MARGINS: The Explosive Volcanic History of the Central Oregon Cascades: Probing the Changing State of the Neogene Cascade Arc

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Subduction zones ? volcanically active regions where one tectonic plate is pushed beneath another ? are home to most of the Earth?s explosive volcanic eruptions. In large part this is because the magmas that are produced and erupted in subduction zones tend to be both richer in water and in silica, both of which increase the propensity for eruptions to be explosive. The Cascade margin, running from Mount Lassen in northern California to southern British Columbia is host to a number of well-known volcanoes such as Mount Hood, Mount St Helens and Mount Rainier as well as numerous other volcanic edifices, and much of the region is also heavily populated. Explosive eruptions have occurred in the recent history of the Cascades, examples include Mount St Helens in 1980 and the prehistoric eruption of Mount Mazama at Crater Lake ~7700 years ago. However, although these younger eruptions are well documented, less is known about the long-term explosive record of the Cascade subduction zone over longer timescales, and the forces that influence such explosive behavior. This project will help establish the long-term eruptive history of the Cascades in Central Oregon over the last 15 million years, by looking at volcanic rocks preserved within the Deschutes and Simtustus Formations of central Oregon. Many examples of ash fall and ash flow deposits that result from large explosive eruptions are preserved in these sequences, and they can be used to estimate the frequency, composition, age and size of large explosive eruptions in this section of the subduction zone. This research will use a combination of approaches, including field, geochronological, geochemical and petrological studies and will enable us to study changes in eruption rate, eruption size and chemical and isotopic composition through time and to compare this to the more recent behavior of the subduction zone.