Two long-dormant “supervolcanoes” on two separate continents appear to be stirring to life. Well, maybe.
In recent months, more than a thousand minor earthquakes have rattled the area around the Campi Flegrei volcano in southern Italy, stoking fears that it may soon erupt again after nearly five centuries. Some 6,000 miles away, scientists have for decades recorded similarly small earthquakes and instances of ground deformation at the Long Valley Caldera, a volcano in eastern California that sits adjacent to Mammoth Mountain.
But does all this seismic unrest really portend a volcanic eruption? It sort of depends on whom you ask.
Most experts say there is no immediate threat of an eruption at either Long Valley or Campi Flegrei. Both volcanoes are calderas — sprawling depressions created long ago by violent “super-eruptions” that essentially collapsed in on themselves — which are often more challenging to forecast compared to the large mountain-shaped features that people typically imagine when they think of volcanoes.
Seismic unrest can be a sign that a volcano is waking up, but the full story is much more complex.
Both Campi Flegrei and the Long Valley Caldera are known as supervolcanoes, a term used to describe a volcano that at one time has erupted more than 240 cubic miles of material. Michael Poland, a geophysicist at the U.S. Geological Survey and the scientist-in-charge of the Yellowstone Volcano Observatory, said that while Campi Flegrei and Long Valley are capable of huge explosions, the supervolcano moniker can be misleading.
“The first thing people think is that there’s going to be a civilization-ending eruption,” Poland said. “You can have an impactful explosion at these places, but the vast majority are smaller eruptions with less explosive lava flows.”
That hasn’t quieted concern in the communities that border the caldera systems. The Italian city of Naples and its surrounding towns are all in close proximity to Campi Flegrei, and local government officials have been planning how to evacuate tens of thousands of people from the area, if needed.
The last time Campi Flegrei erupted was in 1538, and one of the system’s biggest explosions occurred around 39,000 years ago.
In September, the former head of the Vesuvius observatory at Italy’s National Institute of Geophysics and Volcanology told Reuters that the earthquake swarms were causing ground uplift in the area, which could lead to structural damage in the port town of Pozzuoli, located roughly 20 miles outside of Naples.
Christopher Kilburn, a professor of volcanology and geophysical hazards at University College London, said the last period of seismic unrest at Campi Flegrei was in the 1980s. Kilburn said the ground in the town of Pozzuoli was lifted nearly 2 meters, or almost 6.5 feet, over two years.
Still, there was no big eruption.
Fast-forward to today, and Kilburn said there are some key differences with the seismic unrest that has been observed.
“The difference is that today, the uplift has been a bit more than 1 meter, but over 20 years, not two,” he said. “And so this whole uplift has lasted 10 times longer and it has been about 10 times slower.”
Still, Kilburn thinks the current activity at Campi Flegrei indicates that the structure of the volcano’s crust is changing. In a study published in June in the journal Communications Earth & Environment, Kilburn and his colleagues used a model to analyze the volcano’s behavior and found that the crust of Campi Flegrei is becoming weaker, making it more prone to rupturing.
But even if the crust reaches its breaking point, that wouldn’t necessarily have catastrophic consequences, Kilburn said.
“If there’s a rupture, there is no guarantee that magma is going to erupt,” he said. “And that’s why with the observatory there, the official releases cover anything from just an increase in seismicity…
Read More: Two supervolcanoes, a world apart, have the attention of scientists