The asthenosphere is the highly viscous mechanically weak region of the upper mantle of the Earth on which “float” the continental plates. It lies below the lithosphere, at depths between 60 and 120 miles below the surface, but perhaps extending as deep as 400+ miles. The lithosphere is a complex mixture of layers. For example the North American continent is not one thick, rigid slab, but a layer cake of ancient, 3 billion-year-old rock on top of much newer material probably less than 1 billion years old, according to a new study by seismologists at the University of California, Berkeley.