HR: 16:45h
AN: C34A-04    [Abstracts]
TI: Glaciology and Geomorphology at the Mountain Crest: Cirque Development
AU: * MacGregor, k R
EM: macgregor@macalester.edu
AF: Macalester College, Geology Department, St. Paul, MN 55105 United States
AU: Sanders, J W
EM: jws@seismo.berkeley.edu
AF: UC Berkeley, Department of Earth & Planetary Science, Berkeley, CA 94720 United States
AU: Mathers, G
EM: gmathers@macalester.edu
AF: Macalester College, Geology Department, St. Paul, MN 55105 United States
AU: Dwyer, C
EM: cdwyer@macalester.edu
AF: Macalester College, Geology Department, St. Paul, MN 55105 United States
AB: The geomorphic signatures of glacial erosion are ubiquitous in alpine landscapes. Cirques are particularly intriguing features in that they are often used as proxies for past climate, e.g., cirque elevations are assumed to represent elevation of the zero-degree isotherm during the LGM. The detailed physical processes acting to form cirques, including cirque glacier ice dynamics, subglacial erosion, and headwall backwearing, are not well understood. We initiated a field study at Grinnell Glacier in Montana to examine the relevant glacial and geomorphic processes driving cirque development. In July 2004, we installed several temperature sensors and snow stakes to monitor snow and ice melt across Grinnell Glacier, as well as a GPS unit to measure ice motion. During July and early August, air temperature and snowmelt correlate well, with typical melt rates between 3 and 6 cm d$^{-1}$ (water equivalent). Average velocity near the center of the glacier, where ice thickness was $\sim$44 meters, was 4.5 to 5.5 cm d$^{-1}$ during this time. Diurnal variability in ice surface velocity, in addition to calculations of expected velocity due to internal deformation only, suggesting sliding is an important component of measured surface motion. Significant debris cover on the glacier, as well as active rockfalls during our field excursions, imply rapid headwall backwearing, perhaps on par with estimated rates of subglacial erosion.
DE: 1815 Erosion and sedimentation
DE: 1827 Glaciology (1863)
DE: 1863 Snow and ice (1827)
DE: 1625 Geomorphology and weathering (1824, 1886)
SC: Cryosphere [C]
MN: 2004 AGU Fall Meeting