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Abstract: To understand the relationship between volcano deformations, magma overpressure and flank instability, the results of analog experiments of slope instability and ground deformations, recorded by the GBInSAR system on the western flank of the Stromboli volcano during the period 2009–2011 have been analyzed. Analog experiments that consider both the external (accumulation on the slope) and the endogenous (intrusion-related bulging) trigger phenomena. The effect of accumulation on the sub-aqueous slope on the initiation of sub-marine mass movements has been analyzed. By combining the monitoring data with analog modeling, the observed deformations from the combined action of overpressure in the volcanic system and gravity have been related. The results suggest that the superficial movements observed by the GBInSAR system represent the response of the Stromboli volcano to overpressure changes in the conduit. The movements observed on the Sciara del Fuoco were slope instability phenomena where the gravitational component produced a constant creep, while changes in the magma overpressure explain why certain periods are characterized by accelerations, which induce instability on the external flanks of the crater area and in the Sciara del Fuoco and eventually promote failure of the volcanic slopes. [Copyright &y& Elsevier]

Abstract: The 15 March, 2007 Vulcanian paroxysm at Stromboli volcano was recorded by several instruments that allowed description of the eruptive sequence and unraveling the processes in the upper feeding system. Among the devices installed on the island, two borehole strainmeters recorded unique signals not fully explored before. Here we present an analysis of these signals together with the time-lapse images from a monitoring system comprising both infrared and visual cameras. The two strainmeter signals display an initial phase of pressure growth in the feeding system lasting ∼2min. This is followed by 25s of low-amplitude oscillations of the two signals, that we interpret as a strong step-like overpressure building up in the uppermost conduit by the gas-rich magma accumulating below a thick pile of rock produced by crater rim collapses. This overpressure caused shaking of the ground, and triggered a number of small landslides of the inner crater rim recorded by the monitoring cameras. When the plug obstructing the crater was removed by the initial Vulcanian blast, the two strainmeter signals showed opposite sign, compatible with a depressurizing source at ∼1.5km depth, at the junction between the intermediate and shallow feeding system inferred by previous studies. The sudden depressurization accompanying the Vulcanian blast caused an oscillation of the source composed by three cycles of about 20s each with a decreasing amplitude, as well recorded by the strainmeters. The visible effect of this behavior was the initial Vulcanian blast and a 2–3km high eruptive column followed by two lava fountainings displaying decreasing intensity and height. To our knowledge, this is the first time that such a behavior was observed on an open conduit volcano. [Copyright &y& Elsevier]

Abstract: Twelve years of night-time MODIS (Moderate Resolution Imaging Spectroradiometer) observations, has been analysed to detect and quantify the radiative heat power emitted by Stromboli volcano (from March 2000 to September 2011). Using an accurate background subtraction of the MODIS signal at 4μm, we were able to discriminate two main regimes of thermal radiation, related to different levels of volcanic activity. Effusive eruptions (occurred on December 28, 2002 and February 27, 2007) radiated at an average of ~186MW with a frequency of alert detection of 50–95%. Conversely, during the typical strombolian activity, an average of ~9MW is radiated, with a frequency of alert detection of 0–45%. Although during the effusive eruptions the radiative power is basically controlled by the lava discharge rate, our results suggest that during non-effusive periods (strombolian regime) both the intensity and the frequency of MODIS alerts are controlled by the height of the magmatic column feeding the activity at the surface. In particular we found that a radiative power of ~50MW corresponds to a high magma column which is exposed, in the vent area, at the same rate in which the deeper gas-rich magma is typically supplied within the feeding system of Stromboli (~0.3m3 s−1). In this condition the magmatic system approaches steady state regimes. Above this threshold a transition from strombolian to effusive regimes may occur as shown by the detection of ~50MW, 8–10days before the onset of both the last two major flank eruptions. These values were reached after 1–2months of gradual increase of the radiative power which was likely associated the rising of the magma column within the shallowest portion of the conduit. In addition our data suggest that over the years 2000–2011 several cycles of rise and fall of the magma column have occurred, which however did not culminate into an effusive eruption but only into recurrent episodes of sustained spattering or fountaining and summit overflows. These fluctuations has substantially increased in frequency and intensity after the 2007 eruption thus suggesting that this event has perturbed in some way the shallow plumbing system of Stromboli. We stress that the detection of a radiative power higher than 50MW is a clear evidence of a very high magma column, which may prelude the onset of an effusive eruption and/or periods of sustained vent activity. In conclusion, we suggest that systematic analysis of MODIS data can be used to detect variations in the intensity of strombolian activity and may considerably improve volcano surveillance at Stromboli, as well as at other open-system volcanoes. [Copyright &y& Elsevier]

In situ Sr-isotope data by microdrilling, coupled with major and trace element analyses, have been performed on plagioclase and clinopyroxene from seven samples collected during the 2002-2003 eruptive crisis at Stromboli volcano (Aeolian Islands, Italy). On 28 December 2002, the persistent moderate explosive activity was broken by an effusive event lasting about 7 months. A more violent explosion (paroxysm) occurred on 5 April 2003. Two magma types were erupted, namely a volatile-poor and highly porphyritic magma (HP-magma) poured out as scoria or lava and a volatile-rich, phenocryst-poor magma (LP-magma) found as pumice. LP-magma differs from the HP-magma also for its slightly less-evolved chemistry, the groundmass composition and the lower Sr-isotope ratios. Micro-Sr-isotope data show the presence of zoned minerals in strong isotope disequilibrium, as previously found in products erupted in 1984, 1985 and 1996 AD, with Sr/Sr values generally decreasing from cores to rims of minerals. Only some outer rims testify for equilibrium with the host groundmass. The internal mineral zones with high Sr-isotope ratios (0.70665-0.70618) are interpreted as 'antecrysts', crystallised during the previous activity and recycled in the present-day system since the opening shoshonitic activity of the Recent Period, which occurred at about 2.5 ka ago. This result has implications for the dynamics of the present-day plumbing system of Stromboli at intermediate pressure (about 2-3 km depth) and allows us to propose a model whereby an HP-magma reservoir is directly interconnected at the bottom with a cumulate crystal much reservoir. Efficient mixing between residing HP- and input LP-magmas can occur in this reservoir, due to more similar rheological characteristics of the two magmas than in the conduit, where crystallisation is enhanced by degassing. Antecrysts (and possibly melts) re-enter in the HP-magma reservoir both from the bottom, recycled by ascending LP-magmas crossing the crystal mush, and from the top, recycled by descending degassed and dense HP-magma, residual of the periodic Strombolian explosions at the surface. The isotope variation measured in the groundmasses allows calculating the proportion of the LP-magma entering the shallow HP-magma reservoir at ~20%. From this proportion, we estimate that the total volume of LP-magma input during 2002-2003 closely matches the magma volume erupted in the effusive event, suggesting a steady-state system at broadly constant volume. The comparison with estimates of the LP-magma volume ejected by the paroxysm indicates that the LP-magma amount directly reaching the surface during the 5 April paroxysm is minimal with respect to that entering the system. [ABSTRACT FROM AUTHOR]

Magma degassing processes are commonly elucidated by studies of melt inclusions in erupted phenocrysts and measurements of gas discharge at volcanic vents, allied to experimentally constrained models of volatile solubility. Here we develop an alternative experimental approach aimed at directly simulating decompression-driven, closed-system degassing of basaltic magma in equilibrium with an H–C–O–S–Cl fluid under oxidized conditions (fO2 of 1·0–2·4 log units above the Ni–NiO buffer). Synthetic experimental starting materials were based on basaltic magmas erupted at the persistently degassing volcanoes of Stromboli (Italy) and Masaya (Nicaragua) with an initial volatile inventory matched to the most undegassed melt inclusions from each volcano. Experiments were run at 25–400 MPa under super-liquidus conditions (1150°C). Run product glasses and starting materials were analysed by electron microprobe, secondary ion mass spectrometry, Fourier transform infrared spectroscopy, Karl-Fischer titration, Fe2+/Fe3+ colorimetry and CS analyser. The composition of the exsolved vapour in each run was determined by mass balance. Our results show that H2O/CO2 ratios increase systematically with decreasing pressure, whereas CO2/S ratios attain a maximum at pressures of 100–300 MPa. S is preferentially released over Cl at low pressures, leading to a sharp increase in vapour S/Cl ratios and a sharp drop in the S/Cl ratios of glasses. This accords with published measurements of volatile concentrations in melt inclusion and groundmass glasses at Stromboli (and Etna). Experiments with different S abundances show that the H2O and CO2 contents of the melt at fluid saturation are not affected. The CO2 solubility in experiments using both sets of starting materials is well matched to calculated solubilities using published models. Models consistently overestimate H2O solubilities for the Stromboli-like composition, leading to calculated vapour compositions that are more CO2-rich and calculated degassing trajectories that are more strongly curved than observed in experiments. The difference is less acute for the Masaya-like composition, emphasizing the important compositional dependence of solubility and melt–vapour partitioning. Our novel experimental method can be readily extended to other bulk compositions. [ABSTRACT FROM AUTHOR]

h fallout collected during 4 days of sampling at Stromboli confirms that a crystal-rich (HP) degassed magma erupts during the Strombolian explosions that are characteristic of the normal activity of this volcano. We identified 3 different types of juvenile ash fragments (fluidal, spongy and dense), which formed through different mechanisms of fragmentation of the low-viscosity, physically heterogeneous (in terms of the size and spatial distribution of bubbles) shoshonitic magma. A small amount (less than 3 vol%) of volatile-rich magma with low porphyricity (LP), erupted as highly vesicular ash fragments, has been collected, together with the HP magma, during normal strombolian explosions. Laboratory experiments and the morphological, textural and compositional investigations of ash fragments reveal that the LP ash is fresh and not recycled from the last paroxysm (15 March 2007). We suggest that small droplets of LP magma are dragged to the surface by the time-variable but persistent supply of deep derived CO-rich gas bubbles. This coupled ascent of bubbles and LP melts is transient and does not perturb the dynamics of the HP magma within the shallow reservoir. This finding provides a new perspective on how the Stromboli volcano works and has important implications for monitoring strategies. [ABSTRACT FROM AUTHOR]

Abstract: Our ability to forecast volcanic hazards at active volcanoes stems from our knowledge of parameters affecting eruption dynamics. Persistent activity displayed by basaltic volcanic systems, such as Etna and Stromboli, is governed mainly by the textural evolution and rheological properties of eruptive products. Here, we investigate the high temperature (1131°C< T <1187°C) rheological behavior and textural evolution of remelted natural basaltic magmas from Stromboli and Etna volcanic systems upon cooling and crystallization at atmospheric conditions. The pure liquid and subliquidus isothermal viscosities are investigated using a concentric cylinder apparatus by varying strain rate applied to the investigated materials. Detailed textural analysis has been performed in order to evaluate the effect of crystal distribution on the rheological evolution of magmas. At subliquidus conditions, the mineralogical assemblage is dominated by the presence of spinel and plagioclase with the occurrence of stable clinopyroxene only at the lowest temperatures (1157 and 1131°C for the Stromboli and the Etna samples, respectively). The overall crystal fraction (ϕ) varies between 0.06 and 0.27. In the high T regime the viscous response to applied stress and strain rate is strongly affected by the presence of even small amounts of crystals. Large discrepancies between measured and predicted viscosities obtained using the Einstein–Roscoe (ER) equation are also found at low crystal fractions. We find here that, at the investigated conditions, the physical effect caused by the presence of elongated crystals is orders of magnitude higher than that predicted by existing models which only account for spherical particles. It also appears that a weak shear thinning behavior occurs at the lowest temperatures investigated. Crystal shape and, with it, the strain-rate dependence of the rheological properties appear to play primary roles in influencing the transport properties of these basaltic magmas. [Copyright &y& Elsevier]

The article discusses the evidence for the carbon dioxide (CO2)-driven origin of the explosion of Stromboli volcano in Italy. It states that the explosions are systematically preceded by a brief phase of increasing CO2/ S02 weight ration. It cites that non-linear magma flow could account to the observed C02 degassing patterns. It also mentions that C02-rich gas triggers demonstrates possibility to forecast explosions.

Pyrometamorphic ejecta erupted during recent paroxysmal explosions and hydrothermally altered volcanic and subvolcanic rocks from Stromboli volcano (Aeolian Islands, Italy) were studied by combined polarizing microscopy, electron microprobe, scanning electron microscope, X-ray diffraction, transmission electron microscope and whole-rock inductively coupled plasma optical emission spectrometry–mass spectrometry analysis. The pyrometamorphic ejecta are compositionally heterogeneous at the macro- and micro-scale, and are classified as buchites; that is, glass coupled with a high-temperature, low-pressure modal mineralogy. Three distinct facies (i.e. cordierite–plagioclase, mullite–plagioclase palisade and tridymite–clinopyroxene) are recognized among the buchites, whose bulk-rock chemistry is compatible with aluminous- to silica-rich protoliths. Abundant mineral growth (e.g. plagioclase palisades and clinopyroxene coronae) occurs at the contact between the ejecta and the crystal-rich host shoshonitic basalt, which typically coats the pyrometamorphic blocks. Mineral constraints, pseudoporphyritic microstructures, plagioclase relict phenocrysts and major and trace element compositions strongly support the origin of the buchite ejecta as hydrothermally altered rocks from the Vancori edifice (a period of activity spanning >10 kyr), which underwent high-temperature, low-pressure contact metamorphism. In the uppermost magmatic system of an active island arc volcano, such as Stromboli, volcanic and subvolcanic rocks usually undergo alteration processes induced by acid–sulphate hydrothermal systems. The circulation of acidic fluids results in leaching of the Stromboli rocks (in the upper part of the volcano edifice) and, depending on the pH, temperature and exposure time to the hydrothermal fluids, production of different alteration facies. The hydrothermally altered Stromboli rocks show facies consistent with argillic to silicic alteration, containing abundant hydrous sulphate and hydroxy-sulphate minerals such as the alunite group minerals, hexahydrite, millosevichite, pickeringite and kieserite. The hydrothermally altered wall-rocks, heated by contact with the basaltic magma of the feeder dyke system of Stromboli, can easily undergo pyrometamorphic processes, inducing partial melting and subsolidus crystallization of the volcanic protoliths. This results in the development of buchite rocks at the magma–hydrothermal system interface. [ABSTRACT FROM AUTHOR]

Abstract: The 2007 effusive eruption of Stromboli followed a similar pattern to the previous 2002–2003 episode. In both cases, magma ascent led to breaching of the uppermost part of the conduit forming an eruptive fissure that discharged lava down the Sciara del Fuoco depression. Both eruptions also displayed a ‘paroxysmal’ explosive event during lava flow output. From daily effusion rate measurements retrieved from helicopter- and satellite-based infrared imaging, we deduce that the cumulative volume of lava erupted before each of the two paroxysms was similar. Based on this finding, we propose a conceptual model to explain why both paroxysms occurred after this ‘threshold’ cumulative volume of magma was erupted. The gradual decompression of the deep plumbing system induced by magma withdrawal and eruption, drew deeper volatile-rich magma into the conduit, leading to the paroxysms. The proposed model might provide a basis for forecasting paroxysmal explosions during future effusive eruptions of Stromboli. [Copyright &y& Elsevier]

Abstract: Stromboli is considered one of the most active volcanoes in the world, and its persistent but moderate explosive activity is only interrupted by occasional episodes of more vigorous activity accompanied by lava flows. A new effusive eruption began in late February 2007 and was characterized by intense seismic activity throughout the whole period. The accurate seismic signals analysis showed the presence of families of events with similar waveform signatures (i.e., multiplets) located beneath the crater region. Since traditional location techniques do not allow obtaining reliable hypocenters, our analysis focused on high precision locations of the seismicity in order to better define the source geometry of the events. Hypocenters, therefore, have been relocated considering two steps: the first, based on a robust probabilistic approach, is used to find the absolute position of the clusters and the second exploits a master-event concept for the relative location of the events. Finally, the shape of the clusters and the temporal migration of the foci were correlated with the eruptive phases. The results show that the occurrence of a cluster of events is related to the opening and closure of a vent opened in the Sciara del Fuoco slope and, in particular, to the intrusion of a dike injected by central conduit in a radial direction, whereas another cluster lies in a narrow vertical volume positioned under the crater area. The geometry of the clusters suggests a source region depicting the shallower feeding system. Overall, the results highlight that the high precision locations method is an efficient and quick tool to obtain a better understanding of the magmatic processes occurring during an ongoing eruption. [ABSTRACT FROM AUTHOR]

Abstract: Gas mobility plays an important role in driving volcanic eruptions and controlling eruption style. The explosivity of an eruption depends, among other factors, on how easily gas can escape from the magma. Many magmatic systems have high concentrations of suspended crystals that inhibit gas migration through the melt. We use suspensions of plastic beads in corn syrup to investigate interactions between rising bubbles and particles. We observe different interaction styles as the ratio ψ of bubble to particle size is varied. Large bubbles (ψ>1) deform and sometimes break up as they move around particles. Small bubbles (ψ<1) are frequently trapped within the suspension, increasing the concentration of gas held within the system. We compare our experiments to bubble populations in tephra from Stromboli volcano, Italy. We show that these samples typically have bubbles and crystals of similar sizes and suggest that crystals might play a role in controlling bubble size in this natural system as well as in our experiments. Because small bubbles (ψ<1) get trapped within the suspension, and can be formed by breakup of larger bubbles, we expect that an increase in gas flux will result in an increase in the population of small bubbles. Changes in bubble number density and vesicularity in tephra erupted during periods of different eruptive intensity may thus provide a way of tracking changes in gas flux through the magma prior to eruption. [ABSTRACT FROM AUTHOR]

Abstract: Explosive activity at Stromboli is explained in terms of dynamics of large gas bubbles that ascend in the magma conduit and burst at the free surface generating acoustic pressure that propagates as infrasonic signals in the atmosphere. The rate and the amplitude of the infrasonic activity is directly linked to the rate and the overpressure of the bursting gas bubbles and thus reflects the rate at which magma column degasses under non-equilibrium pressure conditions. We investigate the link between explosive degassing and magma vesiculation by comparing the rate of infrasonic activity with the bubble size distributions (BSDs) of scoria clasts collected during several days of explosive activity at Stromboli. BSDs of scoria show a characteristic power law distribution, which reflect a gas bubble concentration mainly controlled by a combined process of bubble nucleation and coalescence. The cumulative distribution of the infrasonic pressure follows two power laws, indicating a clear separation between the frequent, but weak, bursting of small gas bubbles (puffing) and the more energetic explosions of large gas slugs. The exponents of power laws derived for puffing and explosive infrasonic activity show strongly correlated (0.96) changes with time indicating that when the puffing rate is high, the number of energetic explosions is also elevated. This correlation suggests that both puffing and explosive activity are driven by the same magma degassing dynamics. In addition, changes of both infrasonic power law exponents are very well correlated (0.92 with puffing and 0.87 with explosions) with variations of the BSD exponents of the scoria clasts, providing evidence of the strong interplay between scoria vesiculation and magma explosivity. Our analysis indicates that variable magma vesiculation regimes recorded in the scoria correlate with the event number and energy of the explosive activity. We propose that monitoring infrasound on active volcanoes may be an alternative way to look at the vesiculation process in open conduit systems. [Copyright &y& Elsevier]

Abstract: In addition to rhythmic slug-driven Strombolian activity, Stromboli volcano occasionally produces discrete explosive paroxysms (2 per year on average for the most frequent ones) that constitute a major hazard and whose origin remains poorly elucidated. Partial extrusion of the volatile-rich feeding basalt as aphyric pumice during these events has led to consider their triggering by the fast ascent of primitive magma blobs from possibly great depth. Here I examine and discuss the alternative hypothesis that most of the paroxysms could be triggered and driven by the fast upraise of CO2-rich gas pockets generated by bubble foam growth and collapse in the sub-volcano plumbing system. Data for the SO2 and CO2 crater plume emissions are used to show that Stromboli''s feeding magma may originally contain as much as 2wt.% of carbon dioxide and early coexists with an abundant CO2-rich gas phase with high CO2/SO2 molar ratio (≥60 at 10km depth below the vents, compared to ∼7 in time-averaged crater emissions). Pressure-related modelling indicates that the time-averaged crater gas composition and output are well accounted for by closed system decompression of the basalt–gas mixture until the volcano–crust interface (∼3km depth), followed by open degassing and crystallization in the volcano conduits. However, both the low viscosity and high vesicularity of the basaltic magma permit bubble segregation and bubble foam growth at deep sill-like feeder discontinuities and at shallower physical boundaries (such as the volcano–crust interface) where the gas-rich aphyric basalt interacts with the unerupted crystal-rich and viscous magma drained back from the volcano conduits. Gas pressure build-up and bubble foam collapse at these boundaries will intermittently trigger the sudden upraise of CO2-rich gas blobs that constitute the main driving force of the paroxysms. Deeper-sourced gas blobs, driving the most powerful explosions, will be the richest in CO2 and have highest CO2/SO2 ratios. This mechanism is shown to account well for the dynamic, seismic and petrologic features of Stromboli''s paroxysms and, hence, to provide a potential alternative interpretation for their genesis and their forecasting. Enhanced bubble foam leakage prior to a paroxysm, or foam emptying in several steps, should lead indeed to precursory upstream of CO2-rich gas and increasing CO2/SO2 ratio in crater plume emissions. The recent detection of such signals prior to two explosions in December 2006 and March 2007 strongly supports this expectation and the model proposed in this study. [Copyright &y& Elsevier]

Abstract: The Secche di Lazzaro (SDL) phreatomagmatic activity, with the associated Neostromboli sector collapse, represents the most powerful activity of the last 6ky at Stromboli. As revealed by its present-day activity, Stromboli is one of the most eruptive volcanoes in Italy, and flank instability, along its NW flank, is a common process. Volcano instability is often dramatised by explosive eruptions, thus it is of crucial importance to understand the linking between volcano collapse and the plumbing system itself. The possible role of pre-eruptive magmatic processes as triggers of explosive eruptions can be mainly preserved by minerals and revealed by petrochemical studies. We studied the juvenile components (scoria and pumice) of the pyroclastic deposits from the SDL phreatomagmatic activity with the aim to understand the eruption–collapse link. The SDL pyroclastic sequence has been investigated in three different outcrops: at Secche di Lazzaro (SDL-SW sequence), at Vallone del Monaco (SDL-S) and along the old path from Stromboli village to the active craters (SDL-N). Juvenile components are KS-shoshonites similar to the most evolved lavas erupted at the end of the Neostromboli potassic period. Our study indicates that the SDL system is strongly heterogeneous and made up by at least two different sequences (SDL-N and SDL-SW/SDL-S) with different textural characteristics of the deposits and composition of the juvenile components. Mineral chemistry data show a variable presence of recycled crystals (antecrysts), possibly deriving from the previous Neostromboli activity (up to 13ky ago). Whole rocks are characterised by small but significant differences between the SDL-N and the southern samples, pointing out the presence of compositionally different potassic magmas and the crystallization and recycling of slightly different mineral assemblages. A clear isotope heterogeneity has been also found and explained by the presence of 10% minimum of less radiogenic antecrysts, whereas crustal contamination would have only affected the higher Sr-radiogenic residual melt. These data suggest that the SDL phreatomagmatic eruptions were fed by compositionally distinct potassic magmas, which rested and evolved in two shallow magma conduits when the magma-supply ceased in the waning phase of the Neostromboli activity. Our results also rule out the occurrence of new arrivals of refreshing magmas in the SDL feeding system, as well as other internal magma-related causes, as triggers for the SDL explosive eruptions. Accordingly, external collapse-related causes seem effectively more suitable for triggering the SDL phreatomagmatic eruptions by decompression of the magmatic systems. We also suggest the SDL-N and SDL-SW/SDL-S sequences are generated by two respective explosive events, separated in time and/or space. They can be respectively related to multiple failures of Sciara del Fuoco which successively depressurised the SDL-N and the SDL-SW/SDL-S magma conduits. [Copyright &y& Elsevier]

Abstract: Somma-Vesuvius is considered one of the highest-risk volcanic systems in the world due to its high population density and record of highly destructive explosive activity. Eruptive style at Vesuvius varies greatly, alternating between effusive and explosive activities, and is likely strongly controlled by the evolution of the physical and chemical properties of the magma. Nevertheless, with the exception of the 1631 eruption, the rheological properties of Vesuvius magmas remain largely unconstrained. Here, we investigate the Newtonian shear viscosity (η) of dry and hydrous melts from the Mercato (plinian) and 1906 (violent strombolian) eruptions. These eruptions differ in size, eruptive style and magma chemistry (from phonolite to phono-tephrite). To evaluate the dry liquid viscosity variation covered by the eruptive products of the recent activity at Vesuvius, we measured the melt viscosities of bulk rock compositions and, for highly crystalline samples, of the separated groundmasses of tephras from the Pollena and 1906 eruptions. Hydrated samples with up to 4.24wt% dissolved water were synthesised in a piston cylinder apparatus at confining pressure up to 10kbar. The dry high temperature and the dry and hydrous low-temperature viscosities were obtained by combining the concentric cylinder and micropenetration techniques. The measured viscosities were parameterized by a modified Vogel–Fulcher–Tammann equation, accounting for the effect of water content, and were compared with previous measurements and models. At magmatic temperatures, the viscosities of Mercato samples are about four orders of magnitude higher than that of the least viscous investigated products from the 1906 eruption. Complex numerical models to forecast eruptive scenarios and their environmental impact are extremely sensitive to the accuracy of the input parameters and constitutive equations of magma properties. As a consequence, the numerical expressions obtained here are of particular relevance in the context of hazard assessment related to the different possible eruptive scenarios at Vesuvius through numerical simulation tools. The effect of composition on the liquid viscosities is compared to other high-Na (e.g., samples from Teide and Etna) and high-K (e.g., samples from Stromboli and Phlegrean Fields) alkaline magmas. [Copyright &y& Elsevier]

Abstract: Violent strombolian (transitional) eruptions are common in mafic arc settings and are characterized by simultaneous explosive activity from scoria cone vents and lava effusion from lateral vents. This dual activity requires magma from the feeder conduit to split into vertical and lateral branches somewhere near the base of the scoria cone. Additionally, if the flow is separated, gas and liquid (+crystals) components of the magma may be partitioned unevenly between the two branches. Because flow separation requires bubbles to move independently of the liquid over time scales of magma ascent separation is promoted by low magma viscosities and by high magma H2O content (i.e. sufficiently deep bubble nucleation to allow organization of the gas and liquid phases during magma ascent). Numerical modeling shows that magma and gas distribution between vertical and horizontal branches of a T-junction is controlled by the mass flow rate and the geometry of the system, as well as by magma viscosity. Specifically, we find that mass eruption rates (MERs) between 103 and 105 kg/s allow the gas phase to concentrate within the central conduit, significantly increasing explosivity of the eruption. Lower MERs produce either strombolian or effusive eruption styles, while MER>105 kg/s prohibit both gas segregation and lateral magma transport, creating explosive eruptions that are not accompanied by effusive activity. These bracketing MER constraints on eruptive transitions are consistent with field observations from recent eruptions of hydrous mafic magmas. [Copyright &y& Elsevier]

Abstract: Structural field data, including parameters of 109 sheets (strike: mostly N10-55°E, average dip angle: mostly 70–90°; average thickness: two peaks at 1.2 m and 4.2 m), and of 14 eruptive fissures and vents, as well as bathymetric and numerical data, suggest that Stromboli experienced the development of a volcanic rift zone across the cone summit, with a tendency to widen towards the lower submarine flanks, during the last 100 ka. Inside this rift zone, intrusive sheets mostly strike NE and become more abundant, more steeply dipping and thicker on approaching the rift axis. This rift persistence reflects the edifice''s state of stress, with a NW–SE horizontal least principal stress, on the magma intrusion paths. After 13 ka ago, sheets mainly intruded with a horseshoe-shaped geometry in plan view along the boundaries of the northwestern unstable volcano flank, where four main lateral collapses occurred; during the last 13 ka they also intruded with a NE-strike across the volcano summit and the northeastern volcano flank. The horseshoe-shaped geometry is influenced by the lateral creep of the volcano flank, which provides space for intrusion along the transtensional boundaries of the unstable sector where thinner sheets were emplaced. When the collapse depression was filled up by new deposits, NE-striking thicker dykes emplaced across the northwestern volcano flank indicating a rapid re-establishment of regional tectonic forces across the cone. Unbuttressing due to the present Sciara del Fuoco depression, and ongoing flank creeping, can account for intrusions also inside the Sciara del Fuoco (the latter in 2007). [Copyright &y& Elsevier]

Abstract: Stromboli is well known for its persistent, normal explosive activity, consisting of intermittent, mild to moderate, Strombolian explosions that typically occur every 10–20 min. All tephras erupted during this activity usually fall back into the crater terrace, and consist of volatile-poor scoriae fed by Highly Porphyritic (HP) magma. More occasionally, large explosions or “paroxysms” eject a greater quantity of tephra, mainly consisting of HP scoriae and pumice clasts of Low Porphyritic (LP) magma, but also including large lithic blocks. In addition to this activity, between 2004 and 2006 high energy explosions, displaying an intermediate eruptive style between that of normal and paroxysmal explosions in terms of column height, duration and tephra dispersal, were observed to occur at a frequency of one to eight events per year. While many volcanological, geochemical and geophysical studies have focused in the last few years on the two end-members of activity, i.e. normal or paroxysmal, a detailed investigation on these intermediate types of events has not been carried out yet. Here we report of a study on the 9 January 2005 explosion, one of the high energy explosions during which the main fountaining phase lasted nearly a minute causing ejection of coarse bombs up to a height of 120 m, and of ash and lapilli to >200 m. An accompanying ash plume rose up to 500 m at the end of the explosion. We present a multidisciplinary approach that integrates the results from analysis of live-camera images with compositional and textural characterization of the erupted products. Major element composition of glassy groundmass and 3D views of textures in the erupted scoriae support the hypothesis based on volcanological observations that this explosion falls between normal and paroxysmal activity, for which we use the term “intermediate”. By comparing the video-camera images of the 9 January 2005 explosion with volcanological features of other high energy explosions that occurred at Stromboli between June 2004 and October 2006, we find that three additional events can be considered intermediate explosions, suggesting that this type of activity may be fairly common on this volcano. The results of this study, although preliminary given our limited dataset, clearly indicate that the methodology used here can be successfully applied to better define the range of eruptive styles typifying the normal explosive activity, potentially improving our capability of eruption forecasting and assessing volcanic hazard at Stromboli. [Copyright &y& Elsevier]

Abstract  Volcanic degassing is directly linked to magma dynamics and controls the style of eruptive activity. To better understand how gas is transported within basaltic magma we perform a 3D investigation of vesicles preserved in scoria from the 2005 activity at Stromboli volcano (Italy). We find that clasts are characterized by the ubiquitous occurrence of one to a few large vesicles, exhibiting mostly irregular, tortuous, channel-like textures, orders of magnitude greater in volume than all the other vesicles in the sample. We compare observations on natural samples with results from numerical simulations and experimental investigations of vesicle size distributions and demonstrate that this type of vesicle invariably forms in magmas with vesicularities > 0.30 (and possibly > 0.10). We suggest that large vesicles represent pathways used by gas to flow non-explosively to the surface and that they indicate the development of an efficient system that sustains persistent degassing in basaltic systems. [ABSTRACT FROM AUTHOR]

Strombolian eruptions are caused by the bursting of large gas bubbles through essentially stagnant melt that resides in the uppermost part of the conduit. We investigate the physical properties of this shallow melt via a detailed analysis of vesicularity textures in lapilli ejected during a period of moderate-intensity activity at Stromboli volcano (Italy) in 2002. The lapilli show clear evidence that the erupted material is the product of a late- stage dynamic mingling of melts that are distinct in terms of density and rheology. Vesicle- volume distributions for two end-member melts indicate contrasting degrees of outgassing that can be linked to different residence times in the shallow conduit. We propose a model in which actively vesiculating melt rises with the gas phase and mingles with more mature stagnant melt en route to the magma's free surface. We suggest that the complex rheology of this mingled melt feeds back to strongly influence eruption dynamics. [ABSTRACT FROM AUTHOR]

The persistent explosive activity of Stromboli volcano (Italy) ceased in December 2002 and correlated with the onset of a seven-month-long effusive eruption on the volcano flank from new vents that opened just below the summit craters. We intensively monitored this effusive event, collecting and interpreting, in real time, an extensive multiparametric geophysical data set. The resulting data synergy allowed detailed insights into the conduit dynamics that drove the eruption and the transition back to the typical Strombolian activity. We present a direct link between gas flux, magma volume flux, and seismicity, supporting a gas driven model whereby the balance between gas flux and gas overpressure determines whether the system will support effusive or explosive activity. This insight enabled us to monitor the migration of the magma column up the conduit and to explain the onset of explosive activity. [ABSTRACT FROM AUTHOR]

Crystal-rich materials (scoriae and lava flows) emitted during the 1985-2000 activity of Stromboli were taken into consideration for systematic study of bulk rock/matrix glass chemistry and in particular for the study of chemical and textural zoning of plagioclase, the most abundant mineral phase. Over the considered time period, bulk rock composition remained fairly constant in both major (SiO2 49.2-50.9 wt% and K2O 1.962.18 wt%) and trace elements. The quite constant chemistry of matrix glasses also indicates that the degree of crystallization of magma was maintained at around 50 vol%. Plagioclase ranges in composition between An62 and An88 and is characterized by alternating, < 10100 µm thick, bytownitic and labradoritic concentric layers, although the dominant and representative plagioclase of scoriae is An68. The labradoritic layers (An62-70) show small-scale (1-5 µm), oscillatory zoning, are free of inclusions, and appear to record episodes of slow crystal growth in equilibrium with a degassed liquid having the composition of the matrix glass. In contrast the bytownitic layers (An70-An88) are patchy zoned, show sieve structure with abundant micrometric glass inclusions and voids, and are attributed to rapid crystal growth. A key to understanding the origin of bytownitic layers can be retrieved from the texture and composition of the coronas of plagioclase xenocrysts, inherited from crystal-rich magma, in nearly aphyric pumice which are erupted during more energetic events and represent a deep, volatile-rich, HK-basaltic magma. They show a continuum from fine-sieve to evident skeletal texture from the inner to the outer part of the corona associated with normal compositional zoning from An90 to An75. In the light of these observations, we propose that input of H2O-rich melt blobs, and their mixing with the residing magma, causes partial dissolution of the labradoritic layers followed by the growth of bytownitic composition whose sieve texture attests of rapid crystallization occurring under undercooling conditions mainly induced by degassing. As a whole, the zoning of plagioclase in the scoriae records successive and discrete intrusions of volatile-rich magma blobs, its degassing and mixing with the resident degassed magma at shallow level. [ABSTRACT FROM AUTHOR]

Stromboli, in the Aeolian Archipelago, is famous for its persistent volcanic activity. The ‘normal’ activity, consisting of rhythmic explosions ejecting crystal-rich scoriae, is periodically interspersed with more energetic explosions during which, in addition to crystal-rich scoriae, crystal-poor pumices are also emitted. The scoriae contain ∼50 vol. % crystals (Plag ∼65; Cpx ∼25; Ol ∼10) whereas the pumices display <10 vol. % crystals (Plag ∼42–50; Cpx ∼47–30; Ol ∼11–20). The bulk rocks, mainly ranging between K-rich basalts and shoshonitic basalts, surprisingly exhibit only slight variations in major and trace element contents, and rare earth element patterns. Systematic studies of melt inclusions (MI) and their host minerals were performed on three scoria–pumice pairs erupted together during the violent explosions. The MI cover a compositional range (CaO/Al2O3 = 0·99–0·29) far wider than that of the whole rocks (CaO/Al2O3 = 0·69–0·52) and attest to the presence of rather primitive melts not yet identified as erupted magmas. On the basis of MI analyses, the crystal-poor magmas contain between 2·3 and 2·8 wt % H2O, 894–1689 ppm CO2, 2250–1660 ppm S and 2030–1660 ppm Cl, with the S/Cl ratio close to unity. In contrast, the crystal-rich magma is extensively degassed. We propose that this degassed magma, which sustains the ‘normal’ activity, results from the crystallization of volatile-rich magmas within the cone itself, driven by decompression and H2O exsolution at low pressure. The crystallization is accompanied by S and Cl fractionation into the gas phase, consistent with partition coefficients DS and DCl between fluid and melt of 40 and 10, respectively. The most violent explosions appear to result from the uprising and emission of volatile-rich magma blobs. [ABSTRACT FROM PUBLISHER]