Re-establishing the beast : an investigation into the spatiotemporal evolution of the Y5 phase of the Taupō 232 ± 10 CE eruption, New Zealand : a thesis submitted in partial fulfilment of a Philosophiae Doctor degree in Earth Science, Volcanic Risk Solutions, School of Agriculture and Environment, Massey University, New Zealand
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2023
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Massey University
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Abstract
Plinian eruptions are sustained, high-energy explosive eruptions that generate buoyant plumes that reach >20 km into the atmosphere. They often produce devastating pyroclastic density currents (PDC) along with widespread tephra fall out, with significant hazards to communities around the volcanoes. Current computational modelling of Plinian eruptions considers generalized steady versus unsteady column regimes as the explanation for the formation of coeval buoyant Plinian plumes and intraplinian PDCs; however, natural eruption scenarios indicate that these regimes can oversimplify the interpretation of both PDC and plinian fall deposits.
The large-Plinian Y5 phase of the Taupō 232 ± 10 CE eruption has an exceptionally widespread and well-preserved deposit that incorporates fall and coeval PDCs. Despite an extensive dataset in place for the Y5, there remain conflicting views on the interpretation of its deposit regarding eruption and sedimentation dynamics. Original studies by Walker (1980) considered the Y5 as a single eruptive unit from the perspective of the widespread fall deposit, without consideration of the intraplinian, coeval Early Flow Units (EFU) identified by Wilson & Walker (1985). Walker’s study determined that the Y5 phase involved a Plinian plume ~50 km high. Bedding characteristics in the fall deposit were considered in detail by Houghton et al (2014), who used their qualitative observations to propose the presence of 26 subunits within the Y5 fall deposited by a fluctuating plume influenced by strong changes in wind direction. Houghton et al.’s study brought the plume height down to (35 – 40 km) and denoted a vent location ~6 km SW of that proposed by Walker (1980). This Ph.D. research presents a comprehensive quantitative dataset of the deposit characteristics in the vertical stratigraphy of the upper phreatoplinian Y4 deposit, and the coeval fall and PDC deposits of the Plinian Y5 phase of the Taupō eruption. The dataset is used to reconstruct the spatiotemporal evolution of the Y5 phase and improve our understanding of Plinian eruption dynamics and sedimentation.
Detailed sample collection and analysis was conducted on proximal to medial deposit exposures, whose vertical stratigraphy encompass the final stage of the Y4 (Y4-G), the Y5 fall deposit and its coeval Early Flow Units (EFU). Samples were analysed for grain size distributions, componentry, and juvenile textural characteristics. It is demonstrated that foreign lithic lithologies and their time-relative abundance in relation to other deposit characteristics play an important role in informing vent location, the evolution of the conduit and the nature of generation of erupted facies (i.e., PDC and fall). In this study, foreign lithics were subdivided by their inferred stratigraphic depth of origin below the lake floor into: F1) pre-232 ± 10 CE volcanic material (~0 – 400 m), F2) predominantly Huka Group sediments, minor Whakamaru ignimbrite and hydrothermally altered material (~400 – 3000 m), and F3) plutonic microdiorites and granitoids (>4000 m) At the boundary between the Y4-G and Y5 deposits, a decrease in obsidian abundance of c. 30 wt.%, along with an increase in F2 lithics of c. 20 wt.%, and a drop in pumice vesicularity by c. 30 % indicate a distinct change in vent location between the Y4 and Y5 phases. F2 lithologies in the Y4 differ significantly from those in the Y5, but coincide with those of the plinian Y2 deposit, suggesting similar regions of crustal excavation for Y5 and Y2 and imply a vent location comparable to that of the Y2 phase.
Vertical variations in the abundance and relative proportions of different juvenile and lithic pyroclasts, in pyroclast textures and pumice densities identified in the Y5 fall deposit, following the initial clearing of the vent, define three successive stages within a relatively steady, continuous eruption. These stages are: 1) the continuous excavation of the conduit at relatively low mass eruption rate shown through higher lithic:pumice ratios, finer overall grain size and higher pumice densities compared to later stages of the Y5; 2) increasing mass eruption rate towards a climax with relatively steady conduit erosion coinciding with deepening fragmentation, exhibited in increasingly larger grain sizes and relatively lower total lithic abundances, yet higher relative proportions of F2 and F3 lithics; and 3) a moderate decrease in mass eruption rate and the acceleration of conduit erosion (shown through a rapid increase in F1 abundance and decreasing grain size), promoting the potential early onset of caldera collapse that led to the Y6 ignimbrite producing blast event. Vertical bedding features in the Y5 fall deposit are shown to be laterally discontinuous and pinch out over length scales of 101-103 m. This precludes the possibility that coarse-fine fluctuations were caused by mass partitioning of material during partial column collapse, or by variations in wind direction. Instead, I suggest that the bedform features identified in the Y5 deposit result from gravitational instabilities in the umbrella cloud, sedimenting as tephra swathes.
Additionally, the intraplinian EFUs were differentiated by their characteristics into two main types: Type 1 centimetre to metre thick, massive, pink-orange to cream coloured, coarser grained deposits that are topographically confined; and Type 2 decimetre to centimetre thick, massive to moderately stratified, white-grey, finer grained deposits that have mounted topography. The anomalously high proportion of ash (<10 µm at 4 – 27 wt.%) in the EFU deposits, in conjunction with a lack of evidence for enrichment of dense clasts (i.e., lithics and crystals), indicates that there was minimal to no mass partitioning that would be expected in the case of partial column collapse. In addition, the inferred high particle concentration of the Type 1 flows and their high temperature emplacement indicates that the materials that propagated to form the EFU PDCs is likely to have originated from lower heights around the jet where entrained air had limited effect to cool the mixture. A lack of variation in the proportion of lithic types and juveniles between Type 1 and Type 2 with relative height compared to the Y5 fall suggests that the EFU are a product of one generation mechanism and that the deposit types 1 and 2 represent contrasts in relative volume, runout distance, and/or topographical constraints on runout of individual flows. The EFU are entirely contained within fall activity and become more abundant, voluminous and/or increase in flow mobility with increasing mass eruption rate during the Y5 phase. The generation mechanism for the EFU PDCs strongly aligns with the modelling and field observations for gargle dynamics, where a dense sheath formed by recycled pre-existing material in a basin-like vent structure develops on an eruptive jet. This dense sheath produces PDCs simultaneous with a sustained Plinian column that occurs seemingly without interruption. Similarities can be drawn with deposits from other, historical large-Plinian eruptions such as the Bishop Tuff, 0.76 Ma and Novarupta, 1912, which also involved phases of coeval fall and PDC deposition analogous to the Y5 and EFUs, and were likely produced through gargle dynamics.
This study has shown that through the detailed, quantitative characterisation of deposit features in plinian eruption deposits involving coeval fall and PDCs, the temporal changes in eruptive behaviour, conditions at source and the nature of sedimentation can be identified. Interpretations indicate that the Y5 phase of the Taupō 232 ± 10 CE eruption was a large, steady, and extremely powerful eruption beyond the general depiction of a ‘standard’ Plinian event. Using quantitative analysis such as this may help build upon our knowledge base of the eruption and sedimentation dynamics of large Plinian eruptions by providing a field-based foundation for the reconstruction of the spatiotemporal evolution of such events. This is intended to provide a pathway for the amalgamation of field data and computational eruption models, ultimately improving our ability to forecast and mitigate explosive eruption hazards at similar volcanoes globally.
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Volcanic eruptions, Volcanology, New Zealand, Taupo Volcanic Zone, Volcanic hazard analysis