Plate tectonics provides a modern explanation for the presence of the Hawaiian volcanoes and their age progression from young in the southeast to old in the northwest. The lithosphere consists of the crust and uppermost mantle, both of which are rigid, and together can be divided into sections called plates. Beneath the lithosphere is the asthenosphere, a hot plastic layer on which the lithospheric plates can slide. Somewhere beneath the asthenosphere, and possibly as deep as the core-mantle boundary, is a hotspot, and the Hawaiian volcanoes are formed because of it. There are approximately 42 hotspots on earth (Duncan & Richards 1991).