HR: 1340h
AN: V53D-1594    [Abstracts]
TI: Geophysical Constraints on Lava Rheology and Edifice Strength During Emplacement of the 2004-2005 Dome at Mount St. Helens, Washington
AU: * Denlinger, R P
EM: roger@usgs.gov
AF: Cascades Volcano Observatory, 1300 SE Cardinal Court, Building 10, Suite 100, Vancouver, WA 98683 United States
AU: Schilling, S
EM: sschilli@usgs.gov
AF: Cascades Volcano Observatory, 1300 SE Cardinal Court, Building 10, Suite 100, Vancouver, WA 98683 United States
AU: Lisowski, M
EM: mlisowski@usgs.gov
AF: Cascades Volcano Observatory, 1300 SE Cardinal Court, Building 10, Suite 100, Vancouver, WA 98683 United States
AU: LaHusen, R
EM: rlahusen@usgs.gov
AF: Cascades Volcano Observatory, 1300 SE Cardinal Court, Building 10, Suite 100, Vancouver, WA 98683 United States
AB: Abrupt onset and rapid increase in shallow seismicity in August and September, 2004 heralded the intrusion of lava into layers of ice and debris several hundred meters deep (crater glacier) that once smoothly covered the southern crater floor of Mount St. Helens, Washington. The intrusion initially developed a welt around the southern margin of the 1980-1986 lava dome during the last week of September and the first week of October 2004. As the welt grew, new lava emerged near the vent and propagated S25E, forming a leading wedge of debris and ice on the surface similar to the wake from the prow of a ship. By November 19, the distal margin of uplift from this wedge had intersected the crater wall, S45E displacement of the GPS station P697 on the outer flank of the crater wall began, and lava erupting from the vent was riding up and over previously extruded lava. In December, as the leading wedge of ice and debris was compressed and uplifted against the SE crater wall, movement of P697 increased, and deformation of the crater glacier and emplacement of new lava was diverted eastward as the growing lava dome pushed its way around existing lava on the south crater floor. About January 1, P697 achieved a maximum outward displacement of 6 cm and an increase in elevation of 1 cm. In contrast, 4 other flank GPS stations encircling the rim and 2 to 4 km away from P697 showed only gradual subsidence and movement towards the south crater floor concomitant with growth of the new dome. Subsequently the motion of P697 reversed, regaining its October 2004 position by May 2005, while the volume of new lava and deformed ice and debris adjacent to the southeast crater wall remained unchanged. These observations are remarkable in many respects. Inward and downward motion of the crater rim as recorded by a ring of 4 GPS stations is consistent with loading of the crater floor by the weight of the growing dome. The exception is the outward and upward motion of the SE rim at P697. This motion requires a local source of deformation nearby, since expansion of the centrally-located conduit sufficient to produce 6 cm of displacement at P697 would have displaced the other GPS stations outward as well. Onset of P697's SE motion was concurrent with the merging of crater glacier deformation with the crater wall, and is consistent with an outward force applied to the wall by that deformation. From November to January, increased force applied to the SE crater wall is required to increase the outward and upward displacement of P697, yet the flux of lava from the vent remained nearly constant. There cannot be a viscous component to the flow of the 2004 lava, as a constant flux of this lava against the crater wall apparently increased the force on the wall. The subsequent abrupt reversal in P697 motion around January 1 indicates either that the force against the wall decreased as the direction of lava emplacement diverted to the east, or that there was an added contrary displacement from an increasing gravitational load as the dome continued to grow. Finally, finite element results imply that a shear strain of 4*10-5 within the SE portion of the crater wall is required to accommodate the motion of P697 without moving adjacent GPS stations, and this amount of distortion requires either weak elastic constants or frictional failure within this portion of the edifice.
DE: 8159 Rheology: crust and lithosphere (8031)
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8419 Volcano monitoring (7280)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005