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Composite volcanoes can progressively weaken through hydrothermal alteration, which may lead to volcano collapse, forming far-reaching debris avalanches. This work reviews and synthesises the type and extent of hydrothermal alteration on Mt. Ruapehu, New Zealand, from rock mechanical, mineralogical, hyperspectral imaging and aero-magnetic studies for input into the first debris avalanche models of Mt. Ruapehu. Mt. Ruapehu shows surface weathering processes, forming goethite, hematite and phyllosilicate assemblages, and hydrothermally altered rock, with phyllosilicate, Fe- oxides and sulphates, pyrite, jarosite, alunite, anhydrite and sulphur. The surface alteration mapped by hyperspectral remote sensing is linked to the deep-seated (≤500 m) alteration by aero-magnetic inversion modelling. Textural analysis and mapping show that alteration weathering and porosity distribution are a result of the both the primary lithology, age, and location relative to active or fossil hydrothermal systems. The synthesis of the aero-magnetic data, previous magnetotelluric models, hyperspectral and hydrothermal mapping, and rock mechanics studies enabled the construction of a schematic model to provide inputs for modelling potential flank failure zones. These models suggest that the southwest and west flanks of Mt. Ruapehu have the highest probability of collapsing and generating debris avalanches. [ABSTRACT FROM AUTHOR]

Volcaniclastic diamictons, while found in all ancient and modern volcanic settings across New Zealand, are most commonly associated with stratovolcanoes. Lahar refers to a volcanic mass flow that consists of variable amounts of water and particles of different-sized volcanic lithologies; such flows can exhibit a range of rheologies that rapidly change at temporal and spatial scales. At Mt. Taranaki time packages of lahar deposits were originally grouped into chronostratigraphic formations, such as the 22,500–11,000 yrs B.P. Warea Formation, which consists of lahar deposits that display a range of sediment-water interactions reflecting variable rheological regimes. In historic times, lahars have been rare at Mt. Taranaki with the most recent events occurring in 1999 and 2008. In contrast, Mt. Ruapehu has been extremely active over the last 170 years with regular small-volume eruption-induced events and larger, more significant Crater-Lake breakout lahars. The Onetapu Formation comprises flows of the last 2,000 years that are 1–2 orders of magnitude greater in volume, stage and discharge. To better understand the hazard potential of such events, a range of numerical modelling and simulations have been created, with computational simulations being applied to hindcast and forecast the potential hazards of lahars from New Zealand's stratovolcanoes. [ABSTRACT FROM AUTHOR]

Mt Ruapehu is an andesitic to dacitic composite volcano active over the past 250 ky. Prolonged volcanic activity can induce surface and deep-seated hydrothermal alteration that turns primary pyroxene and plagioclase-bearing rocks into secondary clay minerals. Such clay-dominated alteration minerals have unique spectral response in the Near- to Shortwave Infrared (1000 to 2500 nm) of the electromagnetic spectrum. This study utilizes airborne hyperspectral imagery of Mt Ruapehu to map surface hydrothermal alteration zones. The hyperspectral image was acquired with an AisaFENIX full-spectrum (370-2500 nm) push-broom sensor in 2018. The hundreds of narrow spectral bands on this sensor allows detailed discrimination of surface mineralogy on a spatial scale of 1-2 m. The hydrothermal alteration was mapped using band ratios (e.g. bands centred around 2120 nm) and non-parametric supervised image classification approaches, such as support vector machine and random forest with recursive feature elimination. The resulting geological map is validated using existing geological maps and field validation during 2018. The lab-based spectroscopy identified kaolinite and smectite groups as the main mineral phase, using USGS spectral libraries and spectral angle mapper matching algorithm. This is interpreted to be due to shallow surface-related, low-temperature argillic alteration due to meteoric and hydrothermal fluids. The type and distribution of hydrothermal alteration can highlight potential source areas that are structurally weak and might provide material for mass flows (e.g. debris avalanches and landslides). The mapped surface alteration can change at depth, highlighting the limitation of airborne remote sensing approaches. Mt Ruapehu does not have any obvious shallow hydrothermal activity beyond the crater rim; however, older parts of the current volcanic edifice might be altered considerably, causing potential areas of weakness. This can be overcome in the future with the newly acquired aero-magnetic data that allows us to model the sub-surface distribution of hydrothermal alteration through the destruction of magnetite. The mapped source areas will provide inputs for future numerical simulations of mass flows hazards, contributing to long-term land-use planning, volcano monitoring, and natural hazard mitigation at Mt. Ruapehu. [ABSTRACT FROM AUTHOR]