Polar wandering in mantle convection models

We calculate polar motion in models of 3-D spherical mantle convection at R ayleigh numbers up to 10(8) which include internal heating, radial viscosit y variations, and an endothermic phase change. Isoviscous models yield rapi d polar motion of order 3 degrees/Myr, but a factor of 30 increase in visco sity with depth reduces the rate of polar motion to about 0.5 degrees/Myr d ue to stabilization of the large-scale pattern of convection. Avalanching d ue to an endothermic phase change causes pulsating inertial interchange pol ar excursions of order 80-110 degrees and of duration 20-70 Myr. A layered viscosity model with an endothermic phase change yields only one inertial i nterchange event in 600 million years. These models show that the slow obse rved rate of post-Paleozoic true polar wander is not incompatible with high er rates inferred for earlier times.