Associated Press
PORTLAND, Ore. – Federal researchers say they may place new seismic equipment near an Oregon volcano known as the South Sister because of evidence that molten rock has continued to seep into the area over the past year.
Scientists say the influx of volcanic rock doesn’t necessarily mean that South Sister will erupt, but magma three to four miles below the surface could trigger earthquakes as it moves.
New measuring devices would allow further study of the volcanic area, as well as protect the public, said Daniel Dzurisin of the U.S. Geological Survey’s Cascades Volcano Observatory.
“If it continues, it could eventually culminate in an eruption, and we need to be prepared for that,” said Dzurisin, who works in Vancouver, Wash.
South Sister, the youngest of the Three Sisters volcanoes in central Oregon, erupted twice about 2,000 years ago. Volcanic vents near South Sister last spilled magma more than 1,200 years ago and lie in a region that has spewed more volcanic debris than any other in the Pacific Rim.
Satellite images earlier this year revealed that a region nine to 12 miles across rose about 4 inches from 1996 to 2000. Geologists said it was the most striking geological change in the Cascade Range since the 1980 eruption of Mount St. Helens.
At the time, there were no current satellite images to show whether the swelling was continuing. In September, however, a European satellite circling about 500 miles above Earth snapped a new picture that includes precise measurements of the surface topography.
Comparing it with earlier images, researchers found that the area centered three miles west of South Sister had continued bulging upward at more than an inch a year. That brings its total swelling to about 5 inches in five years – a breakneck pace in geologic terms.
Other volcanoes are known to swell and shrink without erupting, but in some cases small surges of molten rock have triggered eruptions.
A relatively small infusion of 30 million cubic yards of magma could have caused the swelling, but it could also come from a larger reservoir of molten rock that has been under the Cascades for hundreds of years.
“We know from other places that there can be magma bodies present for some time, where a small increase causes the original body to erupt,” Dzurisin said. “If the process were to continue, it would be like stretching a rubber band. Eventually it’s going to break. It may take five years or it may take 10 or 20.”
William Scott, scientist-in-charge of the Cascades Volcano Observatory, emphasized that more research would help unravel details of the uplift. He said scientists need to find out if the rate of the swelling is increasing, decreasing or holding steady.
In addition to monitoring ground movement, researchers have sampled springs in the area for volcanic gases that would be a signature of an underground magma reservoir. Those gases act as the driving force of eruptions.
Stephen Ingebritsen, a hydrologist with the Geological Survey, said carbon and helium isotopes in samples collected this summer “showed pretty conclusively” that magma is present.
A small amount of carbon dioxide also has been detected in the atmosphere above the region, which is consistent with the theory that magma is responsible for the uplift.
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