Understanding the provenance and trigger mechanisms of lahars at Mt. Ruapehu, NZ to construct new hazard analysis approaches : 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

Lahars (volcanic flows of mixed water and sediment) represent one of the most frequent and significant volcanic hazard at Mt. Ruapehu, New Zealand. However, modern hazard assessments primarily rely on short historical records (≤150 yr) despite the prehistoric record containing events that are orders of magnitude larger than those observed historically. This study addresses key knowledge gaps by integrating advanced stratigraphic and sedimentological understanding of the depositional characteristics of historic and modern deposits, spectral geochemistry to determine the provenance and triggers of the flows, and numerical modelling to better understand the source conditions of lahars to refine the late Holocene lahar history (approximately the last 2,000 years) of Ruapehu, ultimately improving hazard risk assessment.

The research focused initially on the Onetapu Formation on the eastern ring plain, involving a reassessment of lahar deposition to refine stratigraphy and define the relationships between discrete deposits. Detailed grain-size and lithofacies analyses were used to characterize deposits and revealed evidence of downstream debulking and contributions from the Wahianoa catchment following the 232 CE Taupo Ignimbrite emplacement.

A key methodological contribution to better understand the provenance and triggers of lahars was the development of a multi-criteria analytical approach, employing Short-Wave Infrared Reflectance (SWIR) spectroscopy supported by X-ray fluorescence (XRF), to successfully "fingerprint" lahar provenance and infer triggering mechanisms. SWIR proved capable of resolving hydrothermal alteration signatures (e.g., smectite and kaolinite) at abundances below the detection limits of bulk X-ray diffraction (XRD) methods, offering a sensitive, globally applicable forensic toolset for distinguishing flow origins.

Complementary numerical simulations investigated the dynamics of Crater Lake eruption induced waves, a frequent lahar trigger. Modelling underwater explosions using different cavity size formulations demonstrated that even individually small, bore-generating eruptions can rapidly escalate lahar hazard through cumulative lake drainage, particularly within Ruapehu’s multi-phase eruptive sequences.

The integrated results produce the most comprehensive analysis of Ruapehu's late Holocene lahar record to date. This work highlights the critical need for next-generation multi-trigger probabilistic models that explicitly account for changing physical conditions, such as lake volume, rainfall, eruptive state, and sediment availability, to provide more accurate evaluations of expected flow size and downstream impact for hazard management at Ruapehu and other lahar-prone volcanoes