The lithosphere, Greek for “rocky sphere,” is the outermost shell of the Earth. The term is also used to refer to the outermost rocky shell of other solid planets. It is a relatively thin layer, about 31 to 62 miles (50 to 100 km) thick under the oceans and 93 miles (150 km) thick on the continents. This layer is composed of the upper crust, about 3 miles (5 km) thick in the oceans and 40.3 mles (65 km) thick on the continents, and the upper mantle, which makes up the remainder. Separating the crust and the upper mantle is the Mohorovičić discontinuity, the point at which rocks become plastic rather than solid. Beneath the lithosphere is the asthenosphere, which continues the upper mantle, and is approximately the point at which the mantle becomes liquid.
This planetary shell consists of lithospheric plates, otherwise known as the tectonic plates, which drift slowly over time periods of millions of years. Their rate of drift is comparable to the rate at which a person's fingernails grow. Over long periods of time, however, they can create mighty structures, such as the Himalayan Mountains in Tibet. Mt. Everest and the other mountains were formed when the tectonic plate under India forced its way upwards into Asia.
Using careful measurement methods and the study of strata, paleontologists have determined that the tectonic plates have drifted all over the surface of the planet since at least 600 million years ago, when diverse fossils appeared. During this time, the continents started off separate, then merged together into the giant continent Pangaea, only to split up after a few hundred million years and create the continents people are familiar with today.
Lithospheric activity can have a profound effect on the surface above it. When Australia finally separated from Antarctica 50 million years ago, it allowed a new oceanic current — the circumpolar current — to flow around Antarctica and reinforce its own cooling. This froze the continent, which was previously covered in forests, killing all but the hardiest of life there. It also decreased the average global temperature by several degrees.
Although humans cannot dig very deeply into the Earth’s crust with current technology, geoscientists can study the properties of the deep lithosphere by examining special rocks, or xenoliths, brought up through deep volcanic pipes.