Fracking magma : A field and experimental investigation of hydrofracturing in volcanic systems

Unwin, Holly and Tuffen, Hugh and James, Michael and Wadsworth, Fabian and Phillips, Emrys and Cuss, Robert (2023) Fracking magma : A field and experimental investigation of hydrofracturing in volcanic systems. PhD thesis, Lancaster University.

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Abstract

Rhyolite eruptions typically begin with the explosive ejection of pyroclastic material, with the potential to form widespread ash plumes that can have worldwide impacts. These eruptions then transition to effusive activity, characterised by the gentler emission of lava. Due to the scarcity of rhyolite eruptions, the processes that control the dynamics of these eruptions are poorly understood, despite the potentially significant hazards. Tuffisites — pyroclast-filled fractures that form within and adjacent to silicic volcanic vents at different stages of an eruption — are thought to provide insights into the processes that control the formation and evolution of silicic volcanic vents, and therefore influence the dynamics of a resulting eruption. Tuffisites are more permeable than the surrounding country rock, leading to suggestions that tuffisites may be able to allow significant volumes of gas to escape the conduit, potentially reducing sufficient excess pressure to moderate the explosivity of an eruption and change its hazards. This thesis aims to uncover new details about the formation of tuffisites and constrain whether tuffisite-enabled outgassing might be significant on the timescale of an eruption (Animation 1, Appendix A.1). By extrapolating this knowledge of tuffisite formation to the evolution of silicic vents, this thesis then aims to use tuffisites to gain insights into the processes that control eruption dynamics. This work finds that tuffisites form throughout the evolution of silicic vents, above the level of fragmentation in the conduit. Tuffisites can form by the injection of multiple pulses of pyroclastic material into a fracture, and therefore can be interpreted as records of fluctuations of the fluid pressure within the volcanic conduit during an eruption. By combining particle-size distributions with porosity and permeability measurements, this thesis finds that the opening of the fractures that host tuffisites can allow for the pulsed escape of large volumes of gas from the volcanic conduit, potentially influencing the dynamics of an eruption.