Overlapping craters of all sizes pockmark the Campi Flegrei volcanic region, which is nestled at the western edge of Naples, Italy, and stretches out into the Mediterranean Sea. Today, more than half a million people have settled near the slumbering volcano, whose rumblings and effusive gasses reflect the heat that still brews below. Twice in the last 60,000 years, large blasts of volcanic ash and rock have blanketed the region, and a smattering of smaller bursts have happened before and after each big eruption, including the most recent event in 1538.

For clues on what's happening in the magma chamber below, a team of researchers have now examined the chemistry of volcanic rocks and glass from historical eruptions, using that data to create a computer model to simulate the conditions leading to an eruption. Their study, published today in the journal Science Advances, could help scientists get a grip on the waking and sleeping cycles of these cataclysm-causing volcanoes.

One particular takeaway will likely make a bang: The researchers conclude that magma under Campi Flegrei may be entering a building phase, “potentially culminating, at some undetermined point in the future, in a large-scale eruption,” the team writes in their study.

To be clear, the volcano is not currently heading for catastrophe. Researchers actively monitor the system and understand the cues that could signal pending eruptions. Any hypothetical massive blast would likely be far in the future, possibly thousands of years or more. (Also find out why Yellowstone's supervolcano may rumble to life faster—but not sooner—than previously thought.)

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“We can hypothesize it can occur, but we don't know when,” says lead author Francesca Forni of ETH Zürich in Switzerland. She stresses that the work focuses on chemical cycles, “not really when or if Campi Flegrei is going to erupt soon.” What's more, many researchers not associated with the work expressed concern about the application of the results to modern conditions.

“As often happens in scientific research, the data collection and analysis may be more important than the immediate interpretations, which in fact are not so well constrained,” volcanologist Claudia Troise of Vesuvius Observatory, the team tasked with monitoring Campi Flegrei's activity, says via email.

So before you go running to the nearest underground bunker, let's break down this new study.

What did the researchers do?

As magma crystallizes within the planet, the tiny mineral flakes take on chemical clues that can reveal information about its parent magma, like temperature and water content. As Forni explains, these characteristics can hint at what's happening in the volcano before each eruption—and may be a key to why eruptions are sometimes tiny and other times tremendous.

To tease through the complexities of the region, Forni and her colleagues examined volcanic material from 23 eruptions, analyzing their elemental chemistry to identify signatures of their formation. The researchers then incorporated this information into a computer model to simulate Campi Flegrei's eruptions since its last big blast 15,000 years ago.

Overall, the samples of erupted materials point to cycles of magma cooling and heating, similar to those previously suggested for other large volcanoes. But the latest results use modern methods to paint a much more detailed picture.

What does such a cycle look like?

In broad strokes, the cycle starts with the slow buildup of magma in the chamber, leading to a massive crater-forming blast. The eruptions that follow this big event are relatively small and frequent blasts of hot, dry magma that likely originates from deep inside Earth and doesn't linger as it heads out.

Over time, magma begins to accumulate in the chamber, cooling and slightly crystallizing. Like salt freezing out of seawater as ice forms, the magma's dissolved water doesn't get incorporated as much into these crystals, increasing its presence in the leftover melt. This means eruptions have more water and decreasing temperatures.



At some point, the eruption frequency slows, but magma continues to build. The exact trigger for the start of a new cycle—which could build to a major eruption—is unclear. But Forni and her colleagues point to one key change: the formation of a crystal mush with so much dissolved water that some of it is forced into the magma as bubbles. The team's composition analysis suggests that the volcano's last eruption in 1538 represents this bubbly magma stage. But the next step is a mystery.

“We actually don’t know for sure what the next step is going to be,” Forni says, noting that the chamber could continue to cool and permanently go quiet, or a new build-up cycle could start. The chemistry of the last eruption, she says, “indicate[s] that the magmatic reservoir might be 'ready' for accommodating magmas from recharge without erupting frequently.”

What does this all mean?

“It's really made a huge contribution having this data together to kind of see how things are changing over time,” says Victoria Smith, a geochronologist and volcanologist at the University of Oxford. “But there's not enough evidence to say the next eruption is going to be caldera-forming.”

Smith emphasizes the complexity of the Campi Flegrei system. Each layer of erupted rock and ash did not form tidy, even layers. What's more, the two most massive eruption events blanketed wide swaths of the surrounding region in rock and ash, burying traces of earlier blasts. Because of this, it's nearly impossible to retrieve material from every historical eruption, leaving inevitable gaps in the record.

“I wonder if you had all of those samples, how clear the picture would be,” she says.

Volcanologist Christopher Kilburn at the University College London adds that the paper is a good example of how to use geochemistry to better understand the lifecycle of this large volcanic system. “Indeed, the results may be valuable for understanding large calderas worldwide,” he says in an email.

But he too warns against drawing too many conclusions about the current state of affairs at Campi Flegrei. In the paper, Forni and her team point to recent unrest, such as ground deformation or changes in emitted gas, as signals for potential magma recharge.

“We have to be cautious, though, about connecting the long-term behavior over thousands of years to short-term changes over years to decades, and their implications about the potential for eruption,” Kilburn says.

These rumbles have also sparked some debate over their cause, and recent work suggests that the tremors don't reflect a shallow chamber refilling with magma, Troise points out.