asthenosphere

Concise Oxford English Dictionary © 2008 Oxford University Press:
asthenosphere /əsˈθɛnəsfɪə/

▶noun Geology the upper layer of the earth's mantle, below the lithosphere, in which there is relatively low resistance to plastic flow and convection is thought to occur.

– derivatives
asthenospheric adjective.

– origin early 20th cent.: from Gk asthenēs ‘weak’ + sphere.