Eruptive scenario of ash-dominated events at Vesuvius: the AP3 eruption (2,710±60 years B.P.)

The deposits of several eruptions dominated by ash emission have been recently described in the stratigraphic record of the last 4000 years of activity of Vesuvius. A peculiarity of this type of eruptions is the persistence of repeated phases dominated by abundant emission of ash and the sporadic occurrence of violent strombolian activity, they are recorded by very thick, massive to laminated, ash layers, interlayered with minor scoria-lapilli beds. These eruptions have a strong impact on the environment and on human and animal health, and their hazard implications assume an important role in the definition of the scenario for a short-term, mid-magnitude, expected event at Vesuvius, in particular when considering their long duration and the large dispersal of the products. These types of events are associated at Vesuvius both with processes of magma-water interaction, or with mechanisms dominated by pure magmatic fragmentation. We discuss here the results of a new morphological and textural study on the juvenile material of the 2700 years old, phreatomagmatic AP3 eruption (Andronico and Cioni 2002). The eruption was produced during discontinuous, explosive events of ash emission (pulsating activity) alternated with minor violent strombolian activity. Fragments of gas-rich, crystal-poor, magma are ubiquitous, and their presence suggests that the primary fragmentation process was mainly driven by magmatic explosivity. The interaction of external water with magma (phreatomagmatic activity), certainly occurred during the AP3 eruption, acting to increase (but not to trigger) the explosive fragmentation of the magma, due to water vaporization. Textural data on juvenile clasts can be interpreted in terms of episodes of conduit filling, degassing and ash production, and give a time constraint on the total duration of the eruption. All these data provide a useful reference frame to depict the eruption scenario of this type of activity to be used for the definition of contingency plans.