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| Mount St. Helens growls mysteriously |
As John Pallister circled above Mount St. Helens on Sunday afternoon, a sharply defined line of steam caught his attention.
"It was interesting enough to take some pictures," said Pallister, a U.S. Geological Survey scientist and private pilot.
He didn't know it, but his USGS colleagues back in Vancouver had already noticed a 2.9-magnitude earthquake followed by a small but unusually long tremor at the steadily erupting volcano. The tremor, in fact, continued for almost an hour and a half, punctuated by a 2.7-magnitude quake.
Such tremors typically signal that magma or gases are flowing underground like water in a pipe. The last tremor of note - a 55-minute stemwinder big enough to register on seismometers from Bend, Ore., to Bellingham on Oct. 2, 2004 - prompted the Forest Service to hastily evacuate the Johnston Ridge Observatory five miles north of the crater's mouth.
Sunday's tremor wasn't nearly as powerful.
Cynthia Gardner, scientist in charge of the USGS Cascades Volcano Observatory in Vancouver, said scientists have nonetheless decided to put off any scientific excursions to the crater itself.
"We're going to back off of that for the next couple of days," she said Tuesday.
Sunday's tremor was accompanied by a period of minor ground inflation and deflation near the ever-expanding lava dome. Remote tiltmeters, capable of measuring millimeters or less, registered tiny but significant movements - potentially indicating the volcano was pressurizing for an explosion.
Two days of relatively placid seismic activity have scientists less concerned about an explosive steam and ash blast, the last of which occurred on March 8, 2005.
In his overflight, Pallister noticed steam fuming along a fracture line that runs like a zipper atop the actively erupting portion of the volcano's lava dome. Because the volcano is already freely erupting lava to the surface, there isn't as much risk of the volcano's pressurizing as when it was bottled up in the fall of 2004.
Scientists aren't sure what to make of Sunday's rumblings.
"The settling of the growing lava dome might have caused some fracturing and might have changed the subsurface openings so that water was either being squeezed out of openings or opening new areas," Gardner said.
The mountain has been erupting steadily since the fall of 2004, totaling 123 million cubic yards of material as of the last precise measurement from images taken in July. Although the rate of extrusion has slowed considerably since the eruption started, Sunday's activity underscores the volcano's inherent unpredictability.
"Rumors of an early end of this eruption are once again shown not to be the case," Pallister said. "It's still got some surprises." |
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Mount St. Helens growls mysteriously
2008-01-17 09:19:17
Article by ERIK ROBINSON
Source: 
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