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Phreatic explosions (or eruptions) represent very common and dangerous phenomena occurring in active volcanic and geothermal regions. In spite of this, the understanding of their dynamics and the assessment of their hazards are still scarcely known and quantified. In the present work we try to simulate the dynamics of a phreatic explosion by using a transient, two-dimensional, three-phase flow model. Simulations predict the effects of an explosion of a mixture of compressed gases and ground particles into the atmosphere. The governing transport equations of mass, momentum, and energy are solved by a finite difference multifield scheme for both solids and gas phases. We performed several simulations, assuming different initial conditions and system parameters. The model allows analysis of the timewise and spatial distribution of the groundhugging surge generated by the explosion and estimation of its potential hazards in terms of dynamic pressure, ash-in-air concentration, and temperature.

The net effect of liquid magma composition and water content on the behavior of explosive volcanic eruptions was investigated by the use of numerical simulation. The two-phase flow models employed allow description of the eruptive process from the conduit entrance at the top of the magma chamber up to the pyroclastic dispersion processes in the atmosphere. Results indicate a strong influence of magma composition on eruption intensity, whereas the eruptive style appears to be mainly controlled by the total water content.

The role of anhydrous magma composition, water content, and crystal content on the dynamics of explosive eruptions is investigated by modeling the ascent of magma along volcanic conduits and the subsequent pyroclastic dispersion in the atmosphere, described in a companion paper [Neri, A., Papale, P., Macedonio, G., 1998. The role of magma composition and water content in explosive eruptions: 2. Pyroclastic dispersion dynamics. J. Volcanol. Geotherm. Res., 87, 95–115.]. The conduit model used is based on the solution of the fundamental transport equations assuming steady-state and isothermal flow conditions, and includes a composition-based description of magma properties and their variations along the conduit. This study stems from the well-documented vertical compositional variation of many pyroclastic deposits, often associated with reconstructed variations in initial water content. The results of the modeling show complex and sometimes non-intuitive dependence of the distribution of the flow variables on magma composition, crystal and water contents. In general, a water content decrease is expected to produce a decrease in mass flow-rate, decrease in pressure and velocity along the conduit, an increase in the exit gas volume fraction, and a decrease in velocity, pressure, and mixture density at the conduit exit. Reverse variations are expected to occur by decreasing the degree of chemical evolution of the liquid at a constant water content, apart from exit velocities which show more complex variations. The overall effect of increasing crystals is in general similar to that of increasing the degree of chemical evolution of the liquid, or decreasing the water content. The above results are to a large extent interpreted in terms of variations in magma viscosity, which is recognized as the critical magma property besides water content in the dynamics of magma ascent. The common compositional trend of explosive eruptions characterized by chemically evolved, water-richer and crystal-poorer magma erupted first is predicted to be associated with variations in the evolution of the eruption dynamics, depending on the relative magnitude of the changes. However, the exit velocity always decreases in the above trend, and the mass flow-rate increases in most relevant cases, comparing well with the results of chemical and stratigraphic studies of the deposits from explosive eruptions.

International audience; The MU-RAY detector has been designed to perform muon radiography of volcanoes. The possible use on the field introduces several constraints. First the electric power consumption must be reduced to the minimum, so that the detector can be solar-powered. Moreover it must be robust and transportable, for what concerns the front-end electronics and data acquisition. A 1 m2 prototype has been constructed and is taking data at Mt. Vesuvius. The detector consists of modules of 32 scintillator bars with wave length shifting fibers and silicon photomultiplier read-out. A dedicated front-end electronics has been developed, based on the SPIROC ASIC. An introduction to muon radiography principles, the MU-RAY detector description and results obtained in laboratory will be presented.

[1] We apply a novel computational approach to assess, for the first time, volcanic ash dispersal during the Campanian Ignimbrite (Italy) super-eruption providing insights into eruption dynamics and the impact of this gigantic event. The method uses a 3D time-dependent computational ash dispersion model, a set of wind fields, and more than 100 thickness measurements of the CI tephra deposit. Results reveal that the CI eruption dispersed 250–300 km 3 of ash over 3.7 million km 2 . The injection of such a large quantity of ash (and volatiles) into the atmosphere would have caused a volcanic winter during the Heinrich Event 4, the coldest and driest climatic episode of the Last Glacial period. Fluorine-bearing leachate from the volcanic ash and acid rain would have further affected food sources and severely impacted Late Middle-Early Upper Paleolithic groups in Southern and Eastern Europe. Citation: Costa, A., A. Folch, G. Macedonio, B. Giaccio, R. Isaia, and V. C. Smith (2012), Quantifying volcanic ash dispersal and impact of the Campanian Ignimbrite super-eruption, Geophys. Res. Lett., 39, L10310, doi:10.1029/2012GL051605.

The flow of gas, magma and pyroclasts through a volcanic conduit produces erosion of the conduit wall. Erosion may be produced by the impact of pyroclasts on the conduit wall, fluid shear stress at the wall, conduit wall collapse, and volcanic tremor. Using a two-phase flow non-equilibrium model of magma ascent along the volcanic conduits demonstrated that the erosion due to the impact of particles on the wall can occur only above the magma fragmentation level of the conduit where the particles or pyroclasts remove the wall material by an abrasion process. This abrasion process was found to be the largest near the conduit exit where the gas-magma velocities are the largest. The erosion due to the fluid shear stress at the wall can be produced along the entire length of a conduit, depending on the wall roughness and yield strength of wall rocks. This shear stress is the largest near the magma fragmentation level where the gas-magma viscosity and velocity gradients are very large. The collapse of conduit wall due to the difference between the gas-magma and lithostatic pressures can occur below and above the magma fragmentation level, causing the production of lithics directly when the wall collapses inward, and indirectly when the wall collapses outward. The effectiveness of different erosion mechanisms was tested with the magma characteristics, conduit geometry, and wall rock properties pertaining to the AD 79 eruption of Vesuvius. It was found that during the white and gray magma plinian eruption phases the lithics should had come from the deep as well as from the shallow regions of the conduit. The conclusions from erosion modeling are also consistent with the limited field data whereby the gray magma phase deposits are associated with larger lithic content and larger proportion of deep limestone fragments.

A complex thermo-fluid dynamic model was employed to model collapsing volcanic columns. The two-phase flow model accounts for the mechanical and thermal nonequilibrium between the gas and solid particles. The gas phase involves water vapor and air, and the solids phase involves only one particle size class. The particle collisions, which produce particle viscosity and pressure, were modeled by a kinetic theory model in terms of the granular temperature, whereas the gas phase turbulence was modeled by a turbulent subgrid scale model. The partial differential equations of conservation of mass, linear momentum, energy, and granular temperature were numerically solved for an axisymmetric flow configuration with different vent diameters and two-phase flow conditions. The numerical solutions involved different grid sizes and computational domains in order to assess the adequacy of the model and computational procedure. The results from simulations of collapsing volcanic columns show how after an initial period of fountain building the columns collapse and build radially spreading pyroclastic flows and inward moving column material which is recycled by the columns. For a low-height collapsing column it was found that the fountain reaches a steady state height, whereas for columns with collapsing heights of several kilometers and fine computational grids the fountain heights vary cyclically with periods which are influenced by the dynamics of material recirculation into the columns. The radially spreading pyroclastic flows of the collapsed columns were found to develop convective instabilities whereby rising clouds of gas and particles are developed on the top of the flows. In very large scale volcanic eruptions the numerical results predicted multiple rising clouds on very thick pyroclastic flows. The results from simulations were shown to be consistent with simple column modeling approaches, laboratory experiments, and field observations.

[1] We develop a model to describe ash aggregates in a volcanic plume. The model is based on a solution of the classical Smoluchowski equation, obtained by introducing a similarity variable and a fractal relationship for the number of primary particles in an aggregate. The considered collision frequency function accounts for different mechanisms of aggregation, such as Brownian motion, ambient fluid shear, and differential sedimentation. Although model formulation is general, here only sticking efficiency related to the presence of water is considered. However, the different binding effect of liquid water and ice is discerned. The proposed approach represents a first compromise between the full description of the aggregation process and the need to decrease the computational time necessary for solving the full Smoluchowski equation. We also perform a parametric study on the main model parameters and estimate coagulation kernels and timescales of the aggregation process under simplified conditions of interest in volcanology. Further analyses and applications to real eruptions are presented in the companion paper by Folch et al.

An unsteady, open-channel model is used to simulate lahars (discharge, flow depth, peak arrival time, etc.), under the restriction of negligible volume and solid fraction variations and of channelled flow. The model is able to deal with the propagation of steep fronts and transitions from a supercritical to a subcritical flow regime, and vice versa, conditions frequently occurring on account of the rapidity of lahar-triggering mechanisms (large water-debris volumes are often mobilized in a very short time) and of topographical characteristics. The model is based on equations governing balance of mass and momentum, while the energy dissipation term is expressed as a power function of the flow depth and discharge. Two past lahars at Mount St. Helens, the Pine Creek and Muddy River lahars of May 18, 1980, are simulated and the results obtained are discussed. For these lahars the triggering mechanism was the melting of snow and ice due to pyroclastic flows and surges. Many millions of cubic meters of water (mostly from the melting of ice and snow) and sediments (eroded material and pyroclastic products) were mobilized in only a few minutes. The simulation begins at some kilometers from the vent, where lahar volumes reach an almost constant value and the flow is channelled. The triggering mechanism plays a fundamental role in the downvalley destructive power of a lahar. Initial hydrographs characterized by steep slopes, such as those occurring as a consequence of pyroclastic flows and surges, rapidly flatten downvalley, as predicted by the model. Conversely, hydrographs with a prolonged peak discharge, even if the peak value is smaller, undergo little changes over distances of lens of kilometers, with disastrous consequences, as in the case of the 1980 North Fork lahar at Mount St. Helens, triggered by the seismic shaking of an avalanche debris.

The active crater of Guagua Pichincha volcano is located only about 10 km west of Quito, the capital city of Ecuador. Archeological and historical records confirm that, in the last few millennia, human communities were repeatedly affected by volcanic eruptions in this region. Tephrostratigraphic studies and 14 C datings indicate that the eruptive history of Pichincha in the last 40,000 years has been characterized mostly be explosive eruptions with an average frequency of one event every few centuries. These eruptions produced relatively small volumes of dacitic pumice fallout and pyroclastic surge and flow. Eruptions of the last 10,000 years has also included lava dome growth and lateral blasts by dome explosions. Based upon the history of the volcano, the explosive eruptions of 970 and 1660 A.D. were selected as those representing, in the case of reactivation, the maximum phenomena to be expected. The eruption behaviour (ash fallout and pyroclastic surge) has been simulated by physical-numerical models on digitized topographic map, and the results have been used to evaluate the harzardous areas.

Magma flow during explosive volcanic eruptions has been described assuming rigid conduits with simple cylindrical or planar geometries. Here we study the dynamics of explosive volcanic flows to take account of the role of elastic deformation of the conduit influenced by local magmatic pressure. Three cases are investigated: a dyke with elliptical cross-section, a cylindrical conduit and a deep dyke connected to a shallow cylinder. The model CPIUC (Macedonio et al., 2005) was used for simulations and generalized to account for elastic deformations of the conduit cross-section area due to magmatic overpressure. Fragmentation level is typically deeper in a dyke than in a cylinder. For flows in wide dykes pressure at the fragmentation depth can be lower than the surrounding lithostatic pressure by several tens of MPa, indicating that the wall-rocks of the dyke will be unstable, constraining the dyke width and eventually blocking the eruption. On the other hand, when the fragmentation level is shallow the corresponding lithostatic pressure is not large enough to close the dyke and eruptions from wide dykes are possible. The behaviour changes drastically when we assume the conduit is a dyke at depth that evolves to a cylinder near the surface. In this case even very wide dykes can be stable because the fragmentation level moves into the cylindrical region where deformation is negligible.

An algorithm (and the relative C program) for the triangulation of a set of distinct arbitrarily distributed points is proposed. The set of triangles satisfies the condition of maximum smallest angles (MAX-MIN angle criterion). Some applications are presented about the reconstruction of the volume between geological horizons and about the interpolation of bivariate data, when function values are available at irregularly-spaced data points.

A 2D1/2 model of volcanic ashes (tephra) dispersion, based on an analytical solution of the advection-diffusion equation, is presented. By this model the correlation between a double maximum in thickness and a punctual bimodal size distribution in plinian deposit of volcanic eruptions is outlined for the first time. Both the features are explained by the combined effect of vertical wind profile and of mass distribution along the eruptive volcanic cloud. The model is applied to 1980 Mt.St.Helens plinian eruption. The good reproduction of field data suggests the effective utilization of this model for hazard evaluation from volcanic ash fall.

One of the major problems concerning the assessment of volcanic hazard at Vesuvius is to determine the type and size of the eruptive event that will characterize the volcano when it becomes active once again. During its history, Somma-Vesuvius has exhibited different types of activity, ranging from quiet lava emission to moderate strombolian activity, to catastrophic plinian eruptions. Available data support a behavior model characterized by the increasing size and explosiveness of the eruptions with increasing repose time, as a consequence of a roughly constant periodic supply of deep basic magma to a shallow magma chamber and differentiation and mixing in the chamber. After the A.D. 79 eruption, a homogeneous HK (high potassium) nature of erupted products was reflected by a magma alimentation rate roughly estimated at 1.5–2.0 millions of cubic meters per year. Assuming no major changes have occurred in the feeding system of the volcano after its last eruption in 1944, a volume of 40–70 × 10 6 m 3 magma could be considered presently available for a renewal of activity at Vesuvius. The emission of such a mass of magma during a single eruption would result into the largest event since the highly disruptive 1631 subplinian eruption. Presently, no possibility exists to forecast the eruptive character of such an eruption, and either a “ultrastrombolian” or a “subplinian” case appear equally possible. The latter possibility implies the highest potential hazard. This paper provides the numerical simulations of the main eruptive phenomenon that probably will occur during this “maximum expected event”: the fallout of tephra from a high, sustained eruption column. After the initial explosive opening of the vent, the scenario consists of the formation of a high convective column with lee-side fallout of pumice and lithic fragments, accompanied and followed by column collapses generating pyroclastic flows and surges. The column behavior was numerically simulated by using the Wilson and Walker column model with the following input parameters: mass eruption rate between 0.4 and 2.0 × 10 7 kg s −1 , gas content between 5 and 10 wt.%, lithic content 20 wt.%, basal radius of the column larger than the minimum value for subsonic initial velocity of the jet. The resulting maximum column height and maximum gas thrust height were respectively 11–16 km and below 1000 m. The tephra fallout was modeled by a continuity equation solved on a nonuniform three-dimensional grid. The input parameters to the model are: seasonal wind velocity profiles; eddy diffusion coefficients of 3000 m 2 s −1 (horizontal) and 50 m 2 s −1 (vertical); particles whose nature and grain size are inferred after data on pyroclastic deposits from ancient Vesuvius eruptions distributed along a vertical linear source. The hazard implications of the different modeled cases are briefly discussed.

Risks and damages associated with lava flows propagation (for instance the most recent Etna eruptions) require a quantitative description of this phenomenon and a reliable forecasting of lava flow paths. Due to the high complexity of these processes, numerical solution of the complete conservation equations for real lava flows is often practically impossible. To overcome the computational difficulties, simplified models are usually adopted, including 1-D models and cellular automata. In this work we propose a simplified 2D model based on the conservation equations for lava thickness and depth-averaged velocities and temperature which result in first order partial differential equations. The proposed approach represents a good compromise between the full 3-D description and the need to decrease the computational time. The method was satisfactorily applied to reproduce some analytical solutions and to simulate a real lava flow event occurred during the 1991-93 Etna eruption., 4 pages, 4 figures

[1] Phlegrean Fields is an active and densely populated caldera near Naples (Italy). Two major unrest episodes characterized its recent history, each leading to remarkable ground uplift and followed by slow subsidence. Fumaroles near the caldera centre underwent important chemical changes during these volcanic crises. Based on published data we show that a correlation exits between ground displacement and gas composition. Numerical modelling of hydrothermal circulation shows that periods of enhanced fluid injection at the base of the hydrothermal system, are consistent with the observed chemical variations. The model predicts an average increase in pore pressure and temperature within the system, suggesting potential effects on ground deformation. Literature data and simulation results show that periods of intense magmatic degassing could explain most of the features characterizing recent bradyseismic crises and should be considered a potential trigger for the unrest at Phlegrean Fields, as well as at other calderas in the world.

[1] A multiparticle thermofluid dynamic model was developed to assess the effect of a range of particle size on the transient two-dimensional behavior of collapsing columns and associated pyroclastic flows. The model accounts for full mechanical and thermal nonequilibrium conditions between a continuous gas phase and N solid particulate phases, each characterized by specific physical parameters and properties. The dynamics of the process were simulated by adopting a large eddy simulation approach able to resolve the large-scale features of the flow and by parametrizing the subgrid gas turbulence. Viscous and interphase effects were expressed in terms of Newtonian stress tensors and gas-particle and particle-particle coefficients, respectively. Numerical simulations were carried out by using different grain-size distributions of the mixture at the vent, constitutive equations, and numerical resolutions. Dispersal dynamics describe the formation of the vertical jet, the column collapse and the building of the pyroclastic fountain, the generation of radially spreading pyroclastic flows, and the development of thermal convective instabilities from the fountain and the flow. The results highlight the importance of the multiparticle formulation of the model and describe several mechanical and thermal nonequilibrium effects. Finer particles tend to follow the hot ascending gas, mainly in the phoenix column and, secondarily, in the convective plume above the fountain. Coarser particles tend to segregate mainly along the ground both in the proximal area close to the crater rim because of the recycling of material from the fountain and in the distal area, because of the loss of radial momentum. As a result, pyroclastic flows were described as formed by a dilute fine-rich suspension current overlying a dense underflow rich in coarse particles from the proximal region of the flow. Nonequilibrium effects between particles of different sizes appear to be controlled by particle-particle collisions in the basal layer of the flow, whereas particle dispersal in the suspension current and ascending plumes is determined by the gas-particle drag. Simulations performed with a different grain-size distribution at the vent indicate that a fine-grained mixture produces a thicker and more mobile current, a larger runout distance, and a greater elutriated mass than the coarse-grained mixture.

Received 12 April 2001; revised 2 April 2002; accepted 7 May 2002; published 1 Feburary 2003. [1] A general methodology of pyroclastic fall hazard assessment is proposed on the basis of integrated results of field studies and numerical simulations. These approaches result in two different methods of assessing hazard: (1) the ‘‘field frequency,’’ based on the thickness and distribution of past deposits and (2) the ‘‘simulated probability,’’ based on the numerical modeling of tephra transport and fallout. The proposed methodology mostly applies to volcanoes that, by showing a clear correlation between the repose time and the magnitude of the following eruptions, allows the definition of a reference ‘‘maximum expected event’’ (MEE). The application to Vesuvius is shown in detail. Using the field frequency method, stratigraphic data of 24 explosive events in the 3–6 volcanic explosivity index range in the last 18,000 years of activity are extrapolated to a regular grid in order to obtain the frequency of exceedance in the past of a certain threshold value of mass loading (100, 200, 300, and 400 kg/m 2 ). Using the simulated probability method, the mass loading related to the MEE is calculated based on the expected erupted mass (5 10 11 kg), the wind velocity profiles recorded during 14 years, and various column heights and grain-size populations. The role of these factors was parametrically studied performing 160,000 simulations, and the probability that mass loading exceeded the chosen threshold at each node was evaluated. As a general rule, the field frequency method results are more reliable in proximal regions, provided that an accurate database of field measurements is available. On the other hand, the simulated probability method better describes events in middle distal areas, provided that the MEE magnitude can be reliably assumed. In the Vesuvius case, the integration of the two methods results in a new fallout hazard map, here presented for a mass loading value of 200 kg/m 2 . INDEX TERMS: 8404 Volcanology: Ash deposits; 3210 Mathematical Geophysics: Modeling; 1035 Geochemistry: Geochronology; 5480 Planetology: Solid Surface Planets: Volcanism (8450); 9335 Information Related to Geographic Region: Europe; KEYWORDS: Vesuvius, ash fallout, explosive eruptions, volcanic hazard, numerical modeling, hazard mapping

The thermofluid dynamics of pyroclastic flows down the slopes of Vesuvius (Italy) were investigated using physical modeling of the magma ascent and pyroclastic dispersal processes. The expected properties and conditions of the magma, such as its anhydrous composition, water content, and temperature, were based on the present knowledge of the magmatic system and were used as input data for the magma ascent model. The predicted vent conditions were used to define the boundary conditions for the simulation of pyroclastic flow dispersal along selected two-dimensional axisymmetric profiles, representative of the southern and northern slopes of Vesuvius. The model employed describes the temporal evolution of a three-phase mixture composed of a continuous gas phase and two solid phases representative of fine and coarse particles. The specific terrain roughness of the slopes of Vesuvius, caused by the presence of pine woods and urban settlements, was also estimated and accounted for by the model. Several simulations were carried out by assuming different magmatic compositions (in terms of water content and temperature), eruption intensities, topographic profiles, and flow duration. Pyroclastic flow dynamics appear to be strongly influenced by the fountain and atmospheric dynamics showing complex, unsteady, and, in some cases, non-intuitive behaviors. The mass flow-rate per unit angle of propagation of the flow proves to be the most critical parameter controlling the run-out and, therefore, the hazard on the slopes of Vesuvius. The two-dimensional topographic profiles employed also appear to significantly affect the flow propagation. Simulation outputs allow the quantification of the spatial and temporal evolution of several flow variables that are critical in hazard mitigation studies. The analysis of these variables is extensively described in a companion paper (Esposti Ongaro et al. 2002, this volume).

Several analytical and numerical eruption models have provided insight into volcanic eruption behaviour1,2,3,4,5, but most address plinian-type eruptions where vent conditions are quasi-steady. Only a few studies have explored the physics of short-duration vulcanian explosions6,7,8,9 with unsteady vent conditions and blast events10,11. Here we present a technique that links unsteady vent flux of vulcanian explosions to the resulting dispersal of volcanic ejecta, using a numerical, axisymmetric model with multiple particle sizes. We use observational data from well documented explosions in 1997 at the Soufriere Hills volcano in Montserrat, West Indies, to constrain pre-eruptive subsurface initial conditions and to compare with our simulation results. The resulting simulations duplicate many features of the observed explosions, showing transitional behaviour where mass is divided between a buoyant plume and hazardous radial pyroclastic currents fed by a collapsing fountain12. We find that leakage of volcanic gas from the conduit through surrounding rocks over a short period (of the order of 10 hours) or retarded exsolution can dictate the style of explosion. Our simulations also reveal the internal plume dynamics and particle-size segregation mechanisms that may occur in such eruptions.

THE goal of evaluating the risks related to the reactivation of a quiescent volcano requires the reconstructions of the eruptive history of the volcano, the construction therefrom of a behavioural model of the volcano so as to define the 'maximum expected event9 and the subsequent quantitative models allowing reliable simulation of such an event to be set up and hazard and risk maps to be developed. We have followed this approach to infer the size and character of the explosive eruption of Vesuvius to be expected after a 45-year rest period1,2, and we used a numerical model, tested on the recent Mount St Helens eruption and the Vesuvius eruption in AD 79 3,4, to simulate tephra transport and fallout. By combining the fallout model with a statistical analysis of the wind regime and with the density of urban settlement, we were able to assess quantitatively the tephra fallout risk.

Muon Radiography allows to map the density of a volcanic cone. It is based on the measurement of the attenuation of the flux of muons present in the cosmic radiation on the ground. The MU-RAY project has developed an innovative detector designed for the muon radiography. The main features are the low electric power consumption, robustness and transportability, good spatial resolution and muon time of flight measurement. A 1 m2 detector prototype has been constructed. and collected data at Mt. Vesuvius for approximately 1 month in spring 2013. A second campaign of measurement has been performed at the Puy de Dome, France, in the last four months of 2013. In this article the principles of muon radiography, the MU-RAY detector and the first results from the collected data will be described.

[1] Magma viscosity is strongly temperature-dependent. When hot magma flows in a conduit, heat is lost through the walls and the temperature decreases along the flow causing a viscosity increase. For particular values of the controlling parameters the steady-flow regime in a conduit shows two stable solutions belonging either to the slow or to the fast branch. As a consequence, this system may show an hysteresis effect, and the transition between the two branches can occur quickly when the critical points are reached. In this paper we describe a model to study the relation between the pressure at the inlet and the volumetric magma flow rate in a conduit. We apply this model to explain an hysteric jump observed during the dome growth at Soufriere Hills volcano (Montserrat), and described by Melnik and Sparks [1999] using a different model.

Viscous heating can play an important role in the dynamics of fluids with strongly temperature-dependent viscosities because of the coupling between the energy and momentum equations. The heat generated by viscous friction produces a local temperature increase near the tube walls with a consequent decrease of the viscosity and a strong stratification in the viscosity profile. The problem of viscous heating in fluids was investigated and reviewed by Costa & Macedonio (2003) because of its important implications in the study of magma flows. Because of the strong coupling between viscosity and temperature, the temperature rise due to the viscous heating may trigger instabilities in the velocity field, which cannot be predicted by a simple isothermal Newtonian model. When viscous heating produces a pronounced peak in the temperature profile near the walls, a triggering of instabilities and a transition to secondary flows can occur because of the stratification in the viscosity profile. In this paper we focus on the thermal and mechanical effects caused by viscous heating. We will present the linear stability equations and we will show, as in certain regimes, these effects can trigger and sustain a particular class of secondary rotational flows which appear organised in coherent structures similar to roller vortices. This phenomenon can play a very important role in the dynamics of magma flows in conduits and lava flows in channels and, to our knowledge, it is the first time that it has been investigated by a direct numerical simulation., 18 pages manuscript, 10 figures, to be published in Journal of Fluid Mechanics (2005)

The tephra transport and fallout during the Plinian phase of the 79 A.D. eruption of Vesuvius has been numerically simulated by using an advection-diffusion model based on a continuity equation for the mass concentration in which wind field, atmospheric diffusion, and gravity settling are considered. The solution of the equation has been obtained on a nonuniform three-dimensional grid to optimize the computation time and to reduce the errors. The input data are (1) the total erupted mass, (2) the variation with time of the column height, (3) the particle population in terms of the settling velocity, and (4) the wind field. The vertical mass distribution of the particles which leave the column is given by an empirical formula (Suzuki, 1983), whose parameters are obtained from data of Carey and Sigurdsson (1987) and best fitting with field data. Two distinct phases have been distinguished in the eruption: (1) related to white phonolitic pumice fallout and (2) dominated by the emission of gray tephritic-phonolite pumice. According to the literature the total duration of the pumice fallout was about 19 hours with column height ranging between about 14 and 32 km. The “white phase” has been estimated to have had 9 hours duration, while the “gray phase” lasted 10 hours. Magma discharge rate has been calculated assuming a fourth power relationship with column height. Total erupted volumes of 1.0 and 2.6 km3 of dense rock equivalent products have been assumed for the white and the gray phases, respectively (Sigurdsson et al., 1985). The particle population of the cloud has been assumed to correspond to that of a pyroclastic flow deposit related to a nearly total column collapse. The wind field results from a 60° clockwise rotation of the presently most frequent wind distribution field during the summer reduced in speed module of about 40%. The simulated ground thicknesses generally agree with the actual ones. The computed granulometric spectra at specific localities are in excellent agreement with the measured ones. As a whole, the simulation was successful. This fact emphasizes the significant consequences that the used model with a suitable choice of the input parameters can have for the establishment of a truly probabilistic air fall hazard map of Vesuvius.

The MU-RAY project has the challenging aim of performing muon radiography of the summit cone of Mt. Vesuvius. The muon telescopes developed for this purpose will be available for the radiography of other volcanoes, in particular Stromboli. The scientific goals, the strategy for their implementation and the baseline detector design are discussed in detail. A tentative time schedule for the project is drawn.

Muon radiography represents an innovative tool for investigating the interior of active volcanoes. This method integrates the conventional geophysical techniques and provides an independent way to estimate the density of the volcano structure and reveal the presence of magma conduits. The experience from the pioneer experiments performed at Mt. Asama, Mt. West Iwate, and Showa-Shinzan (Japan) are very encouraging. Muon radiography could be applied, in principle, at any stratovolcano. Here we focus our attention on Vesuvius and Stromboli (Italy).

Eruption source parameters (ESP) characterizing volcanic eruption plumes are crucial inputs for atmospheric tephra dispersal models, used for hazard assessment and risk mitigation. We present FPLUME-1.0, a steady-state 1-D (one-dimensional) cross-section-averaged eruption column model based on the buoyant plume theory (BPT). The model accounts for plume bending by wind, entrainment of ambient moisture, effects of water phase changes, particle fallout and re-entrainment, a new parameterization for the air entrainment coefficients and a model for wet aggregation of ash particles in the presence of liquid water or ice. In the occurrence of wet aggregation, the model predicts an effective grain size distribution depleted in fines with respect to that erupted at the vent. Given a wind profile, the model can be used to determine the column height from the eruption mass flow rate or vice versa. The ultimate goal is to improve ash cloud dispersal forecasts by better constraining the ESP (column height, eruption rate and vertical distribution of mass) and the effective particle grain size distribution resulting from eventual wet aggregation within the plume. As test cases we apply the model to the eruptive phase-B of the 4 April 1982 El Chichón volcano eruption (México) and the 6 May 2010 Eyjafjallajökull eruption phase (Iceland). The modular structure of the code facilitates the implementation in the future code versions of more quantitative ash aggregation parameterization as further observations and experiment data will be available for better constraining ash aggregation processes. This work was partially supported by the MED-SUV Project funded by the European Union (FP7 grant agreement no. 308665). We acknowledge C. Bonadonna for providing grain size data for the Eyjafjallajökull test case. We thank T. Esposito Ongaro and two anonymous reviewer for their constructive comments.

We carry out a parametric study in order to identify and quantify the effects of uncertainties on pivotal parameters controlling the dynamics of volcanic plumes. The study builds upon numerical simulations using FPLUME, an integral steady-state model based on the Buoyant Plume Theory generalized in order to account for volcanic processes (particle fallout and re-entrainment, water phase changes, effects of wind, etc). As reference cases for strong and weak plumes, we consider the cases defined during the IAVCEI Commission on tephra hazard modeling inter-comparison study (Costa et al., 2016). The parametric study quantifies the effect of typical uncertainties on total mass eruption rate, column height, mixture exit velocity, temperature and water content, and particle size. Moreover, a sensitivity study investigates the role of wind entrainment and intensity, atmospheric humidity, water phase changes, and particle fallout and re-entrainment. Results show that the leading-order parameters that control plume height are the mass eruption rate and the air entrainment coefficient, especially for weak plumes. This work was partially supported by the MED-SUV Project funded by the European Union (FP7 Grant Agreement n.308665). AC acknowledges a grant for visiting researchers of Earthquake Research Institute, Japan. The authors warmly thank the Guest Editors of JVGR Yujiro J. Suzuki (Univ. of Tokyo, Japan) for handling the paper and for the useful suggestions. Mattia de' Michieli Vitturi and Wim Degruyter are thanked for their constructive comments that have improved the manuscript.

An Eulerian model for passive gas dispersion based on the K-theory for turbulent diffusion, coupled with a mass consistent wind model is presented. The procedure can be used to forecast gas concentration over large and complex terrains. The input to the model includes the topography, wind measurements from meteorological stations, atmospheric stability information and gas flow rate from the ground sources. Here, this model is applied to study the distribution of the CO2 discharged from the hot sources of the Solfatara Volcano, Naples, Italy, where the input data were measured during a 15 day campaign in June 2001 carried out to test an Eddy Covariance (EC) station by Osservatorio Vesuviano-INGV, Naples.

A numerical model for the computation of tephra fall accumulation resulting from Plinian or sub-Plinian eruptions is presented. Mass accumulation at the ground level is found by solving a continuity equation which describes the transport of tephra in the atmosphere. The treatment incorporates horizontal advection due to wind, vertical gravitational settling, and dispersion due to atmospheric turbulence. Aspects of the parameters which enter the model, such as settling velocity, diffusion coefficients, deposition velocity, and source shape, are considered and discussed. Particular attention is devoted to settling velocity dependence on particle size and density, to deposition velocity behavior in weak and strong wind fields, and to eddy diffusion coefficient dependence on atmospheric conditions, particle size, and density. The results for some theoretical cases are presented to illustrate the behavior of the model. The validity of the model is tested by comparing observed and calculated deposits for the May 1980 Mount St. Helens eruption. The model predicts a double thickness maximum, the first near the volcano, the second at a distance of about 300 km, as was observed in the actual deposit.

Volcanic ash clouds produced by explosive eruptions represent a strong problem for civil aviation, road transportation and other human activities. Since Etna volcano produced in the last 35 years more the 200 explosive eruptions of small and medium size. The INGV, liable for its volcano monitoring, developed since 2006 a specific system for forecasting and monitoring Etna’s volcanic ash plumes in collaboration with several national and international institutions. Between 12 January 2011 and 31 December 2013 Etna produced forty-six basaltic lava fountains. Every paroxysm produced an eruption column ranging from a few up to eleven kilometers of height above sea level. The ash cloud contaminated the controlled airspace (CTR) of Catania and Reggio Calabria airports and caused tephra fallout on eastern Sicily sometime disrupting the operations of these airports. In order to give prompt and detailed warnings to the Aviation and Civil Protection authorities, ash plumes monitoring at Osservatorio Etneo, the INGV department in Catania, is carried out using multispectral (from visible to infrared) satellite and ground-based video-surveillance images; seismic and infrasound signals processed in real-time, a Doppler RADAR (Voldorad IIB) able to detect the eruption column in all weather conditions and a LIDAR (AMPLE) for retrieving backscattering and depolarization values of the ash clouds. Forecasting is performed running tephra dispersal models using weather forecast data, and then plotting results on maps published on a dedicated website. 24/7 Control Room operators were able to timely inform Aviation and Civil Protection operators for an effective aviation safety management. A variety of multidisciplinary activities are planned in the MED-SUV project with reference to volcanic ash observations and studies. These include: 1) physical and analogue laboratory experiments on ash dispersal and aggregation; 2) integration of satellite data (e.g. METEOSAT, MODIS) and ground-based measurements (e.g., RADAR, LIDAR) of Etna’s volcanic plumes to quantify mass eruption rate, grain-size distribution at source, and ash cloud concentration; 3) improvement of tools and automatic procedures for the short-term forecasting of volcanic ash dispersal by adopting a multi-model and multi-scenario approach; 4) development of short-term forecasting tools able to use direct measurements of the plume and ash cloud in almost real time (now-casting); 5) development of long-term probabilistic ash fallout maps at the supersite volcanoes.