# Deciphering unrest at Campi Flegrei: Volcanology

Bruno Scaillet, Raffaello Cioni, Clive Oppenheimer

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Bruno Scaillet, Raffaello Cioni, Clive Oppenheimer. Deciphering unrest at Campi Flegrei: Volcanology. Nature Geoscience, 2025, 18, pp.115-116. 10.1038/s41561-025-01640-4. insu-04963087

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Submitted on 6 Oct 2025

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# Deciphering unrest at Campi Flegrei

Bruno Scaillet, Raffaello Cioni & Clive Oppenheimer

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The cause of episodes of unrest at caldera volcanoes is often unclear. Analysis of the sulfur composition of gas emissions at Campi Flegrei in Italy suggests a magmatic origin of the recent unrest at this hazardous caldera.

Eruption forecasting is a key objective of volcanology. For frequently erupting volcanoes, such as Kilauea in Hawaii and the recently active vents on Iceland's Reykjanes Peninsula, extensive ground-based, airborne, and satellite monitoring has underpinned operational hazard assessments¹,². Conversely, forecasting is more challenging for less active or dormant volcanoes, where the absence of recent eruptive activity can result in diverse explanations for new episodes of unrest, each with distinct hazard implications. This challenge is especially pronounced for calderas, large depressions formed by the discharge of vast volumes of magma. The potential for voluminous eruptions means that sound interpretation of caldera unrest, particularly distinguishing magmatic from non-magmatic causes, is critical. The challenge is particularly pressing at Campi Flegrei, which is adjacent to Naples, Italy, home to around half a million people. Writing in *Nature Geoscience*, Caliro et al.³ provide new insights from an analysis of venting gases into the escalating unrest at this caldera.

Campi Flegrei has not erupted since 1538, but has experienced multiple periods of unrest⁴. One notable episode occurred between 1982 and 1984, marked by earthquakes and ground deformation that led to the evacuation of parts of the city of Pozzuoli⁴. Unrest resumed in 2006 with ground uplift and geochemical anomalies, and since 2021, increased seismic activity has alarmed residents. These episodes have sparked debate over their causes⁵,⁶: are they because of shallow magma intrusion, or do they result from pressurization and heating of the hydrothermal system by fluids from deeper sources?

Recent gas observations (Fig. 1) are helping to address this question. As reported by Caliro et al., since 2018, fumarole emissions in the caldera have shown increasing sulfur content and evolving sulfur isotope ratios. While similar signals were observed during the 1982–84 crisis, the current changes are more pronounced, suggesting different underlying processes. Although a contribution from the hydrothermal system cannot be ruled out, Caliro et al. argue that these changes are indicative of decompression-driven degassing of a magma body at ≥ 6 km depth. This interpretation is consistent with reported evidence of stress accumulation in the crust⁷, but need not imply an eruption is imminent. Instead, it may signify the latest of many iterations of magma recharge that would be needed to trigger an eruption.

Interpreting such signals requires high-quality data and robust understanding of magma degassing processes. Caliro et al. highlight the importance of sustained monitoring of Campi Flegrei's fluid emissions. Without a continuous baseline dataset — made possible by the continued operation of the Vesuvius Observatory, founded in 1841 — recognizing anomalous sulfur degassing would be difficult;

<figure><img src="image_3.png" /><figcaption>**Fig. 1** | Gas emissions from a fumarole in the Solfatara crater in the Campi Flegrei caldera. Caliro et al.³ present multi-decadal measurements of the sulfur composition of such emissions showing unprecedented changes since 2018, which is consistent with degassing of basaltic magma beneath the caldera.</figcaption></figure>

hence long-term surveillance is essential. Understanding the complex physico-chemical processes governing magma degassing is equally crucial. While sulfur is a key focus because of its detectability, its behaviour is difficult to constrain, owing to its complex chemistry and the challenges of high-pressure experiments⁸. Current models, though insightful, fall short of reliably describing sulfur behaviour across varying magmatic conditions (including composition, pressure, temperature, and oxidation state), highlighting the need for further experimental research.

Geochemical signals must also be integrated with geological and geophysical data⁴. Decades of surveys and soundings at Campi Flegrei provide important context. For example, boreholes reveal high temperatures (>400 °C) at depths of ~3 km, indicative of proximity to magma. Seismic data suggest a melt-dominated region at ~7.5 km depth⁹, but no shallower magma bodies have been detected. This discrepancy either reflects the genuine absence of shallow magma or the limitations of current geophysical methods, underscoring the interpretative challenges.

Ultimately, our understanding of subsurface magmatic and hydrothermal processes is limited by models and assumptions. Even if we are eventually able to instrument a magma reservoir itself¹⁰, we will never truly 'see' what is going on inside it. One key question at Campi Flegrei is the composition of any magma in the upper crust, which is a starting point for modelling the gas emissions. Is it a more silica-rich, cooler body like the one responsible for the 1538 eruption, or is it a more recent recharge of basaltic magma? Caliro et al. favour the latter, based on the concentration of volatile species in erupted products. Nevertheless, as the mechanisms by which crystallizing magma bodies release these volatiles remain unclear, degassing of more silica-rich magma cannot be excluded as the source of the current
emissions. If the basaltic scenario is correct, it calls to mind the volu-
minous effusive-explosive 2021 Tajogaite eruption on La Palma in the
Canary Islands, which involved sulfur- and CO2-rich basalt¹¹. Such an
eruption at Campi Flegrei would represent a major departure from the
relatively minor explosive eruption in 1538, with important implica-
tions for hazard assessment.

It remains unclear whether the recent unrest at Campi Flegrei
signifies the final recharge events preceding an eruption or increments
within a longer sequence, but Caliro et al. show that the recent gas
emissions are indicative of magmatic activity. Continuous monitor-
ing, coupled with advanced numerical simulations and laboratory
analyses of past Campi Flegrei magmas, is essential to obtain a more
definitive answer.

Bruno Scaillet Ⓓ¹, Raffaello Cioni Ⓓ² & Clive Oppenheimer Ⓓ³,⁴

¹Institut des Sciences de la Terre d'Orléans, UMR 7327, CNRS-
Université d'Orléans-BRGM – Université d'Orléans, Orléans, France.

²Dipartimento di Scienze della Terra, Università degli Studi di Firenze,
Florence, Italy. ³Department of Geography, University of Cambridge,

Cambridge, UK. ⁴Istituto Nazionale di Geofisica e Vulcanologia,
Osservatorio Etneo, Catania, Italy.

e-mail: bscaille@cnrs-orleans.fr

Published online: 24 January 2025

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**Competing interests**

The authors declare no competing interests.