New perspectives on the style and dynamics of the 79AD plinian eruption at Vesuvius

The 79AD eruption of Mt. Vesuvius is one of the most studied volcanic explosive events in history. It ejected ∼3 km3 of material (dense rock equivalent DRE) in the form of thick layers of pumice and ashfall interbedded with numerous large pyroclastic density current deposits, and is responsible for the destruction of Pompeii and Herculaneum. This eruption represents a type-example of plinian eruption, where stable activity abruptly shifted to column collapse and/or phreatomagmatism.;Vesicle and crystal textures have been shown to aid greatly in deciphering some of the key physical processes that lead to their formation (e.g. storage conditions, ascent styles and rates). A new matlab-based tool (FOAMS) was developed and used to investigate the products of the 79AD eruption, which contain abundant small leucite crystals and vesicles that cover several orders of magnitude in size and number. First, laboratory experiments designed to mimic leucite crystallization and vesiculation in pumice from the opening and one of the main plinian phases (Eruption Units EU1 and EU2 of the phonolitic white magma) were performed at conditions thought relevant to the eruption. Experiments were quenched at different final pressures to investigate the evolution of textures throughout the entire decompression path. It was found that leucites do not crystallize readily during ascent, and that experimental vesicle textures do not match size distributions but do approach number densities measured in natural pumices from EU1 and EU2. Through additional isobaric-isothermal experiments, it was determined that leucites probably crystallized at depth in the reservoir, perhaps during slow decompression prior to the eruption. In particular, the causes for discrepancies between experimental and natural textures are explored and possible ways to improve future experiments are suggested. Kinetics of crystallization and vesiculation in K-phonolites are also discussed.;The last portion of this dissertation is dedicated to finding the causes for major shifts between stable and collapsing eruptive plumes that caused the formation of multiple pyroclastic density currents (PDCs) during the magmatic phase of the eruption (EU1-EU3). PDC and pumice fall clast textures are compared to determine whether differences in conduit degassing behavior can explain the changes in column dynamics after fragmentation.