ANALYZING THE ERUPTIVE HISTORY AND TRENDS OF MOUNT ST. HELENS, CASCADIA SUBDUCTION ZONE

Mount St. Helens (MSH) is one of the most active volcanoes in the continental U.S. Although previous publications document the patterns and trends of eruptions in the past 4,000 years, a complete summary is lacking. In this study, the nature and composition of eruptions and products over the last 40 thousand years was compiled from published literature to document and compare the eruptive history over time. These data are important in understanding the types and nature of previous eruptions, changes in eruption cycles, magma sources, and predicting the behavior of future eruptions.

The record of MSH eruptions over the past 40 ka reveal cyclic patterns defined by initial Plinian-style eruptions that progress into smaller pyroclastic events. The latter stages involve lahars followed by dacitic to rhyolitic lava dome or flows. In the past 40 ka, the repose intervals of eruptions have decreased over time.

Over the 40 ka history there is a general progression from early dacitic to andesitic compositions followed by andesitic to basaltic compositions. Some models propose that this progression is indicative of different magma batches sourced from a progressively graded magma chamber. Other models posited that this progression was a result of magma sourced from a rising pluton of vertically graded, mantle-derived magma. The latter hypothesis is supported by Nd and Sr isotopic signatures that reveal mantle contributions to felsic and intermediate MSH magmas.

The results of this study show long-term shifts in the eruptive patterns and products at MSH that are linked to the evolution of magma reservoirs and structural controls. This record serves as a potential predictive tool for future eruptions from Mount St. Helens.