Consequences of volcano sector collapse on magmatic storage zones: insights from numerical modeling

International audienceMajor volcano flank collapses strongly affect the underlying magmatic plumbing system. Here, we consider the magma storage zone as a liquid pocket embedded in an elastic medium, and we perform numerical simulations in two-dimensional axisymmetric geometry as well as in three dimensions in order to evaluate the consequences of a major collapse event. We quantify the pressure decrease induced within and around a magma reservoir by a volcano flank collapse. This pressure reduction is expected to favor replenishment with less evolved magma from deeper sources. We also estimate the impact of the magma pressure decrease, together with the stress field variations around the reservoir, on the eruptive event associated with the edifice failure. We show that, for a given magma reservoir geometry, the collapse of a large strato-volcano tends to reduce the volume of the simultaneous eruption; destabilization of large edifices may even suppress magma emission, resulting in phreatic eruptions instead. This effect is greater for shallow reservoirs, and is more pronounced for spherical reservoirs than for vertically-elongated ones. It is reduced for compressible magmas containing a large amount of volatiles. Over a longer time scale, the modification of pressure conditions for dyke initiation at the chamber wall may also explain an increase in eruption rate as well as an apparent change of magma storage location

Dynamics of magmatic intrusion: what can we learn from the comparison of analog and numerical models?

This study investigates the dynamics of magmatic intrusions based on the joint analysis of analog and numerical models. By injecting different fluids from the bottom of a solidified gelatin block, we simulate the propagation of magmatic intrusions through the crust and record their shapes, trajectories, and velocity as they rise towards the surface. Additionally, we make use of a 2D fluid-filled crack propagation model constrained by our experimental observations. The numerical simulations demonstrate that our viscous fluid-filled crack experiments, conducted with silicon-oil injections, propagate in the same regime as typical basaltic intrusions. The comparison between analog and numerical results allow us to define the domain of validity of the numerical model and its limit of applicability. This study provides new insights into the processes that control the propagation of magmatic intrusions and our ability to reproduce them using analog and numerical models

Multiple effects of ice load changes and associated stress change on magmatic systems

Ice retreat on volcanoes reduces pressure at the surface of the Earth and induces stress changes in magmatic systems. The consequences can include increased generation of magma at depth, increased magma capture in the crust, and modification of failure conditions of magma chambers. We review the methodology to evaluate each of these effects, and consider the influence of ongoing ice retreat on volcanoes at the Mid-Atlantic divergent plate boundary in Iceland. Evaluation of each of these effects requires a series of assumptions regarding the rheology of the crust and mantle, and the nature of magmatic systems, contributing to relatively large uncertainty in response of a magmatic system to climate warming and associated ice retreat. Pressure release melting due to ice cap retreat in Iceland may at present times generate a similar amount of magma as plate tectonic processes; larger than realized previously. However, new modelling shows that part of this magma may be captured in the crust, rather than being erupted. Gradual retreat of ice caps do steadily modify failure conditions at magma chambers, which is highly dependent on their geometry and depth, as well as the details of ice load variations. A model is presented where long-term ice retreat at Katla volcano decreases the likelihood of eruption, as more magma is needed in the magma chamber to cause failure than in the absence of the ice retreat

Large-scale sliding induced by sill intrusions at Réunion Island: insights from numerical modeling

International audienceRecent structural analysis of outcrops at Piton des Neiges, the eroded shield volcano of La Réunion Island, show that co-and/or post-intrusive sliding occurs at sills, located at deep level, along a detachment. We performed a 2D numerical study to quantify the perturbation induced by sill injection within a volcanic edifice. Magma is considered as an inviscid and pressurized fluid injected in an initially stable edifice under gravity-controlled extension. Displacements along the detachment are controlled by a friction law. We show that sill injection always induces tangential displacements. This effect is strongly increased when sills are emplaced along a detachment rather than in an homegeneous edifice. In this case, the magma injection produces extension behind the intrusion and potentially ends up in a large-scale flank collapse. Sill injections can thus explain the co-intrusive shear deformation observed at Piton des Neiges and the tangential displacements measured at Piton de la Fournaise during the 2007 eruption

Large-scale sliding induced by sill intrusions at Réunion Island: insights from numerical modeling

International audienceRecent structural analysis of outcrops at Piton des Neiges, the eroded shield volcano of La Réunion Island, show that co-and/or post-intrusive sliding occurs at sills, located at deep level, along a detachment. We performed a 2D numerical study to quantify the perturbation induced by sill injection within a volcanic edifice. Magma is considered as an inviscid and pressurized fluid injected in an initially stable edifice under gravity-controlled extension. Displacements along the detachment are controlled by a friction law. We show that sill injection always induces tangential displacements. This effect is strongly increased when sills are emplaced along a detachment rather than in an homegeneous edifice. In this case, the magma injection produces extension behind the intrusion and potentially ends up in a large-scale flank collapse. Sill injections can thus explain the co-intrusive shear deformation observed at Piton des Neiges and the tangential displacements measured at Piton de la Fournaise during the 2007 eruption

Mexico City Subsidence Measured by InSAR Time Series: Joint Analysis Using PS and SBAS Approaches

International audienceIn multi-temporal InSAR processing, both the Permanent Scatterer (PS) and Small BAseline Subset (SBAS) approaches are optimized to obtain ground displacement rates with a nominal accuracy of millimeters per year. In this paper, we investigate how applying both approaches to Mexico City subsidence validates the InSAR time series results and brings complementary information to the subsidence pattern. We apply the PS approach (Gamma-IPTA chain) and an ad-hoc SBAS approach on 38 ENVISAT images from November 2002 to March 2007 to map the Mexico City subsidence. The subsidence rate maps obtained by both approaches are compared quantitatively and analyzed at different steps of the PS processing. The inter-comparison is done separately for low-pass (LP) and high-pass (HP) filtered difference maps to take the complementarity of both approaches at different scales into account. The inter-comparison shows that the differential subsidence map obtained by the SBAS approach describes the local features associated with urban constructions and infrastructures, while the PS approach quantitatively characterizes the motion of individual targets. The latter information, once related to the type of building foundations, should be essential to quantify the relative importance of surface loads, surface drying and drying due to aquifer over-exploitation, in subsoil compaction

Transport et stockage de magma: suivi par interférométrie radar satellitaire et modélisation.

In volcanic hazard assessment, it is crucial to be able to detect, as early as possible, shallow magma emplacement as well as magma migration towards the Earth's surface. Then it becomes important to be able to foresee the future eruption location, its amplitude and its potential impact on the volcanic edifice stability. In this manuscript, I present my research concerning magma migration spatio-temporal tracking by satellite radar remote sensing as well as development of models taking into account the mechanical coupling between a fluid, the magma, and a solid, the encasing medium. These models aim at providing a way to jointly interpret various observations, among which deformation data, in order to obtain predictive information of magma progression towards the surface.Dans une perspective de gestion de l'aléa volcanique, il est essentiel d'être capable de détecter, suffisamment tôt, un apport superficiel de magma et sa migration éventuelle vers la surface. Il est ensuite important de pouvoir prévoir la localisation de l'éruption à venir, son amplitude et le risque de déstabilisation de l'édifice volcanique associé. Dans ce manuscrit, je mets en perspective mes travaux de recherche qui concernent, à la fois, le suivi spatio-temporel de la migration du magma par mesure radar satellitaire et le développement de modèles mécaniques dynamiques prenant en compte le couplage entre un fluide, le magma, et un solide, le milieu encaissant. Ces modèles ont pour objectif de permettre l'interprétation conjointe de différents observables, dont les mesures de déformation, en vue d'obtenir une information prédictive sur la progression du magma en profondeur

Influence of pre-existing volcanic edifice geometry on caldera formation

Volcanic edifice construction at the Earth's surface significantly modifies the stress field within the underlying crust with two main implications for caldera formation. First, tensile rupture at the Earth's surface is favored at the periphery, which enables ring fault formation. Second, edifice formation amplifies the amount of pressure decrease occurring within a magma reservoir before the eruption stops. Taking into account both of these effects, caldera formation can be initiated during a central eruption of a preexisting volcano even when assuming elastic behaviour for the surrounding crust. Providing the roof aspect ratio is small enough, conditions for caldera formation by reservoir withdrawal can be reached whatever the reservoir shape is. However ring fault initiation is easier for laterally elongated reservoirs. Citation: Pinel, V. (2011), Influence of pre-existing volcanic edifice geometry on caldera formation, Geophys. Res. Lett., 38, L11305, doi: 10.1029/2011GL047900

Some consequences of volcanic edifice destruction for eruption conditions

International audienceDestruction of a volcanic edifice by landslides or phreatic explosions unloads the upper crust. Induced changes of stress field around, and of magmatic pressure within, a magma reservoir are investigated with an analytical model for the deformation of a liquid-filled cavity within an elastic half-space. Unloading affects the reservoir pressure, and hence the net result depends on how the liquid-filled reservoir responds to a change of remote stress. Magma compressibility is taken into account and may dampen changes of internal pressure in small volatile-rich reservoirs. The main consequence of edifice destruction is a decrease of magmatic pressure and stresses on the reservoir walls. In some cases, this may be responsible for dyke closure at the reservoir walls, which stops magma withdrawal and may prevent eruption. If an eruption does occur, edifice destruction affects the volume of magma erupted. Depending on edifice size, magma reservoir size and depth, the erupted volume may be smaller or larger than that which would be erupted with no damage to the edifice. These results suggest that major phreatic explosions may prevent magmatic eruptions

Eau et parasites (Giardia et Cryptosporidium) (les difficultés de gestion des risques hydriques en France)

Le risque parasitaire d origine hydrique n est pas uniquement un problème des pays en développement comme l ont montré de récentes épidémies survenues dans les pays industrialisés. Les parasites les plus souvent impliqués étant Giardia et Cryptosporidium spp. La gestion de ce risque doit donc resté un sujet de préoccupation majeur. Celle-ci étant d autant plus compliquée que plusieurs difficultés se posent. D une part, les indicateurs couramment utilisés pour établir la qualité de l eau ne permettent pas toujours d évaluer le risque parasitaire, d autre part la mise en évidence des formes parasitaires dans l eau est compliquée, onéreuse et difficilement interprétable. Et enfin, les formes infestantes résistent aux traitements classiques de désinfection chimique de l eau. La prévention du risque, une amélioration des connaissances ainsi que l évaluation de méthodes simples et efficaces de traitement sont donc bien des sujets d actualité.GRENOBLE1-BU Médecine pharm. (385162101) / SudocSudocFranceF