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Item Variable controlling factors lead to contrasting patterns of volcanism in the Changbaishan volcanic area (Tianchi-Longgang), China-North Korea: Insights from morphometry and spatial-temporal analyses(Elsevier B V, Amsterdam, 2024-07-01) Zhang R; Brenna M; White JDL; Kereszturi GThe coexistence of monogenetic and polygenetic volcanoes is a common phenomenon in volcanic areas. However, the genetic relationship between monogenetic and polygenetic systems and the factors controlling their distinct eruptive styles are not well understood. In active volcanic areas, analysing the clustering and vent alignment of monogenetic volcanoes, as well as examining the geomorphology and relative ages of scoria cones, offers quantitative insights into magma supply rates, volcano type distribution, and volcanic development trends. Our study presents geomorphological and spatio-temporal analyses of the co-existing monogenetic volcanoes in the Longgang Volcanic Field (LVF) and those associated with a polygenetic volcano (Tianchi) in the Changbaishan Volcanic Area, China. The distance between the two volcanic areas is around 150 km. Monogenetic vents in the LVF exhibit greater density compared to the dispersed system associated with Tianchi. The LVF vents also show better alignment, particularly in the direction of pre-existing basement faults (NE-SW, NW-SE and EW). By using scoria cone morphometric parameters and features, we estimated the relative ages and erupted volumes of monogenetic volcanoes in the LVF and the Tianchi area. We classified the cones of the two volcanic systems into five eruptive periods and found that, despite similar magma sources and output rates over approximately 870 kyr, differing numbers of scoria cones across age classes suggest that Tianchi's magma system influences its associated monogenetic volcanic plumbing. Furthermore, the continuous rise in output rates of monogenetic volcanoes in the Tianchi area highlights the increasing magma supply sustaining Tianchi volcano. Together, these interpretations are consistent with the two systems being controlled by different factors: the Tianchi monogenetic volcanic system is more controlled by magmatism, whereas the LVF is more strongly controlled by local tectonic structures, alongside an increasing magma supply causing the formation of progressively larger individual volcanoes. In volcanic areas, analysing monogenetic volcanoes' spatial-temporal distribution, volumes and recurrence rate provides a framework to evaluate magma supply rates and tectonic associations, which are key to the development of different volcano types.Item 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 HolgerPyroclastic 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.Item 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, BoxinPliocene 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.Item 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 RodolfoThe 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.Item 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, AndreaRhyolitic 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.Item Understanding magmatic processes and their timescales beneath the Tongariro Volcanic Centre through microanalytical investigations of the tephra record : a thesis 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, 2020) Lormand, CharlineThe Tongariro Volcanic Centre (TgVC) is a complex volcanic system located at the southern end of the Taupo Volcanic Zone in New Zealand, and has produced historical explosive eruptions of different eruptive styles. Its three ski fields and its iconic Tongariro Alpine Crossing attract more than 130,000 visitors annually. The last eruption occurred in 2012 on the northern flank of Tongariro, at the Te Maari vent. Due to the lack of precursory activity, this eruption could have turned into a tragedy if it had happened during day time. Previous studies have focused on the TgVC phenocrysts, which do not provide insights into shallow magmatic processes, essential to mitigate the resulting volcanic hazards. To understand magma ascent processes and their associated timescales, the textures and compositions of the micrometre-sized crystal cargo (i.e. microlites and micro-phenocrysts) carried during explosive eruptions are investigated, along with their conditions of crystallisation [i.e. P-T-X(H₂O)], which are constrained using hygrothermobarometry and MELTS modelling. Glass shards from five tephra formations spanning from c. 12 ka BP to 1996 AD, associated with explosive eruptions ranging from Strombolian to Plinian in style, are studied here. High resolution images and chemical maps of the tephras and the crystals are acquired using scanning electron microscopy (SEM) and secondary ion mass spectrometry. The variety of disequilibrium textures and compositions found in the micro-phenocrysts (< 100 μm) indicates multiple events of magma mixing, magma recharge, pressure fluctuations, and suggests an antecrystic origin. Crystal size distribution (CSD) of 60,000 microlites (< 30 μm) of plagioclase and pyroxene are generated from back-scattered-electron (BSE) images using a semi-automatic method developed here to undertake this study, employing the Weka Trainable Segmentation plugin to ImageJ. Combined with a well-constrained growth rate, crystallisation times are derived and indicate that microlites crystallised 2 to 4 days before the eruption, regardless of the eruption style. Microlite crystallisation occurred between mid-crustal depths and the surface (average of c. 4 km), at unusually high temperature for arc magmas of intermediate composition (average of 1076 °C), and at low water contents (average of 0.4 wt%). Considering the inferred depths and the crystallisation times of 2 to 4 days, ascent rates of only up to 9 cm s⁻¹ prior to shallow water exsolution are calculated. Vent exit velocities are not exceeding 27 m s⁻¹ after complete water exsolution, too slow to feed explosive eruptions characterised by supersonic exit velocities. This research proposes a new conceptual model for the magmatic plumbing system beneath TgVC, where the microlitic crystal cargos result from multiple intrusions of aphyric melts through dykes, which most of the time stall and evolve at depth as deep as the mid-crust. Eventually, a magma injection percolates through previous intrusions and entrains crystals of differing textures and histories. Dykes feeding volcanism funnel into a narrow cylinder towards the surface, allowing acceleration and triggering explosive eruptions. Therefore, the conduit geometry at TgVC is a key controlling factor on the explosivity, with narrower conduits resulting in more explosive eruptions, suggesting that volatile-poor magmas can still trigger explosive eruptions. This study supports that vertical foliation of the igneous upper crust is consistent with dyking and thus may be more common than typically acknowledged.Item Melt generation, storage and ascent below Tongariro Volcanic Complex, Southern Taupo Volcanic Zone : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Science at Massey University, Manawatu, New Zealand(Massey University, 2018) Arpa, Maria Carmencita B.The study of Tongariro Volcanic Complex in New Zealand gives an opportunity to view arc magmatism from a setting where the classic arc structure is overprinted by the regional tectonic setting. Instead of viewing the volcano (and associated magmatic processes) as a component of a volcanic arc to determine the origin of andesitic magmas, focus was given on magmatic processes within the volcanic complex. Processes within the plumbing system of the volcanic complex and their implications on andesitic magmatism and volcanic hazards were determined by tracking magma, of selected eruptive products, from their reservoirs to the surface. By focusing on processes that may determine the petrological characteristics of specific deposits (from known eruptions), the influence of local structures associated with eruptive centres within the complex and the diversity of resultant eruption styles may be interpreted as magmatic processes are evaluated. The deposits for this study are from the last 16 ka history of Tongariro, majority are from the last 10 ka. These are from known eruptions and the deposits were mapped, dated and studied by previous researchers. Lava flow eruptions are from Te Maari and Red Crater, and Plinian to vulcanian eruptions are represented by the Mangamate Tephra and Ngauruhoe deposits. For each eruptive deposit, whole rock major, trace and isotope compositions were determined. Groundmass and mineral components were analysed for major elements. Major element and volatile (H2O, CO2, S, Cl) compositions of melt inclusions in component olivine and pyroxene crystals were also determined. The deposits from the recent history of Tongariro Volcano can be related to a common source. The basalts can differentiate to more evolved andesitic to dacitic compositions by crystallization and/or melting. Magmatic differentiation takes place in different reservoirs, at different depths, within the complex. Differences were observed in the volatile contents of the magmas and these may be related to magma storage and ascent processes. Magmatic processes for the deposits in this study, interpreted from compositions, considered and are consistent with eruption styles.Item Violent phreatomagmatic eruptions that formed maars in an intra-mountain basin at Arxan-Chaihe volcanic field, Inner Mongolia, China : a thesis presented in partial fulfilment of the requirements for the degree of Masters in Earth Science, School of Agriculture and Environment, Massey University, Palmerston North, New Zealand(Massey University, 2018) Li, BoxinTwo large depressions contain Wusulangzi Lake and Tongxin Lake in the Arxan-Chaihe Volcanic Field (ACVF), which preserves at least 27 volcanoes in NE China in about a 1000 km2 area. Due to the preliminary research on both Wusulangzi and Tongxin, two sample groups (Sample 1 and Sample 2) were collected and field observations were held on both sites. Sample 1, representing Wusulangzi, was collected from the SE part of the lake where the lava flow is suspected to cover and preserve medial to distal sections of the tuff ring. Sample 2, representing Tongxin, was collected from the SW rim of the crater from the proximal area, as well as the eastern side in regard to the distal region. Specifically at Tongxin Lake, the pyroclastic successions and beddings contain a series of horizontal and laminated structures, with dune beddings, cross-beddings, as well as a chute-and-pool structure. Pyroclastic deposits of the tuff rings can be traced from the crater rim about 3 km. The various methods of microscopy reveal that glass shards are distributed differently in both sample groups. Mineral diversity is shown to a large extent, and the mineralogical alteration can be observed under petrographic microscopy. SEM and BSE for 2D and 3D images indicate a relatively high fragmentation of juvenile particles. The grain-size distribution also implies medium-to-high explosive energy. Geochemistry data of both major and trace elements reveals a diversity of magma in relation to fractional crystallisation (olivine, clinopyroxene and plagioclase crystallisation) and magma evolution processes, which are depicted by Harker variation diagrams. The AFM plot reveals a primitive stage of magma evolution. The multi-element diagram shows uranium as abnormal, which is suspected to be a U-rich mantle source. Keywords: phreatomagmatic eruption, maar crater, major and trace elements, SEM, BSE, petrographic microscopy, grain-size distribution, ternary plot, Harker diagramItem Kora : a study of a miocene, submarine arc-stratovolcano, North Taranaki Basin, New Zealand : a thesis submitted in partial fulfilment of the requirements for the degree of Master of Science with Honours at Massey University(Massey University, 1998) Adams, Joshua Donald BusbyKora is a relict submarine arc-stratovolcano buried offshore in north Taranaki Basin. New Zealand. Kora was active on the seafloor in middle to upper bathyal water depths from the late Early Miocene to Late Miocene times. Post-eruptive burial of the volcanic edifice by Mohakatino Formation and Giant Foresets Formation sediments has preserved the edifice and its flanking volcaniclastic deposits. Arco Petroleum New Zealand Inc. drilled the Kora feature in 1987 and 1988. Core recovered from the Kora-1A, Kora-2, and Kora-3 wells contain lithologies derived entirely from fragmented volcanic rocks, with no evidence for massive lavas or pillow lavas. Typical lithologies are interbedded tuffs, hyaloclastite tuffs, volcanic conglomerates, and tuff breccias. The framework clasts in the tuff breccias and conglomerates are porphyritic andesite lithic clasts and andesite eruptives. The lithics were derived from subvolcanic intrusions that formed prior to the main period of edifice construction between 16 Ma and 12 Ma. The round porphyritic conglomerate framework clasts were shaped in transit through the volcanic conduit during volcanic eruptions. Conglomerates lack a planar clast fabric and have a polymodal matrix. They were deposited as density modified grain flows. The tuff breccias are the suspended tails of these deposits. The interbedded tuffs and sparse pebble trains are interpreted to be suspension deposits derived from primary subaqueous eruptions. The fragmental volcaniclastic rocks erupted from Kora were formed entirely at the water-magma interface from fuel-coolant interactions, and cooling-contraction granulation. In contrast, modern volcaniclastic rocks on the southern Kermadec submarine arc-volcanoes, Rumble IV and Rumble V, commonly form from collapsing proximal pillow lava outcrops and small eruptive vents. Like Kora, epiclastic redeposition of volcaniclastic debris on Rumble IV and Rumble V include avalanche slides, debris flows, and grain flows, with little evidence for large-scale channel deposits. Seismic facies comprising the Kora edifice were determined from seismic reflection profiles. The individual apron facies reflectors are identified. These comprise a downlapping terminal wedge that marks the downslope limit of volcaniclastic debris, or the surface along which they travelled. Long continuous, subparallel, individual apron facies reflectors typify northwestern aspects of Kora; these reflectors can be traced laterally from the crest of the edifice to the long thin terminal wedge at the toe of the edifice. The southeastern aspect consists of individual apron facies reflectors that are hummocky, discontinuous and intertwined, with short thick terminal wedges. The edifice has been subject to a sector collapse on NW slopes, where a slump scar occurs. The eastern slopes dip more steeply than the western slopes. The edifice has a conical morphology and is some 10 - 12 km in diameter. The major element geochemical analyses from Kora have been compared to geochemical anlayses from the Coromandel, Waitakere, Rumble IV, Wairakau, Egmont, Titiraupenga, Alexandra, Kiwitahi, and Tongariro volcanic centres using discriminant function analysis. Results have identified four assemblages of volcanic centres with comparable major element geochemistry. Kora, which fits in to the Waitakere, Wairakau and Alexandra volcanic assemblage is a southward extension of the Northland volcanic "trend".Item Understanding the largest-scale explosive volcanism at Mt. Taranaki, New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Science, Massey University, Palmerston North, New Zealand(Massey University, 2017) Torres-Orozco, RafaelOver the last 5000 years B.P., at least 53 explosive eruption episodes occurred at Mt. Taranaki, (western North Island, New Zealand) from either the summit-crater (2500 m), or a satellite vent on Fanthams Peak (1966 m). These eruptions are represented in wellpreserved pyroclastic successions on the upper volcano flanks. At least 16 episodes produced deposits with lithostratigraphic characteristics comparable to those of the last sub-Plinian eruption at AD 1655, suggesting an average recurrence of one Plinian/sub-Plinian eruption episode every 300 years. Several large-scale mafic-intermediate (~48-60 wt.% SiO2) eruption episodes sourced from the two vents were studied in detail to determine the “maximum” intensity, magnitude and eruptive styles from this volcano. These episodes comprised climactic phases with sustained and steady, 14-29 km-high eruption columns, often starting and ending with unsteady pulsating, oscillating and collapsing plumes. The columns erupted 0.1-0.5 km3 DRE at mass and volume discharge rates of 107-108 kg/s and 103-104 m3/s, respectively, indicating magnitudes of 4.1-5.1. The unsteady initial, pre- and post-climactic eruptive phases were dominated by domecollapse, column-collapse and lateral-blast pyroclastic density currents (PDCs), with runout distances of 3-19 km and volumes of up to 0.02 km3 DRE. The steadiest phases were associated with eruption of rheologically homogeneous magmas producing homogenous pumice textures. Unsteady phases produced density and porosity pumice gradients by magma stalling in upper conduit levels. Three eruption onset scenarios were developed from this work: a) initial closed-conduit decompression by vent unroofing and domecollapse, b) transient open and clogged conduits produced by repeated plugging-and bursting of chilled or gas-depleted magma, and c) rapid conduit opening with more mafic eruptives. In all scenarios, the climactic phases are comparable, with pyroclastic fallouts covering 1500-2500 km2. The most violent phases of these events, however, are lateralblast PDCs that could reach a broad arc between 14-19 km from source. This reappraisal of the hazardscape at Mt. Taranaki integrates many new details that enable a more realistic hazard management and provides a range of findings that can be applied to other similar andesitic volcanoes prior to reawakening.