COMPLEX RUNUP TO A LARGE ERUPTION: A NEW LOOK AT THE C.7700 YBP MAZAMA ERUPTION, OREGON

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 conditions that lead to very large eruptions. What does the runup to a M7+ eruption look like? How can we identify, uniquely, precursory activity to a very large event? Here we 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, deposited ash over >1 million km² of north-western North America and disrupted native American communities throughout the region, possibly for 100s of years. We focus particularly on the pre-climactic and climactic eruptive sequences, which 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 two eruptions (the lower and upper pumice units) separated by a thin divider ash. We provide evidence from pyroclast textures and phase compositions that suggests instead that the latter sequence involved at least three eruptions, two of which came from separate vents and, like the Cleetwood, were precursory to the climactic phase, although perhaps occurring only days earlier. In this way, the eruptive sequence appears to resemble the 4 month buildup to the 1883 eruption of Krakatau, Indonesia. Compositional analysis of tephra components further show that at least one of these eruptions tapped a distinct, although related, magma reservoir, 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.