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

Abstract

Hydrothermal alteration is generally associated with the weakening of volcanic rocks. Here we evaluate the possible role of hydrothermal alteration in lava dome collapses at Mt. Taranaki by evaluating the petrophysical (mineralogy, fluid pathways, crystallinity) and elastic properties (stiffness) of variably altered lavas from its summit dome area and block and ash flow deposits. Our results show that acid-sulfate alteration changes the mineralogy of the lavas by dissolving primary feldspars, pyroxenes, amphiboles, Fe-Ti oxides, and volcanic glass and precipitating secondary alunite and silica. These changes alter the fluid pathways and crystallinity of the lavas. However, despite these alteration-related petrophysical changes, we find that altered lavas are stiffer, and inferentially stronger, than fresh lavas of similar porosity. We attribute this at least partially to the precipitation of relatively strong secondary minerals like alunite and silica instead of weaker minerals like clays. We discuss the implications of these findings for dome stability at Mt. Taranaki. We suggest that the role of hydrothermal alteration in weakening volcanic rocks is not merely dependent on the alteration intensity but also on the type of alteration. Altered lavas, without extensive dissolution and with precipitation of strong secondary minerals, are unlikely to weaken the dome.