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As continents are stretched apart, deep rift valleys form and volcanoes can erupt both inside and outside of the valley. Numerical modelling suggests that gravitational unloading, caused by thinning of the stretched crust, can deflect rising magma towards the edges of the rift valley, causing off-rift eruptions. When continents are stretched over a long period of time, deep elongated rift valleys form at Earth’s surface and zones of ponded magma, centred beneath the rift, form at the crust–mantle boundary 1 , 2 . Ascending magma sometimes erupts within the rift valley 3 , 4 or, counterintuitively, at volcanic fields away from the rift valley that are offset by tens of kilometres from the source of magma at depth 5 , 6 , 7 , 8 . The controls on the distribution of this off-rift volcanism are unclear. Here we use a numerical model of magmatic dyke propagation during rifting to investigate why some dykes reach the surface outside the rift valley, whereas others are confined to the valley. We find that the location of magmatism is governed by the competition between tectonic stretching and gravitational unloading pressure, caused by crustal thinning and faulting along the rift borders. When gravitational unloading dominates over tectonic stretching forces, dykes ascending from the ponded magma are steered towards the rift sides, eventually causing off-rift eruptions. Our model also predicts the formation of stacked magma sills in the lower crust above the magma-ponding zone, as well as the along-rift propagation of shallow dykes during rifting events, consistent with observations of magmatism and volcanism in rift zones globally. We conclude that rift topography-induced stress changes provide a fundamental control on the transfer of magma from depth to the surface.

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. Following a large caldera creating volcanic eruption, caldera resurgence may occur as magma accumulation takes place, but this rarely leads to another a major eruption. Here, the authors using thermal and experimental models show that caldera resurgence is driven by magma viscosity contrasts.

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. Magmas may migrate through hydrothermal fluids, but magma-hydrothermal interactions are poorly understood. Here, Chiodini et al . use physical and volatile models showing that at a critical degassing pressure the release of magmatic gases can heat hydrothermal fluids triggering deformation leading to eruption.

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.

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.

Shallow magma transfer is difficult to detect at poorly monitored volcanoes. Magma transfer before the last 1538 eruption at Campi Flegrei caldera (Italy) was exceptionally tracked using historical, archeological, and geological data. Here, we extend that data set to 1650 to uncover any magma transfer during post‐eruptive subsidence. Results show two post‐eruptive subsidence phases, separated by a previously undocumented uplift during 1540–1582. Uplift highlights the pressurization of the central (∼3.5 km depth) and peripheral (∼1 km depth) pre‐eruptive sources, suggesting an aborted eruption. The subsidence events mainly require the depressurization of the central source and pressurization of a deeper magmatic layer (∼8 km depth). Therefore, despite the overall post‐eruptive deflation, after 1538 the deeper reservoir experienced continuous magma supply, with magma almost erupting between 1540 and 1582, challenging the common assumption of post‐eruptive deflation. This underlies the importance of monitoring the deeper magmatic systems, also after eruptions, to properly assess their eruptive potential. Plain Language Summary Today, volcanic activity is monitored by satellite and ground networks. However, very little is known about the pre‐ and post‐eruptive behavior of volcanoes before the instrumental era. Here, we present a unique set of archeological records related to the elevation changes at Campi Flegrei caldera (Italy) during 1515–1650, mainly focusing on its behavior after the last 1538 eruption at Monte Nuovo. After the eruption, subsidence occurred from 1538 to 1540, followed, from 1540 to 1582, by a previously unreported uplift; the latter was followed by renewed subsidence until 1650 at least. Modeling the sources responsible for the deformation, we find that, despite the depressurization of shallow sources, after 1538, a deeper magmatic layer (∼8 km depth) experienced continuous magma supply, with magma almost erupting between 1540 and 1582. This questions the common notion of post‐1538 deflation at Campi Flegrei, where the depressurization of the shallower sources masks the pressurization of the deeper magmatic system for more than a century. Key Points 20 archeological sites at Campi Flegrei show two post‐1538 eruption subsidence phases, separated by an undocumented uplift in 1540–1582 During 1540–1582, a central sill‐like source (∼3.5 km depth) transfers magma below Monte Nuovo, representing an aborted eruption From 1538 to 1650 a deeper magmatic layer (∼8 km depth) experienced continuous magma supply, also during caldera subsidence

Volcanic activity focuses along plate boundaries. However, large volcanoes are also found in intraplate settings. For these volcanoes, geodynamic processes responsible for magma generation and structural controls on magma rise and extrusion need to be evaluated. We merge original (field and remote sensing) and available (geodetic, geophysical, and petrological) data to consider the tectono‐magmatic relationships of three large intraplate volcanoes in the E‐Anatolian‐Iranian plateau; Sar'akhor (NE Iran), Damavand (Central Alborz) and Ararat (E Anatolia). In NE Iran, a Miocene‐Pliocene NW‐trending compression activated E‐W dextral faults to the NW of Sar'akhor and N‐S sinistral faults to the SE, creating an extruding wedge to the west of this volcano. Since Quaternary, NE‐trending compression inverted fault movement, hindering further block extrusion and volcanism terminated. The adakitic composition of the Sar'akhor rocks suggests post‐collisional melting of oceanic slab and/or mafic lower crust, possibly triggered by an asthenospheric rise after slab break‐off or intramantle delamination. For the active Damavand and Ararat volcanoes, available data suggest magma generation due to rising hot asthenosphere, following lithospheric delamination or slab break‐off in a transtensional environment. The features common to Sar'akhor, Damavand and Ararat allow proposing a model, where transtension focuses the rise of magma in intraplate settings overlying hot asthenosphere produced by delamination or slab break‐off. Key Points Sar'akhor volcano formed due to tectonic extrusion in Mio‐Pliocene stress field Rising hot asthenosphere due to slab break‐off or delamination generated magmas Magma extruded in transtension or transition of transpression to transtension

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.

Volcano deformation may occur under different conditions. To understand how a volcano deforms, as well as relations with magmatic activity, we studied Mt. Etna in detail using interferometric synthetic aperture radar (InSAR) data from 1994 to 2008. From 1994 to 2000, the volcano inflated with a linear behavior. The inflation was accompanied by eastward and westward slip on the eastern and western flanks, respectively. The portions proximal to the summit showed higher inflation rates, whereas the distal portions showed several sectors bounded by faults, in some cases behaving as rigid blocks. From 2000 to 2003, the deformation became nonlinear, especially on the proximal eastern and western flanks, showing marked eastward and westward displacements, respectively. This behavior resulted from the deformation induced by the emplacement of feeder dikes during the 2001 and 2002–2003 eruptions. From 2003 to 2008, the deformation approached linearity again, even though the overall pattern continued to be influenced by the emplacement of the dikes from 2001 to 2002. The eastward velocity on the eastern flank showed a marked asymmetry between the faster sectors to the north and those (largely inactive) to the south. In addition, from 1994 to 2008 part of the volcano base (south, west, and north lower slopes) experienced a consistent trend of uplift on the order of ∼0.5 cm/yr. This study reveals that the flanks of Etna have undergone a complex instability resulting from three main processes. In the long term (103–104 years), the load of the volcano is responsible for the development of a peripheral bulge. In the intermediate term (≤101 years, observed from 1994 to 2000), inflation due to the accumulation of magma induces a moderate and linear uplift and outward slip of the flanks. In the short term (≤1 year, observed from 2001 to 2002), the emplacement of feeder dikes along the NE and south rifts results in a nonlinear, focused, and asymmetric deformation on the eastern and western flanks. Deformation due to flank instability is widespread at Mt. Etna, regardless of volcanic activity, and remains by far the predominant type of deformation on the volcano.