The rate at which heat is converted into mechanical work by a convective system can be estimated from the thermal expansion coefficient, the specific heat, and the depth of the layer if the convective heat flux is known. The heat flux from the mantle is reasonably well known, as is the rate of plate creation. The depth of the layer involved is more controversial, as is the thickness of the brittle layer beneath island arcs. Reasonable estimates for these two parameters yield mean stresses on plate boundaries of between 100 and 200 bars with a probable upper bound of about 500 bars. These estimates are consistent with seismically determined stress drops.