GEOLOGIC AND PETROLOGIC FRAMEWORK FOR THE 2004-2005 ERUPTION OF MOUNT ST. HELENS, WASHINGTON

Cliff Hopson's geologic map of pre-1980 Mount St. Helens (USGS OFR02-468), stratigraphic work by Crandall, Hoblitt and Mullineaux, and numerous post-1980 investigations provide a foundation for understanding the petrology and hazards of the ongoing (2004-2005) eruption. Mount St. Helens (MSH) is the most active of the Cascade volcanoes. During its >40,000-year history eruptions have ranged widely in explosivity (VEI 0-6) and eruptive periods have lasted for decades to centuries. Eruptive products range from basalt to high-silica dacite and show evidence of heating and magma mixing. It is in this context that we must assess the ongoing eruption. The 2004-5 eruption began with intense seismic unrest, uplift of crater floor and deformation of glacial ice during late September 2004. During mid- to late-October spines of juvenile lava extruded in the deformed area and since November 4th dacite has extruded continuously. On January 16th, the first explosion since early October blanketed the crater floor with ash and ballistic fragments. For safety, a dredge sling loaded to a helicopter and short-duration landings allowed rock collections on 10/20/04, 10/27/04, 11/04/04, 01/03/05 and 01/14/05. November and January samples are juvenile lava. The composition of the 2004-2005 is most like 1986 dacite, although 1-2% higher in SiO₂ (65%), consistent with remobilization from the 1980-86 reservoir. It has lower Fe-Ti oxide equilibrium temperatures (850°C), higher abundance of small plagioclase microlites, thin or no reaction rims on amphiboles, and partly to fully devitrified matrix glass. Low abundance of volatiles in glass inclusions and matrix glass and low emissions of SO₂, H₂S and CO₂ indicate early and extensive degassing. These petrologic data favor slow cooling, degassing and phenocryst growth at depth, ascent within the stability field of amphibole to the base of the 1992-2004 shallow seismic zone (3 km depth), then more rapid ascent to the surface -- a multi-stage history that led to an extremely viscous, gas-poor, and crystal-rich extrusion. However, zoned Fe-Ti oxides in one sample record apparent temperatures of 850° to 950°C. These oxide data and a minor glassy component in the ash emitted on 01/16/05 suggest that hotter and more mafic magma is also involved in the current eruption.