AVALANCHE EFFECTS IN-PHASE TRANSITION MODULATED THERMAL-CONVECTION - A MODEL OF EARTHS MANTLE

We describe the development and application of an anelastic, multiphase model of the mantle convection process in axisymmetric spherical shell geometry. The radial structure of the anelastic reference state has been determined on the basis of elastic wave propagation data, primarily those used to construct the preliminary reference Earth model (PREM). The multiphase model is employed to examine the extent to which the pressure-induced phase transitions in the planetary mantle may conspire to cause the flow to become radially layered. We find that the endothermic phase transition at 670 km depth profoundly influences the radial mixing process in the high Rayleigh number regime. In the Earth-like region of parameter space the flow exhibits a low-frequency quasi-periodicity characterized by rather long periods of relative quiescence in which the circulation is predominantly layered followed by short periods of intense radial mixing across the endothermic horizon. These ''avalanche'' events are controlled by the periodic instability of the internal thermal boundary layer that develops on the endothermic horizon when the flow is layered. This hydrodynamic process appears to have important implications for the understanding of a number of characteristics of planetary evolution, especially thermal and chemical history.