On the Features of Pyroclastic Deposits and Post-eruption Natural Hazards

Pyroclastic deposits of volcanic explosive eruptions are associated with both sedimentation of tephra and emplacement of pyroclastic density currents (PDCs) that travel on the flank of volcanoes. Pyroclastic deposits are typically mixtures of ash, lapilli and blocks, very loose and unconsolidated, hence prone to move along a slope. Due to highly porous structure, such deposits are also prone to wetting induced collapse, with a transition from a solid-like behaviour to a fluid-like behaviour. It entails a long run-out of mass-movements (landslides or lahars and debris flows) eventually triggered. Disasters associated with this type of deposit remobilization have been recorded all over the world even decades or centuries after the end of an eruption. Here we provide new insights into remobilization of tephra deposits based on various international case studies, bridging a gap among the different dataset and interpretations. Exponential and power relations are found among the uniformity coefficient, the median diameter and the main physical variables adopted in the physical-based models used for the analysis of the natural hazards in volcanic deposits.