The Season of the Alaska Volcano

There's a volcano in Alaska so dependable you can almost set your watch to its eruptions. Pavlof volcano, located near the spot where the Alaska Peninsula turns into the Aleutian Islands, usually starts rumbling in the fall and late winter. Pavlof, one of the world's most active volcanoes with 41 eruptions since the late 1700s, has erupted in mid-November six times in the past 25 years. A volcano researcher thinks he might know why.

Geophysical Institute Research Professor Steve McNutt works for the Alaska Volcano Observatory. He recently sent a memo to the other members of AVO in Fairbanks and Anchorage telling them to keep an eye on Pavlof during the first few weeks of November. As of mid-November, Pavlof showed no alarming jumps in its seismic heartbeat, signals volcanologists can monitor from their offices in Anchorage and Fairbanks. But they won't be surprised if Pavlof starts shaking soon.

In 1973, 1976, 1980, 1983, 1986, and 1996, Pavlof erupted or was already erupting in early to mid-November. Twelve of fifteen eruptions from 1973 to 1992 happened between September 9 and December 8.

Why would a volcano erupt primarily in the fall? McNutt said weather patterns in the Aleutians and the Gulf of Alaska probably play a big role in Pavlof's annual performance. Low pressure systems tend to hang around the Aleutians in the fall. With less atmospheric pressure pushing on the ocean, the Gulf of Alaska tends to be about a foot deeper in the fall. Fall winds also push large volumes of water into the Gulf of Alaska, tending to "pile" water around the Aleutians and the Alaska Peninsula. All this extra water creates a lot of pressure that is felt deep beneath volcanoes, McNutt said. Molten rock within the earth may be getting squeezed by this extra pressure and forced upward to flow from volcano vents. McNutt compared the process to a tube of toothpaste--squeeze the tube and the paste erupts.

Other researchers recently found that large earthquakes can trigger volcanic eruptions. Alan Linde and Selwyn Sacks, of the Carnegie Institution in Washington, D.C., searched historical records dating to the 1500s and found a correlation between volcanic eruptions and earthquakes with a magnitude greater than 8.

Linde and Sacks' study is published in the October 29 issue of Nature. Their theory is that huge earthquakes may shake the bottoms of magma reservoirs and increase the pressure within the bowels of volcanoes, causing eruptions within two days of a large earthquake. In their study, they only considered the eruptions that happened within 465 miles of earthquake epicenters. They looked at data from 204 earthquakes of magnitude 8 or higher.

McNutt said an earthquake has a more immediate effect on a volcano than does higher sea level, which may apply pressure on volcanoes for months before triggering an eruption. Volcanologists hope someday to predict eruptions using natural clues, such as nearby earthquakes, changes in sea level, and changes in atmospheric pressure.

For now, all eyes at AVO are on the seismometers installed two years ago on Pavlof, a volcano that's more dependable than the weather.