The deformation of ice crystals is dominated by the glide of dislocations on the basal plane. Slip or climb of dislocations on non-basal planes cannot be dismissed completely with regard to dislocation multiplication and the required compatibility between grains. The large anisotropy of ice crystals lies at the origin of the development of a non-uniform state of internal stresses during the transient creep of polycrystalline ice. Large strain inhomogeneities within grains are induced by the incompatibility of deformation between grains. For the low stress conditions prevailing in polar ice sheets, grain-boundary migration
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