Deep clues. Magma extruding into the crater of Mount St Helens bears vertical striations that may bear witness to the source of low-frequency earthquakes.

Volcanoes: Shaken, but Not Stirred

Volcanoes often announce their intentions through low-frequency earthquakes. Now researchers have a new idea how this happens, and the findings may shed light on why some eruptions are much more violent than others.

Volcanic eruptions range from mild to extreme, depending on the kind of molten rock that's underground. When the lava is thick and viscous, it usually bulges out as a harmless dome. But runnier lava that's full of gas can explode like a fiery champagne bottle. Sometimes these devastating eruptions are presaged by low-frequency temblors. The reason, according to a standard geophysical model--developed by geophysicist Bernard Chouet of the United States Geological Survey (USGS) in Menlo Park, California--is that gas in the magma flows in and out of cracks in the surrounding rock, creating low-frequency seismic waves.

That may not be all that's going on, however. Based on work at the Montserrat Volcano in the West Indies and other volcanoes, a new model suggests that the quakes could occur when slabs of highly viscous magma with low gas content stick momentarily and then slip past one another, producing the vibrations. Steve Malone, a geophysicist at the University of Washington, Seattle, and colleagues believe that new measurements and observations over the past year greatly strengthen the case for this new model. When the team observed magma emerging from the Mount St. Helens' crater in late October 2004, there was no evidence of gas. The magma was quite viscous, Malone says, and the emerging stuff bore a clue: striations along the edges, as though chunks had slid past one another on their way to the surface.

Malone, who will describe his work later this month at a meeting of the Geological Society of America, stresses that the new idea does not replace Chouet's model. But it does complicate predictions of eruption violence, he says, because seismologists can no longer take low-frequency earthquakes as a sure sign of high gas content, and thus higher risk. The viscous magma that Malone's model predicts is likely to ooze slowly from the surface, producing a mild dome-building eruption instead.

"Mt. St. Helens certainly makes a stronger case [for the magma sliding model] that seismologists are going to have to confront," says Seth Moran, a geophysicist with the USGS's David A. Johnston Cascades Volcano Observatory in Vancouver, Washington.

Related sites
Cascade Volcano Observatory
USGS Mount St Helens news page
Photo gallery of Mt St Helens from the past year
Smithsonian Institution/USGS Weekly Volcanic Activity Report (worldwide)