Convection experiments in a centrifuge and the generation of plumes in a very viscous fluid

Abstract

«Plumes» originating from unstable· thermal boundary layers have been proposed to be the preferred mode of small-scale convection in the Earth's mantle. However, doubts have been cast on the validity of the extrapolation from laboratory to mantle-like conditions. In particular, it was feared that inertial effects might be the origin of the observed instabilities. In this paper, experiments are described fbr which inertial effects are negligible. A small aspect-ratio tank filled with a very viscous fluid (Pr = 106) is used to observe the behaviour of convection for Rayleigh numbers up to 6.3 x 10^5. These high values are reached by conducting the experiment in a centrifuge which provides a 130-fold increase in apparent gravity. Rotational effects are small, but cannot be totally dismissed In this geometry thermal boundary layer instabilities are indeed observed, and are found to be very similar to their lower Prandtl number counterparts. It is tentatively concluded that once given a certain degree of «vulnerability», convection can develop «plume»-like instabilities, even when the Prandtl number is infinite. The concept is applied to the earth's mantle and it is speculated that «plumes» could well be the dominant mode of small-scale convection under the lithospheric plates.