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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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    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, Boxin
    Two 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 diagram
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    The role of substrate hydrogeology and surface hydrology in the construction of phreatomagmatic volcanoes on an active monogenetic field (Auckland, New Zealand) : a thesis presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Earth Science at Massey University, Palmerston North, New Zealand
    (Massey University, 2015) Agustín Flores, Javier
    Phreatomagmatic activity is pervasive in the Auckland Volcanic Field (AVF) with more than two thirds of the erupted volcanoes showing this type of activity at different degrees, dominantly at the onset of their eruptive histories. In general, the volcanoes built in the northern AVF rest on Late Miocene Waitemata Group rocks (turbiditic siltstone and sandstone succession), whereas in the southern AVF the Waitemata rocks are overlain by tens of metres of Plio-Pleistocene, water-saturated sediments (Tauranga Group and Kaawa Formation). Identifying the control exerted by the type of substrate in the eruption dynamics of the phreatomagmatic phases of three volcanoes in the AVF is the objective of this study. The stratigraphic, sedimentary, and pyroclast characteristics of the phreatomagmatic sequences of Maungataketake, Motukorea, and North Head volcanoes, together with supplementary information on the geology and hydrogeology of the area, were investigated to solve the problem. Three phreatomagmatic eruptive scenarios were outlined. Scenario 1 (Maungataketake eruption) and Scenario 2 (Motukorea eruption) depict the formation of maar-diatreme volcanoes in the southern and northern AVF, respectively. The dominant presence of lithics from the upper part of the substrate in their tephra rings suggests the construction of their tephra rings from shallow-seated explosions. Due to the water-saturated sediments filling the diatreme in Scenario 1, the eruption appears to have remained relatively wet throughout. Conversely, the drier Waitemata rocks involved in Scenario 2 promoted a progressive drying of the eruption. Scenario 3 (North Head eruption) represents a Surtseyan eruption scenario in which the rising magma erupted to the shallow sea floor (a few metres-water depth), promoting rapid chilling and explosive fragmentation. This study shows that the characterization of lithics within the tephra ring and the geological and hydrogeological information provide valuable clues to envisage the degree of influence of the substrate in the phreatomagmatic eruption dynamics. Other studies in the AVF appear to confirm this view. It is proposed that any future phreatomagmatic eruption in the AVF will be strongly influenced by the substrate hydrogeology and geology, as well as the surface hydrological conditions.
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    A sedimentological and geochemical approach to understanding cycles of stratovolcano growth and collapse at Mt Taranaki, 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, 2008) Zernack, Anke Verena
    The long-term behaviour of andesitic stratovolcanoes is characterised by a repetition of edifice growth and collapse phases. This cyclic pattern may represent a natural frequency at varying timescales in the growth dynamics of stratovolcanoes, but is often difficult to identify because of long cycle-timescales, coupled with incomplete stratigraphic records. The volcaniclastic ring-plain succession surrounding the 2 518 m Mt. Taranaki, New Zealand, comprises a wide variety of distinctive volcanic mass-flow lithofacies with sedimentary and lithology characteristics that can be related to recurring volcanic cycles over >190 ka. Debrisflow and monolithologic hyperconcentrated-flow deposits record edifice growth phases while polylithologic debris-avalanche and associated cohesive debris-flow units were emplaced by collapse. Major edifice failures at Mt. Taranaki occurred on-average every 10 ka, with five events recognised over the last 30 ka, a time interval for which stratigraphic records are more complete. The unstable nature of Mt. Taranaki mainly results from its weak internal composite structure including abundant saturated pyroclastic deposits and breccia layers, along with its growth on a weakly indurated and tectonically fractured basement of Tertiary mudstones and sandstones. As the edifice repeatedly grew beyond a critical stable height or profile, large-scale collapses were triggered by intrusions preceding magmatic activity, major eruptions, or significant regional tectonic fault movements. Clasts within debris-avalanche deposits were used as a series of windows into the composition of previous successive proto-Mt Taranaki edifices in order to examine magmatic controls on their failure. The diversity of lithologies and their geochemical characteristics are similar throughout the history of the volcano, with the oldest sample suites displaying a slightly broader range of compositions including more primitive rock types. The evolution to a narrower range and higher-silica compositions was accompanied by an increase in K2O. This shows that later melts progressively interacted with underplated amphibolitic material at the base of the crust. These gradual changes imply a long-term stability of the magmatic system. The preservation of similar internal conditions during the volcano’s evolution, hence suggests that external processes were the main driving force behind its cyclic growth and collapse behaviour and resulting sedimentation pattern.