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The dynamics of the 2-12 January 2010 effusive eruption at Piton de la Fournaise volcano were examined through seismic and infrasound records, time-lapse photography, SO2 flux measurements, deformation data, and direct observations. Digital elevation models were constructed for four periods of the eruption, thus providing an assessment of the temporal evolution of the morphology, the volume and the extrusion rate of the lava flow. These data were compared to the continuous recording of the seismic and infrasonic waves, and a linear relationship was found between the seismic energy of the tremor and the lava extrusion rate. This relationship is supported by data from three other summit eruptions of Piton de la Fournaise and gives total volume and average lava extrusion rate in good agreement with previous studies. We can therefore provide an estimate of the lava extrusion rate for the January 2010 eruption with a very high temporal resolution. We found an average lava extrusion rate of 2.4m(3)s(-1) with a peak of 106.6m(3)s(-1) during the initial lava fountaining phase. We use the inferred average lava extrusion rate during the lava fountaining phase (30.23m(3)s(-1)) to estimate the value of the initial overpressure in the magma reservoir, which we found to range from 3.7x10(6)Pa to 5.9x10(6)Pa. Finally, based on the estimated initial overpressure, the volume of magma expelled during the lava fountaining phase and geodetic data, we inferred the volume of the magma reservoir using a simple Mogi model, between 0.25 km(3) and 0.54 km(3), which is in good agreement with previous studies.

Piton de la Fournaise (PdF), in the western Indian Ocean, is a very active hot spot basaltic volcano, with 1-2 fissure eruptions per year on average. Its magmas have been widely studied and its eruptions are well anticipated by the local seismic-geodetic monitoring network. However, no datum was yet available for its magmatic gas emissions (restricted to only eruptive phases and hardly accessible). Here we report on the first measurements of the chemical composition and mass flux of magmatic gases emitted during a PdF eruption in October 2010. Hot gases arising from different eruptive vents were remotely measured with OP-FTIR spectroscopy, using molten lava fragments as IR radiation source, which allowed simultaneous detection of H2O, CO2, SO2, HCl, HF and CO. In situ MultiGas direct analysis of H2O, CO2 and SO2 was also performed in volcanic plumes downwind of the vents. The results reveal a surprisingly H2O-rich (98-97 mol %) and CO2-poor (1.0-1.8%) magmatic gas, whose H2O/CO2 ratio however decreased while C/S (1.2-1.9), S/Cl (13-35) and Cl/F (9.7-12) ratios increased during the course of the eruption. In the same time, SO2 fluxes, measured during walking and helicopter-borne DOAS traverses under the volcanic plume, rapidly dropped from 5 to 1.8 ktons per day as the eruption intensity and lava effusion rate were lowering. From DOAS and OP-FTIR data we derive the emission rate of each gas species and the evolution of total gas flux during that event. Combining these results with available literature data for dissolved volatiles in PdF magmas allows us to draw some preliminary conclusions: (i) Contemporaneous SO2, HCl and HF fluxes and lava effusion rates are compatible with a pure syn-eruptive magma degassing process for S, Cl and F; (b) In contrast, the inferred emission rates for H2O and CO2 suggest some separate CO2 bubble transfer and a water contribution from the hydrothermal system to H2O degassing (steam entrainment during dyke propagation; (c) The changes in gas composition over time is consistent with lower external fluid addition and increasing proportion of truly magmatic volatiles in gas emissions, even though the eruption was decreasing in intensity; and (d) It remains that PdF hot spot mantle source is confirmed to be water-dominated, instead of CO2-rich. Further gas measurements during future PdF eruptions and their cross-correlation with geophysical signals should provide us with increased insight into the conditions of magma ascent and degassing during dyke propagation through the volcanic edifice.

International audience; The dynamics of the 2-12 January 2010 effusive eruption at Piton de la Fournaise volcano were examined through seismic and infrasound records, time-lapse photography, SO 2 flux measurements, deformation data, and direct observations. Digital elevation models were constructed for four periods of the eruption, thus providing an assessment of the temporal evolution of the morphology, the volume and the extrusion rate of the lava flow. These data were compared to the continuous recording of the seismic and infrasonic waves, and a linear relationship was found between the seismic energy of the tremor and the lava extrusion rate. This relationship is supported by data from three other summit eruptions of Piton de la Fournaise and gives total volume and average lava extrusion rate in good agreement with previous studies. We can therefore provide an estimate of the lava extrusion rate for the January 2010 eruption with a very high temporal resolution. We found an average lava extrusion rate of 2.4 m 3 s −1 with a peak of 106.6 m 3 s −1 during the initial lava fountaining phase. We use the inferred average lava extrusion rate during the lava fountaining phase (30.23 m 3 s −1) to estimate the value of the initial overpressure in the magma reservoir, which we found to range from 3.7×10 6 Pa to 5.9×10 6 Pa. Finally, based on the estimated initial overpressure, the volume of magma expelled during the lava fountaining phase and geodetic data, we inferred the volume of the magma reservoir using a simple Mogi model, between 0.25 km 3 and 0.54 km 3 , which is in good agreement with previous studies.

International audience; Subsurface airflow in the unsaturated zone of the soil has been extensively investigated in a variety of disciplines such as mining, nuclear waste or agriculture science. In volcanology, the recent discovery of subsurface airflow close to the terminal cone of Piton de La Fournaise volcano (La Réunion Island, France) provides for the first time insights into the convective behavior of air within the unsaturated layer [1]. The characteristics of the aerothermal system, its occurrence in other volcanoes, its ability to transport heat during quiescent periods and the perturbation of this system before eruptions are the key questions we want to address following this discovery. In this study, we present observations of subsurface convective airflow within opened fractures located at the summit of Miyakejima and Piton de la Fournaise volcanoes from anemometric and temperature data. Two anemometers and thermocouples were placed at the surface and at the center of the fracture at two-meter depth during a diurnal cycle. Six thermocouples also measured the temperature at 1 meter-depth, on a profile set perpendicularly to the fracture. Finally, a thermal camera was used to make punctual measurements of the surface temperature of the fracture. At Miyakejima, two surveys were realized in winter 2010 and summer 2011. During the winter, mild air exit was detected from the fracture with a central vertical velocity of 20 to 50 cm/s. The temperature of the site was constant during the diurnal cycle (~ 22°C), leading to a maximum temperature contrast of 15°C between the fracture and the atmosphere just before sunrise. During summer, a different hydrodynamic behavior was observed: Air inflow was detected during the whole diurnal cycle with a mean velocity of 20 cm/s. The temperature of the fracture followed the temperature of the atmosphere at 2 meters-depth. In the case of Piton de la Fournaise volcano, the same convective behavior was observed at two different fractures during winter 2008 and summer 2010. Moreover, the velocities and temperature contrast between the fracture and the atmosphere were close to the ones recorded at Miyakejima. Finally, the temperature profiles realized across the fractures and confirmed by the infrared thermography data allowed us to define the convective patterns. This study represents the first detection and characterization of air convection at a seasonal scale within fractures on volcanoes. It constitutes a preliminary step to further investigations dedicated to the understanding of the perturbation of such systems before eruptions. [1] Antoine R., Baratoux D., Rabinowicz M., Fontaine F.J., Bachèlery P., Staudacher T., Saracco G., Finizola A., Thermal infrared images analysis of a quiescent cone on Piton de La Fournaise volcano: Evidence for convective air flow within an unconsolidated soil, Journal of Volcanology and Geothermal Research, Volume 183, Issues 3-4, 2009, Pages 228-244.

International audience; Volcanic eruptions impact on societal risk, and volcanic hazard assessment is a necessary ingredient for decision-makers. However, the prediction of volcanic eruptions remains challenging due to the complexity and the non-linearity of volcanic processes. Identified forerunners such as increasing seismicity or deformation of the volcanic edifice prior to eruption are not deterministic. In this study, we use statistical methods to identify and discriminate precursory patterns to eruptions, on three sets of observables of Piton de la Fournaise volcano. We analyzed the short-term (i.e. the inter-eruptive period) time series of the seismicity rate, the deformation and the seismic velocity changes (deduced from seismic noise cross-correlations) over the period 1999-2006, with two main goals. First, we characterize the average pre-eruptive time patterns before 22 eruptions using superposed epoch analysis for the three observables. Using daily rate values, we resolve (1) a velocity change within 100-50 days from the eruptions onsets, then a plateau value up to eruption onset; (2) a power law increase in seismicity rate from noise level 15-10 days before eruption time; (3) an increase of displacement rate on the eruption day. These results support a three step mechanism leading to magma transfers toward the surface. Second we use pattern recognition techniques and the formalization of error diagrams to quantify the predictive power of each forerunner either as used independently or as combined to each other. We show that when seismicity rate alone performs the best prediction in the failure to predict versus alarm duration space, the combination of the displacement and seismicity data reduces the false alarm rate. We further propose a tool which explores the prediction results in order to optimize prediction strategy for decision-makers, as a function of the risk value.

International audience

International audience; We report on the detection of air convection in a couple of quasi circular cavities forming the 300 years old volcanically inactive cone of Formica Leo (Piton de la Fournaise, Reunion Island) [1]. Infrared thermal images of the cone have been acquired in 2006 from a hand held camera at regular time interval during a complete diurnal cycle. During night and dawn, the data display hot rims and cold centers. Both the conductivity contrasts of the highly porous soils filling the cavities and their 30° slopes are unable to explain the systematic rim to center temperature drop. Accordingly, this signal could be attributed to an air convection dipping inside the highly porous material at the center of each cavity, then flowing upslope along the base of the soil layer, before exiting it along the rims. Anemometrical and electrical data acquired in 2007 allow for the first time the direct detection of this air flow on the field: dipping gas velocities are measured at the center of the cone and self-potentials anomalies [2] generated by the humid air flow in the porous medium are detected. To quantify this process, we present 2D/3D numerical models of air convection in a sloped volcanic soil with a surface temperature evolving between day and night and taking into account electrical phenomena created by the air flow. At this present stage, this work constitutes a first step to investigate the deep structure of the active caldera of Bory-Dolomieu. The detection of the air flow at the surface could be of paramount importance for the understanding of volcanic hazards of the Reunion volcano. [1] Antoine et. al, submitted to G-Cubed [2] Darnet, PhD, Université Louis Pasteur (2003)

International audience; We report on the detection of convective air flow in the soil of a terrestrial volcano. Similar processes may operate on Mars as suggested by observations (THEMIS) at Cerberus fractures, motivating the search for subsurface heat source on Mars.

International audience