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Past and present mid-intensity explosive eruptions of Italian volcanoes and their impact on human activity
Abstract and Figures
Active Italian volcanoes are characterized by a large variability of eruptive mechanisms, from quiet lava effusions to catastrophic ignimbritic eruptions. The impact of volcanic activity, and in particular of explosive activity, is clearly related to the intensity and magnitude of the eruptions, varying from an incidental interference with everyday life up to devastating consequences on civilizations. While the largest events have usually monopolized the interest of volcanologists and historians, the modalities and impact of mid intensity eruptions have not been investigated in so much detail. In addition, the frequency of occurrence of mid intensity eruptions is by far higher than that of the largest events, so making their study of primary importance for the assessment of the impact of volcanic activity on environment and human life. In this paper, case histories of mid-intensity explosive activity at Mt. Vesuvius, Phlegrean Fields, Mt. Etna and Stromboli are presented in order to introduce and discuss the hazard and impact related to this type of activity.
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... The mass discharge rate (MDR) calculated assuming a deposit density of ca. 1000 kg/m 3 is of the order of 2-2.6 × 10 5 kg/s and comparable to those calculated for other mid-intensity, violent Strombolian eruptions at Vesuvius and Etna volcanoes (Cioni et al. 2008(Cioni et al. , 2010. ...
... well fragmented and mostly non-welded), the thickness and architecture of the pyroclastic sequence, the nature of particles and the dispersal of products are akin to those of violent Strombolian eruptions, during which rapidly pulsating or steady eruption columns produce the fallout of wellfragmented pyroclasts and very minor ballistic bombs (e.g. Valentine 1998;Arrighi et al. 1999;Valentine et al. 2005;Valentine and Gregg 2008;Cioni et al. 2010;Barsotti et al. 2015). The violent Strombolian phase probably continued for the first 3 days of eruption, with pulsating eruptions until 21 November. ...
The last eruption on Tenerife (Canary Islands, Spain) started on 18 November 1909
from the El Chinyero vent on the northwestern Santiago rift. This fissural eruption was
well documented by scientists and eyewitnesses, but there is a lack of data on the
high-energy phase that produced the most significant emissions of ash and lapilli at the
onset of the eruption. Historical documents (newspapers, dispatches, telegrams, etc.),
eyewitness accounts and scientific reports were reviewed from a volcanological
perspective and integrated with data from the analysis of deposit features, allowing an
accurate reconstruction of the eruption and its dynamics. The 1909 eruption of
Chinyero was fed by a compositionally discrete magma batch that ascended rapidly
within the crust, producing rather violent pulsating Strombolian explosive activity in the
early phases of the eruption. This activity produced a ca. 80 m scoria cone and heavy
fallout of lapilli and ash over the entire northern sector of the island of Tenerife. The
energy of explosive activity waned after three days, giving way to the gentle
Strombolian explosive activity that contributed to a lesser extent to the build-up of the
pyroclastic pile. Eruptions such as those from the Chinyero vent in 1909 are
representative of rift activity on Tenerife and constitute a volcanic hazard for presentday
inhabitants.
... Weakly explosive eruptions produce a large range of volcanic products varying from bombs to fine ash (Cioni et al. 2010). Depending on the energy of each eruption, a large portion of these ejecta falls back into the vent or in the crater area where they are exposed to physicochemical conditions that are close to magmatic (Fig. 1). ...
A set of experiments have been performed on volcanic materials from Etna, Stromboli and Vesuvius in order to evaluate how the exposure to thermal and redox conditions close to that of active craters affects the texture and composition of juvenile pyroclasts. Selected samples were placed within a quartz tube, in presence of air or under vacuum, and kept at T between 700 and 1,130 °C, for variable time (40 min to 12 h). Results show that reheating reactivates the melt, which, through processes of chemical and thermal diffusion, reaches new equilibrium conditions. In all the experiments performed at T = 700–750 °C, a large number of crystal nuclei and spherulites grows in the groundmass, suggesting conditions of high undercooling. This process creates textural heterogeneities at the scale of few microns but only limited changes of groundmass composition, which remains clustered around that of the natural glasses. Reheating at T = 1,000–1,050 °C promotes massive groundmass crystallization, with a different mineral assemblage as a function of the redox conditions. Morphological modifications of clasts, from softening to sintering as temperature increases, occur under these conditions, accompanied by progressive smoothing of external surfaces, and a reduction in size and abundance of vesicles, until the complete obliteration of the pre-existing vesicularity. The transition from sintering to welding, characteristic of high temperature, is influenced by redox conditions. Experiments at T = 1,100–1,130 °C and under vacuum produce groundmass textures and glass compositions similar to that of the respective starting material. Collapse and welding of the clasts cause significant densification of the whole charge. At the same temperature, but in presence of air, experimental products at least result sintered and show holocrystalline groundmass. In all experiments, sublimates grow on the external surfaces of the clasts or form a lining on the bubble walls. Their shape and composition is a function of temperature and fO2 and the abundance of sublimates shows a peak at 1,000 °C. The identification of the features recorded by pyroclasts during complex heating–cooling cycles allows reconstructing the complete clasts history before their final emplacement, during weakly explosive volcanic activity. This has a strong implication on the characterization of primary juvenile material and on the interpretation of eruption dynamics.
Mt. Vesuvius is one of the most studied volcanoes in the world and its proximity to an extremely populated area makes it also one of the most threatening. Violent Strombolian (VS) events have occurred in the most recent history of the volcano, and they are the type most likely to occur in case of reactivation of the volcano in the near future. In order to investigate the dynamics and hazard of this type of eruption, we performed new field and laboratory work and numerical simulations of plume dynamics, fallout and tephra dispersal associated with such eruptions. Attention was specifically focused on the 1906 eruption, a recent and well-studied VS event. Based on new field analyses and historical observations, we reconstructed the temporal evolution of eruption source conditions during the event. The reconstructed explosive phase of the eruption is inferred to have been 8 days long and characterized by two distinct stages: a former short and intense period with sustained convective plume fed by powerful lava fountains (20 h) followed by a prolonged and less intense period of ash emission (172 h). The total grain-size distributions for both phases, used as inputs to the model, were obtained by field work and laboratory analyses. Based on these new volcanological data and reconstruction of wind field direction and intensity, partially derived from historical sources, the 1906 event was numerically simulated and results compared to deposit distributions. The modelling outcomes for the ash emission phase provide a better agreement with the measured tephra mass load for a simulation run in which ash aggregation (described by the models of Cornell et al. J Volcanol Geotherm Res 17:89–109, 1983, and Biass et al. Nat Hazard Earth Syst Sci 14:2265–2287, 2014) is specifically taken into account, confirming the importance of this process during tephra dispersal. The aggregation model that best fits the simulated results to the measured ground loadings has 80 % of particles Φ ≥ 4 that aggregate uniformly in the range Φ = −1 to 3. Two additional simulations of a VS event were carried out by using meteorological data of two specific periods to exemplify weather’s potential on impacts of such eruptions, particularly tephra loading, on the surrounding areas. The model outcomes clearly highlight the major effects of differences in local meteorology on plume dynamics and ash dispersal and the key role of wind shear in determining the cumulative thickness of ground deposits. Results also show that, due to the long duration of this kind of eruption and the large variability in zonal winds at this latitude, ash dispersal and fallout from VS events at Vesuvius represent a probable hazard for all of the territory near Vesuvius, including the city of Naples, where cumulative tephra loadings might reach critical thresholds for roof collapse and infrastructure damage.
Direct observations of mid-intensity eruptions, in which a huge amount of ash is generated, indicate that ash recycling is quite common. The recognition of juvenile vs. recycled fragments is not straightforward, and no unequivocal, widely accepted criteria exist to support this. The presence of recycled glassy fragments can hide primary magmatic information, introducing bias in the interpretations of the ongoing magmatic and volcanic activity. High temperature experiments were performed at atmospheric pressure on natural samples to investigate the effects of reheating on morphology, texture and composition of volcanic ash. Experiments simulate the transformation of juvenile glassy fragments that, falling into the crater or in the upper part of the conduit, are recycled by following explosions. Textural and compositional modifications obtained in laboratory are compared with similar features observed in natural samples in order to identify some main general criteria to be used for the discrimination of recycled material. Experiments were carried out on tephra produced during Strombolian activity, fire fountains and continuous ash emission at Etna, Stromboli and Vesuvius. Coarse glassy clasts were crushed in a nylon mortar in order to create an artificial ash, and then sieved to select the size interval of 1-0.71 mm. Ash shards were put in a sealed or open quartz tube, in order to prevent or to reproduce effects of air oxidation. The tube was suspended in a HT furnace at INGV-Pisa and kept at different temperatures (up to to 1110°C) for increasing time (0.5-12 hours). Preliminary experiments were also performed under gas flux conditions. Optical and electron microscope observations indicate that high temperature and exposure to the air induce large modifications on clast surface, ranging from change in color, to incipient plastic deformation till complete sintering. Significant change in color of clasts is strictly related to the presence of air, irrespective of temperature while sintering is favored by the high temperature and low fO2. Re-heating promotes nucleation and growth of crystals in the groundmass and associated change of glass composition, sometimes accompanied by growth and coalescence of vesicles in the size of 10-50 µm and cracking of the external surface.
