Wednesday, November 01, 2017 at 10:46AM
Benson room 180
Drumlins—the streamlined hills that locally formed in large groups at the beds of Pleistocene ice sheets—have been the subject of over 1300 publications in the last 160 years. Despite this interest, the origin of drumlins is an enduring mystery, in part, because modern case studies are rare. The recently exposed drumlin field in the forefield of the receding surge-type glacier, Múlajökull, provides an unusual opportunity to build a model of drumlin formation based on observations in a modern drumlin-forming environment. These observations indicate that surges deposit till layers that drape the glacier forefield, conform to drumlin surfaces, and are deposited in shear without evidence of compressional thickening. Stratigraphic and geotechnical data indicate that erosion helps create drumlin relief, effective stresses in subglacial till are higher between drumlins than within them, and during quiescent flow crevasses on the glacier surface overlay drumlins while subglacial channels occupy intervening swales.
In a new model of drumlin growth, I consider gentle undulations on the bed bounded by subglacial channels at low water pressure. During quiescent flow, slip of temperate ice across these undulations and basal water flow toward bounding channels create an effective-stress distribution that maximizes till entrainment in ice on the heads and flanks of drumlins. Crevasses amplify this effect but are not necessary for it. During surges effective stresses are uniformly low, and the bed shears pervasively. Vigorous basal melting during surges releases debris from ice and deposits it on the bed, with deposition augmented by transport in the deforming bed. As surge cycles progress, drumlins migrate down-glacier and grow increasingly rapidly, due to positive feedbacks that depend on drumlin height. Drumlin growth can be accompanied by either net aggradation or erosion of the bed, and drumlin heights and stratigraphy generally correspond with observations.
Refreshments and a social hour will follow the talk in Benson 185.