Direct measurement of basal motion at a hard-bedded, temperate glacier: Glacier de Tsanfleuron, Switzerland

A rigid mast, equipped with five multi-turn potentiometers, was bolted to the bedrock floor of a frontal cavity at Glacier de Tsanfleuron, Switzerland. Each potentiometer was linked by a thin cord to an anchor emplaced in the adjacent ice wall, enabling basal ice motion to be measured continually over a 6 day period in the late summer. Results indicate that basal ice velocity increased quasi-linearly away from the glacier bed, rising from 10.66 to 11.82 mm d-1 between 25 and ∼265 mm above the ice-bed interface. Extrapolation of this gradient indicates that ∼10.55 mm d-1 may have been accommodated by pure slip between the ice and the glacier bed. Basal ice motion is temporally variable, generally being characterized by tens of minutes to hours of little or no motion interspersed with rapid-motion events lasting for between <120 s (a single measurement interval) and 360 s. These motion events were responsible for speeds of up to > 400 mm d-1 over individual measurement intervals. The magnitude-frequency distribution of these events is consistent with a pattern of infrequent and large slips initiated at the ice--bed interface, that are manifested as more frequent and smaller motion events some tens of cm above that interface.