Abstract

The Edgecumbe volcanic field (EVF) on Kruzof Island in southeastern Alaska has been the site of explosive eruptive activity within the past 11,000 yr. The EVF is complicated by having multiple vents and a variety of deposits. Early Pleistocene fissure eruptions of basalt flows built a low shield volcano; late Pleistocene eruptions of basaltic andesite, andesite, and low-silica rhyolite from vents aligned along a fissure in the southern part of EVF initiated construction of Mt. Edgecumbe cone (ME) and the domes of Crater Ridge (CR).

A series of explosive eruptions occurred over a period of at least 2,000 yr, ending about 9,200 yr ago. The eruptions (1) produced hot pyroclastic flows that reached the sea on both the west and east coasts of Kruzof Island, (2) bored a crater in the CR domes by ejecting blocks of rock that were as large as 10 cm in diameter 5 km from the vent, and (3) produced a series of ashfalls that eventually totaled 1.5 m in thickness at Sitka, 20 km to the east, and which have been identified 200 km to the north near Juneau and Lituya Bay. Two small eruptions of CR produced airborne ash between about 4,000 and 6,000 yr ago.

There are presently no visible signs of pending eruptive activity at the ground surface; however, periods of quiescence lasting 10,000 yr or more are not unusual for volcanoes, and so EVF cannot be considered “dead.” The geologic record indicates a trend toward explosive activity at EVF in the past, implying that any future eruptions are likely to be explosive. Moreover, a wide variety of eruptive phenomena are probable in the event of future activity. In decreasing order of likelihood, the hazards associated with such phenomena are as follows: (1) ash plumes can affect regional air traffic and ash fallout can affect residents of Sitka; larger and less frequent ash-forming eruptions would affect inhabitants further from the volcano and interstate air traffic; (2) although Kruzof Island is now uninhabited, any persons within EVF during an actual eruption could face extreme hazards such as falling blocks, asphyxiation, incineration by pyroclastic flows, and burns or respiratory irritation resulting from airfall; (3) if eruptive activity includes dome emplacement at ME or production of rhyolitic pyroclasts at CR, then the possibility of producing a volcanogenic tsunami (a sea wave caused by rapid mass movement of material into the sea) would increase.

Despite the absence of eruptive activity more recent than 4,000 yr ago, EVF should be monitored for potential early signs of an eruption because of its proximity to populated areas. One possible method of monitoring would involve installation of a seismometer on southern Kruzof Island.