Plate tectonics 1

Plate tectonics

The tectonic plates of the world were mapped in the second half ofthe 20th century.

Remnants of the Farallon Plate, deep in Earth's mantle. It is thoughtthat much of the plate initially went under North America

(particularly the western United States and southwest Canada) at avery shallow angle, creating much of the mountainous terrain in the

area (particularly the southern Rocky Mountains).

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Plate tectonics (from the Late Latin tectonicus, from the Greek: τεκτονικός "pertaining to building")[2] is ascientific theory that describes the large-scale motion of Earth's lithosphere. The model builds on the concept ofcontinental drift which was developed during the first few decades of the 20th century. The geoscientific communityaccepted the theory after the concepts of seafloor spreading were developed in the late 1950s and early 1960s.The lithosphere, which is the rigid outermost shell of a planet (on Earth, the crust and upper mantle), is broken upinto tectonic plates. On Earth, there are seven or eight major plates (depending on how they are defined) and manyminor plates. Where plates meet, their relative motion determines the type of boundary; convergent, divergent, ortransform. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along these plateboundaries. The lateral relative movement of the plates typically varies from zero to 100 mm annually.[3]

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Tectonic plates are composed of oceanic lithosphere and thicker continental lithosphere, each topped by its own kindof crust. Along convergent boundaries, subduction carries plates into the mantle; the material lost is roughlybalanced by the formation of new (oceanic) crust along divergent margins by seafloor spreading. In this way, thetotal surface of the globe remains the same. This prediction of plate tectonics is also referred to as the conveyor beltprinciple. Earlier theories (that still have some supporters) propose gradual shrinking (contraction) or gradualexpansion of the globe.[4]

Tectonic plates are able to move because the Earth's lithosphere has a higher strength than the underlyingasthenosphere. Lateral density variations in the mantle result in convection. Plate movement is thought to be drivenby a combination of the motion of the seafloor away from the spreading ridge (due to variations in topography anddensity of the crust, which result in differences in gravitational forces) and drag, downward suction, at thesubduction zones. Another explanation lies in the different forces generated by the rotation of the globe and the tidalforces of the Sun and Moon. The relative importance of each of these factors and their relationship to each other isunclear, and still the subject of much debate.

Key principlesThe outer layers of the Earth are divided into the lithosphere and asthenosphere. This is based on differences inmechanical properties and in the method for the transfer of heat. Mechanically, the lithosphere is cooler and morerigid, while the asthenosphere is hotter and flows more easily. In terms of heat transfer, the lithosphere loses heat byconduction, whereas the asthenosphere also transfers heat by convection and has a nearly adiabatic temperaturegradient. This division should not be confused with the chemical subdivision of these same layers into the mantle(comprising both the asthenosphere and the mantle portion of the lithosphere) and the crust: a given piece of mantlemay be part of the lithosphere or the asthenosphere at different times depending on its temperature and pressure.The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates, which rideon the fluid-like (visco-elastic solid) asthenosphere. Plate motions range up to a typical 10–40 mm/year(Mid-Atlantic Ridge; about as fast as fingernails grow), to about 160 mm/year (Nazca Plate; about as fast as hairgrows).[5] The driving mechanism behind this movement is described separately below.Tectonic lithosphere plates consist of lithospheric mantle overlain by either or both of two types of crustal material:oceanic crust (in older texts called sima from silicon and magnesium) and continental crust (sial from silicon andaluminium). Average oceanic lithosphere is typically 100 km (62 mi) thick;[6] its thickness is a function of its age: astime passes, it conductively cools and subjacent cooling mantle is added to its base. Because it is formed atmid-ocean ridges and spreads outwards, its thickness is therefore a function of its distance from the mid-ocean ridgewhere it was formed. For a typical distance that oceanic lithosphere must travel before being subducted, thethickness varies from about 6 km (4 mi) thick at mid-ocean ridges to greater than 100 km (62 mi) at subductionzones; for shorter or longer distances, the subduction zone (and therefore also the mean) thickness becomes smalleror larger, respectively.[7] Continental lithosphere is typically ~200 km thick, though this varies considerably betweenbasins, mountain ranges, and stable cratonic interiors of continents. The two types of crust also differ in thickness,with continental crust being considerably thicker than oceanic (35 km vs. 6 km).[8]

The location where two plates meet is called a plate boundary. Plate boundaries are commonly associated withgeological events such as earthquakes and the creation of topographic features such as mountains, volcanoes,mid-ocean ridges, and oceanic trenches. The majority of the world's active volcanoes occur along plate boundaries,with the Pacific Plate's Ring of Fire being the most active and most widely known today. These boundaries arediscussed in further detail below. Some volcanoes occur in the interiors of plates, and these have been variouslyattributed to internal plate deformation[9] and to mantle plumes.As explained above, tectonic plates may include continental crust or oceanic crust, and many plates contain both. For example, the African Plate includes the continent and parts of the floor of the Atlantic and Indian Oceans. The distinction between oceanic crust and continental crust is based on their modes of formation. Oceanic crust is formed

Plate tectonics 3

at sea-floor spreading centers, and continental crust is formed through arc volcanism and accretion of terranesthrough tectonic processes, though some of these terranes may contain ophiolite sequences, which are pieces ofoceanic crust considered to be part of the continent when they exit the standard cycle of formation and spreadingcenters and subduction beneath continents. Oceanic crust is also denser than continental crust owing to their differentcompositions. Oceanic crust is denser because it has less silicon and more heavier elements ("mafic") thancontinental crust ("felsic").[10] As a result of this density stratification, oceanic crust generally lies below sea level(for example most of the Pacific Plate), while continental crust buoyantly projects above sea level (see the pageisostasy for explanation of this principle).

Types of plate boundariesMain article: List of tectonic plate interactionsThree types of plate boundaries exist,[11] with a fourth, mixed type, characterized by the way the plates move relativeto each other. They are associated with different types of surface phenomena. The different types of plate boundariesare:1. Transform boundaries (Conservative) occur where two lithospheric plates slide, or perhaps more accurately,

grind past each other along transform faults, where plates are neither created nor destroyed. The relative motionof the two plates is either sinistral (left side toward the observer) or dextral (right side toward the observer).Transform faults occur across a spreading center. Strong earthquakes can occur along fault. The San AndreasFault in California is an example of a transform boundary exhibiting dextral motion.

2. Divergent boundaries (Constructive) occur where two plates slide apart from each other. At zones ofocean-to-ocean rifting, divergent boundaries form by seafloor spreading, allowing for the formation of new oceanbasin. As the continent splits, the ridge forms at the spreading center, the ocean basin expands, and finally, theplate area increases causing many small volcanoes and/or shallow earthquakes. At zones of continent-to-continentrifting, divergent boundaries may cause new ocean basin to form as the continent splits, spreads, the central riftcollapses, and ocean fills the basin. Active zones of Mid-ocean ridges (e.g., Mid-Atlantic Ridge and East PacificRise), and continent-to-continent rifting (such as Africa's East African Rift and Valley, Red Sea) are examples ofdivergent boundaries.

3. Convergent boundaries (Destructive) (or active margins) occur where two plates slide toward each other to formeither a subduction zone (one plate moving underneath the other) or a continental collision. At zones ofcontinent-to-continent subduction (e.g., Western South America, and Cascade Mountains in Western UnitedStates), the dense oceanic lithosphere plunges beneath the less dense continent. Earthquakes then trace the path ofthe downward-moving plate as it descends into asthenosphere, a trench forms, and as the subducted plate partiallymelts, magma rises to form continental volcanoes. At zones of ocean-to-ocean subduction (e.g., the Andesmountain range in South America, Aleutian islands, Mariana islands, and the Japanese island arc), older, cooler,denser crust slips beneath less dense crust. This causes earthquakes and a deep trench to form in an arc shape. Theupper mantle of the subducted plate then heats and magma rises to form curving chains of volcanic islands. Deepmarine trenches are typically associated with subduction zones, and the basins that develop along the activeboundary are often called "foreland basins". The subducting slab contains many hydrous minerals which releasetheir water on heating. This water then causes the mantle to melt, producing volcanism. Closure of ocean basinscan occur at continent-to-continent boundaries (e.g., Himalayas and Alps): collision between masses of graniticcontinental lithosphere; neither mass is subducted; plate edges are compressed, folded, uplifted.

4. Plate boundary zones occur where the effects of the interactions are unclear, and the boundaries, usuallyoccurring along a broad belt, are not well defined and may show various types of movements in differentepisodes.

Plate tectonics 4

Three types of plate boundary.

Driving forces of plate motion

Plate motion based on Global Positioning System (GPS) satellite data from NASAJPL [12]. The vectors show direction and magnitude of motion.

Plate tectonics is basically a kinematicphenomenon. Scientists agree on theobservation and deduction that the plateshave moved with respect to one another butcontinue to debate as to how and when. Amajor question remains as to whatgeodynamic mechanism motors platemovement. Here, science diverges indifferent theories.It is generally accepted that tectonic platesare able to move because of the relativedensity of oceanic lithosphere and therelative weakness of the asthenosphere.Dissipation of heat from the mantle isacknowledged to be the original source ofthe energy required to drive plate tectonicsthrough convection or large scale upwelling and doming. The current view, though still a matter of some debate,asserts that as a consequence, a powerful source of plate motion is generated due to the excess density of the oceaniclithosphere sinking in subduction zones. When the new crust forms at mid-ocean ridges, this oceanic lithosphere isinitially less dense than the underlying asthenosphere, but it becomes denser with age as it conductively cools andthickens. The greater density of old lithosphere relative to the underlying asthenosphere allows it to sink into thedeep mantle at subduction zones, providing most of the driving force for plate movement. The weakness of theasthenosphere allows the tectonic plates to move easily towards a subduction zone. Although subduction is believedto be the strongest force driving plate motions, it cannot be the only force since there are plates such as the NorthAmerican Plate which are moving, yet are nowhere being subducted. The same is true for the enormous EurasianPlate. The sources of plate motion are a matter of intensive research and discussion among scientists. One of themain points is that the kinematic pattern of the movement itself should be separated clearly from the possiblegeodynamic mechanism that is invoked as the driving force of the observed movement, as some patterns may beexplained by more than one mechanism.[13] In short, the driving forces advocated at the moment can be divided intothree categories based on the relationship to the movement: mantle dynamics related, gravity related (mostlysecondary forces), and Earth rotation related.

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Driving forces related to mantle dynamicsMain article: Mantle convectionFor much of the last quarter century, the leading theory of the driving force behind tectonic plate motions envisagedlarge scale convection currents in the upper mantle which are transmitted through the asthenosphere. This theory waslaunched by Arthur Holmes and some forerunners in the 1930s and was immediately recognized as the solution forthe acceptance of the theory as originally discussed in the papers of Alfred Wegener in the early years of the century.However, despite its acceptance, it was long debated in the scientific community because the leading ("fixist") theorystill envisaged a static Earth without moving continents up until the major break–throughs of the early sixties.Two– and three–dimensional imaging of Earth's interior (seismic tomography) shows a varying lateral densitydistribution throughout the mantle. Such density variations can be material (from rock chemistry), mineral (fromvariations in mineral structures), or thermal (through thermal expansion and contraction from heat energy). Themanifestation of this varying lateral density is mantle convection from buoyancy forces.[14]

How mantle convection directly and indirectly relates to plate motion is a matter of ongoing study and discussion ingeodynamics. Somehow, this energy must be transferred to the lithosphere for tectonic plates to move. There areessentially two types of forces that are thought to influence plate motion: friction and gravity.• Basal drag (friction): Plate motion driven by friction between the convection currents in the asthenosphere and

the more rigid overlying lithosphere.• Slab suction (gravity): Plate motion driven by local convection currents that exert a downward pull on plates in

subduction zones at ocean trenches. Slab suction may occur in a geodynamic setting where basal tractionscontinue to act on the plate as it dives into the mantle (although perhaps to a greater extent acting on both theunder and upper side of the slab).

Lately, the convection theory has been much debated as modern techniques based on 3D seismic tomography stillfail to recognize these predicted large scale convection cells. Therefore, alternative views have been proposed:In the theory of plume tectonics developed during the 1990s, a modified concept of mantle convection currents isused. It asserts that super plumes rise from the deeper mantle and are the drivers or substitutes of the majorconvection cells. These ideas, which find their roots in the early 1930s with the so-called "fixistic" ideas of theEuropean and Russian Earth Science Schools, find resonance in the modern theories which envisage hotspots/mantle plumes which remain fixed and are overridden by oceanic and continental lithosphere plates over timeand leave their traces in the geological record (though these phenomena are not invoked as real driving mechanisms,but rather as modulators). Modern theories that continue building on the older mantle doming concepts and see platemovements as a secondary phenomena are beyond the scope of this page and are discussed elsewhere (for exampleon the plume tectonics page).Another theory is that the mantle flows neither in cells nor large plumes but rather as a series of channels just belowthe Earth's crust which then provide basal friction to the lithosphere. This theory is called "surge tectonics" andbecame quite popular in geophysics and geodynamics during the 1980s and 1990s.[15]

Driving forces related to gravityForces related to gravity are usually invoked as secondary phenomena within the framework of a more generaldriving mechanism such as the various forms of mantle dynamics described above.Gravitational sliding away from a spreading ridge: According to many authors, plate motion is driven by the higherelevation of plates at ocean ridges.[16] As oceanic lithosphere is formed at spreading ridges from hot mantle material,it gradually cools and thickens with age (and thus adds distance from the ridge). Cool oceanic lithosphere issignificantly denser than the hot mantle material from which it is derived and so with increasing thickness itgradually subsides into the mantle to compensate the greater load. The result is a slight lateral incline with increaseddistance from the ridge axis.

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This force is regarded as a secondary force and is often referred to as "ridge push". This is a misnomer as nothing is"pushing" horizontally and tensional features are dominant along ridges. It is more accurate to refer to thismechanism as gravitational sliding as variable topography across the totality of the plate can vary considerably andthe topography of spreading ridges is only the most prominent feature. Other mechanisms generating thisgravitational secondary force include flexural bulging of the lithosphere before it dives underneath an adjacent platewhich produces a clear topographical feature that can offset, or at least affect, the influence of topographical oceanridges, and mantle plumes and hot spots, which are postulated to impinge on the underside of tectonic plates.Slab-pull: Current scientific opinion is that the asthenosphere is insufficiently competent or rigid to directly causemotion by friction along the base of the lithosphere. Slab pull is therefore most widely thought to be the greatestforce acting on the plates. In this current understanding, plate motion is mostly driven by the weight of cold, denseplates sinking into the mantle at trenches.[17] Recent models indicate that trench suction plays an important role aswell. However, as the North American Plate is nowhere being subducted, yet it is in motion presents a problem. Thesame holds for the African, Eurasian, and Antarctic plates.Gravitational sliding away from mantle doming: According to older theories, one of the driving mechanisms of theplates is the existence of large scale asthenosphere/mantle domes which cause the gravitational sliding of lithosphereplates away from them. This gravitational sliding represents a secondary phenomenon of this basically verticallyoriented mechanism. This can act on various scales, from the small scale of one island arc up to the larger scale of anentire ocean basin.[18]

Driving forces related to Earth rotationAlfred Wegener, being a meteorologist, had proposed tidal forces and pole flight force as the main drivingmechanisms behind continental drift; however, these forces were considered far too small to cause continentalmotion as the concept then was of continents plowing through oceanic crust. Therefore, Wegener later changed hisposition and asserted that convection currents are the main driving force of plate tectonics in the last edition of hisbook in 1929.However, in the plate tectonics context (accepted since the seafloor spreading proposals of Heezen, Hess, Dietz,Morley, Vine, and Matthews (see below) during the early 1960s), oceanic crust is suggested to be in motion with thecontinents which caused the proposals related to Earth rotation to be reconsidered. In more recent literature, thesedriving forces are:1. Tidal drag due to the gravitational force the Moon (and the Sun) exerts on the crust of the Earth2.2. Shear strain of the Earth globe due to N-S compression related to its rotation and modulations;3. Pole flight force: equatorial drift due to rotation and centrifugal effects: tendency of the plates to move from the

poles to the equator ("Polflucht");4. The Coriolis effect acting on plates when they move around the globe;5. Global deformation of the geoid due to small displacements of rotational pole with respect to the Earth's crust;6.6. Other smaller deformation effects of the crust due to wobbles and spin movements of the Earth rotation on a

smaller time scale.For these mechanisms to be overall valid, systematic relationships should exist all over the globe between theorientation and kinematics of deformation and the geographical latitudinal and longitudinal grid of the Earth itself.Ironically, these systematic relations studies in the second half of the nineteenth century and the first half of thetwentieth century underline exactly the opposite: that the plates had not moved in time, that the deformation grid wasfixed with respect to the Earth equator and axis, and that gravitational driving forces were generally acting verticallyand caused only local horizontal movements (the so-called pre-plate tectonic, "fixist theories"). Later studies(discussed below on this page), therefore, invoked many of the relationships recognized during this pre-platetectonics period to support their theories (see the anticipations and reviews in the work of van Dijk andcollaborators).[19]

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Of the many forces discussed in this paragraph, tidal force is still highly debated and defended as a possible principledriving force of plate tectonics. The other forces are only used in global geodynamic models not using plate tectonicsconcepts (therefore beyond the discussions treated in this section) or proposed as minor modulations within theoverall plate tectonics model.In 1973, George W. Moore[20] of the USGS and R. C. Bostrom[21] presented evidence for a general westward drift ofthe Earth's lithosphere with respect to the mantle. He concluded that tidal forces (the tidal lag or "friction") caused bythe Earth's rotation and the forces acting upon it by the Moon are a driving force for plate tectonics. As the Earthspins eastward beneath the moon, the moon's gravity ever so slightly pulls the Earth's surface layer back westward,just as proposed by Alfred Wegener (see above). In a more recent 2006 study,[22] scientists reviewed and advocatedthese earlier proposed ideas. It has also been suggested recently in Lovett (2006) that this observation may alsoexplain why Venus and Mars have no plate tectonics, as Venus has no moon and Mars' moons are too small to havesignificant tidal effects on the planet. In a recent paper,[23] it was suggested that, on the other hand, it can easily beobserved that many plates are moving north and eastward, and that the dominantly westward motion of the Pacificocean basins derives simply from the eastward bias of the Pacific spreading center (which is not a predictedmanifestation of such lunar forces). In the same paper the authors admit, however, that relative to the lower mantle,there is a slight westward component in the motions of all the plates. They demonstrated though that the westwarddrift, seen only for the past 30 Ma, is attributed to the increased dominance of the steadily growing and acceleratingPacific plate. The debate is still open.

Relative significance of each driving force mechanismThe actual vector of a plate's motion is a function of all the forces acting on the plate; however, therein lies theproblem regarding what degree each process contributes to the overall motion of each tectonic plate.The diversity of geodynamic settings and the properties of each plate must clearly result from differences in thedegree to which multiple processes are actively driving each individual plate. One method of dealing with thisproblem is to consider the relative rate at which each plate is moving and to consider the available evidence of eachdriving force on the plate as far as possible.One of the most significant correlations found is that lithospheric plates attached to downgoing (subducting) platesmove much faster than plates not attached to subducting plates. The Pacific plate, for instance, is essentiallysurrounded by zones of subduction (the so-called Ring of Fire) and moves much faster than the plates of the Atlanticbasin, which are attached (perhaps one could say 'welded') to adjacent continents instead of subducting plates. It isthus thought that forces associated with the downgoing plate (slab pull and slab suction) are the driving forces whichdetermine the motion of plates, except for those plates which are not being subducted.[17] The driving forces of platemotion continue to be active subjects of on-going research within geophysics and tectonophysics.

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Development of the theoryFurther information: Timeline of the development of tectonophysics

Summary

Detailed map showing the tectonic plates with their movement vectors.

In line with other previous andcontemporaneous proposals, in 1912the meteorologist Alfred Wegeneramply described what he calledcontinental drift, expanded in his 1915book The Origin of Continents andOceans[24] and the scientific debatestarted that would end up fifty yearslater in the theory of plate tectonics.[25]

Starting from the idea (also expressedby his forerunners) that the presentcontinents once formed a single landmass (which was called Pangea lateron) that drifted apart, thus releasing the continents from the Earth's mantle and likening them to "icebergs" of lowdensity granite floating on a sea of denser basalt.[26] Supporting evidence for the idea came from the dove-tailingoutlines of South America's east coast and Africa's west coast, and from the matching of the rock formations alongthese edges. Confirmation of their previous contiguous nature also came from the fossil plants Glossopteris andGangamopteris, and the therapsid or mammal-like reptile Lystrosaurus, all widely distributed over South America,Africa, Antarctica, India and Australia. The evidence for such an erstwhile joining of these continents was patent tofield geologists working in the southern hemisphere. The South African Alex du Toit put together a mass of suchinformation in his 1937 publication Our Wandering Continents, and went further than Wegener in recognising thestrong links between the Gondwana fragments.

But without detailed evidence and a force sufficient to drive the movement, the theory was not generally accepted:the Earth might have a solid crust and mantle and a liquid core, but there seemed to be no way that portions of thecrust could move around. Distinguished scientists, such as Harold Jeffreys and Charles Schuchert, were outspokencritics of continental drift.Despite much opposition, the view of continental drift gained support and a lively debate started between "drifters"or "mobilists" (proponents of the theory) and "fixists" (opponents). During the 1920s, 1930s and 1940s, the formerreached important milestones proposing that convection currents might have driven the plate movements, and thatspreading may have occurred below the sea within the oceanic crust. Concepts close to the elements nowincorporated in plate tectonics were proposed by geophysicists and geologists (both fixists and mobilists) likeVening-Meinesz, Holmes, and Umbgrove.One of the first pieces of geophysical evidence that was used to support the movement of lithospheric plates came from paleomagnetism. This is based on the fact that rocks of different ages show a variable magnetic field direction, evidenced by studies since the mid–nineteenth century. The magnetic north and south poles reverse through time, and, especially important in paleotectonic studies, the relative position of the magnetic north pole varies through time. Initially, during the first half of the twentieth century, the latter phenomenon was explained by introducing what was called "polar wander" (see apparent polar wander), i.e., it was assumed that the north pole location had been shifting through time. An alternative explanation, though, was that the continents had moved (shifted and rotated) relative to the north pole, and each continent, in fact, shows its own "polar wander path". During the late 1950s it was successfully shown on two occasions that these data could show the validity of continental drift: by

Plate tectonics 9

Keith Runcorn in a paper in 1956,[27] and by Warren Carey in a symposium held in March 1956.[28]

The second piece of evidence in support of continental drift came during the late 1950s and early 60s from data onthe bathymetry of the deep ocean floors and the nature of the oceanic crust such as magnetic properties and, moregenerally, with the development of marine geology[29] which gave evidence for the association of seafloor spreadingalong the mid-oceanic ridges and magnetic field reversals, published between 1959 and 1963 by Heezen, Dietz,Hess, Mason, Vine & Matthews, and Morley.[30]

Simultaneous advances in early seismic imaging techniques in and around Wadati-Benioff zones along the trenchesbounding many continental margins, together with many other geophysical (e.g. gravimetric) and geologicalobservations, showed how the oceanic crust could disappear into the mantle, providing the mechanism to balance theextension of the ocean basins with shortening along its margins.All this evidence, both from the ocean floor and from the continental margins, made it clear around 1965 thatcontinental drift was feasible and the theory of plate tectonics, which was defined in a series of papers between 1965and 1967, was born, with all its extraordinary explanatory and predictive power. The theory revolutionized the Earthsciences, explaining a diverse range of geological phenomena and their implications in other studies such aspaleogeography and paleobiology.

Continental driftFor more details on this topic, see Continental drift.In the late 19th and early 20th centuries, geologists assumed that the Earth's major features were fixed, and that mostgeologic features such as basin development and mountain ranges could be explained by vertical crustal movement,described in what is called the geosynclinal theory. Generally, this was placed in the context of a contracting planetEarth due to heat loss in the course of a relatively short geological time.

Alfred Wegener in Greenland in the winter of 1912-13.

It was observed as early as 1596 that the opposite coasts ofthe Atlantic Ocean—or, more precisely, the edges of thecontinental shelves—have similar shapes and seem to haveonce fitted together.[31]

Since that time many theories were proposed to explain thisapparent complementarity, but the assumption of a solidEarth made these various proposals difficult to accept.[32]

The discovery of radioactivity and its associated heatingproperties in 1895 prompted a re-examination of theapparent age of the Earth.[33] This had previously beenestimated by its cooling rate and assumption the Earth'ssurface radiated like a black body.[34] Those calculationshad implied that, even if it started at red heat, the Earth would have dropped to its present temperature in a few tensof millions of years. Armed with the knowledge of a new heat source, scientists realized that the Earth would bemuch older, and that its core was still sufficiently hot to be liquid.

By 1915, after having published a first article in 1912,[35] Alfred Wegener was making serious arguments for the idea of continental drift in the first edition of The Origin of Continents and Oceans.[24] In that book (re-issued in four successive editions up to the final one in 1936), he noted how the east coast of South America and the west coast of Africa looked as if they were once attached. Wegener was not the first to note this (Abraham Ortelius, Antonio Snider-Pellegrini, Eduard Suess, Roberto Mantovani and Frank Bursley Taylor preceded him just to mention a few), but he was the first to marshal significant fossil and paleo-topographical and climatological evidence to support this simple observation (and was supported in this by researchers such as Alex du Toit). Furthermore, when the rock strata of the margins of separate continents are very similar it suggests that these rocks were formed in the same way,

Plate tectonics 10

implying that they were joined initially. For instance, parts of Scotland and Ireland contain rocks very similar tothose found in Newfoundland and New Brunswick. Furthermore, the Caledonian Mountains of Europe and parts ofthe Appalachian Mountains of North America are very similar in structure and lithology.However, his ideas were not taken seriously by many geologists, who pointed out that there was no apparentmechanism for continental drift. Specifically, they did not see how continental rock could plow through the muchdenser rock that makes up oceanic crust. Wegener could not explain the force that drove continental drift, and hisvindication did not come until after his death in 1930.

Floating continents, paleomagnetism, and seismicity zones

Global earthquake epicenters, 1963–1998

As it was observed early that althoughgranite existed on continents, seafloorseemed to be composed of denser basalt, theprevailing concept during the first half ofthe twentieth century was that there weretwo types of crust, named "sial" (continentaltype crust) and "sima" (oceanic type crust).Furthermore, it was supposed that a staticshell of strata was present under thecontinents. It therefore looked apparent thata layer of basalt (sial) underlies thecontinental rocks.However, based on abnormalities in plumbline deflection by the Andes in Peru, PierreBouguer had deduced that less-dense mountains must have a downward projection into the denser layer underneath.The concept that mountains had "roots" was confirmed by George B. Airy a hundred years later, during study ofHimalayan gravitation, and seismic studies detected corresponding density variations. Therefore, by the mid-1950s,the question remained unresolved as to whether mountain roots were clenched in surrounding basalt or were floatingon it like an iceberg.

During the 20th century, improvements in and greater use of seismic instruments such as seismographs enabledscientists to learn that earthquakes tend to be concentrated in specific areas, most notably along the oceanic trenchesand spreading ridges. By the late 1920s, seismologists were beginning to identify several prominent earthquakezones parallel to the trenches that typically were inclined 40–60° from the horizontal and extended several hundredkilometers into the Earth. These zones later became known as Wadati-Benioff zones, or simply Benioff zones, inhonor of the seismologists who first recognized them, Kiyoo Wadati of Japan and Hugo Benioff of the United States.The study of global seismicity greatly advanced in the 1960s with the establishment of the Worldwide StandardizedSeismograph Network (WWSSN) to monitor the compliance of the 1963 treaty banning above-ground testing ofnuclear weapons. The much improved data from the WWSSN instruments allowed seismologists to map preciselythe zones of earthquake concentration world wide.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 had occurred because continents seemed to have movedthrough different climatic zones during the past. Furthermore, paleomagnetic data had shown that the magnetic polehad also shifted during time. Reasoning in an opposite way, the continents might have shifted and rotated, while thepole remained relatively fixed. The first time the evidence of magnetic polar wander was used to support themovements of continents was in a paper by Keith Runcorn in 1956,[27] and successive papers by him and hisstudents Ted Irving (who was actually the first to be convinced of the fact that paleomagnetism supported continentaldrift) and Ken Creer.

Plate tectonics 11

This was immediately followed by a symposium in Tasmania in March 1956.[36] In this symposium, the evidencewas used in the theory of an expansion of the global crust. In this hypothesis the shifting of the continents can besimply explained by a large increase in size of the Earth since its formation. However, this was unsatisfactorybecause its supporters could offer no convincing mechanism to produce a significant expansion of the Earth.Certainly there is no evidence that the moon has expanded in the past 3 billion years; other work would soon showthat the evidence was equally in support of continental drift on a globe with a stable radius.During the thirties up to the late fifties, works by Vening-Meinesz, Holmes, Umbgrove, and numerous othersoutlined concepts that were close or nearly identical to modern plate tectonics theory. In particular, the Englishgeologist Arthur Holmes proposed in 1920 that plate junctions might lie beneath the sea, and in 1928 that convectioncurrents within the mantle might be the driving force.[37] Often, these contributions are forgotten because:•• At the time, continental drift was not accepted.•• Some of these ideas were discussed in the context of abandoned fixistic ideas of a deforming globe without

continental drift or an expanding Earth.•• They were published during an episode of extreme political and economic instability that hampered scientific

communication.•• Many were published by European scientists and at first not mentioned or given little credit in the papers on sea

floor spreading published by the American researchers in the 1960s.

Mid-oceanic ridge spreading and convectionFor more details on Mid-ocean ridge, see Seafloor spreading.In 1947, a team of scientists led by Maurice Ewing utilizing the Woods Hole Oceanographic Institution's researchvessel Atlantis and an array of instruments, confirmed the existence of a rise in the central Atlantic Ocean, and foundthat the floor of the seabed beneath the layer of sediments consisted of basalt, not the granite which is the mainconstituent of continents. They also found that the oceanic crust was much thinner than continental crust. All thesenew findings raised important and intriguing questions.[38]

The new data that had been collected on the ocean basins also showed particular characteristics regarding thebathymetry. One of the major outcomes of these datasets was that all along the globe, a system of mid-oceanic ridgeswas detected. An important conclusion was that along this system, new ocean floor was being created, which led tothe concept of the "Great Global Rift". This was described in the crucial paper of Bruce Heezen (1960),[39] whichwould trigger a real revolution in thinking. A profound consequence of seafloor spreading is that new crust was, andstill is, being continually created along the oceanic ridges. Therefore, Heezen advocated the so-called "expandingEarth" hypothesis of S. Warren Carey (see above). So, still the question remained: how can new crust becontinuously added along the oceanic ridges without increasing the size of the Earth? In reality, this question hadbeen solved already by numerous scientists during the forties and the fifties, like Arthur Holmes, Vening-Meinesz,Coates and many others: The crust in excess disappeared along what were called the oceanic trenches, whereso-called "subduction" occurred. Therefore, when various scientists during the early sixties started to reason on thedata at their disposal regarding the ocean floor, the pieces of the theory quickly fell into place.The question particularly intrigued Harry Hammond Hess, a Princeton University geologist and a Naval ReserveRear Admiral, and Robert S. Dietz, a scientist with the U.S. Coast and Geodetic Survey who first coined the termseafloor spreading. Dietz and Hess (the former published the same idea one year earlier in Nature,[40] but prioritybelongs to Hess who had already distributed an unpublished manuscript of his 1962 article by 1960)[41] were amongthe small handful who really understood the broad implications of sea floor spreading and how it would eventuallyagree with the, at that time, unconventional and unaccepted ideas of continental drift and the elegant and mobilisticmodels proposed by previous workers like Holmes.In the same year, Robert R. Coats of the U.S. Geological Survey described the main features of island arc subduction in the Aleutian Islands. His paper, though little noted (and even ridiculed) at the time, has since been called

Plate tectonics 12

"seminal" and "prescient". In reality, it actually shows that the work by the European scientists on island arcs andmountain belts performed and published during the 1930s up until the 1950s was applied and appreciated also in theUnited States.If the Earth's crust was expanding along the oceanic ridges, Hess and Dietz reasoned like Holmes and others beforethem, it must be shrinking elsewhere. Hess followed Heezen, suggesting that new oceanic crust continuously spreadsaway from the ridges in a conveyor belt–like motion. And, using the mobilistic concepts developed before, hecorrectly concluded that many millions of years later, the oceanic crust eventually descends along the continentalmargins where oceanic trenches – very deep, narrow canyons – are formed, e.g. along the rim of the Pacific Oceanbasin. The important step Hess made was that convection currents would be the driving force in this process, arrivingat the same conclusions as Holmes had decades before with the only difference that the thinning of the ocean crustwas performed using Heezen's mechanism of spreading along the ridges. Hess therefore concluded that the AtlanticOcean was expanding while the Pacific Ocean was shrinking. As old oceanic crust is "consumed" in the trenches(like Holmes and others, he thought this was done by thickening of the continental lithosphere, not, as nowunderstood, by underthrusting at a larger scale of the oceanic crust itself into the mantle), new magma rises anderupts along the spreading ridges to form new crust. In effect, the ocean basins are perpetually being "recycled," withthe creation of new crust and the destruction of old oceanic lithosphere occurring simultaneously. Thus, the newmobilistic concepts neatly explained why the Earth does not get bigger with sea floor spreading, why there is so littlesediment accumulation on the ocean floor, and why oceanic rocks are much younger than continental rocks.

Magnetic striping

Seafloor magnetic striping.

A demonstration of magnetic striping. (Thedarker the color is the closer it is to normal

polarity)

For more details on this topic, see Vine–Matthews–Morley hypothesis.Beginning in the 1950s, scientists like Victor Vacquier, using magneticinstruments (magnetometers) adapted from airborne devices developedduring World War II to detect submarines, began recognizing oddmagnetic variations across the ocean floor. This finding, thoughunexpected, was not entirely surprising because it was known thatbasalt—the iron-rich, volcanic rock making up the oceanfloor—contains a strongly magnetic mineral (magnetite) and canlocally distort compass readings. This distortion was recognized byIcelandic mariners as early as the late 18th century. More important,because the presence of magnetite gives the basalt measurablemagnetic properties, these newly discovered magnetic variationsprovided another means to study the deep ocean floor. When newlyformed rock cools, such magnetic materials recorded the Earth'smagnetic field at the time.

As more and more of the seafloor was mapped during the 1950s, themagnetic variations turned out not to be random or isolatedoccurrences, but instead revealed recognizable patterns. When thesemagnetic patterns were mapped over a wide region, the ocean floorshowed a zebra-like pattern: one stripe with normal polarity and theadjoining stripe with reversed polarity. The overall pattern, defined bythese alternating bands of normally and reversely polarized rock,became known as magnetic striping, and was published by Ron G. Mason and co-workers in 1961, who did not find,though, an explanation for these data in terms of sea floor spreading, like Vine, Matthews and Morley a few yearslater.[42]

Plate tectonics 13

The discovery of magnetic striping called for an explanation. In the early 1960s scientists such as Heezen, Hess andDietz had begun to theorise that mid-ocean ridges mark structurally weak zones where the ocean floor was beingripped in two lengthwise along the ridge crest (see the previous paragraph). New magma from deep within the Earthrises easily through these weak zones and eventually erupts along the crest of the ridges to create new oceanic crust.This process, at first denominated the "conveyer belt hypothesis" and later called seafloor spreading, operating overmany millions of years continues to form new ocean floor all across the 50,000 km-long system of mid–oceanridges.Only four years after the maps with the "zebra pattern" of magnetic stripes were published, the link between sea floorspreading and these patterns was correctly placed, independently by Lawrence Morley, and by Fred Vine andDrummond Matthews, in 1963[43] now called the Vine-Matthews-Morley hypothesis. This hypothesis linked thesepatterns to geomagnetic reversals and was supported by several lines of evidence:[44]

1.1. the stripes are symmetrical around the crests of the mid-ocean ridges; at or near the crest of the ridge, the rocksare very young, and they become progressively older away from the ridge crest;

2.2. the youngest rocks at the ridge crest always have present-day (normal) polarity;3.3. stripes of rock parallel to the ridge crest alternate in magnetic polarity (normal-reversed-normal, etc.), suggesting

that they were formed during different epochs documenting the (already known from independent studies) normaland reversal episodes of the Earth's magnetic field.

By explaining both the zebra-like magnetic striping and the construction of the mid-ocean ridge system, the seafloorspreading hypothesis (SFS) quickly gained converts and represented another major advance in the development ofthe plate-tectonics theory. Furthermore, the oceanic crust now came to be appreciated as a natural "tape recording" ofthe history of the geomagnetic field reversals (GMFR) of the Earth's magnetic field. Today, extensive studies arededicated to the calibration of the normal-reversal patterns in the oceanic crust on one hand and known timescalesderived from the dating of basalt layers in sedimentary sequences (magnetostratigraphy) on the other, to arrive atestimates of past spreading rates and plate reconstructions.

Definition and refining of the theoryAfter all these considerations, Plate Tectonics (or, as it was initially called "New Global Tectonics") became quicklyaccepted in the scientific world, and numerous papers followed that defined the concepts:• In 1965, Tuzo Wilson who had been a promotor of the sea floor spreading hypothesis and continental drift from

the very beginning[45] added the concept of transform faults to the model, completing the classes of fault typesnecessary to make the mobility of the plates on the globe work out.[46]

• A symposium on continental drift was held at the Royal Society of London in 1965 which must be regarded as theofficial start of the acceptance of plate tectonics by the scientific community, and which abstracts are issued asBlacket, Bullard & Runcorn (1965). In this symposium, Edward Bullard and co-workers showed with a computercalculation how the continents along both sides of the Atlantic would best fit to close the ocean, which becameknown as the famous "Bullard's Fit".

• In 1966 Wilson published the paper that referred to previous plate tectonic reconstructions, introducing theconcept of what is now known as the "Wilson Cycle".[47]

• In 1967, at the American Geophysical Union's meeting, W. Jason Morgan proposed that the Earth's surfaceconsists of 12 rigid plates that move relative to each other.[48]

• Two months later, Xavier Le Pichon published a complete model based on 6 major plates with their relativemotions, which marked the final acceptance by the scientific community of plate tectonics.[49]

• In the same year, McKenzie and Parker independently presented a model similar to Morgan's using translationsand rotations on a sphere to define the plate motions.[50]

Plate tectonics 14

Implications for biogeographyContinental drift theory helps biogeographers to explain the disjunct biogeographic distribution of present day lifefound on different continents but having similar ancestors.[51] In particular, it explains the Gondwanan distribution ofratites and the Antarctic flora.

Plate reconstructionMain article: Plate reconstructionReconstruction is used to establish past (and future) plate configurations, helping determine the shape and make-upof ancient supercontinents and providing a basis for paleogeography.

Defining plate boundariesCurrent plate boundaries are defined by their seismicity.[52] Past plate boundaries within existing plates are identifiedfrom a variety of evidence, such as the presence of ophiolites that are indicative of vanished oceans.[53]

Past plate motionsTectonic motion first began around three billion years ago.Various types of quantitative and semi-quantitative information are available to constrain past plate motions. Thegeometric fit between continents, such as between west Africa and South America is still an important part of platereconstruction. Magnetic stripe patterns provide a reliable guide to relative plate motions going back into the Jurassicperiod. The tracks of hotspots give absolute reconstructions, but these are only available back to the Cretaceous.[54]

Older reconstructions rely mainly on paleomagnetic pole data, although these only constrain the latitude and rotation,but not the longitude. Combining poles of different ages in a particular plate to produce apparent polar wander pathsprovides a method for comparing the motions of different plates through time.[55] Additional evidence comes fromthe distribution of certain sedimentary rock types, faunal provinces shown by particular fossil groups, and theposition of orogenic belts.[54]

Formation and break-up of continentsThe movement of plates has caused the formation and break-up of continents over time, including occasionalformation of a supercontinent that contains most or all of the continents. The supercontinent Columbia or Nunaformed during a period of 2,000 to 1,800 [56] million years ago and broke up about 1,500 to 1,300 [57] million yearsago. The supercontinent Rodinia is thought to have formed about 1 billion years ago and to have embodied most orall of Earth's continents, and broken up into eight continents around 600 [58] million years ago. The eight continentslater re-assembled into another supercontinent called Pangaea; Pangaea broke up into Laurasia (which became NorthAmerica and Eurasia) and Gondwana (which became the remaining continents).The Himalayas, the world's tallest mountain range, are assumed to have been formed by the collision of two majorplates. Before uplift, they were covered by the Tethys Ocean.

Plate tectonics 15

Current platesMain article: List of tectonic platesDepending on how they are defined,there are usually seven or eight"major" plates: African, Antarctic,Eurasian, North American, SouthAmerican, Pacific, andIndo-Australian. The latter issometimes subdivided into the Indianand Australian plates.

There are dozens of smaller plates, theseven largest of which are the Arabian,Caribbean, Juan de Fuca, Cocos,Nazca, Philippine Sea and Scotia.

The current motion of the tectonicplates is today determined by remotesensing satellite data sets, calibrated with ground station measurements.

Other celestial bodies (planets, moons)The appearance of plate tectonics on terrestrial planets is related to planetary mass, with more massive planets thanEarth expected to exhibit plate tectonics. Earth may be a borderline case, owing its tectonic activity to abundantwater [59] (silica and water form a deep eutectic.)

VenusSee also: Geology of VenusVenus shows no evidence of active plate tectonics. There is debatable evidence of active tectonics in the planet'sdistant past; however, events taking place since then (such as the plausible and generally accepted hypothesis that theVenusian lithosphere has thickened greatly over the course of several hundred million years) has made constrainingthe course of its geologic record difficult. However, the numerous well-preserved impact craters have been utilizedas a dating method to approximately date the Venusian surface (since there are thus far no known samples ofVenusian rock to be dated by more reliable methods). Dates derived are dominantly in the range 500 to 750 [60]

million years ago, although ages of up to 1,200 [61] million years ago have been calculated. This research has led tothe fairly well accepted hypothesis that Venus has undergone an essentially complete volcanic resurfacing at leastonce in its distant past, with the last event taking place approximately within the range of estimated surface ages.While the mechanism of such an impressive thermal event remains a debated issue in Venusian geosciences, somescientists are advocates of processes involving plate motion to some extent.One explanation for Venus' lack of plate tectonics is that on Venus temperatures are too high for significant water tobe present.[62] The Earth's crust is soaked with water, and water plays an important role in the development of shearzones. Plate tectonics requires weak surfaces in the crust along which crustal slices can move, and it may well be thatsuch weakening never took place on Venus because of the absence of water. However, some researchers remainconvinced that plate tectonics is or was once active on this planet.

Plate tectonics 16

MarsSee also: Geology of MarsMars is considerably smaller than Earth and Venus, and there is evidence for ice on its surface and in its crust.In the 1990s, it was proposed that Martian Crustal Dichotomy was created by plate tectonic processes.[63] Scientiststoday disagree, and believe that it was created either by upwelling within the Martian mantle that thickened the crustof the Southern Highlands and formed Tharsis[64] or by a giant impact that excavated the Northern Lowlands.[65]

Valles Marineris is a tectonic boundary.Observations made of the magnetic field of Mars by the Mars Global Surveyor spacecraft in 1999 showed patterns ofmagnetic striping discovered on this planet. Some scientists interpreted these as requiring plate tectonic processes,such as seafloor spreading. However, their data fail a "magnetic reversal test", which is used to see if they wereformed by flipping polarities of a global magnetic field.[66]

Galilean satellites of JupiterSome of the satellites of Jupiter have features that may be related to plate-tectonic style deformation, although thematerials and specific mechanisms may be different from plate-tectonic activity on Earth.

Titan, moon of SaturnTitan, the largest moon of Saturn, was reported to show tectonic activity in images taken by the Huygens Probe,which landed on Titan on January 14, 2005.[67]

ExoplanetsOn Earth-sized planets, plate tectonics is more likely if there are oceans of water; however, in 2007, two independentteams of researchers came to opposing conclusions about the likelihood of plate tectonics on largersuper-earths[68][69] with one team saying that plate tectonics would be episodic or stagnant[70] and the other teamsaying that plate tectonics is very likely on super-earths even if the planet is dry.[59]

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Plate tectonics 17

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Plate tectonics 24

External links

Wikimedia Commons has media related to Plate tectonics.

• This Dynamic Earth: The Story of Plate Tectonics (http:/ / pubs. usgs. gov/ gip/ dynamic/ dynamic. html). USGS.• Understanding Plate Tectonics (http:/ / pubs. usgs. gov/ publications/ text/ understanding. html). USGS.• The PLATES Project (http:/ / www. ig. utexas. edu/ research/ projects/ plates/ ). Jackson School of Geosciences.• An explanation of tectonic forces (http:/ / www. tectonic-forces. org). Example of calculations to show that Earth

Rotation could be a driving force.• Bird, P. (2003); An updated digital model of plate boundaries (http:/ / peterbird. name/ publications/

2003_PB2002/ 2003_PB2002. htm).• Map of tectonic plates (http:/ / snobear. colorado. edu/ Markw/ Mountains/ 03/ week3. html).• GPlates (http:/ / www. gplates. org/ ), desktop software for the interactive visualization of plate-tectonics.• MORVEL plate velocity estimates and information (http:/ / www. geology. wisc. edu/ ~chuck/ MORVEL/ ). C.

DeMets, D. Argus, & R. Gordon.• Google Map of the Topography of Plate Tectonics (http:/ / maps. google. com/ maps/

ms?msid=202977755949863934429. 0004f306429540aff190e& msa=0& ll=-2. 811371,-22. 5& spn=139.996839,320. 625) that enables you to zoom in on submarine mid ocean ridges, fracture zones, ocean trenches,thermal vents and submarine volcanoes.

Videos• Khan Academy Explanation of evidence (http:/ / www. youtube. com/ watch?v=6EdsBabSZ4g)• 750 million years of global tectonic activity (http:/ / www. ucmp. berkeley. edu/ geology/ tectonics. html). Movie.

Article Sources and Contributors 25

Article Sources and ContributorsPlate tectonics  Source: http://en.wikipedia.org/w/index.php?oldid=617913110  Contributors: (jarbarf), -- April, 2help, A.M.962, A876, AJR, AManWithNoPlan, Aaron Brenneman, Abeg92,Abune, Academic Challenger, Acalamari, Adashiel, AdjustShift, AeonicOmega, Agathman, Agüeybaná, Ahoerstemeier, Aillema, Aircough, Airhogs777, Ajaxkroon, Alansohn, AlbertBowes,Aldaron, Ale jrb, Aleksander.adamowski, AlexRampaul, AlexiusHoratius, Alison, Allmightyduck, Amplitude101, Ams80, Anastrophe, Anclation, Andkore, Andrew c, Andrew the science guy,Andrewharold, Andrewrost3241981, Andrewrp, Androstachys, Andy M. 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