Sitting within the Bay of Naples in southern Italy is Campi Flegrei, a vast and restless volcanic caldron. The history of this sleeping colossus includes two massive eruptions, 39,000 and 15,000 years ago, that left deep calderas in the landscape. Its last significant volcanic event a 1538 eruption known as Monte Nuovo that spawned a small new mountain. Since then, it has been curiously eruption-free.
Today, 1.5 million people live within the volcano’s caldera and its surroundings, and the cache of magma that could burst from the area’s hellish underbelly makes it one of the most hazardous areas on Earth. In a study published Wednesday in Science Advances, volcanologists report that Campi Flegrei is at the start of an eruption cycle, one that may result in a massive outburst at some point in the likely distant future.
The researchers arrived at this conclusion by unspooling the 60,000-year-long history of the volcano, which revealed what appears to be a rhythm to Campi Flegrei’s upsurges. Fortunately, there are no signs that an eruption is imminent. And because the volcano is one of the world’s most closely monitored, scientists are likely to pick up on any warning signs.
Previous studies of Campi Flegrei have focused on one or a handful of its eruptions. But the new study, led by Francesca Forni, a postdoctoral researcher at Nanyang Technological University in Singapore, who conducted the work at ETH Zurich, used fresh geological samples taken from 23 eruptions across its history, both larger and smaller, to see how the chemistry of the magma changed over deep time. Her team also used computer models to simulate what may have occurred inside the volcano since the last caldera-forming eruption 15,000 years ago.
The team found that Campi Flegrei has gone through stages. First, a massive eruption occurs, resulting in the formation of a caldera. Then the volcano enters a period of regular, small eruptions as magma escapes through new fractures in the crust.
Finally, the volcano enters a pre-caldera phase. Minor eruptions become infrequent, and magma accumulates in the subterranean reservoir. As it pools, the magma evolves into a water-rich, gassy form, and the most buoyant, bubble-rich patches gather at the top. This magma buildup may eventually culminate in another major eruption, and the cycle would begin anew.
But geological cycles can be broken, and the current lack of volcanism at Campi Flegrei doesn’t necessarily portend a major eruption. Christopher Kilburn, a hazards expert at University College London who wasn’t involved in the study, said it cannot be ruled out that the volcano’s sustained period of relative calm could indicate it’s “on the wane.”
But Dr. Forni found that the chemistry of the Monte Nuovo eruption in 1538 is similar to that seen in the deeper past, when the volcano slowly built to caldera-forming blasts. The presence of two calderas suggests that Campi Flegrei has probably completed at least one cycle, and if this pattern is indeed real, then the chemistry hints that “we are potentially at the start of a new cycle,” Dr. Forni said.
But, she said, predicting when a major eruption will occur is beyond the current ability of science: “The best we can do for now is to see how the system behaved in the past.”
One of the study’s co-authors, Gianfilippo De Astis, a researcher at Italy’s National Institute of Geophysics and Volcanology, said that the magmatic reservoir is currently at rest. “It can keep these physical and chemical conditions for a long time,” he said.
Lara Smale, a doctoral student at University College London who is researching Campi Flegrei but was not involved in the study, emphasized that “the chance of a very large eruption at Campi Flegrei in our lifetimes is extremely low.”
Even then, the next eruption probably will be a small event, compared with the massive outbursts tens of thousands of years ago. Most of the eruptions in Campi Flegrei’s past were minor.
The study’s use of geochemistry to understand long-term magmatic evolution, Dr. Kilburn said, “may help to improve our understanding of large calderas worldwide.”.
Campi Flegrei’s apparently cyclical nature is significant. If corroborated, said Ms. Smale, then perhaps large volcanoes elsewhere have cycles too — meaning their futures could also come into sharper focus.