Collaborative Research: The Tumalo Volcanic Center - Probing the Origin of Explosive Pleistocene Silicic Volcanism in Central Oregon

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Why was there a burst of explosive volcanism between 100,000 and 800,000 years ago in the middle of the central Oregon Cascades, a location that over the last 100,000 years has been dominated by basaltic volcanoes like Belknap Crater and stratovolcanoes like South Sister? This is the fundamental question behind this study to examine the Tumalo Volcanic Center (TVC) in central Oregon, where at least three major explosive eruptions that disgorged each at least 1 to 10 cubic kilometers mantle the landscape between the modern Three Sisters volcanoes and the city of Bend. What is the source of the magma that fed these massive eruptions and what is it most closely related to: the volcanism of modern Cascades caused by the collision of North America with the Juan de Fuca plate or ongoing stretching of North America in Oregon High Cascades and the Basin and Range? Without knowing the precise ages of the eruptions themselves and the crystals found in the materials ejected, the TVC can't be placed into the evolution of the Cascade Range or assessed for the likelihood of renewed explosive volcanism in central Oregon. By adding chemical compositional data to the mix, the processes that formed these eruptions can be understood, along with how similar the TVC is to other silicic explosive eruptions around the world. This will be a detailed study of this major volcanic region in the central Oregon Cascades. Although the TVC has been previous mapped, it is poorly known in almost all other respects: the chronology of eruptions; the volumes of individual eruptive units; high quality geochemical or petrochronologic data; and petrological constraints on the processes/conditions of magma genesis. This detailed chronological, petrological and geochemical study will provide a basis for testing models for the formation of these magmas, assist in assessing the future likelihood for major explosive TVC eruptions, and provide important insight into the processes that produce silicic magmas in the Cascades and in other subduction systems. This study will address these points through: field documentation and GIS-based volume estimates; 40Ar/39Ar geochronology; zircon U-Pb (and U-Th) dating and trace element systematics; geochemistry of bulk rocks, glasses, minerals and melt inclusions; and phase equilibria-based estimates of intensive parameters of magma formation and storage. There are also four main Broader Impacts to this study: (1) mutual mentoring and training of graduate and undergraduate students shared across Research 1 and Liberal Arts institutions; (2) increased understanding of volcanic hazards as the population of Deschutes County is over 140,000 and the hazard related to renewed eruption in the TVC is currently unclear; (3) extensive outreach, including blog posts, Twitter, talks, new TVC website and signs in the field that will provide an important opportunity to increase awareness of volcanic hazards in Central Oregon; (4) this research will support a Ph.D. student, a Latino researcher and undergraduate research students, many of which will be recruited from groups underrepresented in STEM fields.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.