The upper (oceanic and continental) layer of the solid Earth, comprising all crustal rocks and the brittle part of the uppermost mantle. It is generally considered to deform by brittle fracture and if subjected to stresses of the order of 100 MPa. It comprises numerous blocks, known as tectonic plates, which have differential motions giving rise to plate tectonics. The concept was originally based on the requirement for a rigid upper layer to account for isostasy. Its rigidity is variable, but much greater than 10²¹ P, which corresponds with the underlying asthenosphere. Its thickness is variable, from 1–2 km at mid-oceanic ridge crests, but generally increasing from 60 km near the ridge to 120–140 km beneath older oceanic crust. The thickness beneath continental crust is uncertain, probably some 300 km beneath the cratonic (see craton) parts of the continental crust, but the absence of the asthenosphere in these regions makes definition difficult. Compare atmosphere; hydrosphere.