Plate Tectonics

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Plate tectonics (from Greek language τέκτων, tektōn "builder" or "mason") is a theory of geology that has been developed to explain the observed evidence for large scale motions of the Earth's lithosphere. The theory encompassed and superseded the older theory of continental drift from the first half of the 20th century and the concept of seafloor spreading developed during the 1960s.

The outermost part of the Earth's interior is made up of two layers: above is the lithosphere, comprising the Crust (geology) and the rigid uppermost part of the Earth's mantle.Below the lithosphere lies the asthenosphere. Although solid, the asthenosphere has relatively low viscosity and shear strength (soil) and can flow like a liquid on geological time scales. The deeper mantle below the asthenosphere is more rigid again. This is, however, not due to cooler temperatures but due to high pressure.

The lithosphere is broken up into what are called tectonic plates—in the case of Earth, there are seven major and many minor plates (#Major plates). The lithospheric plates ride on the asthenosphere. These plates move in relation to one another at one of three types of plate boundaries: Convergent boundary or collision boundaries, Divergent boundary or spreading boundaries, and Transform fault boundaries. Earthquakes, volcano, mountain-building, and oceanic trench formation occur along plate boundaries. The lateral movement of the plates is typically at speeds of 0.65 to 8.50 centimeters per year (the speed at which human nails grow).

Synopsis of the development of the theory In the late ninteenth and early twentieth centuries, geologists assumed that the Earth's major features were fixed, and that most geologic features such as mountain ranges could be explained by vertical crustal movement, as explained by geosyncline. The observations had been made that the opposite coasts of the Atlantic Ocean — or, more precisely, the edges of the continental shelves — have similar shapes and seem once to have fitted together. Since that time many theories were proposed to explain this apparent coincidence, but the assumption of a solid earth made the various proposals difficult to explain.

The discovery of radium and its associated heating properties in 1896 prompted a re-examination of the apparent age of the Earth, since this had been estimated by taking its temperature and assuming that it radiated like a black body. Such calculations assumed that, even if it started at red heat, the Earth would have dropped to its present temperature in a few tens of millions of years. With this new heat source, it was credible that the Earth was much older, and also that its core was still sufficiently hot to be liquid.

Plate tectonic theory arose out of the hypothesis of continental drift first proposed by Alfred Wegener in 1912 and expanded in his 1915 book The Origin of Continents and Oceans, which suggested that the present continents once formed a single land mass which had drifted apart, floating on the molten rocks of the core. But without detailed evidence and calculation of the forces involved, the theory remained sidelined. The Earth might have a solid crust and a liquid core, but there seemed to be no way that portions of the crust could move around -- although later science proved theories proposed by English geologist Arthur Holmes in 1920 that their junctions might actually lie beneath the sea.

The first evidence that crust plates did move around came with the discovery of variable magnetic field direction in rocks of differing ages, first revealed at a symposium in Tasmania in 1956. Initially theorized as an Expanding earth theory,1958: The tectonic approach to continental drift. In: S. W. Carey (ed.): Continental Drift – A Symposium. University of Tasmania, Hobart, 177-363 (expanding Earth from p. 311 to p. 349) later collaborations developed the plate tectonics theory, which accounted for spreading as the consequence of new rock upwelling, but avoided the need for an expanding globe by recognizing subduction zones and conservative translation faults. It was at this point that Wegener's theory moved from radical to mainstream, and became accepted by the scientific community. Additional work on the association of seafloor spreading and magnetic field reversals by Harry Hess and Ron G. Mason Ron Mason's key work pinpointed the precise mechanism which accounted for new rock upwelling.

Following the recognition of Geomagnetic reversal defined by symmetric, parallel stripes of similar magnetization on the seafloor on either side of a mid-ocean ridge, plate tectonics quickly became broadly accepted. Simultaneous advances in early seismic imaging techniques in and around Wadati-Benioff zones collectively with numerous other geologic observations soon solidified plate tectonics as a theory with extraordinary explanatory and predictive power.

Study of the deep ocean floor was critical to development of the theory; the field of deep sea marine geology accelerated in the 1960s. Correspondingly, plate tectonic theory was developed during the late 1960s and has since been accepted all but universally by scientists throughout all geoscientific disciplines. The theory revolutionized the Earth sciences, explaining a diverse range of geological phenomena.

Key principles The division of the outer parts of the Earth's interior into lithosphere and asthenosphere is based on mechanics differences and in the ways that heat is transferred. The lithosphere is cooler and more rigid, whilst the asthenosphere is hotter and mechanically weaker. Also, the lithosphere loses heat by Heat conduction whereas the asthenosphere also transfers heat by convection and has a nearly adiabatic temperature gradient. This division should not be confused with the chemical subdivision of the Earth into (from innermost to outermost) core, mantle, and crust. The lithosphere contains both crust and some mantle. A given piece of mantle may be part of the lithosphere or the asthenosphere at different times, depending on its temperature, pressure and shear strength. The key principle of plate tectonics is that the lithosphere exists as separate and distinct List of tectonic platess, which ride on the fluid-like (visco-elastic solid) asthenosphere. Plate motions range from a few millimeters per year (mm yr-1), to a more typical 10-40 mm yr-1 (Mid-Atlantic Ridge; about as fast as fingernails grow), to about 160 mm yr-1 (Nazca Plate; about as fast as hair grows). Speed of the Continental Plates{{Citation | last1 = Hancock | first1 = Paul L. | last2 = Skinner | first2 = Brian J. | last3 = Dineley | first3 = David L. | title = The Oxford Companion to The Earth | publisher = [Oxford University Press | year = 2000 | isbn = 0-19-854039-6 -->

The plates are around 100 km (60 miles) thick and consist of lithospheric mantle overlain by either of two types of crustal material: oceanic crust (in older texts called Sima (geology) from silicon and magnesium) and continental crust (sial from silicon and aluminium). The two types of crust differ in thickness, with continental crust considerably thicker than oceanic (50 km vs 5 km).

One plate meets another along a plate boundary, and plate boundaries are commonly associated with geological events such as earthquakes and the creation of topographic features like mountains, volcanoes and oceanic trenches. The majority of the world's active volcanoes occur along plate boundaries, with the Pacific Plate's Pacific Ring of Fire being most active and most widely known. These boundaries are discussed in further detail below.

Tectonic plates can include continental crust or oceanic crust, and typically, a single plate carries both. For example, the African Plate includes the continent and parts of the floor of the Atlantic and Indian Oceans. The distinction between continental crust and oceanic crust is based on the density of constituent materials; oceanic crust is denser than continental crust owing to their different proportions of various elements, particularly, silicon. Oceanic crust is denser because it has less silicon and more heavier elements ("mafic") than continental crust ("felsic"). As a result, oceanic crust generally lies below sea level (for example most of the Pacific Plate), while the continental crust projects above sea level (see isostasy for explanation of this principle).

Types of plate boundaries Three types of plate boundaries exist, characterized by the way the plates move relative to each other. They are associated with different types of surface phenomena. The different types of plate boundaries are:

#Transform (conservative) boundaries occur where plates slide or, perhaps more accurately, grind past each other along transform faults. The relative motion of the two plates is either sinistral (left side toward the observer) or dextral (right side toward the observer). The San Andreas Fault in California is one example.

#Divergent (constructive) boundaries occur where two plates slide apart from each other. Mid-ocean ridges (e.g., Mid-Atlantic Ridge) and active zones of rifting (such as Africa's Great Rift Valley) are both examples of divergent boundaries.

#Convergent (destructive) boundaries (or active margins) occur where two plates slide towards each other commonly forming either a subduction zone (if one plate moves underneath the other) or a orogeny (if the two plates contain continental crust). Deep marine trenches are typically associated with subduction zones. Because of friction and heating of the subducting slab, volcanism is almost always closely linked. Examples of this are the Andes mountain range in South America and the Japanese island arc.

Transform (conservative) boundaries John Tuzo Wilson recognized that because of friction, the plates cannot simply glide past each other. Rather, stress (physics) builds up in both plates and when it reaches a level that exceeds the strain threshold of rocks on either side of the fault the accumulated potential energy is released as Strain (materials science). Strain is both accumulative and/or instantaneous depending on the rheology of the rock; the ductile lower crust and mantle accumulates deformation gradually via Shear (geology) whereas the brittle upper crust reacts by fracture, or instantaneous stress release to cause motion along the fault. The ductile surface of the fault can also release instantaneously when the strain rate is too great. The energy released by instantaneous strain release is the cause of earthquakes, a common phenomenon along transform boundaries.

A good example of this type of plate boundary is the San Andreas Fault which is found in the western coast of North America and is one part of a highly complex system of faults in this area. At this location, the Pacific and North American plates move relative to each other such that the Pacific plate is moving northwest with respect to North America. Other examples of transform faults include the Alpine Fault in New Zealand and the North Anatolian Fault in Turkey. Transform faults are also found offsetting the crests of mid-ocean ridges (for example, the Mendocino Fracture Zone offshore northern California).

Divergent (constructive) boundaries valley near Grindavik on the Reykjanes peninsula in southwest Iceland, the boundary of the Eurasian and North American continental tectonic plates.At divergent boundaries, two plates move apart from each other and the space that this creates is filled with new crustal material sourced from molten magma that forms below. The origin of new divergent boundaries at triple junctions is sometimes thought to be associated with the phenomenon known as hotspot (geology). Here, exceedingly large convective cells bring very large quantities of hot asthenospheric material near the surface and the kinetic energy is thought to be sufficient to break apart the lithosphere. The hot spot which may have initiated the Mid-Atlantic Ridge system currently underlies Iceland which is widening at a rate of a few centimeters per year.

Divergent boundaries are typified in the oceanic lithosphere by the rifts of the oceanic ridge system, including the Mid-Atlantic Ridge and the East Pacific Rise, and in the continental lithosphere by rift valleys such as the famous East African Great Rift Valley. Divergent boundaries can create massive fault zones in the oceanic ridge system. Spreading is generally not uniform, so where spreading rates of adjacent ridge blocks are different, massive transform faults occur. These are the fracture zones, many bearing names, that are a major source of submarine earthquakes. A sea floor map will show a rather strange pattern of blocky structures that are separated by linear features perpendicular to the ridge axis. If one views the sea floor between the fracture zones as conveyor belts carrying the ridge on each side of the rift away from the spreading center the action becomes clear. Crest depths of the old ridges, parallel to the current spreading center, will be older and deeper (from thermal contraction and subsidence).

It is at mid-ocean ridges that one of the key pieces of evidence forcing acceptance of the sea-floor spreading hypothesis was found. Airborne Earth's magnetic field surveys showed a strange pattern of symmetrical magnetic reversals on opposite sides of ridge centers. The pattern was far too regular to be coincidental as the widths of the opposing bands were too closely matched. Scientists had been studying Geomagnetic reversals and the link was made by Lawrence Morley, Fred Vine and Drummond Matthews in the Morley-Vine-Matthews. The magnetic banding directly corresponds with the Earth's polar reversals. This was confirmed by measuring the ages of the rocks within each band. The banding furnishes a map in time and space of both spreading rate and polar reversals.

Convergent (destructive) boundaries The nature of a convergent boundary depends on the type of lithosphere in the plates that are colliding. Where a dense oceanic plate collides with a less-dense continental plate, the oceanic plate is typically thrust underneath because of the greater buoyancy of the continental lithosphere, forming a subduction zone. At the surface, the topographic expression is commonly an oceanic trench on the ocean side and a mountain range on the continental side. An example of a continental-oceanic subduction zone is the area along the western coast of South America where the oceanic Nazca Plate is being subducted beneath the continental South American Plate.

While the processes directly associated with the production of melts directly above downgoing plates producing surface volcanism is the subject of some debate in the geologic community, the general consensus from ongoing research suggests that the release of volatiles is the primary contributor. As the subducting plate descends, its temperature rises driving off volatiles (most importantly water) encased in the porous oceanic crust. As this water rises into the mantle of the overriding plate, it lowers the melting temperature of surrounding mantle, producing melts (magma) with large amounts of dissolved gases. These melts rise to the surface and are the source of some of the most explosive volcanism on Earth because of their high volumes of extremely pressurized gases (consider Mount St. Helens). The melts rise to the surface and cool forming long chains of volcanoes inland from the continental shelf and parallel to it. The continental spine of western South America is dense with this type of volcanic orogeny from the subduction of the Nazca plate. In North America the Cascade Range, extending north from California's Sierra Nevada, is also of this type. Such volcanoes are characterized by alternating periods of quiet and episodic eruptions that start with explosive gas expulsion with fine particles of glassy volcanic ash and spongy cinders, followed by a rebuilding phase with hot magma. The entire Pacific Ocean boundary is surrounded by long stretches of volcanoes and is known collectively as The Ring of Fire.

Where two continental plates collide the plates either buckle and compress or one plate delves under or (in some cases) overrides the other. Either action will create extensive mountain ranges. The most dramatic effect seen is where the northern margin of the Indian Plate is being thrust under a portion of the Eurasian plate, lifting it and creating the Himalayas and the Tibetan Plateau beyond. It has also caused parts of the Asian continent to deform westward and eastward on either side of the collision.

When two plates with oceanic crust converge they typically create an island arc as one plate is subducted below the other. The arc is formed from volcanoes which erupt through the overriding plate as the descending plate melts below it. The arc shape occurs because of the spherical surface of the earth (nick the peel of an orange with a knife and note the arc formed by the straight-edge of the knife). A deep undersea trench is located in front of such arcs where the descending slab dips downward. Good examples of this type of plate convergence would be Japan and the Aleutian Islands in Alaska.{] along the collision zone, in addition to subduction.

Not all plate boundaries are easily defined. Some are broad belts whose movements are unclear to scientists. One example would be the Mediterranean-Alpine boundary, which involves two major plates and several micro plates. The boundaries of the plates do not necessarily coincide with those of the continents. For instance, the North American Plate covers not only North America, but also far northeastern Siberia.

Driving forces of plate motion Tectonic plates are able to move because of the relative density of oceanic lithosphere and the relative weakness of the asthenosphere. Dissipation of heat from the mantle is acknowledged to be the original source of energy driving plate tectonics, but it is no longer thought that the plates ride passively on asthenospheric convection currents. Instead, it is accepted that the excess density of the oceanic lithosphere sinking in subduction zones drives plate motions. When it forms at mid-ocean ridges, the oceanic lithosphere is initially less dense than the underlying asthenosphere, but it becomes more dense with age, as it conductively cools and thickens. The greater density of old lithosphere relative to the underlying asthenosphere allows it to sink into the deep mantle at subduction zones, providing most of the driving force for plate motions. The weakness of the asthenosphere allows the tectonic plates to move easily towards a subduction zone.

Two and three-dimensional imaging of the Earth's interior (seismic tomography) shows that there is a laterally heterogeneous density distribution throughout the mantle. Such density variations can be material (from rock chemistry), mineral (from variations in mineral structures), or thermal (through thermal expansion and contraction from heat energy). The manifestation of this lateral density heterogeneity is mantle convection from buoyancy forces. http://www.pnas.org/cgi/content/full/97/23/12409 Toshiro Tanimoto and Thorne Lay, Mantle dynamics and seismic tomography, PNAS, November 7, 2000, vol. 97 no. 23 pp. 12409–12410 How mantle convection relates directly and indirectly to the motion of the plates is a matter of ongoing study and discussion in geodynamics. Somehow, this energy must be transferred to the lithosphere in order for tectonic plates to move. There are essentially two types of forces that are thought to influence plate motion: friction and gravity.

Friction Basal drag: Large scale convection currents in the upper mantle are transmitted through the asthenosphere; motion is driven by friction between the asthenosphere and the lithosphere.Slab suction: Local convection currents exert a downward frictional pull on plates in subduction zones at ocean trenches. Although, one could in effect argue that Slab-suction is actually merely a unique geodynamic setting wherein which basal tractions continue to act on the plate as it dives into the mantle (although perhaps to a greater extent—acting on both the under and upper side of the slab).

Gravitation Gravitational sliding: Plate motion is driven by the higher elevation of plates at ocean ridges. As oceanic lithosphere is formed at spreading ridges from hot mantle material it gradually cools and thickens with age (and thus distance from the ridge). Cool oceanic lithosphere is significantly denser than the hot mantle material from which it is derived and so with increasing thickness it gradually subsides into the mantle to compensate the greater load. The result is a slight lateral incline with distance from the ridge axis.

Casually in the geophysical community and more typically in the geological literature in lower education this process is often referred to as "ridge-push". This is, in fact, a misnomer as nothing is "pushing" and tensional features are dominant along ridges. It is more accurate to refer to this mechanism as gravitational sliding as variable topography across the totality of the plate can vary considerably and the topography of spreading ridges is only the most prominent feature. For example: :1. Flexural bulging of the lithosphere before it dives underneath an adjacent plate, for instance, produces a clear topographical feature that can offset or at least effect the influence of topographical ocean ridges. :2. Mantle plumes impinging on the underside of tectonic plates can drastically alter the topography of the ocean floor.

Slab-pull Plate motion is driven by the weight of cold, dense plates sinking into the mantle at trenches. There is considerable evidence that convection is occurring in the mantle at some scale. The upwelling of material at mid-ocean ridges is almost certainly part of this convection. Some early models of plate tectonics envisioned the plates riding on top of convection cells like conveyor belts. However, most scientists working today believe that the asthenosphere is not strong enough to directly cause motion by the friction of such basal forces. Slab pull is most widely thought to be the greatest force acting on the plates. Recent models indicate that trench suction plays an important role as well. However, it should be noted that the North American Plate, for instance, is nowhere being subducted, yet it is in motion. Likewise the African, Eurasian and Antarctic Plates. The over-all driving force for plate motion and its energy source remain subjects of on-going research.

External forces In a study published in the January-February 2006 issue of the Geological Society of America Bulletin, a team of Italian and U.S. scientists argued that the westward component of plates is from Earth's rotation and consequent tidal friction of the moon. As the Earth spins eastward beneath the moon, they say, the moon's gravity ever so slightly pulls the Earth's surface layer back westward. It has also been suggested (albeit, controversially) that this observation may also explain why Venus and Mars have no plate tectonics since Venus has no moon, and Mars' moons are too small to have significant tidal effects on Mars.Richard A. Lovett, Moon Is Dragging Continents West, Scientist Says, National Geographic News January 24, 2006 http://news.nationalgeographic.com/news/2006/01/0124_060124_moon.html This is not, however, a new argument.

It was originally raised by the "father" of the plate tectonics hypothesis, Alfred Wegener. It was challenged by the physicist Harold Jeffreys who calculated that the magnitude of tidal friction required would have quickly brought the Earth's rotation to a halt long ago. Many plates are moving north and eastward, and the dominantly westward motion of the Pacific ocean basins is simply from the eastward bias of the Pacific spreading center (which is not a predicted manifestation of such lunar forces). It is argued, however, that relative to the lower mantle, there is a slight westward component in the motions of all the plates.

Relative significance of each mechanism

McKnight, Tom (2004) Geographica: The complete illustrated Atlas of the world, Barnes and Noble Books; New York ISBN 0-7607-5974-X
Oreskes, Naomi ed. (2003) Plate Tectonics: An Insider's History of the Modern Theory of the Earth, Westview Press ISBN 0-8133-4132-9
G. Schubert, DL Turcotte, and P. Olson (2001) Mantle Convection in the Earth and Planets, Cambridge University Press, Cambridge, ISBN 0-521-35367-X
Stanley, Steven M. (1999) Earth System History, W.H. Freeman and Company; pages 211–228 ISBN 0-7167-2882-6
Tanimoto, Toshiro and Thorne Lay (2000) Mantle dynamics and seismic tomography, Proc. Natl. Acad. Sci. USA, 10.1073/pnas.210382197 http://www.pnas.org/cgi/content/full/97/23/12409 Accessed 03/29/06.
Thompson, Graham R. and Turk, Jonathan, (1991) Modern Physical Geology, Saunders College Publishing ISBN 0-03-025398-5
Turcotte, DL and Schubert, G. (2002) Geodynamics: Second Edition, John Wiley & Sons, New York, ISBN 0-521-66624-4
Winchester, Simon (2003) Krakatoa: The Day the World Exploded: August 27, 1883, HarperCollins ISBN 0-06-621285-5

External links

Movie showing 750 million years of global tectonic activity
More movies over smaller regions and shorter time scales
Easy-to-draw illustrations for teaching plate tectonics
An explanation of tectonic forces
Bird, P. (2003) An updated digital model of plate boundaries, also available as a large (13 mb) PDF file
Map of tectonic plates
MantlePlumes.org, a website debating the existence of deep mantle plumes
USGS site on plate motions
The geodynamics of the North-American/Eurasian/African plate boundaries
Cenozoic dynamics of the African plate with emphasis on the Africa-Eurasia collision
Speed of the continental plates
ImpactTectonics.org, examines tectonic effects associated with hypervelocity bolide impacts on terrestrial planets

McKnight, Tom (2004) Geographica: The complete illustrated Atlas of the world, Barnes and Noble Books; New York ISBN 0-7607-5974-X
Oreskes, Naomi ed. (2003) Plate Tectonics: An Insider's History of the Modern Theory of the Earth, Westview Press ISBN 0-8133-4132-9
G. Schubert, DL Turcotte, and P. Olson (2001) Mantle Convection in the Earth and Planets, Cambridge University Press, Cambridge, ISBN 0-521-35367-X
Stanley, Steven M. (1999) Earth System History, W.H. Freeman and Company; pages 211–228 ISBN 0-7167-2882-6
Tanimoto, Toshiro and Thorne Lay (2000) Mantle dynamics and seismic tomography, Proc. Natl. Acad. Sci. USA, 10.1073/pnas.210382197 http://www.pnas.org/cgi/content/full/97/23/12409 Accessed 03/29/06.
Thompson, Graham R. and Turk, Jonathan, (1991) Modern Physical Geology, Saunders College Publishing ISBN 0-03-025398-5
Turcotte, DL and Schubert, G. (2002) Geodynamics: Second Edition, John Wiley & Sons, New York, ISBN 0-521-66624-4
Winchester, Simon (2003) Krakatoa: The Day the World Exploded: August 27, 1883, HarperCollins ISBN 0-06-621285-5

External links

Movie showing 750 million years of global tectonic activity
More movies over smaller regions and shorter time scales
Easy-to-draw illustrations for teaching plate tectonics
An explanation of tectonic forces
Bird, P. (2003) An updated digital model of plate boundaries, also available as a large (13 mb) PDF file
Map of tectonic plates
MantlePlumes.org, a website debating the existence of deep mantle plumes
USGS site on plate motions
The geodynamics of the North-American/Eurasian/African plate boundaries
Cenozoic dynamics of the African plate with emphasis on the Africa-Eurasia collision
Speed of the continental plates
ImpactTectonics.org, examines tectonic effects associated with hypervelocity bolide impacts on terrestrial planets

Plate Tectonics
Archive of articles and graphical data related to plate tectonics.

Plate Tectonics
Wonderful animations of Plate Tectonics movement. Including gif, avi and mov formats.

Plate Tectonics: Geological Aspects
Useful General References: PRESS, F. & SIEVER, R. 1998. Understanding Earth (2 nd Ed.) p.504–535 + CD-rom & Internet link: www.whfreeman.com/understandingearth

Tectonic Plates
Plate tectonics: The main features are: The Earth's surface is made up of a series of large plates (like pieces of a giant jigsaw puzzle). These plates are in constant motion ...

Plate tectonics topic
Fowler, C.M.R. 1990. The Solid Earth: An Introduction to Global Geophysics. Cambridge University Press, New York, 472 p. Tarbuck, E.J. and F.K.

BBC - GCSE Bitesize - Plate tectonics
A wide range of exercises on geographical plate tectonics. ... MMVIII. The BBC is not responsible for the content of external internet sites.

Plate Tectonics : Plate Boundaries
Plate Boundaries: There are 3 primary types of Tectonic Plate boundaries: Divergent boundaries;

Earth Science resources: Plate tectonics
Plate Tectonics. From the earliest days of even rudimentary maps and atlases, observers noticed that there seemed to be a remarkable similarity between the coasts of Africa and ...

Plate tectonics - Wikipedia, the free encyclopedia
Plate tectonics (from Greek τέκτων, tektōn "builder" or "mason") is a theory of geology that has been developed to explain the observed evidence for large scale motions of ...

Plate Tectonics: Table of Contents
The Earth's Interior; The Theory of Continental Drift; The Different Tectonic Plates; Sea Floor Spreading; Subduction; Fun Activity: Plasticine Plates! Review Questions