asthenosphere

Etymology

From Ancient Greek ἀσθενής (asthenḗs, “weak”) + -sphere.

noun

1. (geology) The zone of the Earth's upper mantle, below the lithosphere.
   To say that the lithosphere floats on top of the asthenosphere suggests a degree of easy buoyancy that isn't quite right. 2003, Bill Bryson, A Short History of Nearly Everything, BCA, page 192
   The asthenosphere that underlies the surface plates is not liquid in any vernacular sense, although it is relatively plastic. 2004, Richard Fortey, The Earth, Folio Society, published 2011, page 79