An insight into Whakaari’s conduit: How altered tuffs and subsurface pressures can control volcano dynamics

Hydrothermal sealing is one of the mechanisms thought to aid pressure build-up within a volcano. Whakaari (White Island), New Zealand’s most active volcano, has a long history of phreatic and phreatomagmatic eruptions, and is ideally suited for an investigation into seal development from conduit-filling lithologies, where little prior experimental evidence exists. Here we provide an insight into Whakaari’s conduit by studying variably altered rocks ejected as ballistics. We find that hydrothermal alteration, particularly acid sulphate alteration, affects conduit-filling lithologies, lavas and tuffs, differently. In inherently low porosity lithologies like lavas, alteration increases fluid pathways by net dissolution of primary minerals and reduces rock stiffness. Counterintuitively, in tuffs that are inherently porous and permeable, alteration decreases fluid pathways by net precipitation of secondary minerals and increases rock stiffness. Such alteration-related pore filling of tuffs together with pore compaction under subsurface pressures can develop zones of low porosity and permeability within the volcano's conduit. When fluid injection rates are high, these zones could aid pressure build-up and predispose the volcano to eruptions. We discuss these results with observed seismicity at Whakaari and provide implications for ground deformation.