tive determines the type of boundary; convergent, divergent, or transform. , volcanic activity, -building, and oceanic formation occur along these plate boundaries. The lateral relative move- ment of the plates typically varies from zero to 100 mm annually.[2] Tectonic plates are composed of oceanic and thicker continental lithosphere, each topped by its own kind of . Along convergent boundaries, carries plates into the ; the material lost is roughly balanced by the formation of new (oceanic) crust along divergent margins by seafloor spreading. In this way, the total surface of the globe remains the same. This predic- The tectonic plates of the were mapped in the second half of the 20th century. tion of is also referred to as the conveyor belt principle. Earlier (that still have some sup- porters) propose gradual shrinking (contraction) or grad- ual expansion of the globe.[3] Tectonic plates are able to move because the ’s lithosphere has greater strength than the underlying . Lateral variations in the mantle result in . Plate movement is thought to be driven by a combination of the motion of the seafloor away from the spreading ridge (due to variations in topog- raphy and density of the crust, which result in differences in gravitational forces) and drag, with downward suction, at the subduction zones. Another explanation lies in the different forces generated by the rotation of the globe and the tidal forces of the and . The relative im- portance of each of these factors and their relationship to each other is unclear, and still the subject of much debate. Remnants of the , deep in Earth’s mantle. It is thought that much of the plate initially went under (particularly the western and southwest ) at a very shallow angle, creating much of the mountainous 1 Key principles in the area (particularly the southern Rocky ). The outer layers of the Earth are divided into the Plate tectonics (from the Late Latin tectonicus, from [1] lithosphere and asthenosphere. This is based on differ- the Greek: τεκτονικός “pertaining to building”) is a ences in mechanical properties and in the method for the scientific that describes the large-scale motion of transfer of heat. Mechanically, the lithosphere is cooler Earth's lithosphere. This theoretical model builds on and more rigid, while the asthenosphere is hotter and the concept of which was developed flows more easily. In terms of , the litho- during the first few decades of the 20th century. The sphere loses heat by conduction, whereas the astheno- geoscientific community accepted the theory after the sphere also transfers heat by convection and has a nearly concepts of seafloor spreading were later developed in the adiabatic gradient. This division should not late 1950s and early 1960s. be confused with the chemical subdivision of these same The lithosphere, which is the rigid outermost shell of a layers into the mantle (comprising both the asthenosphere (on Earth, the crust and ), is bro- and the mantle portion of the lithosphere) and the crust: ken up into tectonic plates. On Earth, there are seven or a given piece of mantle may be part of the lithosphere eight major plates (depending on how they are defined) or the asthenosphere at different depending on its and many minor plates. Where plates meet, their rela- temperature and pressure.


The key principle of plate tectonics is that the litho- denser because it has less and more heavier ele- sphere exists as separate and distinct tectonic plates, which ments ("mafic") than ("").[9] As a ride on the fluid-like (visco-elastic solid) asthenosphere. result of this density stratification, gener- Plate range up to a typical 10–40 mm/ (Mid- ally lies below level (for example most of the Pacific Atlantic Ridge; about as fast as fingernails grow), to about Plate), while continental crust buoyantly projects above 160 mm/year (; about as fast as grows).[4] (see the page for explanation of this The driving mechanism behind this movement is de- principle). scribed below. Tectonic lithosphere plates consist of lithospheric mantle overlain by either or both of two types of crustal mate- 2 Types of plate boundaries rial: oceanic crust (in older texts called from silicon and ) and continental crust ( from silicon Main article: List of tectonic plate interactions and ). Average oceanic lithosphere is typically 100 km (62 mi) thick;[5] its thickness is a function of its Three types of plate boundaries exist,[10] with a fourth, age: as passes, it conductively cools and subjacent mixed type, characterized by the way the plates move rel- cooling mantle is added to its base. Because it is formed ative to each other. They are associated with different at mid- ridges and spreads outwards, its thickness types of surface phenomena. The different types of plate is therefore a function of its distance from the mid-ocean boundaries are:[11][12] ridge where it was formed. For a typical distance that oceanic lithosphere must travel before being subducted, the thickness varies from about 6 km (4 mi) thick at mid- 1. Transform boundaries (Conservative) occur where ocean ridges to greater than 100 km (62 mi) at subduction two lithospheric plates slide, or perhaps more accu- zones; for shorter or longer distances, the subduction zone rately, grind past each other along transform faults, (and therefore also the mean) thickness becomes smaller where plates are neither created nor destroyed. The or larger, respectively.[6] Continental lithosphere is typi- relative motion of the two plates is either sinistral cally ~200 km thick, though this varies considerably be- (left side toward the observer) or dextral (right side tween basins, mountain ranges, and stable cratonic inte- toward the observer). Transform faults occur across riors of . The two types of crust also differ a spreading center. Strong earthquakes can occur in thickness, with continental crust being considerably along a . The in thicker than oceanic (35 km vs. 6 km).[7] is an example of a transform boundary exhibiting dextral motion. The location where two plates meet is called a plate boundary. Plate boundaries are commonly associ- 2. Divergent boundaries (Constructive) occur where ated with geological events such as earthquakes and two plates slide apart from each other. At zones of the creation of topographic features such as mountains, ocean-to-ocean rifting, divergent boundaries form volcanoes, mid-ocean ridges, and oceanic . The by seafloor spreading, allowing for the formation of majority of the world’s active volcanoes occur along plate new ocean basin. As the splits, the ridge boundaries, with the Pacific Plate’s Ring of being the forms at the spreading center, the ocean basin ex- most active and widely known today. These boundaries pands, and finally, the plate area increases causing are discussed in further detail below. Some volcanoes many small volcanoes and/or shallow earthquakes. occur in the interiors of plates, and these have been var- At zones of continent-to-continent rifting, divergent iously attributed to internal plate deformation[8] and to boundaries may cause new ocean basin to form as mantle plumes. the continent splits, spreads, the central col- As explained above, tectonic plates may include conti- lapses, and ocean fills the basin. Active zones nental crust or oceanic crust, and most plates contain of Mid-ocean ridges (e.g., Mid-Atlantic Ridge and both. For example, the includes the con- East Pacific Rise), and continent-to-continent rifting tinent and parts of the floor of the Atlantic and Indian (such as ’s and , Red . The distinction between oceanic crust and con- Sea) are examples of divergent boundaries. tinental crust is based on their modes of formation. 3. Convergent boundaries (Destructive) (or active mar- Oceanic crust is formed at sea-floor spreading centers, gins) occur where two plates slide toward each other and continental crust is formed through arc and to form either a subduction zone (one plate mov- of through tectonic processes, though ing underneath the other) or a continental colli- some of these terranes may contain sequences, sion. At zones of ocean-to-continent subduction which are pieces of oceanic crust considered to be part of (e.g., Western , and Cascade Moun- the continent when they exit the standard cycle of forma- tains in Western United States), the dense oceanic tion and spreading centers and subduction beneath conti- lithosphere plunges beneath the less dense continent. nents. Oceanic crust is also denser than continental crust Earthquakes then trace the path of the downward- owing to their different compositions. Oceanic crust is moving plate as it descends into asthenosphere, a 3.1 Driving forces related to mantle dynamics 3

trench forms, and as the subducted plate partially melts, rises to form continental volcanoes. At zones of ocean-to-ocean subduction (e.g., the in South America, Aleutian , , and the Japanese arc), older, cooler, denser crust slips beneath less dense crust. This causes earthquakes and a deep trench to form in an arc shape. The upper mantle of the subducted plate then heats and magma rises to form curving chains of volcanic islands. Deep marine trenches are typically associated with sub- duction zones, and the basins that develop along the active boundary are often called “foreland basins”. The subducting contains many hydrous min- Plate motion based on Global Positioning System (GPS) satellite erals which release their on heating. This data from NASA JPL. The vectors show direction and magnitude water then causes the mantle to melt, producing of motion. volcanism. Closure of ocean basins can occur at continent-to-continent boundaries (e.g., and ): collision between masses of granitic con- debate, asserts that as a consequence, a powerful source tinental lithosphere; neither mass is subducted; plate of plate motion is generated due to the excess density edges are compressed, folded, uplifted. of the oceanic lithosphere sinking in subduction zones. When the new crust forms at mid-ocean ridges, this 4. Plate boundary zones occur where the effects of the oceanic lithosphere is initially less dense than the under- interactions are unclear, and the boundaries, usually lying asthenosphere, but it becomes denser with age as it occurring along a broad belt, are not defined and conductively cools and thickens. The greater density of may show various types of movements in different old lithosphere relative to the underlying asthenosphere episodes. allows it to sink into the deep mantle at subduction zones, providing most of the driving force for plate movement. The weakness of the asthenosphere allows the tectonic plates to move easily towards a subduction zone.[13] Al- though subduction is believed to be the strongest force driving plate motions, it cannot be the only force since there are plates such as the which are moving, yet are nowhere being subducted. The same is true for the enormous . The sources of plate motion are a of intensive research and dis- cussion among . One of the main points is that the kinematic pattern of the movement itself should be separated clearly from the possible geodynamic mecha- Three types of plate boundary. nism that is invoked as the driving force of the observed movement, as some patterns may be explained by more than one mechanism.[14] In short, the driving forces advo- cated at the moment can be divided into three categories 3 Driving forces of plate motion based on the relationship to the movement: mantle dy- namics related, related (mostly secondary forces), Plate tectonics is basically a kinematic phenomenon. Sci- and Earth rotation related. entists agree on the observation and deduction that the plates have moved with respect to one another but con- tinue to debate as to how and when. A major question 3.1 Driving forces related to mantle dy- remains as to what geodynamic mechanism motors plate namics movement. Here, diverges in different theories. It is generally accepted that tectonic plates are able to Main article: move because of the relative density of oceanic litho- sphere and the relative weakness of the asthenosphere. For much of the last quarter century, the leading theory of Dissipation of heat from the mantle is acknowledged to the driving force behind tectonic plate motions envisaged be the original source of the required to drive plate large scale convection currents in the upper mantle which tectonics through convection or large scale and are transmitted through the asthenosphere. This theory doming. The current view, though still a matter of some was launched by and some forerunners 4 3 DRIVING FORCES OF PLATE MOTION

in the 1930s[15] and was immediately recognized as the nor large plumes but rather as a series of channels just solution for the acceptance of the theory as originally below the Earth’s crust which then provide basal discussed in the papers of Alfred in the early to the lithosphere. This theory is called “surge tectonics” of the century. However, despite its acceptance, it and became quite popular in and was long debated in the scientific community because the during the 1980s and 1990s.[17] leading (“fixist”) theory still envisaged a static Earth with- out moving continents up until the major break–throughs of the early sixties. 3.2 Driving forces related to gravity Two– and three–dimensional imaging of Earth’s inte- Forces related to gravity are usually invoked as secondary rior () shows a varying lateral den- phenomena within the framework of a more general driv- sity distribution throughout the mantle. Such density ing mechanism such as the various forms of mantle dy- variations can be material (from ), min- namics described above. eral (from variations in structures), or thermal (through thermal expansion and contraction from heat en- Gravitational sliding away from a spreading ridge: Ac- ergy). The manifestation of this varying lateral density is cording to many authors, plate motion is driven by the mantle convection from buoyancy forces.[16] higher elevation of plates at ocean ridges.[18] As oceanic lithosphere is formed at spreading ridges from hot man- How mantle convection directly and indirectly relates to tle material, it gradually cools and thickens with age (and plate motion is a matter of ongoing study and discus- thus adds distance from the ridge). Cool oceanic litho- sion in geodynamics. Somehow, this energy must be sphere is significantly denser than the hot mantle material transferred to the lithosphere for tectonic plates to move. from which it is derived and so with increasing thickness There are essentially two types of forces that are thought it gradually subsides into the mantle to compensate the to influence plate motion: friction and gravity. greater load. The result is a slight lateral incline with in- creased distance from the ridge axis. • Basal drag (friction): Plate motion driven by fric- tion between the convection currents in the astheno- This force is regarded as a secondary force and is often sphere and the more rigid overlying lithosphere. referred to as "". This is a misnomer as noth- ing is “pushing” horizontally and tensional features are • (gravity): Plate motion driven by local dominant along ridges. It is more accurate to refer to convection currents that exert a downward pull on this mechanism as gravitational sliding as variable topog- plates in subduction zones at ocean trenches. Slab raphy across the totality of the plate can vary consider- suction may occur in a geodynamic setting where ably and the of spreading ridges is only the basal tractions continue to act on the plate as it dives most prominent feature. Other mechanisms generating into the mantle (although perhaps to a greater extent this gravitational secondary force include flexural bulging acting on both the under and upper side of the slab). of the lithosphere before it dives underneath an adjacent plate which produces a clear topographical feature that Lately, the convection theory has been much debated as can offset, or at least affect, the influence of topographi- modern techniques based on 3D seismic tomography still cal ocean ridges, and mantle plumes and hot spots, which fail to recognize these predicted large scale convection are postulated to impinge on the underside of tectonic cells. Therefore, alternative views have been proposed: plates. In the theory of plume tectonics developed during the Slab-pull: Current scientific opinion is that the astheno- 1990s, a modified concept of mantle convection currents sphere is insufficiently competent or rigid to directly is used. It asserts that super plumes rise from the deeper cause motion by friction along the base of the lithosphere. mantle and are the drivers or substitutes of the major con- is therefore most widely thought to be the great- vection cells. These ideas, which find their roots in the est force acting on the plates. In this current understand- early 1930s with the so-called “fixistic” ideas of the Euro- ing, plate motion is mostly driven by the weight of cold, pean and Russian Schools, find resonance dense plates sinking into the mantle at trenches.[19] Re- in the modern theories which envisage hot spots/mantle cent models indicate that trench suction plays an impor- plumes which remain fixed and are overridden by oceanic tant role as well. However, as the North American Plate and continental lithosphere plates over time and leave is nowhere being subducted, yet it is in motion presents a their traces in the geological record (though these phe- problem. The same holds for the African, Eurasian, and nomena are not invoked as real driving mechanisms, but Antarctic plates. rather as modulators). Modern theories that continue Gravitational sliding away from mantle doming: Accord- building on the older mantle doming concepts and see ing to older theories, one of the driving mechanisms plate movements as a secondary phenomena are beyond of the plates is the existence of large scale astheno- the scope of this page and are discussed elsewhere (for sphere/mantle domes which cause the gravitational slid- example on the plume tectonics page). ing of lithosphere plates away from them. This gravita- Another theory is that the mantle flows neither in cells tional sliding represents a secondary phenomenon of this 3.4 Relative significance of each driving force mechanism 5 basically vertically oriented mechanism. This can act on invoked many of the relationships recognized during this various scales, from the small scale of one up pre-plate tectonics period to support their theories (see to the larger scale of an entire ocean basin.[20] the anticipations and reviews in the work of van Dijk and collaborators).[23] 3.3 Driving forces related to Earth rota- Of the many forces discussed in this paragraph, is still highly debated and defended as a possi- tion ble principle driving force of plate tectonics. The other forces are only used in global geodynamic models not us- , being a meteorologist, had proposed ing plate tectonics concepts (therefore beyond the discus- tidal forces and pole flight force as the main driving mech- sions treated in this section) or proposed as minor modu- anisms behind continental drift; however, these forces lations within the overall plate tectonics model. were considered far too small to cause continental motion as the concept then was of continents plowing through In 1973, George W. Moore[24] of the USGS and R. C. oceanic crust.[21] Therefore, Wegener later changed his Bostrom[25] presented evidence for a general westward position and asserted that convection currents are the drift of the Earth’s lithosphere with respect to the man- main driving force of plate tectonics in the last edition tle. He concluded that tidal forces (the tidal lag or “fric- of his book in 1929. tion”) caused by the Earth’s rotation and the forces acting upon it by the Moon are a driving force for plate tecton- However, in the plate tectonics context (accepted since ics. As the Earth spins eastward beneath the moon, the the seafloor spreading proposals of Heezen, Hess, Dietz, moon’s gravity ever so slightly pulls the Earth’s surface Morley, Vine, and Matthews (see below) during the early layer back westward, just as proposed by Alfred Wegener 1960s), oceanic crust is suggested to be in motion with the (see above). In a more recent 2006 study,[26] scientists continents which caused the proposals related to Earth ro- reviewed and advocated these earlier proposed ideas. It tation to be reconsidered. In more recent literature, these has also been suggested recently in Lovett (2006) that driving forces are: this observation may also explain why and have no plate tectonics, as Venus has no moon and Mars’ 1. Tidal drag due to the gravitational force the Moon are too small to have significant tidal effects on (and the Sun) exerts on the crust of the Earth[22] the planet. In a recent paper,[27] it was suggested that, on the other hand, it can easily be observed that many plates 2. strain of the Earth globe due to N-S compres- are moving north and eastward, and that the dominantly sion related to its rotation and modulations; westward motion of the Pacific ocean basins derives sim- 3. Pole flight force: equatorial drift due to rotation and ply from the eastward bias of the Pacific spreading cen- centrifugal effects: tendency of the plates to move ter (which is not a predicted manifestation of such lunar from the poles to the ("Polflucht"); forces). In the same paper the authors admit, however, that relative to the , there is a slight westward 4. The effect acting on plates when they move component in the motions of all the plates. They demon- around the globe; strated though that the westward drift, seen only for the past 30 Ma, is attributed to the increased dominance of 5. Global deformation of the due to small dis- the steadily growing and accelerating Pacific plate. The placements of rotational pole with respect to the debate is still open. Earth’s crust;

6. Other smaller deformation effects of the crust due to 3.4 Relative significance of each driving wobbles and spin movements of the Earth rotation force mechanism on a smaller time scale. The actual vector of a plate’s motion is a function of all For these mechanisms to be overall valid, systematic re- the forces acting on the plate; however, therein lies the lationships should exist all over the globe between the problem regarding what degree each process contributes orientation and of deformation and the geo- to the overall motion of each tectonic plate. graphical latitudinal and longitudinal grid of the Earth it- self. Ironically, these systematic relations studies in the The diversity of geodynamic settings and the properties second half of the nineteenth century and the first half of each plate must clearly result from differences in the of the twentieth century underline exactly the opposite: degree to which multiple processes are actively driving that the plates had not moved in time, that the deforma- each individual plate. One method of dealing with this tion grid was fixed with respect to the Earth equator and problem is to consider the relative rate at which each plate axis, and that gravitational driving forces were generally is moving and to consider the available evidence of each acting vertically and caused only local horizontal move- driving force on the plate as far as possible. ments (the so-called pre-plate tectonic, “fixist theories”). One of the most significant correlations found is that Later studies (discussed below on this page), therefore, lithospheric plates attached to downgoing (subducting) 6 4 DEVELOPMENT OF THE THEORY plates move much faster than plates not attached to sub- in the southern hemisphere. The South African Alex du ducting plates. The Pacific plate, for instance, is essen- Toit put together a mass of such information in his 1937 tially surrounded by zones of subduction (the so-called publication Our Wandering Continents, and went further ) and moves much faster than the plates of than Wegener in recognising the strong links between the the Atlantic basin, which are attached (perhaps one could fragments. say 'welded') to adjacent continents instead of subduct- But without detailed evidence and a force sufficient to ing plates. It is thus thought that forces associated with drive the movement, the theory was not generally ac- the downgoing plate (slab pull and slab suction) are the cepted: the Earth might have a solid crust and mantle and driving forces which determine the motion of plates, ex- [19] a liquid core, but there seemed to be no way that portions cept for those plates which are not being subducted. of the crust could move around. Distinguished scientists, The driving forces of plate motion continue to be ac- such as Harold Jeffreys and Charles Schuchert, were out- tive subjects of on-going research within geophysics and spoken critics of continental drift. . Despite much opposition, the view of continental drift gained support and a lively debate started between 4 Development of the theory “drifters” or “mobilists” (proponents of the theory) and “fixists” (opponents). During the 1920s, 1930s and 1940s, the former reached important milestones propos- Further information: Timeline of the development of ing that convection currents might have driven the plate tectonophysics movements, and that spreading may have occurred below the sea within the oceanic crust. Concepts close to the elements now incorporated in plate tectonics were pro- posed by geophysicists and (both fixists and 4.1 Summary mobilists) like Vening-Meinesz, Holmes, and Umbgrove. One of the first pieces of geophysical evidence that was continental / oceanic Laptev Sea 14 (EU) - Yukon continental rift boundary / oceanic spreading ridge 14 16 continental / oceanic subduction zone

30 14 13 velocity with respect to Africa (mm/y) Eurasia (EU) 15 18 19 used to support the movement of lithospheric plates came 8 Alps Okhotsk (OK) 10 western Aleutians 18

11 13 Amur (AM) 59 Eurasia (EU)

7 Pacific (PA) North America (NA)

10 26 JF Alps Anatolia Juan de Fuca from . This is based on the fact that rocks Gorda- 19 5 21 California- AT 92 Pacific (PA) Nevada Persia - Tibet - Burma 22 14 AS 37 10 69 Aegean Sea ATLANTIC 15 48 Yangtze (YA) PACIFIC 15 29 ON of different ages show a variable magnetic field direction, 54 25 24 36 Okinawa 76 Africa (AF) 20 (IN) 51 71 14 west central Atlantic RI MA Arabia (AR) 17 Rivera 69 Rivera- Africa (AF) (CA) Mariana 90 Cocos 10 11 26 Burma (PS) 39 Panama 102 27 84 PA 46 23 19 67 ND 6 Sunda (SU) Cocos (CO) evidenced by studies since the mid–nineteenth century. 14 BU Pacific (PA) Galápagos (GP) North Andes Somalia (SO) 87 Caroline (CL) Manus (MN) Equator BH 92 NB MS 70 96 95 MO 96 32 58 26 40 32 11 15 BS 57 WL SB Ninety East - Sumatra 86 TI SS Peru South America (SA) 83 100 44 103

70 FT BR Altiplano The magnetic north and south poles reverse through time, New Hebrides - Fiji 62 Pacific (PA) Nazca (NZ) 119 NH NI INDIAN CR 26 AP TO OCEAN Easter 51 59 Tonga 51 (AU) 55 OCEAN EA 34 OCEAN 68 69 44 34 Puna- Sierras 51 KE 102 Pampeanas 83 and, especially important in paleotectonic studies, the rel- 13 Kermadec Juan Fernandez JZ



Antarctica (AN) 78

14 Pacific (PA) 70 ative position of the magnetic varies through

31 13 31 10 12 56 82 Sandwich

14 (AN) Scotia (SC) SW 25 47 14 14 time. Initially, during the first half of the twentieth cen- 66 Shetland Antarctica (AN) SL

AUSTRAL OCEAN 12 AUSTRAL OCEAN Antarctica (AN) 13 tury, the latter phenomenon was explained by introduc- ing what was called “polar wander” (see apparent polar Detailed showing the tectonic plates with their movement wander), i.e., it was assumed that the north pole location vectors. had been shifting through time. An alternative explana- In line with other previous and contemporaneous propos- tion, though, was that the continents had moved (shifted als, in 1912 the meteorologist Alfred Wegener amply de- and rotated) relative to the north pole, and each conti- scribed what he called continental drift, expanded in his nent, in fact, shows its own “polar wander path”. During 1915 book The Origin of Continents and Oceans[28] and the late 1950s it was successfully shown on two occasions the scientific debate started that would end up fifty years that these data could show the validity of continental drift: [31] later in the theory of plate tectonics.[29] Starting from the by in a paper in 1956, and by Warren [32] idea (also expressed by his forerunners) that the present Carey in a symposium held in March 1956. continents once formed a single mass (which was The second piece of evidence in support of continental called Pangea later on) that drifted apart, thus releas- drift came during the late 1950s and early 60s from data ing the continents from the Earth’s mantle and likening on the of the deep ocean floors and the na- them to “icebergs” of low density floating on a ture of the oceanic crust such as magnetic properties and, sea of denser .[30] Supporting evidence for the idea more generally, with the development of marine geol- came from the dove-tailing outlines of South America’s ogy[33] which gave evidence for the association of seafloor east and Africa’s west coast, and from the match- spreading along the mid-oceanic ridges and magnetic field ing of the rock formations along these edges. Confirma- reversals, published between 1959 and 1963 by Heezen, tion of their previous contiguous also came from Dietz, Hess, Mason, Vine & Matthews, and Morley.[34] the and , and the Simultaneous advances in early seismic imaging tech- or -like reptile , all widely niques in and around Wadati-Benioff zones along the distributed over South America, Africa, Antarctica, India trenches bounding many continental margins, together and Australia. The evidence for such an erstwhile joining with many other geophysical (e.g. gravimetric) and geo- of these continents was patent to field geologists working 4.3 Floating continents, paleomagnetism, and zones 7 logical observations, showed how the oceanic crust could implied that, even if it started at red heat, the Earth would disappear into the mantle, providing the mechanism to have dropped to its present temperature in a few tens of balance the extension of the ocean basins with shortening millions of years. Armed with the knowledge of a new along its margins. heat source, scientists realized that the Earth would be All this evidence, both from the ocean floor and from the much older, and that its core was still sufficiently hot to continental margins, made it clear around 1965 that conti- be liquid. nental drift was feasible and the theory of plate tectonics, By 1915, after having published a first article in 1912,[39] which was defined in a series of papers between 1965 Alfred Wegener was making serious arguments for the and 1967, was born, with all its extraordinary explana- idea of continental drift in the first edition of The Ori- tory and predictive power. The theory revolutionized the gin of Continents and Oceans.[28] In that book (re-issued Earth , explaining a diverse range of geological in four successive editions up to the final one in 1936), phenomena and their implications in other studies such as he noted how the east coast of South America and the paleogeography and . west coast of Africa looked as if they were once attached. Wegener was not the first to note this (, Antonio Snider-Pellegrini, , Roberto Man- 4.2 Continental drift tovani and Frank Bursley Taylor preceded him just to mention a few), but he was the first to marshal signifi- For more details on this topic, see Continental drift. cant fossil and paleo-topographical and climatological ev- idence to support this simple observation (and was sup- ported in this by researchers such as Alex du Toit). Fur- In the late 19th and early 20th centuries, geologists as- thermore, when the rock strata of the margins of sep- sumed that the Earth’s major features were fixed, and that arate continents are very similar it suggests that these most geologic features such as basin development and rocks were formed in the same way, implying that they mountain ranges could be explained by vertical crustal were joined initially. For instance, parts of movement, described in what is called the geosynclinal and contain rocks very similar to those found theory. Generally, this was placed in the context of a in Newfoundland and . Furthermore, contracting planet Earth due to heat loss in the course of the Caledonian Mountains of and parts of the a relatively short geological time. of North America are very sim- ilar in structure and . However, his ideas were not taken seriously by many geol- ogists, who pointed out that there was no apparent mech- anism for continental drift. Specifically, they did not see how continental rock could plow through the much denser rock that makes up oceanic crust. Wegener could not ex- plain the force that drove continental drift, and his vindi- cation did not come until after his death in 1930.

4.3 Floating continents, paleomagnetism, and seismicity zones

Alfred Wegener in in the winter of 1912-13.

It was observed as early as 1596 that the opposite of the —or, more precisely, the edges of the continental shelves—have similar shapes and seem to have once fitted together.[35] Since that time many theories were proposed to explain this apparent complementarity, but the assumption of a made these various proposals difficult to accept.[36] The discovery of radioactivity and its associated heating properties in 1895 prompted a re-examination of the ap- Global , 1963–1998 parent age of the Earth.[37] This had previously been esti- mated by its cooling rate and assumption the Earth’s sur- As it was observed early that although granite existed on face radiated like a .[38] Those calculations had continents, seafloor seemed to be composed of denser 8 4 DEVELOPMENT OF THE THEORY

basalt, the prevailing concept during the first half of the crust. In this hypothesis the shifting of the continents can twentieth century was that there were two types of crust, be simply explained by a large increase in size of the Earth named “sial” (continental type crust) and “sima” (oceanic since its formation. However, this was unsatisfactory be- type crust). Furthermore, it was supposed that a static cause its supporters could offer no convincing mechanism shell of strata was present under the continents. It there- to produce a significant expansion of the Earth. Certainly fore looked apparent that a layer of basalt (sial) underlies there is no evidence that the moon has expanded in the the continental rocks. past 3 billion years; other work would soon show that the However, based on abnormalities in plumb line deflec- evidence was equally in support of continental drift on a globe with a stable radius. tion by the Andes in Peru, had deduced that less-dense mountains must have a downward projec- During the thirties up to the late fifties, works by Vening- tion into the denser layer underneath. The concept that Meinesz, Holmes, Umbgrove, and numerous others out- mountains had “roots” was confirmed by George B. Airy lined concepts that were close or nearly identical to mod- a hundred years later, during study of Himalayan grav- ern plate tectonics theory. In particular, the English ge- itation, and seismic studies detected corresponding den- ologist Arthur Holmes proposed in 1920 that plate junc- sity variations. Therefore, by the mid-1950s, the question tions might lie beneath the sea, and in 1928 that con- remained unresolved as to whether mountain roots were vection currents within the mantle might be the driv- clenched in surrounding basalt or were floating on it like ing force.[42] Often, these contributions are forgotten be- an iceberg. cause: During the 20th century, improvements in and greater use of seismic instruments such as seismographs enabled sci- • At the time, continental drift was not accepted. entists to learn that earthquakes tend to be concentrated • Some of these ideas were discussed in the context of in specific areas, most notably along the oceanic trenches abandoned fixistic ideas of a deforming globe with- and spreading ridges. By the late 1920s, seismologists out continental drift or an expanding Earth. were beginning to identify several prominent earthquake zones parallel to the trenches that typically were inclined • They were published during an episode of extreme 40–60° from the horizontal and extended several hun- political and economic instability that hampered sci- dred kilometers into the Earth. These zones later became entific communication. known as Wadati-Benioff zones, or simply Benioff zones, in honor of the seismologists who first recognized them, • Many were published by European scientists and at Kiyoo Wadati of and Hugo Benioff of the United first not mentioned or given little credit in the papers States. The study of global seismicity greatly advanced in on sea floor spreading published by the American the 1960s with the establishment of the Worldwide Stan- researchers in the 1960s. dardized Seismograph Network (WWSSN)[40] to mon- itor the compliance of the 1963 treaty banning above- ground testing of nuclear weapons. The much improved 4.4 Mid-oceanic ridge spreading and con- data from the WWSSN instruments allowed seismolo- vection gists to map precisely the zones of earthquake concen- tration world wide. For more details on Mid-ocean ridge, see Seafloor spreading. Meanwhile, debates developed around the phenomena of polar wander. Since the early debates of continental drift, scientists had discussed and used evidence that polar drift In 1947, a team of scientists led by utiliz- had occurred because continents seemed to have moved ing the Woods Hole Oceanographic Institution's research through different climatic zones during the past. Fur- vessel and an array of instruments, confirmed thermore, paleomagnetic data had shown that the mag- the existence of a rise in the central Atlantic Ocean, and netic pole had also shifted during time. Reasoning in an found that the floor of the beneath the layer of opposite way, the continents might have shifted and ro- consisted of basalt, not the granite which is the tated, while the pole remained relatively fixed. The first main constituent of continents. They also found that the time the evidence of magnetic polar wander was used to oceanic crust was much thinner than continental crust. support the movements of continents was in a paper by All these new findings raised important and intriguing Keith Runcorn in 1956,[31] and successive papers by him questions.[43] and his students Ted Irving (who was actually the first to The new data that had been collected on the ocean be convinced of the fact that paleomagnetism supported basins also showed particular characteristics regarding continental drift) and Ken Creer. the bathymetry. One of the major outcomes of these This was immediately followed by a symposium in datasets was that all along the globe, a system of mid- in March 1956.[41] In this symposium, the evi- oceanic ridges was detected. An important conclusion dence was used in the theory of an expansion of the global was that along this system, new ocean floor was being cre- ated, which led to the concept of the "Great Global Rift". 4.5 Magnetic striping 9

This was described in the crucial paper of Bruce Heezen concluded that the Atlantic Ocean was expanding while (1960),[44] which would trigger a real revolution in think- the Pacific Ocean was shrinking. As old oceanic crust ing. A profound consequence of seafloor spreading is is “consumed” in the trenches (like Holmes and others, that new crust was, and still is, being continually created he thought this was done by thickening of the continental along the oceanic ridges. Therefore, Heezen advocated lithosphere, not, as now understood, by underthrusting at the so-called "expanding Earth" hypothesis of S. Warren a larger scale of the oceanic crust itself into the mantle), Carey (see above). So, still the question remained: how new magma rises and erupts along the spreading ridges can new crust be continuously added along the oceanic to form new crust. In effect, the ocean basins are per- ridges without increasing the size of the Earth? In real- petually being “recycled,” with the creation of new crust ity, this question had been solved already by numerous and the destruction of old oceanic lithosphere occurring scientists during the forties and the fifties, like Arthur simultaneously. Thus, the new mobilistic concepts neatly Holmes, Vening-Meinesz, Coates and many others: The explained why the Earth does not get bigger with sea floor crust in excess disappeared along what were called the spreading, why there is so little accumulation on oceanic trenches, where so-called “subduction” occurred. the ocean floor, and why oceanic rocks are much younger Therefore, when various scientists during the early sixties than continental rocks. started to reason on the data at their disposal regarding the ocean floor, the pieces of the theory quickly fell into place. 4.5 Magnetic striping The question particularly intrigued Harry Hammond . Hess, a and a Naval Re- serve Rear Admiral, and Robert S. Dietz, a with Normal magnetic the U.S. Coast and Geodetic Survey who first coined the polarity a term seafloor spreading. Dietz and Hess (the former pub- lished the same idea one year earlier in Nature,[45] but Reversed magnetic polarity b priority belongs to Hess who had already distributed an unpublished manuscript of his 1962 article by 1960)[46] were among the small handful who really understood the broad implications of sea floor spreading and how it would eventually agree with the, at that time, uncon- c ventional and unaccepted ideas of continental drift and the elegant and mobilistic models proposed by previous Litosphere Magma workers like Holmes. In the same year, Robert R. Coats of the U.S. Geological Seafloor magnetic striping. Survey described the main features of island arc subduc- tion in the . His paper, though little noted (and even ridiculed) at the time, has since been called “seminal” and “prescient”. In reality, it actually shows that the work by the European scientists on island arcs and mountain belts performed and published during the 1930s up until the 1950s was applied and appreciated also in the United States. If the Earth’s crust was expanding along the oceanic ridges, Hess and Dietz reasoned like Holmes and others before them, it must be shrinking elsewhere. Hess fol- lowed Heezen, suggesting that new oceanic crust contin- uously spreads away from the ridges in a conveyor belt– like motion. And, using the mobilistic concepts devel- oped before, he correctly concluded that many millions of years later, the oceanic crust eventually descends along A demonstration of magnetic striping. (The darker the color is, the continental margins where oceanic trenches – very the closer it is to normal polarity) deep, narrow – are formed, e.g. along the rim of the Pacific Ocean basin. The important step Hess made For more details on this topic, see Vine–Matthews– was that convection currents would be the driving force in Morley hypothesis. this process, arriving at the same conclusions as Holmes had decades before with the only difference that the thin- Beginning in the 1950s, scientists like Victor Vacquier, ning of the ocean crust was performed using Heezen’s using magnetic instruments () adapted mechanism of spreading along the ridges. Hess therefore from airborne devices developed during World II 10 4 DEVELOPMENT OF THE THEORY to detect , began recognizing odd magnetic in magnetic polarity (normal-reversed-normal, etc.), variations across the ocean floor. This finding, though suggesting that they were formed during different unexpected, was not entirely surprising because it was epochs documenting the (already known from inde- known that basalt—the -rich, making pendent studies) normal and reversal episodes of the up the ocean floor—contains a strongly magnetic min- Earth’s magnetic field. eral () and can locally distort compass readings. This distortion was recognized by Icelandic mariners as By explaining both the -like magnetic striping and early as the late 18th century. More important, be- the construction of the mid-ocean ridge system, the cause the presence of magnetite gives the basalt measur- seafloor spreading hypothesis (SFS) quickly gained con- able magnetic properties, these newly discovered mag- verts and represented another major advance in the devel- netic variations provided another means to study the deep opment of the plate-tectonics theory. Furthermore, the ocean floor. When newly formed rock cools, such mag- oceanic crust now came to be appreciated as a natural netic materials recorded the Earth’s magnetic field at the “tape recording” of the of the geomagnetic field time. reversals (GMFR) of the Earth’s magnetic field. Today, As more and more of the seafloor was mapped during extensive studies are dedicated to the calibration of the the 1950s, the magnetic variations turned out not to be normal-reversal patterns in the oceanic crust on one hand random or isolated occurrences, but instead revealed rec- and known timescales derived from the dating of basalt ognizable patterns. When these magnetic patterns were layers in sedimentary sequences () mapped over a wide region, the ocean floor showed a on the other, to arrive at estimates of past spreading rates zebra-like pattern: one stripe with normal polarity and and plate reconstructions. the adjoining stripe with reversed polarity. The overall pattern, defined by these alternating bands of normally and reversely polarized rock, became known as magnetic 4.6 Definition and refining of the theory striping, and was published by Ron G. Mason and co- workers in 1961, who did not find, though, an explanation After all these considerations, Plate Tectonics (or, as for these data in terms of sea floor spreading, like Vine, it was initially called “New Global Tectonics”) became Matthews and Morley a few years later.[47] quickly accepted in the scientific world, and numerous papers followed that defined the concepts: The discovery of magnetic striping called for an expla- nation. In the early 1960s scientists such as Heezen, • Hess and Dietz had begun to theorise that mid-ocean In 1965, Tuzo Wilson who had been a promotor ridges mark structurally weak zones where the ocean floor of the sea floor spreading hypothesis and continen- [50] was being ripped in two lengthwise along the ridge crest tal drift from the very beginning added the con- (see the previous paragraph). New magma from deep cept of transform faults to the model, completing the within the Earth rises easily through these weak zones classes of fault types necessary to make the mobility [51] and eventually erupts along the crest of the ridges to cre- of the plates on the globe work out. ate new oceanic crust. This process, at first denominated • A symposium on continental drift was held at the the “conveyer belt hypothesis” and later called seafloor Royal Society of London in 1965 which must be re- spreading, operating over many millions of years contin- garded as the official start of the acceptance of plate ues to form new ocean floor all across the 50,000 km-long tectonics by the scientific community, and which system of mid–ocean ridges. abstracts are issued as Blacket, Bullard & Runcorn Only four years after the with the “zebra pat- (1965). In this symposium, and co- tern” of magnetic stripes were published, the link be- workers showed with a calculation how tween sea floor spreading and these patterns was correctly the continents along both sides of the Atlantic would placed, independently by Lawrence Morley, and by Fred best fit to close the ocean, which became known as Vine and , in 1963[48] now called the famous “Bullard’s Fit”. the Vine-Matthews-Morley hypothesis. This hypothesis linked these patterns to geomagnetic reversals and was • In 1966 Wilson published the paper that referred to supported by several lines of evidence:[49] previous plate tectonic reconstructions, introducing the concept of what is now known as the "Wilson [52] 1. the stripes are symmetrical around the crests of the Cycle". mid-ocean ridges; at or near the crest of the ridge, • In 1967, at the American Geophysical Union's meet- the rocks are very young, and they become progres- ing, W. Jason Morgan proposed that the Earth’s sur- sively older away from the ridge crest; face consists of 12 rigid plates that move relative to 2. the youngest rocks at the ridge crest always have each other.[53] present-day (normal) polarity; • Two months later, Xavier Le Pichon published a 3. stripes of rock parallel to the ridge crest alternate complete model based on 6 major plates with their 6.3 Formation and break-up of continents 11

relative motions, which marked the final acceptance by particular fossil groups, and the position of orogenic by the scientific community of plate tectonics.[54] belts.[61]

• In the same year, McKenzie and Parker indepen- dently presented a model similar to Morgan’s using translations and rotations on a sphere to define the 6.3 Formation and break-up of continents plate motions.[55] The movement of plates has caused the formation and break-up of continents over time, including occasional formation of a that contains most or all 5 Implications for of the continents. The supercontinent Columbia or Nuna formed during a period of 2,000 to 1,800 million years Continental drift theory helps biogeographers to explain ago and broke up about 1,500 to 1,300 million years the disjunct biogeographic distribution of present day ago.[64] The supercontinent is thought to have found on different continents but having similar ances- formed about 1 billion years ago and to have embod- tors.[56] In particular, it explains the Gondwanan distri- ied most or all of Earth’s continents, and broken up into bution of and the Antarctic flora. eight continents around 600 million years ago. The eight continents later re-assembled into another supercontinent called ; Pangaea broke up into (which 6 became North America and Eurasia) and Gondwana (which became the remaining continents). Main article: Plate reconstruction The Himalayas, the world’s tallest mountain range, are assumed to have been formed by the collision of two ma- jor plates. Before uplift, they were covered by the Tethys Reconstruction is used to establish past (and ) plate Ocean. configurations, helping determine the shape and make- up of ancient and providing a basis for paleogeography. 7 Current plates 6.1 Defining plate boundaries Main article: List of tectonic plates Current plate boundaries are defined by their Depending on how they are defined, there are usu- seismicity.[57] Past plate boundaries within existing plates are identified from a variety of evidence, such as the presence of that are indicative of vanished oceans.[58]

6.2 Past plate motions

Tectonic motion first began around three billion years ago.[59] Various types of quantitative and semi-quantitative infor- mation are available to constrain past plate motions. The geometric fit between continents, such as between west Africa and South America is still an important part of Plate tectonics map plate reconstruction. Magnetic stripe patterns provide a reliable guide to relative plate motions going back into ally seven or eight “major” plates: African, Antarctic, the period.[60] The tracks of hotspots give abso- Eurasian, North American, South American, Pacific, and lute reconstructions, but these are only available back to Indo-Australian. The latter is sometimes subdivided into the .[61] Older reconstructions rely mainly on the Indian and Australian plates. paleomagnetic pole data, although these only constrain the and rotation, but not the . Combin- There are dozens of smaller plates, the seven largest of ing poles of different ages in a particular plate to produce which are the Arabian, Caribbean, Juan de Fuca, Cocos, apparent polar wander paths provides a method for com- Nazca, Philippine Sea and Scotia. paring the motions of different plates through time.[62] The current motion of the tectonic plates is today de- Additional evidence comes from the distribution of cer- termined by satellite data sets, calibrated tain types,[63] faunal provinces shown with ground station measurements. 12 9 SEE ALSO

8 Other celestial bodies (, entists today disagree, and believe that it was created ei- moons) ther by upwelling within the Martian mantle that thick- ened the crust of the Southern Highlands and formed [69] or by a giant impact that excavated the The appearance of plate tectonics on terrestrial planets is Northern Lowlands.[70] related to , with more massive planets than [71] Earth expected to exhibit plate tectonics. Earth may be may be a tectonic boundary. a borderline case, owing its tectonic activity to abundant Observations made of the magnetic field of Mars by the water [65] (silica and water form a deep eutectic.) spacecraft in 1999 showed patterns of magnetic striping discovered on this planet. Some sci- entists interpreted these as requiring plate tectonic pro- 8.1 Venus cesses, such as seafloor spreading.[72] However, their data fail a “magnetic reversal test”, which is used to see if they See also: were formed by flipping polarities of a global magnetic field.[73] Venus shows no evidence of active plate tectonics. There is debatable evidence of active tectonics in the planet’s 8.3 Galilean satellites of distant past; however, events taking place since then (such as the plausible and generally accepted hypothesis that Some of the satellites of Jupiter have features that may the Venusian lithosphere has thickened greatly over the be related to plate-tectonic style deformation, although course of several hundred million years) has made con- the materials and specific mechanisms may be different straining the course of its difficult. How- from plate-tectonic activity on Earth. On 8 September ever, the numerous well-preserved impact craters have 2014, NASA reported finding evidence of plate tectonics been utilized as a dating method to approximately date on , a satellite of Jupiter - the first sign of such the Venusian surface (since there are thus far no known geological activity on another world other than Earth.[74] samples of Venusian rock to be dated by more reliable methods). Dates derived are dominantly in the range 500 to 750 million years ago, although ages of up to 1,200 8.4 , moon of million years ago have been calculated. This research has led to the fairly well accepted hypothesis that Venus has Titan, the largest moon of Saturn, was reported to show undergone an essentially complete volcanic resurfacing at tectonic activity in images taken by the Huygens Probe, least once in its distant past, with the last event taking which landed on Titan on January 14, 2005.[75] place approximately within the range of estimated sur- face ages. While the mechanism of such an impressive thermal event remains a debated issue in Venusian geo- 8.5 sciences, some scientists are advocates of processes in- volving plate motion to some extent. On Earth-sized planets, plate tectonics is more likely if there are oceans of water; however, in 2007, two in- One explanation for Venus’ lack of plate tectonics is that dependent teams of researchers came to opposing con- on Venus are too high for significant wa- clusions about the likelihood of plate tectonics on larger [66][67] ter to be present. The Earth’s crust is soaked with super-[76][77] with one team saying that plate tecton- water, and water plays an important role in the develop- ics would be episodic or stagnant[78] and the other team ment of shear zones. Plate tectonics requires weak sur- saying that plate tectonics is very likely on super-earths faces in the crust along which crustal slices can move, and even if the planet is dry.[65] it may well be that such weakening never took place on Venus because of the absence of water. However, some researchers remain convinced that plate tectonics is or 9 See also was once active on this planet. • Geological 8.2 Mars • theory • See also: List of plate tectonics topics • Mars is considerably smaller than Earth and Venus, and • Conservation of there is evidence for on its surface and in its crust. • List of topographical features In the 1990s, it was proposed that Martian Crustal Di- chotomy was created by plate tectonic processes.[68] Sci- • Tectonics 10.1 Notes 13

10 References [31] Runcorn 1956. [32] Carey 1956. 10.1 Notes [33] see for example the milestone paper of Lyman & Fleming [1] Little, Fowler & Coulson 1990. 1940.

[2] Read & Watson 1975. [34] Korgen 1995, Spiess & Kuperman 2003.

[3] Scalera & Lavecchia 2006. [35] Kious & Tilling 1996.

[4] Zhen Shao 1997, Hancock, Skinner & Dineley 2000. [36] Frankel 1987.

[5] Turcotte & Schubert 2002, p. 5. [37] Joly 1909.

[6] Turcotte & Schubert 2002. [38] Thomson 1863. [39] Wegener 1912. [7] Turcotte & Schubert 2002, p. 3. [40] Stein & Wysession 2009, p. 26 [8] Foulger 2010. [41] Carey 1956; see also Quilty 2003. [9] Schmidt & Harbert 1998. [42] Holmes 1928; see also Holmes 1978, Frankel 1978. [10] Meissner 2002, p. 100. [43] Lippsett 2001, Lippsett 2006. [11] “Plate Tectonics: Plate Boundaries”. platetectonics.com. Retrieved 12 June 2010. [44] Heezen 1960.

[12] “Understanding plate motions”. USGS. Retrieved 12 June [45] Dietz 1961. 2010. [46] Hess 1962. [13] Mendia-Landa, Pedro. “Myths and Legends on Natural Disasters: Making Sense of Our World”. Retrieved 2008- [47] Mason & Raff 1961, Raff & Mason 1961. 02-05. [48] Vine & Matthews 1963. [14] van Dijk 1992, van Dijk & Okkes 1991. [49] See summary in Heirzler, Le Pichon & Baron 1966 [15] Holmes, Arthur (1931). “Radioactivity and Earth Move- [50] Wilson 1963. ments”. Trans. Geological Society of Glasgow: 559–606. [51] Wilson 1965. [16] Tanimoto & Lay 2000. [52] Wilson 1966. [17] Smoot et al. 1996. [53] Morgan 1968. [18] Spence 1987, White & McKenzie 1989. [54] Le Pichon 1967. [19] Conrad & Lithgow-Bertelloni 2002. [55] McKenzie & Parker 1967. [20] Spence 1987, White & Mckenzie 1989, Segev 2002. [56] Moss & Wilson 1998. [21] “Alfred Wegener (1880-1930)". University of California Museum of . [57] Condie 1997.

[22] Neith, Katie (April 15, 2011). “Caltech Researchers Use [58] Lliboutry 2000. GPS Data to Model Effects of Tidal Loads on Earth’s Sur- face”. Caltech. Retrieved August 15, 2012. [59] Kranendonk, V.; Martin, J. (2011). “Onset of Plate Tectonics”. Science 333 (6041): 413–414. [23] van Dijk 1992, van Dijk & Okkes 1990). doi:10.1126/science.1208766. PMID 21778389.

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[29] Hughes 2001a. [64] Zhao 2002, 2004

[30] Wegener 1966, Hughes 2001b. [65] Valencia, O'Connell & Sasselov 2007. 14 10 REFERENCES

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• Wilson, J. Tuzo (). “A new class of faults 11.1 Videos and their bearing on continental drift”. Nature 207 (4995): 343–347. Bibcode:1965Natur.207..343W. • Khan Academy Explanation of evidence doi:10.1038/207343a0. • 750 million years of global tectonic activity. Movie. • Wilson, J. Tuzo (13 August 1966). “Did the Atlantic close and then re-open?". Nature 211 (5050): 676–681. Bibcode:1966Natur.211..676W. doi:10.1038/211676a0. • Zhen Shao, Huang (1997). “Speed of the Continen- tal Plates”. The Physics Factbook. • Zhao, Guochun, Cawood, Peter A., Wilde, Simon A., and Sun, M. (2002). “Review of global 2.1– 1.8 Ga orogens: implications for a pre-Rodinia su- percontinent”. Earth-Science Reviews 59, pp. 125– 162. 59: 125. Bibcode:2002ESRv...59..125Z. doi:10.1016/S0012-8252(02)00073-9. • Zhao, Guochun, Sun, M., Wilde, Simon A., and Li, S.Z. (2004). “A Paleo- su- percontinent: assembly, growth and breakup”. Earth-Science Reviews, 67, pp. 91–123. 67: 91. Bibcode:2004ESRv...67...91Z. doi:10.1016/j.earscirev.2004.02.003. • Zhong, Shijie; Zuber, Maria T. (2001). “Degree- 1 mantle convection and the crustal dichotomy on Mars”. Earth and Planetary Science Let- ters 189: 75. Bibcode:2001E&PSL.189...75Z. doi:10.1016/S0012-821X(01)00345-4.

11 External links

• This Dynamic Earth: The Story of Plate Tectonics. USGS. • Understanding Plate Tectonics. USGS. • The PLATES Project. Jackson School of Geo- sciences. • An explanation of tectonic forces. Example of cal- culations to show that Earth Rotation could be a driving force. • , P. (2003); An updated digital model of plate boundaries. • Map of tectonic plates. • GPlates, desktop software for the interactive visual- ization of plate-tectonics. • MORVEL plate velocity estimates and information. C. DeMets, D. Argus, & R. Gordon. • Google Map of the Topography of Plate Tectonics that enables you to zoom in on submarine mid ocean ridges, zones, ocean trenches, thermal vents and submarine volcanoes. 19

12 Text and image sources, contributors, and licenses

12.1 Text

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12.2 Images • File:Commons-logo.svg Source: http://upload.wikimedia.org/wikipedia/en/4/4a/Commons-logo.svg License: ? Contributors: ? Original artist: ? • File:Earth_Western_Hemisphere.jpg Source: http://upload.wikimedia.org/wikipedia/commons/7/7b/Earth_Western_Hemisphere.jpg License: Public domain Contributors: http://visibleearth.nasa.gov/view.php?id=57723 Original artist: • Reto Stöckli (land surface, shallow water, ) • Robert Simmon (enhancements: ocean color, compositing, 3D globes, animation) • Data and technical support: MODIS Land Group; MODIS Science Data Support Team; MODIS Group; MODIS Ocean Group • Additional data: USGS EROS Data Center (topography); USGS Terrestrial Remote Sensing Flagstaff Center (Antarctica); Defense Meteorological Satellite Program (city lights). • File:Farallon_Plate.jpg Source: http://upload.wikimedia.org/wikipedia/commons/a/a0/Farallon_Plate.jpg License: Public domain Con- tributors: http://svs.gsfc.nasa.gov/vis/a000000/a002400/a002410/ Original artist: NASA • File:Folder_Hexagonal_Icon.svg Source: http://upload.wikimedia.org/wikipedia/en/4/48/Folder_Hexagonal_Icon.svg License: Cc-by- sa-3.0 Contributors: ? Original artist: ? • File:Global_plate_motion_2008-04-17.jpg Source: http://upload.wikimedia.org/wikipedia/commons/7/7c/Global_plate_motion_ 2008-04-17.jpg License: Public domain Contributors: http://sideshow.jpl.nasa.gov/mbh/all/images/global.jpg Original artist: NASA • File:Grand_Canyon_NP-Arizona-USA.jpg Source: http://upload.wikimedia.org/wikipedia/commons/0/03/Grand_Canyon_ NP-Arizona-USA.jpg License: GFDL Contributors: Own work Original artist: Tobias Alt • File:Oceanic.Stripe.Magnetic.Anomalies.Scheme.svg Source: http://upload.wikimedia.org/wikipedia/commons/7/7e/Oceanic.Stripe. Magnetic.Anomalies.Scheme.svg License: Public domain Contributors: derived from File:Oceanic.Stripe.Magnetic.Anomalies.Scheme.gif Original artist: Chmee2 • File:Padlock-silver.svg Source: http://upload.wikimedia.org/wikipedia/commons/f/fc/Padlock-silver.svg License: CC0 Contributors: http://openclipart.org/people/Anonymous/padlock_aj_ashton_01.svg Original artist: This image file was created by AJ Ashton. Uploaded from English WP by User:Eleassar. Converted by User:AzaToth to a silver color. • File:Plate_tectonics_map.gif Source: http://upload.wikimedia.org/wikipedia/commons/b/b4/Plate_tectonics_map.gif License: Public domain Contributors: ? Original artist: ? • File:Plates_tect2_en.svg Source: http://upload.wikimedia.org/wikipedia/commons/8/8a/Plates_tect2_en.svg License: Public domain Contributors: http://pubs.usgs.gov/publications/text/slabs.html Original artist: USGS • File:Polarityshift.gif Source: http://upload.wikimedia.org/wikipedia/commons/2/23/Polarityshift.gif License: Public domain Contribu- tors: Own work Original artist: Powerkeys • File:Portal-puzzle.svg Source: http://upload.wikimedia.org/wikipedia/en/f/fd/Portal-puzzle.svg License: Public domain Contributors: ? Original artist: ? • File:Quake_epicenters_1963-98.png Source: http://upload.wikimedia.org/wikipedia/commons/d/db/Quake_epicenters_1963-98.png License: Public domain Contributors: http://denali.gsfc.nasa.gov/dtam/seismic/ Original artist: NASA, DTAM project team • File:Tectonic_plate_boundaries.png Source: http://upload.wikimedia.org/wikipedia/commons/4/40/Tectonic_plate_boundaries.png License: Public domain Contributors: [1] Original artist: Jose F. Vigil. USGS • File:Tectonic_plates_boundaries_detailed-en.svg Source: http://upload.wikimedia.org/wikipedia/commons/b/bf/Tectonic_plates_ boundaries_detailed-en.svg License: CC BY-SA 2.5 Contributors: • Background map: Image:Tectonic plates (empty).svg (modified) created by Ævar Arnfjörð Bjarmason under PD and based on an USGS map Original artist: Eric Gaba (Sting - fr:Sting) • File:Wallula-Gap-the-sisters.JPG Source: http://upload.wikimedia.org/wikipedia/commons/6/62/Wallula-Gap-the-sisters.JPG Li- cense: CC-BY-SA-3.0 Contributors: ? Original artist: ? • File:Wegener_Expedition-1930_008.jpg Source: http://upload.wikimedia.org/wikipedia/commons/1/16/Wegener_Expedition-1930_ 008.jpg License: Public domain Contributors: Archive of Alfred Wegener Institute Original artist: Loewe, Fritz; Georgi, Johannes; Sorge, Ernst; Wegener, Alfred Lothar 12.3 Content license 21

12.3 Content license

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