The Effect of a Low Viscosity Layer on Convection in the Mantle

Abstract

Summary In this paper we attempt to find a solution of the following problem. It appears reasonable to expect that if thermal convection occurs in the Earth’s mantle, it may also occur within the Moon and Mars. The dimensions of these latter two bodies are comparable to the thickness of the Earth’s mantle. Presumably the amount of radioactive heat generated per unit mass is similar in all three bodies. Yet the surface morphology of the Earth, which many scientists believe arises ultimately from mantle convection, differs markedly from that of the Moon or Mars. The explanation advanced here for this difference is based on the effect produced on convection in the mantle by the presence of a low ‘viscosity ’ or low creep strength layer. It is assumed that the low velocity layer of the mantle is such a low creep strength layer. A low viscosity layer changes the amount of ‘coupling’ between the outer crust and mantle convection. (The crust is ‘coupled’ if mantle convection produces stresses in the crust which are large enough to deform it plastically. The crust is ‘decoupled’ from the mantle if these stresses are insufficient to produce plastic deformation.) The theory assumes that the viscosity or creep strength is essentially zero in the low viscosity layer. The analysis is similar to that developed earlier for the calculation of stresses within the mantle. We find that the deeper within the mantle lies the low viscosity layer the greater is the coupling of the outer crust to the mantle convection currents. If the low creep strength layer lies close to the surface the outer crust is decoupled from the interior. According to the literature the depth of the low velocity layer is determined by the temperature and pressure profiles within the mantle. If the temperature profile were kept constant but the pressure reduced by changing g, the gravitational acceleration, the low velocity zone would be moved to a level closer to the surface. It is proposed that because of this interdependence between temperature, pressure and depth of the low velocity layer, the low velocity layers in Mars and the Moon (if such layers exist in these bodies) lie much closer to the surface than does the corresponding layer in the Earth. Hence the outer crusts of Mars and the Moon are more nearly decoupled from the interior than is the Earth’s crust.