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Precursory eruptions, magma priming and eruption dynamics associated with the climactic 19th February 1975 eruption of Ngauruhoe

Ngauruhoe is an active central North Island composite volcano, located at the south end of the Taupo Volcanic Zone. This basaltic andesite to andesite cone is one of the most active vents of the larger Tongariro Volcanic Complex; the active summit scoria cone (c. 150 m in diameter) was constructed within the older crater (c. 400 m in diameter) during the eruptions of 1954 to 1975. Approximately 9 m of volcanic material was added to the inner crater rim during the 1975 eruption. Past eruptions from Ngauruhoe have been well-documented in various studies; however, these generally lack detailed observation and analyses of the 1975 eruption deposit meaning that pre-eruptive conditions and explosion dynamics associated with this eruption were left poorly constrained. This research investigates the physical parameters and eruption controls that led up to the 1975 eruption, including the activity observed from 1968. Specifically, this study examines the pre-eruptive conditions, triggers and eruption mechanisms that led to eruption and emplacement of the 1975 inner crater rim deposit. A key part of this study involved identifying the relationship between, and source of, different clast types comprising the deposit, in addition to determining whether a solid plug and/or magma-water interaction contributed to the high explosivity of the 1975 eruption. By improving our understanding of magmatic and volcanic processes associated with this eruption, this study may assist with more reliable interpretation of monitoring data prior to and during such eruptions (not only at Ngauruhoe, but also basaltic-andesitic volcanoes worldwide). Fieldwork involved stratigraphic logging, high-resolution photography, and sample collection, in addition to sub-sample collection of the 1954-75 volcanic ash deposits stored on archive at GNS Science. Outcrop photos were subsequently point-counted to discriminate between the components, and collected samples described and characterised. Further laboratory analyses included optical microscopic petrography, component point counting, vesicularity studies, scanning electron microscopy, electron microprobe analysis, laser sizing, and quantitative software shape analysis. This study has demonstrated the importance of crystal fractionation and magma mixing in the evolution of the magma that erupted during 1975. In particular, the transporting magma may have interacted with, and entrained, exotic crystals from a cooler, more evolved crystal mush zone formed by an antecedent magma. Further, magma ascent was associated with efficient gas segregation and transport to the surface along preferential percolation pathways. Efficient degassing in low viscosity magmas typically results in effusive activity; however, this study shows that the high explosivity of the climactic 19th February 1975 eruption was linked to the presence of a coherent, degassed, and hydrothermally altered caprock resulting in accumulation and coalescence of pressurised gas beneath the plug. This study discounts the previously ascribed theory that magma-water interaction controlled the high explosivity of this event. Instead, limited magma-water interaction likely produced the steam observed in the eruption column. Based on the interpretations outlined above, it is now possible to explain the inner crater stratigraphy. The 1975 inner crater rim deposit is now divided into four facies, A through D, based on textural characteristics and componentry. Facies A (comprising dense sub-angular lapilli and vesicular scoria lapilli) and B (comprising black scoria lapilli/bombs and dense lapilli) were emplaced during vulcanian activity associated with disruption of a caprock, leading up the climactic eruption on the 19th of February 1975. Facies C (comprising loose scoria, dense lapilli, and large irregular domains of densely coalesced/agglutinated pyroclasts) and D (comprising dense lapilli to blocks, scoria lapilli, and bombs) were deposited during the continuous ‘sub-plinian’ eruption on the 19th of February 1975, from strombolian fire-fountaining activity at the base of the large eruption plume. Evidence for this is recorded by the mixture of scoria, agglutinated spatter, and dense clasts throughout these facies; which are indicative of a spatter-fed eruption.
Type of thesis
Geraghty, K. J. (2019). Precursory eruptions, magma priming and eruption dynamics associated with the climactic 19th February 1975 eruption of Ngauruhoe (Thesis, Master of Science (MSc)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/12789
The University of Waikato
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