The Neapolitan Yellow Tuff, a large-magnitude trachytic phreatoplinian eruption: eruptive dynamics, magma withdrawal and caldera collapse

The results of a detailed stratigraphical and volcanological study has allowed the definition of the Neapolitan Yellow Tuff (NYT) as the largest known trachytic phreatoplinian eruption with an estimated volume of not less than 30 km3 DRE. The NYT sequence can be subdivided in a lower and an upper member. Sedimentological and textural variations in time and space indicate that different eruptive and depositional mechanisms operated during the eruption. The lower member eruptive phase was characterized by alternating phreatoplinian and magmatic explosions while the upper member eruptive phase was characterized by a random sequence of phreatomagmatic and magmatic explosions. Particles produced by the same explosion were deposited contemporaneously by fallout and surge mechanisms either at different or at the same distance from the vent. Compositional variations show that the NYT sequence cannot be modeled as an inverted compositionally zoned magma body with magma becoming more basic in the course of the eruption. The chamber, composed of three geochemically distinct magma layers, was tapped at different depths according to the eruption dynamics. Each phreatoplinian explosion tapped concurrently all the magma layers while magmatic explosions tapped only one of them. The onset of the NYT caldera collapse is well constrained between lower and upper member eruptive phases. The collapse, which is only gravimetrically detectable, occurred inside the larger Phlegrean Fields caldera, which therefore can be defined as a nested caldera.