Secondary instabilities developed in upwellings of ultra-high Rayleigh number (2-D and 3-D) convection

(1) Unite Mixte de Recherche, CNRS, Toulouse, France
(2) Department of Geology and Geophysics and Minnesota Supercomputing Institute, University of Minnesota, Minneapolis, MN USA
(3) Department of Geological Sciences, Bolder, CO USA
(4) Department of Geophysics, Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic

Abstract

The idea of secondary instabilities developed from plumes has come from recent high Rayleigh number simulations by Vincent and Yuen (2000). There is also evidence from seismic tomography (Bijwaard and Spakman, 1999, Goes et al., 1999) to suggest that mantle plumes are able to branch to some extent. Skilbeck and Whitehead (1978) and Whitehead (1982) have performed laboratory experiments showing the formation of secondary instabilities from a tilted plume in a set-up in which the conduits were made by injecting oil into a more viscous heavier liquid. The purpose of this work is to show how these secondary instabilities can develop as a consequence of the interaction of the shear flow developed by the large-scale circulation and the rising plume. This will be conducted within the framework of a constant viscosity fluid because of the need to understand the fundamental physics and the parameter space in Rayleigh number before pursuing other types of rheology. First, we will employ a two-dimensional axisymmetric spherical-shell model of Moser et al. (1997) within the framework of a Boussinesq fluid. When scaled to the Earth's mantle, the computational domain of this model will have an aspect-ratio of around six. A large enough aspect-ratio is needed to generate a potent enough large-scale circulation, which is needed to bend the plumes sufficiently for inducing the secondary instabilities found in the Whitehead experiments. Second, for comparing with 3-D situations, we have used a three-dimensional Boussinesq model taken from Dubuffet et al. (2000) with a large-enough aspect-ratio of 44441.

Oral contribution

Extended abstract

References
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