Mantle, region of the interior of the earth that lies between the outer crust and the core. The mantle is the largest of these regions and comprises the bulk of the earth’s mass and volume. Temperatures in the mantle are high, reaching about 3700° C (about 6700° F). Pressures inside the mantle are also high, reaching about 137 gigapascals (1.37 million atmospheres). The mantle is primarily solid, and its density increases with depth. Under these high temperatures and high pressures, however, material in the mantle can deform, or flow slowly like putty. Slow-moving currents, called convection currents, within the mantle are thought to play a part in the movement of the crustal plates on the earth.
The mantle is separated from the crust by a sharp boundary known as the Mohorovi?ic discontinuity, or Moho. It is separated from the core by another sharp boundary known as the Gutenberg discontinuity. Both boundaries are named in honor of the men who discovered them, Croatian seismologist Andrija Mohorovi?ic and German-born American seismologist Beno Gutenberg. The Mohorovi?ic discontinuity lies at a depth of about 8 km (5 mi) under the oceans and an average depth of about 35 km (about 22 mi) under the continents, but it plunges to as deep as 80 km (50 mi) under tall mountain ranges.
The Gutenberg discontinuity lies at a depth of about 2900 km (about 1800 mi). Both men discovered the boundaries by using the fact that when an earthquake, or seismic, wave reaches a sharp boundary between two materials with different densities or elastic properties, some of the wave’s energy bounces, or reflects, off of the surface. In addition, seismic waves may bend, or refract, as they cross a boundary.