Blowing the Top Off Mount St. Helens' Secrets
by Jennifer Donovan and Kara Sokol
Volcanoes are notoriously hard to study. All the action takes place deep inside at enormous temperatures. So geophysicists make models, using what they know to develop theories about what they don’t know. But making those models might also mean getting up close and personal with the real thing.
Greg Waite, an assistant professor of geophysics at Michigan Tech, has studied Mount St. Helens, and, using a new seismic model, he challenges conventional wisdom about the underlying mechanism of the earthquakes caused by an erupting volcano. Waite hopes his research will help scientists better assess the hazard of a violent explosion at Mount St. Helens and similar volcanoes.
Volcanoes don’t always erupt suddenly and violently. The current eruption of Mount St. Helens began in October 2004: a passive eruption with thick and sticky lava squeezing slowly out of the ground like toothpaste from a tube.
When a volcano erupts, it can also cause a series of shallow, repetitive earthquakes at intervals so regular that they’ve been called “drumbeat earthquakes.” Scientists believed that these earthquakes were caused by the jerky movements of a solid plug of molten rock traveling up from the volcano’s core, a process known as the stick-slip model.
Modeling of seismic data collected by Waite and colleagues dispute that explanation. “It suggests a source with a net volume change, such as a resonating fluid-filled crack,” Waite says.
The fluid in the crack is most likely steam, derived from the magma and combined with water vaporized by the heat of the molten rock. A continuous supply of heat and fluid keeps the crack pressurized and the “drumbeats” beating, Waite explains.
“Greg collected a fantastic data set and produced a robust and intriguing model for the process responsible for those earthquakes,” says Seth Moran, the principal USGS seismologist monitoring the current eruption. “We are adjusting our understanding of the mechanics underlying the eruption to incorporate his results.”
As for that up-close research, Waite won’t soon forget the day he was flying directly over the volcano when a large chunk of the dome broke off and kicked up a billowing cloud of ash.
“My heart jumped,” he says. “But then I realized it was just a rock fall and not an explosion.”