In August 2008, a small volcanic island in the central Aleutians erupted for a day. The eruption was a VEI 4, resurfaced the island with pyroclastic flows, increased the area of the island by around 40%, and killed almost everything living on it. Pyroclastic flows entered the surrounding sea and buried kelp beds surrounding most of the island under tens of meters of debris. Incidentally, the eruption also injected the most sulfur dioxide into the atmosphere of any eruption since Pinatubo and Hudson in 1991. Not bad for an island with no significant eruptive history measuring only a mile and a half (three kilometers) across and 1,000’ (300 m) high.
Kasatochi lies 1,200 miles (1,900 km) south and east of Anchorage, AK. It is east of Adak Island. The closest monitored volcano is Great Sitkin on Great Sitkin Island, some 20 miles (35 km) west of the island.
Geologically, the island is a single roundish island with a large crater lake in the middle of it. The crater lake measures around a half a mile (1 km) across and its water level sits about 60’ (20 m) above sea level. The island is surrounded by cliffs, with few if any beaches. Total pre-eruption area of the island was 2 square miles (5 square km). It is also interesting in that it is one of several central Aleutian Islands described as “….the tip of a stratovolcano built of basaltic, andesitic and pyroclastic flows emerging from the ocean.” The last reported eruption of Kasatochi may have been 1826 and 1827. But the report identified a long dormant volcano rather than Kasatochi. There was also a report in 1899 of the crater lake disappearing and steam rising from the crater.
Pre-eruption photos of Kasatochi showed a lush, green, beautiful island. The island was surrounded by cliffs with relatively small beaches. Most of the cliffs on the southern half of the island were filled in by debris after the eruption, though the action of ocean waves was busily re-carving them in the soft remains of the pyroclastic flows in the years following the eruption.
It is part of the Alaska Maritime National Wildlife Refuge, and supported among other birds a colony of a quarter million auklets (birds). There was also Steller sea line rookery that took advantage of abundant sea life growing in the kelp beds surrounding the southern 70% of the island. It was a bird-watcher’s paradise, and the federal birders became central to post-eruption monitoring. The island was also one of the locations that the US Geologic Service (USGS) would use to take test counts of various bird and seal populations. There was a cabin used by US Fish & Wildlife Service employees who lived on the island while they took regular surveys of birds and sea mammals on the island.
A pair of naturalists on the island in the run-up to the eruption was evacuated a short time before the eruption began. Had they not have been evacuated, they would not have survived the eruption or the aftermath. An author, N. Rozell, wrote an article entitled “Escape from Kasatochi” in the February 2010 issue of Alaska Magazine describing the ordeal. I have not found an online copy of the article as of this writing. The following is a description of the festivities from the perspective of the birding community. http://ofafeather.blogspot.com/2008/08/kasatochi-memories.html
The eruption did not come unannounced, with the first reported earthquakes being felt by resident biologists on the island on August 2. The earthquake swarm preceding it was relatively short and quickly built in intensity, peaking out at 5.8 Richter. The Alaska Volcano Observatory issued a warning of impending eruption hours before it finally blew on August 7.
As an aside, the seismic monitoring network for Alaskan volcanoes is funded in part by the Federal Aviation Administration (FAA) as an air traffic safety measure. If you fly the major air traffic routes on the northern part of the Pacific Rim, you want to stay well clear of volcanic ash plumes.
The initial blast was water-rich as the water and mud from the crater lake mixed with debris in and around the crater rim and was lofted into a plume 9 miles (14 km) tall. It lasted less than two hours and was followed with a similar, but shorter plume a couple hours later. Prevailing wind during this time was from the NE, pushing ash down the Aleutians to the west.
The third blast was far more ash-rich and vigorous, lasting 10 hours. There was another pair of blasts during this period. This is when the pyroclastic flows deposited tens of meters of debris on the island and out to sea, growing it. Adak Island got a few inches (cm) of ash on its eastern end.
It was during the main part of the eruption that the SO2 plume was injected into the upper atmosphere.
Several aircraft reported flying through the SO2 plume. None were forced to land after the encounter. Alaska Air Lines cancelled over 40 flights due to the presence of the ash plume in their flight path along the Aleutians.
Local fishing boats reported the expected blackout conditions during daytime under the plume and falls of lapilli and pumice along with finer grained stuff. They also reported lightning in the plume.
Aircraft were flying past the island within a couple days of the eruption. Boats were close behind. Animals did return within weeks to the island, enjoying the warmth of the degassing new dry land.
The first year, the Stellers did manage to breed successfully. The birds did not due to the destruction of their nesting grounds.
Due to the continuing erosion of small particle ash into the sea by water runoff and wind, the recovery of the kelp beds and underwater ecosystem close to the island has not gone quickly at all. Note the runoff in the comparison photo below.
Biologists were able to find some plant life on the island that did survive the eruption, though these were mostly roots, tubers and buried seeds in locations relatively shielded from the main brunt of the eruption.
Eruptions that clear off entire sections of islands are not all that uncommon in the tropics. Given the location in this part of the world and the fact that it is part of a wildlife refuge, the biologists are having a great time watching life on the island return until the next eruption removes it again.
Sometimes interesting things come in small packages. And those packages open very quickly and with surprising violence. A pretty good summary of ongoing research into Kasatochi’s recolonization can be found at: http://alaska.usgs.gov/science/kasatochi/index.php
Downloadable report can be found at: http://doc.nprb.org/web/09_prjs/923_FINAL%20REPORT.pdf