More lava emerged Thursday on the crater floor of Mount St. Helens, expanding a rock formation building on the volcano's old lava dome.

Airborne observers have seen "vigorous jets" of ash and steam near the new growth, said Tina Neal of the U.S. Geological Survey (search). Aerial photographs suggest some uplift on one side of the emerging rock "fin."

But seismic activity remained low Thursday, suggesting that molten rock, or magma, is reaching the surface without significant obstacles, Neal told a news conference at the Gifford Pinchot National Forest (search) headquarters in Vancouver, Wash., about 50 miles south of the mountain.

The new formation has grown to about 400 feet high and 1,600 feet in diameter, Neal said. The red-hot stone, blanketed with feathery ash, exceeds 1,300 degrees Fahrenheit in places.

Because the crater floor slopes downward to the north, the new formation on the higher south side is about level with the 1,000-foot top of the old dome, which was created in the six years following Mount St. Helens' devastating 1980 eruption.

The current dome-building began with intense seismic activity Sept. 23, which indicated magma was breaking through rock. Several steam eruptions followed, and geologists detected lava at the surface late Monday.

Gas-rich magma can cause explosive eruptions, but samples taken this week have detected little carbon dioxide or hydrogen sulfide (search), Neal said. There was no evidence, such as increasing earthquakes or ground deformation, to suggest pressure was building, she said.

So far this fall, the most visible bursts at St. Helens have involved steam and small amounts of ash as rainwater and glacier melt combined with hot rock in the crater of the 8,634-foot peak.

As the dome-building continues, it could produce small explosions with little warning, Neal said.

A large explosion is still possible, but is among the least likely scenarios, she said. Such a blast could send a column of steam and gritty ash tens of thousands of feet up and out, posing potential problems for airplanes and road traffic.

Trails within a five-mile radius of the peak remained closed, with the alert level at mid-range. The Forest Service, however, reopened some roads and trails near the mountain.

Mount St. Helens has been the most active volcano in the lower 48 states and Canada during the past 4,500 years. Its 1980 eruption hurled debris nearly 20 miles north, killed 57 people and paralyzed much of the Northwest with gritty, machine-clogging ash.