That zone of the earth's mantle which lies beneath the relatively rigid lithosphere, between 50 and 300 km below the surface, and representing a mechanical boundary between more rigid regions above and below. It is approximately commensurate with that zone of the mantle which transmits seismic waves at low velocity. The asthenosphere represents the location in the mantle where the melting point is most closely approached, and the transition between a rigid lithosphere and a viscous asthenosphere is gradational (Stüwe (1999) Surveys Geophys. 20). The asthenosphere is composed of hot, semi-molten, and therefore deformable, rock, and may be plate- or density-driven, or both (Hansen et al. (2006) Geology 34, 10).
Subjects: Earth Sciences and Geography.