OREGON.- The west side of the South Sister volcano is still bulging, but not as much.

The rate of expansion of the uplift has slowed to about half the previous rate on the 10,358-foot mountain, geologists say.

According to data collected last summer and released Wednesday, the growth rate of the bulge is about half an inch a year.

Until 2004, the bulge rose an average of just over an inch a year after it began in late 1997 or early 1998 about three miles west of the volcano's summit.

Scientists believe the uplift is caused by magma — hot molten rock and gases — pushing up from deep below the Earth's surface to a depth of about 4 miles below the surface. As the magma intrudes into the rocky crust, it pushes up the ground above it.

The uplift was first discovered in 2001, when a geologist in California was comparing radar images taken from satellites in different years, said Dan Dzurisin, a geologist with the U.S. Geological Survey's Cascades Volcano Observatory in Vancouver, Wash.

Since then, geologists have used a variety of techniques to measure the ground movement — including radar imaging from satellites, Global Positioning System monitoring and leveling techniques.

"The picture that emerged this summer is the uplift is still going on, all three techniques showed that, but it slowed some," said Dzurisin.

So far, magma has pushed the center of the uplift up about 10 inches, Dzurisin said. The bulge covers an area about 10 miles in diameter on the west flank of the volcano.

Geologists are interested in tracking the progress of the uplift because it's the first time they've had a chance to measure action like this in the Cascade Range, Dzurisin said.

There is some information about what happened beneath Mount St. Helens a few months before its May 1980 eruption. But geologists didn't have a chance to extensively monitor its activity in the previous years to see if uplifts occurred.