On March 3, 2015, Chile’s Villarrica volcano launched a mile-high fountain of lava into the inky night sky. In the days leading up to the explosion, the lava lake rose and roiled within the volcano’s crater, bombarding the nearby city of Pucón with a cacophony of over 100 decibels—the intensity of a jackhammer.
But even as the skies shook with the sounds of impending eruption, no one heard a thing.
These mysterious rumblings, produced by Villarrica and many other active volcanoes, fall within the category of infrasound: noises that occur at such a low frequency that they’re imperceptible by human ears. Data on these volcanic “voiceprints” have been collected by researchers for decades. But two recent studies indicate that eavesdropping on this seemingly silent symphony might now help scientists track subtle shifts in a volcano’s internal architecture—including those that foretell an explosion.
With their tube-like interiors and bellowing outputs, volcanoes are not unlike musical instruments. As long as a volcano has a clear opening into the atmosphere—and thus exposed magma—rising gas that’s buffeted against the walls of the volcanic vent can really make some noise. But the quality of the sound emitted into the atmosphere depends on the shape of the path it traverses, including the length of the volcano’s “pipe” and the width of its crater, explains Jeffrey Johnson, a geophysicist and infrasound expert at Boise State University. Trumpet-like volcanoes, for instance, can capitalize on their flared openings to produce strong, resonant blares. If a volcano’s shape isn’t conducive to channeling sound, however, the system is no more effective than straining to hear a solo from a tuba that’s been plugged shut.
“These models can be powerful,” says Dawn Ruth, a volcanologist at Ohio University who was not involved in the new studies. “If we can map the shape of a volcanic conduit, we can hypothesize what an eruption is going to be like… and that’s pretty cool.”
But infrasound monitoring isn’t just about the presence of sound—it’s about how that sound might change over time. Unlike true musical instruments, active volcanoes are constantly resculpting their structure. This in turn modulates the sounds that emerge from their vents, giving researchers an auditory proxy for how the shape of a volcanic crater might be evolving over time. Plenty of that ruckus is audible, but infrasound is a uniquely powerful signal: While earth-shaking seismic waves dissipate quickly, the deep tones of infrasound can really travel, meaning the dynamics of an ongoing eruption can be detected from afar. Johnson and his team usually install sensors a couple of miles away from a crater, but the range of certain eruptions is astounding. Take, for instance, the record-setting 1883 eruption of Krakatoa, which dispatched infrasound reverberations that circled the globe several times.
“There’s this world of invisible energy that humans can’t perceive,” Johnson says. “But with [these sensors], you turn on a certain way of sensing the earth.”
And researchers around the world have been doing exactly that for many decades. But Johnson and a few others are interested in using infrasound-detecting technology to home in on volcanoes haven’t yet blown their tops.
In a study published last year, Johnson and his colleagues monitored infrasound produced by Villarrica in the days before its 2015 eruption to gauge its activity. “For some volcanoes, you can walk over to the edge and peer in to see the lava lake”—though that’s not necessarily advisable, Johnson says. With Villarrica, however, that’s not the case: Even with planes flying overhead, there’s not always an ideal vantage point to observe this volcano’s sweltering innards. That put the onus on infrasound to reveal Villarrica’s inner workings.
As it turned out, though, Villarrica happily spilled its secrets to anyone willing to listen. As lava surged up toward the volcano’s rim, the infrasound notes ringing out from the interior shifted in pitch and duration. Johnson compares the flux of the lava lake to fiddling with the slide of a trombone.
And the predictive power of infrasound has already been put to work. According to a separate 2018 study, an infrasound-based system that monitored Italy’s Mount Etna for eight years successfully detected 57 out of 59 eruptions that occurred between 2008 and 2016. The sensors fed their data to an automated forecasting system—the first of its kind—which alerted officials at least an hour before a possible eruption.
“Infrasound provides complementary data, above and beyond with what you get with seismometers,” Ruth says. “This also allows us to determine where the magma is in the conduit in a more passive way.” More passive, that is, than physically going in to check—which can be risky if that particular volcano’s getting ready to blow.
This tool came in handy last year during the months-long eruption of Hawaiʻi’s Kīlauea, which obliterated entire neighborhoods and uprooted thousands of local residents. Infrasound is only one of many metrics researchers have been measuring to keeping tabs on Kīlauea for the past several decades. But other volcanoes are less closely monitored, with only a few cameras and seismometers in their vicinity. In these situations, infrasound sensors, which are fairly low-cost, could be an important window into what’s happening at an active summit, says Anna Perttu, a geophysicist at the Earth Observatory of Singapore who was not involved in the new studies.
Forecasting a volcanic eruption is not unlike trying to predict a catastrophic weather event, Ruth says: Every clue counts. “Anytime a volcano changes from status quo to something different, we pay attention,” Johnson adds.
The key, Ruth says, will be continuing to combine infrasound with other techniques that measure seismic activity, gaseous output, ground deformations, and more to yield a more comprehensive picture of these dynamic systems.
“We’re not yet at the point where we can completely forecast an eruption,” Ruth says. “But are we moving towards that? Yes.”
For more on Kīlauea and the science of volcanic eruptions, watch “Kīlauea: Hawaiʻi on Fire,” premiering tonight at 9 p.m. ET / 8 p.m. CT on PBS.