ASTHENOSPHERE (from the Greek words "asthenes" (weak) and "sphaira" (ball) * EN: asthenosphere, zone of mobility; DE: Astenosphare; FR: astenosphere; ES: astenosfera; RU: астеносфера) is the layer of the decreased viscosity within the upper mantle of the Earth. The roof of asthenosphere lies under the mainlands at the depth of 80-100 kilometres, under the oceans at 50-70 kilometres (sometimes less), the lower boundary of asthenosphere is at the depth of 250-300 kilometres, it is not sharp. It is distinguished according to the geophysical data as the layer of the decreased velocity of the transverse seismic waves, and of the increased electrical conductivity.

The decreased viscosity of astenosphere is caused, apparently, by the high temperature, which causes, as they believe, the partial smelting of the basaltic magma. There proceeds within asthenosphere the flow of the matter, which causes the vertical and horizontal tectonic movements of the blocks of lithosphere. The fluids and magma, which are penetrating into the Earth's crust from astenosphere, participate in the formation of depositions of useful minerals. Asthenosphere plays the important role during the endogenous processes, which are proceeding within the Earth's crust (magmatism, metamorphism, and others like these).