Understanding the initiation and development of explosive Plinian eruptions: the conduit processes, eruption dynamics and dispersal processes of the ignimbrite and caldera-forming Lower Pumice 1 (184 ka) and Lower Pumice 2 (172 ka) eruptions, Santorini Caldera, Greece

In this thesis, the internal stratigraphy, facies, and facies architecture of the rhyodacitic 184 ka Lower Pumice 1 (LP1) and 172 ka Lower Pumice 2 (LP2) eruption sequences, erupted from Santorini Caldera, Greece, are reassessed as the basis for understanding the initiation and development of caldera-forming Plinian eruptions, including the changing conditions of the magmatic system, the conduit and eruption processes, the transition between eruption phases and ultimately the evolution of the vent system. Key findings from the study of these eruption sequences include the identification of complex precursory eruption phases, evidence for diverse histories of degassing and magma ascent, and for rapid transitions between eruption phases (e.g., precursor to Plinian, plume to flow and catastrophic caldera collapse). 184 ka Lower Pumice 1 eruption The stratigraphy of the LP1 eruption defines two main eruption phases, including an initial precursor phase and a major Plinian phase. The precursor deposit sequence of the LP1 eruption (LP1-Pc) consists of 13 internal stratigraphic subdivisions that incorporate four major lithofacies associations. These include a series of pumice lapillistone deposits, interpreted as fallout of pumice from a buoyant eruption column (LP1-Pc-a1, 2, 3, 4), a finely-laminated tuff, interpreted as a pyroclastic surge deposit (LP1-Pc-b), a sequence of interbedded tuffs and pumice lapillistone horizons, interpreted as phreatomagmatic ash fallout/surge and pumice fallout deposits (LP1-Pc-c1, 2, 3, 4), respectively, and a series of incipiently bioturbated, matrix-supported, lapilli-tuffs, interpreted as phreatomagmatic ignimbrites (LP1-Pc-d1, 2, 3, 4). Incipient bioturbation, variability in unit thickness and lithology in LP1-Pc, attest to multiple time breaks, sufficient for insects and other burrowing organisms to colonise and plant roots to grow. The vesicularity characteristics (collapsed vesicle textures) of dacitic pumice pyroclasts from LP1-Pc-d support these conclusions, and suggests stalling or slow magma rise rates prior to magma evacuation. An interpreted repose period of up to several months preceded the Plinian phase, which commenced with the development of a buoyant convective eruption column (LP1-A1, 2, 3). Vent widening and the increased incorporation of lithics into the eruption column resulted in eruption column collapse and the development of valley-confined pyroclastic flows (LP1-B1, 2, 3-1, 3-2, 4). Lithic-rich lag breccias, which cap the sequence, are indicative of a late-stage catastrophic caldera collapse event (LP1-C1, 2, 3, 4, 5, 6). The vesicularity characteristics of pumice and occurrence of basement-derived lithic assemblages in LP1-C1 (when compared to LP1-B4), indicate a rapid increase in decompression (doubling of decompression rate from 7.4-10.5 MPasˉ¹ during LP1-B to 15.3-28.4 MPasˉ¹ at the onset of LP1-C) and a deepening of the fragmentation surface at the onset of caldera collapse. This data supports previously unsubstantiated numerical models which predict rapid increases in decompression and mass discharge rates at the onset of caldera collapse. We suggest that lithic-lag breccias are indicative of vertical or steeply inwards dipping faults that initially hinder roof block collapse. However, once fractures propagate to the Earth’s surface, sudden decompression of magma system, and deepening of the fragmentation surface because of this, triggers explosive widening of the fractures as vents, so allowing roof block collapse. 172 ka Lower Pumice 2 eruption The stratigraphy of LP2 eruption records two major eruption phases, including an initial precursor event followed by a major Plinian phase. The precursor deposit (LP2-A1) is characterised by a clast-supported, diffusely-stratified framework of rhyodacitic white pumice, reflecting the development of a short-lived volcanic plume. These deposits pass conformably upwards into a massive, moderately to well-sorted, clast-supported, fallout deposit (LP2-A2, 3), reflecting the development of a 36 km high Plinian eruption column. The vesicularity characteristics of rhyodacitic pumice, in addition to the progressive increase in volcanic and basement lithic components vertically within the stratigraphy of LP2-A2, A3, attest to a progressive increase in vent diameter, a deepening of the fragmentation surface and an increase in decompression rate from 13.7 to 18.0 MPasˉ¹ throughout the Plinian phase. Continued vent widening and the inability to effectively transfer air into the eruption column resulted in column collapse and the development of pyroclastic flows. The transition in the stratigraphy from diffusely-stratified to massive pyroclastic flow deposits reflects the progressive increase in depositional and mass discharge rate throughout the eruption phase (LP2-B). The incorporation of water into the magmatic system, possibly associated with incipient caldera collapse or a major vent excavation event, facilitated phreatomagmatism and the development of widespread pyroclastic flows (LP2-C). Late-stage caldera collapse resulted in the explosive breakup of the reservoir roof block, a deepening of the fragmentation surface and an enlargement of the pre-existing LP1 caldera, producing lithic-rich pyroclasts flows and depositing lithic lag breccias (LP2-D).