asthenosphere


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asthenosphere

(ăsthēn`əsfēr), 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 lithospherelithosphere
, brittle uppermost shell of the earth, broken into a number of tectonic plates. The lithosphere consists of the heavy oceanic and lighter continental crusts, and the uppermost portion of the mantle.
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. 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 tectonicsplate tectonics,
theory that unifies many of the features and characteristics of continental drift and seafloor spreading into a coherent model and has revolutionized geologists' understanding of continents, ocean basins, mountains, and earth history.
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). 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 seismologyseismology
, scientific study of earthquakes and related phenomena, including the propagation of waves and shocks on or within the earth by natural or artificially generated seismic signals.
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). 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 earthearth,
in geology and astronomy, 3rd planet of the solar system and the 5th largest, the only planet definitely known to support life. Gravitational forces have molded the earth, like all celestial bodies, into a spherical shape.
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.

asthenosphere

(ass-th'en -ŏ-sfeer) See Earth.

Asthenosphere

 

a layer of low hardness, strength, and viscosity in the earth’s upper mantle. It is identical to Gutenberg’s layer. It is found at depths of about 100 km under the continents and about 50 km under the ocean floor; the lower boundary occurs at 250 to 350 km. The layer may not be continuous. Seismic studies have shown that the rate of propagation of transverse and possibly longitudinal seismic waves within the asthenosphere is somewhat slower than in the covering and underlying layers of the upper mantle. The viscosity of the substance of the asthenosphere is 1019 to 1023 poises, whereas below and above the boundaries of the asthenosphere it is at least 1023 poises. It is conjectured that there is a slow overflow of masses in a horizontal direction within the asthenosphere because of the low yield point. The overflow is caused by uneven stress from the earth’s crust.

The presence of the asthenosphere is explained by the high geothermal gradient, the high temperature of the substance of the asthenosphere (close to the melting point), and processes of relaxation. Volcanic materials usually originate in the asthenosphere, and the subcrustal masses that accomany the main tectonic processes are displaced. The term “asthenosphere” was introduced in 1914 by the American geologist J. Barrell.

V. A. MAGNITSKII

asthenosphere

[as′then·ə‚sfir]
(geology)
That portion of the upper mantle beneath the rigid lithosphere which is plastic enough for rock flowage to occur; extends from a depth of 30-60 miles (50-100 kilometers) to about 240 miles (400 kilometers) and is seismically equivalent to the low velocity zone.
References in periodicals archive ?
These seismic and electrical signatures are supposed to characterize the oceanic asthenosphere below the high-velocity and more resistive lithosphere.
Several authors have established that, due to the lithospheric mantle high strength at low (near surface) temperatures, only a thin layer with intermediate viscosity between the rigid mantle lithosphere and the asthenosphere can be quickly removed (e.
1957, Shadow zone, travel times and energies of longitudinal seismic waves in the presence of an asthenosphere low-velocity layer, Trans.
Even more dramatically, this heat source would also cause mantle rocks to migrate laterally in the asthenosphere hundreds of kilometers out from Iceland ([ILLUSTRATION OMITTED] on page 36).
Where, suffixes a, w, s, c, and m indicates asthenosphere, water, sediments, crust, and mantle respectively, '?
Lying beneath the lithosphere is the asthenosphere, a layer of rock that is slowly deforming and gradually flowing like taffy.
The magma was probably generated as a result of slab break-off and resultant high heat flow associated with upwelling asthenosphere under the extinct Popelogan-Victoria arc following closure of the Tetagouche-Exploits back-arc basin (Salinic collision).
This slow convective overturning, in a region called the asthenosphere, may swallow whole oceans' worth of crust but has hardly touched most cratons.
Froideveaux, Editors, Composition Structure and Dynamics of the Lithosphere Asthenosphere System, AGU, Washington D.
According to the theory the Earth's crust and the uppermost mantle consists of 14 to 16 major lithospheric plates floating on the fluid asthenosphere.
Healy and Kusznir provide an example of how upward divergent flow of asthenosphere can provide admissible models of margin evolution, exemplified by the Goban Spur margin of NW Europe, a little to the north of the Iberia-Newfoundland conjugate margin.