The geomechanical characterization of volcanic material has important implications 16 for geothermal and mineral exploration, engineering design, geophysical signals of volcano 17 unrest, and models of instability and mass flows. Chemical weathering and hydrothermal 18 systems can alter the host rock, leading to changes in mechanical behavior and failure mode. 19 Here, we compare the physical and mechanical properties of lava, autoclastic breccia, and 20 pyroclastic (scoria) samples from Mt. Ruapehu volcano in New Zealand to mineralogical 21 composition determined via infrared spectroscopy and scanning electron microscopy (SEM) with 22 energy-dispersive X-ray spectroscopy (EDS). Laboratory-based spectroscopy shows that the 23 samples contain absorption features indicative of Al-rich hydrous phyllosilicates, Fe-and Mg-24 rich varieties, and sulfates attributed to surface weathering, supergene, and steam-heated 25 alteration. We find that porosity and primary lithology (i.e. sample origin) is the predominant 26 control on physical and mechanical properties, followed by the pervasiveness of 27 weathering/alteration, and then mineralogical composition. Several properties, such as porosity, 28 uniaxial compressive strength, P-wave seismic velocity, density, and Young’s modulus, show 29 strong linear and exponential correlations to other properties, indicating the potential for transfer 30 functions between these properties. Samples near the active hydrothermal system with high 31 intensity hydrothermal alteration do not follow typical physical and mechanical property trends 32 due to high clay content, low permeability, and low strength. The presence of these rocks within 33 the edifice at Mt. Ruapehu implies local barriers to fluid flow and subsequent pore pressure 34 variation, and producing anomalously shallow brittle-ductile transition zones. Additionally, 35 material in the upper conduit area of Mt. Ruapehu could be over three times weaker than typical 36 porosity-strength trends observed in surface rocks, increasing the likelihood of structural 37 collapse. Trends in the pervasiveness of weathering with physical and mechanical properties 38 suggest that it may be possible to extrapolate these properties from imaging spectroscopy, which 39 could be used to create spatially distributed geotechnical maps in volcanic environments. 40 41 Keywords: uniaxial compressive strength, permeability, porosity, triaxial compressive strength, 42 intact rock mi, andesite, failure mode, hydrothermal alteration, weathering, argillic alteration 43 44 45 46 Confidential manuscript submitted to Engineering Geology

Prolonged volcanic activity can induce surface weathering and hydrothermal alteration that is a primary control on edifice instability, posing a complex hazard with its challenges to accurately forecast and mitigate. This study uses a frequently active composite volcano, Mt Ruapehu, New Zealand, to develop a conceptual model of surface weathering and hydrothermal alteration applicable to long-lived composite volcanoes. The rock samples were classified as non-altered, supergene argillic alteration, intermediate argillic alteration, and advanced argillic alteration. The first two classes have a paragenesis that is consistent with surficial infiltration and circulation of the low-temperature (40 degree C) neutral to mildly acidic fluids, inducing chemical weathering and formation of weathering rims on rock surfaces. The intermediate and advanced argillic alterations are formed from hotter (100 degree C) hydrothermal fluids with lower pH, interacting with the andesitic to dacitic host rocks. The distribution of weathering and hydrothermal alteration has been mapped with airborne hyperspectral imaging through image classification, while aeromagnetic data inversion was used to map alteration to several hundred meters depth. The joint use of hyperspectral imaging complements the geophysical methods since it can numerically identify hydrothermal alteration style. This study established a conceptual model of hydrothermal alteration history of Mt Ruapehu, exemplifying a long-lived and nested active and ancient hydrothermal system. This study highlights the need to combine mineralogical information, geophysical techniques and remote sensing to distinguish between current and ancient hydrothermal and supergene alteration systems, to indicate the most likely areas of future debris avalanche initiation.