Alaska Science Center
Small Volcano, Big Eruption
Kasatochi Volcano 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. 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) who have annually deployed scientists to the island to monitor seabirds for 13 years.
The 2008 eruption was unanticipated and 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 1 PM AKDT August 7, a magnitude 5.6 earthquake occurred within a few miles of Kasatochi. Soon after this earthquake, seismic instruments on nearby Great Sitkin Volcano began recording strong volcanic tremors 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 13,700 - 15,250 meters 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 13,700 – 15,250 meters and produced several centimeters of ash fall over the ocean and on islands southwest of Kasatochi, including minor amounts on Adak Island, the closest island with a year-round population. Boats in the vicinity of the volcano reported 10 - 13 centimeters of ash fall, some of it coarse sand and small pebble size, darkening skies, and lightening, likely caused by static electricity in the ash plume. The third event was followed by about 14 hours of continuous ash emission. The cumulative volcanic cloud from Kasatochi (Figure 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. 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, and photographs of what they found can be seen on the photo gallery page. The site visit confirmed that a major eruption had occurred, and thick deposits of grey volcanic debris and ash now covered the formerly lush volcanic island (Photos 1, 2). New deposits exposed along the new coastline, now about 400 meters further into the sea were noticeably warm when visited on August 22 and 23 (Photo 3). These deposits indicate there was a collapse of the vertical eruption column to produce hot avalanches of rock debris, gas, and ash. These 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. Thousands of chicks of nesting seabirds had not 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 meters 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.