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Products and Processes of Cone-Building Eruptions at North Crater, Tongariro

North Crater occupies the north-western quadrant of the Tongariro Volcanic Centre and represents one of at least eleven vents which have been active on Tongariro since the last glacial maximum. The most recent cone-forming activity at North Crater is thought to have occurred between 14-12 ka ago to produce the distinct, wide, flattopped andesite cone. This project focused mainly on the cone-building eruptions at North Crater, including stratigraphic correlations with distal tephra, interpreting eruptive processes, and establishing the sequence of events during cone construction. Detailed field work identified key stratigraphic sections and facies in the proximal, medial and distal environments. These sections allowed stratigraphic correlations to be made between proximal cone-building facies and distal sheet-forming facies at North Crater, and established a complete North Crater eruption stratigraphy. In the proximal environment, welded and non to poorly welded facies formed from fallout of a lava-fountain, pyroclastic flow or as fallout from a convecting plume. In the medial and distal environment, the lithofacies consist of fallout from a convecting plume and minor pyroclastic flow. Convective fall and non to poorly welded pyroclastic flow deposits dominate the lower eruption stratigraphy suggesting explosive eruptions involving a gas-rich magma. A change to welded deposits produced from lava-fountaining occurs later in the cone-building sequence and suggest a change to lower explosively and eruption of gas-poor magma. Grain size, componentry data, density, petrography and SEM analysis were carried out on representative samples to characterise the different facies, and reveal information about eruption processes. The non to poorly welded deposits are typically made up of vesicular pumice, scoria and mingled clasts of sub-rounded bombs and lapilli. The welded facies are relatively dense and clast outlines are often difficult to distinguish. The eruptives are porphyritic with abundant plagioclase gt clinopyroxene gt orthopyroxene gt opaques. Quartzofeldspathic crustal xenoliths are common and indicate crustal assimilation. Mingled clasts of light and dark glass were found to have microlites present in the dark glass, but were absent in the light glass. Electron microprobe analyses found that the dark and light glass components in a single clast had similar compositions, showing that the contrasting physical appearance of the glass is not due to a different chemical composition. Forty three whole rock XRF analyses showed that the magmas ranged from basaltic andesite to andesite, and Harker variation plots display linear trends typical of magma mixing. Magma mixing as the most important magmatic process is supported by disequilibrium of phenocryst compositions and phenocryst textures. Magma viscosity, bulk density and temperature was determined using MAGMA (Kware), and indicate that they fall into the range of typical andesites. Eruptive activity involved vigorous lava-fountaining, minor convecting eruption plumes and dominant collapsing eruption plumes. This activity has produced welded and non-welded pyroclastic flow and fall deposits to form the large cone seen today. There are significant volcanic hazards associated with this style of activity at North Crater, characterised by lava-fountaining, eruption plume fallout, and widespread pyroclastic flows and lahars extending beyond the ring plain. These could all be potentially devastating to the central North Island of New Zealand.
Type of thesis
Griffin, A. M. (2007). Products and Processes of Cone-Building Eruptions at North Crater, Tongariro (Thesis, Master of Science (MSc)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/2235
The University of Waikato
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