There is a buffer belt for seismic wave velocity at the top of mantle. About 5% materials of the belt are in molten state and plastic flow easily occurs. The belt is called asthenosphere. Materials above asthenosphere are all in solid state, called lithosphere. Lithosphere is with high rigidity and breaks up into many blocks, which are called plates. Plates are carried on asthenosphere and move along with it, and this is plate movement, which is also the source of tectonic movement.