Lithosphere

The lithosphere is the rigid outer shell of the Earth, made of the crust plus the cold uppermost mantle, that behaves as the brittle plates of plate tectonics. It rides on the weaker, partly molten asthenosphere below. Oceanic lithosphere is thin and hot at a ridge, then thickens and cools as it ages, reaching 100 km or more by 80 million years; continental lithosphere can exceed 200 km under old cratons. Its thickening with age explains why the seafloor deepens away from a ridge by the square root of crustal age, the basis of the half-space cooling model.