MODELING COMPOSITIONAL VARIATION IN VOLCANIC ROCKS FROM THE NORTH FLANK OF MOUNT ST. HELENS, WASHINGTON

Mount St. Helens is located above a subduction zone in the Pacific Northwest where several volcanoes exist as a result of the convergence between the North American and Juan de Fuca tectonic plates. On May 18, 1980, Mount St. Helens erupted catastrophically scorching over 600 square kilometers of forest and killing 57 people. The products of the eruption included a debris avalanche formed by the failure of the volcano’s north flank. Seventeen rock samples were collected from the hummocks in the debris avalanche deposit during the summer of 2013. Rock samples were described macroscopically and thin sections were made for each sample. Petrographic data, along with whole-rock major and trace element geochemical data were used to characterize compositional variation. These data were then used to test hypotheses that may account for observed compositional variation. Petrologic software was used to model crystal fractionation, magma mixing and combined fractionation and assimilation. The results provide insight into important magmatic processes operating beneath the volcano.