Big Rock Chunk Topples From 'Fin' In Volcano's Crater

May 11, 2006


SEATTLE - 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 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.

"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."

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Giant Rock Fin Rapidly Growing In Mount St. Helens' Crater

Photo USGS

POSTED: 8:15 am PDT May 5, 2006

UPDATED: 6:30 pm PDT May 5, 2006

SEATTLE -- If the skies are clear as forecast, volcano watchers who turn out for the reopening of the Johnston Ridge Observatory on Friday will get a spectacular view of a hulking slab of rock that's rapidly growing in Mount St. Helens' crater. It's jutting up from one of seven lobes of fresh volcanic rock that have been pushing their way through the surface of the crater since October 2004. The fin-shaped mass is about 300 feet tall and growing 4 feet to 5 feet a day, said Dan Dzurisin, a geologist at the U.S. Geological Survey.

The rock in the crater began growing last November, steadily moving west and pushing rock and other debris out of its way as it goes. Mount St. Helens, located in the Cascades of Washington, has been quietly erupting since a flurry of tiny earthquakes began in late September 2004. Scientists initially mistook the quakes as rainwater seeping into the hot interior of the older lava dome. But it soon became clear that magma was on the move, confirmed by the emergence of fire-red lava between the old lava dome and the south crater rim a few weeks after the seismic activity began.

The volcano has continued pumping out lava ever since. Eventually, scientists expect the volcano will rebuild its conical peak that was obliterated in the May 18, 1980 eruption that left 57 people dead. The current growth of the new lava dome has been accompanied by low seismicity rates, low emissions of steam and volcanic gases and minor production of ash, the USGS said. "Given the way things are going now, there's no hint of any sort of catastrophic eruptions," USGS geologist Tom Pierson said. "At any time, however, things can change." Scientists flew a helicopter into the crater late last week to adjust equipment and take photographs that will likely be used to determine just how much the new lava dome has grown the last several months.

The Johnston Ridge Observatory, which closes down every winter, is the closest observatory to the 8,364-foot peak. It is named after David A. Johnston, a volcanologist killed in the 1980 eruption. It sits about five miles north of the mountain and offers the closest views of the volcano's horseshoe-shaped crater.