Dating young lava flows is essential for understanding volcano’s eruption frequency, yet challenging due to methodological limitations of commonly used dating techniques. Ruapehu (Aotearoa New Zealand) has produced many lava flows during the Holocene, but constraints on the timing of these eruptions are scarce. Here, we combine paleomagnetic dating and relative stratigraphy to deliver new eruption ages of 18 lava flows with low-to-high error window ranges between 500 and 2700 years (at the 95% confidence level). Our data indicate that the large lava flow field located on the Whakapapa area (northwest Ruapehu) was emplaced during at least three distinct eruptive episodes between 10.6 and 7.4 ka. Two of these episodes closely followed a large collapse event that affected Ruapehu’s northern edifice, and generated large volumes of lava between 10.6 and 8.8 ka, with the third episode producing less voluminous lava flows between 8.1 and 7.4 ka. Following a smaller collapse of the southeastern sector of the edifice at ca. 5.3 ka, several low-volume lava flows were emplaced during at least two distinct eruptive episodes prior to ca. 1.0 ka, which supplied the Whangaehu valley (east Ruapehu) with lava. The youngest age inferred from our data represents the youngest eruption age provided for a lava flow outside Ruapehu’s summit region. This research provides greater detail to the Holocene effusive chronology at Ruapehu, shedding light on partial cone reconstructions after edifice collapses during the early and late Holocene, and the time relationships between trends observed in its effusive and explosive activity.

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