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3D micro-deformation in hydrothermally altered andesites - Alteration-induced rock fracturing or fracture-induced alteration?

Maia Kidd 1, Mike Heap2, Gabor Kereszturi1, Shannen Mills1, Ben Kennedy3, Andrew Stevenson4,5, Benedicta Arhatari4, Jonathan Procter1

  • Affiliations:  1Volcanic Risk Solutions, School of Agriculture and Environment, Massey University, Palmerston North, New Zealand 2University of Strasbourg, Strasbourg Institute of Earth & Environment, Strasbourg, France 3School of Earth and Environment, University of Canterbury, Christchurch, New Zealand 4Australian Synchrotron, Australian Nuclear Science and Technology Organisation (ANSTO), Clayton, Vic, Australia 5Commonwealth Scientific and Industrial Research Organisation (CSIRO), Clayton, Vic, Australia 

  • Presentation type: Poster

  • Presentation time: Thursday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 34

  • Programme No: 3.8.18

  • Theme 3 > Session 8

Abstract

Volcanoes are inherently unstable, undergoing cycles of rapid construction and destruction due to eruptions and mass movements. Hydrothermal alteration weakens volcanic rocks, promoting phreatic eruptions via clogging outgassing pathways and volcanic landslide activity. This research quantifies the microscale effects of hydrothermal alteration on deformation. Micro-computed tomography (MCT) imaging at the Australian Synchrotron obtained high-resolution (6.5 µm voxel size) 3D maps of rock structures before and after deformation, complemented by scanning electron microscope (SEM) data from undeformed samples. Deformation experiments were conducted under 0, 5, or 10 MPa confining pressures, simulating conditions within a hydrothermal system. 35 samples from three New Zealand volcanoes---Ruapehu, Tongariro, and Whakaari---were analysed, representing a diverse range of alteration mineralogies (unaltered to phyllic), porosities (0.07--0.55), textures (e.g., flow banding, veining, brecciation, grain sizes), and lithologies (dense lava, scoria, breccia, other volcaniclastic). Digital image correlation of before-and-after MCT scans enable a detailed quantification of strain localisation and fracturing. Combined with textural and mineralogical analysis provided by 2D computed tomography images and SEM with energy-dispersive X-ray spectroscopy, the impact of hydrothermal alteration changes to rock properties is contextualised within the 3D deformation space. Our findings provide insight into how alteration influences strain localisation, fracture propagation, and deformation processes. These microscale observations can be upscaled to volcanic rock masses, shedding light on strain localisation in heterogeneous volcanic environments. Ultimately, this work advances understanding of hydrothermal alteration contributing to debris avalanche initiation and over pressurisation leading to phreatic eruptions, improving hazard assessment for volcanic systems.