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lavas. Understanding volcanism provides a background for theories relating to mountain building, the development and evolution of continental and oceanic crt~st,and how the crust is deformed. Our observations of volcanic activity fit nicely into plate-tectonic theory as described in chapter 11. The May 18, 1980 eruption of Mount St. Helens (box 10.1) was a spectacular release of energy from the 's interior. The plate-tectonic explanation is that North America, moving weritwa~d,is overriding a portion of the Pacific Ocean Aoor. Melting of previously solid takes place at depth, just above the subduction zone. (Thii was described briefly in chapter 1 and more thoroughly in chapter 11.) At least some of the (molten rock or liquid that is mostly silica) works its way upward to the earth's surface to erupt. Magma does not always reach the earth's surface before solidifying, but when it does it is called . At Mount St. Helens the lava solidified quickly as it was blasted explosively by gases into the air, producing rock frag- ments known as pyrodasw (from the Greekpyro, "fire," and clart, "broken"). Pyroclastic debris is also known as tcphra. Elsewhere in the world (notably Hawaii) lava extrudes out of Flgulr 10.1 fissures in the ground as lava flows. Pyrodastic debris and Volcanic eruptions in Hawaii, 1969. A lava fountain is supply11 rock formed by solidification of lava are collectively regarded the lava cascading over the cliff. as emwive rock, surface rock resulting from volcanic Photo by D A Swanson, US Geolog~calSurvey acrivity. The most obvious landform created by volcanism is a vol- cano, a hill or mountain formed by the extrusion of lava or fertile soil. Moreover, Hawaii's periodically erupting volc ejection of rock fragments from a vent. However, volcanoes are (which are relatively safe to watch) are great spectacle not the only volcanic landforms. Very fluid lava may flow out attract both tourists and scientists, benefiting the is. of the earth and flood an area, solidifying into a nearly hori- economy (figure 10.1). zontal layer of . Successive layers of lava flows Were it not for volcanic activity, Hawaii would not may accumulate into a lava plateau. The islands are the crests of a series of volcanoes that have Volcanic acrivity is important to for several rea- built up from the bottom of the Pacific Ocean over millic sons. Landforms are created and portions of the earth's surface years (the vertical distance from the summit of Mauna Lo built up. Less commonly, as at Mount St. Helens, landforms can0 to the ocean floor greatly exceeds the height abm are destroyed by violent eruptions. Volcanoes are important to level of Mount Everest). When lava flows into the sc the science of geology because they provide clues about the solidifies, more land is added to the islands. Hawaii is, nature of the earth's inaccessible interior and help us under- literally, growing. stand how the earth's internal processes work. By studying the magma, gases, and rocks from eruptions, we can infer the chemical conditions as well as the temperatures and pressures Geothermal Energy within the earth's crust or underlying mantle. In some other areas of geologically recent volcanic acl underground heat generated by igneous activity is harn for human needs. In Italy, Mexico, New Zealand, Arger Effects on Humans Japan, and California, geothermal installations produce Volcanism is also significant in human affiirs. Its effects can be tric power. Steam or superheated water trapped in lay catastrophic or, surprisingly, beneficial. hot is tapped by drilling and then piped c the ground to power turbines that generate electricity. I The Growth of Hawaii rally heated geothermal fluids can also be tapped for spa domestic water heating or industrial use, as in paper man The overall effects of volcanism have been favorable to turing. humans in Hawaii, although occasionally a field or village is overrun by outpourings of lava. Kilauea volcano is especially active. During the 1980s and 1990s, over 1.5 billion cubic Effect on Climate meters of lava erupted--enough to build a highway that cir- Occasionally, a volcano will spew large amounts of cles the world four times. There were 181 houses destroyed in .volcanic dust and gas into the high atmosphere. U the 1980s and 1990s, but no one was killed or injured. Never- can keep fine particles suspended over the earth for 1 theless, the weathered volcanic ash and lava produce excellent The 1991 eruption of Mount Pinatubo in the Philip

Chapter I0 lrokanirm and Exmiw Rocks

Lake. Collapse of the volcano, as kc11 as ex, sions, accounts for the depression the present- lake occupies (figures 10.3 and 10.4). The southern Cascade Mountains, wt Crater Lake is located, have been built up eruptions over the past 30 to 40 million ye&( urc 10.5; sec also thc geologic map, inside cov Only the youngest peaks (those built within past 2 million years), such as Mount St. Helc Mount Rainier, Mount Shwta, and Mount He still stand out as cones. As we know from Mo St. Helens, any of these could again bccc active. Eruptive Violence and Physica characteristics of Lava What determines the degree of violence associa 8 - r- 10.3 with volcanic activity? Why can we state co Crater Lake, Oregon. Figure 10.4 showi Its geologlc hlstory. dendy that active volcanism in Hawaii poses c Photo by @ Greg VaughnTTom Stack & Assoolates slight danger to humans, but expect violent ex1

C D Flgum 10.4 The development of Crater Lake. (A) Cluster of overlapping volcanoes form. (8)Collapse into the partially emptied magma chamber is accompanied by violent eruptions. (C) Volcanic activity ceases, but steam explosions take place In the . (D)Water fills the calder to become Crater Lake, and minor renewed volcanism builds a (Wizard Island). After C Bacon U S Gsologlcal Survey "greater the dissolved ws content, the more fluid t the lava.) If the lava !&ing extruded is consider- ably hotter than its solidification temperature, the lava is less viscous (more fluid) than when its temperature is near its solidification point. Temperatures at which solidify range from about 700°C for silicic rocks to 1,200°C for rocks. Volcanic rocks, and the magma from which they formed, have a silica content that ranges from 45% to 75% by weight. Sdicic (or felsic) rocks are silica-rich (65% or more SiO,) rocks. is the most abundant silidc volcanic rock. M&c rocks are silica- @n'cnt rocks. Their silica content is close to 50%. Baralt is the most common mafic rock. Intermediate rocks have a chemical content between that of felsic and mafic rocks. The most common intermediate rock is andrrite. A more complete description of the chemistry of igneous rocks and their relation- ship to the mineral content of rocks is given in chapter 11. Mafic lavas, which are relatively low in SiO,, tend to flow easily. Conversely, felsic lavas are very viscous and flow slugg'ihly. Law rich in silica are more viscous because even before they have cooled enough to allow crystalliza- tion of minerals, silicon-oxygen tetrahedrons have begun to form small frameworks in the lava. Although too few atoms are involved for the structures to be considered , the total effect of these silicate structures is to make the liquid lava more viscous, much the way that flour or cornstarch thickens gravy. South Because silicic are the most vis- cous, they are associated with the most violent eruptions. Malic magmas are the least viscous Cascade volcanoes. See also the map in chapter 1 (figure 1.51, and commonly erupt as lava flows (such as in Hawaii). Eruption associated with intermediate magma can be violent or can produce lava to occur in the Cascade Mountains?Whether eruptions are flows. (The Cascade volcanoes are predominantly composed of explosive or relatively "quiet" is largely determined by two intermediate rock.) rs: (1) the amount of gas in the lava or magma and (2)the or difficulty with which the gas can escape to the atmo- re. The vkmiy, or resistance to flow, of a lava determines Extrusive Rocks and Gases easily the gas escapes. The more viscous the lava and the er the volume of gas trying to escape, the more violent the Scientific Investigation of Volcanism ion. Later we will show how these factors not only dcter- Volcanoes and lava flows, unlike many other geologic phenom- the degree of violence of an eruption but also influence the ena, can be observed dimly, and samples can be collected and height of a volcano. without great difficulty (at least for the quiet, Hawaiian-type he two most important factors that influence viscosity of eruption). We can measure the temperature of lava flows, the temperature of the lava relative to the cooler tem- collect samples of gases being given off, obse~ethe lava solidi- re at which it solidifies and (2) the silica (SiO,) content fying into rock, and take newly-formed rock samples into the e lava. (A third factor is gas dissolved in magma-the laboratory for analysis and study. By comparing rocks observed

Volcanirm and Extrusive Rocks Pyrdastlc flow descending Mayon volcano, Philippines, in 1984. Photo by ChrisNewhall, U.S.Geowical Survey Flgwl. 10.7 The ins of St. Pierre in 1902. Mount Pelbe is in the clouds. "lidifying from lava with Ones from other areas of the photo by Underwood& Underwwd,oourtesy Library of Congress world (and even with samples from the moon) where volcan- ism is no longer active, we can determine the nature of vol- canic activity that took place in the geologic past. Extrusive Ro,& Gases steam. Other gases, such as carbon dioxide, sulfur dioxide, hydrogen sulfide (which smells like rotten eggs), and amposi&,n hydrochloric acid, are given off in lesser amounts with the steam. The amount of silica in a lava largely conrrols not only

rocks are generally fine-grained, a specialized micros~ope Gases and Pyroclastics usually needed for precise identification of the compon During an eruption, expanding, hot gases may propel pyro- minerals. In most cases, however, we can guess the proba clastics high into the atmosphere as a column rising from a vol- mineral content by noting how dark or light in color cano. At high altitudes, the pyrodastics ofren spread out into a extrusive rock is. Most silicic rocks are light-colored beca dark, mushroom doud. The fine particles are transported they contain abundant and quartz (both of which are downwind by high atmosphere winds. Eventually debris settles silica-rich) and few dark minerals (which contain iron back to earth under gravity's influence as ahfall (or sometimes magnesium and are silica-deficient). Mafic rocks, on pumicefili) deposits. other hand, tend to be dark because of the abundance of A ppdastic flow is a mixture of gas and pyrodastic romagnesian minerals. debris that is so dense that it hugs the ground as it flows Rhyolite, a silicic rock, is usually cream-colored, tan, or rapidly into low areas (figure 10.6). These turbulent masses ' pink; it is made up mostly of feldspar but always indudes some can travel over 100 kilometers per hour and are extremely dan- quam. Note that the rhyolite (and ) portion of figure 1 1.4 gerous. In 1991, a pyrodastic flow at Japan's Mount Unzen is larger than the areas shown for anhite and . Geologists killed 31 people, including 3 geologists. Far worse was the commonly subdivide this portion of~~cationsystem. For destruction of St. Pierre (figure 10.7) on the Caribbean island example, rtm'tc, the mck associated with the 1980 St. Helens of Martinique where about 28,000 people were killed by a eruptions, contains more ferromagnesian minerals and plagio- pyroclastic flow in 1902 (see box 10.3). clase but less potassium feldspar and quartz than the average rhy-

Chapter I0 iblite. In our classification system, corresponds to the right Two critical factors determine grain size during the solidi- rtion of the area in figure 11.4 assigned to rhyolite. fication of igneous rocks: rate of cooling and viscosity. If lava A basalt has a relatively low amount (about 50% by weight) cools rapidly, the atoms have time to move only a short dis- iO,. Much of that silica is bonded to iron and magnesium to tance; they bond with nearby atoms, forming only small crys- ferromagnesian minerals, such as olivinc orpymxene, which tals. With extremely rapid or almost instantaneous cooling, een or black The remaining silica plus aluminum is bonded individual atoms in the lava are "frozen" in place, forming glass ominantly with calcium to form caldum-rich phgioclarc rather than crystals. r (which tends to be darker gray than the white or pink Grain size is controlled to a lesser extent by the viscosity of um or sodium associated with felsic rocks). Basalt the lava. The atoms in a very viscous lava cannot move as freely

not~ contain~~ auartz.~ because no silica is left over after the r minerals have formed. Because of the preponderance of minerals in basalt, this rock is usually dark gray to black , which crystallizes from an intermediate lava, be recognized by its moderately gray or green color. It is color because a little over half the rock is light- to ium-gray plagioclase feldspar, while the rest is ferromag-

efers to a rock's appearance with respect to the size, d arrangement of its grains or other constituents. ome extrusive rocks (such as and ) are solely on the basis of their textures, but most are clas- by composition and texture. Grain size is a rock's most tant textural characteristic. For the most part, extrusive are fine-grained or else made of glass. fine-grained rock is one in which most of the mineral s are smaller than 1 millimeter. In some, the individual s are distinguishable only with a microscope. Obsidiaa 10.8), which is volcanic glass, is one of the few rocks Figure 10.8 IS not composed of minerals. A fine-grained or glassy tex- Obsidian. distinguishes extrusive rocks from most intrusive rocks. Photo by C. C. plum me^

Whanism and Extrusive Rocks A B Flgun, 10.9 andesite. A few large crystals ()are surrounded by a great number of fine grains. (A)Hand specimen. (8)Photomicrograph (using polarized light) of the same rock. The black and white striped phenocrysts are plagioclase and the green one are ferromagnesian minerals.

Photos by C. C. Plurnrner "- as those in a very fluid lava. Hence, a rock formed from viscous lava is more likely to be obsidian or of finer grains than one formed from more fluid lava. Most obsidian, when chemically analyzed, has a very high silica content and is silicic, and the chemical equivalent of rhyolite. Porphyritic Zxtures Extrusive rock that does not have a uniformly fine-grained tex- ture throughout is described as porphyritic. A porphyitic mck is ax a lunr mound- ''- one in which 1-1 ",cmds endosed in (or~ 0 ':I mars) of much finer-grained minerals or obsidian. The larger crystals are termed phenocryst9. A porphyritic rock looks rather like raisin bread; the or groundmass is the bread, the phe nouysts are the raisins. In the porphyritic andesite shown in fig- ure 10.9, phenocrysts of feldspar and ferromagnesian minerals are endosed in a matrix of uystals too fine-grained to distinguish with the naked eye but visible under a microscope. I. Porphyritic texture usually indicates two stages of solidifica- tion. Slow cooling takes place while the magma is underground. I Minerals that form at higher temperatures crystallize and grow to form phenocrysts in the still partly-fluid magma. If the entire mass is then erupted, the remaining liquid portion cools rapidly Flsum at the earth's surface and forms the fine-grained matrix. Vesicular basalt. Photo by C C Plurnrner Textures Due to Trapped Gas A magma deep underground is under high pressure, generally puhave a piece of ice with small, bubble-shaped holes. Similarly, high enough to keep all its gases in a dissolved state. On eruption, when a lava solidifies while gas is bubbling through it, holes are the pressure is suddenly released and the gases come out of solu- trapped in the rock, creating a distinctive uesicukzr texture. Vcsi- tion. Thii is analogous to what happens when a bottle of beer or des are cavities in extrusive mck resulting from gas bubbles that soda is opened. Because the drink was bottled under pressure, the were in lava A vesicular rock has the appearance of Swiss cheese gas (carbon dioxide) is in solution. Uncapping the drink relieves (whose texture is caused by trapped carbon dioxide gas). Vesidr the pressure, and the carbon dioxide separates from the liquid as bmlt is quite common (figure 10.10). , a highly vesicular gas bubbles. If you freae the newly opened drink very quickly, bdt,actually contains more gas space than rock.

252 Chapter 10 hnp://wmmhhc.cod~a~hr~dplo~/pfummcr Volcanic bombs. Photo by^. C.Plummer

viscous hvas, x.herc.ther Totescape as easily, -ed into a &&(like e head of a glass of beer). led quidds it forms pum& (figure 10.1 1). a frothy th so much void that it floats in water. Powderad is used as an abrasive because it can scratch metal or glass. a, the fragments formed by volcanic explosion, can be PISU~io.ia Photomicrograph of a Fragments of difterent rocks and mkw& are angular and variously colored. PhMo by C C Plummer * -- , vent at the summit of the cone (figure 10.14). Material is not always ejected from thc central vent. In a flank empdat, lava pours from a vent on the side of a volcano. A aMar is a vokanic depression muh &ger than hedgi- nal crater, having a &mew of at kast one Momem. Wemost famous &era in the United Stars is mhwmcd "Craw Lala.") A~cankuPrccdwhenam1cdnd2s~tm~litisblwraoffby explobg gam, as occurred at Mount St.&dem in May 1980, or, as in the cay ofChwL&e, when a volcano (or sevenl volcanoes) wIkpinto a vac~tedmagma chamber (see figure 10.4). The three major types of volcanoes (shield, cinder cone, and composite) discussed Mow are markedly distinct from one another in size, shape, and, usually, composition. Although volcanic domes are not cones, they are associated with volcanoes and are also examined in this section.

Volcanism and hive&ch 253 1 pmddaseriesoflayws~. Hawaiii name k hi piven m two distinctive of basalt flm. Arhonhoc @nono;;acedpab+-boy) is di in#i by a mpy or billowy & Efigurc 10.16). The st I bmdby the quick cvohg and solidification from the hwudof a lava Aaw ~r-~lof lava that was fully lic wnaett,basalt&atiscoolcnoughmhavepardly~( I as a slow. .vasw , mas. Its Mvmli front K fwwnrd as a pile of rubblo. A h;& as this is called r 1 -adM).ndhrasp~~s&(figurelO I A minor feature dl&r rartta cone, aCsm&, me a~lcbuilt from ha qnrfFpd;lg out of a vent (figure 1 *I ocasioa& dndoo-on a solidifnng lava flow, ' d codonef.@ is trapped in-a cooling lay Flgun 10.14 tam is bdchcd out of a vent thmugh the solidified su Crater and caldera in Kamchatka, Russia.- In the foreground is the rhe Falling lava pi- itself onto the dcvelopir crater on Karymsky volcano. In the background is a lake-filled ad aoklifies. The sides ofe6patter cone can be very su caldera. they are dyover 10 meters high. Photo by C Dan Miller, U S,Geological Survey Cinder Cones A cinda mne (less commonly called a py~c~cow) I ~Vdc8floos cano constructed of ppdasts ejected from a central vent SLkkl-prr bd,pdy sbp. ' cons ',2nmucnd of 10.19).In contrast to the gentle slopes of shield volcanoes solislificd~uva During uuptkw, tke''La spreads widely and cones commonly have slopes of about 30". Most of the ,&m"icshdscrjsity. Because rhe lava flows hmacenual material lands near the vent during an eruption, building up much OW davent, the slope am usu- cone to a peak. The steepness of slopes of mulatin *and i . km.& h&&+3, producing a volcano material is limited by gravity to about 33". Cinder cones in&:&apeda&ddgmeorusWc(figure 10.15). ' be very much smaller than shield volcanoes. In fact, cin& The'islkidi &Hawaii arc essentially a series of shield vol- are commonly found on the flanks and in the of I canoes built upward from the ~ceanfloorby intermittent shield volcanoes. Few cinder cones exceed a he& of 500 eruptions over millions of years (figu~10.15B). Although Cinder cones are a product of concentration of spectacular to observe, the eruptions are relatively nonviolent magma rather than of a particular chemical cornposi

New lava

B Figun lala (A)Cutaway vlew of a . (8)The top of Mauna Loa, ash volcano in Hewail, and its summit caldwa. The smaller depressions a1 craters. In the distance is Mauna Kea, another shisld volcano that lar erupted about 3,000 yearn ago. Photo by D W Peterson, US Geolog~calSurvey

Chapter 10 Figure 10.16 Pahoehoe from a 1972 eruption in Hawaii. Photo by D. W. Peterson, U.S. Geological Survey

Figure 10.1 9 Cerro Negro, a cinder cone in Nicaragua. (A)View from the air; .I IV..Y (6)nightthe eruption of pyroclasticsat the summit. tter cone (approximately 1 meter high) erupting in Hawaii. photo A by Mark Hurd Aerial Surveys Corp courtesy California Divislon of y J. 6.Judd, U.S. Geological Survey Mines and Gregory: Photo B by R. W. Decker

Volcanism and Ewmsive Rocks 255 magma. Most cinder cones are associated with mafic or inter- The extrusive material that builds composite cones is mediate lava. Silicic cinder cones, which are made of fragments dominantly of intermediate composition, although there of pumice, are also known as pumice cones. be local minor silicic and dceruptions. Therefore, a The life span of an active cinder cone tends to be short. is the rodc most associated with composite volcanoes. I The local concentration of gas is depleted rather quiddy dur- temperature of an andesite lava is considerably above the ing the eruptive periods. Moreover, as landforms, cinder cones penture at which it would be completely solidified, the are temporary features in terms of geologic time. The uncon- tively nonvismus fluid flows easily from the crater down solidated pyrodasts are eroded relatively easily. slopes. On the other hand, if enough gas pressure exists explosion may litter the slopes with pyroclastic andesite, ticulady if the lava has fully or partially solidified and cl Composite Volcanoes the volcano's vent. A composite volcano (also called ) is one con- The composition as well as eruptive history of indivi structed of alternating layers of pyrodastiw and rock solidified volcanoes can vary considerably For instance, Mount Rain from lava flows (figure 10.2W).The slopes are intermediate in is composed of 90% lava flows and only 10% pyroclas steepness compared with cinder cones and shield volcanoes. ers. On the other hand, Mount St. Helens was built Pyroclastic layers build steep slopes as debris collects near the vent, just as in cinder cones. However, subsequent lava flows partially flatten the profile of the cone as the downward flow builds up the height of the flanks more than the summit axa. The solidified lava acts as a protective cover over the loose pyroclastic layers, making composite volcanoes less vulnerable to erosion than cinder cones. Composite volcanoes are built over long spans of time. Eruption is intermittent, with hundreds or thousands of years of inactivity separating a few years of intense activity. During the quiet intervals between eruptions, composite volcanoes may be eroded by running water, landslides, or glaciers. These surficial processes tend to alter the surface, shape, and form of the cone. But because of their long lives and relative resistance to erosion, composite cones can become very large. Aconcagua, a composite volcano in the Andes, is 6,960 meters (22,835 feet) above sea level and the highest peak in the west- ern hemisphere.

Crater

New lava flow

Figum 10.20 (A)Cutaway view of a composite volcano. Llght-colored layers are pyroclastics. (6)Mount Shasta, a composite volcano in California. Shastina on Mount Shasta's flanks is a subsidiary cone, largely made of pyroclastics. Note the lava flow that originated on Shasta and extends beyond the volcano's base. Photo by B Arnundson

256 Chapter 10 igum 10.21 1"- a' the world showing the major volcanic belts,

m pyroclastic eruptions-reflecting a more violent history. would be expected, the composition of the rocks formed urinp. the 1980 eruptions of Mount St. Helens is somewhat fieiin silica than average for Cascade volcanoes.

listribution of Com~ositeElcanoes

ine of Fire," is t6e larger. The Cascade Range volcanoa

il America, western South America (including NW*

Mount Erebus, Antarctica, the southernmost active volcano Inthe world. , ,

kyon, whose 1993 eruptions killed ov& 30 people, and malp her volcanoes), and Japan. The beautifully symmnrid

Vokanimr and Exmivr Rock Fujiyama, in Japan, is probably the most frequently painted volcano in the world (figure 10.23). The northernmost part of the circum-Pacific belt includes active volcanoes in Russia (see figure 10.14) and on Alaska's Aleutian Islands. The second major volcanic belt is the Mediterranean belt, which includes Mount Vesuvius. An exceptionally vio- lent eruption of Mount Thera, an island in the Mediter- ranean, may have destroyed an important site of early Greek civilization. (Some archaeologists consider Thera the origi- nal "lost continent" of Atlantis.) Mount Etna, on the island of Sicily, has been called the "lighthouse of the Mediter- ranean," because of its frequent eruptions throughout the centuries. The largest eruption in 300 years began in 1991 and lasted for 473 days. Some 250 million cubic meters of lava covered 7 km2 of land. A town was saved from the lava by heroic efforts that included building a dam to retain the lava (the lava quickly overtopped it), plugging some Figure 10.23 natural channels, and diverting the lava into other, newly Mount Fuji, woodblock print by Japanese artist Hiroshige constructed channels. (1797-1858).

258 Chapter 10

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260 Chapter I0 rhc thick, ppl~lava hat squeezes from a vent is to flow, it buildp up a ste~psidcddome or spine 5). Some volcanic darner act cham- carkr, ~YKIh mping. If the plug is removed or bmk uupas rudduJy PA^ viokndy. Some of thc met laic cxplooiona known hws been M

,.. . .

,, > ..,, . . . '. Volcanic dome ~uadwith.~.I*v*.rbsr us and flow dmw.ar cas$.)r.m$v,h~ ammd irs vents. Such haLh;d,m& dsdat a ~~mdt;.~~~ to which it wlidif#h., wid. yiir;an;r :;:; ! hwJo.wcrc pzoducr~d~duriryp.~~ ouringe of Lv&Th* .G&&kdkj#UiJla#d rwki-Sam A voloanle dorna forming In the urtrr~l~~~-

Vokunh and Exmuiw Rocks Figure 10.26 Basalt layers in the Columbia plateau, Washington Photo by P Weis, U.S. Geological Survey

Washington, Idaho, and Oregon (see figure 1.5), for exam- basalt contracts as it cools. The layer of basalt is easily able to ple, is constructed of layer upon layer of basalt (figure accommodate the shrinkage in the narrow vertical dimension; 10.26), in places as thick as 3,000 meters. Each individual but the cooling rock cannot "pull in" its edges, which may be flood of lava added a layer usually between 15 and 100 many kilometers away. The tension fractures the rock into an meters thick and sometimes thousands of square kilometers orderly hexagonal pattern. in extent. Basalt layers give the landscape a striking appearance in most places where they are exposed. Instead of stacked-up slabs Submarine Eruptions- 91 or tablets of solid, unbroken rock, the individual layers appear Submarine eruptions, notably those occurring along mid- to be formed of parallel, vertical columns, mostly six-sided. oceanic ridges, almost always consist of mafic lavas that create This characteristic of basalt is called columnar structure or basalt. As described in chapter 11, basaltic rock, thought to columnar jointing (figure 10.27). The columns can be have been formed from lava erupting along mid-oceanic explained by the way in which basalt contracts as it cools ajer ' ridges, or solidifying underground beneath the ridges, makes solidifying. Basalt solidifies completely at temperatures below up virtually the entire crust underlying the oceans. In a few about 1,200°C. The hot layer of rock then continues to cool to places--Iceland, for example--volcanic islands rise above the temperatures normal for the earth's surface. Like most solids, otherwise submerged system (see box 10.4).

Chapter I0 B

jointing at Devil's Postpile, California. (A)The columns as seen from above. (Scratches were caused by glacial erosion as in chapter 19.) (B)Side view.

Figure 10.29 Pillow basalt on a mid-oceanic ridge. Photo taken from a submersible vessel. Courtesy of Woods Hole Oceanographic institution

breaks out. Each new pillow settles down on the pile, with little space left in between. Newly formed pillow basalt (fig- ure 10.29) along the crests of mid-oceanic ridges (regarded as divergent boundaries) is important evidence supporting plate-tectonic theory. When pillow are found exposed in mountain ranges in various parts of the world, ueezed out like toothpaste, and its surface is chilled to they are usually indications of submarine eruption in the within seconds. A new blob forms as more lava inside geologic past followed, much later, by uplifi.

E&mrimr strdEunuriw Rock

Lava is molten rodt that reaches the eartb's minerals are mostly potassium- and sodium- falls back around the crater. Cinder cones are surhce, having been formed as magma from rich feldspars and quartz. not as large as the other two major types of rock within the eartb's crust or from the A lava with a composition between cones. uppermost part of the mantle. mafic and silicic crystallizes to andaite, a A shield volcano is built up by successive Lava contains 45% to 75% silica (SiO,). moderately dark rock. Andesite contains eruptions of mafic lava. Its slopes are gentle The more silica, the more viscous the lava. about equal amounts of ferromagnesian but its volume generally large. Viscosity is also determined by the tempera- minerals and sodium- and calcium-rich Composite cones are made of alternating ture of the lava. Viscous lavas are associated feldspars. layers of pyroclastic material and solidified with more violent eruptions than are fluid Extrusive rocks are characteristically lava flows. They are not as steep as cinder lavas. Volcanic domes form from the extru- fine-grained. Porphyritic rock contains some cones but steeper than shield volcanoes. tion of very viscous lavas. larger crystals in an otherwise fine-grained Young composite volcanoes, predominantly A mafc lava, relatively low in silica, uys- rock. Rocks that solidified too rapidly for composed of andesite, are aligned along the dliinto basalt, the most abundant extru- crystals to develop form a natural glass called circum-Pacific belt and, less extensively, in rive . Basalt, which is dark in obsidian. Gas trapped in rock forms venkh. the Mediterranean belt. color, is composed of minerals that are rela- Pyroclarts are the result of volcanic Plateau basalts are thick sequences of tively high in iron, magnesium, and calcium. explosions. Tuffis volcanic ash that has con- lava floods. Columnar jointing develops in Rhyolite, a light-colored rock, forms solidated into a rock. If large pyroclastic frag- solidified basalt flows. Basalt that erupts from silicic law that are high in silica but ments have reconsolidated, the rock is a underwater forms a pilbw structure. Pillow contain little iron, magnesium, or calcium. volcanic bwccia. basalts are common along the crests of mid- use potassium and sodium are impor- A cinder cone is composed of loose pyro- oceanic ridges. elements in rhyolite, its constituent clastic material that forms steep slopes as it

lava 244 shield volcano 254 lava flows 244 silicic (felsic) 249 mafic rock 249 spatter cone 254 magma 244 texture 25 1 Mediterranean belt 258 tuff 253 obsidian 251 vent 253 m-Pacific belt 257 252 vesicle 252 umnar structure (columnar jointing) 262 pillow structure (pillow basalts) 263 viscosity 249 ite volcano (stratovolcano) 256 plateau basalts 261 volcanic breccia 253 porphyritic rock 252 volcanic dome 259 pumice 253 volcanism 244 pyroclastic flow 250 volcano 244 pyroclasts 244 rhyolite 250

Testhile Your Wiowledee

4. Name the minerals, and the approximate percentage of each, that you would expect to be present in each of the following rocks: andeaitc, rhyolite, basalt. teristic) of obsidian makes it an loeic definition of rock? I . . I ...... m 6. What determines the viscosity of a lava? I >. One p rypidly not teleased during a volcanic e.mption is 7. What determines whether a series of volcanic eruptions builds a (a) water vapor (b) carbon dioxide (c) sulfur dioxide shield volcano, a wmposite volcano, ot a cinder cone? Describe (d) hydrogen sulfide (e) oxygen each type of volcanic wne. 16. M&c rocks contain about Ohsilica. (a) 10 (b) 25 (c) 50 8. Explain how a vesicutar porphyritic andesite might have (d) 65 (e) 80 17. Silicic rocks wntain about -% silica. (a) 10 (b) 25 (c) 50 9. Why are extrusive igneous rocks fine-grained? (d) 70 (e) 80 10. Why don't flood bas& build volcanic cones? 18. Which is not an extrusive igneous rock? (a) granite (b) rhyoli 11. Mount St. Helens (a) last erupted violently in 1980 (b) is part (c) basalt (d) andesite of the Cascade Range (c) is located in southern Washington 19. Which is not a major type of volcano? (a) shield (b) cinder w (d) all of the above (c) composite (d) stramvolcano (e) spatter cone 12. Volcanic eruptions can affect the dimatc because(a) they heat 20. A typical example of a shield volcano is (a) Mount St. Helens (b) Kilauea in Hawaii (c) El Chich6n (d) Mount Vesuvius 21. An example of a composite volcano is (a) Mount St. Helens (b) El Chich6n (c) Mount Vesuvius (d) all of the above 13. Whether volcanic eruptions iuc very explosive or relative6 quiet 22. Which volcano is not made of basalt? (a) shield is largely determined by (a)the mount of gas in the lava or (b) composite cone (c) spatter cone (d) cinder cone

14. Temperatures at whichlavas solidify range from about C

1. What might explain the remarkable eruption such as that which created 4. Why arc continental igneous rocka alignment of the Cascade volcanoes? Crater Lake? richer in silica than ocanic igneous 2. What would the present-day 3. Why are there no active volcanoes in rocks? environmental effects be for an the eastern oarts of the United States and canad;;?

-- Bullard, F. M. 1984. Volranoes ofthe Harris, S. L. 1989. Fire mountains ofthe Eruption atsea. VHS. Ka'io earth. 2d ed. Austin: University of west: The Carcade and Mono Lake volcanoes, Productions; PO. Box 476, Volcano, Hawaii Texas Press. Missoula, Montana: Mountain Press. 96785. Decker, R. W., and B. B. Decker. 1998. Krafi, M. 1993. Volcanoes: Firefmm the Eruptzvephenomrna ofKihura? East Rift Volcanoes, with CD-ROM. 3d ed. New York: earth. New York: Harry N. Abrams. Zone. VHS. Ka'io Productions; PO. Box W H. Freeman. (Maurice Kr& and his wife Katia were the 476, Volcano, Hawaii 96785. Decker, R. W., and B. B. Decker. 1991. world's foremost photographers of volcanoes In thepath ofa killer vokano. VHS. Films for Mountains offir: The nature of volzanoes. before they were killed during the June 199 1 the Humanities & Sciences, Princeton, NJ. New York: Cambridge University Press. eruption of Unzen Volcano, Japan.) First-rate video on Mt. Pinambo produced Fisher, R. V, G. Heiken, and J. B. Hulen. Simpkin, T., and L. Siebert. 1994. Volcanoex for the NOVA television series. 1997. Volcanoes: Crucibles of change. of the world. 2d ed. Tucsdn, Arizona: Riven ofjre. VHS. Hawaii Natural History Princeton, N.J.: Princeton University Press. Geoscience Press. Association; PO. Box 74, Hawaii Volcanoes Francis, P. 1993. Volcanoes:Apkznetary Wright, T. L., and T. C. Pierson. 1992. National Park, Hawaii 96718. penpective. New York: Oxford University Living with volmnoes. U.S. Geological Press. Survey Circular 1073. rh.edu/-volcano/ learn about volcanoes. At the home page may click on "Volcano of the Week" or Evplore Kihuea Vokano. Fire Work Studios, lcano World Starting Points." "Starting p:llwww.geo.~.edu/voIcanoes/ nts" presents a menu that includes Volcanoes: LZJ$ on the Edge. Corbis, PO.Box 2284, South Burlington, VT 05407. Phone mitigation. Clicking on "volcanic humor" 800-246-2065. will show the lightciride of .

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