The upper mantle of Earth is a very thick layer of rock inside the planet, which begins just beneath the crust (at about under the oceans and about under the continents) and ends at the top of the lower mantle at . Temperatures range from approximately at the upper boundary with the crust to approximately at the boundary with the lower mantle. Upper mantle material that has come up onto the surface comprises about 55% olivine, 35% pyroxene, and 5 to 10% of calcium oxide and aluminum oxide minerals such as plagioclase, spinel, or garnet, depending upon depth. Seismic structure The density profile through Earth is determined by the velocity of seismic waves. Density increases progressively in each layer, largely due to compression of the rock at increased depths. Abrupt changes in density occur where the material composition changes. The upper mantle begins just beneath the crust and ends at the top of the lower mantle. The upper mantle causes the tectonic plates to move. Crust and mantle are distinguished by composition, while the lithosphere and asthenosphere are defined by a change in mechanical properties. The top of the mantle is defined by a sudden increase in the speed of seismic waves, which Andrija Mohorovičić first noted in 1909; this boundary is now referred to as the Mohorovičić discontinuity or "Moho." The Moho defines the base of the crust and varies from to below the surface of the Earth. Oceanic crust is thinner than continental crust and is generally less than thick. Continental crust is about thick, but the large crustal root under the Tibetan Plateau is approximately thick. The thickness of the upper mantle is about . The entire mantle is about thick, which means the upper mantle is only about 20% of the total mantle thickness. The boundary between the upper and lower mantle is a discontinuity. Earthquakes at shallow depths result from strike-slip faulting; however, below about , the hot, high-pressure conditions inhibit further seismicity. The mantle is viscous and incapable of faulting. However, in subduction zones, earthquakes are observed down to . Lehmann discontinuity The Lehmann discontinuity is an abrupt increase of P-wave and S-wave velocities at a depth of (Note that this is a different "Lehmann discontinuity" than the one between the Earth's inner and outer cores labeled in the image on the right.) Transition zone The transition zone is located between the upper mantle