The asthenosphere (from the Greek asthenia, "weakness") is the region of the Earth between 100-200 km below the surface, but may extend as deep as 400 km. It is the weak or "soft" zone in the upper mantle just below the lithosphere that is involved in plate movement and isostatic adjustments. It is plastic, or semi-molten, and has a relatively low density. Seismic waves, the speed of which decrease with the softness of a medium, pass relatively slowly though the asthenosphere, thus it has been given the name low-velocity zone.
The upper part of the asthenosphere is believed to be the zone upon which the great rigid and brittle lithospheric plates of the Earth's crust move about. Due to the temperature and pressure conditions in the asthenosphere, rock behaves plastically, moving at rates of deformation measured in cm/yr over lineal distances of thousands of kilometers. In this way, it flows like a convection current, radiating heat outward from the Earth's interior. Above the asthenosphere, at the same rate of deformation, rock behaves elastically and, being brittle, can break, causing faults. The rigid lithosphere is thought to "float" or move about on the slowly flowing asthenosphere, creating the movement of crustal plates described by Plate Tectonics theory.
Although its presence was suspected as early as 1926, the worldwide occurrence of the asthenosphere was confirmed by analyses of earthquake waves from the Chilean Great earthquake of May 22, 1960.
