Permeability development and degassing timescales in magmas.
Volatile exsolution in magmas is a primary driving force for volcanic eruptions
As magma rises, the decreasing pressure reduces the solubility of volatiles in the melt, leading to saturation. This process results in gas exsolution, followed by bubble nucleation, growth, and coalescence, all of which significantly influence magma ascent through the volcanic conduit. Individual bubbles may ascend within the magma or interact with neighboring bubbles to form connected pathways, enabling wholesale gas escape—a process known as permeability. The critical porosity at which this permeability develops is known as the percolation threshold. It is believed that variations in the development of the percolation threshold may be linked to differences in eruption intensity and transitions in eruption style, such as from strombolian to violent strombolian activity.
Permeability development as a function of crystal-free melt viscosity.
Prior to my research, the relative influence of melt viscosity versus crystal content on the percolation threshold was largely speculative. In Lindoo et al. (2016)*, I conducted decompression experiments on water-saturated rhyolite, rhyodacite, K-rich phonolite, and basaltic andesite melts. Following these experiments, I performed permeability and textural analyses on the quenched samples. My findings revealed that both silicic and mafic melts exhibit similar percolation thresholds (see figure), indicating that melt viscosity does not significantly impact the percolation threshold. However, melt viscosity does influence outgassing timescales by controlling the drainage and rupture of bubble films, which facilitates aperture formation.
In silicic magmas, where bubble nucleation, growth, and coalescence are often delayed, the vesicularity necessary for permeability development may not be achieved until the magma reaches shallow depths. As a result, ascending rhyolite magmas may not outgas quickly enough to relieve overpressure at ascent rates of 10 m/s or greater, leading to explosive eruptions. In contrast, although mafic magmas also require high vesicularities before degassing occurs, bubble coalescence happens more readily, allowing for rapid outgassing.
*Lindoo, A., Larsen, J.F., Cashman, K.V., Dunn, A.L., Neill, O.K., 2016. An experimental study of permeability development as a function of crystal-free melt viscosity, Earth and Planetary Science Letters, vol. 435, 9.
Crystal-controls on permeability development and degassing.
Building on the finding that melt viscosity does not significantly control permeability development, I conducted another series of decompression experiments to investigate the effect of microlites on the percolation threshold. In Lindoo et al. (2017)*, I discovered that mafic magmas containing more than 20 vol.% microlites (white symbols, top figure) became permeable at a lower vesicularity (lower percolation threshold) compared to their crystal-free counterparts (black symbols) and silicic crystal-free melts (gray symbols). The presence of over 20 vol.% crystals significantly alters vesicle shapes and likely enhances vesicle connectivity, as illustrated by the binary images in the figure.
This 20 vol.% crystal content corresponds to the critical volume fraction where prismatic crystals begin to interact—effectively a percolation threshold (bottom figure). At similar crystallinities, other studies have noted the onset of yield strength (hatched regions in the bottom figure), while the threshold for random loose packing (RLP) occurs at relatively higher particle fractions.
The presence of crystals not only affects the bulk rheology of magma but also its degassing efficiency. Understanding these critical thresholds provides valuable insights into changes in eruption dynamics, thereby improving volcanic hazard assessment.
*Lindoo, A., Larsen, J.F., Cashman, K.V., Oppenheimer, J., 2017. Crystal controls on permeability development and degassing in basaltic andesite magma. Geology; 45 (9): 831–834.
August 7-8 2008 eruption of Kasatochi Volcano.
On August 7th and 8th, 2008, Kasatochi Volcano, an island stratovolcano located in the Central Aleutian Islands, erupted explosively and unexpectedly. The eruption comprised three major explosive events and two smaller events over the course of a single day, producing a bulk eruptive volume and ash cloud height consistent with a Volcanic Explosivity Index (VEI) of 4. The eruptive units contained juvenile clasts with a wide range of compositions and textures, spanning from basaltic andesite (52-56 wt.%) to andesite (58-62 wt.%), including some clasts with a banded mix of both endmembers (see figure). The textural and lithological diversity observed during this eruption provided a unique opportunity to correlate these characteristics with eruption dynamics.
To better understand the coupled processes of crystallization and vesiculation during magma ascent, I quantitatively compared vesicle and crystal textures, permeabilities, and electrical tortuosities between the two endmember lithologies erupted from Kasatochi. This study was among the first to combine direct bulk measurements of connected vesicularity, permeability, and tortuosity with calculations of phenocryst and microlite crystallinities. The goal was to determine whether differences in magma composition and textural components influenced permeability development and degassing efficiency.