The viscosity of the mantle

Creep under low stresses is by diffusion and has a linear relation between stress and strain rate; it also obeys the Navier-Stokes equation. Therefore the viscosity of the mantle may be calculated from solid state theory and also from the slow deformations of the Earth. The viscosities derived by these methods are in reasonable agreement, and both show that the viscosity of the lower mantle is ∼ 105 greater than that of the upper. This high viscosity prevents polar wandering and lower mantle convection. Some suggested modifications of the viscosity depth calculations from post glacial uplift may improve their accuracy considerably