Thresholds in the sliding resistance of simulated basal ice L. F. Emerson and A. W. Rempel 1272 University of Oregon, Department of Geological Sciences, Eugene, OR 97403, USA
Abstract. We report laboratory determinations of the shear resistance to sliding
melting ice with entrained particles over a hard, impermeable surface. With
higher particle concentrations and larger particle sizes, Coulomb friction
at particle-bed contacts dominates and the shear stress increases linearly
with normal load. We term this the sandy regime. When either particle
concentration or particle size is reduced below a threshold, the dependence
of shear resistance on normal load is no longer statistically significant.
We term this regime slippery. We use force and mass balance considerations to
examine the flow of melt water beneath the simulated basal ice. At high
particle concentrations, the transition from sandy to slippery behavior
occurs when the particle size is comparable to the thickness of the melt
film that separates the sliding ice from its bed. For larger particle sizes,
a transition from sandy to slippery behavior occurs when the particle concentration drops
sufficiently that the normal load is no longer transferred completely to the
particle-bed contacts. We estimate that the melt films separating the
particles from the ice are approximately 0.1 µm thick at this
transition. Our laboratory results suggest the potential for abrupt
transitions in the shear resistance beneath hard-bedded glaciers with
changes in either the thickness of melt layers or the particle loading.
Citation: Emerson, L. F. and Rempel, A. W.: Thresholds in the sliding resistance of simulated basal ice, The Cryosphere, 1, 11-19, doi:10.5194/tc-1-11-2007, 2007.