Tiny Earthquakes

Scientists have long known that the volcano underneath Yellowstone is alive and well. 

Until a recent study, however, they did not know that the top of the area’s magma reservoir sits about two miles below Yellowstone’s surface, where rock makes space for magma to bubble and flow, creating a magma cap. 

“For decades, we’ve known there’s magma beneath Yellowstone, but the exact depth and structure of its upper boundary has been a big question,” said Brandon Schmandt, a co-author of the study, in a statement. “What we’ve found is that this reservoir hasn’t shut down – it’s been sitting there for a couple of million years, but it’s still dynamic.” 

Yellowstone originated from a volcanic eruption about 631,000 years ago, which created the Yellowstone caldera, a volcanic depression, explained the National Park Service. Since then, there have been about 80 smaller eruptions.  

It was just such a smaller eruption, consisting of “tiny earthquakes” that researchers tried to replicate to see what was under the surface.  

They worked during the night in the northeast Yellowstone caldera in a spot parallel to one of the park’s rivers. They generated the “tiny earthquakes” using a 53,000-pound truck that created seismic vibrations. These waves bounced off many layers below Yellowstone’s surface and were picked up by sensors. Geoscientists then collected data and used seismic imaging and computer models to complete their research. 

They say there is no danger of an eruption anytime soon as the magma cap serves as a seal over the pressure and heat below, while steady gas emissions prevent buildup.  

“Although we detected a volatile-rich layer, its bubble and melt contents are below the levels typically associated with imminent eruption,” Schmandt said. “Instead, it looks like the system is efficiently venting gas through cracks and channels between mineral crystals, which makes sense to me given Yellowstone’s abundant hydrothermal features emitting magmatic gases.” 

Schmandt likened the system to “steady breathing” with bubbles rising and releasing through the porous rock – a natural pressure-release valve that lowers eruption risk. 

When studying volcanoes, it is important to determine if the bubbles are accumulating, if the gas is easily escaping, and what the magma reservoir looks like to assess how likely an eruption is. 

A different study earlier in the year found that the magma will remain active, even if it is unlikely for it to erupt again, given its location and segregation. The new study corroborates this finding. 

By identifying this sharp, volatile-rich cap beneath Yellowstone, Schmandt says the team has established a new benchmark for monitoring the volcano’s activity. Future research could attempt to detect any shifts in melt content or gas accumulation that may serve as early warning signs of unrest. 

Beyond Yellowstone, the study offers broader insights into onshore subsurface imaging with potential applications not only for volcano monitoring but also for carbon storage, energy exploration, and hazard assessment. 

“Being able to image what’s happening underground is important for everything from geothermal energy to storing carbon dioxide,” Schmandt said. “This work shows that with creativity and perseverance, we can see through complicated data and reveal what’s happening beneath our feet.”