Reconstruction and analysis of a sub-Plinian fall deposit from the Astroni volcano (ca. 4100-3800 BP) in the Campi Flegrei area, Italy

Abstract

A simple semi-analytical model presented in Pfeiffer et al. (in press) (“Hazmap”-modified version) was applied to reconstruct the tephra deposit of a Plinian or sub-Plinian phase (called Astroni 6, after Isaia et al., 2004) of an eruption of the Astroni volcano (ca. 4100-3800 BP) within the Campi Flegrei volcanic area in Italy. In this model, the eruption column is assumed to act as a line source in order to neglect complex near/vent interactions. Therefore, the validity of the model is limited to the medium and far areas from the vent (beyond 10-20km), where the assumption of a line source can be justified. The distribution of the particles in the atmosphere is assumed to be only controlled by gravity, wind and eddy diffusion. The model accounts for two different types of particles (juvenile pumice and unspecified dense particles) within a used-defined range of granulometric classes. The numerically calculated deposit was confronted with the observed deposit. Applying a least/squares method it was tried to optimize input variables such as distribution of particles and mass within the eruption column, wind and diffusion parameters by fitting the computed deposit with the observed one. A good correlation between the numerically calculated and the measured deposit could be achieved, although the quality of the input data is poor because of the lack of a sufficient number of distal sample points. Therefore, best fitting input parameters could not be well constrained and the presented results must be seen as a fairly rough estimate on eruption conditions only. IN particular, the erupted mass and eruption column height predicted by the model are considerably smaller than those presented by other authors (Isaia et al., 2004). However, the discrepancy is large enough to raise the question about the precision of other estimates as well.