Conduit, eruption and plume dynamics throughout the 28-29 March 1875 eruption of Askja Volcano, Iceland

Explosive eruptions exhibiting rapid and reversible shifts in eruption style and intensity within a single eruptive event are rare and, hence, very dangerous. The nature and reasons for such shifts are poorly understood, yet are critical to our ability to mitigate future volcanic hazards from these volcanic centers.;The 1875 rhyolitic eruption of Askja Volcano was characterized by intervals of sustained activity, yet also with abrupt shifts in eruption style. The combination of historical records, relative youth of the products, and preservation of both proximal and far distal deposits has presented an unparalleled opportunity to understand such shifts, in addition to further understanding of transport and deposition in wet and dry eruption plumes.;This dissertation project characterizes the products of each eruption phase and their dispersal, which has been used to calculate eruption parameters, such as volume, intensity and vent positions. The amalgamation of these data with quantitative studies of vesicles present in representative pumice clasts has resulted in a comprehensive understanding and evaluation of the relative roles of external factors and inherent characteristics of the magma which produced the 1875 eruptive activity.;All clasts from phases of the main eruption are microvesicular, and quantitative vesicle data suggest that similar deep processes of nucleation and growth occurred throughout the eruption. Subtle differences in vesicle textures suggest that the shallow conduit ascent history was slightly different between and within phases.;The shift between wet and dry phases was due to external factors, principally migration of the vent into, and out of, water sources. The transition between a wet buoyant sustained phreatoplinian plume to collapsing column conditions was also due to external factors, vent widening and availability of water, during maintained high mass discharge rates.;The magma erupted during the phreatomagmatic phases of this eruption was a foam prior to both fragmentation and the interaction of external water. Future studies are required to understand the exact role of external water in driving the phreatoplinian eruption.;Fountaining from multiple separate vents was synchronous with sustained Plinian discharge during the climax of the 1875 eruption, and produced welded fallout deposits in two separate locations. The patterns of regional welding suggest that the accumulation rate and emplacement temperatures were critical controls. More localized welding, associated with meter-sized spatter bombs, appears unique to Askja, and I have termed it 'Local welding'. The textures and distributions of these spatter bombs have important implications for magmatic source, conduit and eruption dynamics.;Proximal exposures up to 1 km from inferred vents have defined, for the first time, a proximal Seg-1 segment on semilog plots of thickness vs. area ½ for each of the fall units, regardless of eruption intensity or style. This is attributed to ephemeral, premature and enhanced sedimentation from the jet and lower convective column margins.