Geology and morphostructural evolution of Piton de la Fournaise

International audienceThe morphology of Piton de la Fournaise volcano results from the succession of construction, destruction and deformation processes that occurred since at least 530 ka. The chaotic surface of the gently dipping submarine flanks indicates that volcaniclastic deposits related to massive flank landslides and erosion cover most of the submarine flanks. Only a few seamounts like Cône Elianne and the submarine continuation of the rift zones are built by lava flows. In the subaerial domain, Piton de la Fournaise exhibits deeply incised canyons evidencing intense erosion and eastward verging scarps whose origin is still controversial. The different interpretations invoking flank landslides and/or summit collapse calderas are summarized. Geological data indicate a twofold construction of Piton de la Fournaise. Between 530 and 60 kyrs, the volcanic centre located in the current Plaine des Sables led to the building of the western part of the massif. The volcanic centre migrated eastwards to its current location, possibly at 60–40 kyrs. Then Piton de la Fournaise experienced caldera collapses and recurrent phreatomagmatic eruptions especially between 4880 and 2340 yr BP as evidenced by the Bellecombe ash deposit. Most of the recent volcanic activity is now currently focused restricted inside the Enclos Fouqué caldera where lava flow accumulation and rare explosive events built the 400-m-high Central Cone

Croissance des volcans: l'effet de la mise en place des systèmes intrusifs

International audienceThe contribution of intrusive systems to the growth and shape of volcanoes has never been studied in a quantitative way. However, it is recognized, from eroded edifices, that intrusions contribute significantly to their volumes at depth. Moreover, permanent deformations are recorded on active volcanoes, during intrusive events. Here, we describe a preliminary approach, applicable to basaltic shield volcanoes, to evaluate the significance of intrusions on edifice shape and height

Transient self-potential anomalies associated with recent lava flows at Piton de la Fournaise volcano (Réunion Island, Indian Ocean)

International audienceSelf-potential signals are sensitive to various phenomena including ground water flow (streaming potential), thermal gradients (thermoelectric potential), and potentially rapid fluid disruption associated with vaporization of water. We describe transient self-potential anomalies observed over recent (< 9 years) lava flows at Piton de la Fournaise volcano (Reunion Island, Indian Ocean). Repeated self-potential measurements are used to determine the decay of the self-potential signals with time since the emplacement of a set of lava flow. We performed a 9 km-long self-potential profile in February 2004 in the Grand Brûlé area. This profile was repeated in July–August 2006. The second repetition of this profile crossed eight lava flows emplaced between 1998 and 2005 during seven eruptions of Piton de la Fournaise volcano. The self-potential data show clear positive anomalies (up to 330 mV) and spatially correlated with the presence of recent lava flows. The amplitude of the self-potential anomalies decreases exponentially with the age of the lava flows with a relaxation time of not, vert, similar 44 months. We explain these anomalies by the shallow convection of meteoric water and the associated streaming potential distribution but we cannot exclude possible contributions from the thermoelectric effect and the rapid fluid disruption mechanism. This field case evidences for the first time transient self-potential signals associated with recent volcanic deposits. It can be also a shallow analogue to understand the variation of self-potential signals in active geothermal areas and transient self-potential signals associated with dike intrusion at larger depths. The empirical equation we proposed can also be used to diagnose the cooling of recent lava flow on shield volcanoes

Inner structure of the Puy de Dˆome volcano: cross-comparison of geophysical models (ERT, gravimetry, muon imaging)

International audienceMuon imaging of volcanoes and of geological structures in general is actively being developed by several groups in the world. It has the potential to provide 3-D density distributions with an accuracy of a few percent. At this stage of development, comparisons with established geophysical methods are useful to validate the method. An experiment has been carried out in 2011 and 2012 on a large trachytic dome, the Puy de Dˆome volcano, to perform such a comparison of muon imaging with gravimetric tomography and 2-D electrical resistivity tomography. Here, we present the preliminary results for the last two methods. North-south and east-west resistivity profiles allow us to model the resistivity distribution down to the base of the dome. The modelling of the Bouguer anomaly provides models for the density distribution within the dome that are directly comparable with the results from the muon imaging. Our ultimate goal is to derive a model of the dome using the joint interpretation of all sets of data

Air shower simulation for background estimation in muon tomography of volcanoes

International audienceOne of the main sources of background for the radiography of volcanoes using atmospheric muons comes from the accidental coincidences produced in the muon telescopes by charged particles belonging to the air shower generated by the primary cosmic ray. In order to quantify this background effect, Monte Carlo simulations of the showers and of the detector are developed by the TOMUVOL collaboration. As a first step, the atmospheric showers were simulated and investigated using two Monte Carlo packages, CORSIKA and GEANT4.We compared the results provided by the two programs for the muonic component of vertical proton-induced showers at three energies: 1, 10 and 100 TeV. We found that the spatial distribution and energy spectrum of the muons were in good agreement for the two codes

History of Volcanic Monitoring and Development of Methods

International audienc

Histoire de la surveillance volcanique et développement des méthodes

International audienc

History of Volcanic Monitoring and Development of Methods

International audienc

Histoire de la surveillance volcanique et développement des méthodes

International audienc

Retrieving self-potential anomalies in a complex volcanic environment: an SP/elevation gradient approach

International audienceThe problem of differentiating self-potential (SP) anomalies from different origins on volcanoes is addressed using the SP/elevation gradient. Surveys on volcanoes classically show the presence of a purely hydrogeological zone on the flanks and a hydrothermal zone in the central area. The hydrogeological zone is characterized by negative SP/elevation gradients, ranging typically between 0 and –2 mV/m, whilst gradients vary in a broader range in the hydrothermal zone. Simple models show how these gradients will be perturbed in the presence of perched water tables, lateral variations of physical properties (e.g. resistivity) or by positive anomalies associated with upward hydrothermal circulations of fluids. The SP/elevation gradient is much more sensitive to the perturbations than the SP and, therefore, it is proposed that the gradient is used to enhance the anomalies. Using digital SP and topographic grids, the gradient can be computed in 2D and a regional SP map of the Piton de la Fournaise volcano (La Réunion, Indian Ocean) is used to illustrate the method. Anomalies are easily identified in the active central area, but are difficult to discern in other areas. Conversely, the anomaly zones are clearly shown on the gradient map as well as the different types of terrain (i.e. with different SP/elevation gradient values) in the hydrogeological zone. In the summit area, where the data coverage is dense, a comparison of the gradient information as a function of the SP map resolution is carried out to illustrate the sensitivity of the method to detect small-scale structures. As a technique, the gradient approach appears to be very promising for the qualitative interpretation of SP surveys in volcanic areas as well as in other environments