Three Sisters May Get a Fourth

The bulge covers 100 square miles -- roughly the size of Fresno -- and is rising at a rate of 1.4 inches a year. The shape resembles a dome, with the highest point about three miles west of the South Sister volcano in the Cascade Range.

Geologists say the bulge represents a unique opportunity to study what could be a volcanic formation in its earliest stages, but officials in this town of 65,000 worry more about the potential hazards, such as lava and ash or flying rocks.

"Is it going to blow up and bury Bend?" City Manager Harold "Andy" Anderson asked. "In the wake of [Hurricane] Katrina, we're trying to assess our biggest natural threats, and the bulge came up in meetings as a possibility."

Scientists have held community forums trying to assuage concerns and educate the public about why the phenomenon should inspire fascination rather than fear. They say nothing is comparable in the Cascades or possibly in all of North America, but the technology that detected the bulge is relatively new.

The bulge, in the Three Sisters Wilderness Area -- named after three volcanic peaks -- was detected in March 2001 by a geologist using a new imaging technology called radar interferometry, which uses satellites to measure changes in Earth's surface.

Since the discovery, scientists have "wired" the bulge with additional measuring equipment. Geologists from the U.S. Geological Survey have also made annual surveying treks to the site, which is not accessible by car. A report on the latest findings -- collected in late August -- is expected this month.

Larry Chitwood, geologist for Deschutes National Forest, which encompasses the area, said the rise probably began in 1997 and has risen about a foot since. The cause of the rise is a matter of speculation.

Chitwood theorizes that a body of magma, or molten rock, could be gathering in a chamber several miles below the surface. There's no way to know the shape of the mass, he said, but the size of the bulge indicates that it would be quite large, equivalent to a lake one mile across and 65 feet deep.

"It gives you an idea of the volume that could be involved," Chitwood said.