Kasatochi Volcano, a small, 3 km diameter, 300 m high, island volcano in the central Aleutian Islands of
Alaska (52.1693 deg N latitude, 175.5113 deg W longitude) erupted violently on August 7, 2008 after an intense period
of precursory seismic activity. Kasatochi has received little study by volcanologists and has had no confirmed historical eruptions;
it is not monitored with seismic instruments on Kasatochi Island. The island is part of the Alaska Maritime National Wildlife Refuge
and has been a long-term study site for the US Fish and Wildlife Service (USFWS), which has annually deployed scientists to the
island to monitor seabirds for 13 years.
The 2008 eruption occurred less than a week after USFWS personnel on the island began feeling small tremors. On August 4, the USFWS
contacted the Alaska Volcano Observatory (AVO) to report these observations. At the time, the reports of earthquake activity were considered normal,
as the area has frequent, and sometimes large, earthquakes and an early analysis suggested that the activity did not appear volcanic in origin.
By early evening August 6, it became clear that a significant volcanic earthquake swarm was occurring in the vicinity of Kasatochi Island and that the
scientists on the island could be endangered should they remain at their camp. At about 7 PM AKDT, AVO issued a formal Volcanic Activity Notice stating
that Kasatochi volcano had become restless and raised the aviation color code and volcano alert level to yellow/advisory. AVO also recommended that the
scientists on the island be evacuated as soon as possible.
Strong seismicity continued throughout the evening and into the next day. At about 1PM AKDT August 7, a magnitude 5.6 earthquake occurred within a few km of
Kasatochi. Soon after this earthquake, seismic instruments on nearby Great Sitkin Volcano began recording periods of strong volcanic tremor usually
indicative of fluid (magma, gas, or both) motion and often associated with eruptive activity. AVO responded by raising the volcano-alert notification to
orange/watch at 1:57 PM AKDT, indicating that an eruption was possible. Just after 2 PM AKDT, satellite images confirmed an eruption of Kasatochi was in
progress and AVO issued a Volcanic Activity Notice announcing an aviation color code and alert level of red/warning. Thankfully, a local fishing boat had
safely evacuated the two scientists less than 30 minutes prior to the opening blast.
The eruption was characterized by three distinct explosions that were detected by the seismic network on Great Sitkin Volcano, at approximately 2:01 PM, 5:50 PM,
and 8:35 PM AKDT. The first two events produced relatively ash-poor, but gas-charged, eruption clouds that reached 45,000 - 50,000 feet above sea level and
apparently no or very little local ash fall. The third event generated an ash- and gas-rich plume that also rose to 45,000 - 50,000 feet and produced several
inches of ash and lapilli fall over the ocean and on islands southwest of Kasatochi, including minor amounts on Adak Island, the closest island with a year-round
population, about 50 miles from the volcano. Boats in the vicinity of the volcano reported 4-5 inches of coarse grained ash fall, darkening skies, and lightning,
likely caused by static electricity in the ash plume. The third event was followed by about 17 hours of continuous ash emission as determined from satellite data.
The cumulative volcanic cloud from Kasatochi (Fig. 1) contained a large amount of sulfur dioxide gas that was detected by the Ozone Monitoring Instrument on NASA's
EOS-Aura satellite for more than a week after the eruption as the cloud circled the globe. The ash and gas cloud drifted east and interfered with air travel between
Alaska and the conterminous US causing at least 40 flight cancellations and stranding many thousands of travelers. The cloud was visible for thousands of miles
downwind and apparently was the cause of some brilliant sunsets over the Midwestern US.
AVO and USFWS scientists visited Kasatochi Island on August 22 and 23. Photographs of what they found can be seen on the AVO web site
(www.avo.alaska.edu). The visit confirmed that a major eruption had occurred, and thick deposits of gray volcaniclastic debris and ash now covered the
formerly lush volcanic island that was home to over a hundred thousand seabirds (Figs. 2, 3). Pyroclastic-flow deposits exposed along the new coastline,
now about 400 m further into the sea were noticeably warm when visited on August 22 and 23 (Fig. 4). These deposits recorded significant collapse of the
vertical eruption column to produce hot avalanches of rock debris, gas, and ash. The pyroclastic flows also initiated a small tsunami that was recorded by
tide gages at Atka, Adak and Amchitka. Attempts to locate the USFWS camp were unsuccessful and the ca. 75-year-old cabin was either swept from the island or
buried beneath the new deposits. A few thousand chicks of nesting seabirds had not yet left their nesting burrows or crevices and were most likely entombed
under the ash. Few signs of life remained on the former major seabird colony. The summit crater had enlarged in diameter by about 100 m and the crater floor
was steaming profusely from a number of circular vents and warm areas on the crater floor.
The 2008 eruption of Kastochi was a significant test of AVO's ability to assess the reawakening of a seismically unmonitored and little-studied,
remote volcano. Fortunately, the earthquake activity was strong enough to be recorded on existing seismic networks on nearby volcanoes and scientists
on the island were able to communicate with local contacts to coordinate a rescue. These seismic networks were installed with funding from the Federal
Aviation Administration to reduce the hazard to aviation from volcanic ash. In this case, the instrumentation was crucial in recognizing the signs of
significant unrest and potential for major eruptive activity, saving the lives of two biologists and providing the aviation community with advance warning
of a possible eruption. AVO and its partner agencies in DOI now have a unique opportunity to evaluate the response of Kasatochi's ecosystem to a major
volcanic event and to address how the landscape evolves following significant physical, chemical, and biological changes.
Article written by Christopher F. Waythomas, Stephanie G. Prejean, and David J. Schneider, USGS, Alaska Science Center, Alaska Volcano Observatory,
Anchorage, Alaska. This article is scheduled for upcoming publication in the DOI magazine People, Land, and Water.