DESTRUCTION OF AN ALEUT VILLAGE BY A CATASTROPHIC FLOOD RELEASE FROM OKMOK CALDERA, UMNAK ISLAND, ALASKA

Okmok caldera, a large basaltic andesite shield volcano with a 10 km wide caldera, is located on the northeastern end of Umnak Island in the Aleutian Islands of Alaska and contained a 5.8 x 10⁹ m³ caldera lake that catastrophically drained when the caldera rim failed sometime between 1560 and 1010 yr. B.P. The catastrophic flood event attained a maximum possible discharge of 1.9 x 10⁶ m³/s, larger than any other flood during the last 10,000 years. Another flood release occurred more recently from the caldera directly affecting human habitation on the island. Historic documentation by Russian explorers during the mid-1800s recorded an explosive eruptive event at Okmok volcano in 1817 A.D. where an Aleut village situated at Cape Tanak was reportedly destroyed by large quantities of ejecta and a “flow of ashes.” During field reconnaissance activities, evidence of a thick ejecta deposit was not found, however a young deposit of boulder-rich alluvium on Cape Tanak was identified and traced upstream to the caldera. Side-channel erosion within the caldera including truncated alluvial fans and a 4-8 m high scarp along the Crater Creek channel margins was also identified. Two radiocarbon dates on a soil horizon directly below the alluvium deposit yielded ages of 190 ± 60 yr. B.P. and 10 ± 60 yr. B.P. Corrected calendar dates for both ages are very close to the timing of the historic eruption of 1817. It is believed the alluvium on Cape Tanak was deposited by a flood event produced by draining of a lake located approximately where Crater Lake is within the caldera today when the 1817 eruption occurred. This second release of water from the crater into the gorge did not exceed a depth of 8 m within the Breach, was confined to the Crater Creek channel as it flowed down the volcano flank, and was most likely the agent that destroyed the Aleut village at Cape Tanak. Flow competence equations indicate the young flood event attained a minimum discharge of 2.0 x 10⁴ m³/s within 5 km of the caldera decreasing to 1.5 x 10³ m³/s on Cape Tanak. The occurrence of two flooding events from Okmok caldera indicates catastrophic floods from intracaldera lakes are more common phenomena than generally thought, and constitute a significant hydrologic hazard in volcanic regions.