What differentiates an active volcano from a dormant one?
While visible lava at the surface is an obvious indicator of activity, the long-standing belief is that active volcanoes have large magma bodies that are expelled during eruptions and then dissipate over time as the volcanoes become dormant. However, new research led by the Department of Earth and Atmospheric Sciences challenges that assumption.
Researchers used seismic waves to identify magma chambers beneath the surface of six volcanoes of various sizes and dormancy within the Cascade Range, which includes half of the U.S. volcanoes designated by the U.S. Geological Survey as "very high threat." The team found that all of the volcanoes, including dormant ones, have persistent and large magma bodies.
The study, led by postdoctoral researcher Guanning Pang, was published Jan. 23 in Nature Geoscience. Geoffrey Abers, the William and Katherine Snee Professor in Geological Sciences, co-authored the study in collaboration with Seth Moran and Weston Thelen at the U.S. Geological Survey's Cascades Volcano Observatory.
Their results are surprising given that some of these volcanoes, such as the Crater Lake volcano in Oregon, have not been active in millennia.
"Regardless of eruption frequency, we see large magma bodies beneath many volcanoes," Pang said. "It appears that these magma bodies exist beneath volcanoes over their whole lifetime, not just during an active state."
The fact that more volcanoes have sustained magma bodies is an important consideration for how researchers may monitor and predict future volcanic activity. The U.S. Geological Survey has been expanding and upgrading its volcanic monitoring networks in the Cascade Range and elsewhere as part of the National Volcano Early Warning System, with the aim of detecting signals of an impending eruption as early as possible.
"We used to think that if we found a large amount of magma, that meant increased likelihood of eruption," Pang said, "but now we are shifting perception that this is the baseline situation."
The results suggest that an eruption does not completely drain a magma chamber, instead, it lets off some of the excess volume and pressure. The chamber can be slowly expanded and refilled over time due to gradual melting of the crust.
"If we had a better general understanding of where magma was, we could do a much better job of targeting and optimizing monitoring," Abers said, noting that there are a "great many volcanoes that are sparsely monitored or have not been subject to intensive study."
Another important aspect of the research is the tools that were used to collect the data - specifically, how few of them were needed. The team utilized very small networks of seismometers around each volcano that they studied and leveraged recent enhancements to seismic broadband stations near Cascade Range volcanoes. By applying a technique that uses scattered wavefields from distant earthquakes, the researchers could create detailed subsurface images.
"Previously, imaging methods required deploying tens to hundreds of seismometers around a volcano, making for a challenging undertaking," Abers said.
Plans are already in the works to expand the magma monitoring system and see if the Cascade discovery translates to other locations, including Alaska.
"Our method is very portable and can be used at many if not most other volcanoes around the world, with just a small number of modern seismographic stations," Abers said. "We think it can help systemize volcano studies, and provide a key piece of global frameworks for volcanic hazard assessment."
Melia Matthews is a freelance writer for Cornell Engineering.
