Identifying pyroclastic density currents from partial outcrop exposure at Mt Ruapehu, New Zealand.
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Mt Ruapehu is one of New Zealand’s most active volcanoes, last erupting in 2007. There have been few studies investigating the pyroclastic density current (PDC) hazard for Mt Ruapehu, despite being a popular tourist attraction. Due to the unpredictability of this hazard, it is crucial to be aware of past events in order to produce an effective plan for future PDCs.
On Mt Ruapehu, poor preservation of pyroclastic material due to past glaciations, erosion, and poor consolidation has led to significant uncertainty in distinguishing PDC deposits. This thesis aims to provide a rapid methodology to classify PDCs when doing whole volcano–scale mapping with limited sampling options. Multiple techniques in the field and laboratory were used to identify and characterise these deposits, and to create broad–scale implications for PDC frequency and magnitude. Comprehensive field-mapping over three months formed the basis for this study by identifying potential PDCs from partial exposures. A confidencebased pyroclastic identification chart developed by Cowlyn (2016) was used to support interpretations based on diagnostic textures of PDCs. Grain size distribution, vesicularity, and geochemical analyses of samples have been used to correlate deposits and infer the eruption style. The magnitude and volume of flows have been approximated using a digital elevation model and estimated flow paths.
This study identified 14 PDCs, adding to the 12 previously characterised by Cowlyn (2016). 50% of the PDCs identified occurred during Plinian to sub-plinian eruptions based on the deposit sizes and textural properties. These formed large volume, pumice-dominated deposits. 36% of the PDCS were likely formed during Strombolian eruptions when accumulated spatter underwent gravitational collapse. This eruption style formed variably welded, thermally altered, spatterrich deposits with denser clasts. The remaining 14% of PDCs formed during smaller, Vulcanian eruptions, creating small volume, scoria-dominated deposits. The results from this study have been integrated into a map with the identified PDC deposits and an updated account of PDC occurrence for Mt Ruapehu. Increasing the awareness of past PDCs can help inform hazard management and provide a foundation for subsequent studies to investigate future PDC scenarios on Mt Ruapehu.