Convection in polar ice sheets as a model for convection in the Earth's mantle

Abstract

Vertical temperature gradients lead to density inversions in polar ice sheets, a situation that may lead to thermal convection in viscous fluids. The conditions necessary to initiate and maintain thermal convection in ice sheets are examined. The principal difficulty arises from the viscoplastic nature of ice and the importance of crystal fabric on ice flow. Available data indicate that critical Rayleigh numbers for convection can be attained by dislocation creep, but not by diffusion creep, the former being strongly dependent on grain fabric whereas the latter is strongly dependent on grain size. At the central ice divide no grain fabric should exist prior to convection, and once begun, convection flow should create a favorable fabric in this region. Once begun, therefore, convection flow should reinforce itself. Polar ice sheets can be considered as laboratories for studying the probability of convection in the earth's mantle. Having no phase changes and being heated from below, polar ice sheets resemble a simplified, miniature mantle, and can duplicate the boundary conditions imposed on the mantle by the various convection models. Convection appears possible for ice thicknesses, which may or may not require basal melting. This is analogous to mantle convection, which may or may not extend to the earth's fluid core. Conditions for convection are most favorable at the central ice divide of the Antarctic ice sheet, and a borehole should be drilled in this region to investigate the mode of heat transport.