asthenosphere

asthenosphere , region in the upper mantle of the earth's interior, characterized by low-density, semiplastic (or partially molten) rock material chemically similar to the overlying lithosphere . 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 (see plate tectonics ). The asthenosphere is generally located between 45-155 miles (72-250 km) beneath the earth's surface, though under the oceans it is usually much nearer the surface and at mid-ocean ridges rises to within a few miles of the ocean floor. Although its presence was suspected as early as 1926, the worldwide occurrence of the plastic zone was confirmed by analyses of earthquake waves from the Chilean earthquake of May 22, 1960. The seismic waves, the speed of which decreases with the softness of the medium, passed relatively slowly though the asthenosphere, thus it was given the name Low Velocity zone, or the Seismic Wave Guide (see seismology ). Deep-zone earthquakes, i.e., those that occur in the asthenosphere or below it, may be caused by crustal plates sinking into the mantle along convergent crustal boundaries. See earth .

asthenosphere A weak layer within a planetary body, beneath the rigid outer layer known as the lithosphere. It is slightly weaker than the lithosphere and deformable over long periods, allowing continental drift to take place on the Earth. On other planets and satellites the asthenosphere allows the topography to smooth out slowly, highlands becoming lower and deep basins shallower. The depth of the asthenosphere varies according to the size, density, composition, and thermal structure of the planet or satellite. On Earth, the top of the asthenosphere is around 100 km below the surface, but on the Moon it lies 800 km below the surface, more than half-way to the centre.