In this chapter we first reviewed the crystal structure of ice, and noted that there are imperfections in this structure, called dislocations, that allow ice (and other crystalline materials) to deform under stresses that are low compared with the strength of individual molecular bonds. Processes that may limit the rate of deformation are those which (1) inhibit motion of a dislocation in a single crystallographic plane (drag), (2) prevent dislocations from climbing from one crystallographic plane to another to get around tangles, (3) impede motion on certain crystallographic planes, and (4) inhibit adjustments of boundaries between crystals.
Experimental data do not, at present, provide a basis for choosing between these possible rate-limiting processes. However, the drag mechanism does provide a theoretical basis for the commonly observed value of the exponent, n, in the flow law (see Equation 4.4). Perhaps equally important, however, are the mechanisms that allow adjustment of grain boundaries.
Because some crystals in a polycrystalline aggregate are not oriented for easy glide, stress concentrations develop. These result in recrystallization by three distinct processes: grain growth, polygoniza-tion, and nucleation of new grains. Recrystallization leads to preferred orientations of c-axes, and hence to more rapid deformation. The principal processes involved in the development of these fabrics appear to be nucleation of new grains and rotation of grains as slip occurs on their basal planes.
To place the creep processes in ice in a more general framework, we introduced a deformation mechanism map in which we displayed the range of temperatures and stresses under which different deformation processes occur. Within the temperature and stress ranges normally found in glaciers, power-law creep is likely to be the dominant process although diffusional creep may occur in some low stress situations.
Next, we introduced Glen's flow law, and related the exponent, n, in the flow law to the creep mechanisms discussed earlier. Then we considered how temperature, pressure, texture, fabric, and water content affect the viscosity parameter, B. Temperature and pressure effects may be incorporated into the flow law by rigorous, physically based modifications, whereas ad hoc procedures based on empirical evidence are used to incorporate the other effects.
Finally, we introduced principles of linear elastic fracture mechanics and demonstrated that these principles can be used to estimate crevasse depths.
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