The upper shell of the solid Earth, comprising the
crust and
upper mantle, which deforms in a
brittle fashion when subjected to a
stress of greater than 100 MPa.
It has a variable depth of around 2 - 3 kilometres at the ocean floor, increasing in thickness to around 180 kilometres underneath old continental crust. Beneath cratonic areas, it is at least 250 kilometres thick, possibly as much as 500 kilometres.