HR: 1340h
AN: V43B-1430    [Abstracts]
TI: Headless Debris Flows From Mount Spurr Volcano, Alaska
AU: * McGimsey, R G
EM: mcgimsey@usgs.gov
AF: U.S. Geological Survey Alaska Volcano Observatory, 4200 University Drive, Anchorage, AK 99508 United States
AU: Neal, C A
EM: tneal@usgs.gov
AF: U.S. Geological Survey Alaska Volcano Observatory, 4200 University Drive, Anchorage, AK 99508 United States
AU: Waythomas, C F
EM: chris@usgs.gov
AF: U.S. Geological Survey Alaska Volcano Observatory, 4200 University Drive, Anchorage, AK 99508 United States
AU: Wessels, R
EM: rwessels@usgs.gov
AF: U.S. Geological Survey Alaska Volcano Observatory, 4200 University Drive, Anchorage, AK 99508 United States
AU: Coombs, M L
EM: mcoombs@usgs.gov
AF: U.S. Geological Survey Alaska Volcano Observatory, 4200 University Drive, Anchorage, AK 99508 United States
AU: Wallace, K L
EM: kwallace@usgs.gov
AF: U.S. Geological Survey Alaska Volcano Observatory, 4200 University Drive, Anchorage, AK 99508 United States
AB: Sometime between June 20 and July 15, 2004-and contemporaneous with an increase of seismicity beneath the volcano, and elevated gas emissions-a sudden release of impounded water from the summit area of Mt. Spurr volcano produced about a dozen separate debris flow lobes emanating from crevasses and bergschrunds in the surface ice several hundred meters down the east-southeast flank from the summit. These debris flows were first observed by AVO staff on a July 15 overflight and appeared to represent a single flooding event; subsequent snow cover and limited accessibility have prevented direct investigation of these deposits. Observed from the air, they are dark, elongate lobate deposits, up to several hundred meters long and tens of meters wide, draping the steep (up to ~45 degree) slopes and cascading over and into crevasses. A water-rich phase from the flows continued down slope of the termini of several lobate deposits, eroding linear rills into the snow and ice down slope. We infer that the dark material composing these flows is likely remobilized coarse lapilli from the June 1992 tephra fall produced by an eruption of Crater Peak, a satellite vent of Mt. Spurr located 3.5 km to the south. Between 1 and 2 meters of basaltic andesite tephra fell directly on the Spurr summit during the 1992 eruption. The exact mechanism for sudden release of water-laden remobilized tephra flows from the summit basin is not clear. However, observations in early August, 2004, of an 80 m x 110-m-wide pit in the summit area snow and ice suggest the possibility of a partial roof collapse of a summit meltwater basin, likely associated with subglacial melting due to recent heat flux. Such a collapse could have led to the hydraulic surge of meltwater, and rapid mixing with tephra to produce slurries. These slurries traveled down slope beneath the ice surface to emerge through existing crevasses and other easy points of exit on the steep inclines. Mount Spurr is an ice- and snow covered, Quaternary andesitic volcanic complex, comprising a centrally located dome (or stratocone) in a breached, 5-km-wide, glacier-filled caldera that dissects ancestral Mt. Spurr volcano. The summit of Mt. Spurr is 130 km west of Anchorage, AK and reaches 3,374 m in elevation. The summit dome complex is topographically asymmetric, with a steeper southwest side and a more gradually sloping northeast flank To our knowledge, this is the first time such debris flows have been observed near the summit of Mt. Spurr. However, the existence of ponded water near the summit may not be unique to 2004. A review of historical photographs and descriptions of the Spurr summit area indicates a dynamic environment that responds to complex variations in snowfall accumulation, solar radiation, and geothermal heat flux. Other authors have noted variations in summit snow pack and the ephemeral appearance of a snow-filled depression and possibly a water-filled pit in 1964 aerial photographs of the summit. The formation of these debris flows near the summit of Mt. Spurr in conjunction with elevated seismicity below the summit and the development of a collapse pit in summit ice cap suggest that increasing geothermal heat flux, possibly in combination with above normal temperatures and long periods of clear, sunny weather in the region is responsible.
DE: 8419 Eruption monitoring (7280)
DE: 8499 General or miscellaneous
DE: 1821 Floods
DE: 1827 Glaciology (1863)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting