Abstract

Dynamics of explosive eruptions is often strongly controlled by temporal changes in conduit geometry. Quantitative constraints to this problem are difficult to define, but basic information on the lithic fraction in pyroclastic deposits can be used as an input of numerical models to infer conduit and crater evolution in terms of shape and dimension. Field data on the 79 CE Pompeii eruption (Vesuvius, Italy) are used here to constrain depth-dependent variations in conduit geometry. The different lithology of the accidental components, resulting from the erosion of a conduit/crater system crosscutting a well-known subsurface stratigraphy, helps in defining the provenance depth of the eroded fragments. We reproduced the eruption evolution by considering three periods of the Plinian phase, associated with the white phonolitic pumice clasts (EU2a) and the tephro-phonolitic gray pumice clasts (EU3a and EU3b). Results constrain the evolution of key eruptive parameters and are consistent with the estimates of mass discharge rate (MDR) and volume of eroded lithic fragments, which require the involvement of conduit geometries with depth-dependent diameters rather than a constant-radius shape. The onset of the Plinian phase (EU2a) was characterized by intense crater excavation processes. The MDR increase during the transition from EU2 to EU3 coincided with a significant increase of conduit diameter at bottom. After the peak of MDR (EU3b), a significant deeping of the fragmentation level and an abrupt inlet pressure drop probably occurred. Exit pressure and velocity would have decreased during all the Plinian phase, consistent with a shift to a collapsing column dynamics.