Superplasticity Owing to Grain Growth in Polar Ices

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Abstract
Deep coring in polar ice sheets has only located the well-known recrystallized ice with a fabric peculiar to tertiary dislocation creep in bottom layers. At lesser depths, anisotropic ice with steady grain-sizes is found; secondary dislocation creep is the dominant process and an anisotropic third-power relation viscosity should ensue. In this paper, ices from the surface down to several hundred metres in depth are considered. Their grain-size increases with time owing to free energy at grain boundaries. This continuous boundary migration appears to be a much more efficient process for relative displacements of the grains than boundary sliding accommodated by diffusional processes between grains of constant size. Locally heterogeneous superplastic deformation leading to moderate viscosities is therefore expected. This deformation mechanism can explain the field data which seem to show a viscosity more than one order of magnitude lower than would result from Nabarro–Herring creep or secondary dislocation creep.