Mount St. Helens puts on a show as part of 'fin' collapses

By Barbara LaBoe

May 31, 2006

Monday's rockfall was the largest at Mount St. Helens since 2004 -- but officials said only its size was unusual.

A large section of the 300-foot tall lava "fin" or "spine" broke loose Monday morning, generating a short-lived ash plume that reached about 20,000 feet into the air. There was no explosion and the plume itself had relatively little ash in it, said Jon Major, a U.S. Geological Survey geologist in Vancouver.

Ash and rock debris from the north and south side of the fin now litter the inside of the volcano's crater and also took out some USGS earthquake-monitoring equipment. Officials hoped to replace those units Tuesday by flying to the crater by helicopter.

Remote cameras showed the volcano is continuing to thrust out solid blocks of hardened lava.

A 3.1 magnitude earthquake also took place at the same time Monday, but scientists are unsure if the earthquake triggered or contributed to the rockfall, Major said. It wasn't a sharp, sudden earthquake that can definitively tied to the avalanche of rock, so its causes are a bit unclear, he said.

In the past few days the number of earthquakes has increased, but seismic activity still is well within normal levels and far below levels of a year ago, Major said.

It's believed the earthquakes are trigged by friction as the nearly-solid magma thrusts its way through the earth's crust and into the dome, said Tom Pierson, a USGS research geologist/hydrologist. The quakes also are a reminder that the volcano remains in continuous eruption mode after rumbling back to life in the fall of 2004. There have been seven new growths or spines since the eruption began and all had rockfalls as the growths become too tall and gravity yanked them down. Officials expected the latest fin would fracture too, just not quite so spectacularly, Major said.

It was still unclear Tuesday just how much rock fell off the fin, but photo comparisons from May 18 and Monday show a significant portion of the fin is gone. Scientists hope to have a volume estimate in the next few days, Major said.

The earthquake Monday set off the monitoring station's alarm and the Federal Aviation Administration was notified as a matter of routine. The FFA in turn reported that pilots could see an ash plume, which caused the USGS to do a larger notification, Major said.

A thin layer of dust was deposited on the volcano's upper northeast flank after steam and dust rose above the crater rim Monday.

The plume dissipated quickly and did not cause any problems for aircraft.