ALBERT

Description: The current paradigm for volcanic eruptions is that magma erupts from a deep magma reservoir through a volcanic conduit, typically modelled with fixed rigid geometries such as cylinders. This simplistic view of a volcanic eruption does not account for the complex dynamics that usually characterise a large explosive event. Numerical simulations of magma flow in a conduit combined with volcanological and geological data, allow for the first description of a physics-based model of the feeding system evolution during a sustained phase of an explosive eruption. The method was applied to the Plinian phase of the Pomici di Avellino eruption (PdA, 3945 ±10 cal yr BP) from Somma–Vesuvius (Italy). Information available from volcanology, petrology, and lithology studies was used as input data and as constraints for the model. In particular, Mass Discharge Rates (MDRs) assessed from volcanological methods were used as target values for numerical simulations. The model solutions, which are non-unique, were constrained using geological and volcanological data, such as volume estimates and types of lithic components in the fall deposits. Three stable geometric configurations of the feeding system (described assuming elliptical cross-section of variable dimensions) were assessed for the Eruptive Units 2 and 3 (EU2, EU3), which form the magmatic Plinian phase of PdA eruption. They describe the conduit system geometry at time of deposition of EU2 base, EU2 top, and EU3. A 7-km deep dyke (length , width ), connecting the magma chamber to the surface, characterised the feeding system at the onset of the Plinian phase (EU2 base). The feeding system rapidly evolved into hybrid geometric configuration, with a deeper dyke (length , width ) and a shallower cylindrical conduit (diameter , dyke-to-cylinder transition depth ∼2100 m), during the eruption of the EU2 top. The deeper dyke reached the dimensions of and at EU3 peak MDR, when the shallower cylinder had enlarged to a diameter of 60 m and a transition depth of 3000 m. The changes in feeding system geometry indicate a partitioning of the driving pressure of the eruption, which affected both magma movement to the surface and dyke growth. This implies that a significant portion of the magma injected from the magma chamber filled the enlarging dyke before it erupted to the surface. In this model, the lower dyke acted as a sort of magma “capacitor” in which the magma was stored briefly before accelerating to the cylindrical conduit and erupting. The capacitor effect of the lower dyke implies longer times of transit for the erupting magma, which also underwent several steps of decompression. On the other hand, the decompression of magma within the capacitor provided the driving pressure to maintain the flow into the upper cylindrical conduit, even as the base of the dyke started to close due to the drop in driving pressure from progressive emptying of the magma chamber. The shallower cylindrical conduit was shaped through the erosion of conduit wall rocks at and above the fragmentation level. Using the lithic volume and duration of EU3, the erosion rate of shallower cylindrical conduit was calculated at ∼5 × 103 m3/s. The outcomes of this work represent an important baseline for further petrologic and geophysical studies devoted to the comprehension of processes driving volcanic eruptions.

Description: Published

Description: 545-555

Description: 5V. Processi eruttivi e post-eruttivi

Description: JCR Journal

Description: The 1913 sub-Plinian eruption of Fuego de Colima volcano (Mexico) occurred after almost 100 years of effusive and (minor) Vulcanian explosive activity, which modulated dome growth and destruction. Dome extrusion persisted from 1869 to 1913. The transition to explosive eruption started on 17 January 1913, and it progressed in three phases: (1) opening, with the generation of block-and-ash flows, (2) vent clearing, with strong explosions that destroyed the summit dome and decompressed the magmatic system, and (3) sustained column (sub-Plinian fallout) with final collapse producing pyroclastic density currents. Because of this succession of events, the 1913 activity represents an excellent case-study for investigating the eruptive style changes at calc-alkaline volcanoes. We investigated the conditions that led to eruptive style transition from effusive (dome growth) to explosive (the final sub-Plinian fallout) through steady-state numerical simulations, using subsurface data and independently inferred (from volcanological data) mass discharge rates as constraints. Results show good matches for hybrid geometrical settings of the shallow conduit-feeding system (i.e., dyke developing into a shallower cylindrical conduit), and the magma chamber top at 6 km of depth. The fragmentation level was shallower than 2 km, as inferred from the lithics contained in the sub-Plinian fall deposits of Phase (3). The most likely solution is represented by a dyke having major axis between 200 and 2000 m and the minor axis of 40 m. The dyke-cylinder transition was set at a depth of 500 m, with a cylinder diameter of 40 m. It emerges that at least two main mechanisms may be responsible for the effusive to explosive transition that led to the Phase (3) of the 1913 eruption: (i) an increase in magma chamber overpressure (magmatic triggering) or (ii) decrease of lithostatic stress acting on the volcano (non-magmatic triggering). The former implies arrival into the magma chamber of a batch of fresh magma, which can have volume between 10 and 200 × 106 m3, depending on the values of magma and host rock compressibility. The latter requires decompression-induced emptying of at least the equivalent of 1000 m of the magma column to produce the necessary unloading of the conduit-feeding system. A sudden jerk in the lateral spreading of the Fuego de Colima cone would be a reliable mechanism for decompressing the upper conduit and driving fragmentation processes over a time period of few hours. The results are not conclusive on an internal (magma chamber overpressure), external (lowering of lithostatic load), or mixed (internal and external) trigger of the observed eruptive style transition. This work highlights how different processes can have non-linear cascade effects on close-to-equilibrium volcanic systems like Fuego de Colima volcano.

Description: Published

Description: id 62

Description: 5V. Processi eruttivi e post-eruttivi

Description: JCR Journal

Description: The emission of volcanic gases can occur during both eruptive and quiescent stages of volcanic activity, affecting air quality when the concentrations exceed species-specific thresholds. Quantitative studies of model validation are essential before applying a simulator for probabilistic volcanic hazard assessment. Here, we provide three examples of model validation aimed at testing the accuracy in providing realistic values of CO〈sub〉2〈/sub〉 concentration, estimating the source gas fluxes using the concentration measurements through resolution of the inverse problem, and identifying potential hazardous gas dispersal scenarios. We selected three case study affected by a persistent diffusive and fumarolic degassing: i) La Solfatara crater, Campi Flegrei caldera, Italy, ii) Caldeiras da Ribeira Grande, São Miguel Island, Azores and iii) Stefanos crater, Nisyros Island, Greece. We used published and original CO〈sub〉2〈/sub〉 flux data as input for numerical simulations run through VIGIL, an open-source workflow for parallel simulations and probabilistic output using two Eulerian models, which account for the passive and gravity-driven gas transport, respectively. Our results provided probabilistic CO〈sub〉2〈/sub〉 concentration maps at 0.5-1.5 m from the ground in order to investigate the potential effects on human and animal health, and statistical tests aimed to infer the best scaling factor for gas flux in reaching hazardous gas concentrations. This kind of methodology has revelad the potential usefulness of the modeling in reproducing the order of magnitude of the observed degassing, therefore, such a testing should be the first logical step to be taken before applying a simulator to assess (gas) hazard in any other volcanic contexts.

Description: The Neapolitan volcanoes are some of the most hazardous in the world because capable of large explosive eruptions releasing threatening quantities of ash in the atmosphere that would reasonably reach highly-populated urban areas, even at considerable distances from the sources. This study aims to show a long-term tephra fallout hazard evaluation posed by the Neapolitan volcanoes (Somma-Vesuvius, Campi Flegrei, Ischia) in Southern Italy, and to track future perspectives to combine a multi-hazard assessment to a more local-scale domain. Through a new HPC workflow for volcanic hazard assessment based on a Bayesian procedure, we provided the mean annual frequency with which the tephra load at the ground exceeds given critical thresholds within 50 years. By performing hazard disaggregation, we also got specific information about the prevalence of different volcanoes and eruptive style in the different target areas. Thousands of numerical simulations were run with the FALL3D model to create a large synthetic dataset of tephra ground loads on a 0.03°-resolution gridded domain and a vertical σ-coordinate system with a linear decay. Each simulation took a randomly sampled set of eruptive source parameters within the ranges established by the knowledge of each volcano and meteorological conditions extracted by the ECMWF ERA5 dataset. In particular, results show that the greater tephra load thresholds are exceeded with longer averaged return times (i.e., 100, 500, 1000 years) in the proximity of the Neapolitan area. This pushes us to do a step forward into a more detailed multi-hazard and risk-ranking quantification in the Neapolitan urban area.

Description: Volcanic and non-volcanic gas emission represent a widespread and frequent hazard. In fact, some gas species (e.g., CO2, H2S, SO2) can affect human health and even threaten life at concentrations and doses above species-specific thresholds. Depending on the starting buoyancy relative to the surrounding atmosphere at the emission location, gas emissions can be classified as dilute passive degassing (e.g. fumaroles) and dense gas flow (e.g., cold CO2 accumulation and flows onto the ground). Here we present the latest version of VIGIL (automatic probabilistic VolcanIc Gas dIspersion modeLling), a Python simulation tool capable of handling the complex and time-consuming gas dispersion simulation workflow and interfaced with two dispersion models: a dilute (DISGAS) and a dense gas (TWODEE-2) dispersion model. VIGIL allows exploring the uncertainty of the meteorological conditions and gas emission location and strength. The post-processing script offers the option to create Empirical Cumulative Distribution Functions (ECDF) of the gas concentrations combining the outputs of multiple simulations. The ECDF can be interrogated by the user to produce maps of gas concentration at specified exceedance probabilities. Tracking points can also be used to produce time series of gas concentration at selected locations and hazard curves if ECDF is produced. Furthermore, the new gas persistence calculation capability creates maps of the probability to overcome specified gas specie-specific concentration thresholds over specified durations. We also present results from applications at different volcanically and tectonically active areas showcasing the various capabilities of VIGIL.