Crevasses on glaciers1

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Abstract
A review of observations on crevasse orientation relative to principal stress or strain rate shows that in many instances crevasses are not aligned in the optimum direction perpendicular to the direction of principal tensile stress. Moreover, a number of studies report the occurrence of strike‐slip motion, with the crevasse walls moving parallel to each other. These observations cannot be explained by the conventional model for crevasse formation, introduced over a century ago by William Hopkins. Instead, it is proposed that crevasses are the manifestation of mixed‐mode fracturing, combining the opening mode usually associated with crevassing, with the shearing mode, which is equivalent to strike‐slip faulting. In a biaxial stress field, the crevasse will tend to orient itself in the optimum direction, thus minimizing the shearing motion, but this direction is achieved asymptotically. This means that on glaciers subject to tension and lateral shear, the orientation of crevasses may differ by up to 20° from the optimum direction. Further, by incorporating the shearing mode, the depth to which fractures can penetrate may be significantly larger than for the opening mode only. The weight‐induced lithostatic stress increases with depth below the surface, so that at some depth, the walls of a crevasse cannot physically separate. However, it is possible that the shearing motion acts over greater depths, thus allowing the fracture to penetrate deeper without separation of the fracture surfaces. This might explain why, on some glaciers, crevasse traces persist well into the ablation area, where surface melting should have obliterated all visible traces of crevasses formed upglacier, if these crevasses penetrated only a few tens of meters, as predicted for the opening mode.

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