Magma is the explosive lifeblood of volcanoes. The Yellowstone supervolcano, centered in northwestern Wyoming, has plenty stored beneath its geyser-laden surface. But magma is a mixture of both solid and liquid parts, and not all of it can erupt.
To find out just how much geologic goulash can potentially come out of the volcano if it erupts, scientists applied a relatively new technique to a 20-year-old catalog of seismic data. Their study, published Thursday in the journal Science, concludes that there is more molten rock in Yellowstone’s upper magma reservoir than previously thought: 16 percent to 20 percent of it is liquid, compared with older estimates of about 10 percent.
“There’s been a really big magmatic system there for two million years,” said Brandon Schmandt, a geophysicist at the University of New Mexico and an author of the study. “It does not look like it’s going away, that’s for sure.”
However, that doesn’t mean Yellowstone is more hazardous than before, said Michael Poland, the scientist-in-charge at the Yellowstone Volcano Observatory who wasn’t involved with the study.
Although the Yellowstone volcano is a little more capable of erupting than previously believed, the new study corroborates that its shallow reservoir is predominantly solid, making the prospect of any forthcoming eruption extremely small.
The study also isn’t implying that more eruptive fuel has been created in recent years. “We’re not saying the system is changing,” Dr. Schmandt said. “This is about getting a clearer image of what is down there and what has been down there for a while.”
As volcanoes go, Yellowstone is no shrinking violet. Since its cataclysmic emergence 2.1 million years ago, there have been two other colossal eruptions, plenty of moderately violent outbursts and countless lava flows. Its most recent eruption came in the form of syrupy flows 70,000 years ago.
Keen on foreseeing the volcano’s future, scientists have tried to ascertain how much molten rock is close to the surface and where it may be. Magma reservoirs are probably more like labyrinths than tanks of permanently liquid rock: Crystalline boundaries trap the melt — a buoyant, hot liquid. The more melt there is, the more capable a volcano is of erupting.
Previous work at Yellowstone revealed two magma reservoirs: one of gloopy magma 3 miles to 10 miles below the surface, and a far more enormous store of runnier magma 12 miles to 30 miles down. It was thought that roughly 2 percent of the deeper reservoir and 5 percent to 15 percent of the shallower reservoir were made of melt.
Volcanologists identify magma reservoirs by monitoring earthquakes. Seismic waves plunge through Earth’s innards before being detected by surface seismometers. They move more slowly through hot and 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 manner, from the quake directly to the seismometer; in reality, seismic waves emanate in all directions, and critical information about Earth’s underbelly 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, the wind and human activity — to zero in on the volcano’s overlooked melt. They dispelled with traditional seismic simplifications and used supercomputers to represent the voyages of seismic waves more accurately.
The team found that seismic waves slowed to a crawl when traveling 2 miles to 5 miles down — corresponding to the upper segment of where the Yellowstone volcano’s shallower magma reservoir is thought to be. This suggests that up to 20 percent of this entire reservoir is molten.
Fortunately, this is nothing to lose sleep over. A rule of thumb is that reservoirs cannot produce eruptions without being 35 percent to 50 percent molten, when things are “kind of like a crystal soup,” said Ross Maguire, a seismologist at the University of Illinois Urbana-Champaign and an author of the study. That value, which volcanologists frequently debate, 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 unearth the secrets of other volcanoes, this study confirms that this relatively new technique “is a really good way to go,” said Diana Roman, a geophysicist at the Carnegie Institution for Science in Washington, D.C., who wasn’t involved with the study.
“It’s a bit like getting a new lens on an old camera,” Dr. Poland said. “It’s the same camera, but you’ve got finer resolution now. You see with more clarity.”