Abstract

Several fall deposits from Glacier Peak and Newberry volcanoes, both located in the Cascade Range, United States, have been studied to determine eruptive column heights, intensities, and volumes. The late Pleistocene eruptions of Glacier Peak ranged from small phreatic explosions to two Plinian eruptions that each erupted more than 1 km3 of magma at intensities >108 kg/s, generating plumes with heights >30 km. At Newberry volcano, the last Plinian eruption (ca. 1300 14C yr B.P.) had an intensity of 2.8 × 107 kg/s and a plume height of 18 to 21 km. About 0.1 km3 of magma was erupted in the Plinian phase, followed by eruption of a pyroclastic flow and an obsidian lava flow. Combined with similar data from Mount St. Helens, Mount Rainier, and Mount Mazama (Crater Lake), these eruptions define the range of Plinian events that have occurred in the Cascade volcanic arc in the past 12 k.y. During this period there have been Plinian eruptions with plume heights between 11 and 55 km, intensities between 106 and 109 kg/s, and volumes between 0.01 and >5 km3 of magma. All eruptions with intensities greater than or equal to 108 kg/s also produced large-volume pyroclastic flows and surges. Monitoring column height (intensity) during eruptions could help mitigate hazards because it may indicate pending generation of pyroclastic flows. In the Cascade Range, there have been at least 12 eruptions of >1 km3 of tephra in the past 12 k.y., suggesting that eruptions of such magnitude occur about once every 1 k.y., although such frequencies vary greatly at each volcano. The volume of magma erupted in each event correlates with both column height (intensity) and magma composition, suggesting that eruptions of these volcanoes relate to the accumulation rate of magma in their reservoirs.

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