Skip to content

Explosive Eruptions at Mount St. Helens: The Influence of Volatile Content in W Tephra Deposits on Eruptive Styles

Oceana Apollo , Rebecca deGraffenried

Abstract

Pre-eruptive H2O content impacts eruptive behavior as greater H2O content is associated with more explosive eruptions. This study examines pre-eruptive H2O content of W tephra deposits from three explosive eruptions (Wn, Wb, and Wd) during the Kalama period (1479--1482 CE) at Mount St. Helens using liquid-plagioclase hygrometry. Differences in pre-eruptive H2O content could potentially explain shifts in eruption style and explosivity preserved by the W tephra deposits that are otherwise chemically similar. Electron probe microanalysis (EPMA) of plagioclase crystals shows a wide range of anorthite component for core compositions, but narrower for rim compositions, indicating that the W tephra eruptions were sourced from similar final depths. Preliminary hygrometer calculations show an overlap in H2O content between the most explosive (Wn) and one of the less explosive eruptions (Wb). However, a distinctly lower H2O concentration is indicated in the sequence's final eruption (Wd), which was also less explosive. H2O solubility indicates the Wn and Wb eruptions equilibrated ~0.5 km deeper than the Wd eruption. These trends are supported by the prevalence of amphibole reaction rims in Wd samples that are missing in Wn and Wb samples. These results indicate that differences in pre-eruptive H2O content can only partially explain differences in eruption behavior. Additional W tephra samples will be studied, including samples from the other major explosive eruption (We), to confirm these preliminary trends. These results have implications for forecasting future eruption behavior at Mount St. Helens and understanding what drives eruption style and intensity at a high-threat volcano.