This study explores a conceptual model for mantle convection in which buoyant and low-viscosity asthenosphere is present beneath the relatively thin lithosphere of ocean basins and regions of active continental deformation, but is less well developed beneath thicker-keeled continental cratons. We start by summarizing the concept of a buoyant plume-fed asthenosphere and the alternative implications this framework has for the roles of compositional and thermal lithosphere. We then describe the sinks of asthenosphere made by forming compositional lithosphere at ridges, by plate cooling wherever the thermal boundary layer extends beneath the compositional lithosphere, and by drag-down of buoyant asthenosphere along...