Onset and impacts of the c.7700 ybp eruption of Mount Mazama, Oregon

Kathy Cashman

University of Oregon

Zoom recording not available

5:30 pm social hour

7:00 pm presentation

Magnitude 7+ eruptions 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. This presentation will 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 square kilometers 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 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.

Please join us at Bridge 99 Brewery:

63063 Layton Avenue, Bend

5:30 pm social hour

7:00 pm presentation

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