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Eruptive dynamics is influenced by gas escape from the ascending magma. Gas pathways form in the magma via bubble coalescence, leading to gas channeling. Magmatic crystals play a key role in gas channel formation. This work constrains experimentally decompression‐induced coalescence in high‐crystallinity silicic magmas without external deformation, focusing on low gas content and bimodal crystal size (microlites and phenocrysts). All percolating samples have permeabilities of 10−14 m2 at bulk porosities of 7–10 vol% and bulk crystallinities up to 75 vol%. Our results demonstrate the possibility of coalescence‐related outgassing at high pressure (120–350 MPa) and without external strain, which corresponds to magma stagnating deep in a volcanic conduit. Channeling at such low gas content implies that bimodal crystallinity favors effusive over explosive volcanic behavior. It may also be the missing physical mechanism explaining gas transfer across magmatic systems despite high melt viscosity and low or absent magma extrusion. Plain Language Summary: The way volcanoes erupt is mainly controlled by the ability of gases to escape from the magma (outgassing). An efficient way to outgas is to connect bubbles together (coalescence) up to the point where channels form in the magma (channeling) in which gas can circulate toward the surface. We investigate experimentally how coalescence and channeling happen in immobile, crystal‐rich viscous magma in conditions (pressure and temperature) similar to those found in deep volcanic conduit, where only a small amount of gas (<10%) is present. Our experiments demonstrate that bubble connections are possible thanks to a large amount of crystals on which bubbles can lean, deform, and join each other. That gas can escape from magma at depth could favor effusive eruption over volcanic explosions and even bring new insights on degassing in immobile magma. Key Points: Gas permeability in silicic magma is possible with gas volumes below 10% thanks to high crystallinityOutgassing at low gas content can happen without external deformation nor fracturation of the magmaVolcanic outgassing in stagnant magmas is explained by deep conduit bubble channeling that may be a key mechanism in eruptive transition [ABSTRACT FROM AUTHOR]

Crystal-rich silicic lavas commonly erupt from hazardous lava dome-forming volcanoes, characterized by both effusive and explosive eruptions. Magma explosivity is inherently dependent on its ability to store pressurized gas, which can be released through permeable pathways like fractures or connected bubbles, yet the role crystals play in regulating gas escape is poorly constrained in crystal-rich systems. We explored the gas storage capacity and outgassing efficiency of crystal-rich magmas through experimental vesiculation of hydrous dacite samples containing crystal volume fractions ( ϕ x ) between 0.5 and 0.8. The maximum unconnected gas volume (isolated porosity) decreases exponentially with increasing crystallinity. We quantify the relative outgassing efficiency as a function of ϕ x using changes in isolated melt porosity during open-system degassing (outgassing). Mean isolated porosity, for ϕ x = 0.5, increases from ~ 0.33 at the start of outgassing to ~ 0.67 by the end, doubling its trapped bubbles. For ϕ x = 0.7, isolated porosity increases from ~ 0.1 to ~ 0.2, implying gas retention and outgassing efficiency are strongly dependent on crystallinity. Outgassing occurs rapidly via fracturing at porosities < 0.1 when ϕ x ≥ 0.7. Fracturing and bubble coalescence are both inefficient outgassing mechanisms at ϕ x = 0.5 due to viscoelastoplastic deformation, which leads to an increase of isolated porosity. Between ϕ x of 0.5 and 0.7, samples sustained a three-fold difference in isolated porosity, implying that gas retention and eruptive behavior of crystal-rich magmas may be controlled by the onset and efficacy of crack-dominated outgassing and can be modulated by relatively small changes in crystallinity. [ABSTRACT FROM AUTHOR]

Fluid inclusion microthermometry on olivines, clinopyroxenes, and amphiboles was used during a volcanic eruption, in combination with real-time seismic data and rapid petrographic observations, for petrological monitoring purposes. By applying this approach to the study of 18 volcanic samples collected during the eruption of Tajogaite volcano on La Palma Island (Canary Islands) in 2021, changes in the magma system were identified over time and space. Magma batches with distinct petrographic and geochemical characteristics emerged from source zones whose depth progressively increased from 27 to 31 kilometers. The rise of magma of deeper origin is attested by fluid inclusions made of N2 and CO, markers of mantle outgassing. Magma accumulation occurred over different durations at depths of 22 to 27 and 4 to 16 kilometers. Time-integrated magma ascent velocities (including ponding times) were estimated at between 0.01 and 0.1 meters per second. This method is cost-effective and quickly identifies changes in the magma system during an eruption, enhancing petrological monitoring procedures. [ABSTRACT FROM AUTHOR]

Silicate vapors play a key role in planetary evolution, especially dominating early stages of rocky planet formation through outgassed magma ocean atmospheres. Our open-source thermodynamic modeling software "VapoRock" combines the MELTS liquid model with gas-species properties from multiple thermochemistry tables. VapoRock calculates the partial pressures of 34 gaseous species in equilibrium with magmatic liquid in the system Si–Mg–Fe–Al–Ca–Na–K–Ti–Cr–O at desired temperatures and oxygen fugacities (f O2, or partial pressure of O2). Comparison with experiments shows that pressures and melt-oxide activities (which vary over many orders of magnitude) are reproduced to within a factor of ∼3, consistent with measurement uncertainties. We also benchmark the model against a wide selection of igneous rock compositions including bulk silicate Earth, predicting elemental vapor abundances that are comparable to (Na, Ca, and Al) or more realistic than (K, Si, Mg, Fe, and Ti) those of the closed-source MAGMA code (with maximum deviations by factors of 10–300 for K and Si). Vapor abundances depend critically on the activities of liquid components. The MELTS model underpinning VapoRock was calibrated and extensively tested on natural igneous liquids. In contrast, MAGMA's liquid model assumes ideal mixtures of a limited set of chemically simplified pseudospecies, which only roughly approximates the nonideal compositional interactions typical of many-component natural silicate melts. Finally, we explore how relative abundances of SiO and SiO2 provide a spectroscopically measurable proxy for oxygen fugacity in devolatilized exoplanetary atmospheres, potentially constraining f O2 in outgassed exoplanetary mantles. [ABSTRACT FROM AUTHOR]

In 1999–2001, Guagua Pichincha volcano, Ecuador, produced a series of cyclic explosive and effusive eruptions. Rock samples, including dense blocks and pumiceous clasts collected during the eruption sequence, and ballistic bombs later collected from the crater floor, provide information about magma storage, ascent, decompression, degassing, repressurization, and regassing prior to eruption. Pairs of Fe-Ti oxides indicate equilibrium within 1.2–1.5 log units above the NNO oxidation buffer and equilibrium temperatures from 805 to 905 °C. Melt inclusions record H2O contents of 2.7–4.6 wt% and CO2 contents (uncorrected for CO2 segregation into bubbles) from 19 to 310 ppm. Minimum melt inclusion saturation pressures fall between 69 and 168 MPa, or equilibration depths of 2.8 and 6.8 km, the lower end of which is coincident with the maximum inferred equilibration depths for the most vesicular breadcrust bombs sampled. Amphibole phenocrysts lack breakdown rims (except for one sample) and plagioclase phenocrysts have abundant oscillatory compositional zones. Plagioclase areal microlite number densities (Na) range over less than one order of magnitude (8.9×103–8.7×104 mm-2) among all samples, with the exception of a dense, low crystallinity sample (Na = 3.0×103 mm−2) and a pumiceous sample erupted on 17 December 1999 (Na = 1.7×103 mm−2). Plagioclase microlite shapes include tabular, hopper, and swallowtail forms. Taken together, the relatively high plagioclase microlite number densities, the high number of oscillatory zones in plagioclase phenocrysts, the presence of CO2 in groundmass glass, seismicity, and time-varying tilt cycles provide a picture of sudden evacuation of magma residing at different levels in the shallow conduit. Explosive eruptions punctuate inter-eruptive repose periods marked by time-varying rates of degassing (volatile fluxing) and re-pressurization. Shallow residence time in the conduit was sufficient to allow precipitation of silica-phase in the groundmass, but insufficient to allow breakdown of hornblende phenocrysts, with the one exception of the final dome sample from 2000, which has the longest preceding repose time. These results support a model of cyclic pressure cycling, volatile exsolution and regassing, and magma decompression decoupled from ascent. [ABSTRACT FROM AUTHOR]

The chemical composition of gases emitted by active volcanoes reflects both magma degassing and shallower processes, such as fluid-rock hydrothermal interaction and mixing with atmospheric-derived fluids. Untangling the magmatic fluid endmember within surface gas emission is therefore challenging, even with the use of well-known magma degassing tracers such as noble gases. Here, we investigate the deep magmatic fluid composition at the Nisyros caldera (Aegean Arc, Greece) by measuring nitrogen and noble gas abundances and isotopes in naturally degassing fumaroles. Gas samples were collected from 32 fumarolic vents at water-boiling temperature between 2018 and 2021. These fumaroles are admixtures of magmatic fluids typical of subduction zones, groundwater (or air saturated water, ASW), and air. The N 2 , He, and Ar composition of the magmatic endmember is calculated by reverse mixing modeling and shows N 2 /He = 31.8 ± 4.5, N 2 /Ar = 281.6, δ 15N = +7 ± 3 ‰, 3He/4He = 6.2 Ra (where Ra is air 3He/4He), and 40Ar/36Ar = 551.6 ± 19.8. Although N 2 /He is significantly low with respect to typical values for arc volcanoes (1,000–10,000), the contribution of subducted sediments to the Aegean Arc magma generation is reflected by the positive δ 15N values of Nisyros fumaroles. The low N 2 /He ratio indicates N 2 -depletion due to solubility-controlled differential degassing of an upper-crustal silicic (dacitic/rhyodacitic) melt in a high-crystallinity reservoir. We compare our 2018–2021 data with N 2 , He, and Ar values collected from the same fumaroles during a hydrothermal unrest following the seismic crisis in 1996–1997. Results show additions of both magmatic fluid and ASW during this unrest. In the same period, fumarolic vents display an increase in magmatic species relative to hydrothermal gas, such as CO 2 /CH 4 and He/CH 4 ratios, an increase of ∼50 °C in the equilibrium temperature of the hydrothermal system (up to 325 °C), and greater amounts of vapor separation. These variations reflect an episode of magmatic fluid expulsion during the seismic crisis. The excess of heat and mass supplied by the magmatic fluid injection is then dissipated through boiling of deeper and peripheral parts of the hydrothermal system. Reverse mixing modeling of fumarolic N 2 -He-Ar has therefore important ramifications not only to disentangle the magmatic signature from gases emitted during periods of dormancy, but also to trace episodes of magmatic outgassing and better understand the state of the upper crustal reservoir. [ABSTRACT FROM AUTHOR]

Water inventories in Martian magmas are poorly constrained. Meteorite‐based estimates range widely, from 102 to >104 ppm H2O, and are likely variably influenced by degassing. Orbital measurements of H primarily reflect water cycled and stored in the regolith. Like water, Th behaves incompatibly during mantle melting, but unlike water Th is not prone to degassing and is relatively immobile during aqueous alteration at low temperature. We employ Th as a proxy for original, mantle‐derived H2O in Martian magmas. We use regional maps of Th from Mars Odyssey to assess variations in magmatic water across major volcanic provinces and through time. We infer that Hesperian and Amazonian magmas had ∼100–3,000 ppm H2O, in the lower range of previous estimates. The implied cumulative outgassing since the Hesperian, equivalent to a global H2O layer ∼1–40 m deep, agrees with Mars' present‐day surface and near‐surface water inventory and estimates of sequestration and loss rates. Plain Language Summary: Past volcanism on Mars has supplied some of the water that carved ancient river valleys and shaped the chemistry of the Martian near‐surface. However, the amount of water carried by Martian magmas is an open question, in part because igneous rocks and meteorites have often lost their original water contents through degassing. The trace element thorium can be used as a proxy for magmatic water, because thorium and water are transferred in similar proportions to magmas during mantle melting, but thorium does not degas. We use regional maps of thorium from the Mars Odyssey spacecraft to track variations in magmatic water through time and across major volcanic provinces. Key Points: Thorium partitions similarly to H2O during Martian mantle melting but does not degas, and is useful as a proxy for primary magmatic H2OGamma Ray Spectroscopy maps of thorium distribution are used to track variations in primary magmatic H2O through Mars' historyWe infer that Hesperian and Amazonian magmas had ∼100–3,000 ppm H2O, implying outgassing of a global H2O layer ∼1–40 m deep [ABSTRACT FROM AUTHOR]

• Textural analysis was performed on basanitic deep submarine lavas from the 2018–2021 Fani Maoré eruption. • Pillow selvages are either highly vesiculated (max 50%) or very massive (<1%). • Heterogeneous textures result from ascent and effusion rate variations with time. • Fani Maoré lavas experienced different degrees of degassing and outgassing. • This submarine lava flow fields was dominantly tube-fed due to high effusion rates (11–200 m3/s). The 2018–2021 Fani Maoré submarine eruption (offshore of Mayotte, Mozambique Channel) extruded a bulk volume of ∼6.5 km3 of basanite magma onto the seafloor at a depth of 3300 m, with effusion rates ranging from 150 to 200 m3/s in the first year of the eruption, to less than 11 m3/s in the final months. Six oceanographic campaigns provided a large sample set covering the entire flow field at high spatial and temporal resolution. These samples allow us to precisely track syn-eruptive degassing processes through quantification of textural parameters including porosity, pore connectivity, vesicle number density (N V) and vesicle size distributions (VSD). Three different textural facies have been distinguished. (1) Vesicular lavas (average porosity of 35%) display unimodal VSDs, high N V (14–214 mm−3), and small and spherical vesicles. (2) Lavas with intermediate porosities (25%) have scarce small vesicles, VSDs shifted towards larger vesicles, and low N V (0.2–39 mm−3). (3) Dense lavas with low porosities (14%) display bimodal VSDs distribution, a dominant mode of small vesicles, and low N V (0–87 mm−3). The early phase of activity (Phase 1, June 2018 – May 2019) built the main edifice and was fed by rapid ascent and closed-system degassing of volatile-rich magma ascending from a deep reservoir to the seafloor (Facies 1). Distal samples collected from lava flows emitted during Phase 2, between June and July 2019, show large and irregular shape vesicles mostly related to bubble growth and coalescence, and outgassing during emplacement (Facies 2). These lavas are interpreted to be emplaced during extension of a lava tube system which began to develop during Phase 1. The final phase (Phase 3, August 2019 – January 2021) was associated with lava effusion located at the northwest lava flow front, 6 km from the summit. Phase 3 involved a more degassed magma due to the increase in the length of the magma pathway (Facies 3). Phase 3 lavas were also extremely outgassed and associated with construction of a new complex lava flow field with tumuli and multiple ephemeral vents (lava breakouts). The heterogeneous textures within the studied samples reflect changing ascent and effusion rates with time, leading to emplacement of lava flows which varied depending on the degree of degassing and effusion rate. We conclude that emplacement of the Fani Maoré large submarine lava flow fields developed through extensive and prolonged tube systems this being supported by the high effusion rates. [ABSTRACT FROM AUTHOR]

Magmas vesiculate during ascent, producing complex interconnected pore networks, which can act as outgassing pathways and then deflate or compact to volcanic plugs. Similarly, in-conduit fragmentation events during dome-forming eruptions create open systems transiently, before welding causes pore sealing. The percolation threshold is the first-order transition between closed- and open-system degassing dynamics. Here, we use time-resolved, synchrotron-source X-ray tomography to image synthetic magmas that go through cycles of opening and closing, to constrain the percolation threshold ΦC at a range of melt crystallinity, viscosity and overpressure pertinent to shallow magma ascent. During vesiculation, we observed different percolative regimes for the same initial bulk crystallinity depending on melt viscosity and gas overpressure. At high viscosity (> 106 Pa s) and high overpressure (~ 1–4 MPa), we found that a brittle-viscous regime dominates in which brittle rupture allows system-spanning coalescence at a low percolation threshold (ΦC~0.17) via the formation of fracture-like bubble chains. Percolation was followed by outgassing and bubble collapse causing densification and isolation of the bubble network, resulting in a hysteresis in the evolution of connectivity with porosity. At low melt viscosity and overpressure, we observed a viscous regime with much higher percolation threshold (ΦC > 0.37) due to spherical bubble growth and lower degree of crystal connection. Finally, our results also show that sintering of crystal-free and crystal-bearing magma analogues is characterised by low percolation thresholds (ΦC = 0.04 – 0.10). We conclude that the presence of crystals lowers the percolation threshold during vesiculation and may promote outgassing in shallow, crystal-rich magma at initial stages of Vulcanian and Strombolian eruptions. [ABSTRACT FROM AUTHOR]

Volatile exsolution is widely considered to be capable of generating magmatic overpressure and triggering volcanic eruptions. Despite its role as an eruption trigger, exsolution-driven overpressurisation is relatively poorly understood. Part of the problem is that thermodynamic models do not consider how the behaviour of small quantities of magma scales up to reservoir level – where variations in temperature and crystallinity become important. In contrast, many thermomechanical models focus only on magma injection, and do not consider how overpressure evolves spatially or temporally, when related to crystallisation and volatile exsolution. Here, we use Rhyolite-MELTS to track exsolution-driven overpressure during cooling for a variety of natural compositions, storage pressures, initial volatile contents and magmatic X H2O (molar H 2 O/ (CO 2 + H 2 O)). We then couple these outputs to a thermal model to determine the timescales and spatial extent of overpressurisation with varying volatile content. We find that the highest overpressures occur in magmas which are initially at their H 2 O solubility limit, with the addition or removal of H 2 O resulting in a decrease in peak overpressure. We also find that maximum overpressure decreases with the addition of CO 2 (decreasing X H2O) at typical magma storage pressures of 100–230 MPa. The higher overpressures generated at the volatile 'sweet spot' have a greater potential to trigger eruptions – or to favour their initiation by making the system more susceptible to other triggers, such as magma injection. The reduction in overpressure with increasing or decreasing initial H 2 O suggests that triggering by volatile exsolution is less likely for these magmas. Peak overpressure at the volatile sweet spot also coincides with an increased incidence of explosive eruptions at water contents ∼4–5.5 wt%. This suggests that higher magmatic overpressures may produce more explosive eruptions, by driving faster initial ascent rates and decreasing outgassing efficiency in the conduit. Our thermal modelling demonstrates that, for small magmatic systems, exsolution-driven overpressurisation can operate on timescales which are much shorter than the crustal relaxation timescale. In these cases, overpressure cannot be dissipated by a visco-elastic crustal response, and therefore has the potential to trigger a volcanic eruption. • Highest overpressure occurs for magmas initially at H 2 O solubility limit. • Overpressure increases with increasing X H2O. • Critical overpressure can be attained within crustal relaxation timescales. • High overpressures may drive faster ascent rates and influence eruption style. [ABSTRACT FROM AUTHOR]

Only a small fraction (approx. 1–20%) of magmas generated in the mantle erupt at the surface. While volcanic eruptions are typically considered as the main exhaust pipes for volatile elements to escape into the atmosphere, the contribution of magma reservoirs crystallizing in the crust is likely to dominate the volatile transfer from depth to the surface. Here, we use multiscale physical modelling to identify and quantify the main mechanisms of gas escape from crystallizing magma bodies. We show that most of the outgassing occurs at intermediate to high crystal fraction, when the system has reached a mature mush state. It is particularly true for shallow volatilerich systems that tend to exsolve volatiles through second boiling, leading to efficient construction of gas channels as soon as the crystallinity reaches approximately 40–50 vol.%. We, therefore, argue that estimates of volatile budgets based on volcanic activity may be misleading because they tend to significantly underestimate the magmatic volatile flux and can provide biased volatile compositions. Recognition of the compositional signature and volumetric dominance of intrusive outgassing is, therefore, necessary to build robust models of volatile recycling between the mantle and the surface. [ABSTRACT FROM AUTHOR]

The permeability of magmas to percolating fluids plays a role in controlling eruption explosivity. While measurements of volcanic rock permeability abound, comparison of these values with models that relate hydraulic properties to random heterogeneous pore structures are few. Here, we present such a comparison. First, we generate numerical materials composed of randomly placed spherical objects in a periodic control volume using two dominant pore structure families: (1) where the space between the spherical objects represents the pore phase that can transmit fluid, and (2) where the spherical objects themselves are the pore phase that can transmit fluid. In the former case, we envisage the system to be analogous to initially granular magmas that undergo welding events that serve to progressively reduce the porosity. In the latter case, we envisage the system to be analogous to vesiculating magmas that evolve to high porosities. In both cases, we constrain relationships for fundamental structural parameters for every porosity including the specific surface area and the percolation thresholds and we compare these with theoretical relationships. Importantly, we run lattice Boltzmann method simulations of fluid flow through these systems at a wide range of porosities to constrain the steady state Darcian fluid permeability using the LBflow code. We finally compare these permeability values with (1) other published simulations, (2) published permeability data for natural volcanic rocks or experimental analogues thereof, and (3) theoretical constraints derived from first principles. In all cases, we obtain a good agreement between our idealized numerical random systems and the natural data, as well as results of other theoretical and experimental approaches. We propose that the simple models that we derive and validate can be of wide utility in building more complex models for the evolution of magmas during shallow ascent with broad implications for outgassing efficiency and the development of pore overpressure immediately prior to eruption. [ABSTRACT FROM AUTHOR]

Magmatic-hydrothermal fluids transport chalcophile metals to the atmosphere as volcanic gases; and to the crust, where they may play a role in the formation of ore deposits. Global volcanic gas datasets show considerable variability in the flux and composition of metals outgassed between volcanoes, but the controls on this variability are unclear. Magmatic chloride is a key ligand for metal transport but magmatic water dominates the exsolved fluid reservoir into which metals partition during crystallisation and decompression. Here we develop models simulating decompression-driven degassing ('first boiling') and isobaric crystallisation-driven degassing ('second boiling') of magmas to show that while moderate concentrations of chlorine are essential for metal partitioning into the magmatic fluids, magmatic water contents have the greatest potential to control the mass yield of metals carried by exsolving fluids. Our models explain why water-rich magmatic-volcanic systems like Mount Etna (Italy) deliver the largest mass fluxes (per unit of degassing magma) of metals to the atmosphere, whereas relatively dry magmatic-volcanic systems like Yasur (Vanuatu) deliver the smaller mass fluxes. Our results establish the important role of magmatic water in generating large reservoirs of metal-rich aqueous fluids in the shallow crust. • Model describing trace metal partitioning into an exsolved magmatic volatile phase. • High magma chlorine contents generate high metal concentrations in MVPs. • High magmatic water contents maximise trace element fluxes in the MVP. • Isobaric fractionation at depth is a major control on chalcophile volatilisation. [ABSTRACT FROM AUTHOR]

The flow dynamics of magmas is controlled by several parameters, including magma rheology, density, surface tension, gas and liquid flow rate, and the geometry of the flow field, which is mainly regulated by the conduit shape. For this reason, magmatic flows along fissures and dikes are significantly different from those along axisymmetric conduits. Basaltic eruptions are typically fed by fractures or dike systems that reach the surface, giving rise to fissure eruptions. Scaled experiments show that in these low-viscosity systems, gas transport and segregation (i.e., the outgassing dynamics) are deeply controlled by the fracture geometry. Rapid bubble clustering and formation of bubble plumes determine the formation of lateral magma convection cells involving up to 70 vol% of the melt. In analogy with outgassing in cylindrical conduits, the average vesicularity and size of bubbles increase with increasing gas flow rate and melt viscosity and density, which also control the lateral extent of the bubble plume and convection cells. [ABSTRACT FROM AUTHOR]

Volcanic eruptions are driven by the ascent of volatile-laden magma. The capacity of a volcano system to outgas these volatiles—its permeability—controls the explosive potential, and fractures at volcanic conduit margins play a crucial role in tempering eruption explosivity by acting as outgassing pathways. However, these fractures are often filled with hot volcanic debris that welds and compacts over time, meaning that these permeable pathways have a finite lifetime. While numerous studies emphasize that permeability evolution is important for regulating pressure in shallow volcanic systems, how and when this occurs remains an outstanding question in volcanology. In this contribution, we show that different pressure evolution regimes can be expected across a range of silicic systems as a function of the width and distribution of fractures in the system, the timescales over which they can outgas (a function of depth and temperature), and the permeability of the host material. We define outgassing , diffusive relaxation , and pressure increase regimes, which are distinguished by comparing the characteristic timescales over which they operate. Moreover, we define a critical permeability threshold, which determines (in concert with characteristic timescales of diffusive mass exchange between the pore and melt phases) whether systems fracture and outgas efficiently, or if a volcano will be prone to pressure increases, incomplete healing, and explosive failure. [ABSTRACT FROM AUTHOR]

Rootless cones form when partially outgassed lava interacts explosively with external water. The explosions represent an end-member system that can elucidate mechanisms of explosive magma-water interactions in the absence of magmatic fragmentation induced by outgassing. The proportion of finely fragmented ejecta (i.e., ash), generated in rootless explosions, may contribute significantly to the energy of the explosion even if the ash volume is small relative to coarser ejecta. Laboratory experiments indicate that the degree of melt-water mixing and energy release are proportional to the abundance of blocky grains, fragmented by brittle disintegration, which effectively contribute thermal energy to the system. To constrain the mechanisms and dynamics of rootless explosive activity, we assess the nature and modes of fragmentation and ejecta characteristics through morphological, textural, and density analysis of rootless tephra associated with a pāhoehoe lava flow in a lacustrine (lake basin) environment. We observe strong correlations between the mean grain size and the mass percentage of both blocky (negative power law trend) and fluidal (positive logarithmic) tephra clasts of all sizes. We interpret these trends as scale-dependent fragmentation behavior due to the decreasing efficacy of hydrodynamic fragmentation as it occurs over finer scales, especially over the ash size range. Additionally, all analyzed beds contain fine ash-sized blocky and mossy clasts, which are thought to be diagnostic of a high transfer rate of thermal to mechanical energy, characteristic of molten fuel-coolant interactions. These results agree with a recent model of rootless cone formation, prior fragmentation theory, and scaled laboratory experiments and therefore provide a field-based analog for future experimental and modeling efforts. [ABSTRACT FROM AUTHOR]

Planetary atmospheres are commonly thought to result from the efficient outgassing of cooling magma oceans. During this stage, vigorous convective motions in the molten interior are believed to rapidly transport the dissolved volatiles to shallow depths where they exsolve and burst at the surface. This assumption of efficient degassing and atmosphere formation has important implications for both the early and long-term planetary evolution, but has never been tested against fluid dynamics considerations. Yet, during a convective cycle, only a finite fraction of the magma ocean can reach the shallow depths where oversaturated volatiles exsolution can occur, and a large-scale circulation can exist for vigorously convecting fluids in the presence of inertial effects. This can prevent a substantial magma ocean volume from rapidly reaching the planetary surface. Therefore, we conducted computational fluid dynamics experiments of vigorous 2D and 3D Rayleigh–Bénard convection at Prandtl number of unity to characterize the ability of the convecting fluid to reach shallow depths at which volatiles are exsolved and extracted to the atmosphere. We find that the outgassing efficiency is essentially a function of the magnitude of the convective velocities. This allows deriving simple expressions to predict the time evolution of the amount of outgassed volatiles as a function of the magma ocean governing parameters. We show that for plausible cases, the time required to exsolve all oversaturated water can exceed the magma ocean lifetime in a given highly vigorous transient stage, leading to incomplete or even negligible outgassing. Furthermore, the planet size and the initial magma ocean water content, through the convective vigor and the exsolution depth, respectively, strongly affect magma oceans degassing efficiency, possibly leading to divergent planetary evolution paths and resulting surface conditions. Overall, despite vigorous convection, for a significant range of parameters, convective degassing appears not as efficient as previously thought. • Magma ocean dynamics governs volatile outgassing efficiency. • Volatile outgassing efficiency mainly depends on the magnitude of convective velocities. • Degassing efficiency decreases with decreasing planet size or initial water content. • Convective dynamics and cooling rate feedback leads to divergent evolution paths. • Magma ocean volatile outgassing is considerably weaker than previously thought. [ABSTRACT FROM AUTHOR]

Large earthquakes sometimes activate volcanoes both in the near field as well as in the far field. One possible explanation is that shaking may increase the mobility of the volcanic gases stored in magma reservoirs and conduits. Here experimentally and theoretically we investigate how sloshing, the oscillatory motion of fluids contained in a shaking tank, may affect the presence and stability of bubbles and foams, with important implications for magma conduits and reservoirs. We adopt this concept from engineering: severe earthquakes are known to induce sloshing and damage petroleum tanks. Sloshing occurs in a partially filled tank or a fully filled tank with density-stratified fluids. These conditions are met at open summit conduits or at sealed magma reservoirs where a bubbly magma layer overlays a newly injected denser magma layer. We conducted sloshing experiments by shaking a rectangular tank partially filled with liquids, bubbly fluids (foams) and fully filled with density-stratified fluids; i.e., a foam layer overlying a liquid layer. In experiments with foams, we find that foam collapse occurs for oscillations near the resonance frequency of the fluid layer. Low viscosity and large bubble size favor foam collapse during sloshing. In the layered case, the collapsed foam mixes with the underlying liquid layer. Based on scaling considerations, we constrain the conditions for the occurrence of foam collapse in natural magma reservoirs. We find that seismic waves with lower frequencies < 1 Hz, usually excited by large earthquakes, can resonate with magma reservoirs whose width is > 0.5 m. Strong ground motion > 0.1 m s − 1 can excite sloshing with sufficient amplitude to collapse a magma foam in an open conduit or a foam overlying basaltic magma in a closed magma reservoir. The gas released from the collapsed foam may infiltrate the rock or diffuse through pores, enhancing heat transfer, or may generate a gas slug to cause a magmatic eruption. The overturn in the magma reservoir provides new nucleation sites which may help to prepare a following/delayed eruption. Mt. Fuji erupted 49 days after the large Hoei earthquake (1707) both dacitic and basaltic magmas. The eruption might have been triggered by magma mixing through sloshing. [ABSTRACT FROM AUTHOR]

Lava lakes provide ideal venues for directly observing and understanding the nature of outgassing in basaltic magmatic systems. Kīlauea Volcano's summit lava lake has persisted for several years, during which seismic and infrasonic tremor amplitudes have exhibited episodic behavior associated with a rise and fall of the lava surface (“gas pistoning”). Since 2010, the outgassing regime of the lake has been tied to the presence or absence of gas pistoning. During normal behavior (no gas pistoning), the lake is in a “spattering” regime, consisting of higher tremor amplitudes and gas emissions. In comparison, gas piston events are associated with an abrupt rise in lava level (up to 20 m), during which the lake enters a “non-spattering” regime with greatly decreased tremor and gas emissions. We study this episodic behavior using long-term multidisciplinary monitoring of the lake, including seismicity, infrasound, gas emission and geochemistry, and time-lapse camera observations. The non-spattering regime (i.e. rise phase of a gas piston cycle) reflects gas bubbles accumulating near the top of the lake, perhaps as a shallow foam, while spattering regimes represent more efficient decoupling of gas from the lake. We speculate that the gas pistoning might be controlled by time-varying porosity and/or permeability in the upper portions of the lava lake, which may modulate foam formation and collapse. Competing models for gas pistoning, such as deeply sourced gas slugs, or dynamic pressure balances, are not consistent with our observations. Unlike other lava lakes which have cyclic behavior that is thought to be controlled by deeply sourced processes, external to the lake itself, we show an example of lava lake fluctuations driven by cycles of activity at shallow depth and close to the lake's surface. These observations highlight the complex and unsteady nature of outgassing from basaltic magmatic systems. [ABSTRACT FROM AUTHOR]

Transitions between effusive and explosive behaviour are routine for many active volcanoes. The permeability of the system, thought to help regulate eruption style, is likely therefore in a state of constant change. Viscous densification of conduit magma during effusive periods, resulting in physical and textural property modifications, may reduce permeability to that preparatory for an explosive eruption. We present here a study designed to estimate timescales of permeability reduction and strength recovery during viscous magma densification by coupling measurements of permeability and strength (using samples from a suite of variably welded, yet compositionally identical, volcanic deposits) with a rheological model for viscous compaction and a micromechanical model, respectively. Bayesian Information Criterion analysis confirms that our porosity–permeability data are best described by two power laws that intersect at a porosity of 0.155 (the “changepoint” porosity). Above and below this changepoint, the permeability–porosity relationship has a power law exponent of 8.8 and 1.0, respectively. Quantitative pore size analysis and micromechanical modelling highlight that the high exponent above the changepoint is due to the closure of wide (∼200–300 μm) inter-granular flow channels during viscous densification and that, below the changepoint, the fluid pathway is restricted to narrow (∼50 μm) channels. The large number of such narrow channels allows porosity loss without considerable permeability reduction, explaining the switch to a lower exponent. Using these data, our modelling predicts a permeability reduction of four orders of magnitude (for volcanically relevant temperatures and depths) and a strength increase of a factor of six on the order of days to weeks. This discrepancy suggests that, while the viscous densification of conduit magma will inhibit outgassing efficiency over time, the regions of the conduit prone to fracturing, such as the margins, will likely persistently re-fracture and keep the conduit margin permeable. The modelling therefore supports the notion that repeated fracture-healing cycles are responsible for the successive low-magnitude earthquakes associated with silicic dome extrusion. Taken together, our results indicate that the transition from effusive to explosive behaviour may rest on the competition between permeability reduction within the conduit and outgassing through fractures at the conduit margin. If the conditions for explosive behaviour are satisfied, the magma densification clock will be reset and the process will start again. The timescales of permeability reduction and strength recovery presented in this study may aid our understanding of the permeability evolution of conduit margin fractures, magma fracture-healing cycles, surface outgassing cycles, and the timescales required for pore pressure augmentation and the initiation of explosive eruptions. [ABSTRACT FROM AUTHOR]

Several mechanisms have been proposed to allow highly viscous silicic magma to outgas efficiently enough to erupt effusively. There is increasing evidence that challenges the classic foam-collapse model in which gas escapes through permeable bubble networks, and instead suggests that magmatic fracturing and/or accompanying localized fragmentation and welding within the conduit play an important role in outgassing. The 2011–2012 eruption at Cordón Caulle volcano, Chile, provides direct observations of the role of magmatic fractures. This eruption exhibited a months-long hybrid phase, in which rhyolitic lava extrusion was accompanied by vigorous gas-and-tephra venting through fractures in the lava dome surface. Some of these fractures were preserved as tuffisites (tephra-filled veins) in erupted lava and bombs. We integrate constraints from petrologic analyses of erupted products and video analyses of gas-and-tephra venting to construct a model for magma ascent in a conduit. The one-dimensional, two-phase, steady-state model considers outgassing through deforming permeable bubble networks, magmatic fractures, and adjacent wall rock. Simulations for a range of plausible magma ascent conditions indicate that the eruption of low-porosity lava observed at Cordón Caulle volcano occurs because of significant gas flux through fracture networks in the upper conduit. This modeling emphasizes the important role that outgassing through magmatic fractures plays in sustaining effusive or hybrid eruptions of silicic magma and in facilitating explosive-effusive transitions. [Display omitted] • Combines observations with a magma ascent model to quantify magmatic fracture outgassing • A significant proportion of outgassing needs to occur through magmatic fractures to produce the Cordón Caulle hybrid eruption • Outgassing through magmatic fractures is likely important for explosive-effusive transitions in many silicic volcanoes [ABSTRACT FROM AUTHOR]

Several features of Earth owe their origin to processes occurring during and shortly following Earth formation. Collisions with planetary embryos caused substantial melting of the growing Earth, leading to prolonged core formation, atmosphere outgassing, and deepening of the magma ocean as Earth grew. Mantle noble gas isotopic compositions and the mantle abundance of elements that partition into the core record this very early Earth differentiation. In contrast, the elements that are not involved in either core or atmosphere formation show surprisingly muted evidence of the fractionation expected during magma ocean crystallization, and even this minimal evidence for early intramantle differentiation appears to have been erased by mantle convection within ∼1.5 billion years of Earth formation. By 4.36 Ga, Earth's surface and shallow interior had reached temperatures similar to those of the present Earth, and mantle melting, and perhaps plate subduction, was producing crustal rock types similar to those seen today. Remnants of early Earth differentiation may still exist in the deep mantle and continue to influence patterns of large-scale mantle convection, sequestration of some trace elements, geomagnetic reversals, vertical motions of continents, and hot-spot volcanism. [ABSTRACT FROM AUTHOR]

The evolution and escape of the martian atmosphere and the planet's water inventory can be separated into an early and late evolutionary epoch. The first epoch started from the planet's origin and lasted ∼500 Myr. Because of the high EUV flux of the young Sun and Mars' low gravity it was accompanied by hydrodynamic blow-off of hydrogen and strong thermal escape rates of dragged heavier species such as O and C atoms. After the main part of the protoatmosphere was lost, impact-related volatiles and mantle outgassing may have resulted in accumulation of a secondary CO atmosphere of a few tens to a few hundred mbar around ∼4-4.3 Gyr ago. The evolution of the atmospheric surface pressure and water inventory of such a secondary atmosphere during the second epoch which lasted from the end of the Noachian until today was most likely determined by a complex interplay of various nonthermal atmospheric escape processes, impacts, carbonate precipitation, and serpentinization during the Hesperian and Amazonian epochs which led to the present day surface pressure. [ABSTRACT FROM AUTHOR]

Abstract: Outgassing, which changes the distribution of volcanic gases in magmas, is one of the most important processes to determine the eruption styles. Shear deformation of ascending bubbly magmas at the vicinity of the volcanic conduit wall has been considered as an efficient mechanism of outgassing. On the other hand, seismological observations of volcanic eruptions reveal the gas bursting associated with long-period (LP) earthquakes and tremors, suggesting the existence of a large void space in the conduit. However both, the quantitative features of shear-induced outgassing and a mechanism to make a large void space, have still remain unknown. Here I perform a series of model experiments in which shear localization of syrup foam causes outgassing by making large bubbles or a crack-like void space, likely a gas bursting source. There is a critical strain, , above which outgassing occurs depending on the Capillary number, Ca, for and for . The width of the region in which outgassing occurs is described as a function of . Outgassing occurs efficiently at the very beginning of the deformation, suggesting that intermittent magma ascent causes effective outgassing such that the eruption style becomes effusive. This hypothesis is consistent with the fact that cyclic activity has been observed during effusive dome eruptions. [Copyright &y& Elsevier]

Abstract: The eruption style of silicic magmas is affected by the loss of gas (outgassing) during ascent. We investigate outgassing using a numerical model for one-dimensional, two-phase, steady flow in a volcanic conduit. By implementing Forchheimer''s equation rather than Darcy''s equation for outgassing we are able to investigate the relative influence of Darcian and inertial permeability on the transition between effusive and explosive eruptions. These permeabilities are defined by constitutive equations obtained from textural analysis of pyroclasts and determined by bubble number density, throat–bubble size ratio, tortuosity, and roughness. The efficiency of outgassing as a function of these parameters can be quantified by two dimensionless quantities: the Stokes number, the ratio of the response time of the magma and the characteristic time of gas flow, and the Forchheimer number, the ratio of the viscous and inertial forces inside the bubble network. A small Stokes number indicates strong coupling between gas and magma and thus promotes explosive eruption. A large Forchheimer number signifies that gas escape from the bubble network is dominated by inertial effects, which leads to explosive behaviour. To provide context we compare model predictions to the May 18, 1980 Mount St. Helens and the August–September 1997 Soufrière Hills eruptions. We show that inertial effects dominate outgassing during both effusive and explosive eruptions, and that in this case the eruptive regime is determined by a new dimensionless quantity defined by the ratio of Stokes and Forchheimer number. Of the considered textural parameters, the bubble number density has the strongest influence on this quantity. This result has implications for permeability studies and conduit modelling. [Copyright &y& Elsevier]

Abstract: To constrain the timing and conditions of syneruptive magma ascent that are responsible for shifting eruption intensity, we have investigated a basaltic sub-Plinian eruption that produced Yufune-2 scoria in Fuji volcano 2200years ago. We deduced magmatic decompression conditions from groundmass microlite textures, including decompression path (i.e. evolution in decompression rate) and approximate decompression rate, in order to relate them to eruption intensity. The microlites revealed decompression conditions after water saturation at 700–1100m depth. The temporal change in scoria size indicates that the magma discharge rate and resultant eruption intensity increased from unit a to unit b, and then declined toward ending units d and e. The overall decompression rate in each eruptive unit has a positive correlation with eruption intensity. The variation in decompression rate was enlarged in the final units, where the maximum remained the same as the peak through the eruption (0.13–0.22MPa/s for units b and c), while the minimum was 0.025MPa/s. The large variation here is due to 1) variation in flow velocity across conduit and 2) part of the erupted magma in unit d experienced remarkably slow decompression (0.002–0.003MPa/s) resulting from decreased overpressure in the reservoir following the major eruption of unit b. Furthermore, crystal size distribution (CSD) of microlites implied that the earliest erupted magma (unit a) had once been decompressed slowly (0.005–0.012MPa/s), having been arrested by material in the conduit–vent system, which was followed by an increase in decompression rate due to removal of the material at the initiation of the eruption. In addition, the magma that had been ascending slowly before the unit-d eruption may record the increase in decompression rate. This increased rate resulted from being pushed up by the successive magma at the start of that eruption. Two factors had a major impact on eruption intensity. First, magma decompression rate determined the degree of gas-phase separation from ascending magma. Judging from CSD, different decompression rates had been generated at least at the start of microlite crystallization. The second factor is the conduit radius that, in combination with magma ascent rate, controlled the magma discharge rate. Before the major eruption of unit b, the conduit radius likely increased, as evidenced by xenoliths of basaltic lava and lithic fragments with the same petrography as the xenoliths in unit a. In unit e, the conduit radius decreased through inward development of high-density magma from the conduit margin. [Copyright &y& Elsevier]

Volcanic gases supplied a large part of Earth's early atmosphere, but constraints on the value of this flux are scarce. Here we model how C-O-H outgassing could have evolved through the late Hadean and early Archean, under the conditions that global plate tectonics had not yet initiated, all outgassing was subaerial, and graphite was the stable carbon phase in the melt source regions. The model fully couples numerical mantle convection, partitioning of volatiles into the melt, and chemical speciation in the gas phase. The mantle oxidation state (which may not have reached late Archean values in the Hadean) is the dominant control on individual species' outgassing rates because it affects both the carbon content of basaltic magmas and the speciation of degassed volatiles. Volcanic gas from mantles more reduced than the iron-wüstite mineral redox buffer would contain virtually no CO 2 because (i) carbonate ions dissolve in magmas only in very limited amounts, and (ii) almost all degassed carbon takes the form of CO instead of CO 2. For oxidised mantles near the quartz-fayalite-magnetite buffer, we predict median CO 2 outgassing rates of less than approximately 5 Tmol yr−1, still lower than the outgassing rates used in many Archean climate studies. Relatively weak outgassing is due in part to the redox-limited CO 2 contents of graphite-saturated melts, and also to a stagnant lid regime's inefficient replenishment of upper mantle volatiles. Our results point to certain chemical and geodynamic prerequisites for sustaining a clement climate with a volcanic greenhouse under the Faint Young Sun. • Stagnant lid C-O-H outgassing rates predicted across mantle oxidation states. • Fully-coupled convection model reveals limiting effect of mantle volatile budget. • Early Archean CO2 outgassing was not necessarily stronger than the modern rate. [ABSTRACT FROM AUTHOR]

The study of medium-large magnitude and intermediate-high intensity eruptions (VEI = 4/5; Column Height 15–20 km) fed by poorly evolved magmas is one of the main topics in volcanology. In this framework, the 472 AD (Pollena) eruption from Somma-Vesuvius (Italy) represents a key case study, as it is one of the subplinian eruptions used to constrain the reference scenario adopted by the Italian Department of Civil Protection in case of renewal activity at Somma-Vesuvius. The Pollena eruption experienced a complex dynamics, with abrupt shifts in eruptive style. This study focused on the fall products (L1-L8) of the magmatic eruptive phases (Phases I and II), which preceded the onset of the final phreatomagmatic phase (Phase III). Phase I was characterized by unsteady magma discharge resulting in an oscillating convective column, whereas Phase II involved pulsating activity with alternation of sustained and collapsing columns. To evaluate the role of textural variability in controlling magma rheology (and therefore variations in magma discharge), a detailed textural analysis of the juvenile products has been performed. Pyroclast textures record a variable degree of outgassing efficiency and lateral textural stratification of magma in the conduit, related to differential magma ascent rates and resulting in variable eruption intensity. Repetitive patterns of magma densification, achieved by progressive outgassing and crystallization, led to stages of decreasing ascent velocity resulting in the end of the eruptive pulse (during the oscillatory Phase I) or anticipating PDC emplacement (at the end of Phase I and during the pulsatory Phase II). Further decompression of the outgassed magma induced intense clinopyroxene microlite crystallization, that markedly increased magma viscosity and promoted the restoration of sustained columns during the eruption climaxes (L3 and L8). Magma densification patterns and consequent unsteadiness in magma discharge at the surface may derive from small heterogeneities in the initial volatile budget and/or represent a natural evolution of low viscosity magmas, as those feeding Pollena eruption, where efficient crystallization in the conduit can deeply and abruptly modify magma rheology, outgassing ability, conduit flow and, ultimately, eruptive style. • A subplinian eruption is a reference scenario in case of Vesuvius reactivation. • Pyroclast textures are used to reconstruct Pollena subplinian eruption dynamics. • Conduit processes control eruption intensity and fall/PDC transition. • Subplinian and Plinian events at Vesuvius show similar eruptive mechanisms. [ABSTRACT FROM AUTHOR]

Fault networks can provide pathways for fluids (water and gas) to the surface. Magma bodies (such as the Socorro Magma body, SMB, in NM) beneath Earth's surface also release CO2 to the surface. Recent work has provided information on CO2 flux and defined background, diffuse, and endogenic sources in the Albuquerque basin and Valles Caldera. Our main focus was to measure the active CO2 flux in the Rio Grande Rift south of any previously measured CO2 emission sites and was specifically across the margin of the Socorro Magma Body. Geological evidence, in the form of numerous travertine deposits associated with bounding rift faults co-located with the SMB, indicates persistent recent outgassing in this region. The method used was an EM-5 with a soil respiration chamber and an infrared detector to enable direct measurement of the flux of CO2 from the ground being studied. After surveying the western part of the Socorro Magma body in the Sevilleta Wildlife Preserve, we found that there was a small amount of CO2 being emitted by non-anthropogenic means (non-biological). To differentiate between anthropogenic and non-anthropogenic we had a minimum flux of 0.084 to 1.01 g/m2/d, which we defined as background flux to a maximum flux of 3.02 to 9.06 g/m2/d, which is substantially above background flux. Compared to previous works on the Valles Caldera, the data we collected for this project is only a small fraction compared to 170,000 g/m2/d. This preliminary work indicates that the Socorro Magma Body is emitting currently emitting diffuse CO2. [ABSTRACT FROM AUTHOR]

Fault networks can provide pathways for fluids (water and gas) to the surface. Magma bodies (such as the Socorro Magma body, SMB, in NM) beneath Earth's surface also release CO2 to the surface. Recent work has provided information on CO2 flux and defined background, diffuse, and endogenic sources in the Albuquerque basin and Valles Caldera. Our main focus was to measure the active CO2 flux in the Rio Grande Rift south of any previously measured CO2 emission sites and was specifically across the margin of the Socorro Magma Body. Geological evidence, in the form of numerous travertine deposits associated with bounding rift faults co-located with the SMB, indicates persistent recent outgassing in this region. The method used was an EM-5 with a soil respiration chamber and an infrared detector to enable direct measurement of the flux of CO2 from the ground being studied. After surveying the western part of the Socorro Magma body in the Sevilleta Wildlife Preserve, we found that there was a small amount of CO2 being emitted by non-anthropogenic means (non-biological). To differentiate between anthropogenic and non-anthropogenic we had a minimum flux of 0.084 to 1.01 g/m2/d, which we defined as background flux to a maximum flux of 3.02 to 9.06 g/m2/d, which is substantially above background flux. Compared to previous works on the Valles Caldera, the data we collected for this project is only a small fraction compared to 170,000 g/m2/d. This preliminary work indicates that the Socorro Magma Body is emitting currently emitting diffuse CO2. [ABSTRACT FROM AUTHOR]

The magma ocean (MO) is a crucial stage in the build-up of terrestrial planets. Its solidification and the accompanying outgassing of volatiles set the conditions for important processes occurring later or even simultaneously, such as solid-state mantle convection and atmospheric escape. To constrain the duration of a global-scale Earth MO, we have built and applied a 1D interior model coupled with either a gray H2O/CO2 atmosphere or with a pure H2O atmosphere treated with a line-by-line model described in a companion paper by Katyal et al. We study in detail the effects of several factors affecting the MO lifetime, such as the initial abundance of H2O and CO2, the convection regime, the viscosity, the mantle melting temperature, and the longwave radiation absorption from the atmosphere. In this specifically multivariable system, we assess the impact of each factor with respect to a reference setting commonly assumed in the literature. We find that the MO stage can last from a few thousand to several million years. By coupling the interior model with the line-by-line atmosphere model, we identify the conditions that determine whether the planet experiences a transient MO or it ceases to cool and maintains a continuous MO. We find a simultaneous dependence of this distinction on the mass of the outgassed H2O atmosphere and on the MO surface melting temperature. We discuss their combined impact on the MO’s lifetime in addition to the known dependence on albedo, orbital distance, and stellar luminosity, and we note observational degeneracies that arise thereby for target exoplanets. [ABSTRACT FROM AUTHOR]

Violent explosive eruptions pose some of the greatest threats to population and infrastructurein the earth system. Many of the largest explosive eruptions, with consequencesthat can reach the global scale, have been fed by rhyolitic magmas. The dynamicsof magma degassing can exert a central influence on the explosivity of rhyoliticvolcanoes via the role it plays in generating the bubble overpressure which is thesource of the explosive energy of fragmentation. Bubbles grow in general via acombination of diffusion-controlled mass transfer from melt to bubbles, viscousrelaxation of the melt, and decompressive volume expansion. The ultimate drivingforce of oversaturation-driven nucleation and growth of water-rich bubbles is thetemperature- composition- and pressure-dependent water solubility in the melt phase.Another key process affecting the explosivity of volcanic eruptions is the formationof impermeable or connected bubble networks; the latter can result in varyinglyefficient outgassing, reducing the explosive energy in an eruption or even may inhibitit. Here, we experimentally investigate the role of heating on rhyolitic magma vesiculationin a shallow conduit. The experiments were conducted using cylindrical (5 mm-diameter and3 mm-height) samples of both natural and rehydrated rhyolitic glasses with low initial watercontents (0.15-0.32 wt% H2O). Samples were heated at variable rates (1-60 ˚ C/min) to finaltemperatures of 750-1000˚ C in an optical dilatometer at 1 bar pressure, during which theevolution in time of sample volume, and therefore vesicularity, was monitored. Duringheating, sample volume increases nonlinearly due to bubble growth. Textural analysesperformed using X-ray computed microtomography (μCT) and Scanning ElectronMicroscope (SEM) reveal low values of bubble number density (BND) for the naturalobsidians and higher values for the rehydrated obsidians. The natural obsidians exhibitisolated bubble growth leading to the development of an impermeable bubble network.The rehydrated samples possess, in contrast, a high level of bubble connectivitythat record significant bulk outgassing as evidenced by SEM and mass differenceanalyses. This starkly contrasting behavior is likely driven by the abruptness of waterexsolution just above the glass transition temperature during heating. The presence ofcrystallinity in some samples also contributes to this thermal history of bubble nucleationand growth and the corresponding development of bubble connectivity yieldingpermeable pathways. Ongoing investigation is aimed at separating these effects. [ABSTRACT FROM AUTHOR]