ByRobin George Andrews
Published October 20, 2023
• 10 min read
Italy’s Campi Flegrei is nothing like your average volcano. It’s not a mountain, but an eight-mile-wide, bowl-shaped caldera centered on the Gulf of Pozzuoli, just outside Naples. The unsubmerged mouth of the volcano is home to more than 360,000 people, and in total, 2.3 million people live on or around it.
Although under constant surveillance, the immense volcanic bowl piqued the curiosity of volcanologists this summer when it began convulsing and shaking more than usual.
Campi Flegrei has been gradually inflating since 2005. But in August, according to a notice from Italy’s National Institute of Geophysics and Volcanology (INGV), the volcano’s seismic activity became more frequent and intense. Then on September 27, a magnitude-4.2 temblor, the most powerful quake in almost 40 years, struck the caldera.
The activity has raised public concerns that an eruption, the first in five centuries, could be on its way. And the threat of more intensive earthquakes has prompted the Italian government to preemptively work on plans to evacuate tens of thousands.
Forecasting the near-term future of Campi Flegrei is fraught with difficulties. With no modern eruptions to compare with the current unrest, volcanologists do not know exactly how the caldera behaves prior to an outburst. But based on decades of scientific observations here and at other calderas across the world, researchers do not believe an eruption is imminent.
“We do not see any kind of pre-eruptive anomaly, any kind of anomaly that indicates that the magma is coming up,” says Warner Marzocchi, a geophysicist and natural hazards researcher at the University of Naples Federico II.
Still, scientists say, an eruption at Campi Flegrei is inevitable, even if the timing of that destined day is unclear.
“Any sort of eruption would be horrendous for the region,” says Mike Cassidy, a volcanologist at the University of Birmingham in England. “The risk of an eruption is low, but not zero. The risk of a large eruption is also not zero. I think that’s an important thing to bring into this discussion.”
Restless infernal fields
Campi Flegrei partly encompasses the city of Naples and stretches out to the west into the Tyrrhenian Sea. Since Roman times, people have noticed the ground in the area has risen and fallen, and by digging into its geologic history, scientists have uncovered this volcano’s violent past. The caldera as it appears today was carved out by two unimaginably massive paroxysms—one 36,000 years ago and another 15,000 years ago—that excavated the earth and smothered the region in volcanic debris.
Since then, Campi Flegrei has hosted countless small eruptions, often involving explosive activity all over its expansive caldera, on land and at sea. Its last outburst, back in 1538, built a small cone over the course of a week. No fresh magma has made it to the surface since.
Starting in the mid-20th century, though, the volcano has been agitated. In the 1950s, 1970s, and 1980s, the caldera engaged in two-year spikes of unrest, changing shape by inflating and subsiding, and shaking in a series of earthquakes. The most unnerving episode was between 1982 and 1984, when the ground rose by six feet and, fearing building damage, 40,000 people from the town of Pozzuoli evacuated.
Back then, “people didn’t know exactly what to expect from Campi Flegrei,” says Marzocchi. With the immense eruption of Mount St. Helens in the United States having taken place just a couple of years earlier, Campi Flegrei’s unrest meant that local volcanologists “were a little bit worried.”
But after the volcano’s unrest, there was no eruption—something that was both puzzling and relieving.
A fourth awakening
Since 2005 the ground of Campi Flegrei has risen by 0.4 to eight inches per year. This rate of inflation is lower than what was observed during the past three periods of unrest, but it has been happening for almost two decades now. Volcanologists have begun to wonder what happens if the crust is stretched beyond its breaking point.
There has also been a jump in the frequency and intensity of quakes in the area since the summer. The number has dropped off in the past few weeks, but potent temblors are still rocking the region, including a magnitude-4.0 event on October 2. What could explain this perturbing activity?
In June a group of scientists published a paper that attempted to decode the behavior of the volcano over the past 70 years. If this study’s interpretation is correct, it could also explain what’s happening in today.
Here’s what they think is going on: At a depth of five to six miles, there is a persistent magma reservoir that is continually loosing noxious gas. Some of that gas is trapped at a barrier of rock roughly two miles below the surface, and some escapes out of small volcanic maws at places like the malodorous Solfatara crater.
The researchers hypothesize that, in the 1950s, a magma batch intruded close to that rock barrier, fracturing it a little and elevating the crust. Once the magma cooled down and the extra gas escaped to the surface, the uplift stopped and the ground subsided—although it never returned to levels measured before the uplift.
The team believes the same process happened again in the 1970s and 1980s. By the time of the 1980s unrest, the repeated fracturing of the crust had created new pathways for trapped gas to escape, which led to a significant period of subsidence afterwards.
Many of those fractures have since healed up. Those past three injections of magma have cooled and frozen, creating a “magma seal, which is also a barrier to the gas’s escape,” says study co-author Nicola Alessandro Pino, senior researcher at the INGV’s Vesuvius Observatory in Naples. But gas has continued to come from the deep magma reservoir, and much of it has accumulated again at the rocky barrier about two miles down, causing the ground to inflate.
Magmatic gas rising from deep below and getting stuck is sufficient to explain the uplift that has been happening since 2005—and the recent and occasionally strong quakes may be coming from the aggressive merging of newly formed fractures.
The coming conflagration
If the uplift continues, eventually something’s going to give. If a big enough fracture (or fractures) reaches the surface, there are three possible outcomes. The first, and likeliest, is that volcanic gas is expelled over a large area, doing no harm. Alternatively, “it could come out in a concentrated zone, in which case there may be some explosions, locally,” says Christopher Kilburn, a volcanologist and geophysical hazards researcher at University College London, and a co-author of the recent study.
The least likely scenario is the most concerning: a shallow intrusion of magma connects with that fracture and quickly erupts to the surface, perhaps with little warning beforehand. It has been suggested that the present uplift and quakes could be caused by recent injections of magma at shallow depths, but measurements of the volcano’s gravity field suggest no major magmatic incursions have happened in recent months, says Pino.
As with all models, the recent one of Campi Flegrei may not be entirely correct. “We don’t have perfect models,” says Mike Burton, a volcanologist at the University of Manchester, because “we don’t have perfect information.”
Campi Flegrei is one of the most thoroughly monitored volcanoes in the world. Its ground deformation, gas output, thermal readings, gravity field, and tremors are carefully and constantly scrutinized. The behavior of other volcanoes can help scientists interpret this data, but interpretations are never unequivocal—especially for a volcano with no contemporary eruptions.
As the INGV recently concluded: “The probability of a volcanic eruption is relatively low, since there is no evidence of magma rising to the surface.” But it adds that “the volcano has its unstoppable natural evolution and, sooner or later, it will erupt again.”
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