HR: 0800h
AN: H51G-0442    [Abstracts]
TI: Effects of Water-Pressure Fluctuations on Glacial Erosion: a Quarrying Experiment Beneath Engabreen, Norway
AU: * Iverson, N R
EM: niverson@iastate.edu
AF: Iowa State University, Department of Geological and Atmospheric Sciences, Iowa State University, Ames, IA 50011 United States
AU: Cohen, D
EM: dcohen@iastate.edu
AF: Iowa State University, Department of Geological and Atmospheric Sciences, Iowa State University, Ames, IA 50011 United States
AU: Hooyer, T S
EM: tshooyer@facstaff.wisc.edu
AF: Wisconsinan Geological and Natural History Survey, 3817 Mineral Point Road, Madison, WI 53705 United States
AU: Thomason, J F
EM: thomason@isgs.uiuc.edu
AF: Iowa State University, Department of Geological and Atmospheric Sciences, Iowa State University, Ames, IA 50011 United States
AU: Jackson, M
EM: mja@nve.no
AF: Norwegian Water Resources and Energy Directorate, P.O. Box 5091 Maj., Oslo, N-0301 Norway
AB: Bedrock erosion by glaciers can play a dominant role in the evolution of high-latitude landscapes. The process that is least understood but probably responsible for most of this erosion is quarrying: the extension of preexisting cracks in subglacial bedrock and dislodgement of resultant rock fragments. On mountain slopes failure of rock masses is commonly triggered by hydraulic transients caused by rainfall or snowmelt. In contrast, the large hydraulic transients that occur beneath glaciers are usually neglected in models of large-scale glacial erosion. Rather in these models erosion rates are assumed to be a simple function of sliding velocity or ice discharge. To study quarrying in real time, a granite step (0.12 m high) was installed under 213 m of ice at the bed of Engabreen, a temperate glacier in Norway. The step protruded upward into sliding ice and was inclined up-glacier. A crack, 2 mm wide and 31 mm deep, was cut across the step, normal to its stoss surface. Acoustic-emission sensors, tested in laboratory rock-fracture experiments, monitored the growth of the crack. Water was pumped under high pressure to the bed to simulate water-pressure fluctuations like those that occur in hydraulically active areas of glacier beds. These pump tests caused a cavity between the ice and bed to open and close down-glacier from the step. Normal stress on the step and the frequency of acoustic emissions increased markedly during closures of the cavity that followed decreases in water pressure. Acoustic emissions emanated from the base of the crack. Over a three-day period the locus of emission sources extended obliquely about 90 mm toward the base of the lee surface of the step, presumably reflecting crack extension. Post-experimental inspection of the step showed that it had indeed been quarried along the trajectory indicated by acoustic emissions. These data indicate that crack growth can occur rapidly as a result of stress differences in the bed caused by decreasing water pressure in zones of ice-bed separation. Fluctuating water pressure may commonly be necessary to exceed stress thresholds required for crack growth. Thus, long-term erosion rates may not be well characterized by models that neglect these fluctuations.
DE: 0720 Glaciers
DE: 1815 Erosion
DE: 1824 Geomorphology: general (1625)
DE: 1827 Glaciology (0736, 0776, 1863)
DE: 1862 Sediment transport (4558)
SC: Hydrology [H]
MN: Fall Meeting 2005