THE DRIVING MECHANISM OF PLATE-TECTONICS

Plate velocities reflect the global mantle circulation which carries the heat from the deep part of the Earth to the surface. Three other measurable parameters put strong constraints on the physical processes involved in mantle dynamics: the surface topography, the geoid and the lateral density heterogeneities mapped throughout the mantle by seismic tomography. Using these internal loads and the Navier-Stokes equations, a first generation of dynamic Earth models have been developed in the last five years. They have produced satisfactory predictions for the divergence of the surface velocities and for the geoid. These predictions are quite sensitive to the assumed radial viscosity variations. The best match with observations has therefore helped to discriminate between various possible viscosity structures. A new set of models taking the existence of plates into account has been particularly successful in predicting the plate velocities, including their vorticity. The driving mechanism of plate tectonics is thus put on a firm physical basis and the computed solutions are quantitatively reasonable.