Understanding the failure of hot rocks and the generation of glowing avalanches

The gravitational instability of incandescent volcaniclastic deposits, whether it involves the collapse of the crater rim or the failure of hot deposits on the flanks of volcanic cones, can give rise to hot rock avalanches, known as deposit-derived pyroclastic density currents (PDCs). These avalanches can remobilize volumes ranging from 104 to 107 m3 and travel several km from the source area. They are deposited at extremely high temperatures, posing a potential danger to nearby communities and tourists in close proximity to volcanoes. Hot avalanches are considerably more widespread than initially hypothesized, and two distinct phenomena have been documented: i) those induced by magma thrust; ii) those resulting from the collapse of hot materials situated on the slopes of the volcano. In the latter scenario, loose volcaniclastic material can collapse due to factors such as surpassing the friction angle or mechanisms that are not fully constrained, like undercutting or overload by volcaniclastic or lava flows. Glowing avalanches have been documented at various locations, including the 1944 eruption at Mt. Vesuvius, and the 1930 and 1944 eruption at Stromboli. In all of these events, avalanches were reported to have formed from the remobilization of poorly welded volcaniclastic agglutinate This study presents the methodological approach and initial findings of a project aimed at understanding the mechanisms of formation and potential impacts of hot avalanches. The project's objectives include identifying the collapse mechanisms of partially welded, still hot volcaniclastic deposits, as well as investigating the rheology of avalanches. The project's approach involves classical geomechanical characterization of volcanic rocks and analysis of rock toughness variations at high temperatures to account for differences in the welding degrees of incandescent deposits, along with variations in porosity and crystallinity. High-temperature (T<1100°C) deformation experiments are designed to investigate the rheology of the multiphase mixture consisting of melt, crystals, and pores. These experiments explore rheological behaviour, ranging from homogeneous to non-homogeneous deformation, with the latter characterized by viscous and/or brittle shear localization. Analysing flow curves enable the determination of high-temperature uniaxial strength (i.e., the transition from viscous to brittle behaviour) and weakening in response to the formation of shear bands and ductile deformation. Field surveys in the study areas provide insights into flow transport and deposition, while geomorphological studies using digital terrain models of varying resolutions investigate both detachment and accumulation areas. Two- and three-dimensional slope stability analyses are conducted to identify parameters with the greatest influence on the stability of volcaniclastic deposits. To gain a better understanding of the dynamics and quantify the potential impact of hot rock avalanches, such as invaded area, velocity, thickness, and dynamic pressure, numerical simulations are carried out, incorporating data from fieldwork and geomorphological analysis. Stromboli and Vesuvius in Italy were chosen as case studies due to recent occurrences of such phenomena, with a significant probability of recurrence. The study will aid in the interpretation of monitoring data from selected volcanoes and inform proper territorial planning in their vicinity.