Scientists watch volcanic bulge on Oregon's South Sister
Hikers and horse-riders cross it Pacific Crest Trail and may see sturdy metallic tripod topped by a white disk. Wires lead to a solar panel. A sign says "Volcano Monitoring Equipment: Do Not Disturb."
Here magma has pushed up the ground, has been rising an inch or so a year since about 1997, a rise invisible to the naked eye.
It began to slow last year, but scientists still watch the area, which has some of the more active magma in the Cascades.
The bulge near the 10,358-foot mountain is the only rising ground along the Cascade range, said Dan Dzurisin, a geologist with the U.S. Geological Survey's Cascade Volcano Observatory in Vancouver, Wash.
"What we think is there is a place where magma is slowly rising from deeper in the Earth," he said. "It's accumulating there, and it's inflating or causing the rocks around it to be pushed aside."
It could go on or some time or be early signs of a possible eruption.
"We don't know, and that's why we keep track of it," Dzurisin said.
While the bulge grew by about inch each year starting in the late-1990s, in 2005, the monitoring experiments gave conflicting results, he said.
Suggestions varied from no change to growth as usual.
"There certainly is an indication that it may have slowed, and that's one reason why we're so anxious for the '06 results," Dzurisin said.
Results from this summer's measurements aren't in.
Some volcanologists think a volcano range could have a series of bulges, said Larry Chitwood, forest geologist with the U.S. Forest Service's Deschutes National Forest.
A bulge would form, rise, then stop, he said. And then later, maybe in a few decades, it will start up again before eventually erupting.
"It looks like volcanic eruptions are usually preceded by bulging of the land surface, and that the more rapidly the bulge rises the more likely there would be an eruption," Chitwood said. "Things are going the opposite way right now."
That area was active as recently as 1,500 years ago, he said, and the geologic record indicates that eruptions happen every 1,000 to 2,000 years.
"It seems like there would be no surprise if we had an eruption up there, it would be right on time," Chitwood said.
He said an eruption near South Sister would probably initially create a cinder cone.
But scientists can't conclude that an eruption is eminent, Dzurisin said.
"More likely it seems that these uplifts happen and magma moves up from depth and finds some cracks, and then in most cases the magma stops and cools and fills the cracks," he said.
Dzurisin's team uses a technique hundreds of years old that measures slight changes in elevation between fixed points and can can give readings accurate within 1/8 of an inch over a mile, he said.
The magma pushes the ground up and stretches it, he said.
Scientists estimate the magma is about four miles deep.
Other scientists are finishing a geologic map of the area that will provide a better understanding of the volcano's long-term history, said Willie Scott, a geologist with the USGS's Cascade Volcano Observatory.
They are identifying and dating the rocks to see how the volcano has grown through time, what kind of eruptions occurred and what kinds of lava were involved. Some have been "quite explosive," he said.
"It helps us to get a better idea of what the volcano is capable of doing," Scott said.
Others are using a new satellite that takes radar images to detect subtle changes in elevation - the method used to first detect the bulge in 2001 when earlier images revealed changes, Scott said.
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