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Open-vent volcanic activity is typically sustained by ascent and degassing of shallow magma, in which the rate of magma supply to the upper feeding system largely exceeds the rate of magma eruption. Such unbalance between supplied (input) and erupted (output) magma rates is thought to result from steady, degassing-driven, convective magma overturning in a shallow conduit/feeding dyke. Here, we characterize shallow magma circulation at Stromboli volcano by combining independent observations of heat (Volcanic Radiative Power; via satellite images) and gas (SO2, via UV camera) output in a temporal interval (from August 1, 2018 to April 30, 2020) encompassing the summer 2019 effusive eruption and two paroxysmal explosions (on July 3 and August 28, 2019). We show that, during the phase of ordinary strombolian explosive activity that preceded the 2019 effusive eruption, the average magma input rate (0.1-0.2 m3/s) exceeds the magma eruption rate (0.001-0.01 m3/s) by ∼2 orders of magnitude. Conversely, magma input and output rates converge to an average of ∼0.4 m3/s during the summer 2019 summit effusion, implying an overall suppression of magma recycling back into the feeding system, and hence of excess degassing. We find that, during the effusive eruption, the peak in SO2 emissions lags behind the thermal emission peak by ∼27 days, suggesting that magma output, feeding the lava flow field, initially dominates over magma input in the conduit. We propose that this conduit mass unloading, produced by this initial phase of the effusive eruption, leads to an overall decompression (of up to 30 Pa/s) of the shallow plumbing system, ultimately causing ascent of less-dense, volatile-rich magma batch(es) from depth, enhanced explosive activity, and elevated SO2 fluxes culminating into a paroxysmal explosion on August 28. Our results demonstrate that combined analysis of thermal and SO2 flux time-series paves the way to improved understanding of shallow magmatic system dynamics at open-vent volcanoes, and of the transition from explosive to effusive activity regimes.

The Miravalles-Guayabo caldera is located in the Guanacaste Cordillera and its genesis and evolution is related with the geodynamic setting of northwestern Costa Rica. The development of a N-S graben, and related NE-trending fracture zones, was essentially coeval with caldera forming eruptions and the multiple collapses which dismembered earlier volcanoes. The lavas of the Miravalles-Guayabo Complex (MGC) are two-pyroxene calc-alkaline andesites and basaltic andesites with subordinate high alumina basalts and rare dacites. Interstitial matrix glasses span from andesite (in basalts) to dacite (in andesite s.l.) and rhyolite (in dacite) that overlap the bulk chemical compositions of the lavas. REE patterns and spider diagrams show typical features of a subduction-related volcanism at convergent plate boundaries with spiked incompatible-element patterns, negative anomalies for Nb and Ti in spider diagrams, enrichment in LILE (with the exclusion of Sr) and LREE, and relative depletion in HFSE. The geochemical signatures of the MGC volcanic rocks are essentially consistent with recent petrogenetic models. Thermobarometric estimates suggest that basaltic and magmas equilibrated with their phenocrystic phases at 1080–1120 °C, and pressures ranges of 380–430 MPa, whereas andesitic melts are confined at 200–230 MPa and temperatures of 1050–1070 °C. In turn, amphibole-biotite dacites equilibrated at 240–290 MPa and temperatures of 920–950 °C approaching water saturated conditions (with H2O contents up to 5.7–7.2 wt%). Thermobarometry indicate that contiguous andesitic-dacitic magma lenses are stored within the subvolcanic region at depths below 10 km, whereas basaltic magmas are stored and degas at slightly higher depths (up to ~15 km). Single eruptions associated with the emplacement of lava flows seem to be related to the tapping of portions of single magma lenses during minor unrest episodes. Conversely, caldera forming eruptions involved the ejection of differentiated rhyolitic pumices (of La Ese member) and likely occurred during recharge and pressurization of this heterogeneous reservoir under critical stress conditions.

International audience; On October 1st, 2005, Santa Ana volcano (El Salvador) underwent a VEI 3 phreatomagmatic eruption after approximately one century of rest. Casualties and damages to some of the local infrastructures and surrounding plantations were followed by the evacuation of the nearby communities. The analysis of MODIS (Moderate Resolution Imaging Spectroradiometer) infrared data reveals that the main explosion was preceded by a one-year-long thermal unrest, associated to the development of a fumaroles field, located at the western rim of the summit crater lake. By combining space-based thermal flux and ground-based measurements (seismicity, sulfur emissions and lake temperatures), we suggest that the activity observed at Santa Ana between 2004 and 2005 was driven by the gradual intrusion of an undegassed magma body at a very shallow depth. Magma injection induced thermal anomalies associated with sustained degassing from the fumaroles field and promoted the interaction between the magmatic-hydrothermal system and the overlying water table. This process culminated into the VEI 3 phreatomagmatic eruption of October 2005 that strongly modified the shallow structure of the crater area. The subsequent three-years-long activity resulted from self-sealing of the fracture system and by the opening of a new fracture network directly connecting the deeper hydrothermal system with the crater lake. Our results show that satellite-based thermal data allow us to detect the expansion of the high-temperature fumarolic field. This may precede an explosive eruption and/or a lava dome extrusion. In particular, we show that thermal records can be analyzed with other geochemical (i.e. SO2 emissions) and geophysical (seismicity) data to track a shallow magmatic intrusion interacting with the surrounding hydrothermal system. This provides a remarkable support for volcano monitoring and eruption forecasting, particularly in remote areas where permanent ground data acquisition is hazardous, expensive and difficult.

In the satellite thermal remote sensing, the new generation of sensors with high-spatial resolution SWIR data open the door to an improved constraining of thermal phenomena related to volcanic processes, with strong implications for monitoring applications. In this paper, we describe a new hot-spot detection algorithm developed for SENTINEL-2/MSI data that combines spectral indices on the SWIR bands 8a-11-12 (with a 20-meter resolution) with a spatial and statistical analysis on clusters of alerted pixels. The algorithm is able to detect hot-spot-contaminated pixels (S2Pix) in a wide range of environments and for several types of volcanic activities, showing high accuracy performances of about 1% and 94% in averaged omission and commission rates, respectively, underlining a strong reliability on a global scale. The S2-derived thermal trends, retrieved at eight key-case volcanoes, are then compared with the Volcanic Radiative Power (VRP) derived from MODIS (Moderate Resolution Imaging Spectroradiometer) and processed by the MIROVA (Middle InfraRed Observation of Volcanic Activity) system during an almost four-year-long period, January 2016 to October 2019. The presented data indicate an overall excellent correlation between the two thermal signals, enhancing the higher sensitivity of SENTINEL-2 to detect subtle, low-temperature thermal signals. Moreover, for each case we explore the specific relationship between S2Pix and VRP showing how different volcanic processes (i.e., lava flows, domes, lakes and open-vent activity) produce a distinct pattern in terms of size and intensity of the thermal anomaly. These promising results indicate how the algorithm here presented could be applicable for volcanic monitoring purposes and integrated into operational systems. Moreover, the combination of high-resolution (S2/MSI) and moderate-resolution (MODIS) thermal timeseries constitutes a breakthrough for future multi-sensor hot-spot detection systems, with increased monitoring capabilities that are useful for communities which interact with active volcanoes.

On January 2008, a “silent” eruption took place on the northeast flank of Nevados de Chillan volcanic complex (Chile). The eruption had an effusive character and persisted for several months during which a small lava field (~ 0.12 km2) was emplaced. Although occurring at < 10 km from several mountain resorts, and < 70 km from Chillan city (~ 150 k inhabitants) this eruption was completely unnoticed by local authorities and was only inferred a year after, based on field reconnaissance. However, the post processing of MODIS-derived thermal images, by means of the MIROVA system (Middle Infrared Observation of Volcanic Activity), revealed that the eruptive episode produced a clear and detectable thermal signal for the whole duration of the effusion. Based on the thermal flux and a sequence of Landsat images, we suggest that the eruption was fed by three distinct segments of an ENE-oriented dike and characterized by two main effusive phases. About 1.4 Mm3 of dacitic lava have been gently erupted (mean output rate of ~ 0.075 m3 s− 1) without producing any evident manifestation (i.e. explosions, volcanic plumes, tremors, sounds) noticeable from distance. The results presented here demonstrate the capacity of thermal satellite data not only to detect but also to quantify the eruptive flux of a “silent” eruption, otherwise unreported.

The Vanuatu archipelago consists of 80 islands and hosts 5 subaerial volcanoes (Yasur, Lopevi, Ambrym, Aoba and Gaua) that have shown sign of activity during the past decade. In this contribution we provide a 15 years-long datasets (2000–2015) of the thermal activity recorded at these active volcanoes by means of MIROVA (Middle InfraRed Observation of Volcanic Activity) a new volcanic hotspot detection system based on MODIS data. The analyzed volcanoes are characterized by a spectrum of volcanic activities whose thermal signature has been tracked and carefully analyzed. These include strombolian–vulcanian explosions at Yasur, lava flows at Lopevi, lava lakes at Ambrym, surtseyan-type eruptions within the Voui crater lake of Aoba and ash-dominated eruptions with strong degassing at Gaua. The collected data reveal several details of the long term eruptive dynamics at single sites such as a monthly long pulse in thermal emissions at Yasur volcano as well as at the two active craters of Ambrym (Benbow and Marum). Heating cycles within Aoba crater lake and intermittent pressurized eruptions at Lopevi volcano has also been detected and shed light in the eruptive dynamics of the analyzed volcanoes. In addition we were able to track a two years long intensification of thermal output at Benbow crater (Ambrym) that preceded the occurrence of the first intra-caldera eruptions of this volcano since 1989. We emphasize how the data provided by MIROVA represent a new, safe and affordable method for monitoring in near-real time a large spectrum of volcanic activities taking place at Vanuatu and other volcanic areas.

Measuring the amount of magma intruding in a volcano represents one of the main challenges of modern volcanology. While in closed-vent volcanoes this parameter is generally assessed by the inversion of deformation data, in open-vent volcanoes its measurement is more complicated and results from the balance between the magma entering and leaving the storage system. In this work we used thermal and SO2 flux data, derived from satellite measurements, to calculate the magma input and output rates of Mt. Etna between 2004 and 2010. We found that during the analysed period more magma was supplied than erupted, resulting into an endogenous growth rate equal to 22.9 ± 13.7 × 106 m3 y−1. Notably, this unbalance was not constant in time, but showed phases of major magma accumulation and drainage acting within a compressible magma chamber. The excellent correlation with the inflation/deflation cycles measured by ground-based GPS network suggests the thermal, SO2 flux and deformation data, can be combined to provide a quantitative analysis of magma transport inside the shallow plumbing system of Mt Etna. Given the global coverage of satellite data and the continuous improvement of sensors in orbit, we anticipate that this approach will have sufficient detail to monitor, in real time, the endogenous growth associated to other world-wide open-vent volcanoes.

Measuring the amount of magma intruding in a volcano represents one of the main challenges of modern volcanology. While in closed-vent volcanoes this parameter is generally assessed by the inversion of deformation data, in open-vent volcanoes its measurement is more complicated and results from the balance between the magma entering and leaving the storage system. In this work we used thermal and SO2 flux data, derived from satellite measurements, to calculate the magma input and output rates of Mt. Etna between 2004 and 2010. We found that during the analysed period more magma was supplied than erupted, resulting into an endogenous growth rate equal to 22.9 ± 13.7 × 106 m3 y−1. Notably, this unbalance was not constant in time, but showed phases of major magma accumulation and drainage acting within a compressible magma chamber. The excellent correlation with the inflation/deflation cycles measured by ground-based GPS network suggests the thermal, SO2 flux and deformation data, can be combined to provide a quantitative analysis of magma transport inside the shallow plumbing system of Mt Etna. Given the global coverage of satellite data and the continuous improvement of sensors in orbit, we anticipate that this approach will have sufficient detail to monitor, in real time, the endogenous growth associated to other world-wide open-vent volcanoes.

The 2014-2015 eruption at Holuhraun has produced more than 1.5 km 3 of basaltic magma and can be considered one of the major effusive events of the last two centuries in the world. During this eruption the MIROVA system (a volcanic hot-spot detection system based on MODIS middle infrared – MIR - data) has been used to detect and locate the active portions of the lava flow(s), and to measure the heat radiated by the growing lava field. According to these data the eruption was characterized by a slow decay of the radiant power, accompanied by a change in the lava transport mechanism that shifted from open channels, at the beginning of the eruption, to lava tubes, during the last months of activity. Despite the evident evolution of lava transport mechanism, we found that the overall decreasing trend of the thermal flux was mainly controlled by the exponential decline of lava discharge rates, while the increasing insulation of the flow field had a strong impact in transporting efficiently the lava at the distal flow front(s). Our results suggest the apparent time averaged lava discharge rates ( TADR ), derived from satellite thermal data, may fluctuate around the real effusion rate at the vent, especially in the case of large lava flows emplacing in nearly flat conditions. The magnitude and frequency of these fluctuations are mainly controlled by the emplacement dynamic, (i.e. occurrence of distinct major flow units), while the transition from channel- to tube-fed lava transport mechanism play only a minor role (±30%) in the retrieval of TADR using MIR data . When the TADR values are integrated to calculate erupted lava volumes, the effects of pulsating emplacement dynamic become smoothed and the eruptive trend become more clear. We suggest that during the Holuhraun ’s eruption, as well as during many other effusive eruptions, the MIR-derived radiant flux essentially mimic the overall trend of lava discharge rates, with only a minor influence due to the emplacement style and evolving eruptive conditions. From a monitoring and operational perspective, MIROVA demonstrates to be a very valuable tool to follow and, possibly, forecast the evolution of on-going effusive eruption.

Batu Tara (Indonesia) and Tinakula (Solomon Island) are two poorly known volcanoes with morphologies and short-term eruptive activity similar to Stromboli (Italy). However, quantitative information about their long-term eruptive behaviour is limited, making the comparisons with Stromboli descriptive and based on short periods of observations. Here, we use over a decade of satellite data to measure and compare the radiant flux (2000–2017) and the SO2 mass (2004–2017) of all three volcanoes. The combined analysis of volcanic radiant power (from MODIS data) and SO2 flux (from OMI data) reveals different long-term eruptive trends and contrasting ratios of SO2/VRP. These data indicate that the eruptive mechanisms operating at each volcano are quite different. The persistent open-vent activity of Stromboli volcano is episodically interrupted by flank eruptions that drain degassed magma stored in the very shallow portion of the central conduit. In contrast, a long-lasting exponential decay of both VRP and SO2 flux observed at Batu Tara is consistent with the eruption of undegassed magma from a deep, closed magma chamber, whilst Tinakula displays multiple year-long eruptive phases, characterised by evolving gas/thermal ratios and an eruptive intensity increasing with time. Magma budget calculations for the latter volcano are consistent with eruption from a volatile-zoned magma chamber, coupled with periods of gas/magma accumulations at depth. Our results suggest that the combined analysis of satellite thermal/gas data provides a valuable tool for decrypting the long-term volcanic dynamics that could remain hidden over shorter timescales.

The thermal signature of Aso Volcano (Nakadake) during unrest episodes has been analyzed by combining the MODIS-MIROVA data set (2000–2017) with high-resolution images (LANDSAT 8 OLI and Sentinel 2) and ground-based thermal observations (2013–2017). The site of major activity (crater 1) is located at the summit of the volcano and is composed by a fumarole field (located in the South Area) and an acidic lake (replaced by a Central Pit during Strombolian phases). The volcanic radiative power (VRP) obtained by nighttime satellite data during the reference period was mainly below 3 MW. This thermal threshold marks the transition from high fumarole activity (HFA) to Strombolian eruptions (SE). However, periods characterized by sporadic phreatic eruptions (PE, eventually bearing phreatomagmatic episodes), which is the prevalent phase during unrest episodes, exhibit very low VRP values, being around 0.5 MW, or below. The statistical analysis of satellite data shows that the transition from HFA to Strombolian activity (which started on August 2014 and ceased in May 2015) occurs when VRP values are above the cited 3 MW threshold. In particular during marked Strombolian phases (November–December 2014), the radiative power was higher than 4 MW, reaching peak values up to 15.6 MW (on December 7, 2014, i.e., 10 days after the major Strombolian explosion of November 27). Conversely, ground-based measurements show that heat fluxes recorded by FLIR T440 Thermo-camera on the fumarole field of the South Area has been relatively stable around 2 MW until February 2015. Their apparent temperatures were fluctuating around 490–575 °C before the major Strombolian explosive event, whereas those recorded at the active vent, named Central Pit, reached their maxima slightly above 600 °C; then both exhibited a decreasing trend in the following days. During the Strombolian activity, the crater lake dried out and was then replenished by early July, 2016. Then, volcanic activity shifted back to phreatic–phreatomagmatic and the eruptive cycle was completed. During this period, the MIROVA system detected very few thermal alerts and the ground-based measurements were fluctuating around 1 MW. The most violent explosion occurred on October 8, 2016, and within the following weeks measured VRP were moderately above 2 MW. This is coeval with a thermal increase at the fumarole field of the South Area, with temperatures well above 300 °C. Thermal monitoring at Aso Volcano is an additional tool in volcano surveillance that may contribute to near-real-time hazard assessment.

Identifying and characterizing strombolian activity from space is a challenging task for satellite-based infrared systems. Stromboli volcano is a natural laboratory that offers a unique opportunity for refining thermal remote-sensing applications that involve transient phenomena and small to moderate hot-spots. A new simple and fast algorithm gave us the opportunity to revisit the MODIS-derived thermal output at Stromboli volcano over the last 13 years. The new algorithm includes both night-time and daytime data and shows high performance with the detection of small-amplitude thermal anomalies ( 1000 MW). The ...

We report additional radon data collected at Stromboli during 2002–2007. The whole data set of periodic measurements has been systematically analyzed to retrieve the values of background, threshold and anomaly for all the stations of the network. Maps of radon concentrations in space and time correlate with changes in volcanic activity. Higher radon emissions are essentially concentrated at summit stations and may also affect some stations located onto the summit-eastern sector of the cone (along the N60°E fracture zone), as well as two stations placed at the NE edge of Sciara del Fuoco. Most of these stations are well above their threshold values during effusive eruptions (namely 2002–2003 and 2007) due to the progressive structural adjustment of the volcano edifice coupled with the opening and resetting of the fracture network. A two-year-long timeseries of automatic 222 Rn measurements (2005–2007) was analyzed together with local environmental parameters and selected geophysical data (on seismic tremor and infrasonic puffing). The data show good correlation between radon and the latter parameters particularly during periods of high Strombolian activity (March–April 2006), and before the onset of the February 27, 2007 lava effusion (with radon emissions being well above 20,000 Bq/m 3 ). Extremely high radon emissions (up to ~ 1.7 × 10 6 Bq/m 3 ) were recorded along the fractures of the SW crater during the first two months of the 2002–2003 effusive eruption. Very high radon emissions (up to ~ 470,000 Bq/m 3 for 222 Rn, and ~ 780,000 Bq/m 3 for 220 Rn, respectively) were also detected by the automatic measurements at the summit station during June 20–early July 2007: i.e., just prior to the resuming of the Strombolian activity at summit craters. These data give us the opportunity to estimate the relative depths of the summit fractures during fracturing (being constrained between 200 and 310 m). This is in good agreement with the hypocentral depths of the so called “hybrid events” and the source of the very long period seismic signals (VLP) localized at a depth of about 500 m a.s.l. below the upper–central part of the Sciara del Fuoco.

Many effusive eruptions are characterized by effusion rates that decay exponentially with time, a trend which is generally ascribed to elastic relaxation of a deep magma chamber. Thermal emissions, detected by satellite during the A.D. 2014–2015 Barðarbunga-Holuhraun eruption (Iceland), indicate that the volume of the erupted magma and effusion rates followed an overall exponential trend that fits the observed major subsidence of the Barðarbunga caldera floor. This trend continued until a critical flow rate was reached. Hence, the subsidence slowed down and the eruption rapidly ceased, reflecting the ultimate closure of the magma path. We present a model of inelastic magma withdrawal that very closely reproduces all the observed phenomena and provides new insights into the caldera collapses and the driving pressure of other effusive eruptions.

Space-based thermal data are increasingly used for monitoring effusive eruptions, especially for calculating lava discharge rates and forecasting hazards related to basaltic lava flows. The application of this methodology to silicic, more viscous lava bodies (such as obsidian lava flows) is much less frequent, with only few examples documented in the last decades. The 2011–2012 eruption of Cordon Caulle volcano (Chile) produced a voluminous obsidian lava flow (~ 0.6 km3) and offers an exceptional opportunity to analyze the relationship between heat and volumetric flux for such type of viscous lava bodies. Based on a retrospective analysis of MODIS infrared data (MIROVA system), we found that the energy radiated by the active lava flow is robustly correlated with the erupted lava volume, measured independently. We found that after a transient time of about 15 days, the coefficient of proportionality between radiant and volumetric flux becomes almost steady, and stabilizes around a value of ~ 5 × 106 J m− 3. This coefficient (i.e. radiant density) is much lower than those found for basalts (~ 1 × 108 J m− 3) and likely reflects the appropriate spreading and cooling properties of the highly-insulated, viscous flows. The effusion rates trend inferred from MODIS data correlates well with the tremor amplitude and with the plume elevation recorded throughout the eruption, thus suggesting a link between the effusive and the coeval explosive activity. Modelling of the eruptive trend indicates that the Cordon Caulle eruption occurred in two stages, either incompletely draining a single magma reservoir or more probably tapping multiple interconnected magmatic compartments.

We present thermal measurements made by high spatial resolution ground-based (a hand-held thermal camera) and low spatial resolution space-based (MODIS) instruments for a lava flow field active during the last phase of the May–July 2003 eruption at Piton de la Fournaise (La Reunion). Multiple oblique ground-based thermal images were merged to provide full coverage of the flow-field. These were then corrected for path length attenuation and orthorectified, allowing the at-surface radiance emitted by the flow-field to be estimated. Comparison with the radiance recorded by the MODIS sensors during the eruption reveals that, for clear-sky conditions and moderate-to-low viewing angles (satellite zenith

Since 1938, Nyamulagira volcano (Democratic Republic of Congo) has operated as a classic pressurized basaltic closed system, characterized by frequent dike-fed flank eruptions. However, on June 24, 2014, an active lava lake was observed in its summit, after a period of 76 years. The small lava lake is now exposed at the bottom of a pit-crater and is rising and growing. Based on satellite-derived infrared (IR) data, SO2 fluxes and periodic field surveys, we provide evidence that the development of the lava lake was gradual and occurred more than 2 years before it was first observed in the field. Notably, this process followed the voluminous 2011-2012 distal flank eruption and was coeval with weakening of the central rock column below the summit. Hence, the opening and development of the pit-crater favoured the continuous rise of fresh magma through the central conduit and promoted the gradual "re-birth" of the Nyamulagira lava lake. Budgeted volumes of magma erupted, and magma degassed at depth indicate that the formation of the lava lake is due to the draining and refilling of a shallow plumbing system (1-2 km depth), probably in response to the rift-parallel 2011-2012 distal eruption. We thus suggest that the transition from lateral to central activity did not result from a substantial change in the magma supply rate but, more likely, from the perturbation of the plumbing system (and related stress field) associated with the distal eruption. The processes observed at Nyamulagira are not unique and suggest that rift-fissure eruptions, in addition to triggering caldera collapses or lava lake drainages, may also induce a progressive resumption of central vent activity. Current activity at Nyamulagira represents a tangible and major hazard for the population living at the base of its southern flank.

Open-conduit volcanic systems are typically characterized by unsealed volcanic conduits feeding permanent or quasi-permanent volcanic activity. This persistent activity limits our ability to read changes in the monitored parameters, making the assessment of possible eruptive crises more difficult. We show how an integrated approach to monitoring can solve this problem, opening a new way to data interpretation. The increasing rate of explosive transients, tremor amplitude, thermal emissions of ejected tephra, and rise of the very-long-period (VLP) seismic source towards the surface are interpreted as indicating an upward migration of the magma column in response to an increased magma input rate. During the 2014 flank eruption of Stromboli, this magma input preceded the effusive eruption by several months. When the new lateral effusive vent opened on the Sciara del Fuoco slope, the effusion was accompanied by a large ground deflation, a deepening of the VLP seismic source, and the cessation of summit explosive activity. Such observations suggest the drainage of a superficial magma reservoir confined between the crater terrace and the effusive vent. We show how this model successfully reproduces the measured rate of effusion, the observed rate of ground deflation, and the deepening of the VLP seismic source. This study also demonstrates the ability of the geophysical network to detect superficial magma recharge within an open-conduit system and to track magma drainage during the effusive crisis, with a great impact on hazard assessment.

Soil CO 2 flux and 222 Rn activity measurements may positively contribute to the geochemical monitoring of active volcanoes. The influence of several environmental parameters on the gas signals has been substantially demonstrated. Therefore, the implementation of tools capable of removing (or minimising) the contribution of the atmospheric effects from the acquired time series is a challenge in volcano surveillance. Here, we present 4 years-long continuous monitoring (from April 2007 to September 2011) of radon activity and soil CO 2 flux collected on the NE flank of Stromboli volcano. Both gases record higher emissions during fall–winter (up to 2700 Bq * m − 3 for radon and 750 g m − 2 day − 1 for CO 2 ) than during spring–summer seasons. Short-time variations on 222 Rn activity are modulated by changes in soil humidity (rainfall), and changes in soil CO 2 flux that may be ascribed to variations in wind speed and direction. The spectral analyses reveal diurnal and semi-diurnal cycles on both gases, outlining that atmospheric variations are capable to modify the gas release rate from the soil. The long-term soil CO 2 flux shows a slow decreasing trend, not visible in 222 Rn activity, suggesting a possible difference in the source depth of the of the gases, CO 2 being deeper and likely related to degassing at depth of the magma batch involved in the February–April 2007 effusive eruption. To minimise the effect of the environmental parameters on the 222 Rn concentrations and soil CO 2 fluxes, two different statistical treatments were applied: the Multiple Linear Regression (MLR) and the Principal Component Regression (PCR). These approaches allow to quantify the weight of each environmental factor on the two gas species and show a strong influence of some parameters on the gas transfer processes through soils. The residual values of radon and CO 2 flux, i.e. the values obtained after correction for the environmental influence, were then compared with the eruptive episodes that occurred at Stromboli during the analysed time span (2007–2011) but no clear correlations emerge between soil gas release and volcanic activity. This is probably due to i ) the distal location of the monitoring stations with respect to the active craters and to ii ) the fact that during the investigated period no major eruptive phenomena (paroxysmal explosion, flank eruption) occurred. Comparison of MLR and PCR methods in time-series analysis indicates that MLR can be more easily applied to real time data processing in monitoring of open conduit active volcanoes (like Stromboli) where the transition to an eruptive phase may occur in relatively short times.

We investigated earthquake–volcano interactions by using a network for radon monitoring at Stromboli volcano. Radon is an alpha emitting radioactive gas produced from the decay of uranium bearing rocks, soils and magmas. Its spatial and temporal variations have been regarded as precursors of earthquakes and volcanic eruptions. Here we provide evidence of how radon emissions at Stromboli can be correlated to high magnitude (ML > 5) regional earthquakes and erupted magma volumes. The Palermo earthquake of September 6, 2002 (ML = 5.6), characterised by shallow hypocentral depth (∼ 15 km) and higher number of energetic aftershocks, enhanced more efficiently postseismic dynamic triggering that may have contributed to triggering the eruptions of Mount Etna and Stromboli by the end of 2002. A viscoeslastic relaxation mechanism seems to be compatible with the onset of both eruptions. The rate of erupted magma volume at Stromboli is positively correlated with the rate of radon degassing, and suggests a possible link between magma chamber volume, gaseous transfer and dynamic response of the volcano to near field seismic triggering. Single and isolated deep earthquakes related to active subduction, such as the Salina event (ML = 5.1) of May 5, 2004, are capable of mobilising fluids (due to the passage of seismic waves at higher depths) but their dynamic effect on the fracture network and the feeding system of the volcanoes seems to be limited, likely due to the near-surface attenuation of seismic waves. It is not excluded that the coupling of two or more earthquakes of similar depth and/or higher magnitude in the near field, could play a more efficient role in the triggering process.

Nowadays there are mainly three types of detectors available for measuring radon concentrations: charcoal canisters, alpha-tracks and E-PERMs. We carried out measurements with the three types of detectors and we tested their performances over a wide range of radon concentrations, temperatures and humidities. We compared the results obtained with the different detectors and we critically evaluated the results.

International audience; After 3 years of mild gases emissions, the Ubinas volcano entered in a new eruptive phase on September 2nd, 2013. The MIROVA system (a space-based volcanic hot-spot detection system), allowed us to detect in near real time the thermal emissions associated with the eruption and provided early evidence of magma extrusion within the deep summit crater. By combining IR data with plume height, sulfur emissions, hot spring temperatures and seismic activity, we interpret the thermal output detected over Ubinas in terms of extrusion rates associated to the eruption. We suggest that the 2013–2014 eruptive crisis can be subdivided into three main phases: (i) shallow magma intrusion inside the edifice, (ii) extrusion and growing of a lava plug at the bottom of the summit crater coupled with increasing explosive activity and finally, (iii) disruption of the lava plug and gradual decline of the explosive activity. The occurrence of the 8.2 Mw Iquique (Chile) earthquake (365 km away from Ubinas) on April 1st, 2014, may have perturbed most of the analyzed parameters, suggesting a prompt interaction with the ongoing volcanic activity. In particular, the analysis of thermal and seismic datasets shows that the earthquake may have promoted the most intense thermal and explosive phase that culminated in a major explosion on April 19th, 2014. These results reveal the efficiency of space-based thermal observations in detecting the extrusion of hot magma within deep volcanic craters and in tracking its evolution. We emphasize that, in combination with other geophysical and geochemical datasets, MIROVA is an essential tool for monitoring remote volcanoes with rather difficult accessibility, like those of the Andes that reach remarkably high altitudes.

The L'Aquila seismic swarm culminated with the mainshock of April 6, 2009 (ML = 5.9). Here, we report and analyze the Large Volume Detector (LVD, used in neutrinos research) low energy traces (∼0.8 MeV), collected during the early-mid stages of the seismic sequence, together with the data of a radon monitoring experiment. The peaks of LVD traces do not correlate with the evolution and magnitude of earthquakes, including major aftershocks. Conversely, our radon measurements obtained by utilizing three automatic stations deployed along the regional NW-SE faulting system, seem to be, in one case, more efficient. In fact, the timeseries collected on the NW-SE Paganica fracture recorded marked variations and peaks that occurred during and prior moderate aftershocks (with ML > 3). The Paganica monitoring station (PGN) seems to better responds to active seismicity due to the fact that the radon detector was placed directly within the bedrock of an active fault. It is suggested that future networks for radon monitoring of active seismicity should preferentially implement this setting.

A yearlong high-resolution radon survey has been carried on at Mount Vesuvius, starting in May 1998. Radon activities were acquired by exposing charcoal canisters and track-etch detectors. Sampling stations were deployed along two major summit faults and around the caldera bottom. Volcanically-related earthquakes, with MD ≥ 2.5, may be discriminated from regional seismic events since their cumulative radon anomalies are recorded from stations located along all the above structural features. On the contrary, radon anomalies correlated to regional earthquakes, with MD ≥ 4, are essentially recorded by the sampling sites located along the two summit faults (whose roots extend deeper into the Tertiary basement rocks that underlay the volcano). Radon migration to the surface is ruled by convection within a porous medium of relatively low porosity (ϕ ≈ 10−5), suggesting that fluid motion is strongly localised along fractures. It is suggested that fluid pressure build up, followed by fluid release and migration during incipient fracturing of the porous medium, precede the onset of volcanically-induced earthquakes.

The origin of Arenal basaltic andesite can be explained in terms of fractional crystallization of a parental high-alumina basalt (HAB), which assimilates crustal rocks during its storage, ascent and evolution. Contamination of this melt by Tertiary calc-alkalic intrusives (quartz–diorite and granite, with 87 Sr / 86 Sr ratios ranging 0.70381–0.70397, nearly identical with those of the Arenal lavas) occurs at upper crustal levels, following the interaction of ascending basaltic magma masses with gabbroic–anorthositic layers. Fragments of these layers are found as inclusions within Arenal lavas and tephra and may show reaction rims (1–5 mm thick, consisting of augite, hypersthene, bytownitic–anorthitic plagioclase, and granular titanomagnetite) at the gabbro–lava interface. These reaction rims indicate that complete `assimilation' was prevented since the temperature of the host basaltic magma was not high enough to melt the gabbroic materials (whose mineral phases are nearly identical to the early formed liquidus phases in the differentiating HAB). Olivine gabbros crystallized at pressure of about 5–6 kbar and equilibrated with the parental HAB at pressures of 3–6 kbar (both under anhydrous and hydrous conditions), and temperatures ranging 1000–1100°C. In particular, `deeper' interactions between the mafic inclusions and the hydrous basaltic melt (i.e., with about 3.5 wt.% H2O) are likely to occur at 5.4 (±0.4) kbar and temperatures approaching 1100°C. The olivine gabbros are thus interpreted as cumulates which represent crystallized portions of earlier Arenal-type basalts. Some of the gabbros have been `mildly' tectonized and recrystallized to give mafic granulites that may exhibit a distinct foliation. Below Arenal volcano a zoned magma chamber evolved prior the last eruptive cycle: three distinct andesitic magma layers were produced by simple AFC of a high-alumina basalt (HAB) with assimilation of Tertiary quartz–dioritic and granitic rocks. Early erupted 1968 tephra and 1969 lavas (which represent the first two layers of the upper part of a zoned magma chamber) were produced by simple AFC, with fractionation of plagioclase, pyroxene and magnetite and concomitant assimilation of quartz–dioritic rocks. Assimilation rates were constant (r1=0.33) for a relative mass of magma remaining of 0.77–0.80, respectively. Lavas erupted around 1974 are less differentiated and represent the `primitive andesitic magma type' residing within the middle–lower part of the chamber. These lavas were also produced by simple AFC: assimilation rates and the relative mass of magma remaining increased of about 10%, respectively (r1=0.36, and F=0.89). Ba enrichment of the above lavas is related to selective assimilation of Ba from Tertiary granitic rocks. Lava eruption occurred as a dynamic response to the intrusion of a new magma into the old reservoir. This process caused the instability of the zoned magma column inducing syneruptive mixing between portions of two contiguous magma layers (both within the column itself and at lower levels where the new basalt was intruded into the reservoir). Syneruptive mixing (mingling) within the middle–upper part of the chamber involved fractions of earlier gabbroic cumulitic materials (lavas erupted around 1970). On the contrary, within the lower part of the chamber, mixing between the intruded HAB and the residing andesitic melt was followed by simple fractional crystallization (FC) of the hybrid magma layer (lavas erupted in 1978–1980). By that time the original magma chamber was completely evacuated. Lavas erupted in 1982/1984 were thus modelled by means of `open system' AFCRE (i.e., AFC with continuous recharge of a fractionating magma batch during eruption): in this case assimilation rates were r1=0.33 and F=0.86. Recharge rates are slightly higher than extrusion rates and may reflect differences in density (between extruded and injected magmas), together with dynamic fluctuations of these parameters during eruption. Ba and LREE (La, Ce) enrichments of these lavas can be related to selective assimilation of Tertiary granitic and quartz–dioritic rocks. Calculated contents for Zr, Y and other REE are in acceptable agreement with the observed values. It is concluded that simple AFC occurs between two distinct eruption cycles and is typical of a period of repose or mild and decreasing volcanic activity. On the contrary, magma mixing, eventually followed by fractional crystallization (FC) of the hybrid magma layer, occurs during an ongoing eruption. Open-system AFCRE is only operative when the original magma chamber has been totally replenished by the new basaltic magma, and seems a prelude to the progressive ceasing of a major eruptive cycle.

Nyamuragira volcano is one of the most active African volcanoes. Eruptions have been occurring every 3–4 years throughout the last century. Here, we analyse satellite infrared data, collected by MODIS sensor to estimate the volcanic radiative power (VRP, in W) and energy (VRE; in J) released during the 2001, 2002, 2004, 2006–2007, 2010 and 2011–2012 eruptions. Based on the statistical distribution of VRP measurements, we found that thermal emissions at Nyamuragira fall into three distinct radiating regimes. The high-radiating regime occurs during the emplacement of poorly insulated lava flows and characterise most of the effusive activity. The moderate-radiating regime is associated with open-vent activity (Strombolian explosions and/or lava lake activity) eventually accompanied by the emplacement of short-lived and well-insulated flows. A third radiating regime (low-radiating regime) occurs during periods, which may last weeks to months, that follow each eruption and are associated with the cooling of the effused lava flows. By applying the radiant density approach to MODIS-derived VRP we also estimated the time-averaged lava discharge rates (TADR; in m3 s−1) and we analysed the effusive trends of the above eruptions. We found that the transition between the effusive and open-vent activity typically takes place when TADR reduces to low values (

Significant changes in both the chemistry of coastal thermal waters and the soil CO 2 and Rn emissions in the crater area were recorded at Stromboli prior to the eruption that began on 28 December 2002. The dissolved CO 2 contents and the δ 13 C and 3 He/ 4 He values were elevated in the thermal aquifer from July 2002. Synchronous variations in the same isotope ratios were recorded in the summit fumarolic gases, with both 3 He/ 4 He and δ 13 C values of gases released from a fumarole in the summit area increasing between May and November 2002. These variations are indicative of early degassing of a new gas-rich magma batch with a 13 C- and 3 He-rich signature. This magma recharge probably fed the intense Strombolian activity recorded during that period. The eruption began with a major explosion that produced a glowing avalanche, immediately followed by a fluid lava overflow from the NE crater and subsequent lava effusion from vents opened in the Sciara del Fuoco depression. Sharp increases in CO 2 soil flux and Rn emissions― to values never observed previously―were recorded in the summit crater area 10 d before the eruption onset. These CO 2 and Rn anomalies are indicative of a high gas-driven magma supply rate and gas overpressure within the conduit. The sudden depressurization of the magma filling the upper conduit probably caused the major explosion that occurred on 28 December, which heralded the effusive phase. These data demonstrate the importance of collecting a wide spectrum of geochemical data from different geological sites when monitoring a volcano.

We report data from a radon survey conducted at Villarrica volcano. Measurements have been obtained at selected sites by E-PERM ® electrets and two automatic stations utilizing DOSEman detectors (SARAD Gmbh). Mean values for Villarrica are 1600 (±1150) Bq/m 3 are similar to values recorded at Cerro Negro and Arenal in Central America. Moderately higher emissions, at measurement sites, were recorded on the NNW sector of the volcano and the summit, ranging from 1800 to 2400 Bq/m 3 . These measurements indicate that this area could potentially be a zone of flank weakness. In addition, the highest radon activities, up to 4600 Bq/m 3 , were measured at a station located near the intersection of the Liquine-Ofqui Fault Zone with the Gastre Fault Zone. To date, the Villarrica radon measurements reported here are, together with those collected at Galeras (Colombia), the sole radon data reported from South American volcanoes. This research may contribute to improving future geochemical monitoring and volcano surveillance.

Monitoring gas emissions from soil allow to get information on volcanic activity, hidden faults and hydrothermal dynamics. Radon activities at Stromboli were collected by means of multi-parametric real-time stations, that measure radon as well as environmental parameters. The last improvements on the detection system are presented and discussed.

During an effusive–extrusive eruption, the capability of an active lava body (flow or dome) to radiate thermal energy depends on how the lava discharge rate is accommodated by the expansion of the magma body and by the cooling of its surface. This feature can be described by a single empirical parameter, defined “radiant density” ( c rad ; in J m − 3 ) that, for a given discharge rate, expresses the efficiency of the lava body to change its area and/or to insulate its inner core, thus modulating the heat radiated from the active surface. By comparing the Volcanic Radiative Energy ( VRE ; in J), detected by MODIS during 28 eruptions at 18 active volcanoes, with their erupted lava volumes ( Vol ; in m 3 ), we show that the radiant density ( c rad = VRE / Vol ) is inversely proportional to the silica content of the erupted lava. Basic lavas (45–52 wt.% SiO 2 ) have the highest observed radiant density (1 to 4 × 10 8 J m − 3 ) while intermediate (52–63 wt.% SiO 2 ) and acidic (> 63 wt.% SiO 2 ) lavas show a gradually lower radiant densities (1.5 to 9 × 10 7 J m − 3 and 0.2 to 1 × 10 7 J m − 3 for intermediate and acidic composition, respectively). We regard this correlation as the result of the control that the rheology of lavas exerts on cooling and spreading processes of related bodies. In particular, we found that for any given compositional group the radiant density is essentially related to a “characteristic thickness” of active lavas, at the time of a satellite acquisition. We suggest that the radiant density of effusive/extrusive lava bodies can be predicted (± 50%) by means of an empirical relationship based on the SiO 2 content of the erupted lava. This makes this parameter very useful in observing volcanic activity, especially in remote regions where access may not be possible. By measuring the energy radiated during an eruption and by assuming a lava composition (based on the tectonic setting or magmatic province), we suggest that the radiant density can be used to estimate lava discharge rates and erupted volumes for volcanoes characterised by effusive or extrusive activity.

We present an overview of the radon surveys at Stromboli island starting from the 2002, with the aim to better understand the relation between in-soil radon degassing and change in volcanic activity. Radon measurements were performed by means of different, passive, methodologies. By using a radon network of 21 sampling sites, placed in the NE sector of the island, we effected periodic surveys by means of two manually techniques. Track-etch detectors and EPERM electrets allowed us to construct a continuous time-series of radon measurements during the period 2002–2007 and to recognize the short-time variations. Radon activity shows, by means of EPERM ® electrets, a wide range of values, from 0.3 to 50 kBq/m 3 . These data confirm the role of the local effects on the in-soil radon emissions and show that the structural alignments represent a preferential pathways for the radon migration from soil to air. Prom the 2007 we developed two real-time multiparametric, fully automated stations where the measurements of radon activity, acquired every 15 minutes, are correlated with the soil temperature and atmospheric pressure data. The first results show the important role of the atmospheric factors ( e.g. soil temperature) in the acquired radon measurements.

Twelve years of night-time MODIS (Moderate Resolution Imaging Spectroradiometer) observations, has been analysed to detect and quantify the radiative heat power emitted by Stromboli volcano (from March 2000 to September 2011). Using an accurate background subtraction of the MODIS signal at 4 μm, we were able to discriminate two main regimes of thermal radiation, related to different levels of volcanic activity. Effusive eruptions (occurred on December 28, 2002 and February 27, 2007) radiated at an average of ~ 186 MW with a frequency of alert detection of 50–95%. Conversely, during the typical strombolian activity, an average of ~ 9 MW is radiated, with a frequency of alert detection of 0–45%. Although during the effusive eruptions the radiative power is basically controlled by the lava discharge rate, our results suggest that during non-effusive periods (strombolian regime) both the intensity and the frequency of MODIS alerts are controlled by the height of the magmatic column feeding the activity at the surface. In particular we found that a radiative power of ~ 50 MW corresponds to a high magma column which is exposed, in the vent area, at the same rate in which the deeper gas-rich magma is typically supplied within the feeding system of Stromboli (~ 0.3 m3 s− 1). In this condition the magmatic system approaches steady state regimes. Above this threshold a transition from strombolian to effusive regimes may occur as shown by the detection of ~ 50 MW, 8–10 days before the onset of both the last two major flank eruptions. These values were reached after 1–2 months of gradual increase of the radiative power which was likely associated the rising of the magma column within the shallowest portion of the conduit. In addition our data suggest that over the years 2000–2011 several cycles of rise and fall of the magma column have occurred, which however did not culminate into an effusive eruption but only into recurrent episodes of sustained spattering or fountaining and summit overflows. These fluctuations has substantially increased in frequency and intensity after the 2007 eruption thus suggesting that this event has perturbed in some way the shallow plumbing system of Stromboli. We stress that the detection of a radiative power higher than 50 MW is a clear evidence of a very high magma column, which may prelude the onset of an effusive eruption and/or periods of sustained vent activity. In conclusion, we suggest that systematic analysis of MODIS data can be used to detect variations in the intensity of strombolian activity and may considerably improve volcano surveillance at Stromboli, as well as at other open-system volcanoes.

The twin Arkenu circular structures (ACS), located in the al-Kufrah basin in southeastern Libya, were previously considered as double impact craters (the ''Arkenu craters''). The ACS consist of a NE (Arkenu 1) and a SW structure (Arkenu 2), with approximate diameters of about 10 km. They are characterized by two shallow depressions surrounded by concentric circular ridges and silica-impregnated sedimentary dikes cut by local faults. Our field, petrographic, and textural observations exclude that the ACS have an impact origin. In fact, we did not observe any evidence of shock metamorphism, such as planar deformation features in the quartz grains of the collected samples, and the previously reported ''shatter cones'' are wind-erosion features in sandstones (ventifacts). Conversely, the ACS should be regarded as a ''paired'' intrusion of porphyritic stocks of syenitic composition that inject the Nubia Formation and form a rather simple and eroded ring dike complex. Stock emplacement was followed by hydrothermal activity that involved the deposition of massive magnetite-hematite horizons (typical of iron oxide copper-gold deposits). Their origin was nearly coeval with the development of silicified dikes in the surroundings. Plugs of tephritic-phonolitic rocks and lamprophyres (monchiquites) inject the Nubian sandstone along conjugate fracture zones, trending NNW-SSE and NE-SW, that crosscut the structural axis of the basin.

Radon emission from soil at Stromboli Island has been monitored since 2002 utilizing a network of 25 stations and exposing two kinds of detectors: E-PERM and calibrated track-etches (LR115). We present and discuss the data from 2002 to 2007, thus including the last two major eruptive cycles. Earthquake-volcano interactions were detected providing evidence that radon emissions are somehow related to the occurrence of regional earthquakes. Single deep earthquakes related to active subduction, such as Salina event (ML =5 .1) of May 5, 2004, may be capable of increasing the ascent of geothermal fluids due to the passage of seismic waves. In addition, we observed major eruptions at Stromboli were preceded by anomalies that occurred at three summit stations that reached values above 20000 Bq m −3 : this is considered an anomalous value and could be regarded, under particular environmental conditions, as a potential precursory signal of a change in volcanic activity.

We present the results of one year of continuous radon monitoring at Stromboli volcano collected at two automated real-time stations. These were deployed on the NE flank (at 520 m a.s.l.) and within the summit area (900 m a.s.l.). Higher daily emissions at the lower station approached 4,200 Bq/m3, with bulk averages around 1,800 (±980) Bq/m3; whereas the summit station reached peak values of 23,000 Bq/m3 and bulk averages of 12,500 Bq/m3 (±4,000). Negative correlations are observed between radon emissions, soil temperature and, to a lesser extent, atmospheric pressure. In contrast, increases in radon concentrations were observed during periods of higher rainfall conditions. Therefore, trends in radon concentrations may be decoupled from those of other geochemical parameters (CO2 fluxes and CO2/SO2 plume ratios) during periods of heavy to moderate rainfalls. Multiple Linear Regression statistics (including the effects of soil temperature, atmospheric pressure and tidal forces) led us to compute the residuals given by the difference of measured and calculated 222Rn concentrations. The cross-check between the daily measured radon activities and the absolute variations in radon residuals, for the data collected at the summit station, give us the opportunity to suggest a methodological approach that can be used in the attempt of predicting some major changes in volcanic activity.

Data collected at Somma-Vesuvius during the 1998–1999 radon surveys have been revisited and reinterpreted in light of recent geophysical and geochemical information. The duration of selected radon anomalies, together with the decay properties of radon, have been used to estimate the permeability and porosity of rocks of the deep hydrothermal system. The current local cyclic seismicity is explained by means of a double convective-cell model. Convective cells are separated by a low-permeability horizon located at about 2–2.5 km below sea level. Fluids convecting within the upper cells show temperatures ranging 300–350°C. Rock permeabilities in this sector are estimated on the order of 10−12 m2, for porosities (ϕ) of about 10−5 typical of a brittle environment where fluid velocities may reach ∼800 m/day. Fluid temperatures within the lower cells may be as high as 400–450°C, consistent with supercritical regimes. The hydrodynamic parameters for these cells are lower, with permeability k ∼ 10−15 m2, and porosity ranging from 10−6 to 10−7. Here, fluid motion toward the surface is controlled by the fracture network within a porous medium approaching brittle–ductile behaviour, and fluid velocities may reach ∼1,800 m/day. The low-permeability horizon is a layer where upper and lower convecting cells converge. In this region, fluids (convecting both at upper and lower levels) percolate through the wallrock and release their brines. Due to self-sealing processes, permeability within this horizon reaches critical values to keep the fluid pressure near lithostatic pressure (for k ∼ 10−18 m2). Deep fluid pressure buildups precede the onset of hydrothermally induced earthquakes. Permeability distribution and rock strength do not exclude that the next eruption at Somma-Vesuvius could be preceded by a seismic crisis, eventually leading to a precursory phreatic explosion. The coupling of these mechanisms has the potential of inducing pervasive failure within rocks of the hydrothermal shell, and may be a prelude to a magmatic eruption. It is finally emphasised that the integrated analysis of seismic and geochemical data, including radon emissions, could be successfully used in testing temperature distributions and variations of porosity and permeability in active geothermal reservoirs.

We analysed the effusive activity of 15 eruptions of Piton de la Fournaise by using the data acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS) during 2003–2007 period. Thermal anomalies were detected by adjusting the values of the Normalised Thermal Index (NTI) threshold [Wright, R.L., Flynn, H., Garbeil, A.J.L., Harris Pilger, E., 2002. Automated volcanic eruption detection using MODIS. Remote Sens. Environ. 82: 135–155.] in accord to the seasonal and topographic variations of the background temperature at Piton de la Fournaise. Each satellite overpass, has been classified according to the presence of clouds and the satellite viewing geometry. The analyzed data indicate that at Piton de la Fournaise only 10% of MODIS overpasses occurred in optimal conditions (clear sky and satellite zenith 20 m3 s− 1) and the following vanishing phase indicate that distal eruptions are triggered when a critical overpressure is reached (>5 MPa). This last seems to be increased during the whole analysed period up to the onset of the April 2007 which led to the collaps of the Dolomieu crater. Finally we propose that the recent activity of PdF may result from a persistent loading of the shallow reservoir with a magma supply rate which increased from 1135 kg s− 1 to 4310 kg s− 1 during the period between 2003 and 2007.

We used a network of stations to perform systematic radon surveys at Stromboli volcano. The time series of periodic measurements show that monthly average 222Rn emissions reflect changes in volcanic activity and exhibit increasing trends prior and during the last major eruptive cycles. Maps of radon emissions indicate that diffuse degassing is operative at Stromboli volcano. Concentrated degassing essentially occurs in the summit area and within a sector proximal to the two major NE trending faults. These sites were chosen for deploying the two real-time stations that are currently operating at Stromboli. In these devices, the 222Rn electronic dosimeters are connected to a radiomodem for wireless data transfer to a receiving station at the volcano observatory. Radon activity, soil temperature and atmospheric pressure data are sampled and instantaneously transferred via web so that they can be checked remotely. Collected time series reveal an overall inverse correlation between radon emissions and seasonal temperature variations. Radon emissions in sectors of diffuse degassing are modulated by tidal forces as well. Radon activities recorded at the summit station, located along the fracture zone where the gas flux is concentrated, are positively correlated with changes in atmospheric pressure and confirm the occurrence of the “atmospheric stack effect”. We finally emphasize that real-time radon monitoring is an innovative technique that may be systematically applied in volcano surveillance.

Several small crater-like structures occur in Gilf Kebir region (SW Egypt). It has been previously suggested that they could be the result of meteoritic impacts. Here we outline the results of our geological and geophysical survey in the area. The proposed impact origin for these structures is not supported by our observations and analyses, and we suggest an alternative interpretation. The crater-like structures in Gilf Kebir area are likely related to endogenic processes typical of hydrothermal vent complexes in volcanic areas which may reflect the emplacement of subvolcanic intrusives.

International audience; [1] A detailed set of thermal images collected during the last day of the August 2003 eruption of Piton de la Fournaise (La Réunion), clearly revealed several dynamic processes associated with a spatter cone containing a lava pond and feeding a channelized lava flow. Periods of steady effusion were interrupted by brief pulses of lava effusion that closely coincide with peaks in seismic tremor amplitude. The thermal measurements show that roofing of a lava channel during steady effusion and cooling of surface flows decrease thermal radiance in two different ways. Here we show that the decrease in thermal radiance because of channel roofing is not related to a decrease in volcanic activity, as might be interpreted from satellite data. In addition, we introduce a new method of representing thermal data (hereby named ''Radiative Thermogramme'') that successfully describes thermal patterns produced by distinct eruptive processes within the same span of time. This graphic solution can be directly correlated with volcanic field processes and provides a useful tool for interpreting a high number of thermal data in a wide range of volcanic activities.