Sometime in late July or early August, scientists will set off a series of underground explosions across a broad area of Southwest Washington. Each of the 24 blasts — separated from each other by about two minutes — will involve up to a ton of TNT placed in holes drilled 200 feet deep.
On the surface, onlookers won’t hear or feel much more than a thud or a muffled bang. But the shock waves will travel hundreds of miles through the earth, then bounce back and jiggle the needles on a thousand seismometers placed strategically around the region.
It will be part of an unprecedented effort to map, or “image,” the deep plumbing system beneath Mount St. Helens. It’s an attempt to help answer some fundamental but elusive questions about one of the world’s most explosive peaks. A few of them:
• Just how does molten rock — magma — rise into the volcano from a melt zone 50 miles below? How long does it take? How many places does it get stored in along the way?
• Why is Mount St. Helens located where it is — farther west than most other Cascade Range volcanoes?
• Why have Oregon’s volcanoes historically produced more lava during eruptions than those in Washington?
• Why is Mount St. Helens so much more violent than, say, Mount Adams, a scant 40 miles to the east? And why has it periodically been more docile — like Hawaiian volcanoes, which send glowing rivers of fluid rock pouring down their sides but don’t explode catastrophically, as Mount St. Helens at 8:32 a.m. 34 years ago today?
”A basic scientific question is, ‘How does magma get from where it is created to the surface?’ An even more fundamental question is, ‘Why do volcanoes form and why is Mount St. Helens where it is?’ ” said Seth Moran, a U.S. Geological Survey seismologist based in Vancouver.
The summer study should help improve long- and short-range forecasting of the volcano because it will give geologists what x-rays and MRIs give doctors: A look at something they can’t see from the outside.
“This will give us a better model of Mount St. Helens. That will be very helpful interpreting monitoring data. It will help us reduce some of the uncertainty” in eruption forecasts, said Moran, a principal investigator on the study.
‘Imaging’ the inner earth
The four-year study, funded by a $2.7 million National Science Foundation grant, is known by an acronym that sounds like the command of a dog-sled racer: iMUSH, which stands for “Imaging Magma Under Mount St. Helens.”
It’s an attempt to plot how molten rock from the earth’s mantle rises into the volcano and where it is stored or pooled before erupting on the surface.
Volcanoes, in a sense, are much like icebergs. Much of what they are is not visible because they’re rooted throughout the entire depth of the earth’s crust. Scientists have pieced together images of the top three or four miles immediately below Mount St. Helens, but the nature of the 40 to 45 miles below is murky, at best, Moran said.
Researchers will use eight separate geological investigatory techniques to create an image of this unknown territory. They include:
• Measuring changes in the earth’s naturally occurring electrical and magnetic fields at 150 locations around the volcano. The technique, used to search for oil and gas, is premised on the fact that electricity flows more freely through hot, liquid rock than solid rock. By tracking how currents move through the ground, researchers hope to make three-dimensional maps of the earth as deep as 200 miles below the surface — well into the molten mantle of the earth. The work started last week and likely will take into next year.
• Detonating dynamite in 24 holes bored around the volcano as far away as the Columbia River Gorge and Upper Cowlitz Valley. Researchers will measure the shock waves using 1,000 seismometers. Seismic waves slow down when they pass through molten rock, so mapping those spots can help pinpoint reservoirs or pockets of magma. The array of explosive charges — in a set pattern on two axes and concentric circles — and the placement of the seismometers is designed to image the earth’s crust right down to its boundary with the mantle — some 50 miles down.
Deploying so many seismometers is a major logistical challenge, but it will yield clearer pictures, said Alan Levander, a Rice University professor of seismology who is leading this part of IMUSH. “You can think of our sensors as something like the pixels making up a digital photograph. The greater the number of pixels, the higher the resolution of the image,” he said.
iMUSH also will involve intensively monitoring naturally occurring earthquakes using 70 seismometers over two years. But most earthquakes at the volcano are shallow, and distant earthquakes aren’t much use charting the deep crust beneath the mountain, Levander said. Setting off explosions to create, in effect, man-made earthquakes will let researchers look more clearly into the deep earth, Levander said.
• Analyzing rocks erupted from the volcano using the latest chemical and laboratory techniques, which can reveal the temperatures, pressures and other conditions under which the rocks formed.
“The rocks will tell you were they were stored in the magmatic system, where they hung out for a while” before erupting, Moran said.
Adam Schultz, a professor of geophysics at Oregon State University, is leading the magnetic and electrical field monitoring part of iMUSH. The work has special meaning for him.
He’s returning to work at Mount St. Helens for the first time since he was a student at the University of Washington in the late 1970s and 1980s. He was in the UW geophysics lab when the first earthquake rumbled through the mountain on March 20, 1980, signalling that the mountain was awakening after more than a century’s nap.
“We went out the next day and instrumented the whole volcano,” Schultz said Thursday. “I have a deep interest in Mount St. Helens.”
Later that spring, Schultz lost his car keys at Forsyth Glacier, where the volcano was bulging 5 feet a day. USGS geologist David Johnston found the keys in a crevasse two days before he became one of 57 people who died when the volcano blew apart on May 18, 1980, Schultz said.
Schultz is particularly excited about evaluating a report published five years ago that suggested Mount St. Helens and Mount Adams may share a giant pool of molten rock 10 or so miles deep. That paper was based on magnetic and electrical conductivity tests — the kind he’s doing.
The report, done by New Zealand researchers, has been hotly debated. Some scientists suggest that the tests mapped ancient sea sediments, which also are effective electrical conductors.
“We’re hoping to resolve the controversy,” Schultz said.
Scientists also will hunt for answers to a newly posed question. Earthquake studies suggest that a fresh batch of molten rock is collecting in an area 6 or 7 miles below the surface of the volcano. It’s an apparent “pool” of magma, Moran said, but scientists have no idea how big it is — or how much molten rock must ooze into it from below before it erupts. In layman’s terms, they don’t know how much air is needed before popping the balloon.
“We don’t even know how many balloons are down there,” said Moran, continuing the metaphor. “There is good reason to believe that there may be multiple pockets of magma at different depths.”
This could explain why Mount St. Helens, which is known for erupting thick, sticky, explosive lava, sometimes erupts more fluid lava, he said. iMUSH may help shed light on the question.
iMUSH will be unprecedented in the history of U.S. volcanic studies, though similar studies have been undertaken at Italy’s Etna, Stromboli and Vesuvius volcanoes, Moran said. One reason Mount St. Helens was chosen for the project, he said, is that it already is much better understood than most other volcanoes, a result of its eruptive activity over the last 34 years.
The power of the study is that so many disciplines are involved, reinforcing one another and helping cover each discipline’s limitations.
“Each of the geophysical measurements tells us something different,” says Levander, the Rice University seismologist.
“We no longer work in isolation. Comparing and interpreting (findings) is a much more powerful method of operating,” Schultz said, adding that iMUSH also is benefitting from new instrumentation developed with economic stimulus funding.
“We now have an incredible capacity to model and image the earth,” he said.
Ideally, iMUSH eventually will contribute toward making more timely, accurate and longer-range eruption forecasts, but that will come in time and will take years beyond the four-year study.
“We’ll be disappointed if there is not follow-up in years and decades to come,” Moran said.
He expects the first papers to appear within a couple years, and perhaps within five years some consensus will emerge about the volcano’s deeper workings.
If that’s the case, Moran said, “the next time Mount St. Helens gives us some shaking and fooling around, we’ll have a better model to work with and guide us about what is going on.”