asthenosphere

asthenosphere (ăsthēnˈəsfēr) [key], 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.

The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.

More on asthenosphere from Fact Monster:

  • lithosphere - lithosphere lithosphere , brittle uppermost shell of the earth, broken into a number of tectonic ...
  • mantle - mantle mantle, portion of the earth's interior lying beneath the crust and above the core. No ...
  • plate tectonics: Development of Plate Tectonics Theory - Development of Plate Tectonics Theory The beginnings of the theory of plate tectonics date to ...
  • plate tectonics: Plate Boundary Conditions - Plate Boundary Conditions There are numerous major plate boundary conditions. When a large ...
  • basalt - basalt basalt , fine-grained rock of volcanic origin, dark gray, dark green, brown, reddish, or ...

See more Encyclopedia articles on: Geology and Oceanography