HR: 1340h
AN: V53D-1590    [Abstracts]
TI: Airborne Measurement of CO₂, SO₂, and H₂S Emission Rates During the 2004-2005 Eruption of Mount St. Helens
AU: * Doukas, M P
EM: mdoukas@usgs.gov
AF: U.S. Geological Survey, Cascades Volcano Observatory 1300 SE Cardinal Ct., 100, Vancouver, WA 98683 United States
AU: McGee, K A
V53D-1590 AF: U.S. Geological Survey, Cascades Volcano Observatory 1300 SE Cardinal Ct., 100, Vancouver, WA 98683 United States
AU: Gerlach, T M
V53D-1590 AF: U.S. Geological Survey, Cascades Volcano Observatory 1300 SE Cardinal Ct., 100, Vancouver, WA 98683 United States
AB: Volcanic gas measurements by helicopter and fixed-wing aircraft began on 27 September 2004 during the initial unrest. Target gases included CO₂, SO₂, and H₂S measured by remote (COSPEC, FLYSPEC) and extraction (LI-COR, Interscan) techniques. These measurements defined periods of negligible, wet, and dry volcanic degassing. Measurements during the period of early seismic unrest on September 2-30 showed little or no CO₂ above atmospheric levels; SO₂ and H₂S were absent. These results suggest involvement of degassed magma, confinement of gases because of reduction in permeability since the 1980-86 dome eruptions, or almost complete gas scrubbing at high water to gas mass ratios. Following the first steam and ash explosion on October 1, a large increase in the number of fumaroles on the 1980-86 dome and frequent detection of CO₂ above atmospheric levels together with more frequent detection of H₂S characterized the period of wet volcanic degassing. Wet degassing of CO₂ and H₂S included observed emission by ejection of large bubbles through pools of water, however the few CO₂ emission rates available for this period were <150 metric tonnes/day (t/d). Dry volcanic degassing commenced on October 5-6 as temperatures rose, steaming increased, and rock adjacent to the invading magma dried out. These conditions enabled a buoyant plume to rise above the crater and enter higher elevation winds, facilitating airborne measurements of gas emission rates. Gas emission rates have been notably low and declined gradually throughout the eruption. Emission rates of CO₂ were mostly in the 500-1000 t/d range until early February 2005, when they fell to 150-500 t/d and remained there. Emission rates of SO₂ rose to 240 t/d during 2004 then declined and remained below 100 t/d (often <50 t/d) after January 2005. Emission rates of H₂S were variable but always <8 t/d. The average emission rates through the end of July 2005, were approximately 650 t/d (CO₂) and 100 t/d (SO₂).
DE: 3640 Igneous petrology
DE: 8419 Volcano monitoring (7280)
DE: 8430 Volcanic gases
DE: 8494 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005