The Campi Flegrei is dangerous. I'm not saying this to scare anyone or fear monger, but any time you have a large, restless caldera volcano with over 3,000,000 people living within ~19 miles (30 kilometers), you should be nervous. It would kind of be like moving Yellowstone to the outskirts of Tampa, Denver or San Diego. You'd want to know as much as you can about what is going on deep below your feet ... and what signs means an eruption might be coming soon.
That is exactly what we have near Naples in Italy, except that we can also add Vesuvius into the mix (so plot Mt. Rainier next to Tampa as well). The Campi Flegrei and Vesuvius are not linked directly, but they are both products of the complicated tectonics of Italy. To distill it down, the African Plate is subjecting (sliding under) the European Plate, creating small amounts of magma under the base of the crust. This melting feeds the volcanoes of Italy, including Campi Flegrei, Vesuvius, Etna, Stromboli and more.
The Notorious Caldera
The Campi Flegrei has produced some of the largest explosive eruptions Europe has seen in the past 50,000 years: the ~36,000 year old Campanian Ignimbrite that dumped almost 70 cubic kilometers of volcanic debris across the continent and the ~15,000 year old Neapolitan Yellow Tuff that added another 40 cubic kilometers on top.
However, the recent history of the Campi Flegrei has been a little more subdued. In 1538, a new cinder cone formed in the area. It was named "Monte Nuovo" or "New Mountain". It was tiny compared to the big explosions I just mentioned, but it still rained volcanic bombs and ash on the region as well as produced lava flows as well.
Since then, the caldera hasn't erupted but it has gone through multiple periods of unrest, mostly in the form of the rising and falling of the land surface. There are also areas of fumaroles (gas vents), mudpots and other hydrothermal features that betray the hot conditions not far underground across the Campi Flegrei.
How Much Stress Is Too Much?
If you are someone living in Pozzuoli, a city of over 350,000 that more or less exists inside the caldera of Campi Flegrei, you likely would love to know when the next eruption will happen. Well, it isn't that simple. Twice over the past 73 years of restlessness, thousands of people were evacuated because there were sufficient signs of a potential eruption. Nothing ended up happening in either case, which can potentially erode the public's trust of emergency managers and volcano monitoring agencies.
A new study in Nature Communications: Earth & Environment by Christopher Kilburn and colleagues tries to better quantify the state of stress in the Campi Flegrei. That's because, as they point out in the first line of the study "volcanoes reawakening after long repose must rupture the crust before magma can erupt".
What does that mean? Well, magma takes the path of least resistance (mostly) to reach the surface. Without pathways to eruption, magma can stall or get stored, but in doing so, it will add stress as the magma body inflates and/or gasses accumulate with the magma. If enough stress builds, cracks and fissures can open, leading to magma moving rapidly to the surface and you have yourself an eruption.
The prevailing evidence suggests that magma is stored on multiple levels beneath the Campi Flegrei. It rises from its formation in the mantle to eventually pond at around 5 miles (8 kilometers) beneath the surface. This isn't likely a big balloon of totally molten rock but rather an area that is a "mush" of crystals and magma. Magma from this zone rises to around 2 miles (3.5 kilometers) where it can stall or cause an eruption. Above this shallower melt zone is the hydrothermal system where water circulates and heats up to drive places like Solfatara.
Rising and Falling
Kilburn and his colleagues examined the changes in the land surface (inflation or deflation) as well as earthquake rates to estimate the stress the crust above the magma is feeling. Since 1950, the Campi Flegrei has seen over 13 feet (4 meters) of uplift with some subsidence in-between. It has also experienced over 30,000 volcano-tectonic earthquakes, most of which occurred since 1982.
Their primary argument is that as stress continues, the crust will go from responding elastically (returning to its original state) to inelastically (not returning to its original shape). After inelastic response comes failure, which means the crust breaks, potentially producing conduits for magma to reach the surface ... in other words: eruption.
Using all the data available about the stress felt in the crust beneath the caldera, Kilburn and others posit that the Campi Flegrei may have reached the point of inelastic response and that since 1984, it has entered the bulk failure phase. The upshot would mean that cracks are forming within the crust.
Failure of Failure or Future Failure?
Kilburn and others say that the initial rupturing of the crust ~2 miles (3 kilometers) down began in 1984 but halted. However, recent changes suggest that the rupturing resumed in 2020, reaching ever closer to the surface. The focus of the deformation is beneath the Solfatara and Pisciarelli, so that could be the potential location of any new eruption.
Does this mean that an eruption is imminent in the Campi Flegrei. The short answer is "no". Kilburn and others see three potential scenarios. First, the crust could establish a new state of fluid flow and all this restlessness could end. Second, fracturing could come and go, meaning the surface could rise and fall regularly, much like Yellowstone. Third, uplift continues as does fracturing and an eruption could occur.
As with all questions of volcanic behaviour, the real sticky wicket is timing. Just how long is it between this phase of "bulk failure" initiating and any potential eruption? A close second might be what are the signs of an imminent eruption brought on by this fracturing?
If we look at past behaviour at the Campi Flegrei, it could be decades before any potential eruption and most likely, it would be an eruption like Monte Nuovo and not a devastating explosive eruption like the Campanian Ignimbrite. However, Kilburn and others point out that our understanding of the rupture at volcanoes is built from stratovolcanoes like Vesuvius, not caldera. The timeline for calderas to go from rupture to eruption might be much shorter ... but how short is still unknown.
The biggest problem in understanding how massive caldera eruptions happen is also our biggest piece of good luck: we have experienced very few eruptions of giant calderas across human history. What each change means, especially with our refined ability to monitor the most minute differences in seismicity, land surface and gas emissions, is challenging to ascertain.
In the past few weeks, a new hydrothermal feature (likely a small new hot spring or geyser) opened near Old Faithful in the Yellowstone Caldera. This started with small explosions almost directly underneath a boardwalk in the National Park. Does this mean that Yellowstone is heating up and heading towards an eruption. Very much "no", but it does show how features and activity can change at these restless calderas.
You've heard may say it before, but it is worth stating again: a new study like this does not change the state of any volcano. It does allow us to have a better understanding of what might be happening, but we already knew that the Campi Flegrei is a highly hazardous volcano. Whether we are closer to an eruption or not is still unknown, but we should still prepare as if one is coming.