The term lithosphere comes from the Greek words for "rocky" and "sphere." It consists of the rigid shell that makes up the outer layer of a rocky planet. On earth, the lithosphere is made up of the crust and the section of the upper mantle that behaves in an elastic fashion over time frames of thousands of years or more.
The earth's lithosphere, the rigid outer layer, consists of both the crust and the strong, uppermost part of the mantle.
The topmost portion of the lithosphere is known as the pedosphere. It reacts chemically with the atmosphere, hydrosphere and biosphere in processes that form the soil layer out of the rocks-igneous, sedimentary and metamorphic-that form the crust.
The lithosphere is governed by the underlying asthenosphere, which is the hotter and weaker layer of rock in the upper mantle. The boundary between the lithosphere and asthenosphere can be defined by a difference in stress response: The lithosphere is more rigid and brittle and will crack, form fault lines and break apart. In contrast, due to heat and pressure, the rock that makes up the asthenosphere is softer and viscous, responding to strain by deforming rather than fracturing.
The lithosphere is composed of tectonic plates that move about on top of the hot, softer and more fluid-like asthenosphere. The plate boundaries, where the tectonic plates converge, spread apart or slide past each other, are the zones where geological occurrences such as volcanoes, thermal springs and earthquakes are most active.
There are two distinct kinds of lithosphere: oceanic and continental.
On average, oceanic lithosphere is about 100 km thick. It becomes thicker with age as well as with distance away from the mid-ocean ridges. These ridges form where the oceanic plates are being pulled apart. Here, the mantle is very thin, and magma from the ashenophere percolates up, spreading outward and gradually cooling. Over time, the cooled oceanic lithosphere becomes more dense. Where the thickened edge of an oceanic plate meets another tectonic plate, it subducts, or sinks down beneath it, where it is consumed by the pressure and heat of the athenophere. The effect is that oceanic lithosphere is constantly recycled, being formed in the mid-oceanic ridges and eventually heading back down into the athenosphere at the subduction zones.
Another characteristic of oceanic lithosphere is that it is dense. The crust is usually composed of basaltic rock that is mafic, or rich in magnesium and iron. This contributes to the tendency for oceanic plates to subduct underneath continental plates. It is also the reason why the ocean floor sits lower than more buoyant continental plates.
Continental lithosphere is characterized as being felsic, or rich in silica, with the crust layer being made largely of granite.
Continental lithosphere is significantly thicker than oceanic lithosphere. The average depth is 200 km, although it varies with the landscape, from low basins to high mountaintops. It is also older than oceanic lithosphere. The oldest oceanic lithosphere averages appropriately 170 million years. In contrast, portions of the continental lithosphere are billions of years old.
The formation of continental lithosphere is basically the opposite of how oceanic lithosphere forms. Instead of forming from the center out, it occurs mostly along the edges by arc volcanism, geological activity that occurs due to interactions of plate boundaries, or by a number of other ways that material will collect at the edge of a landmass. This means that the oldest sections of continental lithosphere are in their interiors, in regions that are more stable and less geologically active called cratons.