Eruption dynamics and frequency-magnitude relationships of explosive eruptions at Mt. Ruapehu, New Zealand over the past 1800 years : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Science at Massey University, Palmerston North, New Zealand

Abstract

Small to moderate explosive eruptions (VEI ≤ 3) constitute the most frequent eruptions and often involve several phases characterised by different eruption dynamics. Deposits associated with small-scale multi-phase eruptions tend to be underrepresented in geological records and the resulting probabilistic eruption forecast models. This PhD research presents a refined high-resolution tephrostratigraphic framework for the 1800-year Tufa Trig Formation at one of New Zealand’s most active volcanoes, Mt. Ruapehu. This framework is used to characterise short- and long-term changes in eruption behaviour aiming to identify time-variable processes in the volcanic system of a long-lived andesite volcano. Systematic mapping and lithosedimentological characterisation of tephra deposits are combined with geochemical fingerprinting and radiocarbon dating to create a detailed frequency-magnitude record of single- and multi-phase eruptions of the last 1800 years. At least 32 eruptions can be identified, ranging from low to mid-intensity single-phase eruptions (1–10 × 10⁶ m³ deposit volumes) to complex multi-phase eruptions up to two magnitudes larger. The largest eruption is the T13-sequence that comprises at least 5 eruption phases. Multi-lobate dispersal pattern and componentry analyses show that individual eruption phases represent multiple fall events of similar eruption style and magnitude. Major and trace element analyses of juvenile glass display limited syn-sequence variability, while heterogeneous pyroclast and textural characteristics suggest that short-term changes in eruption behaviour are predominantly controlled by shallow conduit processes. The frequency-magnitude record is integrated with geochemistry and statistical modelling, identifying time-variable pattern in Mt. Ruapehu’s eruption behaviour: the time span 1718–1300 cal BP involves low-intensity single-phase eruptions every ~40 years and is followed by a low rate regime (one eruption every 125 years). The largest multi-phase eruptions of the last two millennia occur between 610 and 370 cal BP, while the past 370 years are dominated by smaller multi-phase eruptions every ~40 years, suggesting that long-term changes in Mt. Ruapehu’s eruption behaviour are related to changes in magma supply. This research adds critical complexity to the understanding of the processes and timescales controlling eruption behaviour in the southern Taupo Volcanic Zone and provides insights into the dynamic behaviour of small to moderate multi-phase eruptions. These results will constitute the framework for refining dynamic eruption forecast models at Mt. Ruapehu and other similar volcanoes globally.