On May 18, 1980, Mount St. Helens in southwestern Washington state explosively erupted after several months of increasing activity. Much of southern Washington was impacted by pyroclastic flows — superheated clouds of volcanic gas and debris moving hundreds of miles an hour — destroying many trees, damming water sources, and causing ash fall across much of the eastern side of the state. Fifty seven people were killed from the eruption itself, hundreds of houses and infrastructure were destroyed, and the landscape was left wasted, making this the most costly volcanic eruption in the continental US so far. Scientists believe that Mount St. Helens will erupt again, and it will likely produce an event of similar or greater scale to the 1980 eruption someday. Today, Mount St. Helens remains the most recent reminder that active volcanoes exist in the US, capable of significantly disrupting life as we know it. With the right type of instruments and expanded scale of monitoring, we can better understand the warning signs. Since the eruption, NSF-funded geophysical infrastructure has played a role in improving our watch over volcanic activity at Mount St. Helens.
The eruption of 1980
Prior to 1980, GPS geodetic monitoring on Mount St. Helens was sparse. Most observations were done by boots-on-the-ground prospecting or aerial surveying. The last eruptions were in the mid-1800s, when the state was less developed and the extent of volcanic hazards were not yet known. But geologists knew the area was prone to earthquakes due to the deep Cascadia Subduction Zone in the Pacific Northwest, where the Juan de Fuca Plate is swallowed beneath the North America Plate. This process forms the Cascade Volcanic Arc, which spans from the Alaskan peninsula to Northern California. The melting of the Juan de Fuca Plate at depth feeds volcanoes at the surface in the Arc when the magma buoys to the surface. For some volcanoes in Cascadia, it is only ever a question of “when” and not “if” an eruption will happen.
In 1980, signs of an oncoming eruption were near-obvious. Small earthquake swarms triggered surrounding seismometers near the mountain in mid-March, and scientists wondered if the volcano was indeed starting to wake up after a century-long slumber. But without a denser network of sensors in the area, further study would be hard. A chance delivery of four seismometers soon after quakes began allowed scientists to quickly install more monitoring equipment. While network density was boosted, these instruments only managed to record 1% of the timeframe from installation to eruption, potentially meaning that many precursors to the final eruption were not recorded adequately.
Over the next several weeks, clusters of increasingly intense earthquakes and successive steam explosions at the summit fueled both public and scientific excitement. Ramped-up aerial observation of the summit crater commenced with occasional deployment of field volcanologists to test magmatic gas sources, and the surrounding area around Mount St. Helens became a bustling tourist attraction for people seeking a rare glimpse of an active stratovolcano.
Perhaps the most obvious sign of an impending eruption was the development of a large bulge, or cryptodome, on the north flank of Mount St. Helens in April, where magma from below was piping into the upper part of the volcano. Over the next month, the bulge grew at a rate of 5-6 ft per day until it reached an estimated 400 ft of displacement by the middle of May.
As the surrounding community and seismologists awoke on the Sunday morning of May 18, there was nothing obviously different about conditions on the mountain, based on seismic readouts and visual tracking of the cryptodome. Small magnitude cluster earthquakes still swarmed seismographs nearby. There were no new steam explosions. The cryptodome appeared unchanged. But then, at 8:32 am PT, the overlying rock on the bulge suddenly collapsed in a large landslide, releasing overhead pressure on the magma below and resulting in a catastrophic explosion from the flank of the mountain. The 1980 eruption made clear that more intensive monitoring was needed and would be beneficial for public safety.
The Plate Boundary Observatory becomes the eyes and ears of volcano monitoring
The 1980s were a formative period for geodesy-assisted volcanology. Global Positioning System (GPS) instruments were just rolling out, and geologists had begun to hypothesize that strategically placed stations could provide a unique way of tracking plate motion and other tectonic activity over time.
However, these instruments were prohibitively expensive, prompting the formation of UNAVCO to provide a community pool of GPS instruments. Over the next two decades, advancements in GPS geodesy and continuously operating GPS stations led to momentum gained across the geodetic community for a unified, widespread network GPS stations, culminating in the creation of the Plate Boundary Observatory. Alongside the establishment of a transportable fleet of seismometers, called the USArray, both the seismic and geodetic communities could study the West Coast plate boundary in detail. Both PBO and USArray were operated under the original EarthScope project.
Starting with station siting and permitting in 2003, the PBO project constructed nearly 900 permanent GPS stations by 2008, establishing a Western Hemisphere network of geodetic instruments tracking the crustal deformation caused by the collision of the Pacific/Juan de Fuca Plates and North America Plate. Much of this motion occurs incrementally every year, but is not typically felt by people in real-time — unless an earthquake strikes. Since GPS instruments track small changes in position at high precision, they could be leveraged as long-term continuous trackers of the underlying plate movement. But GPS equipment, in tandem with other instruments like seismometers, strainmeters, and tiltmeters, can also be stationed at volcanoes to track how ground deformation is fueled by volcanic activity.
By establishing a well-maintained, dense network of GPS stations co-located with seismometers and other geophysical tools around geologic areas of interest — like the Cascadia volcanoes — scientists can track eruption precursors that manifest as ground deformation, contributing to improved eruption forecasting that mitigates risk for surrounding populations.
A new eruption period begins in 2004
As it turned out, Mount St. Helens wasn’t content to sleep for another century. In September 2004, a series of microearthquakes were detected at the volcano, swiftly kicking scientists into gear. Recon for PBO stations had just wrapped up in August of that year, but the stations weren’t planned to be installed until the following summer.Based on the community guidance to monitor this renewed activity at Mount St. Helens, PBO staff, along with USGS personnel, worked fast to install these stations ahead of schedule. Engineers battled winter weather conditions to install instrumentation, additionally facing increasingly hazardous volcanic activity. By the end of October, seven new permanently-mounted GPS stations taking continuous data complemented the only existing station at Johnston Ridge. As Mount St. Helens entered its next four-year period of eruptive activity, this new network helped study and understand its behavior in greater detail.
It’s true that some future eruption of this volcano could be even larger than the 1980 eruption, but it’s also true that our understanding of the volcano’s behavior will be better and our methods for monitoring and warning will be more robust thanks to the instruments in place.
Since the end of the EarthScope project in 2018, PBO stations have been incorporated into the Network of the Americas (NOTA) and continue to provide great insight into the ground motion of Mount St. Helens by contributing to the USGS Cascades Volcano Observatory. More monitoring means a better window into what volcanoes are doing below the surface.