New Estimate Finds More Magma Under Yellowstone Supervolcano

Volcanologists identify magma deposits by tracking earthquakes. Seismic waves pass through the bowels of the Earth before being detected by surface seismometers. They move slower through hot, partially molten rock, and scientists use their travel times to interpret how molten parts of the subsurface are.

But this traditional seismic imaging technique is imperfect. Seismic waves sometimes bend around molten pockets. This method also assumes that seismic waves travel, in a simplified way, from the earthquake directly to the seismometer; in reality, seismic waves emanate in all directions and critical information about the underbelly of the Earth is lost.

For the new study, the authors turned to a 20-year-long recording of Yellowstone’s background seismic noise, generated by distant ocean waves, wind and human activity, to zero in on the volcano’s overlooked melt. They got rid of traditional seismic simplifications and used supercomputers to represent the travel of seismic waves more accurately.

The team found that the seismic waves slowed down when they traveled 2 to 5 miles down, which corresponds to the upper segment of where Yellowstone volcano’s shallower magma reservoir is thought to be. This suggests that up to 20 percent of all this deposit is molten.

Fortunately, this is nothing to lose sleep over. A general rule of thumb is that reservoirs can’t erupt without being 35 to 50 percent melted, when things are “sort of like crystal soup,” said Ross Maguire, a seismologist at the University of Illinois Urbana-Champaign and study author. That value, which is frequently debated by volcanologists, is likely to vary between volcanoes. Regardless, Yellowstone’s 20 percent is “still well below that critical threshold,” Dr. Maguire said.

For those hoping to unlock the secrets of other volcanoes, this study confirms that this relatively new technique “is a very good way to do it,” said Diana Roman, a geophysicist at the Carnegie Institution for Science in Washington, DC, who was not involved with the study.

“It’s a bit like putting a new lens on an old camera,” said Dr. Poland. “It’s the same camera, but now you have a finer resolution. You see more clearly.”