HR: 10:20h
AN: V52B-01 INVITED     [Abstracts]
TI: Measuring the Dome Growth of the 2004-2005 Eruption of Mount St. Helens
AU: * Schilling, S P
EM: sschilli@usgs.gov
AF: US Geological Survey Cascades Volcano Observatory, 1300 SE Cardinal Court Bldg 10 Ste 100, Vancouver, WA 98681 United States
AU: Denlinger, R
EM: roger@usgs.gov
AF: US Geological Survey Cascades Volcano Observatory, 1300 SE Cardinal Court Bldg 10 Ste 100, Vancouver, WA 98681 United States
AU: Thompson, R A
EM: rathomps@usgs.gov
AF: US Geological Survey, Denver Federal Center, Denver, CO 80225 United States
AU: Messerich, J
EM: jmesser@usgs.gov
AF: US Geological Survey, Denver Federal Center, Denver, CO 80225 United States
AB: In October 2004, a new period of dome growth began that changed the topography of the 1980 crater at Mount St. Helens dramatically. From October 2004 through July 2005, nearly 60 million cubic meters of lava extruded onto the crater floor immediately south of the 1980-1986 lava dome. The eruption intensely deformed and divided the crater glacier on this floor. It created spectacular crevassing and rapid advance of the east arm of the glacier then caused crevassing and broad uplift of the glacier's west arm. Time-sequential vertical aerial photography documents morphologic change and enabled construction of a series of 13 2-m-resolution digital elevation models (DEMs) for the period between October 4, 2004 and July 14, 2005. Vertical aerial photographs flown at a nominal 1:12000 scale were acquired at three-week intervals, scanned at 12-micron resolution, and rectified using a soft-copy (i.e., digital image) photogrammetric workstation. Aerotriangulated models were constructed using ground control outside the area of active deformation, derived from pre-eruption GPS and photogrammetric data, and passed to subsequent model sets. Resulting location accuracy is on the order of decimeters. The DEM data allow us to estimate dome volumes during growth. ˙To extract volumetric changes and calculate extrusion rates, each DEM surface was compared to pre-eruption reference surfaces from 2000 and 2003, as well as to the preceding DEM surface. ˙On July 14, 2005, the new dome was approximately 700 m long (NW-SE) and 560 m wide (SW-NE). ˙The volume of the new dome (including talus), was about 58 million cubic meters, approximately two-thirds the volume of the 1980-1986 dome. ˙The volumetric growth rate in 2004-2005 ranged from a maximum of 9 m³/sec in the early stages of growth to an average of 1-3 m³/sec thereafter. The DEMs also are used to quantify dome height variations, size of the conduit opening, and the mechanics of dome emplacement (growth and collapse) as well as deformation of glacial ice. ˙Between February 1 and June 15, 2005, the highest point of the new dome remained between 2330 m and 2340 m, but reached 2367 m on July 14, nearly the height of the lowest point on the crater rim (2370 m). ˙Over time, migration of the extrusion site limits the diameter of the conduit near the surface to 150-230 m in an east west direction, whereas the diameter in the NS direction is unconstrained. ˙The lava emerging from this conduit appears to move unimpeded through the glacier ice on the south crater floor, but is diverted by deposits from previous extrusions. ˙The extrusion axis was initially southward, and shifted toward the east from October 2004 through April 2005 as successive whaleback-shaped monoliths emerged from the vent area, displacing older parts of the growing dome. The axis reverted to south during May-June, then shifted westward during July-August as extruding material sought out paths of lesser resistance.
DE: 8419 Volcano monitoring (7280)
DE: 8486 Field relationships (1090, 3690)
DE: 8488 Volcanic hazards and risks
DE: 8494 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005