Lithosphere
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The lithosphere (from the Greek for "rocky" sphere) is the solid outermost shell of a rocky planet. On the Earth, the lithosphere includes the crust and the uppermost layer of the mantle (the upper mantle or lower lithosphere) which is joined to the crust.
The distinguishing characteristic of the lithosphere is not composition, but its flow properties. Under the influence of the low-intensity, long-term stresses that drive plate tectonic motions, the lithosphere responds essentially as a rigid shell, while the asthenosphere behaves as a slightly viscous fluid. Both the crust and upper mantle float on the more plastic asthenosphere. The crust is distinguished from the mantle, and hence the upper mantle, by the change in chemical composition that takes place at the Moho discontinuity.
The thickness of the lithosphere varies from around 1.6 km (1 mi) at the mid-ocean ridges to approximately 130 km (80 mi) beneath older oceanic crust. The thickness of the continental lithospheric plates is probably around 150 kilometers (93 mi).
As the cooling surface layer of the Earth's convection system, the lithosphere thickens over time. It is fragmented into relatively strong pieces, called tectonic plates, which move independently relative to one another. This movement of lithospheric plates is described as plate tectonics.
There are two types of lithosphere:
References
- Earth's Crust, Lithosphere and Asthenosphere (http://www.windows.ucar.edu/cgi-bin/tour_def/earth/interior/earths_crust.html)
- Crust and Lithosphere (http://www.geolsoc.org.uk/template.cfm?name=lithosphere)