Mechanical behaviour and failure modes in the Whakaari (White Island volcano) hydrothermal system, New Zealand

Volcanic hydrothermal systems host a prodigious variety of physico-chemical conditions. The physico-chemical state and mechanical behaviour of rocks within is correspondingly complex and often characterised by vast heterogeneity. Here, we present uniaxial and triaxial compression experiments designed to investigate the breadth of mechanical behaviour and failure modes (dilatant or compactant) for hydrothermally-altered lava and ash tuff deposits from Whakaari (White Island volcano) in New Zealand, a volcano with a well-documented and very active hydrothermal system. Our deformation experiments show that the failure mode of low porosity lava remains dilatant over a range of depths (up to pressures corresponding to depths of about 2 km). Upon failure, shear fractures, the result of the coalescence of dilatational microcracks, are universally present. The high porosity ash tuffs switch however from a dilatant to a compactant failure mode (driven by progressive distributed pore collapse) at relatively low pressure (corresponding to a depth of about 250 m). We capture the salient features of the dynamic conditions (e.g., differential stress, effective pressure) in a schematic cross section for the Whakaari hydrothermal system and map, for the different lithologies, areas susceptible to either dilatant vs. compactive modes of failure. The failure mode will impact, for example, the evolution of rock physical properties (e.g., porosity, permeability, and elastic wave velocity) and the nature of the seismicity accompanying periods of unrest. We outline accordingly the potential implications for the interpretation of seismic signals, outgassing, ground deformation, and the volcanic structural stability for Whakaari and similar hydrothermally-active volcanoes worldwide.