- published: 01 Oct 2011
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The asthenosphere (from Greek asthenēs 'weak' + sphere) is the highly viscous, mechanically weak and ductilely-deforming region of the upper mantle of the Earth. It lies below the lithosphere, at depths between 100 and 200 km (~ 62 and 124 miles) below the surface, but perhaps extending as deep as 700 km (430 mi).
The asthenosphere is a portion of the upper layer just below the lithosphere that is involved in plate tectonic movement and isostatic adjustments. In spite of its high temperature, pressures keep it plastic, and it has a relatively low density. Seismic waves pass relatively slowly through the asthenosphere compared to the overlying lithospheric mantle, thus it has been called the low-velocity zone (LVZ), although the two are not exactly the same. The lower boundary of the LVZ lies at a depth of 180–220 km, whereas the base of the asthenosphere lies at a depth of about 700 km. This was the observation that originally alerted seismologists to its presence and gave some information about its physical properties, as the speed of seismic waves decreases with decreasing rigidity.
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