YELLOWSTONE NATIONAL PARK, Wyo. — A geologist says some parts of the continually shifting, collapsed volcano at the center of the park are swelling unusually fast.

Bob Smith, one of the leading researchers into Yellowstone's geology, presented some of his findings at the American Geological Union meeting in San Francisco this week.

Smith and other geologists from the University of Utah are particularly interested in two subterranean lava domes deep beneath the Yellowstone caldera.

"We've gone to this really pronounced, and I would say unprecedented, uplift of the caldera," he said.

But he said any talk of an eruption would be far-fetched.

In fact, it's typical for Yellowstone's caldera to rise and fall. Scientists say it's been happening for at least 15,000 years, with some shifts of more than 10 feet.

Portions of the caldera rose more than 3 feet between 1923 and 1984 and then dropped nearly 8 inches from 1985 to 1995. Measurements in 1995 and 1996 showed the caldera rising again before starting to fall in 1997.

The latest upward motion has been unusual for its speed.

Using data collected on the ground and from satellites, scientists say the Mallard Lake Dome, west of Yellowstone Lake's West Thumb, has risen 4 centimeters a year since the middle of 2004. Meanwhile, the Sour Creek Dome north of Fishing Bridge has risen about 6 centimeters a year.

Smith said an infusion of magma may be heating up groundwater and causing the ground to bulge.

"It's like inflating the balloon, but the balloon is capped," he said.