HR: 1340h
AN: V53D-1597    [Abstracts]
TI: The roles of magmatic and external water in the March 8 tephra eruption at Mount St. Helens as assessed by a 1-D steady plume-height model
AU: * Mastin, L G
EM: lgmastin@usgs.gov
AF: U.S. Geological Survey, 1300 SE Cardinal Court, Bldg. 10, Suite 100, Vancouver, WA 98683 United States
AU: Sherrod, D R
EM:
AF: U.S. Geological Survey, 1300 SE Cardinal Court, Bldg. 10, Suite 100, Vancouver, WA 98683 United States
AU: Vallance, J W
EM:
AF: U.S. Geological Survey, 1300 SE Cardinal Court, Bldg. 10, Suite 100, Vancouver, WA 98683 United States
AU: Thornber, C T
EM:
AF: U.S. Geological Survey, 1300 SE Cardinal Court, Bldg. 10, Suite 100, Vancouver, WA 98683 United States
AU: Ewert, J W
EM:
AF: U.S. Geological Survey, 1300 SE Cardinal Court, Bldg. 10, Suite 100, Vancouver, WA 98683 United States
AB: The dome-building eruption at Mount St. Helens has occurred through glacial ice and snow that would be expected to substantially affect the character of the eruption. Nevertheless, the role of water in the eruption to date has not always been clear. For example, on March 8, 2005, a half-hour-long tephra blast sent a plume to a maximum of ~9 km above the vent (based on pilot reports); seismicity and plume heights were greatest during the first ~10 minutes, then persisted for another ~15 minutes at a lower level before the eruption stopped. Tephra volume within 5 km² downwind of the vent was ~5x10⁴ m³ DRE, but trace amounts were reported at least to Ellensburg, WA (150 km NE), suggesting a total areal coverage >5,000 km² and total volume >1x10⁵ m³. Assuming that most of this material was expelled in the first ten minutes and had a density of 2500 kg/m³, the mass flow rate (M) during the vigorous phase was >~4x10⁵ kg/s. The tephra, composed primarily of non-pumiceous broken and decrepitated dome rock, could have been expelled either by groundwater and steam at relatively modest (boiling-point) temperatures, or by magmatic gas at much higher temperatures. The high plume, however, suggested significant buoyancy, perhaps driven by temperatures closer to magmatic. To assess the effect of magmatic heat on plume height, we employ a 1-D steady volcanic plume model that uses specified vent diameter, exit velocity, eruption temperature, mass fractions of gas and added external water, and profiles of atmospheric temperature and humidity, to calculate plume height and plume properties as a function of elevation. The model considers the enthalpy of equilibrium water condensation and of ice formation. Model results show that, under atmospheric temperature and humidity profiles measured near Mount St. Helens on the afternoon of March 8, 2005, a plume height (h) of 7-9 km could have developed with eruption temperatures (T) as low as 100° C, provided the mass fraction of water vapor in the plume (n) exceeded ~0.25 (mixtures containing less gas at this temperature would collapse before reaching 7-9 km height). At T=100° C and n=0.25, however, a mass flux of 0.6-1x10⁵ kg/s will generate the observed 7-9km plume height, whereas a mass flow rate >~4x10⁵ kg/s, in line with observations, results in h>~11 km. Under more typical magmatic temperatures (900° C) and gas mass fractions (0.02-0.03), plume heights of 7-9 km require M=4-6x10⁵ kg/s -- nearly an order of magnitude greater than the vapor-rich, boiling point mixture, but more in line with the estimated mass flow rate of this eruption. These results, though not definitive, suggest that magmatic heat may have been important in driving the March 8 eruption. Relationships between mass flow rate and plume height may be useful in assessing the roles of magmatic and phreatic sources during other small eruptions.
DE: 8409 Atmospheric effects (0370)
DE: 8411 Thermodynamics (0766, 1011, 3611)
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8428 Explosive volcanism
DE: 8430 Volcanic gases
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005