Deposits and physical properties of pyroclastic density currents during complex Subplinian eruptions: the AD 472 (Pollena) eruption of Somma-Vesuvius, Italy

Small-scale pyroclastic density currents (PDCs) associated with the AD 472 (Pollena) eruption of Somma-Vesuvius, Italy, were generated by both magmatic and phreatomagmatic explosive fragmentation. The resulting deposits were emplaced under flow boundary conditions dominated by varying combinations of grain interaction, fluid escape and traction processes. Stratigraphic and lithofacies analysis of these PDCs offers a new perspective on the en masse versus progressive aggradation debate for PDC deposition. In particular, the analyses indicate that PDCs were density stratified with a basal underflow dominated by grain interactions. The underflows comprised trains of self-organized granular pulses of variable thickness and magnitude, depending on the overall particle concentration and fluid turbulence. A change in gradient between the upper and lower slopes of the volcano promoted deposition and the different pulses aggraded sequentially (stepwise). In this model each pulse stops en masse and the whole deposit aggrades progressively. Particle concentration, density, mean velocity, and flow height were assessed for the studied PDCs using differaent methods for massive and stratified deposits. The calculated mobility of the flows was 0.2 to 0.3, in the expected range for small-scale PDCs.