AN EXPERIMENTAL STUDY OF GLACIER SLIP OVER HARD AND SOFT BEDS

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Movement of glaciers and ice sheets affects their stability and attendant sea-level rise as the climate warms. In addition, ice-sheet movement during the Pleistocene left a spectacular imprint on landscapes. Glaciers that move fastest and modify landscapes most severely move primarily by slip over their rock (hard) or sediment (soft) beds. Increasingly sophisticated theories of this slip have been advanced over the last half-century, but these theories are largely untested, owing to the inaccessibility of glacier beds and the spatial and temporal variability of conditions there. The objective of this project is to use a newly constructed laboratory device to study relationships among glacier slip velocity, basal drag (slip resistance) and effective pressure (ice pressure minus water pressure at the bed). The device drags a ring of melting ice (0.9 m outside diameter, 0.2 m wide, 0.15 m thick) across a hard or soft bed. Features of the device include unlimited slip displacement, temperature control to a few hundredths of degree, and continuous observation of ice sliding and flow separation at the bed. One set of experiments will test the theory that as slip velocity increases, steady drag on a rough, hard bed attains a maximum value and then either remains constant or decreases, depending upon the bed geometry. Basal drag, slip velocity, and effective pressure will be individually varied during slip of ice at its melting temperature over stepped and sinusoidal bed surfaces. Numerical modeling of ice flow under the geometric constraints of the experiment will allow theoretical results to be compared directly with experimental data. A second set of experiments will provide relationships among slip velocity, drag, and effective pressure for soft beds. These experiments will also reveal both the extent to which ice invades the pore spaces of soft beds by melting and refreezing and the degree to which soft beds deform to accommodate slip?problems central to evaluating sediment transport by glaciers. Robust quantitative models of glacier slip are needed to forecast the speeds of glaciers and ice sheets. Glaciers have been observed surging, i.e. moving forward at unusually high velocities, and retreating at various rates, apparently in response to changing climate. The experimental studies of this project will examine glacier movement in a a way that can not be determined from field observations. A more complete analysis of the sliding process will allow glacier slip to be treated more accurately and less arbitrarily in computer models of glacier and ice-sheet motion. This will permit better characterizations of environmental change in the near future and of landscape evolution in the distant past.