The asthenosphere is a part of the upper mantle just below the lithosphere that is involved in plate tectonic movement and isostatic adjustments. The lithosphere-asthenosphere boundary is conventionally taken at the 1300 °C isotherm, above which the mantle behaves in a rigid fashion and below which it behaves in a ductile fashion. and flows very slowly, in a manner similar to the ice at a bottom of a glacier.