This column is provided as a public service by the Geophysical Institute, University of Alaska Fairbanks, in cooperation with the UAF research community. Ned Rozell is a science writer at the institute.

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Pavlof, an Alaska volcano with a fondness for erupting in fall, is not alone. Three other volcanoes from around the globe seem to share Pavlof's preference, erupting mostly from September to December. This routine behavior in a very unpredictable world intrigues Steve McNutt.

McNutt, a volcano seismologist with the Geophysical Institute at the University of Alaska Fairbanks, spoke of the volcanoes' curious timing at the recent fall meeting of the American Geophysical Union. More than 8,500 scientists attended the San Francisco meeting, which is held at a cavernous convention center for an entire week each December.

At a meeting similar to this in 1999, McNutt first received a clue that other volcanoes share Pavlof's tendency to erupt in sync with a season. While in Birmingham, England, McNutt gave a talk on Pavlof's eruptions. Makato Murakami of Japan approached McNutt after the presentation and told him of two volcanoes in Japan that showed a similar pattern of fall eruptions. Sure enough, Oshima and Mikakejima volcanoes mirrored Pavlof's pattern.

McNutt, who did his Ph.D. research on Pavlof, thought changes in sea level may have been a reason the Alaska Peninsula volcano erupts predictably-with 12 of 15 of its latest eruptions occurring between September and December. Low-pressure systems that hang around the Aleutian Islands in the fall cause the Gulf of Alaska to be about one foot higher than in other seasons, he said. This extra water tends to push with greater force upon the Aleutians and the Alaska Peninsula, squeezing molten rock and forcing it to flow upward through the volcano vent. McNutt compares the action to squeezing a tube of toothpaste.

McNutt's Japanese colleague had a different explanation for the similar actions of the Japan volcanoes. A global-positioning-system network on the islands of Japan has recorded a pattern in which Earth's plates squeeze the area around the volcanoes during the fall and stretch the same area in other seasons. McNutt thinks this mechanism may be adding to the sea level affect he noted at Pavlof and may also be the reason a fourth volcano, Villarica in Chile, acts the same as Pavlof and the two volcanoes in Japan.

How could four volcanoes -one in a different hemisphere- erupt at the same time of year? The four are alike in that they have similar plumbing systems, they erupt similar material, and their eruptions tend to be of the same size, McNutt said. The gigantic slabs of Earth's plates beneath the volcanoes may be bending up and down slightly in response to some seasonal trigger. The first step in finding out more about the mechanism will be to install GPS stations near Pavlof and the Chile volcano. GPS uses satellites to track the subtle movements of Earth's plates, which often move at the speed fingernails grow. With GPS receivers in place, McNutt could see if Earth's plates are putting the seasonal squeeze on Pavlof and the volcano in Chile, as the plates are in Japan.

Volcanoes are not the only geological phenomena that seem to respond to the seasons, McNutt said. Swarms of earthquakes beneath Mt. Etnain Italy and a volcano at Montserrat occur in both May in November. The earthquake swarms may be related to the volcanic eruptions, but that's a mystery still unsolved, as are the coincidental eruptions of four scattered volcanoes.

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