Products and Processes of Cone-Building Eruptions at North Crater, Tongariro
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 http://hdl.handle.net/10289/2235
Permanent Research Commons link: http://hdl.handle.net/10289/2235
North Crater occupies the north-western quadrant of the Tongariro Volcanic Centreand represents one of at least eleven vents which have been active on Tongariro sincethe last glacial maximum. The most recent cone-forming activity at North Crater isthought to have occurred between 14-12 ka ago to produce the distinct, wide, flattoppedandesite cone. This project focused mainly on the cone-building eruptions atNorth Crater, including stratigraphic correlations with distal tephra, interpretingeruptive 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 tobe made between proximal cone-building facies and distal sheet-forming facies atNorth Crater, and established a complete North Crater eruption stratigraphy. In theproximal environment, welded and non to poorly welded facies formed from falloutof a lava-fountain, pyroclastic flow or as fallout from a convecting plume. In themedial and distal environment, the lithofacies consist of fallout from a convectingplume and minor pyroclastic flow. Convective fall and non to poorly weldedpyroclastic flow deposits dominate the lower eruption stratigraphy suggestingexplosive eruptions involving a gas-rich magma. A change to welded depositsproduced from lava-fountaining occurs later in the cone-building sequence andsuggest a change to lower explosively and eruption of gas-poor magma.Grain size, componentry data, density, petrography and SEM analysis were carriedout on representative samples to characterise the different facies, and revealinformation about eruption processes. The non to poorly welded deposits are typicallymade up of vesicular pumice, scoria and mingled clasts of sub-rounded bombs andlapilli. The welded facies are relatively dense and clast outlines are often difficult todistinguish. The eruptives are porphyritic with abundant plagioclase gt clinopyroxene gtorthopyroxene gt opaques. Quartzofeldspathic crustal xenoliths are common andindicate crustal assimilation. Mingled clasts of light and dark glass were found tohave microlites present in the dark glass, but were absent in the light glass. Electronmicroprobe analyses found that the dark and light glass components in a single clasthad similar compositions, showing that the contrasting physical appearance of theglass is not due to a different chemical composition.Forty three whole rock XRF analyses showed that the magmas ranged from basalticandesite to andesite, and Harker variation plots display linear trends typical of magmamixing. Magma mixing as the most important magmatic process is supported bydisequilibrium of phenocryst compositions and phenocryst textures. Magmaviscosity, bulk density and temperature was determined using MAGMA (Kware), andindicate that they fall into the range of typical andesites.Eruptive activity involved vigorous lava-fountaining, minor convecting eruptionplumes and dominant collapsing eruption plumes. This activity has produced weldedand 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 NorthCrater, characterised by lava-fountaining, eruption plume fallout, and widespreadpyroclastic flows and lahars extending beyond the ring plain. These could all bepotentially devastating to the central North Island of New Zealand.
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