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Lava tubes are powerful heat insulators, allowing lava to practically keep the initial temperature and travel longer distances than when freely flowing on the ground surface. It is thus extremely important to recognize how, when and where these structures form within a lava flow field for hazard assessment purposes, in order to plan possible interventions should a lava flow approach inhabited areas. Often being formed within thick and complex lava flow fields, lava tubes are difficult to detect, study and explore. In this study, we analyse the 1792-93 Etna lava flow field emplaced on a steep slope (>4°) which comprises several lava tubes located at different distances from the eruptive fissure, at different levels within the lava flow field, and showing various inner morphologies, with peculiar inner features related to their maturity and eruptive history. Our aim is to verify whether it is possible to connect the underground features with features observed on the lava flow surface in order to reconstruct the extension of the tube network and unravel the genetic processes. Our results show that, in the studied lava flow field, a clear correspondence is possible between shallow tubes emplaced late during the lava flow field growth and surface textures. In addition, vertical and horizontal tube capture is very widespread, and might be the primary process for lava tube persistence and long life. Our results might be applicable to other lava tubes on Earth and other rocky planets. [ABSTRACT FROM AUTHOR]

Lava tubes are powerful heat insulators, allowing lava to practically keep the initial temperature and travel longer distances than when freely flowing on the ground surface. It is thus extremely important to recognize how, when and where these structures form within a lava flow field for hazard assessment purposes, in order to plan possible interventions should a lava flow approach inhabited areas. Often being formed within thick and complex lava flow fields, lava tubes are difficult to detect, study and explore. In this study, we analyse the 1792–93 Etna lava flow field emplaced on a steep slope (>4°) which comprises several lava tubes located at different distances from the eruptive fissure, at different levels within the lava flow field, and showing various inner morphologies, with peculiar inner features related to their maturity and eruptive history. Our aim is to verify whether it is possible to connect the underground features with features observed on the lava flow surface in order to reconstruct the extension of the tube network and unravel the genetic processes. Our results show that, in the studied lava flow field, a clear correspondence is possible between shallow tubes emplaced late during the lava flow field growth and surface textures. In addition, vertical and horizontal tube capture is very widespread, and might be the primary process for lava tube persistence and long life. Our results might be applicable to other lava tubes on Earth and other rocky planets.

Abstract Etna is a highly active volcano with frequent eruptions and is closely observed with different monitoring techniques. One of the monitoring methods is a network of short-base tiltmeters that provides continuous control of ground deformation. Since 1996, this network has been integrated with a long-base fluid tiltmeter installed in two 80 m long connected tunnels at the high-altitude (2850 m a.s.l.) volcanological observatory of Pizzi Deneri (PDN) located on the volcano’s summit. The instrumentation was devised with an innovative configuration composed of mercury, free to move along the entire length of the tiltmeter in response to the ground tilt, and laser sensors to measure the changes in the mercury levels. In this study, for the first time, we present the entire 25-year series of data recorded by this instrumentation. During this long interval, the Etna volcano was characterized by numerous major eruptions due to dyke intrusions. The tilt variations recorded in the short-term during the rapid intrusive phases are presented and discussed. These signals contributed both to the modeling of eruptive processes, and in particular in the case of the 2002–2003 eruption, to the real-time understanding of the ongoing eruptive phenomenon, supporting the correct hazard assessment. In the medium-long term (years to decades), we show and discuss that the PDN signal is mainly related to the marked sliding of the entire eastern sector of the volcano. Graphical Abstract

Etna is a highly active volcano with frequent eruptions and is closely observed with different monitoring techniques. One of the monitoring methods is a network of short-base tiltmeters that provides continuous control of ground deformation. Since 1996, this network has been integrated with a long-base fluid tiltmeter installed in two 80 m long connected tunnels at the high-altitude (2850 m a.s.l.) volcanological observatory of Pizzi Deneri (PDN) located on the volcano's summit. The instrumentation was devised with an innovative configuration composed of mercury, free to move along the entire length of the tiltmeter in response to the ground tilt, and laser sensors to measure the changes in the mercury levels. In this study, for the first time, we present the entire 25-year series of data recorded by this instrumentation. During this long interval, the Etna volcano was characterized by numerous major eruptions due to dyke intrusions. The tilt variations recorded in the short-term during the rapid intrusive phases are presented and discussed. These signals contributed both to the modeling of eruptive processes, and in particular in the case of the 2002–2003 eruption, to the real-time understanding of the ongoing eruptive phenomenon, supporting the correct hazard assessment. In the medium-long term (years to decades), we show and discuss that the PDN signal is mainly related to the marked sliding of the entire eastern sector of the volcano. [ABSTRACT FROM AUTHOR]

Mt. Etna, in Italy, is one of the most active volcanoes in the world, producing several explosive events in recent years. Those eruptions form high eruption columns that often reach the top of the troposphere (and sometimes even the lower part of the stratosphere) and create several disruptions to air traffic, mainly to the Fontanarossa International Airport in Catania, which is about 20 NM (~ 37 km; NM = Nautical Miles) away from the summit craters and is located in the main wind direction. In Italy, the institution responsible for volcano monitoring is the Istituto Nazionale di Geofisica e Vulcanologia (INGV). In 2007, the INGV, Osservatorio Etneo (INGV-OE) in Catania was appointed as “State Volcano Observatory” (SVO) and, in 2014, sent the first Volcano Observatory Notice for Aviation (VONA) message. Since that moment, several VONA messages have been sent, mainly due to the high frequency of Etna activity. In order to facilitate and speed in the generation and the dispatch of the VONA messages, a computer-assisted procedure has been designed and built to help the work done by the volcanologist on duty and by the two shift workers of the 24/7 Control Room of INGV-OE. Consequently, information on the explosive activity can be quickly provided to the Volcanic Ash Advisory Center (VAAC) in Toulouse and national air traffic offices, reducing risks to aviation operations. In this work, we describe how the computer-assisted procedure works, addressing the main advantages and possible improvements. We retain that a similar approach could be easily applied to other volcano observatories worldwide. [ABSTRACT FROM AUTHOR]

Timely estimations of magma volumes emitted during an eruption or a sequence of explosive events are vital for investigating the eruptive activity and evaluating the associated hazard. A reliable method for estimating erupted volumes is based on the analysis of digital surface models that nowadays can be obtained subsequently using stereo or tri‐stereo optical satellite imagery. However, the real‐time estimation of the erupted volumes is still an open challenge. Here, we explore the capacity of extracting volume estimates from continuous measurements of volumetric strain changes recorded by borehole dilatometers. We compare the volumes derived from numerous high spatial resolution satellite images with high precision strain records at Etna during 2020–2022, when more than 60 lava fountains occurred. The good correlation between the two data sets shows that strain changes can be used as a proxy to estimate the emitted volumes both over time and in real‐time. Plain Language Summary: Quantifying erupted volumes is fundamental in volcanology to provide a robust characterization of eruptions. To date, estimates of erupted volumes have been calculated after the eruptions have ended and the real‐time estimation of the erupted volumes is still an on‐going challenge. In recent decades, the sequences of lava fountain‐type eruptions at Etna, with dozens of episodes close in time and more than 100 episodes occurring since 2011, has made the task of estimating the volumes in real‐time during each single eruption ever more essential. This would enable providing precise information to Civil Protection authorities and contribute toward hazard evaluation. In this study, we took on this new challenge by exploring the potential to extract erupted volume estimates from continuous measurements of strain changes, recorded by high‐precision borehole instruments installed on the volcano's flanks. We compared and validated the volumes deriving from numerous high spatial resolution satellite images with high‐precision strain records at Etna during 2020–2022, when more than 60 lava fountains occurred. The good correlation between the two data sets shows that strain changes can be used effectively as a proxy to estimate the emitted volumes, both over time and, more importantly, in real‐time. Key Points: Detection of strain changes during sequences of lava fountain eruptions from continuous recording high‐precision borehole instrumentsQuantifying erupted volumes from strain changes, comparison and validation with the volumes calculated from high spatial resolution satellite measurementsNew method to estimate the erupted volumes in real‐time by using the continuous strain recording [ABSTRACT FROM AUTHOR]

At active volcanoes recurring eruptive events, erosive processes and collapses modify the edifice morphology and impact monitoring and hazard mitigation. At Etna volcano (Italy) between February and October 2021, 57 paroxysmal events occurred from the South-East Crater (SEC), which is currently its most active summit crater. Strombolian activity and high lava fountains (up to 4 km) fed lava flows towards the east, south and south-west, and caused fallout of ballistics (greater than 1 m in diameter) within 1–2 km from the SEC. The impacted area does not include permanent infrastructure, but it is visited by thousands of tourists. Hence, we rapidly mapped each lava flow before deposits became covered by the next event, for hazard mitigation. The high frequency of the SEC paroxysms necessitated integration of data from three remote sensing platforms with different spatial resolutions. Satellite (Sentinel-2 MultiSpectral Instrument, PlanetScope, Skysat and Landsat-8 Operational Land Imager) and drone images (visible and thermal) were processed and integrated to extract digital surface models and orthomosaics. Thermal images acquired by a permanent network of cameras of the Istituto Nazionale di Geofisica e Vulcanologia were orthorectified using the latest available digital surface model. This multi-sensor analysis allowed compilation of a geodatabase reporting the main geometrical parameters for each lava flow. A posteriori analysis allowed quantification of bulk volumes for the lava flows and the SEC changes and of the dense rock equivalent volume of erupted magma. The analysis of drone-derived digital surface models enabled assessment of the ballistics' distribution. The developed methodology enabled rapidly and accurate characterisation of frequently occurring effusive events for near real-time risk assessment and hazard communication. [ABSTRACT FROM AUTHOR]

Abstract Timely estimations of magma volumes emitted during an eruption or a sequence of explosive events are vital for investigating the eruptive activity and evaluating the associated hazard. A reliable method for estimating erupted volumes is based on the analysis of digital surface models that nowadays can be obtained subsequently using stereo or tri‐stereo optical satellite imagery. However, the real‐time estimation of the erupted volumes is still an open challenge. Here, we explore the capacity of extracting volume estimates from continuous measurements of volumetric strain changes recorded by borehole dilatometers. We compare the volumes derived from numerous high spatial resolution satellite images with high precision strain records at Etna during 2020–2022, when more than 60 lava fountains occurred. The good correlation between the two data sets shows that strain changes can be used as a proxy to estimate the emitted volumes both over time and in real‐time.

Volcanic plume height is one the most important features of explosive activity; thus, it is a parameter of interest for volcanic monitoring that can be retrieved using different remote sensing techniques. Among them, calibrated visible cameras have demonstrated to be a promising alternative during daylight hours, mainly due to their low cost and low uncertainty in the results. However, currently these measurements are generally not fully automatic. In this paper, we present a new, interactive, open-source MATLAB tool, named 'Plume Height Analyzer' (PHA), which is able to analyze images and videos of explosive eruptions derived from visible cameras, with the objective of automatically identifying the temporal evolution of eruption columns. PHA is a self-customizing tool, i.e., before operational use, the user must perform an iterative calibration procedure based on the analysis of images of previous eruptions of the volcanic system of interest, under different eruptive, atmospheric and illumination conditions. The images used for the calibration step allow the computation of ad hoc expressions to set the model parameters used to recognize the volcanic plume in new images, which are controlled by their individual characteristics. Thereby, the number of frames used in the calibration procedure will control the goodness of the model to analyze new videos/images and the range of eruption, atmospheric, and illumination conditions for which the program will return reliable results. This also allows improvement of the performance of the program as new data become available for the calibration, for which PHA includes ad hoc routines. PHA has been tested on a wide set of videos from recent explosive activity at Mt. Etna, in Italy, and may represent a first approximation toward a real-time analysis of column height using visible cameras on erupting volcanoes. [ABSTRACT FROM AUTHOR]

Gravimetric observations were carried out in 2015, to image the uppermost portion of the volcanic plumbing system of Mt. Etna (Italy). Gravity measurements were performed using two relative gravimeters, along a profile that crosses the summit craters area (elevations between 2,820 and 3,280 m a.s.l.). Accurate positioning of the gravity observation points was determined through GPS measurements. After applying elevation and terrain corrections, the reduced gravity data were used to build a 2D density model of the uppermost part of the volcano edifice. This model was constrained using to-date knowledge of the structural setting of the area and the available volcanological data. We highlighted the presence of low-density material below the summit craters, down to the depth of about 2.1 km, interpreted as highly altered, fumarolized and structurally weakened material. It is also likely that the close presence of the conduits feeding the summit craters of the volcano contributes to the gravity low in the SW half of the measurement profile. Conversely, the gravity low observed at the northern edge of the profile could reflect the high concentration of faults and eruptive fissures in the Pizzi Deneri area, in correspondence of the Ellittico caldera rim.

Volcano ground deformation is a tricky puzzle in which different phenomena contribute to the surface displacements with different spatial–temporal patterns. We documented some high variable deformation patterns in response to the different volcanic and seismic activities occurring at Mt. Etna through the January 2015–March 2021 period by exploiting an extensive dataset of GNSS and InSAR observations. The most spectacular pattern is the superfast seaward motion of the eastern flank. We also observed that rare flank motion reversal indicates that the short-term contraction of the volcano occasionally overcomes the gravity-controlled sliding of the eastern flank. Conversely, fast dike intrusion led to the acceleration of the sliding flank, which could potentially evolve into sudden collapses, fault creep, and seismic release, increasing the hazard. A better comprehension of these interactions can be of relevance for addressing short-term scenarios, yielding a tentative forecasting of the quantity of magma accumulating within the plumbing system. [ABSTRACT FROM AUTHOR]

Understanding lava flow dynamics during major effusive events is of paramount importance in volcanic areas characterized by a high risk of lava invasion. Mount Etna volcano (Sicily, Italy) has a long history of eruptions characterized by the emplacement of kilometer-wide lava fields, which have often reached the distal parts of the volcanic edifice, nowadays the location of numerous population centers. The 1669 eruption was one of the volcano's most important events in historic times due to the low altitude of the eruptive vent and the high volume of emitted products (607 ± 105 × 106 m3), with lava flows that destroyed numerous villages located along their path. The flows reached the city of Catania, at a distance of >16 km from the emission point. In this work, we investigate the products of the 1669 eruption through geochemical, mineralogical, 2D and 3D textural analyses with the aim of reconstructing the degassing, crystallization and rheological history of the magma and lavas in pre- to post-eruptive conditions. Combining geothermobarometric and hygrometric models allowed us to estimate magmatic water content (4.1 wt.%) before the eruption, whereas the syn-eruptive crystal content (10 vol.% at the onset of flowing) was retrieved through the textural analysis of pyroclasts sampled from the near-vent fallout. Finally, crystallization and degassing occurring at surface conditions were reconstructed using the textural and mineralogical analysis of lavas. Results were integrated in a three-phase (melt + crystal + bubble) rheological model indicating that lava viscosity, at the onset of the eruption, was low enough (<3.51 log Pa s) to permit the development of a complex and extensive lava field. Flows reached numerous kilometers in length, thanks to the joint effects of lava tunneling, delayed crystal nucleation and growth, and the presence (up to 21.35 vol.%) of deformed bubbles. The combination of these processes maintained the high fluidity of the melt suspension, allowing the flow to arrive at considerable distances from the vent. Lastly, the results of this study highlight the necessity of an accurate real-time multi-analytical petrological characterization of active lavas during the monitoring of effusive eruptions, essential for reliable viscosity modeling of advancing flow units and therefore vital in predicting the direction of lava flows, especially in densely populated areas such as the southern flank of Mount Etna. [ABSTRACT FROM AUTHOR]

During the last two decades, the Etna volcano has undergone several sequences of lava fountaining (LF) events that have had a major impact on road conditions, infrastructure and the local population. In this paper, we consider the LF episodes occurring between 2011 and 2022, calculating their erupted volumes using the images recorded by the monitoring thermal cameras and applying a manual procedure and a dedicated software to determine the lava fountain height over time, which is necessary to obtain the erupted volume. The comparison between the results indicates the two procedures match quite well, the main differences occurring when the visibility is poor and data are interpolated. With the aim of providing insights for hazard assessment, we have fitted some probabilistic models of both the LF inter-event times and the erupted volumes of pyroclastic material. In more detail, we have tested power-law distributions against log-normal, Weibull, generalised Pareto and log-logistic. Results show that the power-law distribution is the most likely among the alternatives. This implies the lack of characteristic scales for both the inter-event time and the pyroclastic volume, which means that we have no indication as to when a new episode of LF will occur and/or how much material will be erupted. What we can reasonably say is only that short inter-event times are more frequent than long inter-event times, and that LF characterised by small volumes are more frequent than LF with high volumes. However, if the hypothesis that magma accumulates on Etna at a rate of about 0.8 m 3 s − 1   holds, the material accumulated in the source region from the beginning of the observation period (2011) to the present (2022) has already been ejected. In simple terms, there is no accumulated magma in the shallow storage that is prone to be erupted in the near future. [ABSTRACT FROM AUTHOR]

Between 13 December 2020 and 21 February 2022, Etna volcano produced a sequence of 66 paroxysmal explosive eruptions, with Strombolian activity at the summit craters climaxing in lava fountains and eruption columns extending several kilometers above the craters, accompanied by minor and short-lasting lava flows from the crater rim. We selected three of these episodes that occurred within a short space of time, between 13 December 2020 and 12 March 2021, of different magnitude (i.e., erupted volume) and intensity (i.e., mass eruption rate or instantaneous eruption rate), and analyzed them from a multidisciplinary perspective. The aim was to gain insights into those parameters that mostly reveal the eruptive process for hazard assessment purposes. The multidisciplinary data consist of calibrated visible images, thermal images, seismic and infrasound data, ground deformation detected from the strainmeters, as well as satellite SEVIRI images. From these data, we obtained the timing of each paroxysmal event, the erupted volume in terms of tephra and lava flows, and the corresponding deflation of the source region, together with the development of the lava fountains and eruption columns with time. The results enabled determining that the smallest episode was that of 13 December 2020, which comprised three distinctive pulses but did not produce an eruptive column detectable from either monitoring cameras or satellites. The 28 February 2021 episode was remarkable for the short amount of time required to reach the climax, and was the most intense, whereas the 12 March 2021 event showed the longest duration but with an intensity between that of the previous two. Our results show that these three paroxysmal events display a typical trend, with the first event also being the smallest in terms of both erupted volume and intensity, the second being the most intense, and the third the one of greatest magnitude but less intense than the second. This is coherent with the end of the first paroxysmal phase on 1 April 2021, which was followed by 48 days of eruptive pause before starting again. In this context, the end of the paroxysmal phase was anticipated by a more effusive episode, thus heralding a temporary decline in the gas content within the feeding magma batch. [ABSTRACT FROM AUTHOR]

Volcanic plume height is one the most important features of explosive activity; thus, it is a parameter of interest for volcanic monitoring that can be retrieved using different remote sensing techniques. Among them, calibrated visible cameras have demonstrated to be a promising alternative during daylight hours, mainly due to their low cost and low uncertainty in the results. However, currently these measurements are generally not fully automatic. In this paper, we present a new, interactive, open-source MATLAB tool, named ‘Plume Height Analyzer’ (PHA), which is able to analyze images and videos of explosive eruptions derived from visible cameras, with the objective of automatically identifying the temporal evolution of eruption columns. PHA is a self-customizing tool, i.e., before operational use, the user must perform an iterative calibration procedure based on the analysis of images of previous eruptions of the volcanic system of interest, under different eruptive, atmospheric and illumination conditions. The images used for the calibration step allow the computation of ad hoc expressions to set the model parameters used to recognize the volcanic plume in new images, which are controlled by their individual characteristics. Thereby, the number of frames used in the calibration procedure will control the goodness of the model to analyze new videos/images and the range of eruption, atmospheric, and illumination conditions for which the program will return reliable results. This also allows improvement of the performance of the program as new data become available for the calibration, for which PHA includes ad hoc routines. PHA has been tested on a wide set of videos from recent explosive activity at Mt. Etna, in Italy, and may represent a first approximation toward a real-time analysis of column height using visible cameras on erupting volcanoes.

The Etna volcano is renowned worldwide for its extraordinary lava fountains that rise several kilometers above the vent and feed eruptive columns, then drift hundreds of kilometers away from the source. The Italian Istituto Nazionale di Geofisica e Vulcanologia-Osservatorio Etneo (INGV-OE) is responsible for the monitoring of Mt. Etna, and for this reason, has deployed a network of visible and thermal cameras around the volcano. From these cameras, INGV-OE keeps a keen eye, and is able to observe the eruptive activity, promptly advising the civil protection and aviation authorities of any changes, as well as quantifying the spread of lava flows and the extent of pyroclastic and ash plumes by using a careful analysis of the videos recorded by the monitoring cameras. However, most of the work involves analysis carried out by hand, which is necessarily approximate and time-consuming, thus limiting the usefulness of these results for a prompt hazard assessment. In addition, the start of lava fountains is often a gradual process, increasing in strength from Strombolian activity, to intermediate explosive activity, and eventually leading to sustained lava fountains. The thresholds between these different fields (Strombolian, Intermediate, and lava fountains) are not clear cut, and are often very difficult to distinguish by a manual analysis of the images. In this paper, we presented an automated routine that, when applied to thermal images and with good weather conditions, allowed us to detect (1) the starting and ending time of each lava fountain, (2) the area occupied by hot pyroclasts, (3) the elevation reached by the lava fountains over time, and (4) eventually, to calculate in real-time the erupted volume of pyroclasts, giving results close to the manual analysis but more focused on the sustained portion of the lava fountain, which is also the most dangerous. This routine can also be applied to other active volcanoes, allowing a prompt and uniform definition of the timing of the lava fountain eruptive activity, as well as the magnitude and intensity of the event. [ABSTRACT FROM AUTHOR]

We present a new model called GPUFLOW for the modeling and simulation of geophysical flows. GPUFLOW, which is based on the cellular automaton paradigm, features a physical model for the thermal and rheological evolution of lava flows (including temperature-dependent emissivity and cooling by radiation and air convection), support for debris flows without thermal dependency, a parallel implementation on graphic processing units (GPUs), and a simpler and computationally more efficient solution to the grid bias problem. Here, we describe the physical–mathematical model implemented in GPUFLOW and estimate the influence of input data on the flow emplacement through different synthetic test cases. We also perform a validation using two real applications: a debris flow that occurred in July 2006 in the Dolomites (Italy) and the December 2018 lava flow from the Etna volcano. GPUFLOW's reliability prediction is accomplished by fitting the simulation with the actual flow fields, obtaining average values between ~55% and 75%. [ABSTRACT FROM AUTHOR]

In this body of work, we showcase a historic virtual geotrail on the eastern flank of the iconic Mt. Etna volcano (Italy), along a series of outstanding geological sites and features subsequent to an important eruption that took place in 1928. A geohistoric account of such a major eruption, is of great interest, since it is the only event since 1669 to have caused the destruction of a town (Mascali) in the Etna region. Volcanologists, educators, the lay public, tourists and volcano explorers can now access a series of "virtual geostops" belonging to this virtual geotrail, such that "visitors" can virtually fly above these sites by scanning a QR code on the printed or electronic version of the present manuscript, as well as on the poster provided as additional material for this manuscript. The virtual geostops that comprise the virtual geotrail were developed using the structure-from-motion (SfM) photogrammetry technique from images captured by using unmanned aerial vehicles (UAVs). The main result of our work is the virtual geotrail, subdivided in two parts and composed of eight geostops, each showing outstanding examples of geological features resulting from volcanic phenomena that took place also during 1979. Our approach is designed to support classical field trips, and it can undoubtedly become complementary to traditional field teaching in earth sciences, both now and in the future. [ABSTRACT FROM AUTHOR]

Volcano ground deformation is a tricky puzzle in which different phenomena contribute to the surface displacements with different spatial–temporal patterns. We documented some high variable deformation patterns in response to the different volcanic and seismic activities occurring at Mt. Etna through the January 2015–March 2021 period by exploiting an extensive dataset of GNSS and InSAR observations. The most spectacular pattern is the superfast seaward motion of the eastern flank. We also observed that rare flank motion reversal indicates that the short-term contraction of the volcano occasionally overcomes the gravity-controlled sliding of the eastern flank. Conversely, fast dike intrusion led to the acceleration of the sliding flank, which could potentially evolve into sudden collapses, fault creep, and seismic release, increasing the hazard. A better comprehension of these interactions can be of relevance for addressing short-term scenarios, yielding a tentative forecasting of the quantity of magma accumulating within the plumbing system.

The structure of an active volcano is highly dependent on the interplay between the geodynamic context, the tectonic assessment as well as the magmatic processes in the plumbing system. This complex scenario, widely explored at Etna during the last 40 years, is nevertheless incomplete for the recent historical activity. In 1763 two eruptions occurred along the west flank of the volcano. There, an eruption started on 6th February and formed the scoria cone of Mt. Nuovo and a roughly 4-km-long lava flow field. Another small scoria cone, known as Mt. Mezza Luna, is not dated in historical sources. It is located just 1 km eastward of Mt. Nuovo and produced a 700 m long flow field. We focused on the activity of Mts. Nuovo and Mezza Luna for several reasons. First, the old geological maps and volcanological catalogues indicate that Mt. Mezza Luna and Mt. Nuovo cones were formed during the same eruption, while historical sources described Mt. Nuovo’s activity as producing a single scoria cone and do not give information about the formation of Mt. Mezza Luna. Second, petrologic studies highlight that the products of Mt. Mezza Luna are similar to the sub-aphyric Etna basalts; they preserve a composition relatively close to Etna primitive magma which were also erupted in 1763, during La Montagnola flank eruption, which took place along the South Rift of the volcano. Third, the two scoria cones built up along the so-called West Rift of Etna, which represents one of the main magma-ascent zones of the volcano. We applied a multidisciplinary approach that could prove useful for other volcanoes whose past activity is still to be reconstructed. Critical reviews of historical records, new field surveys, petrochemical analyses and petrologic modelling of the Mts. Nuovo and Mezza Luna eruptions have been integrated with literature data. The results allowed improving the stratigraphic record of historical eruptions reported in the Mount Etna Geological map, modelling the sub-volcanic magmatic processes responsible for magma differentiation, and evidencing recurrent mechanisms of magma transfer at Etna. Indeed, the intrusion of a deep primitive magma along the South Rift is often associated with the activation of other rift zones that erupt residual magma stored in the shallow plumbing system.

During the last two decades, the Etna volcano has undergone several sequences of lava fountaining (LF) events that have had a major impact on road conditions, infrastructure and the local population. In this paper, we consider the LF episodes occurring between 2011 and 2022, calculating their erupted volumes using the images recorded by the monitoring thermal cameras and applying a manual procedure and a dedicated software to determine the lava fountain height over time, which is necessary to obtain the erupted volume. The comparison between the results indicates the two procedures match quite well, the main differences occurring when the visibility is poor and data are interpolated. With the aim of providing insights for hazard assessment, we have fitted some probabilistic models of both the LF inter-event times and the erupted volumes of pyroclastic material. In more detail, we have tested power-law distributions against log-normal, Weibull, generalised Pareto and log-logistic. Results show that the power-law distribution is the most likely among the alternatives. This implies the lack of characteristic scales for both the inter-event time and the pyroclastic volume, which means that we have no indication as to when a new episode of LF will occur and/or how much material will be erupted. What we can reasonably say is only that short inter-event times are more frequent than long inter-event times, and that LF characterised by small volumes are more frequent than LF with high volumes. However, if the hypothesis that magma accumulates on Etna at a rate of about 0.8 m3s−1 holds, the material accumulated in the source region from the beginning of the observation period (2011) to the present (2022) has already been ejected. In simple terms, there is no accumulated magma in the shallow storage that is prone to be erupted in the near future.

A main challenge in open conduit volcanoes is to detect and interpret the ultra-small strain (

We present a new three‐dimensional (3D) image of attenuation beneath Mt. Etna volcano based on the coda normalization method. Mt. Etna is an ideal natural laboratory for the application of new or unconventional tomography techniques owing to high levels of seismicity spanning a wide range of epicentral distances and depths. We retrieved seismic waveforms from the database generated in the 2014 TOMO‐ETNA seismic experiment and performed a joint interpretation of tomographic and geophysical inversion models to better constrain interpretations of the volcanic structure. We compared the attenuation tomography results with seismic inversion models (two P wave seismic models and a 3D coda wave seismic attenuation model) and the literature to highlight and interpret structural elements and their impact on the volcano dynamics. We created a new image of the inner structure of Mt. Etna that will help to constrain present and future volcanic behavior. In particular, we focused on magma storage below the summit area and identified a large high‐attenuation volume that is characterized by physical properties compatible with the presence of magma and other fluids. The existence of such a large volume of magma in the shallow crust below Mt. Etna has implications for the eruptive potential of the volcano. Plain Language Summary: Mount Etna, Italy, is one of the world's most active volcanoes and presents an almost constant state of activity. To correctly interpretate potential volcanic hazards, there is a need to better understand the complex inner structure of the volcano. To this end, Mt. Etna is an ideal natural laboratory for the application of new or unconventional tomography techniques. We analyzed seismic attenuation, which measures the loss of energy of the seismic waves, to obtain a three‐dimensional attenuation model of Mt. Etna. We compared our attenuation tomography results with other geophysical data in order to interpret key structural elements and their impact on volcano dynamics. We identified a large volume below the summit area that is compatible with the presence of magma and other volcanic fluids, highlighting the continued eruptive potential of the volcano. Key Points: Three‐dimensional attenuation images over the first 10 km below Etna volcano obtained by coda normalization methodAttenuation images show partial melt body below Etna volcanoJoint interpretation of different models confirm the high heterogeneity below Etna volcano [ABSTRACT FROM AUTHOR]

Between 13 December 2020 and 21 February 2022, Etna volcano produced a sequence of 66 paroxysmal explosive eruptions, with Strombolian activity at the summit craters climaxing in lava fountains and eruption columns extending several kilometers above the craters, accompanied by minor and short-lasting lava flows from the crater rim. We selected three of these episodes that occurred within a short space of time, between 13 December 2020 and 12 March 2021, of different magnitude (i.e., erupted volume) and intensity (i.e., mass eruption rate or instantaneous eruption rate), and analyzed them from a multidisciplinary perspective. The aim was to gain insights into those parameters that mostly reveal the eruptive process for hazard assessment purposes. The multidisciplinary data consist of calibrated visible images, thermal images, seismic and infrasound data, ground deformation detected from the strainmeters, as well as satellite SEVIRI images. From these data, we obtained the timing of each paroxysmal event, the erupted volume in terms of tephra and lava flows, and the corresponding deflation of the source region, together with the development of the lava fountains and eruption columns with time. The results enabled determining that the smallest episode was that of 13 December 2020, which comprised three distinctive pulses but did not produce an eruptive column detectable from either monitoring cameras or satellites. The 28 February 2021 episode was remarkable for the short amount of time required to reach the climax, and was the most intense, whereas the 12 March 2021 event showed the longest duration but with an intensity between that of the previous two. Our results show that these three paroxysmal events display a typical trend, with the first event also being the smallest in terms of both erupted volume and intensity, the second being the most intense, and the third the one of greatest magnitude but less intense than the second. This is coherent with the end of the first paroxysmal phase on 1 April 2021, which was followed by 48 days of eruptive pause before starting again. In this context, the end of the paroxysmal phase was anticipated by a more effusive episode, thus heralding a temporary decline in the gas content within the feeding magma batch.

A key issue on active volcanoes is to investigate the position and characteristics of the magma reservoirs over time. The aim is to better understand the crustal magma transfer, therefore also to define the volcanic hazard and plan the mitigation strategies. Mt. Etna volcano is characterized by a lively eruptive activity with frequent major flank eruptions that can be both purely effusive and explosive-effusive. This volcano has been monitored over 40 years by ground deformation measurements. The studies and modeling of the eruptive processes through these data have mainly concerned single eruptions and the recharge phases that preceded them. In this study, for the first time, we present four decades of numerous recharge periods modeled over time by using the same typology of measurements (geodetic baselines) and the same modeling method. This uniform approach enables tracking the location of magma storage in a robust and unambiguous way during its recharging, which causes the volcano to inflate. In particular, the recharging periods that preceded the main eruptive activities were investigated. The tracking of the source positions contributes to update the representation of the shallow-intermediate plumbing system (last 10 km). Moreover, as a new result, we highlight that the recharges preceding the explosive eruptions are accompanied by a deepening over time of the centroid of the pressure source. This result opens up new scenarios on the relationship between the position of the recharging storage and the subsequent eruptive style.

The Etna volcano is renowned worldwide for its extraordinary lava fountains that rise several kilometers above the vent and feed eruptive columns, then drift hundreds of kilometers away from the source. The Italian Istituto Nazionale di Geofisica e Vulcanologia-Osservatorio Etneo (INGV-OE) is responsible for the monitoring of Mt. Etna, and for this reason, has deployed a network of visible and thermal cameras around the volcano. From these cameras, INGV-OE keeps a keen eye, and is able to observe the eruptive activity, promptly advising the civil protection and aviation authorities of any changes, as well as quantifying the spread of lava flows and the extent of pyroclastic and ash plumes by using a careful analysis of the videos recorded by the monitoring cameras. However, most of the work involves analysis carried out by hand, which is necessarily approximate and time-consuming, thus limiting the usefulness of these results for a prompt hazard assessment. In addition, the start of lava fountains is often a gradual process, increasing in strength from Strombolian activity, to intermediate explosive activity, and eventually leading to sustained lava fountains. The thresholds between these different fields (Strombolian, Intermediate, and lava fountains) are not clear cut, and are often very difficult to distinguish by a manual analysis of the images. In this paper, we presented an automated routine that, when applied to thermal images and with good weather conditions, allowed us to detect (1) the starting and ending time of each lava fountain, (2) the area occupied by hot pyroclasts, (3) the elevation reached by the lava fountains over time, and (4) eventually, to calculate in real-time the erupted volume of pyroclasts, giving results close to the manual analysis but more focused on the sustained portion of the lava fountain, which is also the most dangerous. This routine can also be applied to other active volcanoes, allowing a prompt and uniform definition of the timing of the lava fountain eruptive activity, as well as the magnitude and intensity of the event.

We present a new model called GPUFLOW for the modeling and simulation of geophysical flows. GPUFLOW, which is based on the cellular automaton paradigm, features a physical model for the thermal and rheological evolution of lava flows (including temperature-dependent emissivity and cooling by radiation and air convection), support for debris flows without thermal dependency, a parallel implementation on graphic processing units (GPUs), and a simpler and computationally more efficient solution to the grid bias problem. Here, we describe the physical–mathematical model implemented in GPUFLOW and estimate the influence of input data on the flow emplacement through different synthetic test cases. We also perform a validation using two real applications: a debris flow that occurred in July 2006 in the Dolomites (Italy) and the December 2018 lava flow from the Etna volcano. GPUFLOW’s reliability prediction is accomplished by fitting the simulation with the actual flow fields, obtaining average values between ~55% and 75%.

In this body of work, we showcase a historic virtual geotrail on the eastern flank of the iconic Mt. Etna volcano (Italy), along a series of outstanding geological sites and features subsequent to an important eruption that took place in 1928. A geohistoric account of such a major eruption, is of great interest, since it is the only event since 1669 to have caused the destruction of a town (Mascali) in the Etna region. Volcanologists, educators, the lay public, tourists and volcano explorers can now access a series of “virtual geostops” belonging to this virtual geotrail, such that “visitors” can virtually fly above these sites by scanning a QR code on the printed or electronic version of the present manuscript, as well as on the poster provided as additional material for this manuscript. The virtual geostops that comprise the virtual geotrail were developed using the structure-from-motion (SfM) photogrammetry technique from images captured by using unmanned aerial vehicles (UAVs). The main result of our work is the virtual geotrail, subdivided in two parts and composed of eight geostops, each showing outstanding examples of geological features resulting from volcanic phenomena that took place also during 1979. Our approach is designed to support classical field trips, and it can undoubtedly become complementary to traditional field teaching in earth sciences, both now and in the future.

Dikes are the primary mechanisms to transport magma and feed eruptions. Investigations of the surface deformation and seismicity caused by a dike can potentially provide useful information to forecast the expected propagation and associated hazard. On December 24, 2018, a dike intrusion reached the summit of Mt. Etna, feeding an effusive fissure. The intrusion was accompanied by a seismic swarm, with hypocenters beneath the summit craters and eruptive fissure, and by ground deformation. The seismicity continued the following day, with the hypocenters deepening to 3 km b.s.l. due to the propagation of a deeper and thicker dike. This situation generated the fear of feeding a more dangerous eruption in the medium-low flank. Recently it was found an equation that relates the average thickness and dimension of the dike with the expected released mechanical energy and, therefore, to the seismic moment. By using this updated application, it is shown that the observed seismicity could not be accounted for by the first dike. Instead, the cumulative effect of both dikes indicates a total moment from available energy expected that balances the moment recorded by the seismicity. The proposed approach proved very useful in the specific case of Etna volcano eruptions, resulting an effective tool to monitor the state of the intrusion of the magma and, therefore, to predict if a dike has enough energy to continue propagating or to stop.

For detecting below surface sources of deformation, strain measurements offer a very large advantage (orders of magnitude) in sensitivity over displacement measurements. On active volcanoes an intriguing open challenge is to measure the strain variations caused by the different types of eruptive activity with the highest possible precision in order to obtain advantages on the clear detecting of phenomena, their modeling and understanding. We present the updated main results obtained from the high precision strain recorded by the borehole dilatometer network on Mt. Etna volcano. The instruments, installed from the end of 2011, detected significant changes during different types of eruptive activity: several lava fountains during 2011–2014; two explosive sequences in 2015 and 2016; moderate effusive activity in 2017 and a dike intrusion in 2018. The strain changes provided powerful diagnostic information on the different ongoing processes, and allowed us to add key information on the different eruptive styles and sources. We also highlight how the recorded signals, with the associated modeling and interpretation, provide a powerful contribution to surveillance requirements on an active volcano. This report demonstrates that the borehole dilatometer network represents a useful tool both for the understanding of the volcano processes and for surveillance needs.

Only a few high precision studies of lava and tephra during simultaneous explosive and effusive activity have so far been undertaken. We carried out such measurements by analysis of a unique and homogeneous multi-temporal dataset of high-spatial resolution satellite optical images. Digital Elevation Models (DEMs) and orthophotos (with 1- and 0.5-m-pixel resolutions respectively) were extracted from six specifically tasked Pleiades tri-stereo pairs of Mt. Etna volcano, between 2011 and 2016. During this period, frequent effusive and explosive events formed lava flow fields and built up the new south-east crater pyroclastic cone. The volumes of lava fields and proximal pyroclastic deposits were measured by comparing the Pleiades DEMs with an aerial photogrammetric DEM updated in 2007. The volumes of all distal deposits were estimated using lava and tephra partitioning from the literature for an Etnean lava fountain. The dense rock equivalent volume of lava and tephra, calculated to be 248.4 ± 2.1 × 106 m3 in total, corresponds to an average output rate of 0.98 m3/s over the analysed 8-year period (May 2008–May 2016) and to a multi-event eruption rate of 5.53 m3/s for 520 days of activity. The multi-temporal analysis of high-spatial resolution satellite DEMs, here successfully applied to the well-monitored Etna volcano, demonstrated that the tasking of high-spatial resolution satellite images is crucial for fast and effective monitoring during intense volcanic activity (frequent and overlapping eruptive events). This methodology could be used for the monitoring of remote or hazardous volcanoes that are difficult to study by means of repeated field surveys. [ABSTRACT FROM AUTHOR]

Pyroclastic cones are a typical feature on volcanoes characterized by flank activity. Their distribution and orientation are important markers to obtain information on the maximum horizontal compressional stress acting on a volcano. A geophysical survey was performed on the pyroclastic cone of Mt. Vetore (Mt. Etna volcano, Southern Italy) to obtain information on its internal structural setting and to support the standard morphometric analysis. Results highlighted evident frequency peaks at 1.0 Hz inside the cone, which are attenuated away from it. The random decrement method was applied to this peak to compute damping and then to exclude links with anthropogenic sources. Moreover, time-frequency polarization analysis revealed that ambient vibrations are strongly polarized in a narrow frequency band, centered at a frequency of 1.0 Hz, with a preferred oscillation azimuth of 70–90° N. Array measurement of ambient vibrations was also used to obtain a shear wave velocity profile and then to retrieve the main interfaces with high seismic impedance. Results suggest a cone structure having a feeder dike consisting of fractured rocks with thickness of about 50 m surrounded by pyroclastic material lying on a high-velocity substrate. Finally, a 3D model of Mt. Vetore cone was built employing the finite element method to reproduce an experimental modal frequency of the cone itself. The numerical results successfully reproduced the experimental ones collected by the geophysical survey. [ABSTRACT FROM AUTHOR]

On 13 December 2020, Etna volcano entered a new eruptive phase, giving rise to a number of paroxysmal episodes involving increased Strombolian activity from the summit craters, lava fountains feeding several-km high eruptive columns and ash plumes, as well as lava flows. As of 2 August 2021, 57 such episodes have occurred in 2021, all of them from the New Southeast Crater (NSEC). Each paroxysmal episode lasted a few hours and was sometimes preceded (but more often followed) by lava flow output from the crater rim lasting a few hours. In this paper, we use remote sensing data from the ground and satellite, integrated with ground deformation data recorded by a high precision borehole strainmeter to characterize the 12 March 2021 eruptive episode, which was one of the most powerful (and best recorded) among that occurred since 13 December 2020. We describe the formation and growth of the lava fountains, and the way they feed the eruptive column and the ash plume, using data gathered from the INGV visible and thermal camera monitoring network, compared with satellite images. We show the growth of the lava flow field associated with the explosive phase obtained from a fixed thermal monitoring camera. We estimate the erupted volume of pyroclasts from the heights of the lava fountains measured by the cameras, and the erupted lava flow volume from the satellite-derived radiant heat flux. We compare all erupted volumes (pyroclasts plus lava flows) with the total erupted volume inferred from the volcano deflation recorded by the borehole strainmeter, obtaining a total erupted volume of ~3 × 106 m3 of magma constrained by the strainmeter. This volume comprises ~1.6 × 106 m3 of pyroclasts erupted during the lava fountain and 2.4 × 106 m3 of lava flow, with ~30% of the erupted pyroclasts being remobilized as rootless lava to feed the lava flows. The episode lasted 130 min and resulted in an eruption rate of ~385 m3 s−1 and caused the formation of an ash plume rising from the margins of the lava fountain that rose up to 12.6 km a.s.l. in ~1 h. The maximum elevation of the ash plume was well constrained by an empirical formula that can be used for prompt hazard assessment.

Volcanic plume height is a key parameter in retrieving plume ascent and dispersal dynamics, as well as eruption intensity; all of which are crucial for assessing hazards to aircraft operations. One way to retrieve cloud height is the shadow technique. This uses shadows cast on the ground and the sun geometry to calculate cloud height. This technique has, however, not been frequently used, especially not with high-spatial resolution (30 m pixel) satellite data. On 26 October 2013, Mt Etna (Sicily, Italy) produced a lava fountain feeding an ash plume that drifted SW and through the approach routes to Catania international airport. We compared the proximal plume height time-series obtained from fixed monitoring cameras with data retrieved from a Landsat-8 Operational Land Imager image, with results being in good agreement. The application of the shadow technique to a single high-spatial resolution image allowed us to fully document the ascent and dispersion history of the plume–cloud system. We managed to do this over a distance of 60 km and a time period of 50 min, with a precision of a few seconds and vertical error on plume altitude of ±200 m. We converted height with distance to height with time using the plume dispersion velocity, defining a bent-over plume that settled to a neutral buoyancy level with distance. Potentially, the shadow technique defined here allows downwind plume height profiles and mass discharge rate time series to be built over distances of up to 260 km and periods of 24 h, depending on vent location in the image, wind speed, and direction.

The Etna flank eruption that started on 24 December 2018 lasted a few days and involved the opening of an eruptive fissure, accompanied by a seismic swarm and shallow earthquakes, significant SO2 flux release, and by large and widespread ground deformation, especially on the eastern flank of the volcano. Lava fountains and ash plumes from the uppermost eruptive fissure accompanied the opening stage, causing disruption to Catania International Airport, and were followed by a quiet lava effusion within the barren Valle del Bove depression until 27 December. This was the first flank eruption to occur at Etna in the last decade, during which eruptive activity was confined to the summit craters and resulted in lava fountains and lava flow output from the crater rims. In this paper, we used ground and satellite remote sensing techniques to describe the sequence of events, quantify the erupted volumes of lava, gas, and tephra, and assess volcanic hazards.

In active volcanoes a main challenge is to identify and characterize the dynamics of magmatic sources from deformation and strain data. This task is of primary importance in frequently eruptive volcanoes, such as Etna. After the main flank eruption of 2008–2009 and until 2017, Etna volcano was characterized by a lively eruptive activity of different phases. These comprised 44 lava fountain episodes from the New South East Crater (NSEC), two sequences of close episodes of lava fountains from the Voragine crater (VOR), as well as some periods of summit effusive activity with a more prolonged supply of lava flows. Several studies have described and modeled single lava fountains episodes of the NSEC and VOR, in particular through high precision data from borehole strain-meters. In this study, we broaden the analysis also considering the medium-term volcano recharging/discharging periods preceding/accompanying the different eruptive phases during 2009–2017 by constraining the source positions through deformation recorded by the permanent GPS network. Together with the modeling deduced from the strain-meter data we produce a more complete representation of the different sources that characterized the different periods both in the medium-term (i.e., the preparatory phases showing inflation and the eruptive phases showing deflation) and in the short-term (i.e., the fast discharge associated with eruptive events). Our modeling explains the pathway of magma from the intermediate-shallow plumbing system to the surface and highlights a clear separation between the inflation and the deflation source depths, coherently with petrological constraints on the spatio-temporal evolution of magma transfer and storage.

Lava fountains have a major impact on the local population since they cause ash plumes that spread several kilometers above and hundreds of kilometers away from the crater. Ash fallout is responsible for disrupting airports and traffic on the motorways well beyond the area of the volcano itself, as well as affecting the stability of buildings and causing public health issues. It is thus a primary scientific target to forecast the impact of this activity on local communities on the basis of parameters recorded by the monitoring network. Between 2011 and 2015, 49 paroxysmal explosive episodes occurred at two of Mt Etna's five summit craters: the New South-East Crater (NSEC) and the Voragine (VOR). In this paper, we examine the features of the 40 episodes occurring at the NSEC during 2011–2013, and of the 4 events at VOR in December 2015. We study these paroxysms using geophysical monitoring data, characterize the episodes, and analyse all available data statistically. Our main results are two empirical relationships allowing us to forecast the maximum elevation of the ash plume from the average height of the lava fountain, useful for hazard assessment and risk mitigation. For Etna, and using the examples described in this paper, we can infer that wind speed 10 m s−1 is normally associated with weak plumes, bent-over along the wind direction and reaching lower elevations.

Estimates of lava volumes provide important data on the lava flooding history and evolution of a volcano. For mapping volcanic deposits, including lava flows, the advancement of satellite remote sensing techniques offers a great potential. Here we characterize the eruptive events occurred at Mt Etna between January 2011 and December 2015 leading to the emplacement of numerous lava flows and to the formation of a new pyroclastic cone (NSEC) on the eastern flank of the South East Crater. The HOTSAT system is used to analyze remote sensing data acquired by the SEVIRI sensor in order to detect the thermal anomalies from active lava flows and calculate the associated radiative power. The time-series analysis of SEVIRI data provides an estimation of event magnitude and intensity of the effusive material erupted during each event. The cumulative volume estimated from SEVIRI images from 2011 to 2015 adds up to ~106 millions of cubic meters of lava, with a time-averaged rate of ~0.68 m3 s−1. This estimate is independently supported and bounded using a topographic approach, i.e., by subtracting the last topography of Etna updated to 2005 from a 2015 digital elevation model (DEM), produced using tri-stereo Pléiades satellite images acquired on December 18, 2015. The total volume of products erupted from 2005 to 2015, calculated from topography difference by integration of the thickness distribution over the area covered, is about 287 × 106 m3, of which ~55 × 106 m3 is the volume of the NSEC cone. This 10-year volume is below the typical erupted volumes on decadal scale at Mt Etna, interrupting its stable and resilient output trend.

Archaeoseismology can provide a useful chronological tool for constraining earthquakes and documenting significant evidence that would otherwise be lost. In this paper, we report a case of surface faulting on ancient man-made structures belonging to the archaeological site of Santa Venera al Pozzo situated along the eastern flank of Mt. Etna volcano in eastern Sicily (southern Italy), which is affected by well-developed tectonic faults. Geological surveys highlight a set of fractures affecting the archaeological ruins, suggesting the occurrence of a capable fault zone across the area. An integrated geophysical survey was carried out in order to identify the main subsurface tectonic discontinuity ascribable to the fault zone. The information derived from different geophysical techniques, such as electrical resistivity tomography, seismic refraction tomography, ground-penetrating radar, and magnetic surveys allowed us to infer that the fractures observed at the surface could have been produced by coseismic rupture. They are conceivably linked to a strong earthquake that probably occurred in the Roman period, around mid-end of the third-century AD; time constraints are inferred through the dating of buildings of the archaeological site. [ABSTRACT FROM AUTHOR]