Brittle magmatic fragmentation plays a crucial role in explosive eruptions. It represents the starting point of hazardous explosive events that can affect large areas surrounding erupting volcanoes. Knowing the initial energy released during this fragmentation process is fundamental for the understanding of the subsequent dynamics of the eruptive gas-particle mixture and consequently for the forecasting of the erupting column’s behavior. The specific kinetic energy (SKE) of the particles quantifies the initial velocity shortly after the fragmentation and is therefore a necessary variable to model the gas-particle conduit flow and eruptive column regime. In this paper, we present a new method for its determination based on fragmentation experiments and identification of the timings of energy release. The results obtained on compositions representative for basaltic and phonolitic melts show a direct dependence on magma material properties: poorly vesiculated basaltic melts from Stromboli show the highest SKE values ranging from 7.3 to 11.8 kJ/kg, while experiments with highly vesiculated samples from Stromboli and Vesuvius result in lower SKE values (3.1 to 3.8 kJ/kg). The described methodology presents a useful tool for quantitative estimation of the kinetic energy release of magmatic fragmentation processes, which can contribute to the improvement of hazard assessment.

Abstract View references (35) A new drag law for irregularly shaped particles is presented here. Particles are described by a shape factor that takes into account both sphericity and circularity, which can be measured via the most commonly used image particle analysis techniques. By means of the correlation of the drag coefficient versus the particle Reynolds number and the shape factor, a new drag formula, which is valid over a wide range of Reynolds number (0.03–10,000), is obtained. The new model is able to reproduce the drag coefficient of particles measured in terminal velocity experiments with a smaller scatter compared to other laws commonly used in multiphase flow engineering and volcanology. Furthermore, the new formula uses only one equation, whereas previous models, to insure validity over an ample range, made use of a step function that introduces a discontinuity at the switch of equations. Finally, this drag law works in the whole range of variation, from extremely irregular particles to perfect sphere. A code of the iterative algorithm for the drag coefficient calculation and terminal velocity is included in the supporting information both as a Fortran routine and a Matlab function.

A systematic analysis of the physical parameters that influence the aerodynamics of ash, i.e. the attitude of a particle to be transported and/or settled throughout a fluid, is presented. We investigate juvenile particles from eruptions of Somma-Vesuvius, Campi Flegrei and Vulcano (southern Italy), which encompass a wide range of particle characteristics. The analysed samples were selected from dilute pyroclastic density current (DPDC) and fall deposits, and cover an ample spectrum of magma composition and fragmentation mechanisms. Data show that particles have often highly irregular shapes, as determined by the shape factor Psi. The more irregular is the shape the higher the drag coefficient. C(d), and the lower the terminal velocity. The C(d) of DPDC particles is lower than that of fall particles, as due to rounding by attrition at the base of a density current. As a consequence of the irregular shape, the terminal velocity of ash (0.5 mm) can be less than half of the value that results by hypothesising a spherical shape, as it is frequently done in volcanology. In the fall deposits, for the same size fraction, the settling velocity can be different for samples extracted at different locations along the main dispersal axis, especially if the clast population shows heterogeneity of vesicularity. Particle shape becomes more irregular as grain size decreases down to 025 mm, whereas at finer sizes the values are almost constant. This study has important implications for how long and how far volcanic particles can be dispersed aloft; this is crucial for dispersal models quantifying risk, including for international air traffic. (C) 2011 Elsevier B.V. All rights reserved.

Pyroclastic density currents (PDCs) are gas-particle flows generated during explosive eruptions, which are often erupted over the flanks of stratovolcanoes. These volcanoes may have different shapes, which can affect the flow aerodynamics and hence the depositional processes. Here, multiphase numerical simulations are carried out in order to define semiquantitative relationships among the PDC behavior, particle response, and deposit formation. Three stratovolcano shapes are used: straight, convex and concave, and, by means of numerical simulations, their effects both on the flow structure and depositional processes are highlighted. The current starts moving as a homogeneous flow, and then it rapidly evolves to a turbulent boundary layer moving in contact with the ground, overlaid by a companion wake region. Results show that thin boundary layers produce thick deposits of massive layers, whereas thick boundary layers produce thin laminated deposits. Moreover, concave wake regions would produce thick massive deposits of fine ash, whereas convex wake regions would produce thin ash deposits.

The fragmentation process and aerodynamic behavior of ash from the Eyjafjallajökull eruption of 2010 are investigated by combining grain-size, Scanning Electron Microscopy (SEM), and quantitative particle morphology. Ash samples were collected on land in Iceland at 3–55 km distance from the volcanic vent, and represent various phases of the pulsating eruption. The grain size is fine even for deposits close to the vent, suggesting that the parent particle population at fragmentation consisted of a substantial amount of fine ash. SEM investigation reveals that ash produced during the first phase of the eruption consists of juvenile glass particles showing key features of magma-water interaction, suggesting that phreatomagmatism played a major role in the fragmentation of a vesicle-poor magma. In the last phase of the eruption, fragmentation was purely magmatic and resulted from stress-induced reaction of a microvesicular, fragile melt. The shape of ash, as determined by quantitative morphology analysis, is highly irregular, rendering the settling velocity quite low. This makes transportation by wind much easier than for other more regularly shaped particles of sedimentary origin. We conclude that the combination of magma’s fine brittle fragmentation and irregular particle shape was the main factor in the extensive atmospheric circulation of ash from the mildly energetic Eyjafjallajökull eruption.

Explosive volcanic eruptions are characterized by the rapid fragmentation of a magmatic melt into ash particles. In order to describe the energy dissipation during fragmentation it is important to understand the mechanism of material failure. A quantitative description of fragmentation is only possible under controlled laboratory conditions. Industrial silicate glasses have a high structural affinity with magmatic melts and have the advantage of being transparent, which allows the study of the evolution of fractures by optical methods on a time scale relevant for explosive volcanism.With this aim, a series of low speed edge-on hammer impact experiments on silicate glass targets has been conducted, leading to the generation of fragments in the grain-size spectra of volcanic ash. In order to verify the general transferability of the experimentally generated fragmentation dynamics to volcanic processes, the resulting products were compared, by means of statistical particle-shape analyses, to particles produced by standardized magma fragmentation experiments and to natural ash particles coming from deposits of basaltic and rhyolitic compositions from the 2004 Grimsvötn and the Quaternary Tepexitl tuff-ring eruptions, respectively. Natural ash particles from both Grimsvötn and Tepexitl show significant similarities with experimental fragments of thermally pre-stressed float glasses, indicating a dominant influence of preexisting stresses on particle shape and suggesting analogous fragmentation processes within the studied materials.

It is currently impractical to measure what happens in a volcano during an explosive eruption, and up to now much of our knowledge depends on theoretical models. Here we show, by means of large-scale experiments, that the regime of explosive events can be constrained on the basis of the characteristics of magma at the point of fragmentation and conduit geometry. Our model, whose results are consistent with the literature, is a simple tool for defining the conditions at conduit exit that control the most hazardous volcanic regimes. Besides the well-known convective plume regime, which generates pyroclastic fallout, and the vertically collapsing column regime, which leads to pyroclastic flows, we introduce an additional regime of radially expanding columns, which form when the eruptive gas-particle mixture exits from the vent at overpressure with respect to atmosphere. As a consequence of the radial expansion, a dilute collapse occurs, which favors the formation of density currents resembling natural base surges. We conclude that a quantitative knowledge of magma fragmentation, i.e., particle size, fragmentation energy, and fragmentation speed, is critical for determining the eruption regime.

Explosive activity and lava dome collapse at stratovolcanoes can lead to pyroclastic density currents (PDCs; mixtures of volcanic gas, air, and volcanic particles) that produce complex deposits and pose a hazard to surrounding populations. Two-dimensional computer simulations of dilute PDCs (characterized by a turbulent suspended load and deposition through a bed load) show that PDC transport, deposition, and hazard potential are sensitive to the shape of the volcano slope (profile) down which they flow. We focus on three generic volcano profiles: straight, concave-upward, and convex-upward. Dilute PDCs that flow down a constant slope gradually decelerate over the simulated run-out distance (5 km in the horizontal direction) due to a combination of sedimentation, which reduces the density of the PDC, and mixing with the atmosphere. However, dilute PDCs down a concave-upward slope accelerate high on the volcano flanks and have less sedimentation until they begin to decelerate over the shallow lower slopes. A convex-upward slope causes dilute PDCs to lose relatively more of their pyroclast load on the upper slopes of a volcano, and although they accelerate as they reach the lower, steeper slopes, the acceleration is reduced because of the upstream loss of pyroclasts (lower density contrast with the atmosphere). Dynamic pressure, a measure of the damage that can be caused by PDCs, reflects these complex relations.

Pyroclastic flows represent the most hazardous events of explosive volcanism, one striking example being the famous historical eruption of Vesuvius that destroyed Pompeii (AD 79). Much of our knowledge of the mechanics of pyroclastic flows comes from theoretical models and numerical simulations. Valuable data are also stored in the geological record of past eruptions, including the particles contained in pyroclastic deposits, but the deposit characteristics are rarely used for quantifying the destructive potential of pyroclastic flows. By means of experiments, we validate a model that is based on data from pyroclastic deposits. The model allows the reconstruction of the current's fluid-dynamic behaviour. Model results are consistent with measured values of dynamic pressure in the experiments, and allow the quantification of the damage potential of pyroclastic flows.

Long-range dispersal of volcanic ash can disrupt civil aviation over large areas, as occurred during the 2010 eruption of Eyjafjallajökull volcano in Iceland. Here we assess the hazard for civil aviation posed by volcanic ash from a potential violent Strombolian eruption of Somma-Vesuvius, the most likely scenario if eruptive activity resumed at this volcano. A Somma-Vesuvius eruption is of concern for two main reasons: (1) there is a high probability (38 %) that the eruption will be violent Strombolian, as this activity has been common in the most recent period of activity (between AD 1631 and 1944); and (2) violent Strombolian eruptions typically last longer than higher-magnitude events (from 3 to 7 days for the climactic phases) and, consequently, are likely to cause prolonged air traffic disruption (even at large distances if a substantial amount of fine ash is produced such as is typical during Vesuvius eruptions). We compute probabilistic hazard maps for airborne ash concentration at relevant flight levels using the FALL3D ash dispersal model and a statistically representative set of meteorological conditions. Probabilistic hazard maps are computed for two different ash concentration thresholds, 2 and 0. 2 mg/m 3, which correspond, respectively, to the no-fly and enhanced procedure conditions defined in Europe during the Eyjafjallajökull eruption. The seasonal influence of ash dispersal is also analysed by computing seasonal maps. We define the persistence of ash in the atmosphere as the time that a concentration threshold is exceeded divided by the total duration of the eruption (here the eruption phase producing a sustained eruption column). The maps of averaged persistence give additional information on the expected duration of the conditions leading to flight disruption at a given location. We assess the impact that a violent Strombolian eruption would have on the main airports and aerial corridors of the Central Mediterranean area, and this assessment can help those who devise procedures to minimise the impact of these long-lasting low-intensity volcanic events on civil aviation. © 2012 Springer-Verlag.

Subaqueous ash flows are gravity currents consisting of a mixture of sea water and ash particles. Also called volcaniclastic turbidity currents (VTCs), they can be generated because of remobilization of pyroclastic fall deposits, which are emplaced into the sea around a volcanic island, as well as far away, during an explosive eruption. The VTC upper part is the turbulent transport system for the flow, whereas the viscous basal one is the depositional system. Typical sequences of VTC deposits are characterized by cross-laminations, planar and convolute laminations, and massive beds, which reflect the stratified nature of the flow. Here, the analysis of some VTC hydraulic parameters is presented in order to depict flow behavior and sedimentation during deposition. A reverse engineering approach is proposed, which consists of calculating hydraulic parameters by starting from deposit features. The calculated values show that a VTC is homogeneouslyturbulent for most of the thickness, but is viscous at its base. First, cross-laminations are directly acquired over the rough pre-existing seafloor, then planar or convolute laminations aggrade over the newly formed substrate. Finally, fine-grained suspended particles gently settle and cap the flow deposit.

Turbidity currents are the most common flows of water, and suspended and bed load sediment occurring in the relatively deep sea and lakes, and act as large systems of sediment distribution. Due to the turbid nature of the flow, they may impact subaqueous infrastructures, as well as ecosystems, during motion, so quantifying the turbidity degree is important to predict the current impact. A particular type of these currents is the so-called volcanidastic turbidity currents or subaqueous ash flows, which are mostly composed of fine-grained volcanic particles, and are used here as synonymous (sensu hydraulic) with the meaning of secondary current. In this paper, a method to estimate the water entrainment (column condition model), as well as the sedimentation and deposition rates (conveyer model), in volcaniclastic turbidity currents is proposed, by starting from the physical features of the deposits or inverse procedure. Some criteria of sediment mechanics are used to approximate the flow hydraulic parameters needed to quantify the water entrainment, as well as the shear velocity in volcaniclastic turbidity currents. The deposits used as case study are the impressive, meters thick, well-sorted rhyolitic ash turbidites of Late Pliocene cropping out in Southern Italy, particularly in the Craco area, Matera. The water entrainment coefficient of the currents is calculated in a range of particle concentration and slope angle, whereas the slope angle giving the sedimentation rate is calculated in a range of flow shear velocity, which in turns gives the deposition rate. The results have a general validity for depositional turbidity currents laden with well-sorted sediment, and they show that the water entrainment is low for relatively dense, slow-moving, subcritical flows, but it increases as the Richardson number decreases for relatively dilute, fast, supercritical flows. Moreover, the sedimentation and deposition rates are high for relatively intense flows moving over gentle slopes, but they decrease for relatively weak flows moving over steep slopes. The deposit sedimentary structures thus infer sedimentation, and particularly depositional processes. Finally, the conveyer model is extended conceptually to pyroclastic density currents, when approximating, but not exclusively, the column condition model in the flow above the flow boundary zone.

The eruptive columns of explosive volcanism are fed by a gas-particle conduit flow, which characteristics determine the eruptive regime and are important for assessing the hazard of active volcanoes. In this paper, by means of the combined use of large-scale experiments and numerical modeling, a study on some parameters of the gas-particle conduit flow of explosive eruptions is carried out. A 1D two-phase non-homogeneous Eulerian-Eulerian model has been developed for checking the influence of some crucial quantities: interphase drag, particle-wall friction and particle shape factor. Hundreds of different parameters combinations are tested and used for the simulation of controlled experimental runs. The parameter combination that best fits the whole set of experiments, including both column collapses and convective plumes, results into an average error of about 10%. A further analysis has been carried out to determine the sensitivity of solutions to model parameters. The choice of the interphase drag does not influence dramatically the solution, except for highly concentrated flows. The particle shape factor severely affects gas and particle velocities. The influence of various particle-wall friction laws, which were originally obtained in pneumatic engineering, is thoroughly investigated, as the suitability of these laws has never been proved in volcanology. A detailed parametric analysis allowed the re-calibration of two of these laws, which are now specifically tailored for the case of highly concentrated conduit flows that feed collapsing columns, and dilute flows that feed convective plumes.

The interaction between pyroclastic density currents and buildings is investigated by means of numerical simulation and large-scale experiments. Numerical simulation is performed with the Euler-Lagrange approach using a two-way coupling between gas and particles of three sizes. The collapse of an eruptive column consisting of a mixture of gas and pyroclasts is produced experimentally, and the impact of the resulting shear current with mock-ups representing buildings is monitored. A combination of results from simulations and experiments shows that, upon impact with a building, the multiphase current develops strong turbulence intensity, which significantly affects particle dispersion. The flow recirculation around the building induces forced deposition at the front and wake in the back wall, with flow reattachment farther away. These changes produce a variation in the dynamic pressure, which is the most important parameter for assessing the impact of pyroclastic density currents moving over inhabited areas.

Pyroclastic density currents (PDCs) vary between two end members, concentrated and dilute. When a PDC interacts with an uneven topography, the flow field variables (velocity, pressure, bulk density, particle concentration) may drastically change near the flow-substrate boundary. These changes may significantly affect the sediment flux and the resulting deposits can record the effects in their facies architecture. Here we show, by means of numerical simulations, how a dilute pyroclastic density current interacts with four different types of simple topography, namely: flat, one hill, one valley and two hills. Our numerical scheme treats the very fine particles as being in full thermo-mechanical equilibrium with the volcanic gas, i.e. a dusty gas. A dusty gas-air mixture is defined as a mixture of dusty gas and atmospheric air. The trajectories of the coarser particles or discrete phase (three grain-size classes of 1 mm, 5 mm and 10 mm and density of 1500 kg/m(3)) are tracked as Lagrangian particles that interact with the dusty gas-air mixture through two-way momentum and energy coupling. Numerical results are used to analyze the local effects of topography on the deposition of the Lagrangian particles, by monitoring with time and space the local changes at the boundary between the current and the substrate. The results show that the sediment flux in the flow boundary zone increases near the stoss sides of hills and in the valleys, relative to the flat reference case, whereas it decreases along the lee flanks and on top of the hills. We use the sediment flux in the flow boundary zone and the grain-size distribution of the Lagrangian particles as proxies of the deposit features, and by these parameters we qualitatively compare simulations with deposits of known eruptions.

Pyroclastic density currents are ground hugging, hot, gas-particle flows representing the most hazardous events of explosive volcanism. Their impact on structures is a function of dynamic pressure, which expresses the lateral load that such currents exert over buildings. Several critical issues arise in the numerical simulation of such flows, which involve a theologically complex fluid that evolves over a wide range of turbulence scales, and moves over a complex topography. In this paper we consider a numerical technique that aims to cope with the difficulties encountered in the domain discretization when an adequate resolution in the regions of interest is required. Without resorting to time-consuming body-fitted grid generation approaches, we use Cartesian grids locally refined near the ground surface and the volcanic vent in order to reconstruct the steep velocity and particle concentration gradients. The grid generation process is carried out by an efficient and automatic tool, regardless of the geometric complexity. We show how analog experiments can be matched with numerical simulations for capturing the essential physics of the multiphase flow, obtaining calculated values of dynamic pressure in reasonable agreement with the experimental measurements. These outcomes encourage future application of the method for the assessment of the impact of pyroclastic density currents at the natural scale.

The detailed analysis of stratigraphy allowed the reconstruction of the complex volcanic history of La Fossa di Vulcano. An eruptive activity mainly driven by superficial phreatomagmatic explosions emerged. A statistical analysis of the pyroclastic Successions led to the identification of dilute pyroclastic density currents (base surges) as the most recurrent events, followed by fallout of dense ballistic blocks. The scale of events is related to the amount of magma involved in each explosion. Events involving about 1 million cm(3) of magma occurred during recent eruptions. They led to the formation of hundreds of meters thick dilute pyroclastic density currents, moving down the volcano slope at velocities exceeding 50 m/s. The dispersion of density currents affected the whole Vulcano Porto area, the Vulcanello area. They also overrode the Fossa Caldera's rim, spreading over the Piano area. For the aim of hazard assessment, deposits from La Fossa Cone and La Fossa Caldera were studied in detail, to depict the eruptive scenarios at short-term and at long-term. By means of physical models that make use of deposit particle features, the impact parameters have been calculated. They are dynamic pressure and particle volumetric concentration of density currents, and impact energy of ballistic blocks. A quantitative hazard map, based on these impact parameters, is presented. It could be useful for territory planning and for the calculation of the expected damage.

New volcanological studies allow reconstruction of the eruption dynamics of the Pomici di Mercato eruption (ca 8,900 cal. yr B.P.) of Somma-Vesuvius. Three main Eruptive Phases are distinguished based on two distinct erosion surfaces that interrupt stratigraphic continuity of the deposits, indicating that time breaks occurred during the eruption. Absence of reworked volcaniclastic deposits on top of the erosion surfaces suggests that quiescent periods between eruptive phases were short perhaps lasting only days to weeks. Each of the Eruptive Phases was characterised by deposition of alternating fall and pyroclastic density current (PDC) deposits. The fallout deposits blanketed a wide area toward the east, while the more restricted PDC deposits inundated the volcano slopes. Eruptive dynamics were driven by brittle magmatic fragmentation of a phonolitic magma, which, because of its mechanical fragility, produced a significant amount of fine ash. External water did not significantly contribute either to fragmentation dynamics or to mechanical energy release during the eruption. Column heights were between 18 and 22 km, corresponding to mass discharge rates between 1.4 and 6 x 10(7) kg s(-1). The estimated on land volume of fall deposits ranges from a minimum of 2.3 km(3) to a maximum of 7.4 km(3). Calculation of physical parameters of the dilute pyroclastic density currents indicates speeds of a few tens of m s(-1) and densities of a few kg m(-3) (average of the lowermost 10 m of the currents), resulting in dynamic pressures lower than 3 kPa. These data suggest that the potential impact of pyroclastic density currents of the Pomici di Mercato eruption was smaller than those of other Plinian and sub-Plinian eruptions of Somma-Vesuvius, especially those of 1631 AD and 472 AD (4-14 kPa), which represent reference values for the Vesuvian emergency plan. The pulsating and long-lasting behaviour of the Pomici di Mercato eruption is unique in the history of large explosive eruptions of Somma-Vesuvius. We suggest an eruptive scheme in which discrete magma batches rose from the magma chamber through a network of fractures. The injection and rise of the different magma batches was controlled by the interplay between magma chamber overpressure and local stress. The intermittent discharge of magma during a large explosive eruption is unusual for Somma-Vesuvius, as well as for other volcanoes worldwide, and yields new insights for improving our knowledge of the dynamics of explosive eruptions.

On 7 September 2008 a major ash explosion occurred from the SW summit crater of Stromboli volcano. This explosive event lasted similar to 2 min and consisted of three discrete eruptive pulses, forming an eruptive ash cloud similar to 500-600 m high and similar to 300 m wide, rising with speed of 20-27 m s(-1). The event was recorded by our camera and seismic networks, as well as by two electric stations installed at a 500 m mean distance from the SW crater. The electric signals recorded by the two stations during this event were 10(6) times greater than signals recorded during the persistent Strombolian activity, and the seismic trace had a bigger amplitude and a longer duration. Camera image analysis allowed us to infer that a partial obstruction took place at the SW crater three days before the explosive event, suggesting that a constriction within the upper conduit could have likely led to magma overpressure. Data analysis, combined with previous experimental investigations, revealed that the higher energy output of the ash explosion, when compared to the persistent Strombolian activity, resulted in a greater magma fragmentation and erupted mass. Integration of the different parameters allowed us to classify the event as a Vulcanian type, and electric signal analysis enabled retrieval of the total volume of erupted ash and of the amounts of the juvenile, phreatomagmatic, and lithic components.

The stratigraphic succession of the Pomici di Avellino Plinian eruption from Somma-Vesuvius has been studied through field and laboratory data in order to reconstruct the eruption dynamics. This eruption is particularly important in the Somma-Vesuvius eruptive history because (1) its vent was offset with respect to the present day Vesuvius cone; (2) it was characterised by a distinct opening phase; (3) breccia-like very proximal fall deposits are preserved close to the vent and (4) the pyroclastic density currents generated during the final phreatomagmatic phase are among the most widespread and voluminous in the entire history of the volcano. The stratigraphic succession is, here, divided into deposits of three main eruptive phases (opening, magmatic Plinian and phreatomagmatic), which contain five eruption units. Short-lived sustained columns occurred twice during the opening phase (H(t) of 13 and 21.5 km, respectively) and dispersed thin fall deposits and small pyroclastic density currents onto the volcano slopes. The magmatic Plinian phase produced the main volume of erupted deposits, emplacing white and grey fall deposits which were dispersed to the northeast. Peak column heights reached 23 and 31 km during the withdrawal of the white and the grey magmas, respectively. Only one small pyroclastic density current was emplaced during the main Plinian phase. In contrast, the final phreatomagmatic phase was characterised by extensive generation of pyroclastic density currents, with fallout deposits very subordinate and limited to the volcano slopes. Assessed bulk erupted volumes are 21 x 10(6) m(3) for the opening phase, 1.3-1.5 km(3) for the main Plinian phase and about 1 km(3) for the final phreatomagmatic phase, yielding a total volume of about 2.5 km(3). Pumice fragments are porphyritic with sanidine and clinopyroxene as the main mineral phases but also contain peculiar mineral phases like scapolite, nepheline and garnet. Bulk composition varies from phonolite (white magma) to tephri-phonolite (grey magma).

Pyroclastic density currents (PDCs) generated during the Plinian eruption of the Pomici di Avellino (PdA) of Somma-Vesuvius were investigated through field and laboratory studies, which allowed the detailed reconstruction of their eruptive and transportation dynamics and the calculation of key physical parameters of the currents. PDCs were generated during all the three phases that characterised the eruption, with eruptive dynamics driven by both magmatic and phreatomagmatic fragmentation. Flows generated during phases 1 and 2 (EU1 and EU3pf, magmatic fragmentation) have small dispersal areas and affected only part of the volcano slopes. Lithofacies analysis demonstrates that the flow-boundary zones were dominated by granular-flow regimes, which sometimes show transitions to traction regimes. PDCs generated during eruptive phase 3 (EU5, phreatomagmatic fragmentation) were the most voluminous and widespread in the whole of Somma-Vesuvius' eruptive history, and affected a wide area around the volcano with deposit thicknesses of a few centimetres up to more than 25 km from source. Lithofacies analysis shows that the flow-boundary zones of EU5 PDCs were dominated by granular flows and traction regimes. Deposits of EU5 PDC show strong lithofacies variation northwards, from proximally thick, massive to stratified beds towards dominantly alternating beds of coarse and fine ash in distal reaches. The EU5 lithofacies also show strong lateral variability in proximal areas, passing from the western and northern to the eastern and southern volcano slopes, where the deposits are stacked beds of massive, accretionary lapilli-bearing fine ash. The sedimentological model developed for the PDCs of the PdA eruption explains these strong lithofacies variations in the light of the volcano's morphology at the time of the eruption. In particular, the EU5 PDCs survived to pass over the break in slope between the volcano sides and the surrounding volcaniclastic apron-alluvial plain, with development of new flows from the previously suspended load. Pulses were developed within individual currents, leading to stepwise deposition on both the volcano slopes and the surrounding volcaniclastic apron and alluvial plain. Physical parameters including velocity, density and concentration profile with height were calculated for a flow of the phreatomagmatic phase of the eruption by applying a sedimentological method, and the values of the dynamic pressure were derived. Some hazard considerations are summarised on the assumption that, although not very probable, similar PDCs could develop during future eruptions of Somma-Vesuvius.

Large-scale experiments generating ground-hugging multiphase flows were carried out with the aim of modelling the rate of sedimentation, of pyroclastic density currents. The current was initiated by the impact on the ground of a dense gas-particle fountain issuing from a vertical conduit. On impact, a thick massive deposit was formed. The grain size of the massive deposit was almost identical to that of the mixture feeding the fountain, suggesting that similar layers formed at the impact of a natural volcanic fountain should be representative of the parent grain-size distribution of the eruption. The flow evolved laterally into a turbulent suspension current that sedimented a thin, tractive layer. A good correlation was found between the ratio of transported/sedimented load and the normalized Rouse number of the turbulent current. A model of the sedimentation rate was developed, which shows a relationship between grain size and flow runout. A current fed with coarser particles has a higher sedimentation rate, a larger grain-size selectivity and runs shorter than a current fed with finer particles. Application of the model to pyroclastic deposits of Vesuvius and Campi Flegrei of Southern Italy resulted in sedimentation rates falling inside the range of experiments and allowed definition of the duration of pyroclastic density currents which add important information on the hazard of such dangerous flows. The model could possibly be extended, in the future, to other geological density currents as, for example, turbidity currents. © 2018 International Association of Sedimentologists.

Volcanic ash produced during explosive eruptions can have very severe impacts on modern technological societies. Here, we use reconstructed patterns of fine ash dispersal recorded in terrestrial and marine geological archives to assess volcanic ash hazards. The ash-dispersal maps from nine Holocene explosive eruptions of Italian volcanoes have been used to construct frequency maps of distal ash deposition over a wide area, which encompasses central and southern Italy, the Adriatic and Tyrrhenian seas and the Balkans. The maps are presented as two cumulative-thickness isopach maps, one for nine eruptions from different volcanoes and one for six eruptions from Somma-Vesuvius. These maps represent the first use of distal ash layers to construct volcanic hazard maps, and the proposed methodology is easily applicable to other volcanic areas worldwide.