Single pyroclastic beds deposited by simultaneous fallout and surge processes: Roccamonfina volcano, Italy

Abstract

A sequence of fine-grained, hydrovolcanically generated pyroclastic surge deposits in the White Trachytic Tuff, Roccamonfina volcano, Italy, contains two layers of coarse pumice clasts. Pumice fragments in these layers show no evidence for abrasion during lateral transport, unlike pumice in the enclosing deposits, and are well sorted. In most locations they have textural and grain size characteristics typical of Plinian fall deposits. Bedding of these pumice layers ranges from planar with gradual thickness variation over a few meters distance, to highly variable with strong thickening in depressions. The layers are interpreted to record deposition of pumice falling from a Plinian eruption column through laterally moving pyroclastic surges. Depending on the local vigor of the surges during the fallout event, bedding characteristics range from those approaching pure fallout to those approaching pure surge. The simultaneous Plinian and hydrovolcanic, pyroclastic surge-producing activity may have come from two (or more) separate vents, or from a single vent that experienced rapid variations in magma-water interaction.