Chiominto, Giulia Plinian eruption and their impact in urban context: dynamics and effects on the territory of the fall phases of the 79 A.D. vesuvian eruption. [Tesi di dottorato]

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Abstract

Plinian eruptions are particularly dangerous eruptions that often have long-lasting consequences. They produce high eruptive columns that inject considerable volumes of pyroclastic fragments and volcanic gases into the stratosphere. Pyroclastic clasts fall back to the ground forming widespread tephra sheets in which different components are distinguished: juvenile clasts, lithic clasts and crystals. This ‘classical’ behaviour is well represented by the main sustained phase of the AD 79 Vesuvius eruption, which deposited a thick white to gray pumice lapilli fallout deposit. This phase was followed by a post-Plinian, column collapse phase, which mainly emplaced several pyroclastic density currents (PDCs) with minor sustained column pulses that emplaced thin, lithic-rich layers. The current study focuses of the fall deposits recognized within the entire AD 79 stratigraphic sequence (i.e., belonging to both the Plinian and the post-Plinian phases of the eruption) with the aim to estimate their eruptive parameters (e.g., volumes, column height, mass discharge rate, tephra mass etc.), reconstruct a model of withdrawal from the vertically-stratified AD 79 magma chamber, and define the impact of the fall products on some roman towns (Pompeii, Stabiae). Several stratifications were recognised within the Plinian fallout deposit. Stratification is due to granulometric and lithological contrast between successive layers. Based on grain size, component and chemical analyses results eight units have been recognised within white pumice lapilli deposit and six within grey pumice lapilli deposit. All AD 79 fall layers display Plinian dispersal. Variation of grain size with stratigraphic height suggests that the convective plume was far from steady state. During the first white pumice phase, the plume height reached its maximum plume height of 23 km. During the grey pumice phase, the eruption is characterised by the alternation of sustained column phases and partial column collapse phases, with the emplacement of several PDC ash deposits. The maximum height of the plume is 34 km. Mass discharge rates, varies from 6 x 107 to 4.3 x 108 kg/s. The total volume of the pumice lapilli deposit accumulated during the Plinian fall phase (white + grey) is 6.2 km3 (1.65 km3 DRE). Five lithic-rich lapilli fallout layers interstratified with the pyroclastic density current deposits were emplaced after the total collapse of the Plinian column. During this “late” post-Plinian phase, the column resumes heights from 17 to 19 km and mass discharge rates from 1.8 to 4.2 x 107 kg/s. These data highlight the oscillating behaviour of the sustained eruptive column of the AD 79 eruption. The duration of the Plinian phase of this eruption is estimated to have been six hours and thirty-six minutes for the white pumice fall deposit, eleven hours for grey pumice fall deposit and several ten minutes for the first three post-Plinian sustained column pulses. The results of the petrochemical analyses provide new insights into the withdrawal of the AD 79 magma chamber. Finally, I present the new timing of the destruction at Pompeii during the Plinian phase of the eruption. During the first phase of the eruption part of the population tried to take shelter inside houses, but after 3 hours from the beginning of the eruption, the roofs begin to collapse under the load of the thick fallout deposit. In addition, several roofing-tiles were recovered up to the highest units of the Plinian fallout succession. Consequently, roof have continued to collapse. The lack of archaeological evidence led us to suppose that the post-Plinian fallout phases were not dangerous or destructive for the inhabitants.

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