Laboratory Simulations Of Glacial Abrasion: Comparison With Theory

Abstract

AbstractGlacial abrasion was simulated in experiments in which a small artificial glacier bed was pushed beneath a fixed ice block under pressure. The experiments provide a means of testing theoretical models of abrasion, particularly those factors that govern the magnitude of stress concentrations beneath abrading rock fragments. In preliminary experiments, vertical ice flow around a sphere mounted on the bed was studied. In subsequent experiments, marble tablets were pushed beneath granitic rock fragments frozen into the base of the ice block. Unlike previous abrasion experiments, the sliding velocity was realistic (25 mm d−1), and ice near the bed was at the pressure-melting temperature. Resultant striations closely resemble those observed on glaciated bedrock.As predicted by Hallet (1979), the component of the ice velocity towards the bed strongly influenced stresses beneath fragments, and classical regelation and creep theory provided an approximate estimate of the downward drag force on fragments. Half of the rock fragments rotated significantly, accounting for 10–50% of their motion relative to the bed and influencing abrasion rates and the shear stress supported along the ice-bed interface. Striation patterns indirectly suggest that fragment rotations were inhibited by increases in ice pressure, which presumably increased the drag on roughness elements on fragment surfaces. This may have resulted from a reduction in the thickness of the water film around fragments, facilitated by leakage of water from the bed.