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Much of Earth’s magma is stored as extensive crystal mush systems, yet the prevalence of physical processes operating within mushes and their importance in volcanically active regions remain enigmatic. In this Review, we explore the physical properties and key processes of crystal mush systems. The initiation, evolution and decline of volcanic systems, modulated by heat supply and loss, could generate differences in the prevalence of mush processes through space and time. Additionally, regional tectonics alter mush properties, with mushes in cool wet settings having persistent residual melt, permitting more effective melt segregation than in hot dry settings. Disaggregation of mushes results in crystal mush material being mobilized or entrained into lavas and erupted, presenting opportunities to define the timescales and chemistry of some mush processes in volcanically active regions. Mush systems can be observed on length scales ranging from kilometres (using geological mapping) to micrometres (using crystal textures). Therefore, it is difficult to integrate data and interpretations across different fields. Improved integration of thermodynamics, textural analysis, geochemistry, modelling and experiments, alongside inputs from adjacent fields such as porous media dynamics, engineering and metallurgy will help to advance understanding of mush systems and ultimately improve hazard evaluation at active and dormant volcanic systems. 

While the effects of volcanism on Earth’s climate are well understood, the volcano-ice sheet system hosts a two-way feedback. Volcanic activity promotes ice melting, which in turn affects the internal dynamics of the magma chamber below. At present, accurate forecasts of sea-level rise hinge on the stability of the West Antarctic Ice Sheet, and thus require consideration of subglacial volcano-deglaciation feedbacks. The West Antarctic Ice Sheet, grounded below sea-level, is particularly vulnerable to collapse, yet its position atop an active volcanic rift is seldom considered. Ice unloading raises the geotherm and alters the crustal stress field, impacting dike propagation. However, the consequences on internal magma chamber dynamics and thus long-term eruption behavior remain elusive. Given potential for unloading-triggered volcanism in West Antarctica to accelerate ice retreat, we adapt the thermomechanical magma chamber model of Scholz et al. (2023) for West Antarctic Rift basalts, simulating a shrinking ice load through a prescribed decrease of lithostatic pressure. Examining different unloading scenarios, we investigate the impacts on volatile partitioning within the magma and eruptive trajectory across a wide range of initial magma chamber conditions. Pressurization of a magma chamber beyond a critical threshold results in eruption, delivering enthalpy to the ice. Considering the removal of km-thick ice sheets, we demonstrate the rate of unloading is dominant in influencing the cumulative mass erupted and consequently, heat released to the ice. These findings provide fundamental insights into the complex volcano-ice interactions in West Antarctica and other subglacial volcanic settings. 

Key Points Changes in hydraulic diffusivity and pressurization factor during thermal pressurization (TP) balance each other in low permeability and low porosity fault rocks Hydraulic diffusional length scales as time 0.7 when considering TP parameters that depend on temperature and pressure The constant case model should be considered with ambient initial conditions and not time‐averaged ones 

SUMMARY Seismic tomography is a cornerstone of geophysics and has led to a number of important discoveries about the interior of the Earth. However, seismic tomography remains plagued by the large number of unknown parameters in most tomographic applications. This leads to the inverse problem being underdetermined and requiring significant non-geologically motivated smoothing in order to achieve unique answers. Although this solution is acceptable when using tomography as an explorative tool in discovery mode, it presents a significant problem to use of tomography in distinguishing between acceptable geological models or in estimating geologically relevant parameters since typically none of the geological models considered are fit by the tomographic results, even when uncertainties are accounted for. To address this challenge, when seismic tomography is to be used for geological model selection or parameter estimation purposes, we advocate that the tomography can be explicitly parametrized in terms of the geological models being tested instead of using more mathematically convenient formulations like voxels, splines or spherical harmonics. Our proposition has a number of technical difficulties associated with it, with some of the most important ones being the move from a linear to a non-linear inverse problem, the need to choose a geological parametrization that fits each specific problem and is commensurate with the expected data quality and structure, and the need to use a supporting framework to identify which model is preferred by the tomographic data. In this contribution, we introduce geological parametrization of tomography with a few simple synthetic examples applied to imaging sedimentary basins and subduction zones, and one real-world example of inferring basin and crustal properties across the continental United States. We explain the challenges in moving towards more realistic examples, and discuss the main technical difficulties and how they may be overcome. Although it may take a number of years for the scientific program suggested here to reach maturity, it is necessary to take steps in this direction if seismic tomography is to develop from a tool for discovering plausible structures to one in which distinct scientific inferences can be made regarding the presence or absence of structures and their physical characteristics. 

Abstract Mafic volcanic activity is dominated by effusive to mildly explosive eruptions. Plinian and ignimbrite-forming mafic eruptions, while rare, are also possible; however, the conditions that promote such explosivity are still being explored. Eruption style is determined by the ability of gas to escape as magma ascends, which tends to be easier in low-viscosity, mafic magmas. If magma permeability is sufficiently high to reduce bubble overpressure during ascent, volatiles may escape from the magma, inhibiting violent explosive activity. In contrast, if the permeability is sufficiently low to retain the gas phase within the magma during ascent, bubble overpressure may drive magma fragmentation. Rapid ascent may induce disequilibrium crystallization, increasing viscosity and affecting the bubble network with consequences for permeability, and hence, explosivity. To explore the conditions that promote strongly explosive mafic volcanism, we combine microlite textural analyses with synchrotron x-ray computed microtomography of 10 pyroclasts from the 12.6 ka mafic Curacautín Ignimbrite (Llaima Volcano, Chile). We quantify microlite crystal size distributions (CSD), microlite number densities, porosity, bubble interconnectivity, bubble number density, and geometrical properties of the porous media to investigate the role of magma degassing processes at mafic explosive eruptions. We use an analytical technique to estimate permeability and tortuosity by combing the Kozeny-Carman relationship, tortuosity factor, and pyroclast vesicle textures. The groundmass of our samples is composed of up to 44% plagioclase microlites, > 85% of which are < 10 µm in length. In addition, we identify two populations of vesicles in our samples: (1) a convoluted interconnected vesicle network produced by extensive coalescence of smaller vesicles (> 99% of pore volume), and (2) a population of very small and completely isolated vesicles (< 1% of porosity). Computed permeability ranges from 3.0 × 10⁻¹³to 6.3 × 10⁻¹²m², which are lower than the similarly explosive mafic eruptions of Tarawera (1886; New Zealand) and Etna (112 BC; Italy). The combination of our CSDs, microlite number densities, and 3D vesicle textures evidence rapid ascent that induced high disequilibrium conditions, promoting rapid syn-eruptive crystallization of microlites within the shallow conduit. We interpret that microlite crystallization increased viscosity while simultaneously forcing bubbles to deform as they grew together, resulting in the permeable by highly tortuous network of vesicles. Using the bubble number densities for the isolated vesicles (0.1-3⁻³ × 10⁴ bubbles per mm³), we obtain a minimum average decompression rate of 1.4 MPa/s. Despite the textural evidence that the Curacautín magma reached the percolation threshold, we propose that rapid ascent suppressed outgassing and increased bubble overpressures, leading to explosive fragmentation. Further, using the porosity and permeability of our samples, we estimated that a bubble overpressure > 5 MPa could have been sufficient to fragment the Curacautín magma. Other mafic explosive eruptions report similar disequilibrium conditions induced by rapid ascent rate, implying that syn-eruptive disequilibrium conditions may control the explosivity of mafic eruptions more generally. 

Abstract Before large volumes of crystal poor rhyolites are mobilized as melt, they are extracted through the reduction of pore space within their corresponding crystal matrix (compaction). Petrological and mechanical models suggest that a significant fraction of this process occurs at intermediate melt fractions (ca. 0.3–0.6). The timescales associated with such extraction processes have important ramifications for volcanic hazards. However, it remains unclear how melt is redistributed at the grain‐scale and whether using continuum scale models for compaction is suitable to estimate extraction timescales at these melt fractions. To explore these issues, we develop and apply a two‐phase continuum model of compaction to two suites of analog phase separation experiments—one conducted at low and the other at high temperatures, T, and pressures, P. We characterize the ability of the crystal matrix to resist porosity change using parameterizations of granular phenomena and find that repacking explains both data sets well. A transition between compaction by repacking to melt‐enhanced grain boundary diffusion‐controlled creep near the maximum packing fraction of the mush may explain the difference in compaction rates inferred from high T + P experiments and measured in previous deformation experiments. When upscaling results to magmatic systems at intermediate melt fractions, repacking may provide an efficient mechanism to redistribute melt. Finally, outside nearly instantaneous force chain disruption events occasionally recorded in the low T + P experiments, melt loss is continuous, and two‐phase dynamics can be solved at the continuum scale with an effective matrix viscosity. 

Abstract Magmatic volatiles drive pressure, temperature, and compositional changes in upper crustal magma chambers and alter the physical properties of stored magmas. Previous studies suggest that magmatic H₂O content influences the growth and longevity of silicic chambers through regulating the size and frequency of eruptions and impacting the crystallinity‐temperature curve. However, there has been comparatively little exploration of how CO₂impacts the evolution of magma chambers despite the strong influence of CO₂on H₂O solubility and the high concentrations of CO₂often present in mafic systems. In this study, we integrate the thermodynamic effects of dissolved and exsolved H₂O and CO₂with the mechanics of open‐system magma chambers that interact thermally and mechanically with the crust. We applied this model to investigate how intrinsic variations in magmatic H₂O‐CO₂content influence the growth and longevity of silicic and mafic magma chambers. Our findings indicate that even with a tenfold increase in CO₂content (up to 10,000 ppm), CO₂plays a minimal role in long‐term chamber growth and longevity. While CO₂content affects the magma compressibility, the resulting changes in eruption mass are balanced out by a commensurate change in eruption frequency so that the time‐averaged eruptive flux and long‐term chamber behavior remain similar. In contrast, H₂O content strongly influences chamber growth and longevity. In silicic systems, high H₂O contents hinder magma chamber growth by increasing the total eruptive flux and steepening the slope of the crystallinity‐temperature curve. In mafic systems, high H₂O contents promote magma chamber growth by flattening the slope of the crystallinity‐temperature curve. 

Abstract We present a model for a coupled magma chamber–dike system to investigate the conditions required to initiate volcanic eruptions and to determine what controls the size of eruptions. The model combines the mechanics of dike propagation with internal chamber dynamics including crystallization, volatile exsolution, and the elastic response of the magma and surrounding crust to pressure changes within the chamber. We find three regimes for dike growth and eruptions: (1) below a critical magma chamber size, eruptions are suppressed because chamber pressure drops to lithostatic before a dike reaches the surface; (2) at an intermediate chamber size, the erupted volume is less than the dike volume (“dike-limited” eruption regime); and (3) above a certain chamber size, dikes can easily reach the surface and the erupted volume follows a classic scaling law, which depends on the attributes of the magma chamber (“chamber-limited” eruption regime). The critical chamber volume for an eruption ranges from ∼0.01 km3 to 10 km3 depending on the water content in the magma, depth of the chamber, and initial overpressure. This implies that the first eruptions at a volcano likely are preceded by a protracted history of magma chamber growth at depth, and that the crust above the magma chamber may have trapped several intrusions or “failed eruptions.” Model results can be combined with field observations of erupted volume, pressure, and crystal and volatile content to provide tighter constraints on parameters such as the eruptible chamber size.