Dictionary Meaning and Definition on 'Asthenosphere'
n : the lower layer of the crust
Wikipedia Meaning and Definition on 'Asthenosphere'
The asthenosphere (from Greek asthenēs 'weak' + sphere) is the highly viscous mechanically weak 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 (~ 435 miles).
The asthenosphere is a portion of the upper mantle just below the lithosphere that is involved in plate tectonic movements and isostatic adjustments. In spite of its heat, 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, 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.[See more about Asthenosphere at Dictionary 3.0 Encyclopedia]