Mount St. Helen's Has Something In Her Throat
By modelling the volcano, Richard Iverson of the Cascades Volcano Observatory in Vancouver, Washington, and his colleagues conclude that the most likely explanation for these unusually regular drumbeat earthquakes is the jerky movements of a giant rock plug, wedged in the throat of the volcano. "Magma pressure in the conduit below the plug pushes it upward," says team member Daniel Dzurisin. As the plug moves, at the rate of about 3 to 5 metres per day, it triggers tremors in the area surrounding the volcano (Nature, vol 444, p 439).
Right now there is no sign that Mount St Helens is about to clear its throat in the spectacular way it did in 1980, but changes in the drumbeat could signal the build-up to some kind of eruption. "We're making progress, but we don't have the answers yet," says Dzurisin.
From issue 2579 of New Scientist magazine, 25 November 2006, page 18