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Volcanology of the Raventhorpe and Pokeno West Volcanic Complexes, South Auckland Volcanic Field

The South Auckland volcanic field hosts at least 82 volcanic centres throughout the Pukekohe, Bombay, Tuakau, Pukekawa and Onewhero regions. The monogenetic basaltic volcanic field was active between 1.59 to 0.51 Ma, producing tuff rings, maar craters, scoria cones, and basaltic lava flows covering an area of 300 km2. Two volcanic complexes have been studied in this thesis: the Raventhorpe volcanic complex, and the Pokeno West volcanic complex. The Raventhorpe complex hosts six interconnected and overlapping tuff rings (the Raventhorpe and five Ingram Road tuff rings) and the Rutherford Road scoria cone with several lava flows. The Pokeno West volcanic complex hosts a single tuff ring and surrounding lava flows. Each complex has been mapped and stratigraphic logs of exposed sections have been constructed. Measurements have been made of componentry (juveniles, lithics, crystals) and vesicularity of juvenile clasts. Petrography of the tuff rings, mineralogy (including selected electron microprobe analyses) and geochemistry by XRF has been undertaken to characterise the deposits. These data have been compiled and used for facies analysis, and to determine styles of eruption and their controlling factors. The tuff ring facies of Raventhorpe include finely fragmented laminated to thinly bedded surge beds, coarse planar fall beds, and a massive magmatic block and bomb bed. The data show evidence for eruption drying out in the early stages moving to a Strombolian phase before reverting back to a phreatomagmatic style. Petrographic data indicates the Kaawa Formation aquifers were very important in the eruption dynamics, with the localisation of vents controlled by the St. Stevens Fault. The facies of the Pokeno West tuff ring include finely fragmented laminated or thinly bedded surge beds, cross bedded surge beds, planar bedded fall beds, and massive magmatic coarse lapilli beds. The data suggest variable eruption styles occurred alternating between phreatomagmatic and Strombolian. Petrographic analysis of lithic clasts indicates the Kaawa Formation aquifers were important in the eruption dynamics and the localisation of the tuff ring could be related to the Waikato or Pukekawa faults. The presence of the Kidnappers Ignimbrite allows for a new age constraint based on stratigraphy (>1.0 Ma). GIS analysis of the South Auckland volcanic field indicates fault location is the main control on the distribution of volcanic centres. The type of volcanism occurring is related to the hydrogeological parameters of underlying strata, which throughout the Manukau Lowlands is the Kaawa Formation. However, there are several dry magmatic volcanic centres in the Manukau Lowlands which could indicate another control by rate of magma flux which if rapid enough could prevent water-magma interaction. The distribution of age constrained centres has been statistically verified to be random.
Type of thesis
Taylor, S. N. (2012). Volcanology of the Raventhorpe and Pokeno West Volcanic Complexes, South Auckland Volcanic Field (Thesis, Master of Science (MSc)). University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/6516
University of Waikato
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