Nervousness is necessary in central Oregon these days.
Recent measurements taken from space show that a known volcanic region in the middle of the state is becoming restless, perhaps preparing for an eruption. The Three Sisters region, just west of the city of Bend, has been inflating, lifting the ground upward by about four inches in the past five years, scientists report.
The most likely explanation is incipient volcanism, with a glob of molten rock magma migrating up from deep in the Earth's interior, trying to get to the surface. As molten rock fills an underground magma chamber, the ground's surface is gradually rising. But volcanologists can't yet tell if it will actually lead to an eruption.
"The single most important thing is whether the activity is still going on," whether the bulge is still rising or not, said geologist Dan Dzurisin. "We should know that in the next two months."
Dzurisin, at the U.S. Geological Survey's Cascades Volcano Observatory, in Vancouver, Wash., said if the two instruments recently placed in the Three Sisters region show the ground is still moving, then research will intensify.
"It's a little bit like a piece of detective work," he explained. "We know that something happened we're absolutely sure of that but there's a lot we don't know yet, and we're in the process of finding out."
If the two instruments recently put in place a seismometer to listen for ground tremors, and a GPS receiver to detect changes in ground altitude show activity, then more studies will begin.
"We'll try to see if the rate is changing, and what other manifestations (of volcanism) might be there," he said. "Are there earthquakes? Any unusual gas emissions attributable to shallow magma? Or hydrothermal (hot spring) changes?"
The change in ground level was detected by the European Earth Resources satellite. The bulge is seen on the flanks of a local family of volcanoes, North Sister, Middle Sister and South Sister, and nearby peaks known as Husband, Wife and Little Brother. The last major volcanic activity in that region was about 1,200 years ago, when molten lava erupted just to the north of the Three Sisters.
According to geophysicist Charles Wicks, in that eruption a large amount of relatively runny basaltic lava poured out across the ground. That lava deposit is still young enough to resemble a rough and barren lunar landscape. A few hundred years before that, a more explosive type of lava, rhyolite, had also burst out during an eruption in that area.
Wicks, at the U.S. Geological Survey in Menlo Park, Calif., said a relatively small amount of fresh magma seems to have moved up from below to touch off the new episode of uplift in the Sisters region.
Before 1996, Wicks added, there had been no history of movement on record.