diffraction pat- tidal waves, and long waves over 150 km terns that result from, for example, the in length. Many types of waves are wind- passage of sea waves through a restricted induced, directly or indirectly; others arise harbor entrance, can be calculated mathe- because of submarine earthquake distur- matically provided the characteristics of bances or submarine sediment slides. See the waves and the receiving basin are also constructive wave; destructive wave; known. Diffraction may allow some of the ocean wave; sea wave; tsunami. energy of incident waves to affect the water area in the lee of a breakwater. wave base The lowest limit of orbital motion beneath waves. Below the sea sur- wave forecasting The prediction of fu- face in deep water, the orbits of water par- ture wave conditions using empirical ob- ticles remain, as with surface particles, servations of wave conditions that have almost circular, but the diameter of the or- already occurred or theoretical calcula- bits decreases with depth. For every 1/9th tions. Such calculations take into account of the wavelength in depth, the size of the predicted meteorological conditions and water particle orbits are roughly halved. At such factors as FETCH distance. Of the var- a depth that is equivalent to the wave- ious methods of wave forecasting that have length, the orbit size is 1/535th of the orbit been devised, two main types have size at the surface. Where orbital motion emerged: those that take account of the sig- becomes almost negligible, wave base is nificant height and period of the predicted reached. The concept of wave base as- waves (see significant wave), and those sumes importance in problems of sediment that take account of the total WAVE SPEC- transport beneath waves, because bed TRUM. Wave forecasting is of great impor- movements due to waves are influenced by tance to navigation and to studies of the wave-induced oscillatory currents ex- expected coastal changes. perienced close to the seabed. wave hindcasting /hÿnd-kast-ing, -kahst-/ wave-cut platform (abrasion platform; A consideration of wave conditions that shore platfrom) An irregular gently slop- may have applied at an earlier period and ing bare rock platform extending out to sea in a particular area. The method employed and usually backed by cliffs. The mecha- usually involves synoptic wind charts and nism of platform erosion is still not totally other wind data for the period in question, understood: it is not clear whether the and (if available) hydrographic charts waves are themselves responsible for the covering that same period. The method is rock breakdown or merely for the removal useful if coastal changes have been care- of debris weathered by subaerial processes. fully documented and have to be ac- Whichever processes are active, however, it counted for. is the wearing back of the cliffs that causes the enlargement of the platform. Because wave ray See orthogonal. these forms will not develop where there is a covering of beach material, the most fa- wave refraction (in oceanography) The vorable sites are headlands. turning of wave crests so that they become more parallel to the coastline as they ap- wave cyclone A low-pressure system proach it. The process is due to the effects developing as a wave along a FRONT. Most of the shallowing of water: as the depth of mid-latitude DEPRESSIONS form in this way. water decreases so the wave velocity decreases; consequently, a wave crest ap- wave diffraction (in oceanography) proaching the land obliquely will be The transfer of the energy of wave crests as slowed initially at the end nearest the they negotiate a structure, such as a break- shore, while the other end will continue

372 weathering moving faster until the crest has become way-up structures Sedimentary struc- almost parallel to the coastline. Wave en- tures that can be used to determine the way ergy can be represented diagrammatically up of a series of unfossiliferous beds. Typ- by lines drawn perpendicular to the crests. ical way-up structures are cross bedding, As refraction takes place on an indented mud cracks, ripple marks, graded bedding, coast more energy becomes concentrated included fragments of older beds, convo- on the headlands than the bays, thereby fa- lute bedding, burrows, roots, and geopetal voring coastal straightening. cavities. wave spectrum 1. The complete array weather The state of atmospheric condi- of waves present on a water surface in a tions at any one place and time. The total particular place at a particular time. Care- synthesis of these individual weather states ful measurements at sea or in a scale model is CLIMATE. The state of the weather has enable graphic wave records to be pro- been classified into a code form for use in duced, which show such an array of waves. SYNOPTIC METEOROLOGY. In this way, cur- A typical wave spectrum in the open sea rent and past weather can quickly be iden- will nearly always consist of waves of tified, although greatest stress is placed on many different heights and periods super- precipitation or other weather factors imposed one upon the other. The measured likely to be considered important in fore- wave records enable their size and fre- casting or in their impact on humans. quency distributions to be determined. Efforts have been made to modify the 2. A graphic classification of waves, vary- weather, but without much success. The ing from capillary waves with a period of most publicized work has been on CLOUD less than 0.1 second to the transtidal waves SEEDING, but hurricane steering and fog with periods of 24 hours or more. Most of clearance have been attempted. Because of the energy of such a spectrum would lie in the vast amounts of energy involved in the two of the period ranges: the ordinary tides weather and the large area affected, the and the ordinary gravity waves. prospects for large-scale modifications are not hopeful. Unintentional modifications are far more numerous, such as the effects of towns on weather (see urban climate), or waxing slope the effect of irrigating arid lands.

weather forecast See forecast. free face weathering The process of breakdown and alteration of rock on the Earth’s surface in response to the changes in environ- waning slope mental conditions since the time of their formation. During formation many rocks were subjected to great pressures or high Waxing slope temperatures, away from the effects of atmospheric air and water. Under present- waxing slope The convex slope at the day conditions these rocks are at upper part of a hill-slope profile. Accord- comparatively low temperatures and pres- ing to the South African geologist Lester sures and will naturally be influenced by King, the waxing slopes are most promi- the presence of air and water. The resulting nent at the early stages of the cycle of ero- products of weathering are materials more sion, when the main processes are the nearly in equilibrium with their environ- incision, rather than widening, of valleys. ment than those from which they were de- These slopes are said to be dominated by rived. See also chemical weathering; deep CREEP. See also slope convexity; waning weathering; differential weathering; me- slope. chanical weathering; organic weathering.

373 weathering front weathering front The junction between stancy, and is often characterized by very sound and weathered rock within a weath- large sea waves and swell. ering profile. Certain rocks, especially dense types with well-developed jointing, wet-bulb depression The temperature such as granite and basalt, display ex- difference between the dry- and wet-bulb tremely sharp transitions, whereas in THERMOMETERS in a Stevenson screen. It is porous or fissile rocks the junction is fre- a measure of the amount of cooling result- quently so unclear that there is no real ing from evaporation of water on the wet- weathering front. bulb thermometer. Relative humidity of the air can be calculated from this value weather station An installation using tables. equipped with instruments for making meteorological observations. See also auto- wet day In Britain, a period of 24 hours matic weather station. commencing 9 a.m. GMT in which 1.0 mm or more of rainfall is recorded. The concept websterite /web-ster-ÿt/ An ultramafic has its limitations because 1 mm could fall rock consisting of hypersthene and diop- from a single heavy shower or be the result side. of prolonged drizzle throughout the day. See also rain day. wedge (in meteorology) A region of high pressure that extends from an ANTICY- wetland See marsh; swamp. CLONE. It is narrower than a RIDGE OF HIGH PRESSURE. wetting-and-drying weathering The mechanical breakdown of fine-grained wehrlite /weer-lÿt/ An ultramafic rock rocks through alternate wetting and dry- consisting largely of olivine with accessory ing, causing surface FLAKING or major split- augite. ting of blocks into two or more large pieces. A possible explanation involves the welded tuff See pyroclastic rock. fact that water is a polar liquid, with positive charge on the hydrogen atoms and Wentworth scale See Udden–Wentworth negative charge on the oxygen atom. Lines scale. of mutually attracted water molecules may develop within the rocks on wetting and westerlies The main winds blowing be- drying and these may subsequently exert tween 40° and 70° latitude. It is in the the expansive forces that cause fracture. westerly circulation that depressions form and maintain the vital meridional heat ex- whirlpool A small rotating area of change, and in this zone the strongest water in a lake, river, or the sea; larger undisturbed wind flows are found. whirlpools may become funnel-shaped. It may be caused by the shape of rocks on the West Wind Drift A circumpolar cur- bed or result when two opposed currents rent that constitutes one of the Earth’s meet and circulate around each other. large and significant permanent ocean currents. In the S part of the Pacific Ocean, it whirlwind A small revolving vertical occupies a wide tract of water and gener- eddy of air, which whirls around a local ally flows in an easterly direction to the low-pressure area. Stronger whirlwinds south of the subtropical convergence zone. may be heard, but they are more frequently At about latitude 55°S, the Antarctic Con- noticed because they pick up small pieces vergence cuts across the West Wind Drift. of litter or debris. They are formed during In the S Atlantic Ocean, the West Wind periods of local heating and instability, ap- Drift occupies the wide tract of water that pearing as small-scale TORNADOES. lies approximately between latitudes 35°S and 63°S. It moves with surprising con- white alkali soil See solonchak.

374 wind-chill index white ice Ice that contains trapped air, the continents converge in a collision zone located at or near the surface of the to form a continental suture. See also plate ground, often on a glacier. As it descends tectonics. under further layers of ice, the pressure forces out the air and the ice appears blue. wilting point The point at which a This change is often accompanied by melt- plant has extracted all the available water ing (see regelation). from a soil and can no longer exert enough force to remove the remaining water. Once white-out A meteorological condition this point is reached the cells of the plant that occurs in polar regions when the sky is lose their turgor and it wilts. The wilting uniformly overcast, or there is drifting point varies between different soils accord- snow or blizzard conditions, in combina- ing to particle size; fine particles hold water tion with complete snow cover: the hori- more tightly so it is less easily removed by zon disappears and everything then plants. becomes white with no shadows or identifiable landmarks. wind The horizontal movement of air relative to the Earth’s surface. Air move- white smoker See hydrothermal vent. ment results from thermal differences, which produce pressure variations, or by Wilson cycle A theory for the cyclical dynamic factors such as divergence of the development of ocean basins that relates air flow itself. Wind is one of the basic el- SEA-FLOOR SPREADING to continental geol- ements of weather. It is measured in knots ogy. It is named for the Canadian geo- and its direction is that from which it physicist John Tuzo Wilson (1908–93) blows. Wind speeds at the ground surface who first formulated it in the1960s. The se- can be very variable from absolute calm quence begins with the opening of the during anticyclones up to 200 knots in a ocean, then progresses with its develop- tornado. ment, through to its destruction, a process that may be repeated throughout history. windbreak Any barrier having the de- In the opening stage a continental plate be- liberate effect of reducing surface wind gins to split or rupture and form rift valleys speeds. It can be artificial or natural, al- (e.g. the Great Rift Valley of East Africa). though the latter is more frequently called In the following stage there is subsidence a shelter-belt. The degree of shelter that a accompanied by sea-floor spreading with barrier offers depends upon its height, its the formation of a narrow sea (e.g. the Red degree of permeability, and distance. For a Sea separating the Arabian Peninsula from dense barrier, wind speeds fall dramati- Africa). The next stage sees the develop- cally immediately to the leeward, but soon ment of a wide ocean basin (e.g. the At- recover to the prevailing wind speed. With lantic) with continued expansion as molten a barrier that offers about 50% permeabil- rock erupts to create new oceanic lithos- ity, the effect reaches a maximum in terms phere along the mid-ocean ridges; sedimen- of overall reduction of wind speed and dis- tary wedges form on the continental tance affected downwind. Even at a dis- shelves. In the following stage the ocean tance of forty times the height of the basin begins to close: subduction bound- barrier there is still some reduction. aries form and the oceanic lithosphere is subsumed down into the asthenosphere wind-chill index An index of the effects forming a trench and associated island arc. upon living creatures of cold winds. The The closing continues with subduction stronger a cold wind is blowing the more zones close to the continents and the for- rapid will be the rate of heat removal from mation of volcanic arcs; the compression a mammal. Consequently, cold but calm and uplift of accretionary wedges forms conditions are more physiologically ac- new mountain ranges. The stage is reached ceptable than warmer temperatures in a in which the ocean basin finally closes and strong wind. A number of wind-chill in-

375 wind drift current dices have been used worldwide. That used Such rocks are exposed as an enclosed out- in North America was based on the crop, as a result of erosional processes cut- Siple–Passel equation, which was devised ting down through the overlying rock mass from experiments conducted in the 1940s to the younger rock beneath. in Antarctica by the American geographers Paul Siple and Charles Passel. In 2001 a wind rose A diagram for illustrating the new revised Wind-chill Temperature Index frequency of wind directions over a speci- was implemented in the USA and Canada. fied time period. It is normally split into This index: calculates wind speed at an av- eight or sixteen points of the compass and erage height of 1.5 m (typical height of the can also give an indication of the frequency adult human face) based on readings from of wind speeds within specified ranges by the standard anemometer height of 10 m; direction, as shown in the diagram over- uses modern heat-transfer theory; is based leaf. on a human face model; has the threshold for calm wind at 4.8 km per hour; and as- wind set-down The lowering of the sumes clear night sky conditions. still-water level on the down-wind flank of a body of water because of wind stresses wind drift current A current that reacting on the water surface, or the differ- sults from wind stresses on the sea’s sur- ence in levels between the up-wind and face, part of the wind energy producing down-wind flanks resulting from wind surface waves and part generating these stress. Like WIND SET-UP the term is usually currents. Indirectly, the wind is responsible applied to the phenomenon as it affects for most of the currents in the surface and reservoirs, lakes, estuaries, and other bod- near-surface waters. Because of the Earth’s ies of water that are relatively limited in rotation and the resulting Coriolis effect, size. One example is the Plate River estuary the wind-induced currents in the open of Argentina: here winds blowing up (or ocean have a mean direction approxi- down) the estuary cause set-up (or set- mately 45° to the wind direction (some- down) and also change the current pat- what less than this near the surface, and terns. Statistical analyses of wind data greater than this at depth). The currents show that for some 220 days per year, tidal flow to the right of the wind in the N hemi- levels differ by up to a half a meter or more sphere and to the left of the wind in the S from the predicted values. hemisphere. A wind blowing fairly hard for a long period may produce a thick ho- wind set-up The raising of the still- mogeneous layer of isothermal water, for water level on the down-wind flank of a example, under the influence of the steady body of water because of wind stresses act- trade winds. In the shallower water near ing on the water surface, or the difference the coast, onshore winds tend to drag the in levels between the up-wind and down- surface water shoreward, while offshore wind flanks resulting from wind stress. winds cause the reverse to happen; in both cases, the related currents near to the wind shear The local rate of change of seabed may be flowing in the reverse direc- wind velocity, usually at right angles to the tion to the surface wind-induced current. horizontal air flow. Vertical shear may also be considered. wind erosion See eolian erosion. windward Describing the side of a hill wind gap A gap (col) or notch in a ridge or other feature that faces into the prevail- or hilltop, through which the wind can ing wind. blow and may whistle. It may have been formed by RIVER CAPTURE or glacial action. winter solstice See solstice. window (fenster) An outcrop of rock witherite /with-er-ÿt/ A white, yellow- lying beneath a recumbent fold or thrust. ish, or gray mineral form of barium car-

376 WWW

variable and calm 9.0%

0–3 knots

4–10 knots

11–21 knots 22–23 knots S

0 5 10 15 % of total wind flow

Wind rose

bonate, BaCO3. It crystallizes in the or- land of mixed deciduous trees (replaced by thorhombic system, and generally occurs evergreens in the Mediterranean area), in veins or in association with the lead ore which grow between the tropical forests GALENA. It is used as a source of barium. farther south and the coniferous forests See also carbonate minerals. farther north. wold In E or NE England, a chalk hill. World Weather Watch (WWW)A program planned by the World Meteoro- wolframite /wucirc;l-frăm-ÿt/ A red- logical Organization to provide a world- brown to black mineral form of iron man- wide coverage of surface and upper ganese tungstate, (Fe,Mn)WO4. It atmospheric data. This is then used to pro- crystallizes in the monoclinic system, and vide an initial detailed data set for numeri- occurs mainly in pegmatites and hy- cal models of the atmosphere. drothermal veins in quartz. It is the chief source of tungsten. See also scheelite. worm cast See earthworm. wollastonite /wucirc;l-ă-stŏn-ÿt/ A white wrench fault See strike–fault. triclinic calcium silicate mineral of composition CaSiO3. It is found in metamor- wulfenite /wucirc;l-fĕn-ÿt/ A yellow or phosed limestones and in some alkaline orange mineral form of lead molybdate, igneous rocks, such as nepheline SYENITES. PbMoO4. It crystallizes in the tetragonal system, and occurs as earthy or granular woodland An area where the dominant aggregates in lead deposits. It is used as a vegetation is trees that form a canopy, but source of molybdenum. not as dense as a forest. In temperate climates, the mild moist climate favors wood- WWW See World Weather Watch.

377 XYZ

xanthophyllite /zan-th’ŏ-fil-ÿt/ A brit- Yardangs are caused by erosion by wind- tle yellowish form of hydrated calcium borne sand (corrasion), and often lie pa- magnesium aluminosilicate. It crystallizes rallel to the direction of the prevailing in the monoclinic system, and is a member wind. of the MICA group of minerals. yazoo stream /yă-zoo, ya-zoo/ A tribu- xenoblastic /zen-ŏ-blas-tik/ Describing tary stream that flows for a consider- crystals in metamorphic rocks that are an- able distance parallel to the main stream, hedral, exhibiting no crystal faces. from which it is separated by a natural levée, before joining the main stream. xenocryst /zen-ŏ-krist/ A crystal superfi- Yazoo streams are named for the Yazoo cially resembling a PHENOCRYST, which is River, which flows alongside the Mis- not in equilibrium with the other minerals sissippi River for 320 km before joining in an igneous rock. It is commonly an- it near Vicksburg, Mississippi. hedral, having suffered resorption due to reaction with the magma. yellow ocher See ocher. zen-ŏ-lith xenolith / / An inclusion within yield point The point at which a rock an igneous rock body. The inclusion may ceases to deform elastically to an applied be a block of country rock that has been force, and beyond which further force will caught up in the intrusion, but has not been cause it to fracture. completely assimilated. It may alternatively be a block of the igneous body itself Young’s modulus A stretch modulus that solidified at an earlier period and (E), equal to the ratio of the stress on a therefore has a slightly different composition. cross-sectional area of a rod of material to the longitudinal strain. It is named for the xenomorphic /zen-ŏ-mor-fik/ Describ- British physicist, physician, and Egyptolo- ing an igneous or metamorphic rock whose gist Thomas Young (1773–1829). mineral crystals do not have characteristic crystal faces, caused by disturbance during zenith The point in the sky that is verti- crystallization. cally above an observer, It is frequently used in radiation geometry for calculating xerophyte /zeer-ŏ-fÿt/ A plant that can solar radiation input. tolerate drought or extremely dry climatic conditions. Their adaptations include long zenithal projection See azimuthal pro- roots (to reach underground water), small jection. or thick leaves (to reduce water loss during transpiration), water storage cells in fleshy zeolites /zee-ŏ-lÿts/ A large group of stems, and dense hairs (to trap moist air). minerals with a general formula Cacti are typical xerophytic plants. (Na2,K2,Ca,Ba,Sr) ((Al,Si)O2)n.xH2O. Ze- yardang /yar-dang/ A narrow steep- olites have a structure similar to feldspars sided ridge that occurs in arid regions. and feldspathoids and consist of a very

378 zone open framework of linked (Si,Al)O4 tetra- zoisite /zoh-ă-sÿt/ A member of the EPI- hedra, with the metal cations filling the DOTE group of minerals that crystallizes in large cavities in the structure, which also the orthorhombic system. contains loosely held water molecules. The compositions of some of the commonly oc- zonal circulation Wind circulation curring zeolites are as follows: with a dominant west–east directional natrolite Na2(Al2Si3O10).2H2O component. The main zonal circulations mesolite Na2Ca2(Al2Si3O10)3.8H2O are the WESTERLIES of the mid-latitudes and scolecite Ca(Al2Si3O10).3H2O the EASTERLIES of the tropical oceans. East- thomsonite NaCa2(Al5Si5O20).6H2O erly winds are normally defined as being heulandite (CaNa2)(Al2Si7O18).6H2O negative. Compare meridional circulation. phillipsite ½ ( Ca,Na,K)3(Al3Si5O16).6H2O zonal index A measure of the strength harmatome Ba(Al2Si6O16).6H2O of the zonal circulation in a specified area stilbite (Ca,Na2,K2)(Al2Si7O18).7H2O and time period. The index most com- chabazite Ca(Al2Si4O12).6H2O monly used is that for the N Atlantic ° ° laumontite Ca(Al2Si4O12).4H2O Ocean between latitudes 35 N and 55 N Most zeolites are colorless or white and based on the differences in mean pressure are relatively soft, varying in hardness along these lines across the ocean. from 3.5 to 5 on Mohs’ scale. Natrolite, mesolite, scolecite, and thomsonite have fi- zonal soil A soil occurring over a wide brous habits. Zeolites are commonly found area because of the dominance of the bio- as late-stage minerals in AMYGDALES in climatic factor in soil formation, which de- basic lavas. They also occur as alteration termines soil-forming processes. Where the products after feldspars and feldspathoids. bioclimatic influence is locally replaced by The occurrence of zeolites on a regional relief and drainage, INTRAZONAL SOILS scale in volcanic rocks and sediments is occur. Over a century ago the Russian N. considered to indicate a very low-grade M. Sibertsev (1860–1900) conceived soil metamorphism termed the zeolite facies. as a climatic succession running from the tundra soils to laterite. This simplification zeugen /tsoi-gĕn/ A type of PEDESTAL is increasingly regarded as outmoded be- ROCK whose column comprises tabular cause of the location of many soils outside slabs. The variation in profile is the result their zone through the local influence of of differential erosion of the rock by wind- some other factor (e.g. podzols on sandy borne particles. parent materials in the brown earth zone) and the climatic change of the recent past, zinc blende See sphalerite. which has varied zonal boundaries. zincite /zink-ÿt/ An orange to deep-red zone 1. (in metamorphism) A spatial di- mineral oxide of zinc and manganese, vision in an area that has undergone META- (Zn,Mn)O. It crystallizes in the hexagonal MORPHISM, based on the first appearance of system, and occurs as masses in metamor- successive index minerals within the meta- phosed limestone. It is used as a source of morphosed rocks in the progression to- zinc. ward rocks of the highest metamorphic GRADE. The progressive nature of meta- zinnwaldite /zin-wawld-ÿt/ See mica. morphism was developed by the British geologist George Barrow (1853–1932) zircon /zer-kon/ A zirconium silicate working in the Scottish Highlands, where mineral of composition ZrSiO4, found as he identified a number of zones in pelitic an accessory mineral in intermediate and rocks. The index minerals occurring in acid igneous rocks. Being very hard and re- PELITES in order of increasing grade are: sistant to weathering, it is a common detri- chlorite, biotite, almandine garnet, stauro- tal mineral in sediments. lite, kyanite, and sillimanite. A line drawn

379 zone fossil on a map representing the first appearance consists of the strata in which the ranges of of a particular index mineral is termed an particular fossil species or groups overlap. isograd. These are lines of equal grade and In some situations a zone in biostratigra- link rocks originating under similar physi- phy may correspond to a CHRONOZONE in cal conditions of metamorphism. The al- the chronostratigraphic scale. mandine zone, for instance, is the zone 3. See seismic zone. occurring between the almandine and stau- 4. See crystal zoning. rolite isograds. However, in rocks of other 5. See morphogenetic zone. compositions, an index mineral may appear at a higher or lower grade than that zone fossil (index fossil) A fossil species corresponding to its first appearance in that is used in BIOSTRATIGRAPHY to delimit pelitic rocks. Different mineral zones have a ZONE. Ideally a zone fossil should have a been correlated with different rock types limited vertical range in the succession and and this correlation is embodied in the con- should in life have been rapidly and widely cept of metamorphic FACIES. distributed to permit correlation over a Sequences of metamorphic zones other large area. Several species, forming a whole than those proposed by Barrow have been zonal assemblage, may be used in the defi- identified in other areas. In NE Scotland, nition of a biostratigraphical division. the Buchan zones (named for this Scottish Highland region) are defined by the index zone of aeration The zone below the minerals staurolite, cordierite, andalusite, ground surface and above the water table and sillimanite. The Buchan zones repre- in which the pore spaces and openings sent a different metamorphic gradient involving relatively lower pressures than within the soil, sediments, and rock con- those represented by the Barrow zones. tain mainly air. 2. (in stratigraphy) The fundamental division used in biostratigraphical methods of zone of saturation (phreatic zone)A calibrating and correlating rock succes- zone below the ground surface where all sions. A zone is demarcated by the fossils it cracks and pore spaces in sediments or contains and is usually named after a ZONE rock are filled with water. The zone’s FOSSIL; when applied to a zone the name of upper boundary forms the water table (see the fossil should be italicized. Examples of groundwater); phreatic water. various types of zone are as follows. An assemblage zone is characterized by the oc- zooplankton /zoh-ŏ-plank-tŏn/ See plank- currence of a particular assemblage of ton. fossils; an acme zone is characterized by the particular abundance of a single species zweikanter /tsvÿ-kan-ter/ A pebble with or fossil group; a total-range zone com- two facets, formed by the erosive action of prises the entire thickness of rocks in which windblown sand in desert regions. See also a fossil occurs; a concurrent-range zone dreikanter.

380 APPENDIXES

Appendixes

The Chemical Elements (* indicates the nucleon number of the most stable isotope)

Element Symbol p.n. r.a.m Element Symbol p.n. r.a.m actinium Ac 89 227* fluorine F 9 18.9984 aluminum Al 13 26.982 francium Fr 87 223* americium Am 95 243* gadolinium Gd 64 157.25 antimony Sb 51 112.76 gallium Ga 31 69.723 argon Ar 18 39.948 germanium Ge 32 72.61 arsenic As 33 74.92 gold Au 79 196.967 astatine At 85 210 hafnium Hf 72 178.49 barium Ba 56 137.327 hassium Hs 108 265* berkelium Bk 97 247* helium He 2 4.0026 beryllium Be 4 9.012 holmium Ho 67 164.93 bismuth Bi 83 208.98 hydrogen H 1 1.008 bohrium Bh 107 262* indium In 49 114.82 boron B 5 10.811 iodine I 53 126.904 bromine Br 35 79.904 iridium Ir 77 192.217 cadmium Cd 48 112.411 iron Fe 26 55.845 calcium Ca 20 40.078 krypton Kr 36 83.80 californium Cf 98 251* lanthanum La 57 138.91 carbon C 6 12.011 lawrencium Lr 103 262* cerium Ce 58 140.115 lead Pb 82 207.19 cesium Cs 55 132.905 lithium Li 3 6.941 chlorine Cl 17 35.453 lutetium Lu 71 174.967 chromium Cr 24 51.996 magnesium Mg 12 24.305 cobalt Co 27 58.933 manganese Mn 25 54.938 copper Cu 29 63.546 meitnerium Mt 109 266* curium Cm 96 247* mendelevium Md 101 258* darmstadtium Ds 110 269* mercury Hg 80 200.59 dubnium Db 105 262* molybdenum Mo 42 95.94 dysprosium Dy 66 162.50 neodymium Nd 60 144.24 einsteinium Es 99 252* neon Ne 10 20.179 erbium Er 68 167.26 neptunium Np 93 237.048 europium Eu 63 151.965 nickel Ni 28 58.69 fermium Fm 100 257* niobium Nb 41 92.91

383 Appendixes Element Symbol p.n. r.a.m Element Symbol p.n. r.a.m nitrogen N 7 14.0067 silver Ag 47 107.868 nobelium No 102 259* sodium Na 11 22.9898 osmium Os 76 190.23 strontium Sr 38 87.62 oxygen O 8 15.9994 sulfur S 16 32.066 palladium Pd 46 106.42 tantalum Ta 73 180.948 phosphorus P 15 30.9738 technetium Tc 43 99* platinum Pt 78 195.08 tellurium Te 52 127.60 plutonium Pu 94 244* terbium Tb 65 158.925 polonium Po 84 209* thallium Tl 81 204.38 potassium K 19 39.098 thorium Th 90 232.038 praseodymium Pr 59 140.91 thulium Tm 69 168.934 promethium Pm 61 145* tin Sn 50 118.71 protactinium Pa 91 231.036 titanium Ti 22 47.867 radium Ra 88 226.025 tungsten W 74 183.84 radon Rn 86 222* ununbium Uub 112 285* rhenium Re 75 186.21 ununtrium Uut 113 284* rhodium Rh 45 102.91 ununquadium Uuq 114 289* roentgenium Rg 111 272* ununpentium Uup 115 288* rubidium Rb 37 85.47 ununhexium Uuh 116 292* ruthenium Ru 44 101.07 uranium U 92 238.03 rutherfordium Rf 104 261* vanadium V 23 50.94 samarium Sm 62 150.36 xenon Xe 54 131.29 scandium Sc 21 44.956 ytterbium Yb 70 173.04 seaborgium Sg 106 263* yttrium Y 39 88.906 selenium Se 34 78.96 zinc Zn 30 65.39 silicon Si 14 28.086 zirconium Zr 40 91.22

384 Appendixes

Web Sites

An important general site for information about the Earth Sciences is that of the US Geological Survey. It has sections covering all branches of the subject.

US Geological Survey www.usgs.gov

Other useful sites are given below.

Geology American Geophysical Union www.agu.org American Geological Institute (AGI) www.agiweb.org Earth Sciences on the Web http://geology.com The Geological Society of America www.geosociety.org International Union of Geological Sciences www.iugs.org

Minerals Athena Mineralogy http://un2sg4.unige.ch/athena/mineral/ mineral.html Mindat www.mindat.org Web Mineral http://webmineral.com

Oceanography NOAA Ocean Explorer http://oceanexplorer.noaa.gov Ocean Biogeographic Information System http://www.iobis.org United Nations Atlas of the Oceans www.oceansatlas.com

Meteorology, Climate, and the Atmosphere American Meteorological Society, Boston www.ametsoc.org/AMS Climate Prediction Center www.cpc.ncep.noaa Hadley Centre for climate prediction and www.met-office.gov.uk/research/ research, UK hadleycentre/index Mount Washington Observatory www.mountwashington.org NASA's Earth Observatory www.earthobservatory.nasa.gov National Oceanic and Atmospheric www.noaa.gov Administration (NOAA) University Corporation for Atmospheric www.edu/ucar/index Research US Environmental Protection Agency www.epa.gov World Meteorological Organization www.wmo.ch WW2010 (weather world 2010 project) www.2010.atmos.uiuc.edu/(Gh)/home University of Illinois

385 Appendixes

El Niño El Niño site www.pmel.noaa.gov/toga-tao/el-nino-story

Satellites European Space Agency (ESA) www.esa.int EUMETSAT www.eumetsat.de NASA www.nasa.gov NESDIS www.nesdis.noaa.gov

386 Bibliography

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