Understanding the rheological behaviour of ice, particularly at the low stresses relevant to flow within the Earth's cryosphere, is a longstanding problem in glaciology. For purposes of glacier and ice-sheet modelling, the flow of ice is most often described by the Glen flow law e = Bon (1)

where e is strain rate, B is a constant at constant temperature, o is differential stress, and n is the stress exponent. Glen-law flow is and dynamic recrystallization are efficient recovery processes. The low stress exponent corresponding to the deformation of ice at low stresses is in accordance with the primordial role of recrys-tallization in the accommodation of basal slip. Fabrics in glaciers and ice sheets are strain-induced as long as grain growth and rotation recrystallization prevail and stress-induced when migration recrystallization occurs. The enhancement factor in LGM ice appears to be due to the effect of fabric and crystal size. Solid impurities from atmospheric input impede grain growth, but they do not directly affect the ice viscosity.

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