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[[File:Plates tect2 en.svg|thumb|250px|The tectonic plates of the world were mapped in the second half of the 20th century.]]
[[File:Tectonicplates Serret.png|thumb|250px|Tectonic plates on a different map projection]]
[[File:Quake epicenters 1963-98.png|thumb|250px|Global earthquake [[epicenter]]s, 1963–1998]]
[[File:Global plate motion 2008-04-17.jpg|thumb|250px|right|Global tectonic plate movement]]

'''Plate tectonics''' is a [[theory]] of [[geology]]. It explains movement of the [[Earth]]'s [[lithosphere]]: this is the [[earth's crust]] and the upper part of the [[Earth's mantle|mantle]]. The lithosphere is divided into plates, some of which are very large and can be entire [[continent]]s.

Heat from the [[Mantle (geology)|mantle]] is the source of energy driving plate tectonics. Exactly how this works is still a matter of debate.<ref name='Turcotte-and-Schubert_2002'/>

== Earth's crust ==
The outermost part of the [[structure of the Earth]] is made up of two layers. The lithosphere, above, is solid. It includes the [[Crust (geology)|crust]] and the uppermost part of the mantle.

Below the lithosphere is the [[asthenosphere]]. The asthenosphere is like a solid or a hot [[viscosity|viscous]] liquid. It can flow like a liquid on long time scales. Large [[convection]] currents in the asthenosphere transfer heat to the surface, where plumes of less dense [[magma]] break apart the plates at the spreading centers. The deeper mantle below the asthenosphere is more rigid again. This is caused by extremely high pressure.

=== Continental and oceanic plates ===
There are two types of tectonic plates: oceanic and continental.

An [[ocean]]ic plate is a tectonic plate at the bottom of the oceans. It is primarily made of [[mafic]] rocks, rich in [[iron]] and [[magnesium]]. It is thinner than the continental crust (generally less than 10 kilometers thick) and [[density|denser]]. It is also younger than continental crust. When they collide, the oceanic plate moves underneath the continental plate because of its density. As a result, it melts in the mantle and reforms. The oldest oceanic rocks are less than 200 million years old.

Continental plate is the thick part of the earth's crust which forms the large land masses. Continental rock has lower [[density]] than oceanic rock. They are mostly made of [[felsic]] rocks. These have [[granite]], with its abundant [[silica]], [[aluminum]], [[sodium]] and [[potassium]]. Continental plates are rarely destroyed. Their oldest rocks seem to be 4 [[billion]] years old. Oceanic plates cover about 71 percent of Earth’s surface, while continental plates cover 29 percent.

=== Thickness of plates ===
Ocean lithosphere varies in thickness. Because it is formed at [[mid-ocean ridge]]s and spreads outward, it gets thicker as it moves further away from the mid-ocean ridge. Typically, the thickness varies from about {{convert|6|km|mi}} thick at mid-ocean ridges to greater than {{convert|100|km|mi}} at [[subduction]] zones.<ref name=Turcotte-and-Schubert_2002/>

Continental lithosphere is about {{convert|200|km|mi}} thick. It varies between basins, mountain ranges, and the stable [[craton]]ic interiors of continents. The two types of crust differ in thickness, with continental crust being much thicker than oceanic: {{convert|35|km|mi}} vs. {{convert|6|km|mi}}.<ref name=Turcotte-and-Schubert_2002>{{Cite book |last1=Turcotte |first1=D.L. |last2=Schubert |first2=G. |title=Geodynamics |url=https://archive.org/details/geodynamics00dltu |edition=2 |chapter=Plate Tectonics|year=2002 |publisher=Cambridge University Press |pages=[https://archive.org/details/geodynamics00dltu/page/1 1]–21 |isbn=0-521-66186-2}}</ref>
[[File:Tectonic plate boundaries.png|thumb|center|500px|Three types of plate boundaries and a [[Hotspot (geology)|hot spot]]]]

=== Movement of plates ===
The lithosphere consists of ''tectonic plates''. There are seven major and many minor plates. The lithospheric plates ride on the [[asthenosphere]] ([[aesthenosphere]]). The plate boundary is where two plates meet. When movement occurs, the plates may create [[mountain]]s, [[earthquake]]s, [[volcano]]es, [[Seafloor spreading#Mid-ocean ridge|mid-oceanic ridge]]s and oceanic trenches, depending on which way the plates are moving.<ref>Rolf Meissner 2002. ''The little book of planet Earth''. New York: Copernicus Books. p202 {{ISBN|978-0-387-95258-1}}.</ref><ref name = 'platetectonics.com'>{{cite web|title=Plate Tectonics: Plate boundaries|url=http://www.platetectonics.com/book/page_5.asp|publisher=platetectonics.com|accessdate=12 June 2010|archive-date=16 June 2010|archive-url=https://web.archive.org/web/20100616062513/http://www.platetectonics.com/book/page_5.asp|url-status=dead}}</ref><ref name=Tackley>{{cite journal
| last=Tackley | first=Paul J.
| title=Mantle convection and plate tectonics: towards an integrated physical and chemical theory
| journal=Science | date=2000-06-16
| volume=288 | issue=5473 | pages=2002–2007
| doi=10.1126/science.288.5473.2002
| pmid=10856206 | bibcode=2000Sci...288.2002T
}}</ref><ref name=book1>{{cite book| last = Seyfert | first= Carl K. |title = The encyclopedia of structural geology and plate tectonics| url = https://archive.org/details/encyclopediaofst0000unse |year=1987|isbn=9780442281250}}</ref><ref name="Or">Oreskes, Naomi ed. 2003. ''Plate tectonics: an insider's history of the modern theory of the Earth''. Westview Press {{ISBN|0-8133-4132-9}}</ref>
# [[Convergent boundary|Convergent boundaries]]: two plates move toward each other. Sometimes one plate will move under the other. This is called [[subduction]]. When an oceanic plate collides with a continental plate, the oceanic plate will move underneath the continental because it is denser. Convergent boundaries can create mountains and volcanoes. The [[Andes]] mountain range in [[South America]] and the [[Japan]]ese island [[arc]] are examples, along with the [[Pacific Ring of Fire]].
# [[Divergent boundary|Divergent boundaries]]: two plates move apart. As shown in the diagram, the place where the boundary occurs is called a rift. Magma from the mantle pushes up and cools off forming new land. They create earthquakes and trenches. The [[Mid-ocean ridge]]s and [[Africa]]'s [[Great Rift Valley]] are examples.
# [[Fault (geology)|Transform fault boundaries]]: two plates move side to side. They make earthquakes. The [[San Andreas Fault]] in [[California]] is an example of a transform boundary. [[New Zealand]] is another, more complex, example.

[[Earthquake]]s, [[volcano|volcanic activity]], [[mountain]]-building, and [[Ocean trench|oceanic trench]] formation occur along plate boundaries. The lateral movement of the plates varies from:
* {{convert|1|-|4|cm|in}} per year (Mid-Atlantic Ridge). This is as fast as fingernails grow.
* {{convert|10|cm|in}} per year (Nazca Plate). This is as fast as hair grows.<ref>{{Cite web |url=http://hypertextbook.com/facts/ZhenHuang.shtml|title=Speed of the continental plates |author=Huang Zhen Shao |year=1997 |work=The physics factbook}}</ref><ref>{{Cite book |last1=Hancock |first1=Paul L |last2=Skinner |first2=Brian J |last3=Dineley |first3=David L |title=The Oxford Companion to the Earth |url=https://archive.org/details/oxfordcompaniont00hanc |publisher=Oxford University Press |year=2000 |isbn=0198540396}}</ref>

== Major plates ==
Depending on how they are defined, seven or eight major plates are usually listed:
* [[African plate]]
* [[Antarctic plate]]
* [[Indo-Australian plate]], sometimes subdivided into:
** [[Indian plate]]
** [[Australian plate]]
* [[Eurasian plate]]
* [[North American plate]]
* [[South American plate]]
* [[Pacific plate]]

==See also==
* [[List of tectonic plates]]
* [[Tectonics]]
*[[Alfred Wegener]]

== References ==
{{Reflist}}
* McKnight, Tom 2004. ''Geographica: the complete illustrated atlas of the world'', Barnes and Noble; New York. {{ISBN|0-7607-5974-X}}
* Stanley, Steven M. 1999. ''Earth system history''. Freeman, p211–228. {{ISBN|0-7167-2882-6}}
* Thompson, Graham R. & Turk, Jonathan 1991. ''Modern physical geology''. Saunders. {{ISBN|0-03-025398-5}}
* Turcotte D.L. & Schubert G. 2002. ''Geodynamics''. 2nd ed, Wiley, New York. {{ISBN|0-521-66624-4}}

== External links ==
{{commonscat|Plate tectonics}}
* [http://www.ucmp.berkeley.edu/geology/tecall1_4.mov Movie] showing 750 million years of global tectonic activity.
* [http://www.ucmp.berkeley.edu/geology/tectonics.html More movies] over smaller regions and smaller time scales.
* [http://geology.com/nsta/ Easy-to-draw illustrations for teaching plate tectonics]
* [http://snobear.colorado.edu/Markw/Mountains/03/week3.html Map of tectonic plates]
* [http://en.citizendium.org/wiki/Plate_tectonics Plate tectonics] -Citizendium

[[Category:Plate tectonics| ]]

Plate tectonics - Revision history

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