Scenario-based forecast of the evolution of 75 years of unrest at Campi Flegrei caldera (Italy)

About 2 million people live in the city of Naples and in close proximity to a large active volcano: Campi Flegrei (no, not Vesuvius). This volcano is currently under unrest and since 1950 this is the 4th episode of ground uplift, heighten seismicity, and enhanced gas emissions.
Luca CARICCHI Jan 12, 2026

The challenges of applying science to an active volcano

Our capacity to identify and quantify processes occurring at depths that are unreachable and over timescales that are often much longer than a human life, is for me the most fascinating aspect of being an Earth Scientist. We are trained to extract information on the functioning of our planet using field observations and measurements, geophysics, and a wide variety of microanalytical instruments. It goes without saying that any model we construct based on our data is far from perfect, but it must account for all available observations. While this is a solid scientific principle, using this approach to provide useful insights on an ongoing crisis at an active volcano around which 2 million people live, was the most challenging effort of my career and compels some initial considerations. First, we could not have even designed our research without the expertise of the colleagues at the Italian National Institute for Geophysics and Volcanology (INGV) in Naples, the monitoring data they collect since decades, historical data, and a large amount of scientific literature existing on this fascinating volcano. Second, we structured our research to further stimulate the essential ongoing scientific debate and determine whether some of the scenarios which are plausible based on the monitoring data, could be excluded.

The monitoring data collected for the ongoing unrest are best explained by an inflating source at about 4 km below the apex of the uplifting region located in Pozzuoli (municipality of Naples). What remains unclear is whether the inflation is related to the ascent of magma or fluids from a large magma reservoir located at depth greater than 8 km (Refs.1–3). The importance of this difference is that a volcanic eruption would be more likely if the cause of inflation is magma. Thus, our study focused on the question: “Can we categorically exclude the presence of eruptible magma (i.e. hot enough to reach the Earth’s surface4) at 4 km depth?”.

 

Making an extreme assumption

Because we do not have direct access to the inflating source at 4 km depth, and geophysical methods cannot resolve the difference between magmatic fluids and magma, our question could only be answered by making an extreme and strong assumption. We consider the “worst case scenario” and assumed that the 4 episodes of inflations recorded since 1950, were all associated with the injection of magma at 4 km depth. We performed thermal modelling calculations exploring a variety of parameters within ranges that are plausible based on existing data and measurements collected in the field. As an example, the aspect ratio of the single pulses of magma we assumed were injected at 4 km depth cannot be resolved and we used measurements collected on magma intrusions today’s exposed at the surface by plate tectonic. The thickness of the magma pulses turned out to be fundamental as a value of 15 m would result in the absence of eruptible magma while if the pulses of magma potentially injected since 1950 would be 25 m thick, about 100 million cubic meters of eruptible magma would be present today at 4 km depth (Fig. 1).

Thus, the first important result of our analysis is that we cannot categorically exclude the presence of magma at 4 km depth. Importantly, even if eruptible magma is present today at 4 km depth, this magma might never reach the surface to feed a volcanic eruption. This depends primarily on the volume of the reservoir where the magma is stored5. The pressure within the reservoir must be sufficiently high to push the magma to the surface, however, if the reservoir is too small, as the surrounding rocks fracture and magma starts to rise, the pressure drops rapidly and magma ascent stops. To better understand the control of reservoir size on the rate at which pressures drop you can imagine the difference in the rate of deflation that would be achieved by piercing a kids’ ballon or a passenger air balloon with a needle.

 

Proposing scenarios

Our results show that the reservoir potentially present today at 4 km depth below Campi Flegrei can be pressurised sufficiently to crack the surrounding rocks and allow the escape of magma from the reservoir, but it is too small to allow the magma to reach the surface. Under our extreme assumption, at the current rate of magma input, the reservoir will reach a size sufficient to feed a volcanic eruption in few decades. A caveat to this model is that would one of this magma filled crack reach the well-developed hydrothermal system at Campi Flegrei, the explosive interaction between magma and water could lead to a so-called “phreatomagmatic” eruption even if the reservoir is currently not ideally suited to feed volcanic activity. Coincidentally, the last eruption of Campi Flegrei (Monte Nuovo 1538), started with the explosive interaction between magma and water6. 

The involvement in this study made it clear to me that when applying science to active volcano, caution in the interpretation of the results is necessary. What is of outmost importance, and a professional obligation, is to generate as many plausible scenarios as possible and exclude those that do not match existing data. The scientists in charge of monitoring and advising decision makers, can use the continuously collected monitoring data together with theoretical models to assess the evolution of the ongoing crisis. Finally, to be sure all options have been considered, it is very important not to be afraid to be wrong.

REFERENCES

  1. Astort, A. et al. Tracking the 2007–2023 magma-driven unrest at Campi Flegrei caldera (Italy). Communications Earth & Environment 5, 506 (2024).
  2. Isaia, R. et al. 3D magnetotelluric imaging of a transcrustal magma system beneath the Campi Flegrei caldera, southern Italy. Communications Earth & Environment 6, 213 (2025).
  3. Caliro, S. et al. Escalation of caldera unrest indicated by increasing emission of isotopically light sulfur. Nature Geoscience https://doi.org/10.1038/s41561-024-01632-w (2025) doi:10.1038/s41561-024-01632-w.
  4. Marsh, B. D. On the Crystallinity, Probability of Occurrence, and Rheology of Lava and Magma. Contribution to mineralogy and petrology 78, 85–98 (1981).
  5. Townsend, M. & Huber, C. A critical magma chamber size for volcanic eruptions. Geology 48, 431–435 (2020).
  6. Di Vito, M., Lirer, L., Mastrolorenzo, G. & Rolandi, G. The 1538 Monte Nuovo eruption (Campi Flegrei, Italy). Bull Volcanol 49, 608–615 (1987).