Scientists don't expect a repeat of the catastrophic eruption that blew off the top of the mountain in southwestern Washington state, killing 57 people and destroying more than 200 homes.

Instead, the lava will continue to add mass to the lava dome, accompanied by small steam and ash eruptions, similar to a dome-building period that lasted until 1986, said Tina Neal, a USGS geologist.

"The volcano remains restless," Neal said.

Some areas of the hot rock were observed to be up to 1,270 degrees Fahrenheit (690 Celsius) and could be seen glowing at night, Neal said. During the day, the rock is mainly gray in color.

So far, the dome has risen at least 250 feet, or the size of a 30-story building, since Mount St. Helens woke from its slumber nearly three weeks ago with small eruptions spewing steam and ash.

The U.S. Forest Service kept the Johnston Ridge Observatory, the nearest lookout point five miles from the crater, closed. Tourists and media have flocked to the Coldwater Ridge Visitor Center north of the mountain.

The 1980 eruption reduced the summit of the volcano to 8,364 feet from 9,677 feet, devastating hundreds of square miles of surrounding evergreen spruce forest.

Ash from that eruption billowed across much of the United States and the explosion was heard as far away as Canada.

The biggest concern in the event of an eruption is the impact of ash on air traffic as Mount St. Helens is 50 miles north of a busy airport near Portland, Oregon.