Hydrothermal alteration impacts on composite volcanoes rock mechanics: a New Zealand perspective

Abstract:

Volcanic hydrothermal systems are common to andesitic composite volcanoes in New Zealand and internationally. The hydrothermal systems and deposits on each volcano are unique, with different mineralogy, spatial distributions, and pervasiveness. Hydrothermal systems induce complex physical and chemical changes to the primary volcanic host rock. The feedback between hydrothermal alteration and rock properties are vital to constrain volcanic hazards, such as phreatic eruptions and volcanic flank stability.

In this study, we present mechanical and mineralogical data from Whakaari, Tongariro, and Ruapehu, selected to complement existing data on Whakaari and Ruapehu while providing the first mechanical data on Tongariro. The samples cover a range of alteration type, including (advanced) argillic and phyllic alteration with varying pervasiveness. Laboratory measurements of porosity, permeability, elastic wave velocity, and uniaxial compressive strength were used jointly with SEM-EDS and reflectance spectroscopy to quantify hydrothermal alteration history and physical properties. Our data is consistent with published data, showing trends of decreasing rock strength and elastic velocity waves with increasing alteration, while porosity and permeability increases with alteration. For the same porosity (~10%) and lithology (lava), we found a reduction in strength from ~100 MPa unaltered samples, ~80 MPa intermediate argillic samples, and ~30 MPa advanced argillic samples. Some samples show opposite trends, with increasing rock strength from ~100 MPa to ~150 MPa due to silicic alteration, but these are only found on Tongariro. While the overall trends in mechanical and alteration data are consistent across all three volcanoes, the alteration pervasiveness and styles differ. Ruapehu and Whakaari have crater-hosted systems, with highly pervasive alteration within these selected areas, and Tongariro has a dispersed hydrothermal system resulting in lower alteration degrees over a wider area. Our data has implications for how hazards such as phreatic eruptions and flank stability are modelled and highlights the importance of having individualised data for each volcano.