A microstructural and micromechanical investigation into shear dynamics during volcanic edifice collapse on Ascension Island: An experimental approach

During gravitational collapse flows, shear forces are expressed through localised or diffuse, brittle or ductile strain. Understanding material responses to shear within gravitational collapse flows can be achieved through microstructural and micromechanical investigation using established experimental techniques. This thesis investigates a shear zone generated during a volcanic debris avalanche following the collapse of a scoria cone on Green Mountain, Ascension Island through (1) quantitative data on microstructural evolution within the shear zone through Scanning Electron Microscope imaging, and (2) experimental work using rotary shear apparatus to constrain the mechanical behaviour of the material under stress, and its influence on internal microstructure. Microstructural analysis of the Green Mountain shear zone reveals a decrease in grain size and porosity, as well as clast morphology evolution toward the principal slip zone in the centremost region. Such observations are mirrored in experimental shear zones presented herein. Mechanical data provide evidence that material saturation promotes dynamic velocity weakening behaviour at seismic velocities. Based on observations and evidence presented in this thesis, a model for shear dynamics during the Green Mountain volcanic debris advance is proposed. It is suggested that (1) a brittle cataclastic regime dominated within the shear zone, resulting in the microstructural characteristics observed and (2) processes to facilitate velocity weakening behaviour may include pore pressure fluidisation and nanoparticle lubrication. Overall, this work contributes to the understanding of shear localisation, internal microstructure, and facilitators of mechanical behaviour within the Green Mountain volcanic debris avalanche deposit. Application of these findings to other deposits and associated shear zones may help to better constrain collapse behaviour and to mitigate associated risks.