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Sulfate aerosols are a primary driver of climate impacts during and following volcanic eruptions and form from erupted SO2 gas. However, the amount of SO2 that is delivered to the stratosphere is not clearly related to the amount dissolved in the magma (the ‘sulfur excess problem’). Therefore, magma properties and eruption magnitude are not necessarily predictive of climate impacts from eruptions, which is exacerbated by the as-yet unknown importance of the insulated, hot transport pathway. During a magnitude 6 explosive volcanic eruption there is up to 100 seconds of transport between the magma fragmentation depth – where volcanic ash is formed and the mixture accelerates – and the Earth’s surface. Here, we present a numerical implementation of a theoretical framework which predicts the rapid reactions between gases and volcanic ash in this transport interval, which include: (1) iron oxidation state changes; (2) SO2 uptake via calcium sulfate surface crystallization; (3) HCl uptake via NaCl surface crystallization; and (4) incipient nanolite crystallization that may be related to (1). In all cases, these processes are rate-limited by a suite of diffusive exchanges between the ash bulk and surface, for which our model solves. To demonstrate the upscaled importance of these processes, we couple our models to volcanic plume simulations (using a 1991 Pinatubo baseline simulation), and output the bulk SO2 that can be captured by ash. We find that depending on the source parameters of the eruption, anywhere between 30 and 100 wt.% of the total erupted SO2 can be removed from the plume gas and captured by ash. This effectively changes the sink of SO2 from the stratosphere to the hydrosphere, as CaSO4 crystals are soluble and ultimately wash into the environment following ash deposition. We propose that these hot sulfur scrubbing processes may be crucial in mediating SO2 delivery to the atmosphere, and therefore may explain much of the complexities associated with correlating eruption magnitude with climate impacts in the recent past or back into the Last Glacial period.

Magma ascent pathways open when pressurised gas-ash mixtures overcome the strength of the surrounding rock to form fractures. Gas-ash mixtures are injected into the propagating fractures, and if ash is deposited this early stage of dyke or conduit evolution is preserved as a tuffisite vein. Once a conduit has become established, fractures formed in the country rock adjacent to the volcanic conduit are also injected with gas-ash mixtures to form tuffisites. If tuffisites allow for the significant escape of magmatic gases from the main conduit zone, tuffisites might dissipate sufficient pressure to moderate eruption style from explosive to effusive. The hot particles within the tuffisite, however, sinter together through time, reducing tuffisite permeability until gas no longer flows. Despite their potentially important role in controlling eruption dynamics, the length of time that a tuffisite may remain permeable and the flux of gas that tuffisites can allow to escape are poorly constrained.The dimensions of tuffisites vary, but fractures are typically tens of centimetres to tens of metres in length. The internal structure of tuffisites can be complex, often consisting of multiple units of pyroclastic material, varying from massive lithic breccias to stratified tuffs. Erosion and deposition, due to the repeated injection of ash-laden fluid, produces a variety of sedimentary structures, from cross-lamination and graded bedding to soft-sediment deformation and internal injections. These structures record how the velocity and particle volume fraction of the injected fluid fluctuated through time, controlled by the fluid pressure gradient along the fracture. Tuffisites can therefore be interpreted as a fossil record of the fluid pressure fluctuations occurring during the opening of magmatic pathways.We aim to quantify and reconstruct the fluid overpressure at different stages of tuffisite evolution. A large sub-horizontal tuffisite (0.9 m wide, >40 m long) formed at 500 m depth at the dissected rhyolitic Húsafell volcano, Iceland, has been used to constrain the pressures of tuffisite formation1. The pressure required to open fractures within the Húsafell tuffisite host rocks (basalt, friable ignimbrite, densely welded ignimbrite) has been constrained experimentally by injecting samples with pressurised water. The dimensions of different units within the Húsafell tuffisite suggest overpressures of 1.9-3.3 MPa would be needed for the emplacement of the largest units seen (0.1 cm thick and 40 m long), using a simple fracture opening model1.Sintering of hot particles within the tuffisite fill reduces tuffisite permeability through time, hindering outgassing. The porosity, permeability, and particle sizes of different units within the Húsafell tuffisite allow us to constrain the possible gas flux carried by tuffisite and how this would have evolved through time. By combining constraints on the fluid pressure, permeability, and sintering timescales of the Húsafell tuffisite we aim to gain insight into the processes controlling tuffisite formation, and whether tuffisites might permit sufficient outgassing to moderate eruption explosivity.1. Unwin et al. (2021) doi:10.3389/feart.2021.668058

Crystal bearing magmas have long been known to be non-Newtonian, exhibiting clear shear-thinning features when exposed to shear stresses. However, the micromechanical origin of this shear thinning remains enigmatic and attempts to describe how shear thinning arises in magmas have been equivocal. Here, we demonstrate that in controlled experimental systems, shear thinning is a non-local and scale-dependent artefact of crystal organisation during flow, and is not therefore an intrinsic property of the crystal-bearing magmas sensu stricto. Furthermore, we show that most experimental approaches to crystal-bearing magma rheology sit in a regime in which crystal migration effects will be dominant, explaining why the experimental evidence is that non-Newtonian effects are observed. We use a numerical conduit model for crystal migration physics to demonstrate that in nature, volcanic systems sit in another regime altogether and will not organise crystal cargo on the time- and length-scales of magma ascent. This leads us to tentatively conclude that crystal-bearing magmas on Earth are Newtonian, and that the only non-Newtonian effects of concern relate to bubbles at moderate capillary number, and the melt phase at moderate Weissenberg number. Finally, we note that this goes some way to unify the mismatch in effective viscosity between low-temperature analogue experiments and experiments on natural crystal-bearing melts.

The material behaviours that underpin artistic work with glass have striking similarities to those of volcanic glasses, silicate melts, and magmas. This similarity presents a compelling opportunity for collaboration between glass artists and volcanologists to better understand silicate melts, and thereby to elucidate volcanic processes. Using silicate melts as an experimental material is advantageous, because it allows us to investigate coupled thermorheological effects that are not well captured by the more widely used low-temperature magma analogue materials such as syrups and oils. Here we present work focussed on adapting and refining the artistic, kiln-based process of bubble entrapment and the precision manipulation of gas phases in glass. These techniques have previously been used to create glass art pieces involving complex bespoke figures formed from gas trapped within layers of glass. We expand the use of this technique to investigate bubble rise dynamics in soda-lime-silicate glass under non-isothermal conditions; varying temperature to slow or accelerate the bubble rise.We present a scaled mathematical model that provides an excellent description of the experimental data even in case of complex changing temperature environments similar to the long, slow annealing stages of glass art production. Scaling analysis is used to show how our experiments using artistic soda-lime-silica glass can be scaled to volcanic environments. This allows a better understanding of bubble motion in magmas under variable temperature conditions, such as those in shallow basaltic lava lakes, flows, or the uppermost parts of magma plumbing systems. This art-science collaboration used artistic skills and knowledge to validated our model, and tested the limits of the artistic technique: the use of kiln-based heating programs and the glass used by artists. Overall, the advantages of using kiln-based art techniques and skills for volcanology is clear. But we also find that there is a deep mutuality to glass art collaborations, such that the art techniques can be refined via the scientific model development. Ultimately, when working to better understand the behaviours of a complex material such as glass, the approaches of the artist and the scientist are very similar, and our project shows that the presumed disciplinary divide may be less divisive than expected.

The tensile strength of volcanic rock exerts control over several key volcanic processes, including fragmentation, magma chamber rupture, and dyke propagation. However, and despite this importance, values of tensile strength for volcanic rocks are relatively rare. It is also unclear how their tensile strength is modified by rock physical properties such as porosity, pore size, and pore shape and ongoing processes such as hydrothermal alteration. We present here the results of systematic laboratory and numerical experiments designed to better understand the influence of porosity, microstructural parameters (pore size, shape, and orientation), and hydrothermal alteration on the tensile strength of volcanic rocks. Our data show that tensile strength is reduced by up to an order of magnitude as porosity is increased from 0.01 to above 0.3, highlighting that porosity exerts a first-order control on the tensile strength of volcanic rocks. Our data also show that pore diameter, pore aspect ratio, and pore orientation can also influence tensile strength. Finally, our data show that hydrothermal alteration can decrease tensile strength if associated with mineral dissolution and weak secondary minerals, or increase tensile strength if associated with pore- and crack-filling mineral precipitation. We present a series of theoretical and semi-empirical constitutive models that can be used to estimate the tensile strength of volcanic rocks as a function of porosity or alteration intensity. To outline the implications of our data, we show how tensile strength estimations can influence predictions of magma overpressures and assessments of the volume and radius of a magma chamber, and we explore the influence of alteration using discrete element method modelling in which we model the amount and distribution of damage within variably-altered host-rock surrounding a pressurised dyke. It is our hope that the experiments, models, and understanding provided by our study prove useful for modellers that require the tensile strength of volcanic rocks for their models.

Ash emission in explosive silicic eruptions can have widespread impacts for human health, agriculture, infrastructure, and aviation. Estimates of the total grainsize distribution (TGSD) generated during explosive magma fragmentation underpins eruption models and ash dispersal forecasts. Conventionally, the TGSD constrained via erupted deposits is assumed to match the TGSD produced at explosive fragmentation. Here we present observations from within the vent of a recent rhyolitic eruption (Cordón Caulle, Chile, 2011–2012), demonstrating that fine and ultra-fine ash particles are captured and sintered to fracture surfaces, and thus sequestered in the shallow subsurface, rather than emitted. We establish a conceptual model—uniquely contextualised through a combination of syn-eruptive observations and detailed post-eruption field investigation—in which turbophoresis (particle migration towards zones of lower turbulence) and rapid sintering create an inverse relationship between particle size and the probability of its subsurface capture. Such size-dependent capture efficiency preferentially removes submicron-diameter ash from the erupted componentry, decoupling the erupted size distribution from magmatic source conditions and potentially playing an important role in modulating eruption dynamics.

Formation of surficial sulfate– and halide–bearing salts by syn–eruptive ash–gas interactions is known to occur during volcanic eruptions. For reactions between aluminosilicates and the gas SO2, at...