Understanding the largest-scale explosive volcanism at Mt. Taranaki, New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Science, Massey University, Palmerston North, New Zealand

Abstract

Over the last 5000 years B.P., at least 53 explosive eruption episodes occurred at Mt. Taranaki, (western North Island, New Zealand) from either the summit-crater (2500 m), or a satellite vent on Fanthams Peak (1966 m). These eruptions are represented in wellpreserved pyroclastic successions on the upper volcano flanks. At least 16 episodes produced deposits with lithostratigraphic characteristics comparable to those of the last sub-Plinian eruption at AD 1655, suggesting an average recurrence of one Plinian/sub- Plinian eruption episode every 300 years. Several large-scale mafic-intermediate (~48-60 wt.% SiO2) eruption episodes sourced from the two vents were studied in detail to determine the “maximum” intensity, magnitude and eruptive styles from this volcano. These episodes comprised climactic phases with sustained and steady, 14-29 km-high eruption columns, often starting and ending with unsteady pulsating, oscillating and collapsing plumes. The columns erupted 0.1-0.5 km3 DRE at mass and volume discharge rates of 107-108 kg/s and 103-104 m3/s, respectively, indicating magnitudes of 4.1-5.1. The unsteady initial, pre- and post-climactic eruptive phases were dominated by domecollapse, column-collapse and lateral-blast pyroclastic density currents (PDCs), with runout distances of 3-19 km and volumes of up to 0.02 km3 DRE. The steadiest phases were associated with eruption of rheologically homogeneous magmas producing homogenous pumice textures. Unsteady phases produced density and porosity pumice gradients by magma stalling in upper conduit levels. Three eruption onset scenarios were developed from this work: a) initial closed-conduit decompression by vent unroofing and domecollapse, b) transient open and clogged conduits produced by repeated plugging-andbursting of chilled or gas-depleted magma, and c) rapid conduit opening with more mafic eruptives. In all scenarios, the climactic phases are comparable, with pyroclastic fallouts covering 1500-2500 km2. The most violent phases of these events, however, are lateralblast PDCs that could reach a broad arc between 14-19 km from source. This reappraisal of the hazardscape at Mt. Taranaki integrates many new details that enable a more realistic hazard management and provides a range of findings that can be applied to other similar andesitic volcanoes prior to reawakening.