Magma Overpressure Research Articles

Aspect ratios and magma overpressures of non‐feeder dikes observed in the Miyake‐jima volcano (Japan), and fracture toughness of its upper part

AbstractWe present a new method for estimating the length and maximum thickness (aperture) of a dike from the observed opening at one dike tip. We apply the method to 15 arrested non‐feeder dikes (where the upper tip is known, the lower tip unknown) in the caldera walls of Miyake‐jima, Japan, to estimate the length‐thickness ratio, as well as the magma overpressure and fracture toughness. The calculated length‐thickness ratio ranges from 61 to 246, with an average of 136. The ratios are low because the dikes are emplaced close to the surface in comparatively compliant (soft) rocks. Using these ratios and the appropriate elastic constants, the calculated magmatic overpressures of the dikes are between 2.3 and 8.9 MPa, and the stress intensity factors between 38 and 117 MPa m1/2. All these values are within the range of typical in situ estimates, supporting the validity of this new method.

Deformations and slope instability on Stromboli volcano: Integration of GBInSAR data and analog modeling

To understand the relationship between volcano deformations, magma overpressure and flank instability, the results of analog experiments of slope instability and ground deformations, recorded by the GBInSAR system on the western flank of the Stromboli volcano during the period 2009–2011 have been analyzed. Analog experiments that consider both the external (accumulation on the slope) and the endogenous (intrusion-related bulging) trigger phenomena. The effect of accumulation on the sub-aqueous slope on the initiation of sub-marine mass movements has been analyzed. By combining the monitoring data with analog modeling, the observed deformations from the combined action of overpressure in the volcanic system and gravity have been related. The results suggest that the superficial movements observed by the GBInSAR system represent the response of the Stromboli volcano to overpressure changes in the conduit. The movements observed on the Sciara del Fuoco were slope instability phenomena where the gravitational component produced a constant creep, while changes in the magma overpressure explain why certain periods are characterized by accelerations, which induce instability on the external flanks of the crater area and in the Sciara del Fuoco and eventually promote failure of the volcanic slopes.

The association of cone–sheets and radial dykes: Data from the Isle of Skye (UK), numerical modelling, and implications for shallow magma chambers

We present field data and numerical models that aid understanding of fundamental volcano-tectonic processes of inclined sheet and dyke propagation, and more in general of volcano internal growth, under a stress field resulting from the inflation of a shallow magma chamber. Structural field data from the classical Cuillins cone–sheet complex (Isle of Skye) show that sheets have a constant average dip angle (45°), with pure dilational or hybrid shear-dilational kinematics. Our elasto-plastic finite element models, which consider the total stress field including gravity, tectonics and magma overpressure, suggest that only in the case of an inflating oblate shallow magma chamber cone–sheets can be predicted. They do not develop with spherical or prolate magma chambers and/or under deflation. Radial dykes dominate beyond a critical distance of 1–1.2 diameters of the magma chamber, whilst cone–sheets are confined immediately above it. These results may be used to infer, from observations on the sheet and dyke pattern at surface, the geometry and tensional state of magma chambers under active volcanoes.

Complex plumbing of monogenetic scoria cones: New insights from the Lunar Crater Volcanic Field (Nevada, USA)

The complexity of monogenetic basaltic eruptions is largely governed by the shallow plumbing of the volcanoes that develop in the upper hundreds of meters of the crust, as well as within the growing edifice during the eruption. We present data pertaining to geometry and dynamics of plumbing features associated with two monogenetic volcanoes observed within in the Lunar Crater Volcanic Field (Nevada; USA) that have been eroded to varying degrees, exposing numerous intrusive bodies. The majority of the intrusive bodies observed within the eroded scoria cone and cone remnant are radial dikes and dike sets, many of which have irregular shapes. We infer that the radial patterns of the dikes resulted from magma overpressure in central conduits and from the stresses induced by the topographic load of the cones, with the regional tectonic stresses playing a negligible role. Internally, the dikes display textural features such as multiple chilled margins and/or distinct bandings of vesicle populations, suggesting multiple injections or pulses of magma originating from the vent/conduit area were incrementally added to the dikes. Pressure fluctuations within the magma columns capable of producing such features can be associated with a variety of dynamic processes including; the ascent of slugs of gas through the magma column, variations in the source pressure of the ascending magma, as well as the weight of relatively degassed magma and temporary blockages of the conduit. Irregular shapes of some dikes are likely related to the interaction between viscous intruding magma and weakly consolidated scoria cone deposits. The geometry of the dikes and the relationship to a central magma column have important implications for eruption processes, degassing patterns, and gas–melt coupling associated with monogenetic eruptions.

The shapes of dikes: Evidence for the influence of cooling and inelastic deformation

Research Article| July 01, 2012 The shapes of dikes: Evidence for the influence of cooling and inelastic deformation Katherine A. Daniels; Katherine A. Daniels † 1School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol BS8 1RJ, UK †E-mail: k.a.daniels@bristol.ac.uk Search for other works by this author on: GSW Google Scholar Janine L. Kavanagh; Janine L. Kavanagh 2School of Geosciences, Monash University, Clayton Campus, Wellington Road, Clayton, Victoria 3800, Australia Search for other works by this author on: GSW Google Scholar Thierry Menand; Thierry Menand 3Clermont Université, Laboratoire Magmas et Volcans, Université Blaise Pascal, BP 10448, F-63000 Clermont-Ferrand, France4Centre National de la Recherche Scientifique, UMR 6524, Laboratoire Magmas et Volcans, F-63038 Clermont-Ferrand, France5Institut de Recherche pour le Développement, R 163, Laboratoire Magmas et Volcans, F-63038 Clermont-Ferrand, France Search for other works by this author on: GSW Google Scholar J. Sparks R. Stephen J. Sparks R. Stephen 1School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol BS8 1RJ, UK Search for other works by this author on: GSW Google Scholar Author and Article Information Katherine A. Daniels † 1School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol BS8 1RJ, UK Janine L. Kavanagh 2School of Geosciences, Monash University, Clayton Campus, Wellington Road, Clayton, Victoria 3800, Australia Thierry Menand 3Clermont Université, Laboratoire Magmas et Volcans, Université Blaise Pascal, BP 10448, F-63000 Clermont-Ferrand, France4Centre National de la Recherche Scientifique, UMR 6524, Laboratoire Magmas et Volcans, F-63038 Clermont-Ferrand, France5Institut de Recherche pour le Développement, R 163, Laboratoire Magmas et Volcans, F-63038 Clermont-Ferrand, France J. Sparks R. Stephen 1School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol BS8 1RJ, UK †E-mail: k.a.daniels@bristol.ac.uk Publisher: Geological Society of America Received: 24 Apr 2011 Revision Received: 25 Aug 2011 Accepted: 05 Oct 2011 First Online: 08 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 © 2012 Geological Society of America GSA Bulletin (2012) 124 (7-8): 1102–1112. https://doi.org/10.1130/B30537.1 Article history Received: 24 Apr 2011 Revision Received: 25 Aug 2011 Accepted: 05 Oct 2011 First Online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Katherine A. Daniels, Janine L. Kavanagh, Thierry Menand, J. Sparks R. Stephen; The shapes of dikes: Evidence for the influence of cooling and inelastic deformation. GSA Bulletin 2012;; 124 (7-8): 1102–1112. doi: https://doi.org/10.1130/B30537.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract We provide detailed observations on the shape of dikes from well-exposed field locations in the Isle of Rum, Scotland, and Helam Mine, South Africa. The basaltic Rum dikes crop out on a smaller scale than the Helam kimberlite dikes and have a smaller length to thickness ratio (∼100:1 Isle of Rum, ∼1000:1 Helam Mine). We compare the dike thickness field measurements with the geometry predicted by elastic theory, finding best-fit models to estimate magma overpressure and regional stress gradients at the time of dike emplacement. Most of the dike shapes fit poorly with elastic theory, being too thick at the dike ends and too narrow in the middle. Even for dikes where the model fit is acceptable, the calculated overpressures and stress gradients are very large and much larger than independent estimates based on rock strength, particularly for the small-scale basaltic dikes on Rum, where calculated overpressures average 687 MPa, and calculated stress gradients average 622 MPa m−1. The Swartruggens dikes have calculated overpressures of between 4 and 40 MPa and calculated stress gradients in the range of 15–87 kPa m−1. Dike shape can be explained by a combination of host-rock inelastic deformation prior to and coeval with magma emplacement, and by magma chilling at the dike's tapering edges, which prevented its closure as magma pressure declined during emplacement; this sequence provides the most complete explanation for the mismatches between the data and the model. The permanent wedging of the dike edges due to chilling has implications for crustal magma transport and strain response in the crust due to dike emplacement. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Effects of mechanical layering of host rocks on dike growth and arrest

Many dikes intruding the layered host rock of Miyakejima and Piton de la Fournaise are arrested, segmented, and show variation in thickness. These geometric factors can be largely attributed to the mechanical layering of the host rocks that constitute these volcanoes. Some dikes are arrested at the base of stiff (high Young's modulus) rock layers, whereas others are segmented in the layers. We use finite-element models (FEM) to simulate a typical dike arrest at the base of a stiffer layer. The dike may become arrested when it reaches at the base of the stiffer layer for several reasons. First, the dike-induced tensile stress may not be high enough to break the rock. Second, the stiff layer may act as a “stress barrier”. Third, the material toughness of the layers may be unfavorable for vertical dike propagation and result either in dike arrest or dike deflection. When the magmatic overpressure (driving pressure) is sufficiently high dike can intrude into the overlying layer. Some deflected dikes resume their vertical propagation, thereby generating offset segments. The layering of a volcano thus commonly controls the dike propagation paths and if and where the dikes become arrested. Our measurements show that the dikes in these volcanoes tend to be comparatively thick where they dissect “soft” pyroclastic layers, and thin where they dissect stiff lava flows and sills. Numerical FEM simulations indicate that the variation of dike thickness observed in Miyakejima and Piton de la Fournaise can be broadly explained in terms of layer-stiffness differences of one or two orders of magnitude.

Imaging the multi-level magma reservoir at Mt. Etna volcano (Italy)

[1] The continuous GPS network operating on Mt. Etna with its 36 stations is currently one of the largest worldwide. The aim of this network is the evaluation of volcanic hazard and the modelling of the active sources. In this paper, we propose an in-depth analysis and modelling of continuous GPS data collected at Mt. Etna from May 2008 to December 2010. The analyzed period has been divided into four different coherent phases: 1) 14 May 2008–02 August 2008 (deflation of the entire GPS network); 2) 02 August 2008–14 June 2009 (deflation of the summit area and inflation at lower heights); 3) 14 June 2009–21 May 2010 (inflation of the entire GPS network); 4) 21 May 2010–31 December 2010 (inflation at medium and low heights and end of the inflation in the summit area). Analytical models indicate a non-uniform deformation style revealing spaced sources acting at different time on different segments of a multi-level magma reservoir. The Etnean plumbing system imaged here is depicted as an elongated magma reservoir that extends from the volcano body downwards to about 6.5 km below sea level (b.s.l.), sloping slightly towards the North-West, with storage volumes located at about 6.5, 2.0 and 0.0 km (b.s.l.). The changes in position of the modelled pressure sources during the analyzed time intervals indicate that, throughout the 2008 eruptive period, the deformation field was mostly driven by the upward migration of magma. On the other hand, the pattern of deformation recorded after the end of the eruption strongly suggests a significant contribution of the magma overpressure generated by the gas boiling, thus outlining the importance of volatiles content in magma.

Insights of dyke emplacement mechanics from detailed 3D dyke thickness datasets

Abstract: Dyke thickness datasets offer new insights into the detailed 3D geometry of dyke swarms and an exceptional opportunity to evaluate theoretical emplacement models. The Swartruggens kimberlite dyke swarm extends over 7 km along strike and intrudes a dolerite, quartzite, shale and andesitic lava succession. The Star kimberlite dykes cut shales and sandstones, intersect a large dolerite sill and extend 15 km along strike. Both dyke swarms comprise anastomosing en echelon segments, each several hundred metres long. In total 1532 Swartruggens dyke thickness measurements were taken, to 750 m below the surface over a 250 m depth range, and 3354 Star dyke thickness measurements were taken over a 520 m depth range. The Swartruggens dyke thicknesses are 0.05–1.95 m (mean 0.64 m), whereas the Star kimberlites range from <0.01 to 1.6 m thick (mean 0.40 m). Two-dimensional models of elastic cracks in a homogeneous, isotropic material subject to constant magmatic over-pressure poorly describe the dyke thickness variation, which is complex and varies along both breadth and length. The Swartruggens dyke segments thicken toward their terminations along strike, which represent regions where stresses were focused. Towards the surface, rock deformation is increasingly difficult to recover as inelastic processes such as host-rock brecciation, stoping, and magma solidification become important.

Magmatic-metering controls the stopping and restarting of eruptions

[1] We use time-series of magma efflux and GPS-derived surface deformation observations to constrain the transfer of compressible magma within the crustal plumbing of the Soufriere Hills volcano for three cycles of effusion and pause. Our system model has two vertically-stacked spherical chambers. Deep melt supply to the system is constrained to be continuous and steady, yielding a rate of 1.2 m3/s, which fixes the geometry of the dual interconnected chambers to depths of about 5 and 19 km. The eruptive volume change of the shallow chamber is in-phase and an order of magnitude smaller than the deep chamber. Significantly, the shallow chamber seems to control the periodic system behavior: surface magma efflux resumes when the shallow chamber reinflates to its initial threshold pre-eruptive volume (triggering re-opening of an eruptive feeder dike), and ceases when it has lost 14–22 Mm3 (106 m3) of its volume (sealing the conduit and staunching magma flow). These observations are consistent with eruption re-initiation and re-cessation controlled by magma overpressure thresholds.

Conditions for the arrest of a vertical propagating dyke

Magma ascent towards the Earth’s surface occurs through dyke propagation in the vast majority of cases. We investigate two purely mechanical effects unrelated to cooling or solidification that lead to the arrest of propagation, so that no eruption occurs. The first is that the input of magma from the source is not maintained continuously, such that a fixed volume of magma is released. Laboratory experiments show that, in this case, the dyke stops at a finite distance from the source. This behaviour is specific to the fracturing process in 3-D. We derive a relationship for the minimum magma volume required for an eruption as a function of magma buoyancy and source depth. When large magma volumes are available, eruption may also be prevented by a thick low density layer in the upper crust. Numerical studies of dyke propagation show that the dyke continues to rise even though it is negatively buoyant. Magma accumulates in a swollen nose region at the interface between the low density layer and the dense basement. Magma overpressure is largest at this interface and increases with increasing penetration into the upper layer. It may become large enough to induce horizontal fractures in the dyke walls and lateral intrusion of a sill, which prevents eruption. This requires that the thickness of the low density layer exceeds a threshold value that depends on the density contrast between magma and host rock. If the magma volume is smaller than a threshold value, neither sill intrusion nor eruption are possible and magma gets stored in a horizontal blade-shaped dyke straddling the interface. Scaling laws for variations of ascent rate and for the minimum magma volume allow diagnosis of a failed eruption.

Emplacement of magma at shallow depth: insights from field relationships at Allan Hills, south Victoria Land, East Antarctica

AbstractAllan Hills nunatak, south Victoria Land, Antarctica, exposes an exceptional example of a shallow depth (< 500 m) intrusive complex formed during the evolution of the Ferrar large igneous province (LIP). Dyke distribution, geometries and relationships allow reconstruction of its history and mechanics of intrusion. Sills interconnect across host sedimentary layers, and a swarm of parallel inclined dolerite sheets is intersected by a radiating dyke-array associated with remnants of a phreatomagmatic vent, where the dolerite is locally quenched and mixed to form peperite. Intrusion geometries, and lack of dominant rift-related structures in the country rock indicate that magma overpressure, local stresses between mutually interacting dykes and vertical variations of host rock mechanical properties controlled the intrusive process throughout the thick and otherwise undeformed pile of sedimentary rocks (Victoria Group). Dolerite sills connected to one another by inclined sheets are inferred to record the preferred mode of propagation for magma-carrying cracks that represent the shallow portions of the Ferrar LIP plumbing system.

Dike emplacement and flank instability at Mount Etna: Constraints from a poro-elastic-model of flank collapse

Many volcanic edifices are subject to flank failure, usually produced by a combination of events, rather than any single process. From a dynamic point of view, the cause of collapse can be divided into factors that contribute to an increase in shear stress, and factors that contribute to the reduction in the friction coefficient μ of a potential basal failure plane. We study the potential for flank failure at Mount Etna considering a schematic section of the eastern flank, approximated by a wedge-like block. For such geometry, we perform a (steady state) limit equilibrium analysis: the resolution of the forces parallel to the possible basal failure plane allows us to determine the total force acting on the potentially unstable wedge. An estimate of the relative strength of these forces suggests that, in first approximation, the stability is controlled primarily by the balance between block weight, lithostatic load and magmatic forces. Any other force (sea load, hydrostatic uplift, and the uplift due to mechanical and thermal pore-fluid pressure) may be considered of second order. To study the model sensitivity, we let the inferred slope α of the basal surface failure vary between −10° and 10°, and consider three possible scenarios: no magma loading, magmastatic load , and magmastatic load with magma overpressure. We use error propagation to include in our analysis the uncertainties in the estimates of the mechanics and geometrical parameters controlling the block equilibrium. When there is no magma loading, the ratio between destabilizing and stabilizing forces is usually smaller than the coefficient of friction of the basal failure plane. In the absence of an initiating mechanism, and with the nominal values of the coefficient of friction μ = 0.7 ± 0.1 proposed, the representative wedge will remain stable or continue to move at constant speed. In presence of magmastatic forces, the influence of the lateral restraint decreases. If we consider the magmastatic load only, the block will remain stable (or continue to move at constant speed) , unless the transient mechanical and thermal pressurization significantly decrease the friction coefficient, increasing the instability of the flank wedge for α > 5° (seaward dipping decollement) . When the magma overpressure contribution is included in the equilibrium analysis, the ratio between destabilizing and stabilizing forces is of the same order or larger than the coefficient of friction of the basal failure plane, and the block will become unstable (or accelerate), especially in the case of the reduction in friction coefficient. Finally, our work suggests that the major challenge in studying flank instability at Mount Etna is not the lack of an appropriate physical model, but the limited knowledge of the mechanical and geometrical parameters describing the block equilibrium.

Mechanics of active magmatic intraplating in the Rio Grande Rift near Socorro, New Mexico

We investigate long‐term deformation due to the Socorro Magma Body (SMB), one of the largest active intrusions in the Earth's continental crust, using interferometric synthetic aperture radar (InSAR) observations and finite element simulations. InSAR data spanning 15 years (1992–2007) indicate that the magma body is associated with a steady crustal uplift at a rate of about 2 mm yr−1. Previous work showed that while the pattern of surface uplift is consistent with an elastic inflation of a large sill‐like magma body, the SMB could not have formed via steady elastic inflation because the latter would be outpaced by magma solidification. We resolve this problem using coupled thermo‐visco‐elastic models, and place constraints on the intrusion history as well as the rheology of the ambient crustal rocks. We demonstrate that observations rule out the linear Maxwell response of the ductile crust, but are consistent with laboratory‐derived power law rheologies. Our preferred model suggests that the age of the SMB is of the order of 103 years, and that the apparent constancy of the present‐day uplift may be due to slow heat transfer and ductile deformation in a metamorphic aureole of a giant sill‐like magma intrusion, rather than due to a steady increase in the magma overpressure. The SMB is a contemporaneous example of “magmatic intraplating,” a process by which large volumes of mafic melt stall and spread at midcrustal depths due to density or rheology contrasts.

Geometric difference between non-feeder and feeder dikes

Feeder dikes bring magma to the surface; non-feeder dikes become arrested and never reach the surface. The differences, if any, between these dike types remain largely unexplored because in the field it is normally unknown if a particular dike is a feeder or non-feeder. Here we present measurements of feeder and non-feeder dikes exposed from depths of >200 m to the surface in the walls of the A.D. 2000 caldera collapse of the Miyakejima Volcano, Japan. A typical feeder thickness reaches a maximum of 2–4 m at the surface, decreases rapidly to ∼1 m at a depth of 20–40 m, and then remains constant to the bottom of the exposure. By contrast, a typical non-feeder thickness reaches a maximum of 1.5–2 m at 15–45 m below the tip, and then decreases slowly with depth to 0.5–1 m at the bottom of the exposure. We propose that free-surface effects and magmatic overpressure (driving pressure) changes during the eruption cause the overall shape of a feeder to differ from that of a non-feeder.

Local stresses, dyke arrest and surface deformation in volcanic edificesand rift zones

Field studies indicate that nearly all eruptions in volcanic edifices and rift zones are supplied with magma through fractures (dykes) that are opened by magmatic overpressure. While (inferred) dyke injections are frequent during unrest periods, volcanic eruptions are, in comparison, infrequent, suggesting that most dykes become arrested at certain depths in the crust, in agreement with field studies. The frequency of dyke arrest can be partly explained by the numerical models presented here which indicate that volcanic edifices and rift zones consisting of rocks of contrasting mechanical properties, such as soft pyroclastic layers and stiff lava flows, commonly develop local stress fields that encourage dyke arrest. During unrest, surface deformation studies are routinely used to infer the geometries of arrested dykes, and some models (using homogeneous, isotropic half-spaces) infer large grabens to be induced by such dykes. Our results, however, show that the dyke-tip tensile stresses are normally much greater than the induced surface stresses, making it difficult to explain how a dyke can induce surface stresses in excess of the tensile (or shear) strength while the same strength is not exceeded at the (arrested) dyke tip. Also, arrested dyke tips in eroded or active rift zones are normally not associated with dyke-induced grabens or normal faults, and some dykes arrested within a few metres of the surface do not generate faults or grabens. The numerical models show that abrupt changes in Young's moduli(stiffnesses), layers with relatively high dyke-normal compressive stresses (stress barriers), and weak horizontal contacts may make the dyke-induced surface tensile stresses too small for significant fault or graben formation to occur in rift zones or volcanic edifices. Also, these small surface stresses may have no simple relation to the dyke geometry or the depth to its tip. Thus, for a layered crust with weak contacts, straightforward inversion of surface geodetic data may lead to unreliable geometries of arrested dykes in active rift zones and volcanic edifices.

Effects of wall-rock elasticity on magma flow in dykes during explosive eruptions

Abstract Magma flow during explosive volcanic eruptions has been described assuming rigid conduits with simple cylindrical or planar geometries. Here we study the dynamics of explosive volcanic flows to take account of the role of elastic deformation of the conduit influenced by local magmatic pressure. Three cases are investigated: a dyke with elliptical cross-section, a cylindrical conduit and a deep dyke connected to a shallow cylinder. The model CPIUC (Macedonio et al., 2005) was used for simulations and generalized to account for elastic deformations of the conduit cross-section area due to magmatic overpressure. Fragmentation level is typically deeper in a dyke than in a cylinder. For flows in wide dykes pressure at the fragmentation depth can be lower than the surrounding lithostatic pressure by several tens of MPa, indicating that the wall-rocks of the dyke will be unstable, constraining the dyke width and eventually blocking the eruption. On the other hand, when the fragmentation level is shallow the corresponding lithostatic pressure is not large enough to close the dyke and eruptions from wide dykes are possible. The behaviour changes drastically when we assume the conduit is a dyke at depth that evolves to a cylinder near the surface. In this case even very wide dykes can be stable because the fragmentation level moves into the cylindrical region where deformation is negligible.

Numerical simulation of stress induced piezomagnetic fields at Etna volcano

3-D magneto-elastic models based on finite element method (FEM) have been developed to compute piezomagnetic fields caused by magmatic overpressure sources. We solved separately (i) the elastostatic equation for the stress field and (ii) the coupled Poisson's equation for magnetic potential field. The numerical computations were focused on a more realistic modelling of Etna volcano, where remarkable magnetic changes have been observed during eruptive events. We evaluated the effects of topography and medium heterogeneities considering different multilayered crustal structures constrained by seismic tomography and geological evidences. When topography and magneto-elastic heterogeneities are included in the numerical models, the piezomagnetic field changes show significant deviations from the homogeneous half-space solution. Generally, it was found that the analytical model overestimates the expected magnetic changes. The numerical models of the piezomagnetic field showed significant differences in presence of strong magnetization contrasts and in proximity of steep cliffs, that is, at the volcano summit. The FEM approach presented here allows considering a picture of a fully 3-D model of Etna volcano, which could advance the reliability of model-based assessments of magnetic observations.

Organization of volcanic plumbing through magmatic lensing by magma chambers and volcanic loads

The development of discrete volcanic centers reflects a focusing of magma ascending from the source region to the surface. We suggest that this organization occurs via mechanical interactions between magma chambers, volcanic edifices, and dikes and that the stresses generated by these features may localize crustal magma transport before the first eruption occurs. We develop a model for the focusing or “lensing” of rising dikes by magma chambers beneath a free surface, and we show that chambers strongly modulate dike focusing by volcanic edifices. We find that the combined mechanical effects of chambers, edifice loading, and dike propagation are strongly coupled. Chambers deeper than ∼20 km below the surface with magmatic overpressure in the range of 20–100 MPa should dominate dike focusing, while more shallow systems are affected by both edifice and chamber focusing.

The role of magma chamber-fault interaction in caldera forming eruptions

This paper examines the role of the position and orientation of a regional fault in the roof of a magma chamber on stress distribution, mechanical failure, and dyking using 2D finite element numerical simulations. The study pertains to the magma chamber behavior in the relatively short time intervals of several hundreds to thousand of years. The magma chamber is represented as an elliptical inclusion (eccentricity, a/b = 0.12) at a relative depth, H/a, of 0.9. The fault has a 45° dip and is represented by a frictionless fracture. The temperature field in the host rock is calculated assuming a quasisteady-state thermal regime that develops through periodic episodes of magma supply. The rheology of the surrounding rocks is treated using viscoelasticity with temperature activated strain-rate dependent viscosity. Strain weakening of the rocks in the ductile zone is described within the frame of the Dynamic Power Law model . The magma pressure is coupled with the deformation of the rock mass hosting the chamber, including the fault. The variation of magma pressure in response to magma supply and chamber deformation is calculated in the elastic and viscoelastic regimes. The latter corresponds to slow filling, while the former represents a filling time much less than the viscous relaxation time scale. The resulting “equation of state” for the magma chamber couples the magma pressure with the chamber volume in the elastic regime, and with the filling rate for the viscoelastic regime. Analysis of stresses is used to predict dyke propagation conditions, and the mechanical failure of the chamber roof for different fault positions and magma overpressures. Results show that an outward dipping fault located on the periphery of the chamber roof hinders the propagation of dykes to the surface, causing magma to accumulate under the footwall of the fault. At high to moderate overpressures (30–40 MPa), the fault causes localized shear failure and chamber roof collapse that might lead to the first stage of a caldera-forming eruption.

A model for episodic degassing of an andesitic magma intrusion

Episodic magmatic degassing has been observed at numerous volcanoes, especially those of intermediate composition. It can span timescales from years to decades. Here we propose a physical model for the degassing of a shallow magma intrusion to explain this phenomenon. The magma cools by convection, which leads to melt crystallization, volatile exsolution, and magma overpressure. When the pressure reaches a critical value, wall rocks fracture and the exsolved gas escapes. The intrusion then returns to the initial lithostatic pressure and a new cooling‐crystallization‐degassing cycle occurs. A series of such cycles leads to episodic degassing. The trend and timescale of the degassing process are mainly governed by magma cooling. Two degassing regimes are exhibited: an early phase with a high frequency of gas pulses and a later phase with a lower gas pulse frequency. The transition between these two regimes is caused by the viscosity increase when the magma crystallinity exceeds the crystal percolation threshold. We find that the time to this transition is dependent on magma volume, to a first approximation. Where observations are available from sustained geochemical surveillance, the model provides constraints on key aspects of the subsurface magmatic system, with estimation of the volume of an intrusion and tensile strength of the surrounding rocks. It therefore represents a relevant tool for volcanic surveillance and hazard assessment.