Satellite Gravity Measurements and a Laminar Viscous Flow Model of the Earth's Mantle

The existence of nonhydrostatic low-degree harmonic terms in the earth's gravitational field has been proved by satellite observations. The associated variations of density in the deep mantle might have arisen during the earth's formation and been maintained since by the finite strength of the mantle. Alternatively they may result from convection, in which case there should be a relation between the gravity anomalies and the surface features from which the existence of convection has been inferred, for example, the oceanic rises and regions of mountain building. If a Newtonian viscosity is attributed to the mantle and if laminar flow is assumed, the density distribution and the flow pattern in the mantle can be simply related. It is shown that the tractions exerted by the convection currents on the crust can be computed directly from the coefficients of the spherical harmonic terms of the gravity data. The patterns so derived show an approximate relationship to the oceanic rises and zones of compression.