ABSTRACT
In a grounded ice sheet, three mechanisms may cause the ice to flow in the direction of decreasing surface elevation. The first mechanism is internal deformation as described by Glen’s flow law discussed in Chapter 2. The ice deforms under its own weight to relieve the internal stresses. The associated deformational velocity is zero at the glacier base and reaches a maximum at the upper surface (Section 4.2), and is determined by the geometry-through the driving stress-and ice temperature. Typically, these velocities range from less than a few meters per year to a few hundred meters per year and change only slowly over time as the glacier geometry adjusts to mass imbalances. The second flow mechanism, basal sliding, becomes important where basal temperatures have reached the pressure melting temperature and subglacial water is present and is responsible for the dynamic behavior exhibited by outlet glaciers of the Greenland and Antarctic ice sheets. The transition from a frozen bed (cold-based glacier) to one that is lubricated by basal water (warm-based glacier) can lead to an increase in discharge by a factor of 10 or more. The third process that may contribute to glacier flow is deformation of subglacial sediments. As noted by Clarke (2005), processes that act in the layer extending a few meters above and below the ice-bed interface can have a greater impact on glacier dynamics than processes operating within the ice itself (such as fabric development and temperature changes), yet this layer remains poorly studied and understood.
