Convection studies of temperature-dependent Newtonian and non-Newtonian rheology in aspect-ratio seven boxes have been carried out for volume averaged Rayleigh numbers between O(105) and O(107). Large lateral viscosity variations, O(104) are found for non-Newtonian cases with total temperature-dependent viscosity variations up to 250. Flow structures for both rheologies are distinguished by large-scale circulations with relatively stable descending limbs. Much larger hot thermal anomalies are found near the surface for non-Newtonian rheology. Lithospheric thinning is facilitated by non-Newtonian rheology because of stress-softening and the lubrication of the descending limbs by hot diapirs. ¿ American Geophysical Union 1993