HR: 0800h
AN: C51B-0278    [Abstracts]
TI: Dynamics of the Mount St. Helens Crater Glacier During the Eruption of 2004-05
AU: * Walder, J S
EM: jswalder@usgs.gov
AF: US Geological Survey, Cascades Volcano Observatory, 1300 SE Cardinal Ct., Bldg. 10, Suite 100, Vancouver, WA 98683 United States
AU: LaHusen, R G
EM: rlahusen@usgs.gov
AF: US Geological Survey, Cascades Volcano Observatory, 1300 SE Cardinal Ct., Bldg. 10, Suite 100, Vancouver, WA 98683 United States
AB: Following the cataclysmic 1980 eruption of Mount St. Helens, a lava dome grew in the crater until 1986, at which time the crater contained only discontinuous patches of perennial snow and ice. By Sept. 2004, when a new dome began to grow, the crater hosted a glacier with a surface area of ~1 km² and a thickness locally as much as ~200 m. This extraordinarily rapid growth occurred because the glacier sits in a north-facing amphitheater-like basin and is fed by avalanches from the crater walls. During the first two months of the 2004-05 eruption, the glacier was cut in half as the new lava dome grew toward the south crater wall. Subsequently this lava dome grew toward the east, with the dome/glacier contact migrating at a rate of ~1 m/d. The east crater glacier was thus squeezed between the growing lava dome and the east crater wall. East/west trending crevasses formed and the previously smooth surface bowed upward. The intense squeeze caused ice thickness locally to more than double (from ca. 100 m to 200 m), the rate of increase being as much as ~0.5 m/d. Single-frequency GPS stations slung by helicopter onto the east-glacier surface recorded surface displacement from Feb.-Aug. 2005. Displacement rate dropped from about 1.4 m/d to 0.4 m/d when eastward lava-dome growth stopped in mid-April 2005. Displacement records show neither diurnal velocity fluctuations nor an overall acceleration with the onset of the ablation season, in contrast to virtually all previous data from temperate glaciers. The explanation of this unusual behavior probably follows from the fact that the crater glacier grew atop several tens of m of highly permeable rubble (mainly rock-avalanche debris) that had accumulated on the crater floor following the 1980 eruption. The volcanic edifice beneath this rubble is highly fractured. Thus water at the glacier bed penetrates downward into the volcano's groundwater system. Further evidence for this interpretation is the absence of outlet streams at the termini. Thus there is apparently no pressurized drainage system that conveys water along the glacier bed and modulates the basal stress state. Since early Aug. 2005, lava extrusion on the west side of the new dome has been squeezing part of the west crater glacier. Crevasses have formed normal to the expanding dome margin; surface speed is locally >1 m/d and uplift rate up to 0.6 m/d. Displacement records here again lack any diurnal signal.
DE: 0720 Glaciers
DE: 1294 Instruments and techniques
DE: 8419 Volcano monitoring (7280)
SC: Cryosphere [C]
MN: Fall Meeting 2005