Densification of permeable liquids and magmas

A central question pertinent to our understanding of volcanic eruptions is: how long can magma remain permeable during shallow ascent? The answer to this complex question has implications for whether or not volcanic plugs can form at the top of silicic conduits and for the longevity of overpressure in magma, which is key to understanding the likelihood that a magma will fragment explosively in eruption. In this thesis a conceptual, and then mathematical framework for addressing this problem is established before experimental data is presented. The mathematical treatment of the problem progresses from processes that affect single droplets and that can be explicitly constrained, such as heat and mass transfer and shape changes in volcanic droplets, before applying these concepts to arrays of many droplets and porous liquids in general. In testing experimental data, a step-by-step approach is taken in which (1) a controlled analogue dataset is used to differentiate the model that best describes the data; (2) the chosen model is extended to more volcanically relevant conditions by testing it against experiments performed using natural materials; and (3) the consequences of the densification process for the time dependence of permeability are assessed. A universal scaling is found between the porosity and the permeability of densifying systems and this is used to calibrate a numerical model for the kinetics of permeability decay in volcanic plugs. Finally, a densification map is provided on which the dominant timescales and lengthscales are compared such that specific volcanic conditions or observations can be plotted to assess whether or not they are consistent with the densification process. In conclusion, it is noted that permeable magmas and viscous liquids in general will densify until an equilibrium volume is reached. This densification is driven by either the surface tension stresses internal to the permeable pore network or by additional external stresses and is limited by the liquid viscosity and the lengthscale of the pores. All volcanic eruptions are driven by exsolved gas and the buoyancy and pressure they contribute to the system and must be outgassed in either explosive or passive events. While the explosive contribution of magma outgassing has received much attention, the physical process by which passive outgassing, and the resultant densification, occur remains poorly understood. Future work could constrain the regime in which explosive and passive degassing are coincident and compete to release the gas-pressure built up during the shallowest portions of magma ascent to the Earth's surface

Dehydration‐driven mass loss from packs of sintering hydrous silicate glass particles

Glass sintering involves the densification of packs of particles and the expulsion of the interparticle pore gas. The pore space begins as a convolute interconnected interparticle network, and ends as distributed isolated bubbles; two configurations that are separated by the percolation threshold. Here, we perform experiments in which (i) the particles are initially saturated in H2O at 871 K, and (ii) they are then heated non-isothermally at different rates to temperatures in excess of 871 K. In step (ii), H2O becomes supersaturated and the particles diffusively lose mass as they sinter together. We use thermogravimetry to track the loss of mass with time. We find that the mass loss is initially well predicted by solutions to Fick's second law in spherical coordinates with the appropriate material and boundary conditions. However, as the sintering pack crosses the percolation threshold at a time predicted by sintering theory, we find that the mass loss deviates from simple diffusional solutions. We interpret this to be the result of an increase in the diffusion distance from the particle-scale to the scale of the sintering pack itself. Therefore, we conclude that the open- to closed-system transition that occurs at the percolation threshold is a continuous, but rapid jump for diffusive and other transport properties. We use a capillary Peclet number Pc to parameterize for this transition, such that at low Pc diffusive equilibrium is achieved before the sintering-induced transition to closed system, whereas at high Pcthere is a “diffusion crisis” and disequilibrium may be maintained for longer relative timescales that depend on the system size

A Scaling for the Permeability of Loose Magma Mush Validated Using X‐Ray Computed Tomography of Packed Confectionary in 3D and Estimation Methods From 2D Crystal Shapes

Melt percolation through partially molten “mushy” regions of the crust underpins models for magma migration, accumulation, and processes that prime systems for eruption. Knowledge of the hydraulic properties of magma mush, specifically permeability, is required for accurate predictions of melt migration rates and accumulation timescales. Previous studies, validated for cuboidal crystal analogs, show that crystal shape exerts a first‐order control on the permeability, and is tested here for anisometric natural crystal shapes using X‐ray CT 3D data sets of magma mush analogs made from packed confectionary particles arranged randomly. We use a lattice‐Boltzmann fluid flow simulation tool to determine the permeability of the analogue melt phase network between the packed particles. We find excellent agreement with our data sets to within ∼0.1 log units, when the specific surface area is measured. To extend this to more typical cases where the specific surface area is unknown, we use the shape and size of the objects determined in both 3D and 2D to estimate the specific surface area assuming a cuboid approximation. These approximate solutions give good results to within ∼0.5 log units of the measured permeability and offer a method by which permeability could be estimated from a thin section of a cumulate or pluton sample. Our shape‐sensitive approach is more accurate than existing models for permeability of magma mush, most approximating natural crystal shapes to spheres. We therefore propose that these could be implemented in dynamic magma mush models for melt movement in the crust to produce more accurate flux predictions

Microstructural Controls on the Uniaxial Compressive Strength of Porous Rocks Through the Granular to Non‐Granular Transition

Under uniaxial compression, a porous rock fails by coalescence of stress‐induced microcracks. The micromechanical models developed to analyze uniaxial compressive strength data consider a single mechanism for the initiation and propagation of microcracks and a fixed starting microstructure. Because the microstructure of clastic porous rock transitions from granular to non‐granular as porosity decreases during diagenesis, their strength cannot be captured by a single model. Using synthetic samples with independently controlled porosity and initial grain radius we show that high‐porosity granular samples, where microcracks grow at grain‐to‐grain contacts, are best described by a grain‐based model. Low‐porosity non‐granular samples, where microcracks grow from pores, are best described by a pore‐based model. The switch from one model to the other depends on porosity and grain radius. We propose a regime plot that indicates which micromechanical model may be more suitable to predict strength for a given porosity and grain radius

Introducing Volcanica: The first diamond open-access journal for volcanology

Welcome to Volcanica's first issue! Our community of editors and Volcanica supporters are excited to present you with our inaugural issue of peer-reviewed volcano research. This editorial accompanies the first issue of Volcanica so that we can provide you with some background to the Volcanica initiative, explain some of the evidence-based motivation for starting a new journal, and explore ways in which we can improve universal access to published research. We discuss our model of "diamond open access", which is entirely free for authors to publish and free for everyone to read. We will explain how this model is possible and state explicitly the challenges related to how this project can be sustainable and scalable. Finally, we will signpost the information you may need to publish with Volcanica as we continue to grow

Forecasting Multiphase Magma Failure at the Laboratory Scale Using Acoustic Emission Data

Magmas fracture under high shear stresses, producing radiating elastic waves. At the volcano scale, eruption is often preceded by accelerating seismicity, while at the laboratory scales, sample failure appears to be preceded by similarly accelerating Acoustic Emission (AE). In both cases, empirical relationships between the acceleration and the time of the singular final event have offered tantalizing possibilities for forecast of eruptions and material failure. We explore the success of these tools in the laboratory by briefly reviewing datasets that have been presented previously and comparing the range of errors on forecast times with the range of errors associated with attempts to retrospectively forecast eruptions. We demonstrate that the heterogeneity of a system is crucial to making accurate forecasts on the sample scale, such that homogeneous systems are inherently unpredictable. We then analyse the effect of having an incomplete data sequence, as might be the case for real-time forecasting scenarios. We find that for heterogeneous systems, there is a critical proportion of the sequence that needs to have occurred before a forecast time converges on relatively low errors. As might be expected, the final portion of the sequence is the most important, while uncertainty on the start of the sequence is less important. Finally, we explore the simplest method for scaling the laboratory results to the volcano scenario

The Influence of Grain Size Distribution on Mechanical Compaction and Compaction Localization in Porous Rocks

The modes of formation of clastic rocks result in a wide variety of microstructures, from poorly-sorted heterogeneous rocks to well-sorted and nominally homogeneous rocks. The mechanical behavior and failure mode of clastic rocks is known to vary with microstructural attributes such as porosity and grain size. However, the influence of the grain size distribution, in particular the degree of polydispersivity or modality of the distribution, is not yet fully understood, because it is difficult to study experimentally using natural rocks. To better understand the influence of grain size distribution on the mechanical behavior of porous rocks, we prepared suites of synthetic samples consisting of sintered glass beads with polydisperse grain size distributions. We performed hydrostatic compression experiments and found that, all else being equal, the onset of grain crushing occurs much more progressively and at lower pressure in polydisperse synthetic samples than in monodisperse samples. We conducted triaxial experiments in the regime of shear-enhanced compaction and found that the stress required to reach inelastic compaction was lower in polydisperse samples compared to monodisperse samples. Further, our microstructural observations show that compaction bands developed in monomodal polydisperse samples while delocalized cataclasis developed in bimodal polydisperse samples, where small grains were systematically crushed while largest grains remained intact. In detail, as the polydispersivity increases, microstructural deformation features appear to transition from localized to delocalized through a hybrid stage where a compaction front with diffuse bands propagates from both ends of the sample toward its center with increasing bulk strain

Alternating Subplinian and phreatomagmatic phases during the construction of a phonolitic maar-diatreme volcano (Caldera del Rey, Tenerife, Canary Islands)

The Early Pleistocene, well exposed, Caldera del Rey maar-diatreme volcano, Tenerife, Canary Islands was constructed during a ∼ VEI 4 phonolitic eruption that involved two cycles of magmatic-to-phreatomagmatic activity and resulted in two overlapping craters aligned NE-SW. Magmatic phases fed unsteady Subplinian eruption columns that reached 8–12 km altitude and dispersed tephra to the west and southwest of the volcano and shed pyroclastic density currents. Phreatomagmatic phases, driven by explosive interactions between magma and groundwater, constructed an extensive tephra ring via deposition from ballistic curtains, pyroclastic density currents, and tephra fall. Near-optimal-scaled depth phreatomagmatic explosions (strong and/or shallow) excavated a substantial diatreme beneath the north crater and constructed a substantial tephra ring. This abruptly transitioned to deeper-than-optimal scaled depth explosions (weak and/or deep) that erupted mostly fine ash which was dispersed by dilute pyroclastic density currents and fallout and filled the south crater. At distances of >4 km from the volcano, over a metre of ash and pumice accumulated during the phreatomagmatic phases. The Caldera del Rey volcano provides an instructive study on how interaction between ascending felsic magma and groundwater can modify Subplinian eruptions

The physics of dancing peanuts in beer

In Argentina, some people add peanuts to their beer. Once immersed, the peanuts initially sink part way down into the beer before bubbles nucleate and grow on the peanut surfaces and remain attached. The peanuts move up and down within the beer glass in many repeating cycles. In this work, we propose a physical description of this dancing peanuts spectacle. We break down the problem into component physical phenomena, providing empirical constraint of each: (i) heterogeneous bubble nucleation occurs on peanut surfaces and this is energetically preferential to nucleation on the beer glass surfaces; (ii) peanuts enshrouded in attached bubbles are positively buoyant in beer above a critical attached gas volume; (iii) at the beer top surface, bubbles detach and pop, facilitated by peanut rotations and rearrangements; (iv) peanuts containing fewer bubbles are then negatively buoyant in beer and sink; and (v) the process repeats so long as the beer remains sufficiently supersaturated in the gas phase for continued nucleation. We used laboratory experiments and calculations to support this description, including constraint of the densities and wetting properties of the beer–gas–peanut system. We draw analogies between this peanut dance cyclicity and industrial and natural processes of wide interest, ultimately concluding that this bar-side phenomenon can be a vehicle for understanding more complex, applied systems of general interest and utility