Dissipation of heat from the mantle is the original source of energy driving plate tectonics. Exactly how this works is still a matter of debate. The driving forces of plate motion continue to be active subjects of on-going research.
The outermost part of the Earth's interior is made up of two layers. The lithosphere, above, includes the crust and the rigid uppermost part of the mantle.
Below the lithosphere is the asthenosphere. Although solid, the asthenosphere 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.
Thickness of plates
Ocean lithosphere varies in thickness. Because it is formed at mid-ocean ridges and spreads outwards, it gets thicker as it moves further away from the mid-ocean ridge. Typically, the thickness varies from about 6 kilometres (3.7 mi) thick at mid-ocean ridges to greater than 100 kilometres (62 mi) at subduction zones.
Continental lithosphere is typically about 200 kilometres (120 mi) thick, though this also varies considerably between basins, 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 kilometres (22 mi) vs. 6 kilometres (3.7 mi).
Movement of plates
The lithosphere consists of tectonic plates. There are eight major and many minor plates. The lithospheric plates ride on the asthenosphere. These plates move at one of three types of plate boundaries.
- 1–4 centimetres (0.39–1.57 in) per year (Mid-Atlantic Ridge). This is as fast as fingernails grow.
- 10 centimetres (3.9 in) per year (Nazca Plate). This is as fast as hair grows.
- together = mountains; volcanoes. The Andes mountain range in South America and the Japanese island arc are examples. Also the Pacific Ring of Fire.
- away = earthquakes, trenches. The Mid-ocean ridges and Africa's Great Rift Valley are examples.
- side to side = earthquakes. The San Andreas Fault in California is an example of a transform boundary. New Zealand is another, more complex, example.
Depending on how they are defined, there are usually seven or eight major plates:
- African Plate
- Antarctic Plate
- Indo-Australian Plate, sometimes subdivided into:
- Indian Plate
- Australian Plate
- Eurasian Plate
- North American Plate
- South American Plate
- Pacific Plate
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