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Measurements of the composition-dependent viscosity of rhyolitic magma reveal a tipping point that changes the physical properties of the melt and controls the transition between effusive and explosive eruptions. A change of eruption style Calcalkaline rhyolites produce the largest explosive volcanic eruptions, but these eruptions can switch repeatedly between being effusive and explosive. This is difficult to attribute to the rheological effects of magma water content or crystallinity. Danilo Di Genova and co-authors report the viscosity of a series of melts spanning the compositional range of the Yellowstone rhyolitic volcanic system. They find that, within a narrow compositional zone, melt viscosity increases by up to two orders of magnitude, which they propose to be the consequence of melt structure reorganization. The authors confirm that such a compositional tipping point exists in the global geochemical record of rhyolites, which separates effusive from explosive deposits. They conclude that the anhydrous (water-free) composition of calcalkaline rhyolites is decisive in determining mobilization and eruption dynamics of the Earth's largest volcanic systems. The most viscous volcanic melts and the largest explosive eruptions 1 on our planet consist of calcalkaline rhyolites 2 , 3 . These eruptions have the potential to influence global climate 4 . The eruptive products are commonly very crystal-poor and highly degassed, yet the magma is mostly stored as crystal mushes containing small amounts of interstitial melt with elevated water content 5 . It is unclear how magma mushes are mobilized to create large batches of eruptible crystal-free magma. Further, rhyolitic eruptions 6 , 7 , 8 can switch repeatedly between effusive and explosive eruption styles and this transition is difficult to attribute to the rheological effects of water content or crystallinity 9 , 10 . Here we measure the viscosity of a series of melts spanning the compositional range of the Yellowstone volcanic system and find that in a narrow compositional zone, melt viscosity increases by up to two orders of magnitude. These viscosity variations are not predicted by current viscosity models 11 , 12 and result from melt structure reorganization, as confirmed by Raman spectroscopy. We identify a critical compositional tipping point, independently documented in the global geochemical record of rhyolites, at which rhyolitic melts fluidize or stiffen and that clearly separates effusive from explosive deposits worldwide. This correlation between melt structure, viscosity and eruptive behaviour holds despite the variable water content and other parameters, such as temperature, that are inherent in natural eruptions. Thermodynamic modelling demonstrates how the observed subtle compositional changes that result in fluidization or stiffening of the melt can be induced by crystal growth from the melt or variation in oxygen fugacity. However, the rheological effects of water and crystal content alone cannot explain the correlation between composition and eruptive style. We conclude that the composition of calcalkaline rhyolites is decisive in determining the mobilization and eruption dynamics of Earth’s largest volcanic systems, resulting in a better understanding of how the melt structure controls volcanic processes.

Magma viscosity strongly controls the style (for example, explosive versus effusive) of a volcanic eruption and thus its hazard potential, but can only be measured during or after an eruption. The identification of precursors indicative of magma viscosity would enable forecasting of the eruption style and the scale of associated hazards 1 . The unanticipated May 2018 rift intrusion and eruption of Kīlauea Volcano, Hawai‘i 2 displayed exceptional chemical and thermal variability in erupted lavas, leading to unpredictable effusion rates and explosivity. Here, using an integrated analysis of seismicity and magma rheology, we show that the orientation of fault-plane solutions (which indicate a fault’s orientation and sense of movement) for earthquakes preceding and accompanying the 2018 eruption indicate a 90-degree local stress-field rotation from background, a phenomenon previously observed only at high-viscosity eruptions 3 , and never before at Kīlauea 4 – 8 . Experimentally obtained viscosities for 2018 products and earlier lavas from the Pu‘u ‘Ō‘ō vents tightly constrain the viscosity threshold required for local stress-field reorientation. We argue that rotated fault-plane solutions in earthquake swarms at Kīlauea and other volcanoes worldwide provide an early indication that unrest involves magma of heightened viscosity, and thus real-time monitoring of the orientations of fault-plane solutions could provide critical information about the style of an impending eruption. Furthermore, our results provide insight into the fundamental nature of coupled failure and flow in complex multiphase systems. Rotated fault-plane solutions in earthquake swarms at volcanoes could provide an early indication of relatively viscous magma, and hence of the style and hazard potential of an impending eruption.

The 15 January 2022 submarine eruption at Hunga volcano was the most explosive volcanic eruption in 140 years. It involved exceptional magma and seawater interaction throughout the entire submarine caldera collapse. The submarine volcanic jet breached the sea surface and formed a subaerial eruptive plume that transported volcanic ash, gas, sea salts and seawater up to ~ 57 km, reaching into the mesosphere. We document high concentrations of sea salts in tephra (volcanic ash) collected shortly after deposition. We also discuss the potential climatic consequences of large-scale injection of salts into the upper atmosphere during submarine eruptions. Sodium chloride in these volcanic plumes can reach extreme concentrations, and dehalogenation of chlorides and bromides poses the risk of long-term atmospheric and weather impact. Salt content in rapidly collected tephra samples may also be used as a proxy to estimate the water:magma ratio during eruption, with implications for quantification of fragmentation efficiency in submarine breaching events. The balance between salt loading into the atmosphere versus deposition in ash aggregates is a key factor in understanding the atmospheric and climatic consequences of submarine eruptions.

Catalytic nucleic acids, such as ribozymes, are central to a variety of origin-of-life scenarios. Typically, they require elevated magnesium concentrations for folding and activity, but their function can be inhibited by high concentrations of monovalent salts. Here we show that geologically plausible high-sodium, low-magnesium solutions derived from leaching basalt (rock and remelted glass) inhibit ribozyme catalysis, but that this activity can be rescued by selective magnesium up-concentration by heat flow across rock fissures. In contrast to up-concentration by dehydration or freezing, this system is so far from equilibrium that it can actively alter the Mg:Na salt ratio to an extent that enables key ribozyme activities, such as self-replication and RNA extension, in otherwise challenging solution conditions. The principle demonstrated here is applicable to a broad range of salt concentrations and compositions, and, as such, highly relevant to various origin-of-life scenarios.The correct function of ribozymes in a prebiotic world would be dependent on the presence of optimal salt compositions and concentrations. Now, local heat fluxes have been shown to create an ideal salt habitat for ribozyme activity based on geologically plausible salt-leaching processes.

During volcanic eruptions, solidifying magma ascends through the volcanic conduit, often accompanied by repetitive, drum-beat seismicity. Laboratory experiments on magma samples from Soufrière Hills Volcano, Montserrat, and Mount St Helens Volcano, USA, show that viscous melt formed at the surface between the rising magma and conduit walls can temporarily halt magma ascent, accentuating the cyclical seismicity. During volcanic eruptions, domes of solidifying magma can form at the volcano summit. As magma ascends it often forms a plug bounded by discrete fault zones, a process accompanied by drumbeat seismicity. The repetitive nature of this seismicity has been attributed to stick-slip motion 1 at fixed loci between the rising plug of magma and the conduit wall 2 , 3 . However, the mechanisms for such periodic motion remain controversial 4 , 5 , 6 , 7 . Here we simulate stick-slip motion in the laboratory using high-velocity rotary-shear experiments on magma-dome samples collected from Soufrière Hills Volcano, Montserrat, and Mount St Helens Volcano, USA. We frictionally slide the solid magma samples to generate slip analogous to movement between a magma plug and the conduit wall. We find that frictional melting is a common consequence of such slip. The melt acts as a viscous brake, so that the slip velocity wanes as melt forms. The melt then solidifies, followed by pressure build up, which allows fracture and slip to resume. Frictional melt therefore provides a feedback mechanism during the stick-slip process that can accentuate the cyclicity of such motion. We find that the viscosity of the frictional melt can help define the recurrence interval of stick-slip events. We conclude that magnitude, frequency and duration of drumbeat seismicity depend in part on the composition of the magma.

Seismogenic lavas: Explosive collapse Volcanoes typically undergo cycles of dome growth and — sometimes very explosive — collapse. The nature of the ductile-to-brittle transition in the highly crystalline lavas involved in volcanic dome-building eruptions may hold the key to an accurate description of dome growth and stability. Lavallée et al . have performed rheological experiments combined with continuous micro-seismic monitoring to reveal that such dome lavas are seismogenic and that the character of the seismicity changes markedly across the ductile-to-brittle transition until complete brittle failure occurs at high strain rates. The authors conclude that monitoring such magma seismicity may lead to improved forecasting for some volcanic eruptions. Lavallee and colleagues have performed rheological experiments on the lavas involved in volcanic dome-building eruptions. They find that these dome lavas are seismogenic, the nature of the seismicity changing across the ductile-to-brittle transition, and conclude that monitoring such magma seismicity may lead to improved forecasting for some volcanic eruptions. Volcanic dome-building episodes commonly exhibit acceleration in both effusive discharge rate and seismicity before explosive eruptions 1 . This should enable the application of material failure forecasting methods to eruption forecasting 2 , 3 . To date, such methods have been based exclusively on the seismicity of the country rock 4 . It is clear, however, that the rheology and deformation rate of the lava ultimately dictate eruption style 5 . The highly crystalline lavas involved in these eruptions are pseudoplastic fluids that exhibit a strong component of shear thinning as their deformation accelerates across the ductile to brittle transition 6 . Thus, understanding the nature of the ductile–brittle transition in dome lavas may well hold the key to an accurate description of dome growth and stability. Here we present the results of rheological experiments with continuous microseismic monitoring, which reveal that dome lavas are seismogenic and that the character of the seismicity changes markedly across the ductile–brittle transition until complete brittle failure occurs at high strain rates. We conclude that magma seismicity, combined with failure forecasting methods, could potentially be applied successfully to dome-building eruptions for volcanic forecasting.

Understanding the flow of multi-phase (melt, crystals and bubbles) magmas is of great importance for interpreting eruption dynamics. Here we report the first observation of crystal plasticity, identified using electron backscatter diffraction, in plagioclase in andesite dome lavas from Volcán de Colima, Mexico. The same lavas, deformed experimentally at volcanic conduit temperature and load conditions, exhibit a further, systematic plastic response in the crystalline fraction, observable as a lattice misorientation. At higher stress, and higher crystal fraction, the amount of strain accommodated by crystal plasticity is larger. Crystal plastic distortion is highest in the intact segments of broken crystals, which have exceeded their plastic limit. We infer that crystal plasticity precludes failure and can punctuate the viscous-brittle transition in crystal-bearing magmas at certain shallow magmatic conditions. Since crystal plasticity varies systematically with imposed conditions, this raises the possibility that it may be used as a strain marker in well-constrained systems. The rheological behaviour of magma in shallow conditions may help determine a volcano’s eruptive style. Here, the authors perform deformation experiments on lava from Volcán de Colima to demonstrate that crystal plasticity may preclude failure at certain shallow magmatic conditions.

Radon anomalies are commonly observed prior to dynamic failure in the crust and are interpreted as cracking of the medium, thus attracting considerable attention in understanding the precursory phenomena of earthquakes and volcanic activity. In this study we have compared the starting radon emissions from low porosity crystalline lava (phonolite) samples with those from damaged and failed samples. The damaged sample was loaded up to just beyond the end of the linear elastic phase, as evidenced by the output of AE energy, the increase in total porosity and a decrease in P‐wave and S‐wave velocity relative to the intact sample. Whereas, the failed sample showed deformation behaviour characteristically brittle with increasing values of AE output and porosity as the sample approached macroscopic failure. Radon measurements have evidenced that dilatational microcracking of deformed sample produced no significant variation in radon emanation with respect to the intact sample. In contrast, after macroscopic failure, radon emanation drastically increased. Therefore, major finding from this study is that, in the case of low porosity and relatively high strength crystalline lavas, the development of a macroscopic fracture provides new large exhaling surface resulting in a substantial increase in radon emission rate. Key Points Radon emission from rocks increases by means of macroscopic fractures Radon emission does no change in deformed low‐porosity crystalline lavas Radon emission is governed by the different rock types

Volcanic eruptions can be either slow outpourings of lava or explosive ejections of fragmented rock. In his Perspective, Dingwell discusses the physical and chemical properties of magma that lead to these two styles of eruption. New experiments and computer simulations are pointing to the involvement of solidlike behavior of liquid magma when it makes a transition to a glass.

The transition from Newtonian to non‐Newtonian flow of silicate melts is commonly manifested as shear thinning at conditions of high stress and strain rate. Shear thinning may strongly influence the dynamics of magmatic flows, but the details of its microscopic origins are not fully understood. Here we consider viscous heating and thermomechanical coupling as a potential cause of shear thinning. We compare the results of laboratory, uniaxial compression experiments of a silicate melt with the results of thermomechanical numerical simulations corresponding to the experimental setup. Both the experimental and numerical results concord and indicate that the reduction of the temperature‐dependent viscosity in flowing silicate melts is a result of viscous heating. Viscous heating was quantified for glasses with viscosities ranging from 108 to 1011 Pa s and strain rates from 10−5 to 100 s−1. The results of 48 compression experiments indicate that the transition from Newtonian to non‐Newtonian flow in the silicate melt occurs at a Brinkmann number (i.e., ratio of heat gained to heat lost) around 1 whereas brittle behavior dominates the melt deformation when the Deborah number (i.e., ratio of viscoelastic relaxation time to characteristic deformation time) is larger than around 0.01. The observed viscous heating significantly contributes to the viscosity decrease observed in high stress‐strain rate experiments and questions our current understanding of the non‐Newtonian deformation behavior of silicate melts. Key Points Experimental and numerical confirmation