Petrophysical and elastic properties of altered lavas from Mt. Taranaki: Implications for dome stability

In this project, we investigated the role of hydrothermal alteration in lava dome collapses at Mt. Taranaki., an active stratovolcano in New Zealand. We measured the mineral composition, crystallinity, porosity, permeability, and stiffness of variably altered rocks collected from the summit dome area and dome collapse deposits (block and ash flows) of the volcano. Our results showed that altered lavas were similar or even stiffer (stronger) than fresh lavas of similar porosity, due to precipitation of strong secondary minerals like silica and alunite instead of weak minerals like clays. This finding suggests that the role of hydrothermal alteration in the stability of lava domes is not merely dependent on the degree of hydrothermal alteration but also on the type. Alteration observed in our study shows that the dome weakening due to hydrothermal alteration is unlikely to have been the cause of past collapses. Instead, alteration-related precipitation of secondary minerals in permeable pathways could have caused pressurization by restricting fluid flow.