ROTATION, TRANSLATION, AND VERTICAL DISPLACEMENT OF SUPRAGLACIAL CLASTS DURING SUMMER ABLATION, MATANUSKA GLACIER, CHUGACH MTS., AK

Research at the Matanuska Glacier during an NSF sponsored REU program, operated by Augustana College and CRREL, revealed several phenomena associated with supraglacial clasts. Depending upon their thickness and surface area, clasts were observed to move across the ice surface and either became inset into the glacier surface or elevated on a pedestal of ice. Experiments measured clast movements over a three-week period, and explored factors controlling movement. Fifty-one dark, phyllitic cobbles and boulders displayed translations between 3.7 cm (0.18 cm/d) and 26 cm (1.24 cm/d), and a maximum clockwise rotation of 285°. Ten clasts built pedestals up to 6 cm, six remained at grade, and 35 melted into the ice downward to 9 cm. Other experiments investigated the intriguing debris tails present behind translating clasts. Debris tails were spray painted to monitor the movement of sediment, which was observed entering the tail as a boulder slides off a pedestal, across debris-covered ice. Azimuths of 60 debris tails, taken from random boulders on clean and transition ice near the medial moraine, revealed a mean azimuth of 210°, indicating movement towards the southwest. Another experiment placed 21 boulders and cobbles on constructed sediment beds. Simulated tails revealed a mean azimuth of 213°, a maximum translation of 32 cm (1.5 cm/d), and a maximum pedestal height of 15 cm. Additional experiments determined the effect of sediment thickness on ablation rates. The mean ablation for clean ice during the study period was 184.8 cm. A 5 mm thick cover increased ablation another 7.8 cm. At 1 cm of cover, a threshold is approached; mean ablation exceeded clean ice by 1.7 cm, but positive relief features (reflecting reduced ablation) grew to 12 cm in height. A 2 cm cover increased ablation by 5.1 cm beyond the mean, but also produced positive relief features up to 15 cm tall. Boulders translate towards the southwest, as they slide off their pedestal at an angle of repose affected by the path of the sun. The mechanism entrapping debris into the tail remains debatable, with hypotheses involving sheetwash, spallation, or incorporation by over-riding being considered. This research helps to explain ice surface micro-topography and the mechanisms degrading the surficial expression of debris bands, medial moraines, or colluvium.