Origin of extension. Convection within the asthenosphere may result in divergent ("moving apart") movement of the lithosphere, causing extension (stretching), thinning of the crust and lithosphere, normal faulting, and the formation of rift basins. Convection refers to the action of large-scale circulation cells that transfer heat from the interior of the Earth. Hot material rises to the surface, spreads out laterally, cools, and ultimately sinks (not shown above). The lithosphere is the Earth's outermost shell and includes the crust; it overlies the asthenosphere. Modified from Withjack et al. (1998).