noun Geology.
the region below the lithosphere, variously estimated as being from fifty to several hundred miles (eighty-five to several hundred kilometers) thick, in which the rock is less rigid than that above and below but rigid enough to transmit transverse seismic waves.

1910–15; < Greek asthen(ḗs) frail (see asthenia) + -o- + -sphere Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
asthenosphere (əsˈθiːnəˌsfɪə, -ˈθɛn-)
See also isostasy a thin semifluid layer of the earth (100--200 km thick), below the outer rigid lithosphere, forming part of the mantle and thought to be able to flow vertically and horizontally, enabling sections of lithosphere to subside, rise, and undergo lateral movement
[C20: from astheno-, from Greek asthenēs weak + sphere]

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Word Origin & History

layer of the Earth's upper mantle, 1914, from Gk. asthenos (see asthenia) + sphere
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Science Dictionary
asthenosphere  [%PREMIUM_LINK%]     (ās-thěn'ə-sfîr')  Pronunciation Key 
The upper part of the Earth's mantle, extending from a depth of about 75 km (46.5 mi) to about 200 km (124 mi). The asthenosphere lies beneath the lithosphere and consists of partially molten rock. Seismic waves passing through this layer are significantly slowed. Isostatic adjustments (the depression or uplift of continents by buoyancy) take place in the asthenosphere, and magma is believed to be generated there. Compare atmosphere, hydrosphere, lithosphere.

The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
Below the asthenosphere lies another layer, stronger and more solid than the asthenosphere.
If the latter was the case, it's possible that decompression melting liquefied
  some of the underlying asthenosphere.
These plates move on a hot flowing mantle layer called the asthenosphere, which
  is several hundred kilometers thick.
They move on a hot flowing mantle layer called the asthenosphere, which is
  several hundred kilometers thick.
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