Crevassing

A better understanding of the relationship between crevasses and flow dynamics may allow crevasse patterns to be used as a tool for quick and easy assessment of the surface strain-rate tensor. A rough appraisal of the flow field would thus be possible for portions of nearly every glacier and ice sheet. This project compares detailed measurements of the surface flow field with crevasse patterns on a reach of a temperate valley glacier. The objective is to investigate the relationship between families of crevasses and the regional flow field (not to study the localized flow field associated with any one crevasse).

Crevasses of the study reach have been mapped by photogrammetric techniques. A computer program was written to calculate the magnitude and orientation of the principal axes at the nodes of an orthogonal grid. The kernal of the program utilizes a velocity-gradient method for calcuation of the principal axes. Analysis indicates that the pattern of crevasses contains a great deal of information about values of the strain-rate tensor as averaged over the 20 m length scale:

• Across the entire study area the crevasses show an orientation which is normal to the axis of greatest extending principal strain-rate (parrallel to the axis of least tensile principal strain-rate).
• Crevasses form a splaying pattern where the longitudinal strain-rate is compressional or near zero and surface deformation is dominated by shear strain. Crevasses become transverse where there is longitudinal extension.
• The crevasses appear to be transient features which represent the stress conditions of their present location -- they are not translated and rotated by the flow field.

Crevasses on Worthington Glacier are a useful tool for mapping the strain-rate tensor at the glacier surface. This may be true for many alpine glaciers where flow, being dominated by basal slip, is fast relative to geometric changes of the channel. Tensional features such as crevasses cannot persist for long as the ice is quickly moved through a changing stress field.

Map showing crevasses of the study reach

Map showing the orientation and magnitude of the principal axes of strain rate.