Deposit-Derived Pyroclastic Density Currents from the 1944 Eruption of Mount Vesuvius, Italy: Integrating Field Observations, Historical Accounts, and Morphological Data to Reconstruct Predisposing Factors and Triggering Mechanisms

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.