The products of the opening phase of large explosive eruptions: towards the understanding of the modalities and rates of eruption trigger and magma ascent

The deposits of distinct opening phases related to weak transient activity marking the onset of the eruption have been recognized frequently at the base of Plinian and Sub-Plinian sequences from volcanoes all over the world (e.g.: Minoan Eruption, Santorini; 1815 Tambora Eruption; AD 79 Vesuvius Eruption). In general, these deposits appear as thin ash-bearing beds characterized by largely variable juvenile material and high lithic content. The volcanic products of these phases well record pre- and syn-eruptive processes in the magma reservoir, the processes of eruption trigger, the dynamics of conduit formation as well as they preserve information on the onset of magma ascent. Hence, geological evidence from these basal deposits, together with the compositional and textural features of the juvenile fractions, can also inform about short-term precursory phenomena as seismicity, ground deformation and gas emission, thus assuming a key role for volcanic hazard assessment as well as for monitoring purposes at high-risk volcanoes like Somma -Vesuvius volcano (Italy). In this thesis, a detailed study of the deposits and the products of the opening phases of the Bronze Age Avellino Pumice eruption and of the AD79 Pompeii eruption from Somma-Vesuvius is presented with the aim to define the immediately pre-eruptive state of the magmatic system and the modalities of eruption trigger and onset. By examining the morphology, texture, and composition of the initial eruption products, it has been possible to identify two distinct magma dynamics that are unique to each specific eruption, although certain overall characteristics are recognized as common features in both eruption. These thin basal layers are in fact both associated to small erupted volumes and to a large heterogeneity of the juvenile material, which strongly differ from the products of the following paroxysmal phases, thus suggesting different evolutionary paths. In the case of the Avellino eruption, some evidence confirmed the presence of a magma body emplaced at shallow depth only years before the eruption, that partially cooled and crystallised at different conditions with respect to the main magma reservoir. The timing of eruption dynamics has been revealed through the application of elemental diffusion chronometry on clinopyroxene from the dense juvenile material, suggesting that the shallow magma emplacement occurred over a maximum of about 50 years, while the last mafic input to the magma body has been estimated in a short period of time of only few years. Conversely, the initial phase of the Pompeii eruption was marked by the early destabilization of a diffusive interface placed at the boundary between white and grey magma in the main reservoir, possibly triggered by the arrival of deep mafic magma batch(es). This mechanism induced mixing processes and the contemporaneous withdrawal of magma from the uppermost and the lower 2 portions of the magmatic reservoir, resulting in a large compositional and textural variability of products emitted during the AD 79 opening phase.