The asthenosphere is the mechanically weak and ductile region of the upper mantle of Earth. It lies below the lithosphere, at a depth between c. 80 and 200 km below the surface, and extends as deep as 700 km. However, the lower boundary of the asthenosphere is not well defined.

The asthenosphere is almost solid, but a slight amount of melting (less than 0.1% of the rock) contributes to its