Basal motion

Field research indicates that basal motion is essentially governed by a small number of factors, including basal shear traction, bed temperature and the presence and pressure of water at the basal interface (whether hard or soft). Basal water pressure is the least-well constrained of these controls, being governed by a balance between water delivery to the glacier base and water removal from it. Although our understanding of spatial and temporal variations in basal water pressure at temperate valley glaciers is now fairly well advanced, little is known about these variables beneath larger and/or thermally complex ice masses. There is therefore a pressing need for field programmes that simultaneously record motion components and their principal controls at these ice masses. Indeed, some work has already begun in this arena. Bingham et al. (2003), for example, report systematic annual and seasonal motion variations at polythermal John Evans Glacier in the Canadian Arctic.

This lumped basal motion approach should develop in tandem with further detailed investigations of specific processes of basal motion. For example, the effects of variations in bedrock roughness and the integrity of the basal drainage system are currently neglected in sliding models. The balance between sliding by slip, enhanced deformation and regelation also needs to be formalized, perhaps as functions of stress regime, location and basal temperature. Similarly, controls over processes and rates of subglacial sediment deformation need further investigation. Ultimately, a constitutive relation for subglacial sediments is required. This relation should include terms for processes such as ploughing and perhaps even terms for grain-size evolution and the development of clast fabric that models could calculate in a time-evolving manner. Finally, basal motion at subfreezing temperatures also needs to be investigated more thoroughly and at the glacier-wide scale, both at hard- and soft-bedded glaciers.

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