THE 2004-2005 ERUPTION OF MOUNT ST. HELENS AS VIEWED THROUGH THE LENS OF THE 1980-1986 ERUPTIVE ACTIVITY

When Mount St. Helens awoke in 1980, existing paradigms for volcanic activity focused on large magma storage systems that were zoned in bulk composition and volatile content. The Mount St. Helens eruption issued in a new era of volcanic studies that focused on smaller, more frequently active systems of intermediate composition. In these systems conduits are highly elongate in shape and eruptions are characterized by plinian or subplinian explosions interspersed with dome growth and collapse. Magma in these systems is typically volatile rich (≥ 4-5 wt% H2O) and thus reaches volatile saturation during ascent, with the result that (1) much of the crystallization within these systems is a consequence of decompression and gas exsolution rather than of cooling and (2) both the mode and extent of crystallization are limited by equilibration pressure (effective undercooling) and decompression rate rather than by time scales of cooling. Combined analysis of resulting crystal textures and phase compositions provide detailed information on decompression paths prior to individual eruptions. Studies of the 1980-1986 eruptions of Mount St. Helens suggest that (1) magma ascent prior to the climactic eruption on May 18 occurred in batches that rose quickly from a deep storage area to the edifice, where they crystallized rapidly to form a cryptodome; (2) by mid-June 1980 an intermediate level of magma storage had developed as explosive activity through the summer changed from subplinian to vulcanian; (3) intermittent explosive changed to intermittent effusive eruptions as the conduit connecting the different storage levels developed; (4) very slow but steady magma ascent through this conduit system in 1983 led to continuous endogenous dome growth accompanied by extrusion of a highly crystalline volcanic spine; and (5) eventual cooling of the system created the last intermittent dome lobes of 1985-1986 prior to cessation of activity. From this perspective, the highly crystalline groundmass of the 2004-2005 lava suggests slow magma ascent through the largely open conduit created during the 1980s… the driving force for this ascent remains an open question.