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On the Formation of Cirques by Glaciers


For more than a century, geomorphologists have sought a mechanistic explanation for the formation of alpine cirques, the theater-shaped basins at the heads of valleys and carved into the flanks of mountains like the Matterhorn. It is understood that glaciers in cirques scour their beds and sap their headwalls, but evidence to constrain models has been elusive. Here I present field measurements and numerical analyses of a small alpine cirque glacier that advance our understanding of the formation of cirques. My field site, West Washmawapta Glacier, sits within a cirque carved in Helmet Mountain, and is approximately 1 km long and 1 km wide, with a maximum depth of ~185 m. The glacier and surrounding cirque walls resemble a reclined armchair, with steep head and toe sections connected by a flatter central expanse. I demonstrate that the prevailing view of cirque glaciers - in which the ice mass rotates rigidly above an arcuate bed - is not applicable. Instead, the glacier behaves much like larger temperate glacier systems, with basal stresses that tend toward 105 Pa everywhere. Partitioning between internal deformation and basal sliding is a function of the spatial variation of ice flux and basin geometry. Basal sliding rates are minimum beneath the glacier center, in the deepest part of the cirque bowl. Along the northern margin and above the stoss side of the riegel, however, basal sliding accounts for more than 50% of the surface velocity. The `classic' cirque form found at Helmet Mountain is maintained by erosion of the glacier bed and transport of loose debris away from the headwall by the glacier. Using a sediment budget approach, we show that over the past few centuries, the cirque has elongated and deepened at roughly equivalent rates of order 1 mm/yr. In 2007, we measured a proglacial stream sediment flux between 70 and 1840 tons per year at the basin outlet, a third of which left the cirque in a period of two days early in the meltseason. Using a combination of remotely-sensed and on-site measurements, I estimate that subaerial headward retreat of the headwall occurred at ~1.3 mm/yr (0.2 - 5 mm/yr). I propose that the steep bedrock slopes characteristic of the headwall, which encourage rockfall and snow avalanches, result from undermining by rock fracture and glacial plucking in the bergschrund. A suite of environmental measurements taken in the bergschrund for nearly two years demonstrate subfreezing temperatures, rather than diurnal fluctuations above and below zero, are the norm. I use my temperature measurements, coupled with a numerical description of rock fracture by ice segregation, to show that the bergschrund is a favorable environment for shattering of rock. Only within the bergschrund can periglacial weathering and glacial entrainment conspire to undermine the headwall and thereby play a pivotal role in cirque development.

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