The Pomici di Avellino eruption of Somma–Vesuvius (3.9 ka BP). Part II: sedimentology and physical volcanology of pyroclastic density current deposits

Abstract

Pyroclastic density currents (PDCs) generated during the Plinian eruption of the Pomici di Avellino (PdA) of Somma-Vesuvius were investigated through field and laboratory studies, which allowed the detailed reconstruction of their eruptive and transportation dynamics and the calculation of key physical parameters of the currents. PDCs were generated during all the three phases that characterised the eruption, with eruptive dynamics driven by both magmatic and phreatomagmatic fragmentation. Flows generated during phases 1 and 2 (EU1 and EU3pf, magmatic fragmentation) have small dispersal areas and affected only part of the volcano slopes. Lithofacies analysis demonstrates that the flow-boundary zones were dominated by granular-flow regimes, which sometimes show transitions to traction regimes. PDCs generated during eruptive phase 3 (EU5, phreatomagmatic fragmentation) were the most voluminous and widespread in the whole of Somma-Vesuvius' eruptive history, and affected a wide area around the volcano with deposit thicknesses of a few centimetres up to more than 25 km from source. Lithofacies analysis shows that the flowboundary zones of EU5 PDCs were dominated by granular flows and traction regimes. Deposits of EU5 PDC show strong lithofacies variation northwards, from proximally thick, massive to stratified beds towards dominantly alternating beds of coarse and fine ash in distal reaches. The EU5 lithofacies also show strong lateral variability in proximal areas, passing from the western and northern to the eastern and southern volcano slopes, where the deposits are stacked beds of massive, accretionary lapilli-bearing fine ash. The sedimentological model developed for the PDCs of the PdA eruption explains these strong lithofacies variations in the light of the volcano's morphology at the time of the eruption. In particular, the EU5 PDCs survived to pass over the break in slope between the volcano sides and the surrounding volcaniclastic apron-alluvial plain, with development of new flows from the previously suspended load. Pulses were developed within individual currents, leading to stepwise deposition on both the volcano slopes and the surrounding volcaniclastic apron and alluvial plain. Physical parameters including velocity, density and concentration profile with height were calculated for a flow of the phreatomagmatic phase of the eruption by applying a sedimentological method, and the values of the dynamic pressure were derived. Some hazard considerations are summarised on the assumption that, although not very probable, similar PDCs could develop during future eruptions of Somma-Vesuvius.

Pyroclastic flow hazard assessment at Somma–Vesuvius based on the geological record

“…In the northern, southern, and eastern sectors, the deposits are generally thin (0.5-1 m) and mainly comprise massive, accretionary lapilli-bearing ash. The PDC deposits of the Pomici di Avellino eruption show strong lateral variations in lithofacies and thickness (Sulpizio et al 2009b). Stratified, dune-bedded, and massive, coarsegrained deposits up to 8 to 9-m thick predominantly cover the western and northwestern slopes.…”

The Pomici di Avellino eruption of Somma-Vesuvius (3.9 ka bp). Part I: stratigraphy, compositional variability and eruptive dynamics

“…Eruptive pauses are also clearly demonstrated by the recognition of human and animal footprints at different heights within the EU5 succession in the plain north of Somma-Vesuvius (Mastrolorenzo et al 2006;Di Vito et al 2009). Lithofacies analyses of EU5a-d deposits indicate that the deposits of the first and the last sub-units were emplaced by very dilute PDCs, less energetic than those emplaced during EU5b-c (Sulpizio et al 2010).…”

“…For point (3), it is important to recall that the EU5 PDCs were mainly generated by pyroclastic fountaining (Sulpizio et al 2010). These types of eruption dynamics usually generate thick and coarse-grained deposits close to the collapsing area, with PDCs that spread radially from the source.…”

“…These comprise deposits of a distinct opening phase that preceded the main Plinian phase, coarse-grained (breccia-like) fallout deposits emplaced very close to the vent and widely dispersed, thinly stratified PDC deposits emplaced during the final phreatomagmatic phase. The detailed sedimentological study of the phreatomagmatic PDCs and the assessment of their impact on the territory are the foci of a companion paper (Sulpizio et al 2010). …”

“…PdB = Pomici di Base deposits. Note the 2-m scale; c EU1-5 deposits at section A6 (described in the companion paper;Sulpizio et al 2010) encompassing massive to cross-bedded pyroclastic successions and form a thick wedge-shaped body with its maximum thickness corresponding with the outer rims of Piano delle Ginestre.…”