Asthenosphere, hot, partially molten layer of Earth, lying below the lithosphere (the rocky outer layer of Earth). The term asthenosphere is derived from Greek and means “weak layer.” The asthenosphere extends from approximately 100 to 350 km (60 to 220 mi) below Earth’s surface. In some areas it extends to a depth of 700 km (435 mi). Beneath Earth’s surface, both temperature and pressure increase with depth. In the asthenosphere the effects of temperature dominate those of pressure, causing the rock to be in a semimolten, slowly flowing state.
However, slightly above and slightly below the asthenosphere, pressure dominates temperature and thus maintains the rock as a rigid solid. The difference between the lithosphere and the asthenosphere is thus one of consistency: rigid, solid rock in the lithosphere, and plastic, partially molten rock in the asthenosphere. Rocks in the asthenosphere experience high temperatures and high pressures. The temperatures reach levels at which the rock becomes soft and plastic, or capable of flowing or being deformed while maintaining a solid state (only about 10 percent of the asthenosphere is totally molten).
The soft, plastic, but solid quality of the asthenosphere allows the lithospheric plates to shift and slide on top of the asthenosphere. This shifting of the lithosphere’s tectonic plates is the source of most seismic activity, such as earthquakes and volcanic eruptions (see Plate Tectonics). The tremendous weight of the overlying rock in the lithosphere causes the extremely high pressures in the asthenosphere. Most of the heat in the asthenosphere comes from energy emitted by radioactive decay, or the transformation of radioactive substances into other elements deep within Earth. The radioactive elements that are present in Earth’s interior.