Lucas K. Zoet

A Theoretical Model of Drumlin Formation Based on Observations at Múlajökull, Iceland

U.S. National Science Foundation nThe Icelandic Research Fund nThe University of Iceland Research Fund nThe Energy Research Fund of Landsvirkjun nThe Carlsberg Foundation nThe Royal Physiographic Society in Lund, Sweden nThe Fulbright Foundation

Subglacial drumlins and englacial fractures at the surge-type glacier, Múlajökull, Iceland: SUBGLACIAL DRUMLIMS AND ENGLACIAL FRACTURES

1 The interaction between drumlins and overriding glacier ice is not well studied, 2 largely due to the difficulty of identifying and accessing suitable active subglacial en3 vironments. The surge-type glacier Múlajökull, in central Iceland, overlies a known 4 field of actively forming drumlins and therefore provides a rare opportunity to inves5 tigate the englacial structures that have developed in association with ice flow over 6 the subglacial drumlins. In this study detailed ground penetrating radar surveys are 7 combined with field observations to identify clear sets of up-glacier and down-glacier 8 dipping fractures at Múlajökull’s margin. These are interpreted as conjugate shear 9 planes or Pand R-type Reidel shears that developed and filled with saturated sedi10 ment derived from the glacier bed, during a previous surge. The fracture sets exhibit 11 focused spatial distributions that are influenced by the subglacial topography. In 12 particular, down-glacier dipping fractures are strongly focused over drumlin stoss 13 slopes. These fractures, although well developed at depth, were mostly unable to 14

Rate-weakening drag during glacier sliding: RATE-WEAKENING GLACIER SLIDING

Accurately specifying the relationship between basal drag on a hard, rough glacier bed and sliding speed is a long-standing and central challenge in glaciology. Drag on a rigid bed consisting of steps with linear treads inclined upglacier—a good idealization for the bedrock morphology of some hard-bedded glaciers—has been considered in sliding theories but never studied empirically. Balancing forces parallel to step treads indicates that drag should be independent of sliding speed and cavity size and set by the limit-equilibrium condition sometimes called Ikens bound. In this study we used a large ring-shear device to slide ice at its pressure melting temperature across a stepped bed, over a range of steady sliding speeds (29–348u2009mu2009yr−1), and under a steady effective pressure (500u2009kPa). Contrary to expectation, drag decreased 42% with increasing sliding speed and cavity size. Experimental deviations from theory cannot explain this decrease in drag with increasing sliding speed (i.e., rate weakening). We suggest that stress bridging in ice between ice-bed contact zones and cavities causes stress gradients that require viscous deformation of ice to sustain stress equilibrium, so that contact zones can be at shear stresses below limit-equilibrium values. A parameter—linearly dependent on sliding speed—that scales the extent of ice deformation to areas of ice-bed contact allows the experimental drag relationship to be fitted with a simple sliding model. Rate-weakening drag has now been observed for two contrasting bed morphologies, stepped and sinusoidal, highlighting the need to consider such behavior in glacier flow models.