Добірка наукової літератури з теми "Pyroclastic fountains"

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. A granular multiphase model has been used to evaluate the action of differently sized particles on the dynamics of fountains and associated pyroclastic density currents. The model takes into account the overall disequilibrium conditions between a gas phase and several solid phases, each characterized by its own physical properties. The dynamics of the granular flows (fountains and pyroclastic density currents) has been simulated by adopting a Reynolds-averaged Navier-Stokes model for describing the turbulence effects. Numerical simulations have been carried out by using different values for the eruptive column temperature at the vent, solid particle frictional concentration, turbulent kinetic energy, and dissipation. The results obtained provide evidence of the multiphase nature of the model and describe several disequilibrium effects. The low concentration (≤5 × 10−4) zones lie in the upper part of the granular flow, above the fountain, and above the tail and body of pyroclastic density current as thermal plumes. The high concentration zones, on the contrary, lie in the fountain and at the base of the current. Hence, pyroclastic density currents are assimilated to granular flows constituted by a low concentration suspension flowing above a high concentration basal layer (boundary layer), from the proximal regions to the distal ones. Interactions among the solid particles in the boundary layer of the granular flow are controlled by collisions between particles, whereas the dispersal of particles in the suspension is determined by the dragging of the gas phase. The simulations describe well the dynamics of a tractive boundary layer leading to the formation of stratified facies during Strombolian to Plinian eruptions.

Abstract. A granular multiphase model has been used to evaluate the action of differently sized particles on the dynamics of fountains and associated pyroclastic density currents. The model takes into account the overall disequilibrium conditions between a gas phase and several solid phases, each characterized by its own physical properties. The dynamics of the granular flows has been simulated by adopting a Reynolds Average Navier-Stokes model for describing the turbulence effects. Numerical simulations have been carried out by using different values for the eruptive column temperature at the vent, solid particles frictional concentration, turbulent kinetic energy, and dissipation. The results obtained underline the importance of the multiphase nature of the model and characterize several disequilibrium effects. The low concentration (≤ 5 · 10–4) sectors lie in the upper part of the granular flow, above the fountain, and above the pyroclastic current tail and body as thermal plumes. The high concentration sectors, on the contrary, form the fountain and remain along the ground of the granular flow. Hence, pyroclastic density currents are assimilated to granular flows constituted by a low concentration suspension flowing above a high concentration basal layer (boundary layer), from the proximal regions to the distal ones. Interactions among solid, differently sized particles in the boundary layer of the granular flow are controlled by collisions between particles, whereas particles dispersal in the suspension is determined by the dragging of the gas phase. The simulations describe well the dynamics of a tractive boundary layer leading to the formation of stratified facies during eruptions having a different magnitude.

The Campo de Calatrava Volcanic Region is located in Central Spain (Ciudad Real province, Castilla-La Mancha) where some eruptions of different intensity and spatial location took place throughout a period of more than 8 million years. As a result, more than 360 volcanic edifices spread over 5000 km2. Eruptions of this volcanic system were derived from alkaline magmas with events of low explosivity (Hawaiian and Strombolian). These events are characterized by three different manifestations: the emission of pyroclasts (cinder and spatter cones) and lava flows; some hydromagmatic events, which lead to the formation of wide craters (maars) and pyroclastic flows; and remnant volcanic activity related to gas emission (CO2), hot springs (hervideros) and carbonic water fountains (fuentes agrias). The methods used for this study are based on analytical studies of geography, geomorphology and geoheritage to identify volcanoes and their resources and attractions linked to the historical-cultural heritage. These volcanoes are a potential economic resource and attraction for the promotion of volcano tourism (geotourism), and they are the basis for achieving a UNESCO Global Geopark Project, as a sustainable territorial and economic management model, to be part of the international networks of conservation and protection of nature and, especially, that of volcanoes.

Between December 2020 and February 2022, the South East Crater of Etna has been the source of numerous eruptions, mostly characterized by the emission of lava fountains, pyroclastic material and short-lasting lava flows. Here we estimate the volume and distribution of the lava deposits by elaborating multi-source satellite imagery. SEVIRI data have been elaborated using CL-HOTSAT to estimate the lava volume emitted during each event and calculate the cumulative volume; Pléiades and WorldView-1 data have been used to derive Digital Surface Models, whose differences provide thickness distributions and hence volumes of the volcanic deposits. We find a good agreement, with the total average lava volume obtained by SEVIRI reaching 73.2 × 106 m3 and the one from optical data amounting to 67.7 × 106 m3. This proves the robustness of both techniques and the accuracy of the volume estimates, which provide important information on the lava flooding history and evolution of the volcano.

Several types of eruptions have occurred at Sakurajima volcano in the past 100 years. The eruption in 1914 was of a Plinian type followed by an effusion of lava. The progression of seismicity of volcanic earthquakes prior to the eruption is reexamined and seismic energy is estimated to be an order of 1014 J. Lava also effused from the Showa crater in 1946. Since 1955, eruptions frequently have occurred at the Minamidake or Showa craters at the summit area. Vulcanian eruptions are a well-known type of summit eruption of Sakurajima, however Strombolian type eruptions and continuous ash emissions have also occurred at the Minamidake crater. The occurrence rate of pyroclastic flows significantly increased during the eruptivity of Showa crater, with the occurrence of lava fountains. Tilt and strain observations are reliable tools to forecast the eruptions, and their combination with the seismicity of volcanic earthquakes is applicable to forecasting the occurrence of pyroclastic flows. An empirical event branch logic based on magma intrusion rate is proposed to forecast the scale and type of eruption. Forecasting the scale of an eruption and real-time estimations of the discharge rate of volcanic ash allows us to assess ash fall deposition around the volcano. Volcanic ash estimation is confirmed by an integrated monitoring system of X Band Multi-Parameter radars, lidar and the Global Navigation Satellite System to detect volcanic ash particles with different wave lengths. Evaluation of the imminence of eruptions and forecasting of their scale are used for the improvement of planning and drilling of volcanic disaster measures.

AbstractWe integrate the different contemporary sources together with new field data on the pyroclastic deposits to make a new volcanological reconstruction of the explosive phases of the 1944 Vesuvius eruption. We adopt the four successive phases of the eruption first defined by Imbò (1945), who made the most detailed contemporary description of the eruption: Phase 1 – effusive, Phase 2 – lava fountains, Phase 3 – mixed explosions and Phase 4 – seismic-explosive. Phase 1 consisted of four days of effusive activity. Phase 2 generated eight successive lava fountains which formed agglutinated spatter in a restricted area around the crater. At distances of > 1 km from the crater, reverse graded, well-sorted, scoria lapilli with up to 94 wt % juvenile material and calculations indicate a volume of 8.2 × 106 m3 DRE (Dense Rock Equivalent) for Phase 2. A short pause in scoria fallout was observed that coincides with the transition between Phases 2 and 3 of the eruption. On the crater rim there is clear evidence for the different phases, owing to the stratification of the deposits; however, away from the crater, stratigraphic breaks suggesting any discontinuity in the eruptive activity are absent. The beginning of Phase 3 is marked by the appearance of abundant dense scoria fragments, coincident with the coarsest part of the lapilli. High-density scoria forms 10 wt % of juvenile material in Phase 2, increasing to 45% in the upper part of Phase 3. Isopach maps derived from field measurements indicate a mean volume of 40.2 × 106 m3 DRE for Phase 3. Distal ash, mainly formed during Phase 3, was dispersed to the SE as far as Albania, and calculations yield a volume of 102 × 106 m3 DRE. Intermittent activity associated with Phase 4 generated ash-rich plumes dispersed towards the SW and contemporary thickness descriptions yield a bulk volume of 4.2 × 106 m3 (2.5 × 106 m3 DRE). Small pyroclastic density currents (PDCs) were observed during Phases 3 and 4. The deposits (200 m from the crater rim) of these currents have been identified on the flanks of the cone. Thin, massive and poorly sorted ash layers, that occur up to 2.5 km from the crater rim, are interpreted to represent the distal facies of these PDCs. Mass discharge rate (MDR) estimates for the paroxysmal phase (end of Phase 2 and start of Phase 3) of this event are around 3.5 × 106 kg/s, however, this increases to > 107 kg/s if the mass of distal ash is taken into account. Column height estimates from fallout isopleths associated with the eruption's paroxysmal phase are > 10 km. Based on the contemporaneous chronicles, we were able to define the type and extent of damage associated with the different styles (or temporal phases) of the eruption. Our calculations demonstrate that the present-day population at risk has doubled compared to 1944. The contemporaneous (and also subsequent) scientific literature underestimated the magnitude and intensity of this eruption and very little attention has been dedicated to the damage that occurred. We suggest that this is at least partly related to the extensive destruction of Neapolitan area and the deaths of tens of thousands of civilians related to the Second World War.

On 16 February 2021, an eruptive paroxysm took place at Mt. Etna (Sicily, Italy), after continuous Strombolian activity recorded at summit craters, which intensified in December 2020. This was the first of 17 short, but violent, eruptive events occurring during February–April 2021, mostly at a time interval of about 2–3 days between each other. The paroxysms produced lava fountains (up to 1000 m high), huge tephra columns (up to 10–11 km above sea level), lava and pyroclastic flows, expanding 2–4 km towards East and South. The last event, which was characterised by about 3 days of almost continuous eruptive activity (30 March–1 April), generated the most lasting lava fountain (8–9 h). During some paroxysms, volcanic ash led to the temporary closure of the Vincenzo Bellini Catania International Airport. Heavy ash falls then affected the areas surrounding the volcano, in some cases reaching zones located hundreds of kilometres away from the eruptive vent. In this study, we investigate the Mt. Etna paroxysms mentioned above through a multi-platform satellite system. Results retrieved from Advanced Very High Resolution Radiometer (AVHRR), Moderate Resolution Imaging Spectroradiometer (MODIS), and Spinning Enhanced Visible and Infrared Imager (SEVIRI), starting from outputs of the Robust Satellite Techniques for Volcanoes (RSTVOLC), indicate that the 17th paroxysm (31 March–1 April) was the most powerful, with values of radiative power estimated around 14 GW. Moreover, by the analysis of SEVIRI data, we found that the 5th and 17th paroxysms were the most energetic. The Multispectral Instrument (MSI) and the Operational Land Imager (OLI), providing shortwave infrared (SWIR) data at 20/30 m spatial resolution, enabled an accurate localisation of active vents and the mapping of the areas inundated by lava flows. In addition, according to the Normalized Hotspot Indices (NHI) tool, the 1st and 3rd paroxysm (18 and 28 February) generated the largest thermal anomaly at Mt. Etna after June 2013, when Landsat-8 OLI data became available. Despite the impact of clouds/plumes, pixel saturation, and other factors (e.g., satellite viewing geometry) on thermal anomaly identification, the used multi-sensor approach allowed us to retrieve quantitative information about the 17 paroxysms occurring at Mt. Etna. This approach could support scientists in better interpreting changes in thermal activity, which could lead to future and more dangerous eruptions.

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.

Les colonnes pyroclastiques se forment lors d'éruptions volcaniques explosives au cours desquelles un mélange de gaz et de particules est éjecté à grande vitesse depuis un évent et peut conduire à la formation de panaches convectifs. La stabilité de ces colonnes dépend de divers paramètres qui peuvent varier au cours du temps et causer l'effondrement partiel ou total du mélange pyroclastique. Ces effondrements donnent naissance à des fontaines éruptives à l'origine de courants de densité pyroclastiques (CDPs). L'objectif de cette thèse est double : étudier (1) les mécanismes de sédimentation des particules dans le panache et la partie diluée des CDPs et (2) les mécanismes d'émergence des CDPs dans les zones d'impacts des fontaines. La méthode choisie est l'approche expérimentale.Une première série d'expériences consiste à mettre en suspension des particules de taille variant de 49 à 467,5 µm dans un dispositif cylindrique et à mesurer la concentration locale de particules de chaque mélange. Pour cela, deux approches indépendantes ont été utilisées et ont donné des résultats similaires : une méthode acoustique et l'utilisation des capteurs de pression. Ces expériences mettent en lumière deux mécanismes de sédimentation des particules : la sédimentation améliorée et la sédimentation retardée. Dans les suspensions de petites particules (78 µm), la vitesse de sédimentation augmente avec la concentration locale de particules en raison de la formation de « clusters » qui chutent à une vitesse quatre fois supérieure à la vitesse terminale de sédimentation des particules individuelles (sédimentation améliorée). En revanche, dans les suspensions de plus grandes particules (467,5 µm), la vitesse de sédimentation diminue avec l'augmentation de la concentration de particules malgré la présence de « clusters » et elle est 30 % inférieure à la vitesse de chutes des particules individuelles (sédimentation retardée). Ces résultats suggèrent que les mécanismes de sédimentation en présence de « clusters » et se produisant dans les panaches où la partie diluée des courants de densité pyroclastiques devraient être pris en compte dans les modèles utilisés pour simuler ces phénomènes volcaniques afin de mieux prédire les caractéristiques des dépôts.Une seconde série d'expériences consiste à simuler une fontaine pyroclastique en relâchant dans un chenal des particules de tailles comprises entre 29 et 269 µm et à une hauteur de 3,27 m. Les résultats montrent que les mélanges dilués (1,6 - 4,4 vol.%) en chute libre s'accumulent dans la zone d'impact pour former des écoulements granulaires concentrés (~ 45 - 48 vol.%) dont la pression de fluide interstitiel compense presque totalement le poids des particules pour des tailles < 76 µm. De plus, la pression de fluide maximale mesurée à l'impact, la distance de parcours des écoulements et l'étirement horizontal des dépôts augmentent avec la diminution de taille des particules. En considérant le dimensionnement des expériences, ces résultats indiquent qu'une pression de fluide interstitielle élevée dans les courants de densité pyroclastiques concentrés peut être générée dans la zone d'impact des fontaines pyroclastiques en effondrement. La petite taille des particules, qui cause une faible perméabilité et un long temps de diffusion de la pression de pore, peut être l'un des facteurs principaux qui causent les longues distances parcourues par les écoulements
Pyroclastic columns form during explosive volcanic eruptions in which a mixture of gases and particles is ejected at high speed from a vent and can lead to the formation of convective plumes. The stability of these columns depends on various parameters that can vary over time and cause partial or total collapse of the pyroclastic mixture. These collapses give rise to eruptive fountains, forming density currents called pyroclastic density currents (PDCs). The objective of this thesis is twofold: to study (1) the mechanisms of particle sedimentation in the plume and the dilute part of PDCs, and (2) the mechanisms of PDC emergence in the impact zones of the fountains. The chosen method is the experimental approach.A first series of experiments involves suspending particles ranging in size from 49 to 467.5 µm in a cylindrical device and measuring the local particle concentration for each mixture. For this purpose, two independent approaches were used and provided similar results: an acoustic method and the use of pressure sensors. These experiments highlight two mechanisms of particle sedimentation: enhanced sedimentation and delayed sedimentation. In suspensions of small particles (78 µm), the sedimentation rate increases with the local particle concentration due to the formation of « clusters » that fall at a speed four times higher than the terminal settling velocity of individual particles (enhanced sedimentation). However, in suspensions of larger particles (467.5 µm), the sedimentation rate decreases with increasing particle concentration, despite the presence of « clusters » and it is 30 % lower than the settling speed of individual particles (delayed sedimentation). These results suggest that the sedimentation mechanisms in the presence of « clusters » occurring in plumes or the dilute part of PDC should be considered in models used to simulate these volcanic phenomena to better predict deposit characteristics.A second series of experiments simulates a pyroclastic fountain by releasing particles of sizes ranging from 29 and 269 µm into a channel at a height of 3.27 meters. The results show that dilute mixtures (1.6 - 4.4 vol.%) in free fall accumulate in the impact zone to form concentrated granular flows (~ 45 - 48 vol.%) whose interstitial fluid pressure nearly compensates for the weight of particles for sizes < 76 µm. Furthermore, the maximum fluid pressure measured at the impact, the flow travel distance, and the horizontal stretching of deposits increase with decreasing particle size. Considering the experiment dimensions, these results indicate that a high interstitial fluid pressure can be generated in the impact zone of collapsing pyroclastic fountains. The small particle size, causing low permeability and a long pressure diffusion time, may be one of the main factors leading to the long runout distances covered by the flows

North Crater occupies the north-western quadrant of the Tongariro Volcanic Centre and represents one of at least eleven vents which have been active on Tongariro since the last glacial maximum. The most recent cone-forming activity at North Crater is thought to have occurred between 14-12 ka ago to produce the distinct, wide, flattopped andesite cone. This project focused mainly on the cone-building eruptions at North Crater, including stratigraphic correlations with distal tephra, interpreting eruptive processes, and establishing the sequence of events during cone construction. Detailed field work identified key stratigraphic sections and facies in the proximal, medial and distal environments. These sections allowed stratigraphic correlations to be made between proximal cone-building facies and distal sheet-forming facies at North Crater, and established a complete North Crater eruption stratigraphy. In the proximal environment, welded and non to poorly welded facies formed from fallout of a lava-fountain, pyroclastic flow or as fallout from a convecting plume. In the medial and distal environment, the lithofacies consist of fallout from a convecting plume and minor pyroclastic flow. Convective fall and non to poorly welded pyroclastic flow deposits dominate the lower eruption stratigraphy suggesting explosive eruptions involving a gas-rich magma. A change to welded deposits produced from lava-fountaining occurs later in the cone-building sequence and suggest a change to lower explosively and eruption of gas-poor magma. Grain size, componentry data, density, petrography and SEM analysis were carried out on representative samples to characterise the different facies, and reveal information about eruption processes. The non to poorly welded deposits are typically made up of vesicular pumice, scoria and mingled clasts of sub-rounded bombs and lapilli. The welded facies are relatively dense and clast outlines are often difficult to distinguish. The eruptives are porphyritic with abundant plagioclase gt clinopyroxene gt orthopyroxene gt opaques. Quartzofeldspathic crustal xenoliths are common and indicate crustal assimilation. Mingled clasts of light and dark glass were found to have microlites present in the dark glass, but were absent in the light glass. Electron microprobe analyses found that the dark and light glass components in a single clast had similar compositions, showing that the contrasting physical appearance of the glass is not due to a different chemical composition. Forty three whole rock XRF analyses showed that the magmas ranged from basaltic andesite to andesite, and Harker variation plots display linear trends typical of magma mixing. Magma mixing as the most important magmatic process is supported by disequilibrium of phenocryst compositions and phenocryst textures. Magma viscosity, bulk density and temperature was determined using MAGMA (Kware), and indicate that they fall into the range of typical andesites. Eruptive activity involved vigorous lava-fountaining, minor convecting eruption plumes and dominant collapsing eruption plumes. This activity has produced welded and non-welded pyroclastic flow and fall deposits to form the large cone seen today. There are significant volcanic hazards associated with this style of activity at North Crater, characterised by lava-fountaining, eruption plume fallout, and widespread pyroclastic flows and lahars extending beyond the ring plain. These could all be potentially devastating to the central North Island of New Zealand.