Lead Author

In late 2004, Mount St Helens in Washington, the most active of the US Pacific Northwest volcanoes, started a steady eruption that continues today. Earthquakes occur almost constantly as a new dome is pushed into place by rock and magma below. Richard Iverson and colleagues at the US Geological Survey’s Cascades Volcano Observatory in Vancouver, Washington, and at the University of Washington in Seattle, have spent the past two years trying to uncover the cause. The data led Iverson to propose the unusually simple model of the volcano's underground mechanics found on page 439.

What has been happening at Mount St Helens in the past two years?

Before 2004, nobody thought that eruptions at Mount St Helens would happen again in our lifetime. Since then, there have been more than one million earthquakes accompanying the eruption. That's not unusual, but what's been extraordinary about these earthquakes is their uniquely periodic nature — so much so we dubbed them ‘drum beats'. For months at a time, earthquakes were happening once or twice a minute. Very few things in solid earth sciences are so obviously repetitive — that's what led us to the idea behind this model. The eruption has been a great opportunity to do research because it has been so well behaved.

What's novel about the model detailed here?

The model assumes magma comes in from depth at a steady rate, solidifying and pushing a solid body upwards. The earthquakes result from a lurching motion, called stick-slip behaviour, which accompanies incremental pressure build-up and release as solid rock is pushed out of the ground. Our model demonstrates that something as complicated as a volcano may exhibit surprisingly simple mechanical behaviour.

What led you to a less complex model?

The single most exciting thing about the repetitive earthquakes is that they call for a very simple explanation based on a mechanical process. Whatever is generating the seismicity can happen over and over again without destroying itself. Our data suggest that two solid surfaces are persistently grinding against one another, which can continue if the supply of new rock continues.

How long can Mount St Helens keep erupting?

Most of us expected it to have shut off by now. There is little evidence of systematic changes at depth, implying the magma is being resupplied at depth. Understanding what would shut that off forces us to consider progressively deeper processes, leading us to subduction and melting of tectonic plates.