The lithosphere is the outermost layer of the earth. It is solid and relatively rigid, and is composed of the crust and upper-most solid part of the mantle. Below the lithosphere lies the asthenosphere, which is hotter and is comprised of very viscous, ductile material.

The asthenosphere is plastic and flows, bring the heat from the Earth's core to the surface where it can escape. The lithosphere is more rigid and merely floats like an icy crust on top of the asthenosphere. Both are technically solid rock - the flowing of the asthenosphere takes place over such long time scales that even solid rocks can behave like fluids.