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Resurgent calderas are excellent targets for geothermal exploration, as they are associated with the shallow emplacement of magma, resulting in widespread and long-lasting hydrothermal activity. Resurgence is classically attributed to the uplift of a block or dome resulting from the inflation of the collapse-forming magma chamber due to the intrusion of new magma. The Los Humeros volcanic complex (LHVC; Mexico) consists of two nested calderas: the outer and older Los Humeros formed at 164 ka and the inner Los Potreros formed at 69 ka. The latter is resurgent and currently the site of an active and exploited geothermal field (63 MWe installed). Here we aim to better define the characteristics of the resurgence in Los Potreros by integrating fieldwork with analogue models and evaluating the spatio-temporal evolution of the deformation as well as the depth and extent of the intrusions responsible for the resurgence, which may also represent the local heat source(s). Structural field analysis and geological mapping show that the floor of the Los Potreros caldera is characterized by several lava domes and cryptodomes (with normal faulting at the top) that suggest multiple deformation sources localized in narrow areas. Analogue experiments are used to define the possible source of intrusion responsible for the observed surface deformation. We apply a tested relationship between the surface deformation structures and depth of elliptical sources to our experiments with sub-circular sources. We found that this relationship is independent of the source and surface dome eccentricity, and we suggest that the magmatic sources inducing the deformation in Los Potreros are located at very shallow depths (hundreds of metres), which is in agreement with the well data and field observations. We propose that the recent deformation at LHVC is not a classical resurgence associated with the bulk inflation of a deep magma reservoir; rather, it is related to the ascent of multiple magma bodies at shallow crustal conditions ( km depth). A similar multiple source model of the subsurface structure has also been proposed for other calderas with an active geothermal system (Usu volcano, Japan), suggesting that the model proposed may have wider applicability.

Large calderas are among the Earth's major volcanic features. They are associated with large magma reservoirs and elevated geothermal gradients. Caldera-forming eruptions result from the withdrawal and collapse of the magma chambers and produce large-volume pyroclastic deposits and later-stage deformation related to post-caldera resurgence and volcanism. Unrest episodes are not always followed by an eruption; however, every eruption is preceded by unrest. The Campi Flegrei caldera (CFc), located along the eastern Tyrrhenian coastline in southern Italy, is close to the densely populated area of Naples. It is one of the most dangerous volcanoes on Earth and represents a key example of an active, resurgent caldera. It has been traditionally interpreted as a nested caldera formed by collapses during the 100–200 km3 Campanian Ignimbrite (CI) eruption at ∼39 ka and the 40 km3 eruption of the Neapolitan Yellow Tuff (NYT) at ∼15 ka. Recent studies have suggested that the CI may instead have been fed by a fissure eruption from the Campanian Plain, north of Campi Flegrei. A MagellanPlus workshop was held in Naples, Italy, on 25–28 February 2017 to explore the potential of the CFc as target for an amphibious drilling project within the International Ocean Discovery Program (IODP) and the International Continental Drilling Program (ICDP). It was agreed that Campi Flegrei is an ideal site to investigate the mechanisms of caldera formation and associated post-caldera dynamics and to analyze the still poorly understood interplay between hydrothermal and magmatic processes. A coordinated onshore–offshore drilling strategy has been developed to reconstruct the structure and evolution of Campi Flegrei and to investigate volcanic precursors by examining (a) the succession of volcanic and hydrothermal products and related processes, (b) the inner structure of the caldera resurgence, (c) the physical, chemical, and biological characteristics of the hydrothermal system and offshore sediments, and (d) the geological expression of the phreatic and hydromagmatic eruptions, hydrothermal degassing, sedimentary structures, and other records of these phenomena. The deployment of a multiparametric in situ monitoring system at depth will enable near-real-time tracking of changes in the magma reservoir and hydrothermal system.

Abstract Volcanic islands pose several major types of natural hazards, often interconnected and concentrated in relatively small areas. The quantification of these hazards must be framed from a multi-hazard perspective whilst building on existing single-hazard analyses. Ischia is a densely inhabited volcanic island with a long eruptive history lasting more than 150 ka (last in 1302 AD) characterized by the significant asymmetric resurgence of a caldera block. Here, we review the state-of-art of the natural hazards of Ischia, aiming at building a solid base for future holistic multi-hazard quantifications. We frame our analysis in three steps: i) review of geological, historical and current activity; ii) review of available hazard models and analyses; iii) development of an interpretative framework for the interdependent hazards. The results highlight that volcanic activity has been quite intense and many volcano-related hazardous phenomena have affected the island including in very recent times, both for eruptive (phreatic or magmatic eruptions) and non-eruptive (earthquakes, landslides, and tsunamis) phenomena. The effects of some of them (e.g. tsunamis, tephra) are also relevant beyond the island territory. Quantitative hazard assessments are almost absent and should be developed in the future considering the evident interconnections between hazards. To this end, we propose a conceptual interpretative multi-hazard framework that highlights the fundamental role played by the resurgent block in controlling and connecting the different hazards, in terms of both spatial distribution of the sources and temporal clustering.

Abstract  Dikes within stratovolcanoes are commonly expected to have radial patterns. However, other patterns may also be found, due to regional stresses, magmatic reservoirs and topographic variations. Here, we investigate dike patterns within volcanic edifices by studying dike and fissure complexes at Somma-Vesuvius and Etna (Italy) using analogue models. At the surface, the dikes and fissures show a radial configuration. At depths of tens to several hundreds of metres, in areas exposed by erosion, tangential and oblique dikes are also present. Analogue models indicate that dikes approaching the flanks of cones, regardless of their initial orientation, reorient to become radial (parallel to the maximum gravitational stress). This re-orientation is a significant process in shallow magma migration and may also control the emplacement of dike-fed fissures reaching the lower slopes of the volcano. [ABSTRACT FROM AUTHOR]

Most flank eruptions within a central stratovolcano are triggered by lateral draining of magma from its central conduit, and only few eruptions appear to be independent of the central conduit. In order to better highlight the dynamics of flank eruptions in a central stratovolcano, we review the eruptive history of Etna over the last 100 years. In particular, we take into consideration the Mount Etna eruption in 2001, which showed both summit activity and a flank eruption interpreted to be independent from the summit system. The eruption started with the emplacement of a ~N-S trending peripheral dike, responsible for the extrusion of 75% of the total volume of the erupted products. The rest of the magma was extruded through the summit conduit system (SE crater), feeding two radial dikes. The distribution of the seismicity and structures related to the propagation of the peripheral dike and volumetric considerations on the erupted magmas exclude a shallow connection between the summit and the peripheral magmatic systems during the eruption. Even though the summit and the peripheral magmatic systems were independent at shallow depths (<3 km b.s.l.), petro-chemical data suggest that a common magma rising from depth fed the two systems. This deep connection resulted in the extrusion of residual magma from the summit system and of new magma from the peripheral system. Gravitational stresses predominate at the surface, controlling the emplacement of the dikes radiating from the summit; conversely, regional tectonics, possibly related to N-S trending structures, remains the most likely factor to have controlled at depth the rise of magma feeding the peripheral eruption. [ABSTRACT FROM AUTHOR]

Magmatism accompanies rifting along divergent plate boundaries, although its role before continental breakup remains poorly understood. For example, the magma-assisted Northern Main Ethiopian Rift (NMER) lacks current volcanism and clear tectono-magmatic relationships with its contiguous rift portions. Here we define its magmatic behaviour, identifying the most recent eruptive fissures (EF) whose aphyric basalts have a higher Ti content than those of older monogenetic scoria cones (MSC), which are porphyritic and plagioclase-dominated. Despite these differences, calculations highlight a similar parental melt for EF and MSC products, suggesting only a different evolutionary history after melt generation. While MSC magmas underwent a further step of storage at intermediate crustal levels, EF magmas rose directly from the base of the crust without contamination, even below older polygenetic volcanoes, suggesting rapid propagation of transcrustal dikes across solidified magma chambers. Whether this recent condition in the NMER is stable or transient, it indicates a transition from central polygenetic to linear fissure volcanism, indicative of increased tensile conditions and volcanism directly fed from the base of the crust, suggesting transition towards mature rifting., (© 2021. The Author(s).)

Transient seismicity at active volcanoes poses a significant risk in addition to eruptive activity. This risk is powered by the common belief that volcanic seismicity cannot be forecast, even on a long term. Here we investigate the nature of volcanic seismicity to try to improve our forecasting capacity. To this aim, we consider Ischia volcano (Italy), which suffered similar earthquakes along its uplifted resurgent block. We show that this seismicity marks an acceleration of decades-long subsidence of the resurgent block, driven by degassing of magma that previously produced the uplift, a process not observed at other volcanoes. Degassing will continue for hundreds to thousands of years, causing protracted seismicity and will likely be accompanied by moderate and damaging earthquakes. The possibility to constrain the future duration of seismicity at Ischia indicates that our capacity to forecast earthquakes might be enhanced when seismic activity results from long-term magmatic processes, such as degassing., (©2019. The Authors.)

When a batch of magma reaches Earth's surface, it forms a vent from which volcanic products are erupted. At many volcanoes, successive batches may open vents far away from previous ones, resulting in scattered, sometimes seemingly random spatial distributions. This exposes vast areas to volcanic hazards and makes forecasting difficult. Here, we show that magma pathways and thus future vent locations may be forecast by combining the physics of magma transport with a Monte Carlo inversion scheme for the volcano stress history. We validate our approach on a densely populated active volcanic field, Campi Flegrei (Italy), where we forecast future vents on an onshore semiannular belt located between 2.3 and 4.2 km from the caldera center. Our approach offers a mechanical explanation for the vent migration over time at Campi Flegrei and at many calderas worldwide and may be applicable to volcanoes of any type.

Calderas are impressive volcanic depressions commonly produced by major eruptions. Equally impressive is the uplift of the caldera floor that may follow, dubbed caldera resurgence, resulting from magma accumulation and accompanied by minor eruptions. Why magma accumulates, driving resurgence instead of feeding large eruptions, is one of the least understood processes in volcanology. Here we use thermal and experimental models to define the conditions promoting resurgence. Thermal modelling suggests that a magma reservoir develops a growing transition zone with relatively low viscosity contrast with respect to any newly injected magma. Experiments show that this viscosity contrast provides a rheological barrier, impeding the propagation through dikes of the new injected magma, which stagnates and promotes resurgence. In explaining resurgence and its related features, we provide the theoretical background to account for the transition from magma eruption to accumulation, which is essential not only to develop resurgence, but also large magma reservoirs.

Many calderas show repeated unrest over centuries. Though probably induced by magma, this unique behaviour is not understood and its dynamics remains elusive. To better understand these restless calderas, we interpret deformation data and build thermal models of Campi Flegrei caldera, Italy. Campi Flegrei experienced at least 4 major unrest episodes in the last decades. Our results indicate that the inflation and deflation of magmatic sources at the same location explain most deformation, at least since the build-up of the last 1538 AD eruption. However, such a repeated magma emplacement requires a persistently hot crust. Our thermal models show that this repeated emplacement was assisted by the thermal anomaly created by magma that was intruded at shallow depth ~3 ka before the last eruption. This may explain the persistence of the magmatic sources promoting the restless behaviour of the Campi Flegrei caldera; moreover, it explains the crystallization, re-melting and mixing among compositionally distinct magmas recorded in young volcanic rocks. Our model of thermally-assisted unrest may have a wider applicability, possibly explaining also the dynamics of other restless calderas.

During the reawaking of a volcano, magmas migrating through the shallow crust have to pass through hydrothermal fluids and rocks. The resulting magma-hydrothermal interactions are still poorly understood, which impairs the ability to interpret volcano monitoring signals and perform hazard assessments. Here we use the results of physical and volatile saturation models to demonstrate that magmatic volatiles released by decompressing magmas at a critical degassing pressure (CDP) can drive volcanic unrest towards a critical state. We show that, at the CDP, the abrupt and voluminous release of H 2 O-rich magmatic gases can heat hydrothermal fluids and rocks, triggering an accelerating deformation that can ultimately culminate in rock failure and eruption. We propose that magma could be approaching the CDP at Campi Flegrei, a volcano in the metropolitan area of Naples, one of the most densely inhabited areas in the world, and where accelerating deformation and heating are currently being observed.

Calderas are collapse structures related to the emptying of magmatic reservoirs, often associated with large eruptions from long-lived magmatic systems. Understanding how magma is transferred from a magma reservoir to the surface before eruptions is a major challenge. Here we exploit the historical, archaeological and geological record of Campi Flegrei caldera to estimate the surface deformation preceding the Monte Nuovo eruption and investigate the shallow magma transfer. Our data suggest a progressive magma accumulation from ~1251 to 1536 in a 4.6 ± 0.9 km deep source below the caldera centre, and its transfer, between 1536 and 1538, to a 3.8 ± 0.6 km deep magmatic source ~4 km NW of the caldera centre, below Monte Nuovo; this peripheral source fed the eruption through a shallower source, 0.4 ± 0.3 km deep. This is the first reconstruction of pre-eruptive magma transfer at Campi Flegrei and corroborates the existence of a stationary oblate source, below the caldera centre, that has been feeding lateral eruptions for the last ~5 ka. Our results suggest: 1) repeated emplacement of magma through intrusions below the caldera centre; 2) occasional lateral transfer of magma feeding non-central eruptions within the caldera. Comparison with historical unrest at calderas worldwide suggests that this behavior is common.