HR: 0800h
AN: C31B-0325    [Abstracts]
TI: A Multifaceted View of a Glacier Speed-Up Event
AU: * Harper, J T
EM: joel@mso.umt.edu
AF: Dept. of Geology Univ. of Montana, 32 Campus Drive, #1296, Missoula, MT 59802 United States
AU: Pfeffer, W T
EM: pfeffer@tintin.colorado.edu
AF: INSTAAR Univ. of Colorado, 1560 30th St., Boulder, CO 81309 United States
AU: Humphrey, N F
EM: neil@uwyo.edu
AF: Dept. of Geology and Geophysics Univ. of Wyoming, Dept. 2006 1000 University Ave., Laramie, WY 82071 United States
AU: Lazar, B
EM: brian.lazar@colorado.edu
AF: INSTAAR Univ. of Colorado, 1560 30th St., Boulder, CO 81309 United States
AB: We present a suite of observations that elucidate the subglacial and englacial conditions leading to an eight fold speed-up of a valley glacier. Data were collected on the 7.5 km long temperate Bench Glacier, Chugach Mountains, Alaska. We documented the speed-up event, which lasted for approximately one week of June, with data that include: 1) subglacial water pressure measured in a grid of 16 closely spaced boreholes to the bed; 2) water pressure measured in an additional 31 boreholes at 14 locations spanning the length of the glacier; 3) discharge of the terminus outlet stream; 4) GPS measurements of surface velocity and elevation at 4 locations spaced along the glacier; 5) optical surveys of the velocity and uplift of a network of 24 stakes; 5) vertical strain along two boreholes; 6) borehole slug and pump tests; and, 7) direct observation of englacial and subglacial water flow via borehole video camera. The speed-up event consisted of a two stage velocity increase to a maximum level eight times the background deformational velocity. The speed-up was associated a slight drop in subglacial water pressure (not a pressure rise). Numerous anomalous events occurred in conjunction with the accelerated sliding, for example: pressure variations that were unusually uniform over space, surface uplift due to bed separation, and an episode of massive up-welling and refreezing of englacial waters. Our interpretation of the speed-up cycle is that it began with an initial enhanced sliding event that propagated up-glacier as an insatiability within a thin but widespread layer of subglacial water. After a small displacement, stability was regained and sliding temporarily terminated. This initial event, however, initiated connections between the bed and a large volume of englacial water contained within void spaces, which caused widespread basal flooding, bed separation, and resumed sliding. Sliding accelerated until well developed drainage pathways were established, leading to draining of the bed and a flood at the terminus.
UR: http://research.gg.uwyo.edu/joelh/benchglacier
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