The
asthenosphere (from
Greek ἀσθενής
asthenḗs '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 approximately below the surface. The
Lithosphere-Asthenosphere boundary is usually referred to as LAB. The asthenosphere is generally solid, although some of its regions could be melted (e.g., below
mid-ocean ridges). The lower boundary of the asthenosphere is not well defined. The thickness of the asthenosphere depends mainly on the temperature. In some regions the asthenosphere could extend as deep as . It is considered the source region of mid-ocean ridge
basalt (MORB).