ARTHUR L. DAY MEDAL: BASAL SLIP OF GLACIERS: CREEPING TOWARD BETTER PARAMETERIZATIONS

Glaciers move as creeping viscous fluids but, unlike most fluids, can slip over their beds. The results of that slip are spectacular landscapes of glacial erosion, vast basal till sheets, conspicuous bedforms such as drumlins, and, most importantly, fast glacier flow that helps destabilize ice sheets and raise sea level. Laboratory and field experiments, together with study of basal tills, help reveal the kinematics and mechanics of slip over soft and hard beds. Soft beds generally shear in patches, to shallow depths (O(0.1) m), with Coulomb shearing resistance subject to pore-pressure feedbacks that can cause transient rate-weakening or strengthening. Bed deformation patterns are tied to effective stress variations. Cavity formation during slip over 2D hard beds causes pronounced rate-weakening drag. This effect, however, may be inhibited by debris-bed friction and the 3D variability of bed morphology.

None of these factors is considered in Weertman-style slip parameterizations that are used in large-scale models of glacier flow and bedrock erosion. A path forward involves modeling ice flow in the boundary layer where basal slip is accommodated. Including 3D bed morphology, debris-bed friction, and bed deformation in such models will allow the uncertainty of basal-slip parameterizations to be assessed, with potentially sobering results.