Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Michael Poland, geophysicist with the U.S. Geological Survey and Scientist-in-Charge of the Yellowstone Volcano Observatory.

A common question asked of YVO scientists is “when will Yellowstone erupt?”  And the usual reply is that “an eruption of any sort, either an explosion or a lava flow, is very unlikely in our lifetimes.”  This is a true statement—seismic imaging has shown that the magma chamber beneath Yellowstone is mostly solid, we know that lava flow eruptions occur in clusters separated by tens to hundreds of thousands of years.

A logical, and frequent, follow-up question, then, is “if there is little concern of a volcanic eruption, why monitor Yellowstone at all?”

There are two very compelling answers.

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First, volcanic eruptions are not the only geologic hazards in Yellowstone.  Far more likely on human timescales are damaging earthquakes and hydrothermal explosions.

Strong earthquakes, like the M7.3 Hebgen Lake earthquake in 1959, can happen once or twice per century in the region.  The Hebgen Lake event caused a landslide that swept through a campground, killing over two dozen people, and also dammed the Madison River, creating a new lake, known as Earthquake Lake, that still exists today.  The event was not related to Yellowstone’s magmatic system, but was caused by one of the many faults related to the Basin and Range extensional province, which stretches from eastern California into western Wyoming. 

Similar strong earthquakes will happen in the future.  Tracking seismicity and ground deformation in the region can help geologists better understand which faults are most active, leading to more accurate assessments of seismic hazard.  And when earthquakes do occur, the data collected by monitoring stations throughout the region can provide insights into the processes that drive the seismicity—information that is invaluable to investigating how earthquakes work so that we can be better prepared for future events.

Hydrothermal explosions are an underappreciated hazard in Yellowstone National Park.  These explosions do not involve magma, but rather occur when hot water beneath the surface suddenly flashes to steam.  The volume expansion that accompanies the change from liquid water to gaseous steam can result in an explosion if it occurs in a confined space.

Some of the largest hydrothermal explosion craters known on Earth occur in Yellowstone National Park—for example, Mary Bay, which formed about 13,000 years ago.  Explosions that create craters hundreds of meters (yards) across probably occur once every few centuries or millennia.  Smaller hydrothermal explosions, leaving craters only a few meters (~10 feet) in size, occur more frequently, perhaps every few years, although usually in areas where they are not observed by humans.  A good example of a small explosion that did occur in a heavily visited area and that was witnessed by people was the 1989 event at Porkchop Geyser.  It is not clear if hydrothermal explosions have precursors that could be used to forecast their occurrence, and monitoring data can help to address this lack of knowledge.  In fact, better tracking of seismicity, ground deformation, gas emissions, and other parameters in thermal areas is a major focus of the Yellowstone monitoring plan for 2022–2032.

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A second reason for monitoring Yellowstone is that the region is an exceptional natural laboratory for volcanology, and lessons learned there can be applied elsewhere.  Caldera systems like Yellowstone occur throughout the world.  Taupō, in New Zealand, for instance, experienced a major eruption just 26,500 years ago, and that explosion was larger than the one that formed Yellowstone Caldera!

Fortunately, large caldera-forming eruptions are rare, occurring somewhere on Earth perhaps once or twice every 100,000 years.  But those volcanic systems also experience small eruptions that can be hazardous to humans.  A lava flow eruption occurred at Campi Flegrei caldera, in Italy, in 1538, and the region is famous for high rates of ground deformation—orders of magnitude greater than that occurring in Yellowstone.  Rabaul caldera, in Papua New Guinea, has been the site of several damaging explosive eruptions since 1994.  By studying Yellowstone, we can better understand the sort of activity that might be occurring beneath Campi Flegrei, Taupō, Rabaul, and similar systems.  Likewise, studies of other caldera systems can help us to better understand Yellowstone!

Monitoring at Yellowstone is about more than just tracking potential volcanism—activity that is fortunately rare in the area.  The data from seismic and deformation measurements, gas emissions, water chemistry, and other monitoring can also help us understand other more common geologic hazards in the region, while also teaching us more about how Yellowstone works.  And the lessons we learn aren’t restricted to Yellowstone, but rather have implications for understanding, forecasting, and preparing for volcanic eruptions across the globe.