Mount St. Helens vented more steam Monday as new thermal images revealed that parts of the lava dome in its crater are piping hot, a sign that magma continues to rise within the volcano.

Scientists said an area on the south side of the old dome, where a large uplift of rock has been growing, now appears perforated as if magma has been hammering at the surface.

"The magma is not just pushing up but pushing out," John Pallister, a U.S. Geological Survey geologist, said at a news conference. He said scientists believe the magma is less than a half-mile below the surface.

Fast-moving magma would cause greater concern because explosive gases would not have time to dissipate. A team in Denver is evaluating aerial photos to gauge how quickly the magma is rising.

The alert level remains at "volcano advisory," but scientists have said an eruption could occur with very little warning.

Pallister said the most likely scenario remains an explosion with a few inches of ash spreading within a 10-mile radius of the crater. Such an event could happen in days, weeks or months -- or not at all, he said.

Scientists believe the chances are slim of a larger eruption such as the one on May 18, 1980, that killed 57 people. But Pallister was cautious nonetheless.

"I'm a fairly conservative guy, and I don't like a one in 10 chance," he said.

Any eruption would likely be vertical instead of the devastating horizontal blast that leveled old-growth trees for miles in 1980.

Willie Scott, a U.S. Geological Survey geologist, said earlier Monday that temperatures in some spots on the dome surface could be as high as 400 to 570 degrees Fahrenheit.

Scientists could not get precise temperatures for the hottest parts of the lava dome -- on the south side -- because the instruments were not calibrated high enough, said Jeff Wynn, chief scientist for volcano hazards at the USGS's Cascades Volcano Observatory.

"They didn't expect it to get that hot," Wynn said.