Lessons Learned from the 1980-1986 Eruption of the Mount St. Helens Composite Lava Dome

After its cataclysmic explosive eruptive activity on May 18, 1980, most of the output of Mount St. Helens (MSH) for the next six and a half years was quietly extruding lava, which built up one of the best documented and most instructive lava domes of the twentieth century. The unprecedented amount of data collected about the growth of the dome led to a profusion of new models and concepts. In this paper, we first describe some of the early mechanical models and then focus on three specific aspects of the emplacement of the MSH lava dome that were measurable in particularly great detail: the partitioning between exogenous and endogenous styles of growth; the distribution of vesicular textures and their relationship to volatile contents and eruptive conditions; and the presence of characteristic structural features like fractures, folds, and spines. Taken together, these three overlapping physical manifestations of magmatic behavior, evidence of which may be preserved in the geologic record or observed with remote sensing, can provide insights about whether a quietly extruding dome is likely to exhibit dangerous endogenic pyroclastic behavior