The asthenosphere (from Greek asthenes '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 ~80 and ~200 km (~ 50 and 124 miles) below the surface. The lithosphere-asthenosphere boundary is usually referred to as LAB. Asthenosphere is generally solid although some of its regions could be melted (e.g. below mid-ocean ridge). The lower boundary of the asthenosphere is not well defined. The thickness of the asthenosphere depends mainly on the temperature. For some regions asthenosphere could extend as deep as . It is considered the source region of mid-ocean ridge basalt (MORB).