from geophysical evidence, we have learned that although the mantle consists almost entirely of solid rock, it is hot and weak enough to exhibit fluidlike convective flow. the simplest type of convection is analogous to heating a pot of water on a stove (figure 7.33). Heating the base causes the material to rise in relatively thin sheets or blob that spread out at the surface and cool. Eventually, the surface layer thickens (increase in density) and sink back to the bottom where it is reheated until it achieves enough buoyancy to rise again.