Rheology of basal ice

In comparison with ice higher in a glacier, basal ice may have fewer bubbles, a different solute content, and more sediment. In addition, it is quite likely to have more interstitial water because strain heating is significant here, and there is no way to remove this heat other than by melting ice. Finally, the constant changes in stress field as the ice flows around successive bumps may result in zones of transient creep as the crystal structure adjusts to the changes.

In a unique experiment to study these effects, Cohen (2000) utilized the facilities of the Svartisen Subglacial Laboratory described above. He placed an instrumented flat-topped conical obstacle at the base of the glacier under 210 m of ice. The obstacle was 0.15m high, and was 0.05 m in diameter at its top and 0.25 m at its base. Cohen measured forces on the obstacle, temperatures at many places in it, and the speed with which ice flowed past it. He then modeled the flow with the use of a fully three-dimensional numerical model employing the finite-element method (Chapter 11). Assuming n = 3, he found that the observed forces and ice speeds could be reproduced in the model with values of B ranging from 0.06 to 0.13 MPa a1/3. A normal value for temperate ice with little or no interstitial water would be slightly more than 0.2 MPa a1/3 (Figure 4.18). Because n « 3, the 2- to 4-fold reduction in B results in an 8- to 64-fold increase in e.

Cohen (1998,2000) also studied the structure and texture of the basal ice at the site of the experiment. The ice contained sediment-bearing lamellae, several millimeters thick, interlaminated with clean ice. This is very typical of basal ice from both temperate and polar glaciers. Debris concentrations in the sediment-rich layers at the level of the obstacle were about 20% by volume. The cross-sectional area of the crystals averaged ~7 mm2 compared with ~50 mm2 in the overlying clean ice. There was no preferred orientation of c-axes, so this could not explain the low value of B. Cohen also measured the water content of the basal ice and found that it was ~2%. A nonlinear extrapolation of the data in Figure 4.18 suggests that even this high a water content cannot explain the low viscosity. Instead, Cohen suggested that unbound water at the interface between the ice and the sediment particles acts as a lubricant, enhancing sliding between the sediment-rich layers and the lamellae of clean ice. Such interfacial water layers are nanometers in thickness.

Echelmeyer and Wang (1987) also found that ice in the basal zone of Urumqi Glacier No. 1 in western China deformed much more readily than clean ice. In this case, the material involved was ice-cemented drift with an ice content of ~31% by weight. The temperature was -2 °C. The measured deformation rate would correspond to a value of B of ~0.04 MPa a1/3. They, too, attributed the softness of the drift to liquidlike interfacial water layers.

At lower temperatures, dirt appears to strengthen ice, presumably because the amount of unbound water decreases. In a series of experiments at -9.1 °C, Hooke et al. (1972) found that B was ~85% higher in ice with 20 volume percent dispersed fine sand than it was in clear ice. They also found that B increased approximately linearly with sand content. They concluded that sand particles inhibited movement of dislocations, and attributed the strengthening of the ice to development of dislocation tangles in the vicinity of the particles.

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