Alessandro Frontoni

The renewed start and funding of the CARG project in volcanic areas have enabled new surveys and refinements of data on the volumes and extents of ignimbrites across the Roman Magmatic Province (RMP). To date, the investigation has focused particularly on the Roccamonfina volcano (Sheets 416 Sessa Aurunca and 417 Teano) and the Bracciano caldera (Sheet 364 Bracciano). The project is enhancing field data from areas already surveyed in past decades, while integrating new models and technologies to obtain more accurate quantifications of erupted magma volumes and a consequent re-evaluation of eruption magnitudes. Preliminary results indicate that the volume of some ignimbrites increases by more than one order of magnitude, suggesting that many other ignimbrites within the RMP may have been significantly underestimated, such as the Brown Leucitic Tuff and the White Trachytic Tuff pertaining to the Roccamonfina volcano. This reassessment potentially characterizes the RMP as an ignimbrite flare-up system, comparable to some of the largest and most impactful volcanic provinces worldwide, such as the Taupo Volcanic Zone. In this framework, new field and literature data, borehole stratigraphy, and GIS-integrated methodologies were combined to refine the bulk volume, areal extension, and magnitude of a case-study ignimbrite, with the aim of developing a standardized procedure for computing and integrating field surveys applicable to all ignimbrites.

The gravitational instability of glowing volcaniclastics can lead to the formation of deposit-derived pyroclastic density currents (PDCs). These flows can mobilise volumes of 10³ to 10⁷ m³, travelling several kilometres from their source while maintaining extremely high temperatures, posing a significant risk to nearby communities and visitors. Deposit-derived PDCs are typically formed by two primary mechanisms: (i) those driven by magma thrust, resulting in the collapse of crater rims, and (ii) those triggered by the collapse of hot material on volcanic slopes due to factors such as exceeding the angle of friction, undercutting or overloading by fresh volcaniclastics or lava flows. At Vesuvius, Italy, deposit-derived PDCs were directly observed during the 1822 and 1944 eruptions, both characterized by activity that varied from effusive, fire fountaining, sub-Plinian, to late Vulcanian. During the 1944 eruption, which occurred between 18 and 29 March, deposit-derived PDCs were generated during the fire fountaining phase (Phase 2) following the lava emission phase (Phase 1). This study presents a comprehensive investigation of the formation mechanisms of deposit-derived PDCs during the 1944 eruption. It integrates historical documentation and photographs by Giuseppe Imbò, the Director of the Vesuvius Observatory at the time, with fieldwork aimed at evaluating the geometric relationships between the proximal accumulation, welding and detachment zones. The spatial distribution of PDC deposits around the volcanic cone was mapped using photogrammetry-based digital elevation models (DEMs) and 1943 orthophotos. Volume estimates for deposits in the proximal cone and surrounding areas were derived by comparing the 1943 DEM with a more recent 2012 DEM. This analysis provided new insights into the dynamics of PDC formation and its spatial and volumetric characteristics. The results of this study show that the failure of glowing volcaniclastic deposits and the subsequent generation of deposit-derived PDCs is controlled by both the pre-eruption morphology of the volcano and the distribution of deposits with different degrees of welding. In particular, deposits accumulated near the old crater rim with lower degrees of welding were more susceptible to collapse, whereas zones with higher welding degree were less likely to fail. Conversely, in areas further from the crater rim where material from fire fountains accumulated, a greater thickness of deposits was required to initiate failure. These variations significantly influence the timing and volume of individual PDCs. In proximal areas, smaller flows were generated more quickly, while in distal areas, larger flows developed later, particularly towards the end of the fire fountaining phase. These results provide critical insights into the mechanisms governing the formation of deposit-derived PDCs in volcanoes with comparable eruptive styles. Such volcanoes can produce deposit-derived PDCs that extend over areas considerably larger than those directly affected by the primary eruptive phenomena.

Failure of glowing volcaniclastic rocks can result in hot rock avalanches, commonly referred to as deposit-derived pyroclastic density currents (PDCs). These phenomena are common in volcanoes with low to moderate eruptive activity, where steep slopes and proximal material accumulation near eruptive vents predispose volcanic flanks to instability. To investigate the factors influencing these failures, we studied the welded deposits from the 1944 eruption of Mt. Vesuvius, which produced deposit-derived PDCs along the volcano's slopes. Our analyses include the physical, mechanical and compositional characterization of proximal fire-fountaining deposits, with particular emphasis on the influence of variations in welding degree, porosity and crystallinity at high temperatures. Field and laboratory tests were carried out to investigate the physical (porosity measurements) and mechanical (i.e., sclerometer measurements, point load tests, and uniaxial compression tests) properties. Petrographic observations were carried out using transmitted light microscopy, supplemented by scanning electron microscopy (SEM) to examine textural and morphological characteristics. The composition of mineral phases was obtained through electron microprobe (EPM), while the abundance of major and trace elements in whole rocks was determined using X-ray Fluorescence (XRF) and inductively coupled plasma (ICP) spectroscopy.High-temperature rheological experiments were carried out using a newly developed apparatus, the Volcanological In-situ Deformation Instrument (VIDI), designed to study magma rheology under conditions relevant to volcanic processes. VIDI allows vertical uniaxial deformation experiments on natural silicate melts at temperatures up to 1100°C. The experiments were performed on partially remelted samples with varying welding degrees, including (i) coherent lava blocks or pyroclastic bombs and (ii) partially welded pyroclasts. These investigations explored the rheological response of multiphase materials (comprising melt, crystals and pores) in different regimes ranging from homogeneous to inhomogeneous deformation, the latter characterised by viscous and brittle shear localisation. The flow curves generated from these high temperature deformation experiments defined the uniaxial strength of the materials at elevated temperatures. Additionally, the experiments quantified material weakening caused by shear band formation and ductile deformation. To further constrain the textural and porosity changes experienced by the samples, X-ray microtomography imaging analysis was carried out both before and after the experiments. This analysis provided valuable insights into the microstructural evolution of the materials during deformation. These results elucidate the mechanical processes that contribute to the failure of incandescent volcaniclastic rocks and the generation of deposit-derived PDCs, thereby advancing our understanding of instability dynamics in volcanic systems and provides critical insights into the hazards posed by such phenomena.

The eruptive style of a volcanic system and the energy of the eruption depend on a complex interplay among several internal and external factors, as temperature, pressure, volatile composition and content, and geometry of the volcanic system, all of which concur to affect the rheology of the magma and thus the eruptive strength of a volcano.The comprehension of the rheological behavior of the mixtures of melt, crystals, and bubbles is necessary to understand the migration of magma within the volcanic system and the modality of eruptions. To date, crystal-bearing magmas and their behavior have been the focus of the majority of investigations by the scientific community.The paucity of studies on bubble-bearing magmas may have been partly due to the greater challenges posed by the experimental procedure, mostly related to the outgassing of the gas phase during the experiments. So, while the influence of crystals in magma rheology has been parameterized in detail, a comprehensive model for the rheology of the bubble-bearing magmas has yet to be formulated.The aim of this work is to understand the complex dependence of the viscosity on the bubble content and strain rate, by performing suites of in situ degassing experiments on rhyolitic magma at an experimental temperature of 850 °C, followed by uniaxial deformational experiments (constant strain rates of 5 x 10-5, 10-4 and 10-3 s-1) through a vertical uniaxial press at experimental temperatures varying between 720 and 800 °C. Experimental parameters are selected to investigate the rheology under unsteady flow conditions (i.e., prior to equilibrium deformation of bubbles). Such conditions may be very common during explosive volcanic eruptions, where bubbles are not able to relax in the fast-ascending magmas.For constant strain rates, results show trends of apparent viscosity as a function of bubble content, consisting of an initial increase of viscosity for low amounts of bubbles (0-20%), followed, above the 20% threshold porosity, by a general viscosity decrease. At fixed porosity, a strain rate dependence of viscosity is also observed, with viscosity decreasing as the strain rate increases. Overall, the sample's apparent viscosity appears to be non-linearly controlled by both ambient conditions (Temperature Tdef and Strain rate ) and sample texture (Porosity and Vesicle Number Density VND), whose interplay concurs to define the degree of flow unsteadiness. We, therefore, discuss the weight of each parameter in determining the apparent viscosity of the magmatic mixture.

In this work, we present a comprehensive rheological database including most of the existing data relevant for crystal-bearing magmas collected from the scientific literature, covering the entire range of natural volcanic conditions, in terms of crystal content (1–80%), crystal shape (aspect ratio R from 1 to 13), and strain rate (between 10⁻⁷ and 10²s⁻¹). Datasets were collected and discerned as a function of the information which we considered necessary for building a general systematic model describing relative viscosity of crystal-bearing magmas, such as the apparent and melt viscosity, the crystal concentration, crystal shape, and the strain rate. The selected dataset was then used for modelling the relative viscosity of a liquid-solid mixture having different concentrations of particles with different R, subjected to different strain rates. The proposed model allows us to quantitatively describe the rheological behaviour of crystal-bearing magmatic systems.

In this work, we present a novel rheological database including most of the existing data relevant for crystal-bearing magmas collected from the scientific literature, covering the entire range of natural volcanic conditions, in terms of crystal content (1-80%), crystal shape (aspect ratio R from 1 to 13), and strain rate (between 10-7 and 102 s-1). Datasets were collected and discerned as a function of the information which we considered necessary for building a general systematic model describing relative viscosity of crystal-bearing magmas, such as the apparent and melt viscosity, the crystal concentration, crystal shape, and the strain rate. The selected dataset was then used for modelling the relative viscosity of a liquid-solid mixture having different concentrations of particles with different R, subjected to different strain rates. The proposed model is based on a generalization of the four parameters semi-empirical formula, proposed by Costa et al. (2009). A new model was calibrated for the novel collected database, allowing us to quantitatively describe the rheological behaviour of crystal-bearing magmatic systems.