A burst of seismic activity at the mountain Sunday night likely corresponded to the collapse. "Certainly a big piece fell off - something like 65,000 cubic yards," said geologist Dan Dzurisin at the Cascades Volcano Observatory in Vancouver, Wash., about 50 miles from the mountain and 150 miles south of Seattle.

Bad weather had iced over scientists' cameras on the rim of the volcano, so the rockfall wasn't recorded on film, he said.

Now the fin is about the same height as it was before, but rock that was previously in the middle is now at the top.

"At that height, it becomes unstable and ... begins to collapse under gravity," he said. Boulders and finer rubble from the crumbling top surround the base of the fin.

This is the seventh rock feature formed by lava in the crater since the 8,364-foot mountain reawakened with a drumfire of low-level seismic activity in September 2004.

The crater was formed by the southwest Washington volcano's deadly May 18, 1980, eruption that killed 57 people and blasted about 1,300 feet off the then-9,677-foot peak.

The most recent lava feature started growing in mid-October, Dzurisin said.

The emerging rock takes different shapes, depending on what it meets at the surface.

At the moment it's like toothpaste coming out of a tube. "As it emerges it's having to deal with its own debris, so we're seeing steeper-sided features looking more like spines," the geologist said.

The pace of the lava extrusion has slowed since October 2004, he added. For the past few months, St. Helens has been pushing lava to the surface at a pace of about 1 meter per second, down from 6 meters a second.

It's not yet clear whether it's slowing to a stop, or has reached a pace it can maintain for years or even decades, Dzurisin said.

"St. Helen's loves to build domes," he said. "It's built many in its history and we suspect some of those went on for decades.

"We might be in that situation - and then again we might not."