HR: 1340h
AN: V53D-1596    [Abstracts]
TI: On the possible role of sliding friction in controlling the 2004-2005 Mount St. Helens eruption
AU: * Beeler, N M
EM: nbeeler@usgs.gov
AF: USGS, 345 Middlefield Rd MS977, Menlo Park, CA 94025 United States
AU: Mastin, L G
EM: lgmastin@usgs.gov
AF: USGS-CV0, 1300 Cardinal Ct, Vancouver, WA 98683 United States
AU: Roeloffs, E
EM: evelynr@usgs.gov
AF: USGS-CV0, 1300 Cardinal Ct, Vancouver, WA 98683 United States
AU: Gerlach, T
EM: tgerlach@usgs.gov
AF: USGS-CV0, 1300 Cardinal Ct, Vancouver, WA 98683 United States
AB: The nearly solid dacite emerging at Mount St. Helens (MSH) during the 2004/05 eruption is bounded at the conduit wall by a meter thick gouge layer, indicative of significant brittle deformation. We consider the implications of extrusion controlled by resistance due to frictional sliding along the conduit wall using a 1D model consisting of a magma chamber and conduit at representative pressure P extruding a solid rock plug sliding against gravity and friction. In laboratory experiments, frictional shear resistance depends on the deformation rate, and can be either rate strengthening (always aseismic) or rate weakening (aseismic or seismic). At Mount St. Helens, high temperatures at emergence, very low normal stress and the presence of a thick gouge layer all favor aseismic over seismic slip. A simple accounting of the daily seismic moment release and measured rate of extrusion, assuming a cylindrical plug geometry, also suggests that the extrusion is largely aseismic. So, we allow friction to increase weakly with sliding rate as in the highest temperature gouge sliding experiments (Blanpied et al, JGR, 100,13045,1995; Chester, JGR, 100,13033,1995). The average slip rates are high (> 50 microns/s) and for simplicity we ignore explicit dependence of sliding strength on slip and time. We derive the governing equations and solutions for the cumulative extruded volume and the rate of extrusion when acceleration is assumed negligible, plug mass, circumferential and cross-sectional area are constant, and the magma volume is assumed large relative to the extruded volume. The initial condition is a pressurized magma system. If there is no new influx of magma into the system during the eruption, the volume of extruded rock increases logarithmically with time, consistent with observations at MSH. A fit to the observations from 9/04-2/05 produces a characteristic time of 22 days. Over this 5 month period, the inferred pressure decay within the magma chamber is < 4 MPa. Assuming a conduit radius of 25-75m the estimated length of the plug is 0.6 - 1.0 km. Positive deviation from the logarithmic form at longer times may be interpreted as evidence of magma influx into the system or may indicate violation of the underlying assumptions. For the former, additional information on system volume may be inferred.
DE: 8163 Rheology and friction of fault zones (8034)
DE: 8414 Eruption mechanisms and flow emplacement
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005