Basic Geology/Lithosphere

The lithosphere is the solid shell of a rocky planet called earth. That means the crust and the upper part of the mantle which is joined to the crust (see picture on the right).

Under the lithosphere, there is the asthenosphere, the weaker, hotter, and deeper part of the upper mantle.

Picture 2: The tectonic plates of the Lithosphere on Earth.

The lithosphere is the surface layer of the fluid parts of the Earth's convection system, therefore it thickens over time. It is broken up into pieces called plates (shown in the picture on the left), which move independently relative to one another. This movement of lithospheric plates is described as plate tectonics.

The division of Earth's outer layers into the lithosphere and asthenosphere should not be confused with the chemical subdivision of the outer Earth into mantle, and crust. All crust is in the lithosphere, but the lithosphere generally contains more mantle than the crust.

There are two types of lithosphere:

• Oceanic lithosphere, which is associated with Oceanic crust
• Continental lithosphere, which is associated with Continental crust

Oceanic lithosphere is typically about 50-100 km thick (but beneath the mid-ocean ridges is no thicker than the crust). The continental lithosphere is thicker (about 150 km). It consists of about 50 km of crust and 100 km or more of the uppermost mantle.

The oceanic lithosphere consists mainly of mafic crust and ultramafic mantle and is denser than the continental lithosphere, for which the mantle is associated with a crust made of felsic rocks. The crust is distinguished from the upper mantle by the change in chemical composition that takes place at the Moho discontinuity. The oceanic lithosphere thickens as it ages and moves away from the mid-ocean ridge. This thickening occurs by conductive cooling, which converts hot asthenosphere into the lithospheric mantle, and causes the oceanic lithosphere to become increasingly dense with age. Oceanic lithosphere is less dense than the asthenosphere for a few tens of millions of years, but after this becomes increasingly denser than the asthenosphere. The gravitational instability of the mature oceanic lithosphere has the effect that at subduction zones the oceanic lithosphere invariably sinks underneath the overriding lithosphere, which can be oceanic or continental. The new oceanic lithosphere is constantly being produced at mid-ocean ridges and is recycled back to the mantle at subduction zones. As a result, the oceanic lithosphere is much younger than the continental lithosphere: the oldest oceanic lithosphere is about 170 million years old, while parts of the continental lithosphere are billions of years old.

Another distinguishing characteristic of the lithosphere is 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 and thus deforms primarily through brittle failure, whereas the asthenosphere (the layer of the mantle below the lithosphere) is heat-softened and accommodates strain through plastic deformation.