In response to rapid decompression, porous magma may fragment explosively. This occurs when the melt can no longer withstand forces exerted upon it due to the overpressure in included bubbles. This occurs at a critical pressure difference between the bubbles and the surrounding magma. In this study we have investigated this pressure threshold necessary for the fragmentation of magma. Here we present the first comprehensive, high temperature experimental quantification of the fragmentation threshold of volcanic rocks varying widely in porosity, permeability, crystallinity, and chemical composition. We exposed samples to increasing pressure differentials in a high temperature shock tube apparatus until fragmentation was initiated. Experimentally, we define the fragmentation threshold as the minimum pressure differential that leads to complete fragmentation of the pressurized porous rock sample. Our results show that the fragmentation threshold is strongly dependent on porosity; high porosity samples fragment at lower pressure differentials than low porosity samples. The fragmentation threshold is inversely proportional to the porosity.Of the other factors, permeability likely affects the fragmentation threshold at high porosity values, whereas chemical composition, crystallinity and bubble size distribution appear to have minor effects. The relationship for fragmentation threshold presented here can be used to predict the minimum pressure differential necessary for the initiation or cessation of the explosive fragmentation of porous magma.

The gas permeability of volcanic rocks may influence various eruptive processes. The transition from a quiescent degassing dome to rock failure (fragmentation) may, for example, be controlled by the rock's permeability, in as much as it affects the speed by which a gas overpressure in vesicles is reduced in response to decompression. Using a modified shock-tube-based fragmentation bomb (Alidibirov and Dingwell 1996a,b;Spieler et al. 2003a), we have measured unsteady-state permeability at a high initial pressure differential. Following sudden decompression above the rock cylinder, pressurized gas flows through the sample. Two pressure transducers record the pressure signals above and below the sample. A transient 1D filtration code has been developed to calculate permeability using the experimental decay curve of the lower pressure transducer. Additionally an analytical steady-state method to achieve permeability is presented as an alternative to swiftly predict the sample permeability in a sufficiently precise manner. Over 100 permeability measurements have been performed on samples covering a wide range of porosity. The results show a general positive relationship between porosity and permeability with a high data scatter. Our preferred interpretation of the results is a combination of two different, but overlapping effects. We propose that at low porosities, gas escape occurs predominantly through microcracks or elongated micropores and therefore could be described by simplified forms of Kozeny-Carman relations (Carman 1956) and fracture flow models. At higher porosities, the influence of vesicles becomes progressively stronger as they form an increasingly connected network. Therefore, a model based on the percolation theory of fully penetrable spheres is used, as a first approximation, to describe the permeability-porosity trend. In the data acquired to date it is evident, that in addition to the porosity control, the sample's bubble size, shape and distribution strongly influence the permeability. This leads to a range of permeability values up to 2.5 orders of magnitude at a given porosity.

Fragmentation of porous magma that is subject to gas overpressure is considered to be a crucial process in the generation of explosive volcanic eruptions. A decompressive event (e.g., rapid magma ascent, landslide, dome collapse) disrupts the stress equilibrium between the gas phase and the surrounding melt. When the gas in the pores is exposed to a pressure gradient, it may either fragment the surrounding magma or escape from the magma along an existing pathway of cracks and interconnected bubbles. Therefore, magma permeability can be a decisive parameter in determining if an eruption experiences fragmentation (i.e., whether it is explosive or effusive, or exhibits a temporal transition between the two eruptive styles). Despite the central role that gas permeability may play in the fragmentation of volcanic rocks, previous studies have not experimentally verifi ed or quantifi ed this infl uence. Based on a comprehensive database of combined permeability and fragmentation experiments, we show that high permeability substantially increases the overpressure required to fragment porous volcanic rocks. Our results allow us to deduce a fragmentation criterion that incorporates gas permeability as well as porosity and internal overpressure. This criterion implies that the energy required for fragmentation is less dependent on the actual pore geometry than on the way the void space is interconnected and, thus, on the contribution of permeable gas fl ow to decompression.

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.