Oregon Public Broadcasting

Scientists monitoring Mount St. Helens are looking to put a new instrument array on the volcano.

They want to get to the bottom, literally, of whether or not the mountain is recharging for another blow. Correspondent Tom Banse reports from the scene.

The most recent dome building eruption at Mount St. Helens ended in early 2008.

USGS geologist Dan Dzurisin at Mount St. Helens

Ever since, scientists and just regular folks have wondered how soundly the volcano sleeps.

A key to the answer is figuring out whether the magma reservoir underneath the mountain is refilling.

Dan Dzurisin: “We'd really like to know. It's a challenge to know whether magma is accumulating underneath the volcano.”

Dan Dzurisin is a veteran geologist with the Cascades Volcano Observatory.

Dan Dzurisin: “We have some tools that we use to try to understand that. We look at earthquakes. We now know that earthquakes are associated with magma moving, but sometimes magma can accumulate and not make earthquakes.”

Molten rock also gives off volcanic gases when it rises. But this indicator too has been unreliable at Mount St. Helens.
Then there's the monitors measuring the warping of the ground.

Dan Dzurisin: “Volcanoes swell up when magma accumulates beneath them. We now know, well yes, But ... magma is compressible. It's got a lot of gas in it. You can stuff it into a strong container. You can stuff more of it in without stretching the container very much – at least in theory. It's possible we could be accumulating magma and not deforming the surface.”

So as a result, the U.S. Geological Survey is looking for another tool to divine what's really happening 3 to 5 miles down below. Dzurisin says gravity meters might do the trick. They make very, very precise measurements of earth's gravity.

Dan Dzurisin: “Now you might think earth's gravity is just earth's gravity; it's always the same. It's not. If you put more material underneath your feet, the gravitational pull is actually a little bit more.”

The USGS has started a theoretical study to predict how much the gravity should change if molten rock accumulates under the volcano when nothing else is going on. If the results are encouraging, local scientists hope to borrow gravity meters to do an initial survey yet this year. Dzurisin says additional federal money would be needed to establish a permanent gravity network.

Online:

USGS Cascades Volcano Observatory