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

Abstract

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.