Mechanical Behaviour and Failure of Glowing Volcaniclastic Rocks: Implications for Deposit-Derived Pyroclastic Density Currents

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.