EXPLOSIVE VOLCANIC PROCESSES AND HAZARDS IN THE CASCADES AND ALASKA, USA (Invited Presentation)

A key motivation for studying subduction zone processes is to understand controls on the nature of volcanic activity. Arc volcanism presents important hazards on all scales, from local impacts of pyroclastic density currents to the global effects of ash on aircraft. Yet each eruptive center has unique tectonic, magmatic and environmental factors that influence its hazards. This presentation considers the styles and timescales of volcanism in the Cascade and Aleutian Arcs. Which are the most frequently active volcanoes? Which are the most explosive? Why, for example, are Mount Rainier and Mount Hood >95% effusive by volume, while Mount St. Helens is significantly more explosive? Opportunities exist for more detailed stratigraphy to refine our estimates of eruptive magnitude and frequency by mining the lake sediment record downwind of active volcanoes. The extent and nature of hazards are also related to processes on the Earth’s surface and in the atmosphere. What is the probability that ash from a future Mount St. Helens eruption will reach Boise or Missoula? Atmospheric wind data used in dispersion models such as Ash3d can help examine this question. Another important control on eruption style and hazards is volcano hydrology. For example, the May 18, 1980, eruption of Mount St. Helens was strongly influenced by the groundwater system within the edifice; water expelled by the lateral blast scrubbed fine ash out of the atmosphere as accretionary lapilli, shortening the lifetime of the cloud. Adequate characterization of future eruptive hazards in the Cascades and Alaska requires attention to both the geological record and present-day hydrological systems.