Santangelo, Ileana (2023) Plinian eruptions and their impact in urban contexts: dynamics and effects on the territory of the pyroclastic currents of the AD 79 Vesuvian eruption. [Tesi di dottorato]

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Abstract

Plinian eruptions are amongst the most powerful and destructive volcanic events on Earth. These explosive eruptions generally form a sustained eruptive column which, as a result of abrupt changes in eruptive conditions can collapse producing one of the most hazardous volcanic events: pyroclastic density currents (PDCs), able to reach great distances far from the vent and entirely destroy populated areas. In this scenario, the well-documented AD 79 eruption was selected as representative case study for the current PhD research. It killed thousands of people, devastating the surrounding countryside and destroying many towns, most notably Pompeii and Herculaneum, as well as country villas, suburbs and dwellings over a radius of more than 10-18 km from the volcano. The AD 79 eruption can be said to have given birth to the science of volcanology and to have significantly advanced the science of archaeology. It was characterized by two main phases which can be summarized as follows: a Plinian sustained eruptive column emplacing a southward-distributed widespread tephra fallout concurrent with successive partial column collapses and a second phase dominated by PDCs representing the focus of the present research. Despite the large number of studies concerning the AD 79 eruption, multiple interpretations were reported in literature for the PDC deposits from this event. Therefore, with the aim of resolving the most debated topics and tentatively provide an unified view regarding the AD 79 PDC deposits and the parental PDCs emplacing them, with the current research a detailed field work was perfomed on numerous and well-exposed outcrops around Vesuvius, in the Campanian Plain and on the Lattari Mts, including several archaeological sites. 27 individual PDC stratigraphic units from the opening to the intra- and post-Plinian phases of the eruption have been documented for the first time with this research, emplaced by 16 PDCs. In order to document lateral and vertical variations in grain size, types and amount of components of the AD 79 PDC deposits, all recognized PDC stratigraphic units were carefully sampled (417 samples) with the recognition of 14 different lithofacies. A new reconstruction of the behaviour of the AD 79 PDCs has been established through the merging of all the available data. The very initial phase of the eruption is marked by the formation of a small-volume, slow-moving, ground-hugging ash flow with a limited areal distribution. For the first time, nine partial Plinian column collapses are documented representing a purely magmatic phase of the eruption. Changes in the eruptive dynamics led to the formation of post-Plinian PDCs representing the most powerful and turbulent AD 79 PDCs, even capable to overcome the barrier ridge represented by the Lattari Mountains. In addition, juvenile components (consisting of pumice clasts, both white and grey) were sampled at various stratigraphic heights and 97 of them were selected for major oxides and trace element concentrations analyses adding new information on the geochemical composition of the AD 79 PDC deposits. Finally, the detailed and well constrained stratigraphy of the AD 79 PDC deposits has been a useful tool to define the exact timing of the destruction during the intra-Plinian and post-Plinian phases of the eruption with significative implications on the risk assessment related to high-energetic volcanic events.

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