Calvari, Sonia

Explosive sequences are quite common at basaltic and andesitic volcanoes worldwide. Studies aimed at short-term forecasting are usually based on seismic and ground deformation measurements, which can be used to constrain the source region and quantify the magma volume involved in the eruptive process. However, during single episodes of explosive sequences, integration of camera remote sensing and geophysical data are scant in literature, and the total volume of pyroclastic products is not determined. In this study, we calculate eruption parameters for four powerful lava fountains occurring at the main and oldest Mt. Etna summit crater, Voragine, between 3 and 5 December 2015. These episodes produced impressive eruptive columns and plume clouds, causing lapilli and ash fallout to more than 100 km away. We analyse these paroxysmal events by integrating the images recorded by a network of monitoring cameras and the signals from three high-precision borehole strainmeters. From the camera images we calculated the total erupted volume of fluids (gas plus pyroclastics), inferring amounts from 1.9×109 m3 (first event) to 0.86 × 109 m3 (third event). Strain changes recorded during the first and most powerful event were used to constrain the depth of the source. The ratios of strain changes recorded at two stations during the four lava fountains were used to constrain the pyroclastic fraction for each eruptive event. The results revealed that the explosive sequence was characterized by a decreasing trend of erupted pyroclastics with time, going from 41% (first event) to 13% (fourth event) of the total erupted pyroclastic volume. Moreover, the volume ratio fluid/pyroclastic decreased markedly in the fourth and last event. To the best of our knowledge, this is the first time ever that erupted volumes of both fluid and pyroclastics have been estimated for an explosive sequence from a monitoring system using permanent cameras and high precision strainmeters. During future explosive paroxysmal sequences this new approach might help in monitoring their evolution also to understand when/if they are going to finish. Knowledge of the total gas and pyroclastic fractions erupted during each lava fountain episode would improve our understanding of their processes and eruptive behaviour.

A number of tumuli formed on the aa-dominated lava fan complex which developed in the medial zone of the 1983 flow-field of Mount Etna during the later stages of the eruption. This complex flow-field formed on shallow sloping ground below a scarp between 1900 and 1700 m asl. A major tube system fed a branching tube network in the fan complex. Numerous tumuli and break-outs of lava formed in the fan. Three main types of tumulus are identified: (1) Focal tumuli, which are formed from the break-up and uplift of `old´, thick lava crust and themselves become sustained sites for the distribution of lava both as flows and within distributary tubes. These focal tumuli are significant centres associated with major tubes. (2) Satellite tumuli, which are typically elongate, whale-back shaped features that branch out from focal tumuli. These satellite tumuli were initially lava flows erupted from a focal tumulus. The crust of the flow slowed or came to a halt and the rigid crust became uplifted and fractured, forming a dome-shaped ridge feature. These satellite tumuli continued to be fed from the focal tumulus and became sites of lava emission with numerous break-outs. (3) Distributary tumuli formed on the fan associated with short-lived break-outs from tubes and are relatively simple structures formed from limited effusion of toey lobes and pahoehoe lava. The major tumuli on the fan complex show distinct dilation fractures. The fracture surfaces provide good exposure of the crust and three distinct zones are recognised – an upper zone showing columnar jointing, a middle zone consisting of planar fracture surfaces and a basal zone with distinctive banded planar fracture surfaces showing evidence of both brittle and ductile formation. Using these data a model is proposed for tumulus growth. Field analysis of the fan complex shows how it was fed by a branching tube system, leading to flow thickening, formation of tumuli and numerous ephemeral boccas.

Absolute chronologies of active volcanoes and consequently timescales for eruptive behaviour and magma production form a quantitative basis for understanding the risk of volcanoes. Surprisingly, the youngest records in the geological timescale often prove to be the most elusive when it comes to isotopic dating. Absolute Holocene volcanic records almost exclusively rely on 14C ages measured on fossil wood or other forms of biogenic carbon. However, on volcanic flanks, fossil carbon is often not preserved, and of uncertain origin when present in paleosols. Also, low 14C-volcanic CO2 may have mixed with atmospheric and soil 14C-CO2, potentially causing biased ages. Even when reliable data are available, it is important to have independent corroboration of inferred chronologies as can be obtained in principle using the 40K/40Ar decay system. Here we present results of a 40Ar/39Ar dating study of basaltic groundmass in the products from the Pleistocene e Holocene boundary until the beginning of the historic era for the north-northeastern flank of Stromboli, Aeolian Islands, Italy, identifying a short phase of intensified flank effusive activity 7500 500 a ago, and a maximum age of 4000 900 a for the last flank collapse event that might have caused the formation of the Sciara del Fuoco depression. We expect that under optimum conditions 40Ar/39Ar dating of basaltic groundmass samples can be used more widely for dating Holocene volcanic events.

Effusive activity at Stromboli is uncommon, and the 2002–2003 flank eruption gave us the opportunity to observe and analyze a number of complex volcanic processes. In particular, the use of a handheld thermal camera during the eruption allowed us to monitor the volcano even in difficult weather and operating conditions. Regular helicopter-borne surveys with the thermal camera throughout the eruption have significantly improved (1) mapping of active lava flows; (2) detection of new cracks, landslide scars, and obstructions forming within and on the flanks of active craters; (3) observation of active lava flow field features, such as location of new vents, tube systems, tumuli, and hornitos; (4) identification of active vent migration along the Sciara del Fuoco; (5) monitoring of crater's inner morphology and maximum temperature, revealing magma level changes within the feeding conduit; and (6) detection of lava flow field endogenous growth. Additionally, a new system developed by A. J. L. Harris and others has been applied to our thermal data, allowing daily calculation of effusion rate. These observations give us new insights on the mechanisms controlling the volcanic system.

The 2004–05 eruption of Etna was characterised by outpouring of degassed lava from two vents within Valle del Bove. After three months of eruption lava volumes were estimated to be between 18.5 and 32 × 106 m3, with eruption rate between 2.3 and 4.1 m3/s. Petrological analyses show that magma is resident in the shallow plumbing system, emplaced during the last South-East Crater activity. SO2 flux data show no increase at the onset of the eruption and SO2/HCl ratios in gas emitted from the eruptive fissure are consistent with a degassed magma. No seismic activity was recorded prior to eruption, unlike eruptions observed since the 1980's. The purely effusive nature of this eruption, fed by a degassed, resident magma and the fracture dynamics suggest that magmatic overpressure played a limited role in this eruption. Rather, lateral spreading of Etna's eastern flank combined with general inflation of the edifice triggered a geodynamically-controlled eruption.

Volcanic paroxysmal explosive activity has enormous potential destructive power and usually causes widespread damages to the Society (NAS–National Academies of Sciences Engineering Medicine, 2017). Serious problems can occur even during explosive activity related to modest eruptions, such as the case of the 2010 Eyiafjallajokull eruption in Iceland that paralyzed the air traffic in the European continent and in the North Atlantic. In general, a crucial issue, and also an intriguing challenge, is to evaluate the state of the preparatory phase leading to an eruption. It is important to understand the characteristics of the volcano state both for the long-term preparatory phases, usually anticipating the strongest eruptions, and also for the medium- to short-term phases preceding the more frequent and usually less powerful eruptions, but with more immediate consequences. The first paper of the collection gives an overview of the long-term dynamics of the volcanic paroxysmal activity at andesitic and dacitic volcanoes during 1960–2010 (Zobin). In this study two groups of eruptions with VEI 5–6 and VEI 3–4 were considered. The main tool used was the seismic monitoring of the volcanoes. The eruptions of the first group are characterized by long periods of quiescence (longer than 120 years) and precursory volcano tectonic seismic swarms. The second group develops in more individual styles, each typical of a certain volcano. The study suggests that the eruptions with higher VEI are related to a plugged magmatic conduit, whereas eruptions with lower VEI are usually associated to open conduits. As a specific case of volcano producing frequent explosive eruptions with low VEI, the second paper of this Research Topic considers the Etna eruptive activity during 2009–2017 by using ground deformation and strain data (Aloisi et al.). Etna volcano was characterized over this period by an incredible lively eruptive activity. This comprised 44 lava fountain episodes from the New South East Crater, two sequences of lava fountains from the Voragine crater, as well as some periods of summit effusive activity with a more prolonged supply of lava flows. The authors produced a 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). Ganci et al. investigated the middle-term behavior of Mt. Etna. Detection of the thermal anomalies allowed retrieval of radiative power time-series and associated volumes, and thus characterization of each of the paroxysms in terms of intensity and magnitude. Topographic data derived from satellite imagery gave the total volume of products erupted from 2005 to 2015. Overall, the integration of thermal and topographic data highlighted that the 2011–2012 lava fountains had higher intensity than those of 2013–2015 and that the total volumes erupted from 2005 to 2015 resulted being below those typically erupted by Mt. Etna in a decadal time scale. Recent eruptive activity from Etna offered a great opportunity to measure the SO2 gas flux from ground-based and satellite instruments prior to, during and after the paroxysmal sequences (D’Aleo et al.), allowing to detect the switch from an active crater to another on the basis of increased and decreased SO2 flux. Based on these measurements, the degassed magma volume was estimated, leading to inferring on the fraction of erupted magma compared to the amount emplaced into the shallow feeding system. Another important parameter essential for hazard assessment is Mass Eruption Rate (MER), which has been estimated for 47 paroxysmal episodes using a Doppler Radar installed on Etna (Freret-Lorgeril et al.). The Lidar detection of the volcanic plume, volcanic ash concentration in atmosphere, and characterization of optical properties of volcanic particles, represent the distal characterization of the paroxysmal activity, essential for modeling volcanic ash clouds and their impact (Boselli et al.). Mapping of the erupted products and measurement and retrieval of the main eruptive parameters are crucial to understand the eruptive dynamics and investigate into the volcano shallow feeder system. Nowadays, volcanology can take advantage from different measurement techniques and methodologies spanning from direct field observations to ground- and satellite-based remote sensing. Indeed, the opportunity to inspect volcanic phenomena by multidisciplinary approaches allows getting overall view of the volcanic scenarios. The study of the tephra fallout produced by the 3 March 2015 lava fountain at Villarrica basaltic-andesitic volcano by field surveys, laboratory analysis, and satellite imagery provided characterization and parameterization of the paroxysm (Romero et al.). The integration of the key constraints shaded light in the mechanism that triggered the paroxysmal explosion in a volcano that commonly shows persistent activity associated to lava lake-like dynamics. At Mt. Etna, the multidisciplinary approach permitted the study of the recent paroxysmal activity at the volcano summit at both short- and long-time scale. Andronico et al. focused on the 25–26 October 2013 lava fountain. Using field observations and ground- and satellite-based imagery and photogrammetry, they mapped the lava flow field and estimated the volume of the pyroclastic cone. Mapping of the fallout deposit in field and laboratory analysis gave grain size distribution, composition, and total erupted mass. The hazards posed by explosive activity is a function of the size of the eruption and of the elevation reached by the eruptive column and ash plume, that may impact the atmosphere or even the stratosphere and affect aviation, infrastructures, viability, climate, and health. The Volcanic Explosivity Index (VEI; Newhall and Self, 1982) is a way to estimate the size of an explosive eruption, with a VEI 6 posing severe hazard all over the country and a VEI 2–4 having only local effects. The last two papers of this collection evaluate the hazard posed by two end-member eruptions, such as the VEI 6 potential for Öræfajökull volcano in Iceland (Barsotti et al.) and the VEI 2–4 of Etna volcano in Sicily (Calvari et al.). Barsotti et al. use a numerical model to assess the impact on infrastructures in Iceland caused by a rare but potentially highly destructive activity occurring at the Öræfajökull volcano, which experienced this event only once during the last 1100 years. Etna instead has experienced hundreds of mild VEI 2–4 eruptions during the last decades, and Calvari et al. statistically analyze several of these events to propose an empirical law that allows them to evaluate the maximum elevation of an ash plume as soon as the lava fountain height has reached the peak steady value, measured using the images recorded by the monitoring thermal camera network. Most of the papers comprised in this collection pertain to Mt. Etna, which has been identified as Decade Volcano by the International Association of Volcanology and Chemistry of the Earth’s Interior (IAVCEI), thus confirming itself as an ideal volcano laboratory (Bonaccorso et al., 2004). The empirical laws thus far proposed for predicting the extension and impact of the eruptive clouds (Calvari et al.) are thus applicable just to this volcano, and in the future a greater effort has to be dedicated to multidisciplinary studies involving more volcanoes and cross-analyses of their data (e.g., Zobin) in order to obtain general statements, laws and/or formulas that could be applicable to those volcanoes which are less monitored. More peculiar and still unknown processes could arise from in depth studies of other basaltic volcanoes from the Earth and other planets, increasing our ability to understand and predict their behavior. A growing attention is dedicated to the use of UAV (unmanned aerial vehicles; e.g., Neri et al., 2017; Favalli et al., 2018), remote-sensing (Boselli et al.; Calvari et al.; D’Aleo et al.; Freret-Lorgeril et al.) and satellite measurements (Ganci et al.) for their safety issues, low cost and broad perspective, that result in an important integration of field studies (Andronico et al.; Romero et al.), numerical simulations (Barsotti et al.), and laboratory experiments (e.g., Dellino et al., 2010). It is a general feeling that the innovative methods will grow even more in the future, possibly coupled with virtual reality, allowing results and simulations of volcanic processes impossible to imagine nowadays.

Volcanic crises are complex and especially challenging to manage. Volcanic unrest is characterised by uncertainty about whether an eruption will or will not take place, as well as its possible location, size and evolution. Planning is further complicated by the range of potential hazards and the variety of disciplines involved in forecasting and responding to volcanic emergencies. Effective management is favoured at frequently active volcanoes, owing to the experience gained through the repeated ‘testing’ of systems of communication. Even when plans have not been officially put in place, the groups involved tend to have an understanding of their roles and responsibilities and those of others. Such experience is rarely available at volcanoes that have been quiescent for several generations. Emergency responses are less effective, not only because of uncertainties about the volcanic system itself, but also because scientists, crisis directors, managers and the public are inexperienced in volcanic unrest. In such situations, tensions and misunderstandings result in poor communication and have the potential to affect decision making and delay vital operations. Here we compare experiences on communi- cating information during crises on volcanoes reawakening after long repose (El Hierro in the Canary Islands) and in frequent eruption (Etna and Stromboli in Sicily). The results provide a basis for enhancing commu- nication protocols during volcanic emergencies.

Thermal imagery obtained with portable infrared cameras is widely used to track and measure volcanic phenomena. In the case of explosive eruptions, both air and ground-based thermal monitoring have enabled collection of data streams from relatively safe distances. Analysis of these data have enabled the characterisation of different explosive regimes, parameterisation of eruptive plumes, and assessment on the dynamics occurring in the shallow system. Here we explore the suitability of infrared imagers for investigating the short time scale eruptive behaviour of three basaltic volcanoes. We present high-time resolution thermal image data-sets recorded at Etna, Stromboli and Kīlauea volcano. At the time of observations, all three exhibited pulsed degassing. Signal processing of the mean apparent temperature time-series highlights four broad classes of cyclic temperature changes at the three volcanoes based on characteristic time-scales revealed in the periodograms: (1) <15 s, (2) ~20-50 s, (3) ~1-10 min, and (4) 12–90 min. Based on previous studies and integrating time-series results with qualitative visible and thermal observations and, in case of Kīlauea, also with SO2 column amounts in the plume, we hypothesise that short cycles relate mainly to bursting of overpressured gas bubbles at the magma surface, while long cycles might be associated with mechanisms of gas slug formation and ascent, and to the emplacement and drainage of a lava lake. At Kīlauea, slow fluctuations may reflect periodic variations of the lava lake surface level. The data from all three volcanoes reveal superimposition of degassing cycles of different frequencies, suggesting link through common magmatic processes and physical properties.

Lava flow surface morphologies are like pages of a book. If we are able to read the writing of that book, we can understand its content, and learn, act, and react accordingly. In the same way, if we understand lava surface morphology, recognise how it formed and the hazard it poses while flowing, we can adopt actions to protect from lava flow invasion our villages, infrastructures and local population. The surface of lava is a function of intrinsic and extrinsic qualities, and their combination results in different shapes, sizes, and complexities, as well as in different hazards. Initial sheet flows spreading at high speed have great potential for devastating land, as happened in Hawaii in May-August 2018 (Neal et al., 2018). However, their destructive potential significantly decreases with time and distance from the vent. Conversely, lava oozing from the distal exit of lava tubes moves slowly but allows the tubes to expand, increasing gradually and slowly the potential hazard for invasion of more remote lands. In this paper, I present an overview of diverse lava flow surfaces, morphologies and structures in a framework of their generating eruptive parameters, in order to suggest preliminary but prompt hazard evaluations that could be applied during the initial phases of effusive volcanic crises at basaltic volcanoes worldwide.

Bearing in mind the destructive potential of tsunamis induced by volcanic landslides, the tsunamigenic event occurring at Stromboli volcano in Italy on 30 December 2002 has been reexamined here, by means of visible images and slope stability analysis. This was one of the few examples in the world of a flank collapse occurring at a volcano that was directly observed. We present the results of stability analyses, together with a sequence of photos collected from a helicopter a few minutes before the collapse. The result of this study is that the sequence of landslides triggering the 2002 Stromboli tsunami can be defined as the final stage of a lateral magma intrusion that exerted a high thrust at high altitude, destabilizing the entire slope. This study allows a more complete understanding of the event that took place on Stromboli on 30 December 2002. Furthermore, the approach used here, if appropriately modified, can be used in other contexts, contributing to the understanding of the condition that leads to tsunamigenic landslides