Onset and impacts of the c.7700 ybp eruption of Mount Mazama, OR
by Kathy Cashman
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Magnitude 7+ eruptions (≥ 40km3 DRE) occur globally at a rate of 1-2 per 1000 years and are large enough to cause devastating global impacts. Yet we know little about either the conditions that lead to very large eruptions or the long-term impacts of such events. Here I address these questions using data from the c.7700 ybp eruption of Mount Mazama that produced Crater Lake, Oregon, which had a magnitude M≥7.1 and deposited ash over >1 million km2 of northwestern North America. The pre-climactic and climactic eruptive sequences of this event have been interpreted to comprise a Plinian eruption followed by effusion of a large (0.5 km3) obsidian flow at the Cleetwood vent, followed weeks to a few months later by the climactic eruption (comprising lower and upper pumice units followed by widespread pyroclastic flows). New data from the lower pumice unit, however, suggests that the "climactic" sequence involved at least three eruptions, two of which were, like the Cleetwood, precursory to the climactic phase, and at least one of which tapped a distinct, although related, magma reservoir. In this way, the eruptive sequence resembled the four-month buildup to the 1883 eruption of Krakatau, Indonesia, and lends support to the idea that evacuation of large magma volumes may often require either pre- or syn-eruptive amalgamation of multiple melt lenses within a larger magmatic system. New data on the distal ash deposit provides new constraints on the eruptive volume, conditions of ash transport and deposition, and types of post-eruption redistribution of the widespread ash that supports archeological interpretations of disruption to Native American communities throughout the region, probably for centuries.
About the speaker
Kathy Cashman is a professor in the Department of Earth Sciences at the University of Oregon and the University of Bristol who has studied volcanoes on all seven continents to learn how they erupt and why eruption styles vary. From 1980 to 1982, she was the Public Information Scientist at the Cascades Volcano Observatory in Washington, and has worked with all of the volcano observatories in the U.S. Prior to coming to University of Oregon, she was a Fulbright scholar studying igneous petrology in New Zealand, and was on the faculty at Princeton University. She is a member of the National Academy of Sciences and a Fellow of the Royal Society. Cashman is also a Fellow of the American Geophysical Union and the American Academy of Arts and Sciences, and a member of the Academia Europaea.
Thursday, November 3, 2022 at 4:00pm to 5:00pm
Burt Hall, 193
2651 SW Orchard Avenue, Corvallis, OR 97331
Free
Ernest Colantonio
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