Vertical and lateral variations of grainsize and pyroclast componentry in the Taupo 232 CE Y2 fall deposit : implications for spatiotemporal deposition and conduit conditions in large Plinian eruptions : a thesis presented in partial fulfilment of the requirements for the degree of Magister Scientiarum in Earth Science at Massey University, Manawatū, New Zealand

Abstract

The Taupo Volcanic Zone is one of the most frequent producers of Plinian eruptions globally, which constitute one of the most powerful natural disasters on Earth. These types of eruptions can eject volumes of tephra >10 km³ into the atmosphere. The dispersal and sedimentation of pyroclasts in Plinian eruptions, as well as the high risk of pyroclastic density current production, pose significant environmental and societal threats. Therefore, further understanding of the dynamics of these volcanic events is essential for hazard planning and decision making at many volcanoes across the globe. The Hatepe pumice of the Taupo 232 CE eruption (Y2) is an extremely well-preserved example of a seemingly monotonous plinian fall deposit, and previous studies exhibit a lack of investigation into vertical and lateral deposit variations which would allow the reconstruction of eruption dynamics and deposition history. This study examined three tephra profiles using a combination of grain-size, componentry and clast density measurements, as well as microscopic analysis of juvenile pyroclast textures and petrographic analysis of foreign lithic components to define syn-eruption chronostratigraphic time markers. The juvenile components were grouped into five distinctive textural classes (C1 – C5) which showed a systematic increase in clast density and shear deformed bubbles and glass, respectively. From this, a tentative conduit model for the time immediately prior to magma fragmentation was developed to explain the strong horizontal gradients in magma density and estimate radial extents of the variably vesiculated and shear-deformed proportions of the magma. Identification and distinction of foreign lithic lithologies and broad estimates of their likely depth of origin showed that deep excavation of plutonic country rock above the magma mush reservoir occurred strongly during onset of the eruption, but rapidly declined thereafter. The chronostratigraphic time markers, determined through componentry analysis, can be used in future computational studies as a real-world test for the validation of plume and ash-dispersal models. This research will allow for the possibility of further investigations to add more complexity to the understanding of the Y2 eruption and large Plinian eruptions elsewhere.