The Pomici di Base plinian eruption of Somma-Vesuvius

The Pomici di Base eruption represents the first of a series of plinian eruptions which occurred at Somma-Vesuvius in the period 20,000 yr B.P.–79 A.D. These eruptions led to substantial demolition of the Mt. Somma stratovolcano and the formation of the 4.9×3.4 km E–W-elongated summit caldera. New 14C datings and previous radiometric data constrain the age of the Pomici di Base eruption to between 18,000 and 19,000 yr B.P. Deposits of the Pomici di Base eruption comprise from base to top: (1) plinian fallout with minor surge deposits and (2) a succession of volcanic landslide and lithic-rich fallout, surge and flow deposits. Ballistic block distribution and thickness of tephra deposits indicate that the vent was located in a 50° wide western sector within a distance of 1–2.5 km from the present Vesuvius crater, in a fairly eccentric position with respect to the ancestral Somma cone. The plinian fallout likely blanketed an eastwards elliptical area of 2600 km2 within the 20-cm isopach. Reconstruction of isopachs yields an approximate volume calculation of 4.4 km3. Comparison of maximum thickness of the fallout deposit with other plinian deposits of Somma-Vesuvius suggests that the PB eruption was the largest explosive event of the volcano. The mass discharge rate deduced from clast dispersal models is estimated in the range of 2–2.5×107 kg/s, corresponding to a column height of 16–17 km. Part of the plinian phase was characterized by pulsatory behaviour with repeated partial column collapses (surge emplacement) and concurrent oscillation of the height of the plume (stratified fallout). The plinian phase was followed by a limited slope failure of the Somma cone and by several explosive episodes with a prominent phreatomagmatic nature. We proposed that this activity occurred in connection with a phase of substantial demolition of the Somma edifice due to caldera collapse. The plinian fallout is dominated by strong compositional zoning from white trachytic pumice (SiO2 63.0 wt.%) to black latitic scoriae (SiO2 53.7 wt.%), coupled with a marked decrease of vesicularity of juvenile clasts from 70–80% to 45–55%. The compositional variation reflects strong pre-eruptive zoning of the magma chamber probably associated with volatile zonation.