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Friday, May 12, 2006
Huge rock breaks off St. Helens' rising fin
The sheer rock fin emerging in Mount St. Helens' crater lost about a third of its northern face recently, but because lava keeps pushing to the surface, the height remained the same Thursday -- around 330 feet.
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.
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 volcano's deadly May 18, 1980, eruption, which 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.
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