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Acocella, Valerio
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Acocella, Valerio
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Acocella, V.
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- PublicationRestrictedHow caldera collapse shapes the shallow emplacement and transfer of magma in active volcanoes(2015)
; ; ; ; ; ; ; ; ; ; ;Calderas are topographic depressions formed by the collapse of a partly drained magma reservoir. At volcanic edifices with calderas, eruptive fissures can circumscribe the outer caldera rim, be oriented radially and/or align with the regional tectonic stress field. Constraining the mechanisms that govern this spatial arrangement is fundamental to understand the dynamics of shallow magma storage and transport and evaluate volcanic hazard. Here we show with numerical models that the previously unappreciated unloading effect of caldera formation may contribute significantly to the stress budget of a volcano. We first test this hypothesis against the ideal case of Fernandina, Galápagos, where previous models only partly explained the peculiar pattern of circumferential and radial eruptive fissures and the geometry of the intrusions determined by inverting the deformation data. We show that by taking into account the decompression due to the caldera formation, the modeled edifice stress field is consistent with all the observations. We then develop a general model for the stress state at volcanic edifices with calderas based on the competition of caldera decompression, magma buoyancy forces and tectonic stresses. These factors control: 1) the shallow accumulation of magma in stacked sills, consistently with observations; 2) the conditions for the development of circumferential and/or radial eruptive fissures, as observed on active volcanoes. This top-down control exerted by changes in the distribution of mass at the surface allows better understanding of how shallow magma is transferred at active calderas, contributing to forecasting the location and type of opening fissures.283 1 - PublicationOpen AccessDyke emplacement and related hazard in volcanoes with sector collapse: the 2007 Stromboli (Italy) eruption(2008-08-13)
; ; ; ;Neri, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Catania, Catania, Italia ;Lanzafame, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Catania, Catania, Italia ;Acocella, V.; Università RomaTre; ; In February 2007, two effusive vents opened along the flank of Sciara del Fuoco (SdF) depression at Stromboli. The summit craters collapsed, obstructing the central conduit, choking the vents and increasing the deformation within SdF. Here a new vent opened, releasing the excess magmatic pressure. The eruption continued, after a summit explosion, until April. The vents were fed by laterally propagating dykes. Vent location is similar to that of the 2002-2003 eruption, fed by dykes triggering landslides, which in turn produced a tsunami. However, the 2007 eruption did not develop landslides, suggesting that their triggering also depends on other factors, (i.e. magmatic pressure).180 284 - PublicationRestrictedDike propagation in volcanic edifices: Overview and possible developments(2009-06)
; ; ;Acocella, V.; Dip. Scienze Geologiche Roma Tre. Largo S.L. Murialdo 1, 00146, Roma, Italy ;Neri, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Catania, Catania, Italia; Eruptions are fed by dikes; therefore, better knowledge of dike propagation is necessary to improve our understanding of how magma is transferred and extruded at volcanoes. This study presents an overview of dike patterns and the factors controlling dike propagation within volcanic edifices. Largely based on published data, three main types of dikes (regional, circumferential and radial) are illustrated and discussed. Dike pattern data from 25 volcanic edifices in different settings are compared to derive semi-quantitative relationships between the topography (relief, shape, height, and presence of sector collapses) of the volcano, tectonic setting (presence of a regional stress field), and mean composition (SiO2 content). The overview demonstrates how dike propagation in a volcano is not a random process; rather, it depends from the following factors (listed in order of importance): the presence of relief, the shape of the edifice and regional tectonic control. We find that taller volcanoes develop longer radial dikes, whose (mainly lateral) propagation is independent of the composition of magma or the aspect ratio of the edifice. Future research, starting from these preliminary evaluations, should be devoted to identifying dike propagation paths and likely locations of vent formation at specific volcanoes, to better aid hazards assessment.210 28 - PublicationRestrictedStructural features of an active strike-slip fault on the sliding flank of Mt. Etna (Italy)(2005-02)
; ; ;Acocella, V.; Dipartimento di Scienze Geologiche Roma TRE, Roma, Italy ;Neri, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Catania, Catania, Italia; The strike-slip Pernicana fault system (PFS) was activated along the eastern flank of Mt. Etna during an earthquake in September 2002 and, one month later, during the eruption of the NE Rift. Structural and volcanological data suggest that the PFS was activated as a result of the slide of the NE flank of Etna. This activation produced surface fracturing on walls and on paved and unpaved roads. The segments of the PFS, arranged in a right stepping en échelon configuration, show (a) an inverse proportion between length and frequency; (b) fractal behavior over scales of 10−2 –101 m, between their length, overstep and overlap; (c) consistent strike with regard to their fault array; and (d) a progressive eastward decrease in the displacement, along the smallest faults. The consistent geometric and kinematic features of the PFS, related to the sector collapse of Etna, are similar to those of faults in strike-slip settings.222 77 - PublicationRestrictedLink between major flank slip and 2002–2003 eruption at Mt. Etna (Italy)(2003-12-27)
; ; ; ; ;Acocella, V.; Dip. Scienze Geologiche Roma TRE. L. S.L. Murialdo, Roma, Italy ;Behncke, B.; Dip. Scienze Geologiche, Univ. Catania, Corso Italia, Catania, Italy ;Neri, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Catania, Catania, Italia ;D'Amico, S.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Catania, Catania, Italia; ; ; The 2002–2003 Etna eruption is studied through earthquake distributions and surface fracturing. In September 2002, earthquake-induced surface rupture (sinistral offset 0.48 m) occurred along the E-W striking Pernicana Fault (PF), on the NE flank. In late October, a flank eruption accompanied further ( 0.77 m) surface rupturing, reaching a total sinistral offset of 1.25 m; the deformation then propagated for 18 km eastwards to the coastline (sinistral offset 0.03 m) and southwards, along the NW-SE striking Timpe (dextral offset 0.04 m) and, later, Trecastagni faults (dextral offset 0.035 m). Seismicity (<4 km bsl) on the E flank accompanied surface fracturing: fault plane solutions indicate an overall ESEWNWextension direction, consistent with ESE slip of the E flank also revealed by ground fractures. A three-stage model of flank slip is proposed: inception (September earthquake), climax (accelerated slip and eruption) and propagation (E and S migration of the deformation).241 25 - PublicationRestrictedUnderstanding the origin of magmatic necks: insights from Mt. Etna volcano (Italy) and analogue models(2019-01-11)
; ; ; ; ; ; ; ; ; Magmaticnecksarecommonlyfoundinvolcanicareas,andtheyoftenexhibitahomogeneousstructurewithacylindricalshape and a diameter of up to several hundreds of metres. Their massive and uniform structure poses a space problem for their emplacement in the brittle crust. Here, we use field data and analogue models to investigate how necks may emplace at shallow levels. Field analysis focuses on characterising the geometric, structural and magmatic features of two necks outcropping in the eroded portions of Mt. Etna, Italy. These are homogeneous and massive intrusive bodies, related to a single episode of emplacement at 400–600 m below the paleosurface. We further investigated their possible emplacement mechanism through analogue models, injecting vegetable oil within (a) a flat sand pack and (b) a sand cone. Dikes form with both configurations, erupting to the surface through vents. However, dikes injected within the cone are characterised by a larger thickening at shallow levels, in correspondencewiththe vent, where a neck-like structure forms.Thissuggeststhatthe gravitational loadimposed bya volcanic edificeprovidesthemostsuitableconditionsforthedevelopmentofmagmaticneck,asthedownslopeshearstressesenhancethe deformation of the cone slope during shallow dike emplacement promoting shallow dilation and thickening of the dike. Therefore, topography should be a further factor enhancing the development of necks, in addition to those mechanisms previously proposed. Our results are consistent with natural examples of feeder dikes thickening towards the surface and dikes transitioning to necks, supporting the reliability of the proposed conceptual model.794 3 - PublicationRestrictedPropagation of dikes at Vesuvio (Italy) and the effect of Mt. Somma(2006)
; ; ; ; ; ;Acocella, V.; Dipartimento di Scienze Geologiche, Universita` Roma Tre, Rome ;Porreca, M.; Dipartimento di Scienze Geologiche, Universita` Roma Tre, Rome ;Neri, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Catania, Catania, Italia ;Massimi, E.; Dipartimento di Scienze Geologiche, Universita` Roma Tre, Rome ;Mattei, M.; Dipartimento di Scienze Geologiche, Universita` Roma Tre, Rome; ; ; ; Dikes provide crucial information on how magma propagates within volcanoes. Somma-Vesuvio (Italy) consists of the active Vesuvio cone, partly bordered by the older Mt. Somma edifice. Historical chronicles on the fissure eruptions in 1694–1944 are matched with an analytical solution to define the propagation path of the related dikes and to study any control of the Mt. Somma relief. The fissures always consisted of the downslope migration of vents from an open summit conduit, indicating lateral propagation as the predominant mechanism for shallow dike emplacement. No fissure emplaced beyond Mt. Somma, suggesting that its buttressing hinders the propagation of the radial dikes. An analytical solution is defined to describe the mechanism of formation of the laterally propagating dikes and to evaluate the effect of topography. The application to Somma-Vesuvio suggests that, under ordinary excess magmatic pressures, the dikes should not propagate laterally at depths >240–480 m below the surface, as the increased lithostatic pressure requires magmatic pressures higher than average. This implies that, when the conduit is open, the lateral emplacement of dikes is expectable on the S, Wand E slopes. The lack of fissures N of Mt. Somma is explained by its buttressing, which hinders dike propagation. Citation: Acocella, V., M. Porreca, M. Neri, E. Massimi, and M. Mattei (2006), Propagation of dikes at Vesuvio (Italy) and the effect of Mt. Somma, Geophys. Res. Lett., 33, L08301, doi:10.1029/2005GL025590.213 30 - PublicationOpen AccessFluid Geochemistry Contribution to the Interpretation of the 2011–2012 Unrest of Santorini, Greece, in the Frame of the Dynamics of the Aegean Volcanic Arc(2019-02-20)
; ; ; ; ; ; ; ; ; ; ; Tectonic and magmatic activity may couple at volcanic arcs, even though any relationship is less defined in smaller arcs, experiencing limited activity. Here we use gas geochemistry data collected during the 2011–2012 unrest at Santorini (Greece) to understand better the dynamics of the Aegean Volcanic Arc with regard to its tectonic setting. Since the most recent eruption in 1950 and before the unrest, minor seismicity and CO2 degassing (mainly from the fumaroles of Nea Kameni islet) were observed at Santorini. On January 2011, anomalous seismicity along the NE‐SW trending Kameni Line was accompanied by an inflation north of Nea Kameni. Fumarolic gas composition changed and gas release notably increased. We carried out geochemical study on both Kameni and Thera islands from January 2012 to June 2013. We repeated surveys of diffuse soil CO2 degassing and of in‐soil gas concentration, and we analyzed fumaroles and gas dissolved in thermal waters for chemical and isotopic composition. In agreement with previous studies, our geochemical data, particularly the diffuse soil CO2 flux increase, the increase of H2 content, and of CO2/CH4 and 3He/4He ratios in fumarolic gases, support geophysical data in indicating that unrest was associated with the emplacement of new mafic magma. This unrest had limited effect on the regional setting, with gas emissions focusing along the regional NE‐SW structures, without triggering by any seismic event, conversely to the 1950 eruption, which probably occurred in a frame of general tectonic reorganization of the Aegean microplate.454 53 - PublicationRestrictedStress inversions to forecast magma pathways and eruptive vent locationWhen 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.
424 2 - PublicationRestrictedUnderstanding the link between circumferential dikes and eruptive fissures around calderas based on numerical and analog models(2016)
; ; ; ; ; ; ; ; ;Active calderas are seldom associated with circumferential eruptive fissures, but eroded magmatic complexes reveal widespread circumferential dikes. This suggests that, while the conditions to emplace circumferential dikes are easily met, mechanisms must prevent them from reaching the surface. We explain this discrepancy with experiments of air injection into gelatin shaped as a volcano with caldera. Analog dikes show variable deflection, depending on the competition between overpressure, P e , and topographic unloading, P l; when P l /P e = 4.8–5.3, the dikes propagate orthogonal to the least compressive stress. Due to the unloading, they become circumferential and stall below the caldera rim; buoyancy is fundamental for the further rise and circumferential fissure development. Numerical models quantitatively constrain the stress orientation within the gelatin, explaining the observed circumferential dikes. Our results explain how dikes propagate below the rim of felsic and mafic calderas, but only in the latter they are prone to feed circumferential fissures.272 2