Before May 1980, when you looked northeast from Portland, OR, on a clear day, you would see a picturesque, conical peak towering over the surrounding landscape. With its summit rising more than 5,000 feet above its base and more that 9,600 feet above the nearby Pacific Ocean, Mount St. Helens stood out for its immaculate symmetry. Now, Mount St. Helens does not attract attention for its prominent edifice but rather for its volcanic activity within the last three and a half decades. The volcano erupted cataclysmically on May 18, 1980, discharging ~0.67 cubic miles of material (the approximate volume of Seattle’s Lake Washington). The northern flank of the mountain collapsed, leaving a gigantic, horseshoe-shaped crater. In 2004 the volcano reawoke, extruding thick lava flows onto the crater floor and creating a small dome.
Recently (since March 18, 2016), over 230 tiny earthquakes have popped off beneath Mount St. Helens. Though this still-ongoing earthquake swarm does not mean that the volcano is about to blow its top (again), it does indicate that magma is moving through the volcano’s inner plumbing system. The magma chamber, located about five miles beneath the surface, is slowly refilling. But where is the new magma coming from?
The magma, or liquid rock below the Earth’s surface (as opposed to lava, which is liquid rock at the surface), is formed deep underground. When two tectonic plate push together, like is occurring in the Pacific Northwest, enormous amounts of pressure and heat liquefy rock into magma. Because the magma is lighter, or less dense, than the rock above it, it rises. This process is similar to how oil will float above water. Eventually, the magma finds it’s home somewhere near the Earth’s surface where the rock density is similar to the magma’s density.
One place that magma is stored beneath Mount St. Helens is the shallow chamber beneath the mountain’s crater. But, the magma storage system is likely much more complex than a single chamber. Magma can snake intricately towards the surface and be housed at multiple locations. What does the inside of Mount St. Helens really look like? Without the answer to this question, scientists have no way of quantifying the amount of magma in the ground, and consequently the volume of material that could eventually be erupted.
The iMUSH (Imaging Magma Under St. Helens) project is a four-year project with the goal of mapping the magma’s path from its source, deep in the ground, to where it erupts in the crater of Mount St. Helens. For one portion of the project, Carl Ulberg, a PhD student at the University of Washington, is using earthquakes to create an image of the deep structure beneath the volcano. This method, formally called seismic tomography, works because the energy (a.k.a. seismic) waves generated by earthquakes travels through different materials at different speeds.
In particular, seismic waves travel slower through liquids than solid rock, like how we humans can move faster through open air than water. In the context of Mount St. Helens, seismic waves travel more slowly through the magma chamber (which is filled with liquid rock), than the surrounding rock. By recording the seismic waves from small earthquakes on numerous receivers (seismometers), we can sample waves that travel various paths through the volcano’s interior (e.g. see below cartoon). The variation in wave speeds along those paths is then used to create an image of the St. Helens plumbing system.
During the summer of 2014, a group of scientists (including myself) from the United States Geological Survey, the University of Washington, and Columbia University installed 70 seismometers in a 40-mile radius surrounding the volcano. You can read about details about the install in my previous blogs post. The instruments recorded earthquakes that will be used to image the magma chamber for two years and were removed this August. With these new data, Ulberg hopes to not only map the shallow magma chamber we already know exists, but also to reveal the structure of the deeper portions of the Mount St. Helens system.
“We’ll be able to use the data from the seismometers along with the other iMUSH results to get a better image of the subsurface than we have before, and a greater understanding of how Mount St. Helens works,” says Ulberg.
Stay tuned for both his results and the results from the other portions of the iMUSH project. Updates from last month’s instrument removal, including tales of wasps and washed-out roads from , can be found on the Pacific Seismic Network blog. For more info about the project, you can visit the iMUSH website.
 For more information see: Doughton, Sandi. “It’s not about to blow, but magma is moving under Mount St. Helens.” The Seattle Times 9 May, 2016. Web.