PhD Thesis: Geophysical Signatures of Hydrothermal Alteration for Volcano Monitoring

Abstract

Hydrothermal alteration changes the petrophysical properties of stratovolcanoes world-wide. But, how these changes affect volcano dynamics and manifest themselves in geo-physical data used to monitor and study the internal structure of volcanoes is not as simple as suggested by prior investigations. This thesis aims to address this challenge by systematically measuring, in the laboratory, the petrophysical and geophysical (elastic and magnetic) properties of variably altered volcanic rocks, from Whakaari and Mt. Taranaki, New Zealand. This thesis adds three new ideas to the current understanding of acid-sulfate hydrothermal alteration in volcanoes.

First, in volcanic conduits, acid-sulfate alteration affects the fluid pathways and elastic properties of lavas, tuffs, and breccias differently. In lavas, alteration creates fluid pathways and decreases rock stiffness by net dissolution. In contrast, in the inherently porous and permeable tuffs, alteration reduces fluid pathways and increases rock stiffness by net precipitation of secondary minerals. Compaction of tuffs under subsurface pressures and alteration-related sealing can form low porosity and low permeability zones within the conduit. Such zones could promote fluid-pressure build-up and predispose the volcano to explosive eruptions.

Second, altered lavas can have higher remanent magnetization (NRM) than fresh lavas. For most rocks from Whakaari and Mt. Taranaki, NRM dominates induced magnetization (susceptibility), highlighting the importance of measuring both induced and remanent magnetization of samples used to constrain field-scale data. These findings urge caution in assuming that altered regions in volcanic environments would only manifest as areas of reduced magnetization in field magnetic surveys. Altered regions posing a potential edifice collapse hazard could be associated with high magnetization resulting from high NRM.

Third, the role of hydrothermal alteration in weakening volcanic rocks is not merely dependent on the degree of alteration but also on the type of alteration. Altered rocks with precipitation of strong secondary minerals and without extensive dissolution, are unlikely to have been substantially weakened to cause slope instabilities. The implications of the thesis results are discussed in the context of analyzing and interpreting ground deformation and magnetic survey data for volcano monitoring. Furthermore, the elastic and magnetic properties data presented in this thesis provides needed constraints for geophysical inversions of acid-sulfate altered volcanic regions.