Okmok Volcano on the Rise in the Aleutians

Mount Okmok is bulging like a balloon. Scientists noticed the expansion of Okmok, a volcano on Umnak Island in the Aleutian chain, while using Global Positioning System receivers recently to determine tiny changes in the surface of the mountain. A portion of Okmok’s summit crater inflated by about one inch during the past year, hinting at a possible eruption.

“If we end up having an eruption in the next several months, we’ll say ‘Ah-ha!’” said Jeff Freymueller, who studies the planet’s slow-motion movements with GPS.

A few years ago, he used the same system to discover Seward and Homer were moving away from one another by a few centimeters each year. Freymueller, a professor of geophysics at the University of Alaska’s Geophysical Institute, is also using GPS to see how much the new lake created by Three Gorges Dam in China will press into the Earth, possibly triggering earthquakes. The GPS system Freymueller uses is similar to hand-held GPS units, though his receivers are more complex and expensive. GPS is a system of 24 satellites operated by the U.S. Air Force. As the 24 satellites orbit the globe, they broadcast radio signals to the receivers. A computer within the receiver then performs a quick geometry equation to pinpoint the location of the receiver. By stationing the receivers at the same exact points around Okmok for several years, the researchers were able to see the surface rise like bread in the oven.

Okmok Volcano is in the middle of the Aleutian chain’s sweep from east to west. Green, wet and windy, Umnak Island is home to a cattle ranch based at the old Fort Glenn, which the U.S. Army built in World War II to protect Dutch Harbor from Japanese attack.

The discovery of the deformation of Okmok Volcano came as GPS receivers on Okmok were installed by Freymueller, graduate student Doerte Mann and three scientists from Japan, led by Fumi Kimata of Nagoya University. The Japanese Space Agency is funding the study, in part because millions of people live on the flanks of volcanoes in Japan. GPS measurements on volcano slopes are more common in Japan than in the U.S., where the most active volcanoes are in places inhabited by few people, such as the Aleutian Islands.

Scientists who keep a daily tab on volcanoes at the Alaska Volcano Observatory in Anchorage and Fairbanks use seismometers to detect earthquake activity within Alaska’s volcanoes and satellite images to check for heat and ash clouds. GPS receivers are a new tool in Alaska volcano monitoring, one that can show scientists the inflation and deflation of volcanoes caused by the migration of molten rock within them, which is often the first sign of unrest before an eruption.

Okmok’s expansion intrigues Freymueller because the mountain is inflating at the same rate that it did before the last eruption in 1997. In each of four years before that eruption, the center of the crater on top of Okmok rose at about one inch per year. During its eruption in 1997, when Okmok spit up lava and blew an ash cloud to 30,000 feet, the crater floor deflated by about five feet.

Now, as molten rock flows back in beneath the surface, Okmok is rising again. The Volcano inflated almost as much in one year as Redoubt has in a decade.

“Next year, we’ll either have a nice eruption to look at, or a good glimpse at what happens before an eruption,” Freymueller said.