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In the Aleutian volcanic chain (USA), the 2050 ± 50 BP collapse of Okmok caldera generated pyroclasts that spread over 1000 km2 on Umnak Island. After expelling up to 0.25 km3 DRE of rhyodacitic Plinian air fall and 0.35 km3 DRE of andesitic phreatomagmatic tephra, the caldera collapsed and produced the 29 km3 DRE Okmok II scoria deposit, which is composed of valley-ponding, poorly sorted, massive facies and over-bank, stratified facies with planar and cross bedding. Geological and sedimentological data suggest that a single density current produced the Okmok II deposits by segregating into a highly concentrated base and an overriding dilute cloud. The dense base deposited massive facies, whereas the dilute cloud sedimented preferentially on hills as stratified deposits. The pyroclastic current spread around Okmok in an axisymmetric fashion, encountering topographic barriers on the southwest, and reaching Unalaska Island across an 8-km strait on the east, and reaching the shoreline of Umnak in the other directions. The kinematic model by Burgisser and Bergantz (2002, Earth Planet. Sci. Lett. 202:405-418) was used to show how decoupling of the pyroclastic current was triggered by both sea entrance and interaction with the topography. In the former case, the dense part of the current and the lithics transported by the dilute cloud went underwater. In the latter case, topographical barriers noticeably decelerated both parts of the decoupled current and favored sedimentation by partial or complete blocking. The resulting unloading of the dilute current drastically reduced the runout distance by triggering an early buoyant lift-off

Physical volcanology of the 2,050 bp caldera-forming eruption of Okmok volcano, Alaska