MENLO PARK, Calif., April 2 (UPI) -- The ground near one of the long-dormant Three Sisters volcanoes in the Cascade Mountains of west-central Oregon has risen approximately 4 inches in a 6-by-12-mile parcel since 1996, meaning that magma or underground lava is slowly flowing into the area, according to a research team from the U.S. Geological Survey.
"It hasn't erupted in about 1,500 years, so it's a truly dormant volcano. Yet there's something going on there," said geologist Charles Wicks Jr., lead author of a report in the American Geophysical Journal.
"Right now in the Cascades, the only volcano that we know is restless is Three Sisters, based on the deformation," said co-author, geologist Daniel Dzurisin at the USGS Cascade Volcano Observatory in Vancouver, Wash.
The researchers said it is far too early to tell if this swelling and movement of magma presages a volcanic eruption. It might simply "freeze out," said Wicks, meaning the magma flow might simply stop.
The groundswell was observed by satellite radar images, a new technique Dzurisin said would help scientists track such deformations.
"We now have a tool that allows us to see this kind of deformation around volcanoes, which we've suspected for a long time. But in the past we haven't been able to observe it unless there was accompanying seismicity," said Dzurisin, referring to the earthquakes that occur when the magma breaks through the rocky crust prior to eruptions. Such quakes often are a warning of an impending eruption.
"As far as I know, this type of observation hasn't been made any other place. We don't know if this type of uplift occurs fairly frequently at similar types of places around the world. This type of thing may be relatively common, but we don't know," said University of Washington geophysicist Stephen Malone, who is familiar with the finding but is not connected with the published research.
He noted geologists are installing seismometers in the area to monitor for earthquakes.
The Three Sisters area -- which contains five volcanos -- is only about 170 miles from Mount St. Helens, which erupted in 1980. Both are part of the Cascades Range, a line of 27 volcanoes stretching from British Columbia in Canada to northern California.
Periodic eruptions are generated by movement of the enormous Pacific tectonic plate sliding under the similarly massive North American plate. The grinding of the two plates against each other causes tremendous heat and pressure. Magma from deep in Earth's interior is pushed up through fissures or cracks along a relatively straight line underneath the North American plate. The magma reaches the surface at the sites of the Cascades volcanoes.
While geologists say tracking such groundswells is important for basic understanding of the way volcanoes work, there are important practical benefits as well.
"Ultimately the goal is mitigate volcanic hazards, and the way we can do that is to understand the processes that lead up to eruptions better. We hope that by being able to see ground deformation like this we might be able to extend the warnings that we might give," Dzurisin said.
(Reported by Harvey Black in Madison, Wis.)