International audiencePore connectivity is a measure of the fraction of pore space (vesicles, voids or cracks) in a material thatis interconnected on the system length scale. Pore connectivity is fundamentally related to permeability,which has been shown to control magma outgassing and the explosive potential of magma duringascent in the shallowest part of the crust. Here, we compile a database of connectivity and porosityfrom published sources and supplement this with additional measurements, using natural volcanic rocksproduced in a broad range of eruptive styles and with a range of bulk composition. The databasecomprises 2715 pairs of connectivity C and porosity φ values for rocks from 35 volcanoes as well as 116products of experimental work. For 535 volcanic rock samples, the permeability k was also measured.Data from experimental studies constrain the general features of the relationship between C and φassociated with both vesiculation and densification processes, which can then be used to interpret naturaldata. To a first order, we show that a suite of rocks originating from effusive eruptive behaviour can bedistinguished from rocks originating from explosive eruptive behaviour using C and φ. We observe thaton this basis, a particularly clear distinction can be made between scoria formed in fire-fountains andthat formed in Strombolian activity. With increasing φ, the onset of connectivity occurs at the percolationthreshold φc which in turn can be hugely variable. We demonstrate that C is an excellent metric forconstraining φc in suites of porous rocks formed in a common process and discuss the range of φc valuesrecorded in volcanic rocks. The percolation threshold is key to understanding the onset of permeability,outgassing and compaction in shallow magmas. We show that this threshold is dramatically different inrocks formed during densification processes than in rocks formed in vesiculating processes and proposethat this value is the biggest factor in controlling the evolution of permeability at porosities above φc

Volcanic ash is often deposited in a hot state. Volcanic ash containing glass, deposited above the glass transition interval, has the potential to sinter viscously both to itself (particle‐particle) and to exposed surfaces. Here we constrain the kinetics of this process experimentally under nonisothermal conditions using standard glasses. In the absence of external load, this process is dominantly driven by surface relaxation. In such cases the sintering process is rate limited by the melt viscosity, the size of the particles and the melt‐vapor interfacial tension. We propose a polydisperse continuum model that describes the transition from a packing of particles to a dense pore‐free melt and evaluate its efficacy in describing the kinetics of volcanic viscous sintering. We apply our model to viscous sintering scenarios for cooling crystal‐poor rhyolitic ash using the 2008 eruption of Chaitén volcano as a case example. We predict that moderate linear cooling rates of > 0.1°C min−1 can result in the common observation of incomplete sintering and the preservation of pore networks.

Elastic waves are generated when brittle materials are subjected to increasing strain. Their number and energy increase non-linearly, ending in a system-sized catastrophic failure event. Accelerating rates of geophysical signals (e.g., seismicity and deformation) preceding large-scale dynamic failure can serve as proxies for damage accumulation in the Failure Forecast Method (FFM). Here we test the hypothesis that the style and mechanisms of deformation, and the accuracy of the FFM, are both tightly controlled by the degree of microstructural heterogeneity of the material under stress. We generate a suite of synthetic samples with variable heterogeneity, controlled by the gas volume fraction. We experimentally demonstrate that the accuracy of failure prediction increases drastically with the degree of material heterogeneity. These results have significant implications in a broad range of material-based disciplines for which failure forecasting is of central importance. In particular, the FFM has been used with only variable success to forecast failure scenarios both in the field (volcanic eruptions and landslides) and in the laboratory (rock and magma failure). Our results show that this variability may be explained, and the reliability and accuracy of forecast quantified significantly improved, by accounting for material heterogeneity as a first-order control on forecasting power.

We conduct experiments to investigate the sintering of high-viscosity liquid droplets. Free-standing cylinders of spherical glass beads are heated above their glass transition temperature, causing them to densify under surface tension. We determine the evolving volume of the bead pack at high spatial and temporal resolution. We use these data to test a range of existing models. We extend the models to account for the time-dependent droplet viscosity that results from non-isothermal conditions, and to account for non-zero final porosity. We also present a method to account for the initial distribution of radii of the pores interstitial to the liquid spheres, which allows the models to be used with no fitting parameters. We find a good agreement between the models and the data for times less than the capillary relaxation timescale. For longer times, we find an increasing discrepancy between the data and the model as the Darcy outgassing time-scale approaches the sintering timescale. We conclude that the decreasing permeability of the sintering system inhibits late-stage densification. Finally, we determine the residual, trapped gas volume fraction at equilibrium using X-ray computed tomography and compare this with theoretical values for the critical gas volume fraction in systems of overlapping spheres.

[1] Sintering and densification are ubiquitous processes influencing the emplacement of both effusive and explosive products of volcanic eruptions. Here we sinter ash-size fragments of a synthetic National Institute of Standards and Technology viscosity standard glass at temperatures at which the resultant melt has a viscosity of ∼108–109 Pa.s at 1bar to assess sintering dynamics under near-surface volcanic conditions. We track the strength recovery via uniaxial compressive tests. We observe that volcanic ash sintering is dominantly time dependent, temperature dependent, and grain size dependent and may thus be interpreted to be controlled by melt viscosity and surface tension. Sintering evolves from particle agglutination to viscous pore collapse and is accompanied by a reduction in connected porosity and an increase in isolated pores. Sintering and densification result in a nonlinear increase in strength. Micromechanical modeling shows that the pore-emanated crack model explains the strength of porous lava as a function of pore fraction and size.