SOURCES AND CHARACTER OF DEBRIS COVER ON EMMONS GLACIER, MOUNT RAINIER

Mount Rainier is a 4392-meter stratovolcano located 80 kilometers southeast of Seattle, Washington. Twenty-five glaciers cover the mountain’s upper flanks, and several terminate under thick debris cover at ~1200 m to 1700 m asl. Debris cover is known to impact glacial mass balance and surface morphology, conducting heat when present in thin layers but insulating ice at greater thicknesses. We are characterizing debris cover on Emmons Glacier, the largest glacier on Mount Rainier with a surface area of ~ 11 square kilometers, where ~ 2 square kilometers are covered by debris. We distinguish multiple surface debris units based on weathering extent, moisture content, and sparse estimates of debris cover thickness that indicate multiple debris sources. Debris cover is thinner along the centerline of the glacier near a supraglacial stream, and thickest closer to the surrounding valley walls, which may also source debris. Angular debris is most common, suggesting rockfall dominates the debris supply. Forthcoming petrographic analysis of selected debris samples may provide additional constraints on debris sourcing. Temperature probes installed above and within the debris cover suggest larger diurnal fluctuations in temperature in the first few centimeters of debris, than in probes placed 10 cm or deeper in the debris cover; a thick cover is an efficient insulator of underlying ice. Our work suggests a rockfall-dominated debris cover that is preserving glacial ice, particularly at the glacier margins.