Eruption and emplacement of the 1.3-Ma Ongatiti Ignimbrite, New Zealand: Regional pathways, particle processes, and pumice evolution associated with a large-volume pyroclastic flow deposit
Yousef Zadeh, E. (2020). Eruption and emplacement of the 1.3-Ma Ongatiti Ignimbrite, New Zealand: Regional pathways, particle processes, and pumice evolution associated with a large-volume pyroclastic flow deposit (Thesis, Doctor of Philosophy (PhD)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/14063
Permanent Research Commons link: https://hdl.handle.net/10289/14063
Pyroclastic flows are the most devastating phenomena of explosive volcanic eruptions. These hazardous density currents are able to travel several tens of kilometers radially away from their source as fast, hot density currents. Due to the enveloping ash cloud, it is still impossible to directly study pyroclastic flows, however, their deposits (i.e. ignimbrites) provide useful insight into their internal processes. The Ongatiti Ignimbrite was sourced from Mangakino caldera (1600-950 ka), the oldest volcanic center in the Taupo Volcanic Zone, North Island, New Zealand. The crystal- and pumice-rich ignimbrite offers a unique opportunity to understand an ancient and large-volume pyroclastic flow as well as more details about particle processes and pumice evolution. The minimum deposit volume for the Ongatiti Ignimbrite has been revised to approximately 720 km³, or 512 km³ dense rock equivalent. Previous studies proposed ages from 1.21 to 1.28 Ma by dating methods, such as ⁴⁰Ar/³⁹Ar, K/Ar, and zircon fission-track. This research used the (U-Th)/He method to determine an age of 1.3 Ma. This study has combined field facies analysis and GIS methods to characterize the Ongatiti Ignimbrite. The topographic controls on the spatial distribution of the ignimbrite has been determined to understand pyroclastic flow pathways through valleys and over hills. The Ongatiti Ignimbrite was a landscape-modifying event that covered at least the western North Island and as far away as Auckland and Wellington. It is a welded to non-welded, columnar-jointed, cliff-forming deposit, that has been divided into nine facies based on the variation in pumice and lithic clast abundance, and welding rank. This study also focuses on the texture and components of the ignimbrite matrix to better understand the magma fragmentation process, the transportational and depositional dynamics of the gas-ash fluid, and the post-emplacement processes. The microtextures of ignimbrite matrix were described and quantified by using optical microscopy, scanning electron microscopy, synchrotron x-ray microtomography, electron probe microanalysis, and X-ray powder diffraction (XRD). The matrix is poorly sorted and varies in the size and texture of ash-sized pumice fragments, glass shards, crystals, and lithic clasts. Glass shards range in shape between dominantly platy, Y-shaped, and complex shapes, and in size from a few microns to about 900 microns. The matrix is crystal-rich and comprises primary subhedral to anhedral volcanic crystals (plagioclase, quartz, pyroxene, hornblende, and Ti-Fe oxides and range between 100 to a few 1000 microns in size. The degree of welding in the matrix is defined by crystal-crystal contacts and deformation of glass shards around particles, and the highest degree of welding is shown by moderately deformed shards. The physical characteristics and chemical composition of pumice clasts were determined, and the main pumice textures were recognised as vesicular, fibrous, microvesicular and dense type. Tomographic imaging shows different vesicularities and local crystal arrangement within pumice and ignimbrite. Glass chemical analysis shows two different groups, higher-silica (~80-81 wt%), and high-silica (76-78 wt%). The Ongatiti Ignimbrite was emplaced by a super-eruption with a volcanic explosivity index (VEI) of 7, and eruptions of similar size could catastrophically bury a vast area of the North Island. The matrix of the Ongatiti Ignimbrite suggests complex particle interactions within the ash-gas fluid of pyroclastic flows. Microtomographic imaging of ignimbrite components offers new insights into understanding these complex processes.
The University of Waikato
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