Small-volume volcanism associated with polygenetic volcanoes, 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

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In the past 350 ky, the Taupo Volcanic Zone (TVZ) has been the most productive silicic volcanic region in the world, with 12 silicic large-volume (35-2200 km3 DRE) caldera-forming eruptions and hundreds of predominantly silicic smaller-volume eruptions. These spatially and temporally scattered small-volume events are characterised by relatively short-lasting single eruptive episodes that may have been strongly influenced by location-specific environmental factors. The aim of this study is to evaluate the volcanic hazards associated with the activity of these small-volume volcanoes using two different approaches. At the local scale, possible eruption scenarios were defined through three example localities (Ohakune, Motuoapa, Puketerata) that are characterised by different chemical compositions, eruption styles, as well as distinct environmental conditions representative of the entire TVZ. At the field scale, evaluation of small-volume activity was focused on the spatio-temporal and volumetric distribution of vents and their relationship to the structural elements of the TVZ. The most common small-volume eruptions form lava domes that were originally thought to be rarely associated with explosive activity. In contrast, this study shows that dome emplacement is often accompanied by explosive activity. However, the associated pyroclastic deposits are characterised by a low preservation potential. The most likely scenario for future eruptions is a rhyolitic eruption with an initial phreatomagmatic phase followed by the effusive emplacement of lava with or without associated explosive activity. Based on the average eruptive volumes (0.2-0.3 km3) of single TVZ events and typical eruption rates of dome-forming activity (2-5 m3/s), future eruptions are expected to last for several months to a few years. TVZ-scale spatio-temporal analysis of eruptive vents indicates that small-volume volcanism is not entirely limited to active calderas. Instead, frequent fissural activity indicates a strong linkage to the fault system of the TVZ. The temporal and volumetric pattern of small-volume volcanism displays a sudden increase of activity at 45 ka, producing at least 130 km3 of volcanic material to date. The random occurrence, great variability of eruption styles, small but significant volumes, prolonged duration and relatively high frequency of small-volume volcanism pose a significant threat within the entire TVZ, which New Zealand will have to face in the near future.
Chapter 8 removed from thesis for copyright reasons, but may be accessed from the publisher (Bulletin of Volcanology) upon publication. Chapter 7 published as: Kósik, S., Németh, K., Lexa, J & Procter, J.N. (2017). Understanding the evolution of a small-volume silicic fissure eruption: Puketerata Volcanic Complex, Taupo Volcanic Zone, New Zealand. Journal of Volcanology and Geothermal Research. Chapter 6 published as: Kósik, S., Németh, K., Procter, J.N. & Zellmer, G. (2017). Maar-diatreme volcanism relating to the pyroclastic sequence of a newly discovered high–alumina basalt in the Maroa Volcanic Centre, Taupo Volcanic Zone, New Zealand. Journal of Volcanology and Geothermal Research, 341: 363–370. Chapter 5 published as: Kósik, S., Németh, K., Kereszturi, G., Procter, J.N., Zellmer, G.F. & Geshi, N. (2016). Phreatomagmatic and water-influenced Strombolian eruptions of a small-volume parasitic cone complex on the southern ringplain of Mt. Ruapehu, New Zealand: Facies architecture and eruption mechanisms of the Ohakune Volcanic Complex controlled by an unstable fissure eruption. Journal of Volcanology and Geothermal Research, 327: 99–115.
Volcanoes|zNew Zealand|zTaupo District, Volcanism|zNew Zealand|zTaupo District, Taupo Volcanic Zone (N.Z.)