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An experimental study of ice-bed separation during glacial sliding

Separation of sliding ice from hard beds plays a central role in theories of subglacial hydrology, sediment transport, and quarrying of subglacial bedrock. Despite a half-century of interest in cavities at glacier beds, there are no data establishing relationships among steady cavity size, bed geometry, sliding speed, and effective pressure. Field studies are complicated by unsteady behavior and various factors that are poorly known, including the local effective pressure at the bed, bedrock geometry, and cavity size. A laboratory ring-shear device allows sliding and ice-bed separation to be studied experimentally. The apparatus drags a ring of ice (0.9 m O.D., 0.2 m wide, 0.2 m thick) across a stepped, rigid bed. The steps are 0.18 m long and 0.027 m high along the ice-ring centerline, with treads inclined uniformly 8 ̊ up-flow. Sliding speed and effective pressure are controlled, while cavity volume and bed and wall temperatures are recorded. A glycolwater mixture, which is regulated to ±0.01  ̊C with an external circulator, keeps ice at the melting temperature and melt rates low. Post-experimental measurements of the ice ring's basal topography provide reconstructions of cavity geometries. Ice c-axis orientations are measured throughout the ice ring using a Rigsby (universal) stage. Monotonic cavity growth towards a larger, steady size in response to increased sliding speeds was expected. Instead, cavities initially grew past their steady-state volume, followed by a series of progressively dampened oscillations above and below steady dimensions before reaching a steady size. Steady-state cavities initiated at step edges and had slightly curved roofs. Experimental cavity lengths were accurately predicted by a model based on Kamb’s (1987) theory of glacial surging and a new model incorporating Nye’s (1953) borehole closure theory. The c-axes of ice crystals at shear strains ≥ 1 formed steeply