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SLIDING AND THE GROWTH AND DECAY OF CAVITIES BENEATH AN ALPINE GLACIER: RESULTS FROM BENCH GLACIER, ALASKA

ANDERSON, ROBERT S  University of Colorado.
Anderson, Suzanne P  University of Colorado.
MacGregor, Kelly R  Macalester College.
Waddington, Edwin D  University of Washington.
O'Neel, Shad  University of Colorado.
Riihimaki, catherine A  Bryn Mawr College.
Loso, Michael G  UC Santa Cruz.

We report the spatial and temporal pattern of sliding on the 7-km long Bench Glacier, Alaska. Using five continuously recording GPS receivers at monuments drilled into the surface ice, distributed at 1 km spacing along the glacier centerline, we documented surface ice motion over 50 days during summer 2002. Surface speeds in two previous winters constrain the motion components associated with ice deformation and any steady basal motion, allowing isolation of the sliding speed history. We observed two speedup events bracketing two weeks of steady slow sliding. The first event did not correspond to a meteorological trigger, was more subtle than the second, and propagated up-glacier at a rate of several hundred meters per day. The second event coincides with a warm up-valley wind, which enhanced the melt rate of the glacier surface. Sliding speeds in this event reached 30 cm/day, and began almost simultaneously at all sites in the ablation area. Both the horizontal and vertical displacement time series can be explained with growth by sliding, and collapse by viscous creep, of cavities in the lee of inclined steps in the bedrock bed. We posit that effective pressure, averaged over some large area of the bed, is inversely proportional to the sliding speed. This effective pressure then controls the collapse rate of cavities, whose dimensions are estimated from a plausible, stepped-bed geometry. This model explains well the horizontal and vertical surface displacement history through the first event and beginning of the second event. The vertical record demands a substantial and abrupt drop in water pressure that departs from the posited sliding-effective pressure relationship. We argue that this pressure drop reflects establishment of efficient subglacial drainage, as manifested in a nearly simultaneous step-increase in water discharge in the exit stream. The establishment of an efficient conduit system defeats sliding; its maintenance inhibits further sliding over the remainder of the summer. This behavior is not unique to Bench Glacier. The general relationship between sliding and the state of the hydrological system we have deduced from Bench Glacier is supported by analysis of the trajectories of surface targets on the much larger Kennicott Glacier during its annual outburst flood.


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