asthenosphere

noun

1. A portion of the Earth's upper mantle, below the lithosphere.

   “Holonyms: upper mantle, outer mantle < mantle < Earth”

   “To say that the lithosphere floats on top of the asthenosphere suggests a degree of easy buoyancy that isn't quite right.”

   “The asthenosphere that underlies the surface plates is not liquid in any vernacular sense, although it is relatively plastic.”