Asthenosphere

The asthenosphere (from Greek asthenēs 'weak' + sphere) is the highly viscous, mechanically weak and ductilely-deforming region of the upper mantle of the Earth. It lies below the lithosphere, at depths between 100 and 200 km (~ 62 and 124 miles) below the surface, but perhaps extending as deep as 700 km (430 mi).