Probably the most important mechanism of glacial erosion is quarrying: the growth and coalescence of cracks in subglacial bedrock and dislodgement of resultant rock fragments. Although evidence indicates that erosion rates depend on sliding speed, rates of crack growth in bedrock may be enhanced by changing stresses on the bed caused by fluctuating basal water pressure in zones of ice-bed separation. To study quarrying in real time, a granite step, 12 cm high with a crack in its stoss surface, was installed at the bed of Engabreen, an outlet glacier of the Svartisen Ice Cap, Norway. Acoustic-emission sensors monitored crack-growth events in the step as ice slid over it. Vertical stresses, water pressure, and cavity height in the lee of the step were also measured. Water pressure was manipulated by pumping water to the lee of the step. With repeated pump tests, the crack on the stoss face grew until the step's lee surface was quarried indicating that fluctuating water pressure caused stress thresholds required for crack growth to be exceeded. To fully comprehend quarrying mechanisms, another field effort will measure quarrying rates as a function of sliding speed during the spring melt season when sliding speed is expected to increase by a factor of 2 to 3. Acoustic emissions will be correlated to sliding speed and also to natural water-pressure fluctuations. Results will allow an assessment of the relative effects of sliding speed and water-pressure variability on quarrying. By providing the first measurements of crack growth in subglacial rock, this project will provide the necessary data to correlate glaciological variables to quarrying rates, removing the ambiguity that exists presently in landscape evolution models which are grounded on uncertain erosion rules. The field data will be combined with a theoretical model of glacial quarrying. The culmination of this work will be a quarrying rule that can serve as a basis for large-scale erosion models.
