the lower layer of the earth's crust, below the lithosphere. It is estimated as from fifty to several hundred miles thick. It is less rigid than the lithosphere, but still rigid enough to transmit some transverse seismic waves.
A layer of soft but solid, mobile rock comprising the lower part of the upper mantle from about 100 to 350 kilometers beneath the Earth's surface. See also lithosphere.
The weak zone inside Earth directly below the lithosphere, from 10 to 200 km below the surface. Seismic velocities are distinctly lower in the asthenosphere than in adjacent parts of Earth's interior. The material in the asthenosphere is therefore believed to be soft and yielding to plastic flow.