Asthenosphere flow and plate motions

Summary. A dynamical theory of flow in the asthenosphere is developed by treating it as a thin layer of Newtonian viscous fluid, pushed horizontally by the dense sinking slabs attached to some plates and by the differences in heating under continental and oceanic crust. Two-dimensional lubrication theory is adapted to give the world-wide effects of inflow to the asthenosphere at the slabs, outflow to the plates at mid-ocean ridges, and the horizontal forces applied at the slabs. The resulting torque balance for the plates is better than any previously published, and leads one to conclude that the asthenosphere's viscosity is least at the top.