The influence of pore size distribution on fracturing in lava domes

Dome-building eruptions are a complex expression of hybrid, effusive to explosive volcanism at the Earth’s surface. They are characterized by frequent and rapid shifts in eruptive style that make them highly unpredictable and hazardous. This multifaceted behavior is strongly influenced by the thermal/rheological evolution and mechanical response of lavas and rocks in the dome, and the efficiency of gas release through the edifice. Gas escape or entrapment in lava domes depends primarily on the permeability of the porous networks at different scale. Fracturing is crucial in developing connected and permeable pore pathways and thus in enabling and regulating gas escape efficiency in lava domes. In this study, we use X-ray micro-tomographic data obtained on a dacite from a block-and-ash flow deposit from the 1990-1995 eruption of Mt Unzen (Japan). We simulate pore-controlled fracturing in the 3D volume through a watershed-type image analysis, through which we explore the combined role of fractures and pore size distribution on pore connectivity and permeability. We use two different starting pore networks: one consisting of macro-porosity surrounded by micropores and a second set consisting of macropores only, yielding two distinct starting pore size distributions and pore connectivities. We quantify the influence of crack number density and width on the evolution of pore connectivity with porosity for the two pore networks. We then use Lattice Boltzmann simulations to quantify the porosity-permeability relationships of virtually cracked rocks. Our results show that as cracking progresses in a crystal-rich dome rock, connectivity and permeability strongly increase. The initial pore size distribution has a strong impact on crack propagation as well as connectivity and permeability. Our dataset also suggests that connectivity for a given pore network depends mostly on crack-number density, whereas permeability is more sensitive to crack width. Combining measurements of connectivity and permeability may be key when assessing the extent and mode of fracturing in volcanic edifices such as lava domes. We compare our simulations with mechanical tests of uniaxial compressive strength on similar Unzen dacites in order to link the impact of initial porosity and pore size distribution on crack propagation during failure. We finally discuss the implications of our results for the stability and eruptive style of silicic lava domes.