Fallout Events During the Post-plinian Phase of the AD 79 Vesuvius Eruption

The AD 79 Vesuvius eruption is characterized by two main phases: a sustained column phase, which deposited a thick white–gray pumice lapilli deposit, followed by a column collapse phase, which emplaced several pyroclastic density currents. Here, we present stratigraphic and volcanological evidence for five lithic-rich lapilli fall layers (D, G1, G3, I, and X2, from base to top) interstratified with pyroclastic density current deposits, which have been emplaced after the collapse of the AD 79 Plinian column. These are distributed south of the Vesuvius, at distances between 5 and 20 km from the vent. All sections were sampled to quantify lateral and vertical changes in grain size, and sedimentological parameters were determined. Clasts were separated into three main categories: juvenile, lithic, and crystals. The coarser layers, D and G1, have median diameters similar to those of the finer part of the Plinian deposit at the same locations, indicating a significant plume height during these later sustained column pulses. Our study suggests that the resumption of a sustained column was repeatedly established during the post-Plinian phase of the AD 79 eruption, similar to the ignimbrite phase of the Novarupta and Bishop Tuff Plinian eruptions. The lithic-rich nature of all these late fall phases suggests changes in eruption style (with respect to the magmatic Plinian phase), possibly associated with instabilities in the conduit-vent system.