Effect of a frozen bed

When the temperature at the base of a glacier is below the pressure melting temperature and the ice is frozen to the bed, it is usually assumed that sliding cannot take place. For most purposes, this is a reasonable assumption. However, Shreve (1984) showed that a liquid-like layer, present at interfaces between ice and foreign particles (including a glacier bed), could result in regelation of ice past bumps on the bed at subfreezing temperatures. The presence of the liquid-like layer is commonly attributed to a change in chemical potential of water adjacent to the foreign surface, much as molecules of a solute change the chemical potential of the water in which they are dissolved (Gilpin, 1979). Foradriving stress of0.1 MPa and a bed roughness spectrum measured by Nye (1970, pp. 386-387), Shreve calculated sliding rates ranging from 3.5 mm a-1 at -20 °C to 35 mm a-1 at -5 °C. Echelmeyer and Wang (1987) measured a sliding speed of 180 mm a-1 at -4.6 °C under Urumqi Glacier No. 1 in

Figure 7.28. Hypothetical distribution of frozen and thawed areas within a transition zone from a region of frozen bed to one of thawed bed. (Modified from Hughes, 1992, Figure 14.)

Figure 7.28. Hypothetical distribution of frozen and thawed areas within a transition zone from a region of frozen bed to one of thawed bed. (Modified from Hughes, 1992, Figure 14.)

western China. After adjusting for differences in driving stress and bed roughness, this speed was consistent with Shreve's theory. These sliding speeds are too low to be of significance glaciologically. However, over a period of years they could result in striations, a possibility which should give nightmares to glacial geologists who commonly interpret striations as evidence of a thawed bed.

As ice moves from a region of frozen bed to one of thawed bed, the sliding speed increases, resulting in geomorphic features like the transverse till scarps (Figure 6.17) and ribbed moraine (Figure 6.18) described in Chapter 6. However, as suggested in Chapter 6, the transition from frozen to thawed bed occurs in a broad zone, not along a single line. Within the transition zone, hill tops and areas underlain by materials with lower thermal conductivity may remain frozen while intervening areas reach the pressure melting point. Thus, there is a gradual transition from completely frozen to completely thawed (Figure 7.28). As the fraction of the bed that is wet increases, the sliding speed increases, but the increase is nonlinear. Even when 85% of the bed is thawed, ub is still only 25% of the speed when the bed is completely thawed, ubmax (Figure 7.29). However, as the fraction of the bed that is frozen decreases, frictional heating from high shear tractions on the remaining frozen areas should raise temperatures fairly rapidly.

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