The Lithosphere in a Nutshell

The lithosphere is the brittle outer layer of the solid Earth. The plates of plate tectonics are segments of the lithosphere. (Plate Tectonics in a Nutshell)

The lithosphere includes the crust (the rocks of the continents and the ocean floor) and the uppermost part of the mantle beneath the crust. These two layers are different in mineralogy but very similar mechanically, so for the most part they act as one plate.

The greatest forces that bend and break the lithosphere come from plate tectonics. Where plates collide, the lithosphere on one plate is forced downward into the hot mantle. This process of subduction bends the plate downward as much as 90 degrees. Subducting lithosphere cracks extensively, triggering numerous earthquakes in the descending rock slab. In some cases (such as in northern California) the subducted part can break off completely, sinking into the deep Earth as the plates above it change their orientation.

Underneath the lithosphere is a hotter, softer layer of solid rock, the asthenosphere. Instead of cracking like ordinary surface rocks, the material of the asthenosphere can deform slowly under stress like putty. This allows the lithosphere to move across or through the asthenosphere under the forces of plate tectonics. This also means that earthquake faults extend through the lithosphere, but not beyond it. (more on faults)

The lithosphere is not totally rigid, but slightly elastic. It flexes when loads are placed on it or removed from it. Ice-age glaciers are one type of load. In Antarctica, the ice cap has pushed the lithosphere well below sea level today, for example. In Canada and Scandinavia, the lithosphere is still rising from the removal of the glaciers about 10,000 years ago. Here are some other types of loading:

• Construction of volcanoes
• Deposition of sediment
• Rise in sea level
• Large lakes and reservoirs

Here are other examples of unloading:

• Erosion of mountains
• Excavation of canyons and valleys
• Drying up of large water bodies
• Lowering of sea level

The flexing of the lithosphere from these causes is relatively small (usually much less than a kilometer), but measurable. We can model the lithosphere using simple engineering physics, as if it were a metal beam, and get an idea of its thickness. We can also study the behavior of seismic waves and place the base of the lithosphere at depths where these waves begin to slow down, indicating softer rock.

These flexural and seismic models suggest that the lithosphere ranges from less than 20 kilometers in thickness near the mid-ocean ridges to about 50 km in old oceanic regions. Under the continents the lithosphere is thicker, as much as 150 km.

Models of the Earth's heat and lab studies of rocks suggest that the base of the lithosphere occurs where the temperature reaches about 600ºC. Some specialists argue the other way that the lithosphere must begin where actual melting of the rocks is possible; that would mean about 1200ºC and a "thermal lithosphere" about twice as thick as the mechanical lithosphere.

Geologists of different specialties have made the lithosphere a complex, even tangled subject. One valiant attempt to keep things sorted out is Don Anderson's article "Lithosphere, Asthenosphere and Perisphere" in the February 1995 Reviews of Geophysics. I have followed his view that "lithosphere" should be a strictly mechanical term.

Plate Tectonics in a Nutshell