Multiple scales in mantle convection

The multiple-scales structure of mantle convection remains largely unknown. A 3D model of convection is used to investigate the effect of a viscosity step on the multiple-scales pattern of an infinite Prandtl number convection for Rayleigh numbers (Ra) from 10(5) to 1.75 X 10(7). The planform of the large-scale convection is polygonal and the ascending plumes are quite steady even for a Rayleigh number of 1.75 x 10(7). When the Rayleigh number exceeds a value of 3 X 10(5) the upper thermal boundary layer becomes unstable. Small-scale instabilities appear away from the plume and they are advected by the large-scale flow. The pattern of these small-scale flows evolves with the Rayleigh number. They form circular-like waves centered on the axis of the plume for Ra = 10(6). An increase of the Rayleigh number, above Ra = 5 x 10(6), leads to the generation of cold drops inside the circular waves. Finally, for Ra = 1.75 X 10(7), the circular waves disappear at some distance from the plume axis. Then, the small-scale flow consists of cold drops. As they are advected by the large-scale flow, the cold drops mature inside cold radial 'gutters'. This superimposition of small-scale instabilities on the large-scale flow clearly results from the viscosity jump. The implications of these models appear more straightforward in the case of Venus than of the Earth due to the absence of moving plates in the model. The existence of small-scale convection around the plumes could explain the presence of volcanoes in the plains around the equatorial highlands of Venus. (C) 2000 Published by Elsevier Science B.V. All rights reserved.