HR: 1340h
AN: V13B-1345    [Abstracts]
TI: Ten Years of Monitoring the Eruption of Shrub Mud Volcano, Alaska
AU: * McGimsey, R G
EM: mcgimsey@usgs.gov
AF: U.S. Geological Survey, Alaska Science Center, Alaska Volcano Observatory, 4200 University Drive, Anchorage, AK 99508, United States
AU: Evans, W C
EM: wcevans@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, MS 434, Menlo Park, CA 94025, United States
AU: Bergfeld, D
EM: dbergfel@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, MS 910, Menlo Park, CA 94025, United States
AU: McCarthy, S H
EM: suzanne.mccarthy@pwscc.edu
AF: Prince William Sound Community College, P.O. Box 730, Glennallen, AK 99588, United States
AU: Hagstrum, J T
EM: jhag@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, MS 937, Menlo Park, CA 94025, United States
AB: Shrub mud volcano, one of three in the Klawasi group on the eastern flank of Mount Drum volcano in the Wrangell volcanic field of eastern Alaska, has been erupting warm, saline mud and CO₂-rich gas continuously since at least the summer of 1997, following 40 years of repose. The initial eruption in early summer of 1997, documented by Richter and others (1998), involved violent fountaining of mud, up to 6-8 m high, from nearly a dozen vents located near the summit, and quiet effusion from vents located about mid-way down the north flank of the 100-m-high cone. Guided by topography, early emissions of copious amounts of CO₂ gas flowed in narrow streams through brushy foliage leaving behind stripes of brown, dead vegetation along the flow paths. The hazard posed by the CO₂ emissions was evident from dead birds and mammals found near the vents. Initial surveys of the activity in 1997 recorded water temperatures up to 46°C. A survey in 1999 by Sorey and others (2000) found numerous active vents-many in different locations than those two years earlier-a maximum water temperature of 54°C, and an estimated total discharge of warm water of 50 l/s. Measured CO₂ emissions were extrapolated to a discharge rate of 6-12 tonnes/day. The highest water temperature recorded was 57.3°C in 2000, with temperatures gradually declining since. From year to year, we found that eruptive activity migrated amongst clusters of vents, some new and some continuing from 1997. Between the summer of 2003 and the spring of 2004, the system changed dramatically when a large collapse pit formed a few tens of meters from the main summit vents and all previously active vents became inactive. This water-filled circular pit measured 28 m in diameter, up to 9 m deep, and encompassed an area that had previously been unaffected by the eruptive activity. In July 2004, water temperature and discharge at the outlet channel was 37.2°C and 9.4 l/s, respectively. The total CO₂ discharge from the roiling pool was 140 l/s (about 20 tonnes/day), and the diffuse efflux (0.13 tonnes/day) was comparable to the 0.23 tonnes/day measured in 1999. Based on discharge and δ13C values of the gas and water phases (-4.6‰ and +1.35‰, respectively) carbon in the reservoir was calculated to be -3.1‰, supporting a mixed magmatic and limestone source for the CO₂. Deep pits exposed during the current eruption have provided clues to the possible origin of Shrub, the largest, and highest mud volcano of the Klawasi group. Shrub stands about 100 m high and is an oblong cone in shape. Cross-sections of the material composing the cone reveal a chaotic pile of debris ranging from clay to boulders (angular to subrounded, up to 1.5 m across), the latter too large to have been rafted up by mud and gas venting. Pleistocene glaciers extending into the Copper River Valley from Mount Drum covered the area currently occupied by Shrub. We hypothesize that as the glaciers stagnated and began to retreat, subglacial melting caused by the rising warm mud formed an anomalous moulin that became the receptacle for rock and sediment debris washed in from supraglacier runoff. Thus the constructional form of Shrub is likely a moulin kame.
DE: 8419 Volcano monitoring (7280)
DE: 8426 Mud volcanism
DE: 8430 Volcanic gases
DE: 8488 Volcanic hazards and risks
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting