"The magma is pushing the plug upward. It's going high in the sky," said hydrologist Carolyn Driedger of the U.S. Geological Survey at the Cascades Volcano Observatory in Vancouver, about 50 miles from the southwest Washington mountain.

One section of the new lobe has risen by 330 feet in the past nine days, Driedger said.

"The whole area is lifting or rising," she said. Exact dimensions are not yet known but will be determined from photos taken Thursday.

"It seems like every time you think you know what's going on, (the volcano) twists and does something different," said Jeff Wynn, chief scientist for volcano hazards at the observatory.

Two scientists flew into the crater by helicopter Thursday and landed beside the new structure, under strict orders to stay no longer than 10 minutes to collect samples, he said.

"It's dangerous to go in there. They landed, ran over and grabbed whatever they could," he said.

The new lobe, which began building last month, had grown to roughly the size of an aircraft carrier. Scientists described it as 900 feet long, 250 feet wide and 230 feet high.

"What we have been noticing with this monster is that it was growing at an unusually high rate and it was spreading out horizontally like a big pancake," he said. "And now all of a sudden it's like a huge piston has been thrust up."

Magma, or molten rock, is reaching the surface at the rate of 7 to 8 cubic meters per second - or about one large dump truck load every second, Wynn said.

Like the old lava dome, formed in the six years after St. Helens' devastating May 18, 1980, eruption, the new lobe is made of a type of volcanic rock called dacite, Wynn said. More than 63 percent silica, it tends to be sticky and viscous, unlike the free-flowing lava of Hawaii.

The massive 1980 eruption killed 57 people.

The new piston-like area "actually raises the ante a little bit initially," Wynn said. Big chunks of hot rock could break off, with unpredictable consequences, including possible explosive ash emissions, he said.

Temperatures on the new protrusion can spike as high as 1,200 degrees Fahrenheit. Wynn said that's hotter than a self-cleaning oven, which can reach 900 degrees Fahrenheit.

Scientists want to know if there's newer, fresher magma coming up, which could have more gas and be more explosive.

"We believe there's new magma in the system, but there's still very small amounts of gas coming out," Wynn said. "More gas means more explosive potential and increased danger."

Driedger said the current activity could continue for weeks or months.

Seismic activity has remained at a low level consistent with the free movement of magma.

The volcano rumbled back to life Sept. 23, with shuddering seismic activity that peaked above magnitude 3 as hot magma broke through rocks in its path. Molten rock first reached the surface on Oct. 11, marking the resumption of dome-building activity that had stopped in 1986.

A more explosive eruption, possibly dropping ash within a 10-mile radius of the crater, is possible at any time, scientists have said.