University of Cambridge > Talks.cam > Scott Polar Research Institute - Physical Sciences Seminar > Mechanisms of englacial conduit formation and their implications for subglacial recharge

Mechanisms of englacial conduit formation and their implications for subglacial recharge

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Ideas about the character and evolution of englacial drainage systems have been deeply influenced by the theoretical model developed by Shreve (1972). The Shreve model is based on three main assumptions: (1) englacial drainage is in steady state; (2) englacial water will flow along the steepest hydraulic gradient within the glacier; and (3) pressure head equals the pressure of the surrounding ice minus a small component due to melting of the walls. The Shreve model has been widely adopted as a fundamental component of englacial drainage theory.

To evaluate Shreve’s theory, we used speleological techniques to directly survey englacial conduits. We have explored more than 45 distinct englacial conduits to ice depths of 110 m and mapped a total of 9.5 km of passage in 28 conduits in temperate, polythermal, cold-based and debris-covered glaciers between 2005 and 2009. New information reported here is supplemented by published data on 40 other englacial conduits located worldwide and surveyed to ice depths of 203 m using speleological techniques. In all cases, englacial drainage systems consisted of a single unbranching conduit. Englacial conduit morphologies were found to be intimately linked to the orientation of a glacier’s principal stresses or the presence of pre-existing lines of high hydraulic conductivity. If a sufficient supply of water is available, hydrofracturing forms vertical conduits in zones of longitudinal extension and subhorizontal conduits where longitudinal stresses are compressive. On unfractured glacier surfaces, relatively shallow subhorizontal conduits with migrating nickpoints form by cut-and-closure provided channel incision is significantly faster than surface lowering. Conduits can also form along permeable debris-filled crevasse traces that connect supraglacial lake basins of different potential. Only conduits formed by extensional hydrofracture were found to be connected to glacier beds. Our results suggest that a Shreve-type englacial drainage system probably does not exist and implies that englacial conduits can only penetrate through thick ice to recharge the bed of the Greenland Ice Sheet where supraglacial water bodies either intersect, or are advected through, zones of acceleration.

This talk is part of the Scott Polar Research Institute - Physical Sciences Seminar series.

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