Dynamics of ash eruptions at Vesuvius

In the recent years, continuous ash emission activity, related to mid-low intensity, long lasting eruptions, has been increasingly described to occur at different volcanoes worldwide. Focusing on this type of deposits, a retrospective analysis of the stratigraphic successions at Vesuvius revealed that such type of eruptions have occurred repeatedly in the last 4000 years of activity. This type of activity has been overlooked in the past and the mechanism of ash production in these eruptions is not yet clear. The detailed study of the deposits suggests that these eruptions are dominated by discrete phases of repeated emission of a highly fragmented mixture, alternated with violent strombolian episodes. In this study we present morphological, textural and compositional data on the products of two ash eruptions representative of the whole variability of this activity at Vesuvius (Italy), occurred in the periods between the “Avellino” and “Pompeii” Pumice eruptions (AP3, 2,710±60 years B.P.) and after the 512 A.D. eruption, (AS1a). Juvenile fragments from different ash layers throughout the studied stratigraphic sections were fully characterized in terms of external morphology, particle outline parameterization, groundmass texture (in terms of Crystal and Vesicle Size Distributions) and glass composition. Volcanic ash holds information about magma dynamics within the volcanic conduit, where fragmentation occurs and eruption style is decided. Results of our investigations have been interpreted in terms of fragmentation processes, transport and dispositional mechanisms, dynamics of magma ascent ant timing of the eruption. The methodology of ash analysis used for this study has also important implications for tephrochronological studies, adding a complete parameterization of physical and textural properties of the ash to the routinely used compositional data.