Tomographic velocity models of the upper 1000 km of the Earth beneath the western United States. Orange regions represent low seismic velocities interpreted as warm upwelling regions that may be associated with surface volcanism. Blue regions represent high seismic velocities interpreted as cool downwellings that take the form of more planar curtains sinking into the mantle. The subducting Juan de Fuca slab has been disrupted by an upwelling plume, which appears to have torn the slab from north to south. (Image courtesy of R. Allen.)