Asthenosphere

n. one of the layers of the interior of the Earth, upper part of the Earth's mantle

The asthenosphere (from an invented Greek  a + sthenos "without strength") is the region of the Earth between 100-200 km below the surface — but perhaps extending as deep as 400 km — that is the weak or "soft" zone in the upper  mantle. It lies just below the lithosphere, which is involved in plate movements and isostatic adjustments. In spite of its heat, pressures keep it plastic, and it has a relatively low density. Seismic waves, the speed of which decrease with the softness of a medium, pass relatively slowly through the asthenosphere, the cue that originally alerted seismologists to its presence; thus it has been given the name low-velocity zone.
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Noun
1. the lower layer of the crust
(hypernym) layer
(part-holonym) crust, Earth's crust

A layer of soft but solid, mobile rock comprising the lower part of the upper mantle from about 100 to 350 kilometers beneath the Earth's surface. See also lithosphere.

ASTENOSFERA. MANTELLO ESTERNO. STRATO A BASSA VELOCITÀ