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    Understanding the volcanic response to edifice collapse : a case study of the Poto and Paetahi formations at Mt. Taranaki : 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
    (Massey University, 2024) Mills, Shannen Alice
    Stratovolcanoes are unstable and prone to collapse. Depressurisation from collapse events can possibly impact the subvolcanic plumbing system. This may cause a change in the eruptive size, style and frequency of eruptive activity following a collapse. Mt. Taranaki in New Zealand provides a unique opportunity to investigate the influence of edifice collapse on eruptive behaviour. The extensive ring plain around Mt. Taranaki is dominated by debris-avalanche deposits (DAD) recording the last > 200 kyr of eruptive history including at least fourteen events. The medial ring plain provides a stratigraphic record of the last 30 ka of eruptive history comprising four DADs including two of the largest to have occurred in Mt. Taranaki’s history the 27.3 ka (5.85 km³) Ngaere and 24.8 ka (> 7.5 Understanding the volcanic response to edifice collapse. A case study of the Poto and Paetahi Formations at Mt. Taranaki Understanding the volcanic response to edifice collapse. A case study of the Poto and Paetahi Formations at Mt. Taranaki km³ Pungarehu DAD. The (27.3-23.1 ka) Poto and Paetahi Formations deposited across the eastern and southeastern sector of the ring plain are used to investigate the effects of depressurisation on the magmatic system. A detailed stratigraphic analysis of medial to distal exposures of the Poto and Paetahi Formations was undertaken across the eastern and southeastern sectors of the Taranaki ring plain. Changes in lithosedimentological characteristics were used to identify single and multiphase eruptive events. Isopach and Isopleth mapping of the deposits show a period of increased explosive activity within Mt. Taranaki’s history. The deposits were analysed for grain size distributions, componentry, juvenile and density, as well as X-ray tomography to define vesicle and crystal number densities and volumes. Geochemical analysis on whole rock, glass, feldspar, and pyroxene crystals was conducted to create a detailed account of the changes within the Poto and Paetahi Formations and infer the response to edifice collapse. This study found that the relative abundance of lithics informed processes of conduit stability throughout the eruptive period, with increase abundance reflecting conduit excavation. Components were divided into juveniles, lithics and free crystals with subcategories established for each lithology class. Micro-Computed X-ray tomography indicated the high percentage of small bubbles present within the juvenile deposits. Twenty-eight subplinian eruptions produced at least ~3 km³ of tephra across the eastern and southeastern Taranaki ring plain within a ~4 kyr period, producing single and multiphase eruptive events with eruption column heights between 10-20 km and individual deposit volumes of 0.01-0.26 km³. Variations in the relative abundance of lithic clasts and density analysis of juvenile deposits reflect changing conduit conditions throughout the Poto and Paetahi Formations. Connected porosities and the abundance of juvenile clasts increased during stable conduit conditions due to the formation of gas flow pathways. A decrease in connected porosities and increase in the abundance of lithics indicated conduit excavation through unstable/ widening events, disrupting the formation of gas flow networks. Large populations of small bubbles (2.75 x 10-7 mm-3) are indicated through high vesicle number densities (VND) (9.03 x 1015 – 1.74 x 1016 cm-3), reflecting the domination of late-stage bubble nucleation within the upper conduit by fast ascending magmas occurring throughout the Poto and Paetahi Formations. Vesicle size populations reflect the onset location of bubble nucleation within the system. Changes in vesicle size distributions and oscillatory crystal rims throughout the sequence reflect cycles of magma recharge and storage occurring below Mt. Taranaki. Single stage nucleation events reflect the rapid ascent of magma through the system, while bubble coalescence indicates some magma stalled within the mid-to-upper crustal system. Whole rock compositions from these tephra vary between 3.03 – 5.19 wt.% MgO and reflect an evolution in magmatic composition overtime. Depressurisation from the eastern Ngaere collapse resulted in an increase in MgO wt.% (from 4.06 wt.% to 4.55 wt.%), decrease in VND (from 1.53 x 1016 to 9.76 x 1015 cm-3), but uniform vesicle volumes (VV) (from 4.9 to 4.8 %). This indicates a change in magmatic overpressure and deactivation of the mid-to-upper crustal system. The younger (23.1 – 24.1 Ka) Paetahi Formation is more evolved than the Poto Formation (27.3-25 Ka), reflecting the re-activation of the mid-to-upper crustal system throughout the regrowth period. Continued evolution in magmatic composition (from 3.62 wt.% to 3.14 wt.% MgO), increase in VND (from 1.15 x1016 to 1.29x1016 cm-3) and decrease in VV (from 7.1 to 3.3%) following the western Pungarehu collapse (~2,500 years after the Ngaere) reflects no depressurisation on the shallow volcanic system. The observed differences in response to collapse events is due to the relative height of the edifice and location of the conduit/ vent. The sedimentological, textural, and geochemical analysis of the Poto and Paetahi tephra formations demonstrate the changes to eruptive activity following collapse events. However, these results highlight the relationship between edifice height, lithostatic pressure and the magmatic system. The Ngaere collapse depressurized a fully grown edifice (~ 2500 m), shifting the vent location within the scar and destabilized the remanent cone. The relatively short time between collapse events (~2,500 years) saw the western remanent cone collapse before Mt. Taranaki had fully regrown. This did not cause a significant change on the magmatic system below and allowed for the continued regrowth. This study highlights a need to understand the relationship between hazardous volcanic phenomena to generate more accurate hazard scenarios for stratovolcanoes which are prone to edifice collapse.
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    Analysis of Mount Ruapehu tephra deposits from 4.3 to 6.1 ka : within a transitional timeframe : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Earth Science at Massey University, New Zealand
    (Massey University, 2024) Fleming, Joseph
    Mount Ruapehu is the largest active volcano in New Zealand, having shown a dominant eruptive record within the last 1,800 years. However, it showed a long period of relative dormancy beyond that time frame, extending to ~12 ka. Within this period of time sits the Papakai Formation. This formation is dominated by tephra deposits from the upper reaches of the Tongariro Volcanic Centre with only eight Ruapehu sourced tephras being found within the extents of this formation dating between 11-3.5 ka. This study aimed to analyse tephra deposits from Mount Ruapehu’s eastern ring plain found within the Papakai Formation. To define their geographical extents and physical characteristics within the field as well as their componentry and geochemical composition within the laboratory. Conducting this study allowed for insight into the eruptive processes from Mount Ruapehu throughout a period of dormancy as well as any trends in ashfall deposition and geochemistry over time. Two black ash layers found beneath the Taupō Ignimbrite were sampled and collected alongside the five Papakai Formation tephras for comparison. Four of the five layers had been previously identified by Donoghue (1991) but had not been fully analysed with field observations having only been made at a handful of locations. This study aimed to provide more information of the ashfall deposits. The two black ash layers and three orange lapilli layers within the Papakai Formation were found to be basaltic andesite with each showing definable traits and characteristics within the field. The tephras showed a trend towards higher silica content over time with deposits younger than the Taupō Ignimbrite being dacitic in nature (Voloschina, 2020). This trend could reflect the lower eruptive frequency of Mount Ruapehu within the span of the Papakai Formation and could potentially show the point at which volcanic activity reactivated at the southern crater after a long period of dormancy. Maximum extents to the north, west and south were estimated but access into the Rangipo Desert would be required to provide a more accurate analysis on the geographical extents of each tephra layer within the eastern ring plain stratigraphy.
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    Monitoring unrest from ambient seismic noise recordings : results from Mount Ruapehu, New Zealand for 2022 : a thesis presented in partial fulfillment of the requirements for the degree of Master of Science in Earth Science, Massey University, Palmerston North, New Zealand
    (Massey University, 2024-04) Almassri, Mustafa
    Ruapehu is an active andesitic composite volcano that had two significant eruptions on 4 October 2006 and on 25 September 2007 since the last major eruption in 1995–1996. These were mostly phreatic explosions that happened with minimal precursors. A significant unrest period occurred between March and June at Ruapehu volcano in 2022, with heating of the crater lake and tremor levels consistent with moderate to high levels of volcanic unrest. In this study, velocity changes during the 2022 unrest were explored using seismic interferometry. Analysis of one year of ambient noise data using moving window cross spectral analysis (MWCS) found a nearly 0.5% drop in the East-North edifice’s seismic velocity, while no reduction in velocity was noted in the West-North. This decrease began three weeks before the unrest signals of Crater Lake temperature and tremor were observed. This drop is a reversible process, and several factors, including fractures opening, fluid fluxes like water, gas, or magma, magmatic anomalies, magma intruding into the subsurface without reaching the surface, and environmental factors (such as rainfalls or changes in atmospheric pressure), could cause a low-velocity zone. There is, however, no evidence of the precise reason for the 2022 unrest.
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    Assessing key physical properties of the Rotorua, Kaharoa and Taupō tephras for their potential use in hydroponics : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Earth Science at Massey University, Manawatū, New Zealand
    (Massey University, 2024) McLachlan, Susan
    Food security is an increasing concern as global populations grow and fertile land availability decreases. Hydroponics, where plants are grown in a soilless medium or nutrient solution, is a potential method of securing food supplies. Understanding the physical and chemical properties of growth media is important as they influence plant growth and productivity. Pumice, a vesicular lightweight material produced by volcanic eruptions, is used in some areas as a growth medium due to its ability to support plant growth. This natural resource is abundant in New Zealand but currently underutilised. The Taupō and Okataina Volcanic Centres (TVC and OVC) in the Central North Island have produced vast volumes of pyroclastic material in the last >60,000 years. This study focused on some of their youngest eruptives, the Kaharoa (OVC), Rotorua (OVC) and Taupō Y (TVC) tephras, to assess their suitability as hydroponic growth media. Samples of the Kaharoa, Rotorua, Taupō Y2 and Y5 deposits and the commercially available Daltons pumice were characterised for grain size then split into hydroponic grades of 1-4 mm and 4-8 mm. The physical properties were used to define relationships between volcanological and hydroponic parameters. Componentry of the grades and pumice clast morphology, texture, density, and porosity were characterised. Vertical variations in the Rotorua and Taupō tephra profiles reflect changes in the eruption plume, degassing and/or conduit processes during the eruption. Settling velocities are reflected in lateral changes in pumice clast shape, size, and density in the Kaharoa deposits. The hydroponic parameters water holding capacity (WHC), bulk density and total porosity were found to be closely related to clast density and porosity and generally fall within the range of the tested hydroponic media. However, tephra WHC was generally lower than that of the hydroponic media. The low bulk density of the tephras particularly the 4-8 mm grade make them a relatively light material, however, their low WHC may limit their usefulness as a growing medium or require more frequent or alternate methods of irrigation. The higher bulk density and WHC of the 1-4 mm grade means it is likely to be better suited for many species.
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    Effects of multiphase turbulence on the flow and hazard behavior of dilute pyroclastic density currents : a thesis submitted in partial fulfilment for the degree of Doctor of Philosophy in Earth Science at Massey University, Palmerston North, New Zealand
    (Massey University, 2024-06-16) Uhle, Daniel Holger
    Pyroclastic surges (also dilute pyroclastic density currents or dilute PDCs) are amongst the most hazardous volcanic phenomena associated with explosive volcanic eruptions and hydrothermal explosions. These fast-moving, turbulent, polydisperse multiphase flows of hot volcanic particles and gas occur frequently and have severe impacts on life and infrastructure. This is attributed to a compounding of hazard effects: large flow-internal dynamic pressures of tens to hundreds of kilopascals destroy reinforced buildings and forests; temperatures of up to several hundreds of degrees Celsius pose severe burn hazards; and readily respirable hot fine ash particles suspended inside dilute PDCs cause rapid asphyxiation. Direct measurements inside pyroclastic density currents are largely absent, and previous research has used a combination of detailed field studies on PDC deposits, laboratory experiments on analog density currents, numerical modeling, and theoretical work to interrogate the internal flow structure, gas-particle transport, sedimentation and destructiveness of dilute PDCs. Despite major scientific advances over the last two decades, significant fundamental gaps in understanding the turbulent multiphase flow behavior of dilute PDCs endure, preventing the development of robust volcanic hazard models that can be deployed confidently. Critical unknowns remain regarding: (i) how turbulence is generated in dilute PDCs; (ii) how multiphase processes modify the flow and turbulence structure of dilute PDCs; and (iii) if and how turbulent gas-particle feedback mechanisms affect their destructiveness. To address these gaps in understanding, this PhD research involved high-resolution measurements of velocity, dynamic pressure, particle concentration, and temperature inside large-scale experimental dilute PDCs. It is shown that dilute PDCs are characterized by a wide turbulence spectrum of damage-causing dynamic pressure. This spectrum is strongly skewed towards large dynamic pressures with peak pressures that exceed bulk flow values, routinely used for hazard assessments, by one order of magnitude. To prevent severe underestimation of the damage potential of dilute PDCs, the experimentally determined ratio of turbulence-enforced pressure maxima and routinely estimated bulk pressures should be used as a safety factor in hazard assessments. High-resolution measurements of dynamic pressure and Eulerian-Lagrangian multiphase simulations reveal that these pressure maxima are attributed to the clustering of particles with critical particle Stokes numbers (𝑆𝑡=𝒪(1)) at the margins of coherent turbulence structures. The characteristic length scale and frequency of coherent structures modified in this way are controlled by the availability of the largest particles with critical Stokes number. Through this, spatiotemporal variations in peak pressures are governed by the mass loading and subsequent sedimentation of these clustered particles. In addition to the ‘continuum phase’ loading pressure, the measurements also revealed that the direct impact of clustered margins and high Stokes number particles decoupling from margins with structures generate instantaneous impacts. These piercing-like impact pressures exceed bulk pressure values by two orders of magnitude. Particle impact pressures can cause severe injuries and damage structures. They can be identified as pockmarks on buildings and trees after eruptions. This new type of PDC hazard and the magnitude of pressure impacts need to be accounted for in hazard assessments. Systematic measurements of the evolving experimental pyroclastic surges along the flow runout demonstrate that time-averaged vertical profiles of all flow velocity components and flow density obey self-similar distributions. Variations of the roughness of the lower flow boundary, geometrically scaling ash- to boulder-sized natural substrates, showed the self-similar distributions are independent of the roughness. Mathematical relationships developed from the self-similar velocity and density distribution reveal the self-similar vertical distribution of mean dynamic pressure. This empirical model can inform multi-layer PDC models and estimate the height and values of peak time-averaged dynamic pressure for dilute PDCs of arbitrary scale. Turbulence fluctuations around the mean were investigated through Reynolds decomposition. The large-scale turbulence structure and the dominant source of turbulence generation are shown to be controlled by free shear with the outer flow boundary, while strong density gradients at the basal high-shear flow boundary dampen turbulence generation. The large-scale, shear-induced coherent turbulence structures can be tracked along the runout and were found to be superimposed by smaller turbulence structures. In Fourier spectra of dynamic pressure, flow velocity, and temperature, these sub-structures are observed as discrete frequency bands that correspond to the coarse modes of the spatiotemporally evolving flow grain-size distributions. This can be associated with the preferential clustering of particles at the peripheries of the sub-structures. Following the decoupling of particle clusters, the rapid sedimentation of particle clusters occurs periodically at the characteristic frequency of the turbulence sub-structures. This mechanism of preferential clustering, decoupling and rapid sedimentation of particles with critical particle Stokes numbers is an important mechanism of turbulent sedimentation to explain the spatiotemporally evolving flow grain-size distribution of pyroclastic surges.
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    The Arxan-Chaihe Volcanic Field of monogenetic volcanism in intracontinental settings in NE China : 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, 2024-02) Li, Boxin
    Pliocene to Recent Arxan-Chaihe Volcanic Field (ACVF) is composed of at least 47 vents, representing various types of volcanism, such as Strombolian style, phreatomagmatic explosive, effusive, and lava-fountaining eruptions. These eruptions produced scoria cones, fissure-aligned spatter cones, and tuff rings with a few surrounding maar craters. Field observations imply that the lava-fountaining eruptions are more common on the western side of ACVF, represented by Yanshan (YS)-the Triple Vent, and Daheigou (DHG). In the southwest part of ACVF, lava flows and loose pyroclastic ejecta, such as scoria, mark the eruption events that took place during the Holocene era about 2000 years ago. Dichi (Earth Pond) Lake, with fissures on its eastern side, formed by lava-effusive eruption styles with spatter rows occurring along a fissure, while the low-lying western side of the vent chain is a maar volcano cut into the pre-eruptive lava sheets. Tianchi (Heaven Lake) Lake and Tuofengling (Camel Hump) Lake on the western side of ACVF preserves a range of well-exposed pyroclastic deposits consistent with edifice-building successions. These are composed of scoriaceous pyroclastic materials, yielding construction histories of complex cones (with both "wet" and "dry" explosive eruptive phases). The most significant and largest vent is in the eastern corner of ACVF, Tongxin Lake, a complex phreatomagmatic eruption-style volcano with a maar crater and thick rim deposits. Tongxin Lake is interpreted to be a maar lake that erupted into an intra-montane basin. Intact pyroclastic deposits are preserved within a km from the crater rim and at least 5 meters thick. Stratigraphic and granulometric analyses from five sites around Tongxin Lake indicate the tuff ring of Tongxin was built by processes associated with magma-water interactions that fueled violent explosive eruptions during distinct syn-eruptive stages. Geochemistry is consistent with at least three magma sources contributing to the formation of the complex eruptive products that build the large tuff ring of the maar edifice. Geomorphology terrain analyses performed through GIS-based applications (QGIS) imply that the diverse range of local geology, especially the pre-eruptive topography, was confined and reshaped by the subsequent Pliocene to Recent volcanism in ACVF. Lava flows within ACVF were emplaced over large areas around the two major fluvial systems: Halaha River in the west and Chaoer River in the east of ACVF. The lava flows in the west of ACVF are generally young and can be modelled using the Q-LavHA plug-in of QGIS. The model has been utilized to simulate lava flow inundation and indicates diverse flow along the flow axis as well as lateral and temporal variations during the evolution of the edifice. Other studies of ACVF, e.g., hazard management, concluded that violent phreatomagmatic explosive events had impacted the fluvial valleys that are commonly associated with structural weakness zones in ACVF. In addition, lava-effusive and lava-fountaining eruptions in urban areas and along major utilities (e.g., roads, geopark facilities or powerlines) also could be heavily impacted by fissure-fed lava flows and potential phreatomagmatic explosions controlled by the local hydrology conditions.
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    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
    (Massey University, 2023) Tapscott, Sarah Joanne
    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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    Subduction cycling and its controls on hyperactive volcanism in the Taupo Volcanic Zone, 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, 2023) Corella Santa Cruz, Carlos Rodolfo
    The origin and magmatic evolution of arc magmas are strongly influenced by transcrustal and source processes. Transcrustal processes are often employed to explain the geochemical diversity seen in arc magmas, from mafic to the most felsic endmembers. Source processes are usually used to explain the diversity seen particularly in mafic magmas. Yet, the relative contributions of both processes are highly controversial and difficult to identify. The southernmost volcanic expression of the Tonga-Kermadec-Hikurangi subduction system, the Pleistocene to Holocene Taupo Volcanic Zone (TVZ), is a suitable volcanic area to assess these ideas. Here, the subduction of the unusually thick Hikurangi Plateau has strong effects on tectonic erosion. The TVZ is dominated by rhyolites, which is unusual given the thin (~16 km) basement comprised mostly of the Permian to Early Jurassic Torlesse metasedimentary terrane. In comparison, the southern TVZ, dominated by andesitic volcanism, is located on a thicker (~30 km) crust. The general view of the magmatic evolution of the TVZ corresponds to mafic magmas coming from the mantle, ponding at the base of the crust, where they assimilate crustal material and start to ascend through the crust where more transcrustal processes occur. In this thesis, the impact of assimilation-fractional crystallisation (AFC) on rock composition was assessed by using major and trace element concentrations, Sr-Pb isotope systematics and the Magma Chamber Simulator (MCS), yielding thermodynamically constrained results. It was found that i) variations seen in mafic magmas cannot be reproduced by transcrustal processes alone, ii) some intermediate samples can be explained by AFC and mixing, but others cannot, and iii) large volumes of crustal assimilation (50%) and fractionation (90%) are required to reproduce the signatures of the most felsic endmembers. In Pb isotope space, a broadly linear correlation of the magmas is seen, consistent with the mixing of two endmembers: the mantle and a ‘crustal material’. One possibility would be mixing these two endmembers in the source before the transcrustal ascent of magmas. This idea was examined by analysing samples from the Hikurangi margin provided by the IODP Expedition 375. Through the calculation of the bulk chemical and Sr-Pb-Nd-Hf isotopic compositions of the subducting material, it was found that there is no geochemical correlation between this material and the TVZ. This material is too variable and too radiogenic to generate the broadly linear relation seen in Pb isotopic space, and it is also inconsistent in all other isotopic systems (Sr-Nd-Hf). The material located in the accretionary prism and above the décollement zone is homogenous and strongly correlates in the Sr-Pb-Nd isotopic systems. This material would be subducted if affected by tectonic erosion. Once this material is tectonically eroded, it can contribute to the source from where the magmas are being generated. The isotopic correlations are seen in fluid-mobile and fluid-immobile elements. Thus, the recycled material contributed by releasing fluids and melts or solid material derived from the subducting slab. Whether these interactions occur at the slab-mantle interface and/or during a diapiric ascent remains uncertain. Thus, the isotopic diversity of the TVZ may be controlled by crustal recycling of tectonically eroded material, with subsequent transcrustal processing adding to the diversity generated already in the source. This process would not only limit the amounts of crustal assimilation needed to generate the isotopic signatures of the most felsic endmembers but would also explain the isotopic diversity seen in the most mafic endmembers and the presence of andesites with primitive isotopic signatures. Ultimately, the impacts of crustal recycling in subduction zones can help elucidate the processes of magmatic differentiation, crustal growth, crustal recycling and crustal loss.
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    The interactions of pyroclastic density currents with obstacles : a large-scale experimental study : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Sciences at Massey University, Manawatū, Palmerston North, New Zealand
    (Massey University, 2023) Corna, Lucas
    Pyroclastic Density Currents (PDCs) are hot multiphase flows of volcanic particles and gas that are frequently produced during explosive volcanic eruptions. These fast-moving currents show variable runout distances that range from a few kilometres to more than 100 kilometres from their sources. Through their high velocities, large contents of respirable fine ash, temperatures and dynamic pressures, PDCs constitute extreme suffocation, burn and damage hazards for people living around volcanoes. An important additional source of hazard arises from the ability of PDCs to surmount significant topographic obstacles, such as hills and ridges. Because of their ferocity, to date there are no direct observations and measurements of PDCs interacting with obstacles and current knowledge comes from characterizing PDC deposits and PDC damage features across terrain after eruptions. The interaction of dense granular flows and dilute aqueous and gaseous particle-laden gravity currents with simple topographic barriers has been studied in laboratory experiments. These represent dense, non-turbulent or dilute, fully turbulent end-members of PDC behaviour. How the behaviour of dense and dilute end-member flows interacting with obstacles differs from those of real-world PDCs, which encompass a complete multiphase spectrum from dilute to dense transport regimes, remains unknown. This PhD research aims at bridging this gap in knowledge through synthesizing the behaviour of interaction of PDCs with ridge-shaped obstacles in large-scale experiments. The experiments were designed and conducted at the eruption simulator facility PELE (Pyroclastic flow Eruption Large-scale Experiment) to visualise the processes of PDCs interacting with obstacles, measure the changes in the flow velocity and density structure induced by this interaction, characterise the effects of the interaction on downstream flow behaviour, and study the variations in deposition across obstacles. Three hill-shaped obstacles were designed for these experiments, with the same shape and aspect ratio, but varying sizes compared to the size of the experimental PDC. The experiments reveal strong changes in the vertical velocity and density structures of the currents immediately before and after obstacles and strong losses in flow momentum. This is associated with flow compression and acceleration along the stoss side of the ridge, flow detachment with boundary layer separation behind the ridge crest, and formation of a turbulent wake underneath the detached flow before re-attachment. The amount of flow acceleration, the size of the turbulent wake and the flow re-attachment distance increase with obstacle size. High-speed video footage of the interaction shows evidence for a typical sequence of transient behaviour that could be linked to the time-variant velocity and density structure of the head, proximal body, distal body and a tail regions of the experimental PDCs. Four phases of interactions are noted in all three experiments: (1) during head passage - flow acceleration and compression on the stoss side, followed by detachment after the crest, generation of the wake behind the hills and re-attachment downstream; (2) during passage of the proximal body - the development of an alternatively thickening and collapsing turbulent jet structure along the stoss side that forms the base of the detached flow, and which separates and shields the wake from the detached flow above; (3) during passage of the remaining body - an increase in flow density leads to the blocking of a dense underflow forming thick deposits on the stoss side and to the advection of particles from the lower flow region into the detached flow above the wake; (4) during waning flow and passage of the gravity current tail - the velocity field rotates and the angle of attack of the flow approaches the inclination of the lee side of the obstacle. In this situation, the size of the turbulent wake decreases and eventually flow detachment ceases. The compression and acceleration of material on the stoss side of obstacles allow particles at the base of the flow to be conveyed upward back into the detached flow. The higher the obstacle, the stronger the acceleration and the larger the proportion of the flow that is advected. Ballistic trajectory models, which have been used to predict flow paths of dense and dilute analogues flows across simple obstacles, do not describe well the wake measured in experiments and under-estimate its dimensions. As evidenced by vortex shedding and high detachment angles in a flow with high Reynolds number, PDC-obstacle interactions are instead controlled by boundary layer separation in a turbulent flow. Therefore, they are linked to the drag coefficient of the hill and the drag force exerted by the obstacle onto the flow. A study of the wake dimensions revealed that a lift force assists in maintaining the wake aloft and in countering gravity. The ratio between the drag and lift forces controls the wake dimensions. An empirical scaling relationship between the re-attachment distance of the flow and the height of the obstacle was derived and tested against natural data of preserved tree patches behind hills. The experimental measurements also showed that loss in flow momentum due to obstacle drag is associated with complete blocking of the basal granular-fluid underflow and partial blocking of the upper dilute turbulent part of the experimental PDCs. Data from the three experimental runs, in combination with measurements from experiments with no obstacles, allowed extrapolation of the minimum ridge size that leads to complete flow blocking. This relationship agrees well with results from previous laboratory experiments on dilute gas-particle gravity currents and could find application in volcanic hazard estimates. The increasing loss in flow momentum with increasing obstacle size is associated with a strong reduction in the bulk flow density. Thus, experimental PDCs propagating over larger obstacles show a lower density contrast with the ambient air, and therefore a lower driving force, than currents propagating over smaller obstacles. Despite this, the final runout distances are remarkably similar in experiments with different obstacle sizes. This finding is explained by two different processes. First, flow compression and acceleration on the stoss side of obstacles leads to the acceleration of internal gravity waves. The gravity waves move faster than the surrounding flow, intrude and provide momentum into the PDC head. Initially slower currents behind large obstacles thus accelerate periodically and ‘catch up’ with less compressed and less accelerating currents downstream of smaller obstacles. Second, particles that sedimented below the level of the obstacle crest before the obstacle become advected with the detached flow into flow regions above the height of the crest. Larger obstacles, which induce stronger flow acceleration, advect particles higher into the detached flow than smaller obstacles. The duration and downstream length over which the advected particles re-sediment, deposit and eventually become inactive to drive the flow as excess density therefore increases with obstacle size. With increasing obstacle height, this second process generates increasingly hotter flows with slightly coarser and thicker deposits downstream of obstacles. The experimental results and relationships derived in this research add critical complexity to the understanding of PDCs interacting with topographic obstacles and resulting downstream hazards. The reported compressibility effects in the experimental PDCs are currently not captured in PDC flow and hazard models. The local flow acceleration against gravity on the obstacle stoss side warrants caution for the application of kinetic to potential energy conversion models that are used to estimate bulk velocities of PDCs. These findings encourage further experimental and numerical experiments to investigate, for instance, the more complex situations of three-dimensional obstacles, systematic test of different obstacle geometries and series of obstacles in PDC pathways to help development of predictive PDC flow and hazard models.
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    A new approach to volcanic geoheritage assessment in the Auckland Volcanic Field, 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, 2021) Nemeth, Boglarka
    The concept of conserving geoheritage was announced in 1991 because of concern about the disappearing geological and geomorphological integrity from the Earth's surface. The degradation imposes a high risk on resilience and future scientific work. Geology-related research has primarily been linked to natural hazard mitigation studies and mineral resource exploration and management. In this study, we build a conceptual framework for geoheritage conservation to incorporate geoeducation into land use planning in Auckland. The study's main goal is to assist geoheritage conservation initiatives in the Auckland Volcanic Field (AVF) by providing a synthesis of concepts and their integration into a GIS environment for successful policy implementation. In the lack of clearly defined values, the valuer must rely on their own perception of value that is shaped by cultural, economic and scientific background. The peer-reviewed scientific literature contains a collection of concepts that need to be organised into a framework. The difficulty of quantifying the benefits of protecting scientific value led to the inclusion of cultural, touristic, aesthetic, recreational and biotic values. Scientific value in the light of economic benefits can be easily overviewed as we revealed it through a meta-analysis of influencing factors on the conceptual background of geoheritage implementations. A conceptual framework must reinforce Geoeducation as the essence of geoheritage. Geopreservation Inventory is the result of a collaboration of New Zealand Earth Scientists under the leadership of Bruce Hayward. The recognised geoheritage sites still wait for recognition and inclusion in urban planning under a clarified geoheritage preservation plan. From a broad aspect, the outstanding features are all small-volume volcanoes, but the inventory clearly reflects their diverse nature. It is seen from the map that none of the features that were assigned the highest importance is typical tourist destinations. The landforms are classified by the Topographic Position Index of the area based on the one-meter digital elevation model produced from high-resolution light detection and ranging (LiDAR) point cloud data. The classified landforms, geology map and Geopreservation Inventory are aggregated into a Geoeducational Capacity Map. The high geoeducation capacity areas are compared with the areas receiving high visitation in order to understand the role of scientific value in land use planning. The high indigenous value of geoheritage sites is assessed from a cultural aspect. The communities need to be involved with promoting geoeducation from geoscientific and indigenous aspects to increase the depth of geoheritage and bring the concept closer to the society to create resilience and a sense of respect for the relicts of geology.