Abstract
AbstractA new ring-shear device allows basal slip and related processes to be studied in laboratory experiments for the cases of hard or soft beds. The device rotates a confined ring of ice (0.9 m outside diameter) across a horizontal bed at a constant velocity or drag, while a vertical stress is applied and basal water pressure is controlled. A bath with circulating fluid regulated to ∼0.01°C surrounds the ice chamber and keeps the ice at its pressure-melting temperature. In a first experiment with a stepped rigid bed and zero basal water pressure, steady lengths of step cavities depended upon slip velocity raised to a power of 0.59, in general agreement with the square-root dependence of some models of sliding and linked-cavity hydraulics. Transient cavity growth after slip velocity increases was not monotonic, with damped volume oscillations that converged to a steady value. Once ice separated from lee surfaces, drag on the bed was constant and independent of slip velocity and cavity size, consistent with a shear-stress upper bound like that indicated by sliding models. Shear strains near the bed exceeded 30 and ice developed multiple-cluster c-axis fabrics similar to those of sheared ice in temperate glaciers.
Highlights
Processes at glacier beds have far-reaching effects (Clarke, 2005)
A final water layer was added to the ice ring, which was pushed upward into contact with the platen by raising the ice chamber and base plate with the hydraulic cylinder
Ice separated from vertical lee surfaces at all slip velocities and, after an increase in slip velocity, cavity volume oscillated before attaining a steady value
Summary
Processes at glacier beds have far-reaching effects (Clarke, 2005). Ice slips, enabling fast glacier flow that can trigger global environmental change (Clark and others, 1999). Ice melts, enhancing slip and sometimes depositing vast basal till sheets Dreimanis and Gibbard, 2005). Rempel, 2008) and reducing the supply of water that may help sustain rapid slip (e.g. Tulaczyk and others, 2000). Debris within ice scrapes against the bed, eroding it but inhibiting slip Subglacial sediment layers shear, causing high sediment fluxes, erosion or aggradation of the bed and conspicuous landforms Bedrock erodes, resulting in Alpine landscapes and modulating uplift in orogenic belts (e.g. Egholm and others, 2009)
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