Browsing by Author "Procter, Jonathan"

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    Deciphering magmatic processes in response to growth and destruction at Taranaki Volcano, New Zealand : a dissertation presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Sciences at Massey University (Manawatū campus), Palmerston North, New Zealand
    (Massey University, 2021) Zemeny, Aliz
    Taranaki Volcano is an atypical back-arc andesitic stratovolcano located on the Taranaki Peninsula of the North Island in New Zealand. Volcanism started c. 200 kyr ago and the edifice went through at least 14 sector collapse events with the same number of corresponding growth cycles, expanding the surrounding volcanic apron over its lifetime, which presently is populated and farmed. Previous studies focussed on the modern edifice (<14 ka), the tephra deposits (<29 ka), and the volcaniclastic stratigraphy over the 200 kyr volcanic history. However, there is a significant knowledge gap in relation to the evolution of the Taranaki volcanic system during successive edifice growth cycles (i.e. inter-collapse states) and the response of the magmatic system to unloading of the edifice. In order to unravel the subaerial and subvolcanic aspects of these growth cycles, the sedimentary textures, lithologies and stratigraphy of the volcaniclastic mass-flow deposits were investigated in the southwestern sector of the Taranaki ring plain, which provides a nearly continuous stratigraphic record of the time period of c. 65-34 ka that comprises three edifice regrowth phases. Volcaniclastic mass-flow deposits were the focus of this study, providing an opportunity to explore sedimentological and geochemical characteristics of eruptive periods of Taranaki Volcano, as there are no proximal sites available close to the modern edifice. Due to the well-exposed volcaniclastic successions along the coastline of Taranaki Volcano, a classification framework was established for the globally applicable categorization of volcaniclastic mass-flow deposits in ring plain settings. Additionally, the development of the stratigraphic model of the time period c. 65-34 ka highlighted the high frequency of widely distributed volcanic mass-flow events, approximately occurring 4-5 times in every 4-10 kyr. As these deposits encompass the characteristics of eruptive periods, vesicular pyroclasts were analysed in order to investigate the time related aspects of the Taranaki magmatic system during edifice growth cycles. Based on the analysis of 220 lapilli-sized pyroclasts, whole-rock compositions were reproduced by a mixing model, indicating that the volcanic rocks originate from melt-mush mixing processes. The mixing ratios varied within the individual growth cycles and further revealed that the melt-mush ratios define the produced whole-rock compositions, where the assimilant endmember is a primitive mush and the melt endmember is a trachyandesitic ascending melt. The temporal variation of the pyroclast geochemistry showed that within the inter-collapse states (i.e. growth cycles), the range of bulk rock compositions display a broadening pattern over time towards pre-collapse states. This chemostratigraphic pattern was attributed to edifice loading affecting crustal magmatic processes over time. Whole-rock geochemical results demanded a detailed investigation of the crystal mush, from which Taranaki Volcano is fed, producing the basaltic to trachyandesitic magmas. The textural and chemical analyses of the Taranaki crystal cargo revealed reoccurring and specific crystal patterns and proved the antecrystic origin of the majority of the clinopyroxene, plagioclase and amphibole crystals. The observations highlighted that the Taranaki magmatic plumbing system involved repeated magma recharge, melt-mush mixing and crystal convection processes affecting the produced magmas within the time period of c. 65-34 ka. Crystallisation conditions (i.e. P-T-H₂O) were estimated applying thermobarometric modelling on clinopyroxene and amphibole phenocrysts. Results of the clinopyroxene rim equilibration modelling suggested source depths of approximately 26-12 km (±7.5), which outline the mid- to lower-crustal regions and further indicate polybaric rim crystallization processes. In addition, hygrometry approximations indicated that within the individual growth cycles, melts with various properties (2.9-3.7 to 3.9-4.8 wt% H₂O) arrived at different regions of the crystal mush at mid-crustal depths. Altogether, textural, chemical and hygrothermobarometric analyses outlined the spatiotemporal complexity of the Taranaki magmatic plumbing system and the connected magmatic processes of andesitic volcanism. The interconnected sedimentological and geochemical studies of this research provided an understanding of mid-crustal melt-mush mixing processes producing the Taranaki magmas within a complex, interconnected vertical mush domain affected by the temporal influence of edifice loading and unloading during consecutive edifice growth cycles.
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    Intra-caldera rhyolitic eruptions : lithostratigraphy and pyroclast textures to reconstruct the ~1314 CE Kaharoa eruption of Mt Tarawera, New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Science at Massey University, Palmerston North, New Zealand
    (Massey University, 2022) Todde, Andrea
    Rhyolitic eruptions are commonly sourced from silicic caldera systems and display a great variety of eruptive styles and magnitudes, ranging from the extrusion of lava domes to catastrophic caldera-forming eruptions. The different types of eruptions associated with rhyolitic volcanism can result in severe impacts to the environment and society, varying from a local to a global scale. Yet, due to their typically longer recurrence times compared to volcanic events of less evolved magmas, only a very limited number of rhyolitic eruptions have been documented in historical accounts or recorded through geophysical monitoring. This lack of direct records limits the current understanding of the main processes controlling the dynamics of rhyolitic volcanism and hinders the construction of robust eruption scenarios. This study presents new insights into the eruptive behaviour of rhyolitic eruptions using the Kaharoa eruption from the Taupō Volcanic Zone (TVZ) of New Zealand as a case study. The TVZ is one of the most frequently active regions of rhyolitic volcanism on Earth with the 1314±12 CE Kaharoa eruption being the most recent rhyolitic event in the TVZ. This eruption is sourced from multiple vents along the Tarawera dome complex, within the Okataina caldera system, and erupted up to 9 km3 of magma. By investigating the Kaharoa pyroclastic succession, this research contributes to constraining the key factors controlling the dynamics of moderate- to large-scale rhyolitic eruptions occurring in intra-caldera settings. The approach used in this research combines geological field investigations and quantification of the sedimentological and componentry characteristics of the deposits with the analyses of single-clast features (e.g., bulk density and textures of vesicles in pumice clasts). Field-stratigraphic relationships of 24 lithostratigraphic units for the Kaharoa deposit elucidate the intra-eruption chronology, placing time constraints on the numerous, discrete explosive episodes, while revaluating previous stratigraphic schemes. From variations within the stratigraphy in sedimentary structures, grain size and particle content of the pyroclastic beds, an array of deposit types is identified and linked to temporal changes in transport and depositional patterns as well as eruptive styles. Five distinct explosive eruptive phases are established for the Kaharoa eruption. These include multiple phases of repeated subplinian-type, fall-dominated episodes, alternating with phases characterised by overall sustained pyroclastic density currents and episodes of ash emission. A final sixth phase places the main lava dome building sequence within the proposed reconstruction and eruption model. Following constraints on the eruption from field-derived data, an in-depth investigation of the Kaharoa pumice microtextures is performed using Scanning Electron Microscopy, which revealed complex and anisotropic vesicle textures. To characterise the observed complex vesicle features of the Kaharoa pumices, a methodology is developed providing guidelines for the 2D quantification of tube-like vesicles. The integration and interpretation of the pumice textural results along the stratigraphic sequence indicates that the main processes that regulate the evolution of the magma during ascent in the shallow conduit region are magma shearing, bubble coalescence and outgassing. These factors provide bounding conditions for magma ascent dynamics and indicate cyclical variation in the eruption behaviour. Furthermore, by combining textural, sedimentological and componentry data, this study suggests that the inferred dike-shaped geometry of the conduit, together with conduit-vent wall instabilities, are primary factors in controlling: (i) the intrinsic responses of the magma to ascent and decompression to the surface and (ii) the characteristics of the ejected gas-pyroclasts mixtures, influencing the transportsedimentation regime. The dynamic magma-conduit interrelationships ultimately govern the changes in eruptive styles and overall dynamics of the Kaharoa eruption. This research defines a framework to relate depositional and textural characteristics to eruptive processes and provides critical insights into the types of eruptive sequence and eruptive style changes of intra-caldera rhyolitic eruptions. The scenario depicted for the Kaharoa eruption highlights the complex episodic, multi-phase, explosive to dome-forming nature of dike-fed rhyolitic eruptions. Furthermore, it provides crucial information for scenario-based volcanic hazard assessments of a Kaharoa-type eruption at Okataina and at other rhyolitic centres within the TVZ. Finally, comparing with available datasets from other volcanic events of similar magnitudes, magma composition, and geological settings, this research suggests that this type of rhyolitic eruption behaviour is common at other silicic caldera systems worldwide, making it of great relevance for future volcanic hazard studies.
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    Towards improving volcanic mass flow hazard assessment at New Zealand stratovolcanoes : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Science at Massey University, Palmerston North, New Zealand
    (Massey University, 2009) Procter, Jonathan
    The most common hazards for communities surrounding mountain‐forming stratovolcanoes are mass flows of a range of types. Determining their frequency, characteristics and distribution is a major focus of hazard mapping efforts. Recent improvements in computer power and numerical models have meant that simulation of mass flow scenarios is a new tool available for hazard analysis. Its application to hazard mapping, land use planning and emergency management awaits robust evaluation of the conditions under which simulation tools are effective. This study focuses on this question in attempting to improve mass‐flow hazard assessments at the typical stratovolcanoes of Mts. Taranaki and Ruapehu in New Zealand. On Mt. Ruapehu, Titan2D modelling was applied to forecast behaviour of non‐cohesive lahars in the Whangaehu River, primarily produced by Crater Lake break‐outs, such as on 18 March 2007. The simulations were accurate in predicting inundation area, bifurcation, super‐elevation, hydraulic ponding, velocity and travel times of the lahar to 9‐10 km. A 6 x 10[exponent 6] m³ simulated granular flow had a minimum discharge of 1800‐2100 m³/s at the apex of the Whangaehu Fan, 9‐10 km from source, comparable to all historic information. The modelling implied that it was highly unlikely for a flow of this nature to overtop a lahar training dyke (bund) at the fan‐apex location and avulse northward into a more vulnerable catchment. Beyond this point, the model could not cope with the rapid and complex changes in rheology of these non‐cohesive lahars. At Mt. Taranaki chronostratigraphic grouping of mapped past lahar deposits often clouds the actual series of landscape forming processes and hence variations in hazard that occurred over time. Here, patterns of mass flows following emplacement of a 7 km³ debris avalanche deposit were examined from field geology and Titan2D modelling to define a three‐stage recovery process, where lahars of different types and sources were focused initially beside and later on top of the debris avalanche deposit for up to 10 000 years. Results from Titan2D were used to identify source areas of mass flows at different stages and their probable rheologies. Debris avalanche emplacement at Mt. Taranaki was investigated on the c. 7 ka B.P. Opua Formation with the help of Titan2D simulations to identify initial collapse parameters and major flow paths. Once again, the simulations were reliable in proximal reaches, but could not reproduce the rheological transformations from an initial collapsing/sliding pile through to a cohesive clay‐rich flow with long runout. In a further example, past block‐and‐ash flows (BAFs) and dense pyroclastic flow deposits northwest of the current crater were analysed to define the range of realistic model parameters for Titan2D simulations. These could be incorporated inside aGeographic Information System to produce a gradational map of relative probabilities of inundation by future BAF events that took both modelling and geological variability into account. This study highlights that computational models are now reaching the stage where a holistic approach can be taken to hazard analysis that combines both geological mapping and simulation of mass flow scenarios in a probabilistic framework to provide better tools for decision makers and land‐use planners.