Collaborative Research: Toward an Erosion Rule for Glacial Quarrying: Modeling and Measurements

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Collaborative research toward an erosion rule for glacial quarrying: Modeling and measurementsDenis Cohen and Neal Iverson EAR-0229692 Iowa State UniversityThomas Hooyer EAR-0229526 Wisconsin Geological SurveyAbstractErosion of rock by glaciers results in spectacular landforms, such as horn peaks and U-shaped valleys, and can also have a dramatic effect on rates of rock uplift in mountain belts and associated changes in weathering rates, CO2 production, and climate. Quarrying, the process in which glaciers crack underlying rock and remove resultant rock fragments is thought to be the most important process of glacial erosion. There is at present, however, no comprehensive quantitative model of quarrying that fully links glaciological variables, such as sliding speed and basal water pressure, to quarrying rate. Thus, results of models of glacial erosion applied over broad areas (e.g., mountain belts) and long periods (10^5-10^6 years) are uncertain, both quantitatively and conceptually.With this research, a versatile theoretical model of quarrying will be constructed and then tested, both with real-time measurements of crack growth in bedrock beneath a glacier and with mapping of crack patterns in bedrock recently exposed by glacier retreat. Ice flow will be modeled over bedrock consisting of inclined steps by fully coupling water pressure in cavities in the lee of these steps with sliding speed and cavity size for both steady and transient cases. Results will be used to calculate the stress distribution in the rock bed and the consequent speeds and paths of cracks, as they slowly extend and isolate rock blocks that can be extracted easily by the glacier. The model will be tested by installing inclined rock steps beneath 210 m of sliding ice at Engabreen, a temperate glacier in northern Norway, where the Svartisen Glaciological Observatory provides human access to the glacier-rock interface. Sound waves generated by growth of cracks in the steps will be used to determine the relation between glaciological variables and crack-growth speeds and paths in the steps. To further test aspects of the quarrying model, sizes of former subglacial cavities and distributions and orientations of associated glacially-induced cracks will be mapped on stepped bedrock in front of Castelguard Glacier, Alberta, and compared with model predictions. The overall result of this work will be a quarrying rule that can serve as the basis for large-scale models of glacial erosion.