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Open conduit volcanoes erupt with the highest frequency on Earth. Their activity is characterized by an outgassing flux that largely exceeds the gas that could be released by the erupted magma; and by frequent small explosions intercalated by larger events that pose a significant risk to locals, tourists, and scientists. Thus, identifying the signs of an impending larger explosion is of utmost importance for the mitigation of volcanic hazard. Larger explosive events have been associated with the sudden ascent of volatile rich magmas, however, where and why magma accumulates within the plumbing system remains unclear. Here we show that the interaction between CO2‐rich fluids and magma spontaneously leads to the accumulation of volatile‐rich, low density and gravitationally unstable magma at depth, without the requirement of permeability barriers. CO2‐flushing forces the exsolution of water and the increase of magma viscosity, which proceeds from the bottom of the magma column upward. This rheological configuration unavoidably leads to the progressive thickening of a gas‐rich and low density (i.e., gravitationally unstable) layer at the bottom of the feeding system. Our calculations account for observations, gas monitoring and petrological data; moreover, they provide a basis to trace the approach to deeply triggered large or paroxysmal eruptions and estimate their size from monitoring data. Our model is finally applied to Stromboli volcano, an emblematic example of open conduit volcano, but can be applied to any other open conduit volcano globally and offers a framework to anticipate the occurrence of unexpectedly large eruptions. Plain Language Summary: Open conduit volcanoes erupt with the highest frequency on Earth and release significantly more gas than magma. Their activity is characterized by almost continuous and small explosions that are intercalated by significantly larger explosive events. The relatively continuous explosive activity makes these volcanoes attractive both for scientific research and tourism, however, unexpectedly large explosions pose a significant risk to scientists and tourists. The excess gas these volcanoes release is CO2‐rich and its interaction with magma leads to the accumulation of gas‐rich and low‐density magma at depth, below higher density magma. We suggest that the gravitational destabilization of this deep and gas rich magma is ultimately responsible for the large explosions that punctuate the activity of open conduit volcanoes. The approach to this larger events can be traced with continuous gas monitoring, which therefore provides a unique opportunity to anticipate this otherwise unexpected explosions. Key Points: Continuous CO2 flushing can prime basaltic volcanic systems toward more energetic eruptionsAccumulation of gas‐rich and low‐density magma at depth results from CO2 flushing alone and no ad‐hoc physical barrier is requiredBuild‐up to larger explosions can be monitored through continuous gas monitoring at the surface [ABSTRACT FROM AUTHOR]

Many volcanic eruptions are shortly preceded by injection of new magma into a pre-existing, shallow (<10 km) magma chamber, causing convection and mixing between the incoming and resident magmas. These processes may trigger dyke propagation and further magma rise, inducing long-term (days to months) volcano deformation, seismic swarms, gravity anomalies, and changes in the composition of volcanic plumes and fumaroles, eventually culminating in an eruption. Although new magma injection into shallow magma chambers can lead to hazardous event, such injection is still not systematically detected and recognized. Here, we present the results of numerical simulations of magma convection and mixing in geometrically complex magmatic systems, and describe the multiparametric dynamics associated with buoyant magma injection. Our results reveal unexpected pressure trends and pressure oscillations in the Ultra-Long-Period (ULP) range of minutes, related to the generation of discrete plumes of rising magma. Very long pressure oscillation wavelengths translate into comparably ULP ground displacements with amplitudes of order 10-10 m. Thus, new magma injection into magma chambers beneath volcanoes can be revealed by ULP ground displacement measured at the surface. [ABSTRACT FROM AUTHOR]

Abstract: Determining consistent sets of vent conditions for next expected eruptions at Vesuvius is crucial for the simulation of the sub-aerial processes originating the volcanic hazard and the eruption impact. Here we refer to the expected eruptive scales and conditions defined in the frame of the EC Exploris project, and simulate the dynamics of magma ascent along the volcanic conduit for sub-steady phases of next eruptions characterized by intensities of the Violent Strombolian (VS), Sub-Plinian 2 (SP2), and Sub-Plinian 1 (SP1) scale. Sets of conditions for the simulations are determined on the basis of the bulk of knowledge on the past history of Vesuvius [Cioni, R., Bertagnini, A., Santacroce, R., Andronico, D., Explosive activity and eruption scenarios at Somma–Vesuvius (Italy): towards a new classification scheme. Journal of Volcanology and Geothermal Research, this issue.]. Volatile contents (H2O and CO2) are parameterized in order to account for the uncertainty in their expected amounts for a next eruption. In all cases the flow in the conduit is found to be choked, with velocities at the conduit exit or vent corresponding to the sonic velocity in the two-phase non-equilibrium magmatic mixture. Conduit diameters and vent mixture densities are found to display minimum overlapping between the different eruptive scales, while exit gas and particle velocities, as well as vent pressures, largely overlap. Vent diameters vary from as low as about 5 m for VS eruptions, to 35–55 m for the most violent SP1 eruption scale. Vent pressures can be as low as less than 1 MPa for the lowest volatile content employed of 2 wt.% H2O and no CO2, to 7–8 MPa for highest volatile contents of 5 wt.% H2O and 2 wt.% CO2 and large eruptive scales. Gas and particle velocities at the vent range from 100–250 m/s, with a tendency to decrease, and to increase the mechanical decoupling between the phases, with increasing eruptive scale. Except for velocities, all relevant vent quantities are more sensitive to the volatile content of the discharged magma for the highest eruptive scales considered. [Copyright &y& Elsevier]

The AD 79 eruption of Vesuvius is certainly one of the most investigated explosive eruptions in the world. This makes it particularly suitable for the application of numerical models since we can be quite confident about input data, and the model predictions can be compared with field-based reconstruction of the eruption dynamics. Magma ascent along the volcanic conduit and the dispersal of pyroclasts in the atmosphere were simulated. The conduit and atmospheric domain were coupled through the flow conditions computed at the conduit exit. We simulated two different peak phases of the eruption which correspond to the emplacement of the white and gray magma types that produced Plinian fallout deposits with interlayered pyroclastic flow units during the gray phase. The input data, independently constrained and representative of each of the two eruptive phases, consist of liquid magma composition, crystal and water content, mass flow rate, and pressure–temperature–depth of the magma at the conduit entrance. A parametric study was performed on the less constrained variables such as microlite content of magma, pressure at the conduit entrance, and particle size representative of the eruptive mixture. Numerical results are substantially consistent with the reconstructed eruptive dynamics. In particular, the white eruption phase is found to lead to a fully buoyant eruption plume in all cases investigated, whereas the gray phase shows a more transitional character, i.e. the simultaneous production of a buoyant convective plume and pyroclastic surges, with a significant influence of the microlite content of magma in determining the partition of pyroclast mass between convective plumes and pyroclastic flows. [Copyright &y& Elsevier]

Discusses volcanic eruptions and presents a model in which a fragmentation criterion based on a rate-limited crossing of the glass transition, is incorporated into a multiphase fluid-dynamic description of magma ascent. How the numerical results of this model demonstrate the feasibility of strain-induced brittle fragmentation of magma in volcanic eruptions, and reconcile experimental with theoretical studies as well as with the observed volcanic products of large explosive eruptions.

Explosive eruptions are the final result of complex dynamics occurring within the volcanic plumbing system and involving domains such as one or more magma chambers, their connecting dykes, and the conduit or dyke connecting magma with the surface. We present here a 1D quasi-steady numerical modeling approach that allows the interplay between magmatic reservoirs located at different depths to be taken into account in the analysis of the eruption dynamics. We simulate a set up, common to a large number of real eruption cases, where a shallow, relatively small magma chamber hosting partially degassed, chemically more evolved magma is connected through a dyke to a deeper, much larger, gas-rich, less differentiated magmatic body. The numerical results describe two separate regimes controlled by either the shallow or the deep magma reservoir. While the shallow-controlled regime shows a general increase of eruption magnitude with increasing volume of magma stored at shallow depth, the deep-controlled regime, that we associate to the potential occurrence of a large caldera collapse, lacks such a relationship, resulting in eruption magnitudes that are substantially independent from the volume of the shallow magma reservoir. The efficiency of the deep interconnections between magmatic reservoirs, exemplified in our simulations by the width of the interconnecting dyke, is found to be a major factor controlling the shift from one regime to the other. [ABSTRACT FROM AUTHOR]

Eruption products of individual volcanic eruptions commonly display heterogeneities that are often interpreted as evidence of short times from new magma arrival to eruption. The time evolution of magma mixing and the degree of homogenization are still not well understood. We present the results of a parametric study through numerical simulations of andesite-dacite buoyancy driven magma mixing, by varying viscosity and volatile contents. The results show that highly efficient buoyancy-driven mixing is followed by a phase where stable convective layers develop, and mixing becomes progressively less efficient. Low viscosity and more volatiles favor mixing and achievement of a higher level of homogenization. On the contrary, high viscosity and lower volatiles give rise to long lived heterogeneities, in some cases preserving the end member compositions. Analysis of the results suggest that during the early transient phase of efficient magma mixing, crystal dissolution dominates; however, for cases where the transient phase takes longer, growth and resorption can act together, likely resulting in multiple resorption surfaces and zoning patterns before stable conditions are reached and euhedral crystal growth takes place. Crystals initially in equilibrium with magma can display a range of textural evolutions depending on their initial composition and individual paths during one single event of magma recharge, convection and mixing. Overall, the numerical simulations suggest that complex patterns of growth and dissolution followed by euhedral rims of various composition can be associated with single recharging events; and that short reaction times do not necessarily reflect comparably short times from recharge to eruption. [ABSTRACT FROM AUTHOR]

Sophisticated models of volcanic scenarios are increasingly sensitive to the accuracy of their input parameters and constitutive equations for magma properties. Viscosity is certainly one of the most important magma properties, but only recently systematic investigations on silicate liquids with natural compositions have started. We investigated the Newtonian viscosity of dry and hydrous phonolitic and trachytic melts from Vesuvius and Phlegrean Fields volcanic complexes, respectively. The analysed samples come from the deposits of the AD 1631 (Vesuvius) and ca. 4400 BP Agnano Monte Spina (AMS) (Phlegrean Fields) eruptions that are commonly taken as reference events for the most hazardous scenarios in case of reactivation of the two volcanoes. Samples were hydrated via piston cylinder synthesis at P=10 kbar and T=1600 °C. The dry high temperature and dry or hydrous low temperature viscosities were measured by a combination of micropenetration and concentric cylinder techniques, covering a total temperature range from about 400 to 1500 °C, water content range from virtually dry to 3.8 wt.%, and viscosity range from 102 to 1012 Pa s. The viscosity data for each composition were fitted by a modified Tamman–Vogel–Fulcher equation, allowing viscosity calculations at eruption temperatures and for dissolved water contents in the range of those examined. The viscosity data and model calculations were used for a comparison with other natural or synthetic phonolitic and trachytic melts, as well as with rhyolitic melts, for which viscosities had been measured. At water contents less than 1 wt.%, a trend of increasing viscosity from phonolitic to trachytic to rhyolitic melts is found. At water contents larger than 1 wt.%, the viscosity of trachytic melts is close to that of rhyolitic melts, while the viscosity of phonolitic melts is one to two orders of magnitude lower. A compositional parameter given by the (Na+K+H)/(Si+Al) molar ratio is found to be linearly related to the low-T hydrous viscosity of the trachytic and phonolitic melts considered, either analysed in this work or taken from literature. Differently, the rhyolitic melt shows significant variations from the trend found for phonolitic and trachytic melts. [Copyright &y& Elsevier]

Short-term interaction of magma with crustal carbonates can affect a volcano's eruptive style and drive even low-viscosity magmas toward large explosive eruptions. Only a few studies have focused on short-term magma-carbonate interaction under controlled laboratory conditions and the physical processes behind the experimental observations are still poorly understood. In this work, we present the first numerical modelling study of short-term magma-carbonate interaction and provide an interpretative framework for experimental and field observations. We developed thermodynamic and dynamic models for carbonate dissolution and mixing and mingling between contaminated magma pockets and host magma. We find that mixing and mingling can play a central role in modulating the efficiency of volatile exsolution. The increasing viscosity of the host melt slows down melt mingling and hence the mixing process, limiting volatile exsolution. Less efficient mixing and mingling could allow the fingerprints of short-term magma-carbonate interaction to be preserved in volcanic and intrusive rocks. Finally, we highlight that the mechanism and timescale of magma-carbonate interaction open a key question about the anomalous high mobility of CaO during carbonate dissolution. • Modelling of magma-carbonate interaction to interpret experimental observations. • Anomalous high mobility of calcium during carbonate dissolution. • Mixing and mingling can modulate volatile exsolution. [ABSTRACT FROM AUTHOR]

On December 29, 2013, an isolated vulcanian-type eruption occurred at Chaparrastique volcano (El Salvador) after 12 years of inactivity. The eruption was classified as VEI 2 and produced an ash plume with a maximum height of ~ 9 km. Textural and compositional data from phenocrysts from the erupted products have been integrated with geochemical and isotopic information from bulk rocks to elucidate the magmatic processes responsible for the reawakening of volcanic activity. Phenocrysts consist of Fo-rich poikilitic olivines hosting high-Mg titanomagnetites, and Fo-poor olivines coexisting with low-Mg titanomagnetites. Mineral-melt equilibria suggest an origin for the distinct phenocryst populations by mixing between a high- T (~ 1130–1150 °C), basaltic magma with f O 2 (NNO buffer) typical of the lower crust in arc systems and a low- T (~ 1060–1080 °C), basaltic andesitic magma with f O 2 (NNO + 1 buffer) commonly encountered in shallower, more oxidized crustal reservoirs. Thermobarometry based on Fe-Mg exchange between orthopyroxene and clinopyroxene constrains the crystallization before eruption at relative low- P (~ 150–250 MPa) and low- T (~ 1000–1050 °C). Mixing between two chemically distinct magmas is also evidenced by the occurrence of reverse zoned plagioclase phenocrysts with resorbed sodic cores and re-growth of sieve-textured calcic mantles. Conversely, plagioclase rims exhibit disequilibrium compositions addressed to decompression kinetics (~ 10 − 3 MPa/s) driven by rapid magma ascent to the surface (~ 0.03 m/s). Major and trace element modelling excludes fractional crystallization as the primary mechanism controlling the bulk rock variability, whereas geochemical data align along a mixing trend between two end-members representative of the primitive basalt and the differentiated basaltic andesite. Trace element and isotope data indicate that the primary source of magmatism is an enriched MORB-like mantle with the contribution of fluxes of metasomatic fluids and/or melts produced by the subducted slab. The role played by slab-fluid inputs of carbonate origin and slab-melts from the hemipelagic sediments seems to be minimal. Assimilation/contamination processes of magmas by crustal rocks are also negligible. In contrast, the geochemical signature of magmas is greatly influenced by slab-derived aqueous fluids produced prevalently by progressive dehydration of marine sediments and altered basaltic crust. [ABSTRACT FROM AUTHOR]

Abstract: Persistent degassing of closed-conduit explosive volcanoes may be used to inspect and monitor magmatic processes. After interaction with shallow hydrothermal fluids, volcanic gases collected at surface can differ substantially from those exsolved from magma. We report here on an innovative approach to identify and separate the contribution of variable magmatic components from fumarolic gases, by processing the 30-year-long geochemical dataset from the Campi Flegrei caldera, Southern Italy. The geochemical record shows periodic variations, which are well correlated with geophysical signals. Such variations are interpreted as due to the time-varying interplay of two magma degassing sources, each differing in size, depth, composition, and cooling/crystallization histories. Similar multiple degassing sources are common at explosive volcanoes, with frequent ascent and intrusion of small magma batches. Our innovative method permits the identification of those magma batches, which contributes to the interpretation of unrest signals, forecasting and assessment of volcanic hazards. [Copyright &y& Elsevier]

It is generally accepted, but not experimentally proven, that a quantitative prediction of volcanic eruptions is possible from the evaluation of volcanic gas data. By discussing the results of two years of real-time observation of H2O, CO2, and SO2 in volcanic gases from Mount Etna volcano, we unambiguously demonstrate that increasing CO2/SO2 ratios can allow detection of the pre-eruptive degassing of rising magmas. Quantitative modeling by the use of a saturation model allows us to relate the pre-eruptive increases of the CO2/SO2 ratio to the refilling of Etna's shallow conduits with CO2-rich deep-reservoir magmas, leading to pressurization and triggering of eruption. The advent of real-time observations of H2O, CO2, and SO2, combined with well-constrained models of degassing, represents a step forward in eruption forecasting. [ABSTRACT FROM AUTHOR]