Pore connectivity is a measure of the fraction of pore space (vesicles, voids or cracks) in a material that is 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 during ascent in the shallowest part of the crust. Here, we compile a database of connectivity and porosity from published sources and supplement this with additional measurements, using natural volcanic rocks produced in a broad range of eruptive styles and with a range of bulk composition. The database comprises 2715 pairs of connectivity C and porosity ϕ values for rocks from 35 volcanoes as well as 116 products 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 ϕ associated with both vesiculation and densification processes, which can then be used to interpret natural data. To a first order, we show that a suite of rocks originating from effusive eruptive behaviour can be distinguished from rocks originating from explosive eruptive behaviour using C and ϕ. We observe that on this basis, a particularly clear distinction can be made between scoria formed in fire-fountains and that formed in Strombolian activity. With increasing ϕ, the onset of connectivity occurs at the percolation threshold ϕc which in turn can be hugely variable. We demonstrate that C is an excellent metric for constraining ϕc in suites of porous rocks formed in a common process and discuss the range of ϕc values recorded 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 in rocks formed during densification processes than in rocks formed in vesiculating processes and propose that this value is the biggest factor in controlling the evolution of permeability at porosities above ϕc.
•Connectivity can be used to distinguish between subsets of volcanic materials.•Connectivity allows to identify the percolation threshold ϕc of volcanic rocks.•Pore connectivity evolves differently as a result of vesiculation and cracking.•Shear-deformation reduces the percolation threshold during vesiculation.•Connectivity contains information about permeability at porosities close to ϕc.
Previous studies of Vulcanian eruptive products have shown that the respective volcanic conduits were filled for the most part with low-porosity magma (i.e., < 10 vol%) prior to eruption. Comparison ...with the theoretical porosity distribution expected from closed-system degassing suggests that gas loss must have taken place at depth within the magmatic column (between 3 and 5 km). At such high pressures (between 70 and 110 MPa), however, porosities are low enough (< 20 vol%) to rule out traditional gas loss mechanisms. We tested if channelling, an outgassing mechanism based on bubble connection due to high crystal content (>40 vol%) proposed to occur in mushy magma reservoirs, could also happen in volcanic conduits. We reanalyzed phenocryst, microlite, and porosity data from recent eruptions of Merapi volcano, Indonesia, Soufrière Hills volcano, Montserrat, and Tungurahua volcano, Ecuador. Overall, these magmas had crystal contents high enough for outgassing to occur by channelling. Gases could be channelled out of the magma columns at various levels during ascent to yield mostly gas-depleted magma columns prior to explosive behavior. Such outgassing by channelling thus has the capacity to influence eruptive style. Depending on the phenocryst content, microlite growth during ascent can either foster or impede gas escape by channelling. Considering the pervasive occurrence of microlites and ensuing high crystal contents in volcanic conduits (between 40 and 70 vol%), the high likelihood of channelling implies that other outgassing mechanisms might not be as dominant as previously envisioned.
•Bubbles nucleate more efficiently on Fe-Ti oxides than on other mineral type.•Number density of oxide microlites is more relevant than volume fraction.•Bubble coalescence is enhanced by phenocrysts.
...Silicic magmas span a wide range of eruptive styles between explosive and effusive, and transitions between these styles are commonplace. Yet the triggers of switches in eruptive style remain poorly understood. Eruptions are mostly driven by degassing of magmatic water and their eruption style - effusive or explosive - is likely governed by the efficiency of outgassing as well as magma ascent rate. Microlites and phenocrysts are often purported to promote heterogeneous bubble nucleation and outgassing, both key variables in the degassing dynamics that become crucial in controlling the eruptive style. Here, in order to shed light on the role of nature, size and abundance of crystals on degassing of silicic magma, we experimentally investigate (heating-induced) vesiculation in a multiphase (microlite- and phenocryst-bearing), low-water content, and bubble-free, natural rhyolite. The experiments were conducted at magmatic temperature (∼900-1100°C) and atmospheric pressure in an optical dilatometer. Our results indicate that microlites exert a large influence on bubble nucleation while the effect of phenocrysts is subordinate. Amongst the microlite phases, bubbles nucleate more easily on Fe-Ti oxides than other mineral phases. Bubble coalescence and connectivity are, in contrast, enhanced by the phenocrysts, more than microlites, in low-crystallinity magma. Comparing the bubble textures of the post-experimental samples with those produced in a phenocryst-free and microlite-poor silicic magma, we observe that the phenocryst- and microlite-bearing magma exhibits significantly more bubble coalescence and connectivity, as well as higher bubble number densities. These findings help to constrain the roles that pre- and syn-eruptive crystalline phases may play in degassing processes during ascent of silicic magma.
Despite their generally low volume fraction, FeTi oxides have the potential to greatly influence the eruptive style because they lower the supersaturation pressure for heterogeneous bubble ...nucleation. Once nucleated, bubbles respond fast to pressure changes, fostering rapid expansion and explosive behavior. Yet, oxide microlite quantifications are often absent from data of explosive products. We used new, re-analyzed, and published data to build a compilation of oxide number densities (ONDs) and vesicle number densities (VNDs) of trachytic and calc-alkaline products. Four eruptive styles were selected: 1) Vulcanian explosions from Soufrière Hills volcano, Montserrat, Lascar volcano, Chile, and Kilian volcano, France, 2) blasts from Mt. Pelée volcano, Lesser Antilles, Mount St. Helens, USA, and Merapi volcano, Indonesia, 3) a sub-Plinian explosion from Merapi volcano, and 4) lava dome effusions with intermittent collapse from Soufrière Hills and Mt. Pelée volcanoes. Natural samples were separated into two groups according to the dominant texture of the products of each event: 1) vesicular pumice clasts from explosions with a strong vertical component and 2) dense clasts with diktytaxitic textures from dome collapse event and lateral blast. Group 1 samples are either distributed alongside the 1:1 trend between VND and OND that spans from 1015 to 1017 m−3, or have a constant VND of 1016–16.5 m−3 regardless of OND. A large proportion of oxides (55–100%) touch vesicles. A more variable proportion of vesicles (16–72%) are in contact with oxides because of syn-explosive growth and coalescence. Group 2 samples have ONDs in the same broad range as group 1 samples. We also used new and published data to build a compilation of ONDs and VNDs of five series of experimental decompression of rhyolitic and phonolitic melts. In samples with OND ≈ VND, most bubbles are in contact with more than one oxide and 64–88% of the oxides are in contact with bubbles. Such high levels of connectivity suggest that the role of oxides in controlling bubble nucleation has been underestimated. When VND ≥ OND, nucleation densities of experimental vesicles can be reproduced by heterogeneous nucleation models, which we used to calculate syn-explosive decompression rates from VNDs at Merapi, Soufrière Hills, and Kilian. These rates and textural evidence suggest that the decompression front accompanying these Vulcanian and sub-Plinian explosions is responsible for syn-explosive bubble nucleation. We calculated the average pre-explosive ascent rates necessary to yield the observed ONDs at Soufrière Hills and Merapi volcanoes. The resulting rates, 0.005–20 m/s, overlap considerably with the range of critical ascent rate inferred for the effusive–explosive transition, supporting the hypothesis that this transition is primarily controlled by oxide microlites in the conduit because oxides are a primer for explosive behavior when present in sufficiently high number densities. Focusing on the older eruption of Kilian, for which no observation is available, we infer that pre-explosive ascent rates of >7 × 10−3 m/s were necessary for explosive behavior to occur.
•Measures of oxide microlites number densities in products of silicic eruptions are scarce.•We measured oxide and vesicle number densities in experiments and at five volcanoes.•Heterogeneous nucleation models only work when oxides are at least as numerous as vesicles.•Ascent rates needed to nucleate oxides overlap those characteristic of the effusive–explosive transition.•The appearance of oxides in the conduit is a primer for explosive behavior.
Explosivity in erupting volcanoes is controlled by the degassing dynamics and the viscosity of the ascending magma in the conduit. Magma crystallisation enhances both heterogeneous bubble nucleation ...and increases in magma bulk viscosity. Nanolite crystallisation has been suggested to enhance such processes too, but in a noticeably higher extent. Yet the precise causes of the resultant strong viscosity increase remain unclear. Here we report experimental results for rapid nanolite crystallisation in natural silicic magma and the extent of the subsequent viscosity increase. Nanolite-free and nanolite-bearing rhyolite magmas were subjected to heat treatments, where magmas crystallised or re-crystallised oxide nanolites depending on their initial state, showing an increase of one order of magnitude as oxide nanolites formed. We thus demonstrate that oxide nanolites crystallisation increases magma bulk viscosity mainly by increasing the viscosity of its melt phase due to the chemical extraction of iron, whereas the physical effect of particle suspension is minor, almost negligible. Importantly, we further observe that this increase is sufficient for driving magma fragmentation depending on magma degassing and ascent dynamics.
Accurate predictions of volcanological phenomena, such as the trajectory of blocks accelerated by volcanic explosions, require quantitative skills training. Large outdoor experiments can be useful to ...convey concepts of volcanic processes to students in an exciting way. Beyond the fun aspects, these experiments provide an opportunity to engage with the physics of projectile flight and help promote mathematical learning within the Earth Sciences. We present a quantitative framework required to interpret ballistic trajectories and the outdoor experiment known commonly as “trashcano”, taking a step-by-step approach to the physics of this problem, and deriving a range of mathematical solutions involving different levels of complexity. Our solutions are consistent with the predictions from established computer programs for volcanic ballistic trajectory modelling, but we additionally provide a nested set of simplified solutions for a range of teaching scenarios as well as simulated datasets for use where the full experiment may not be possible.
Optical and microscopical analyses of surficial and internal textures are key elements of volcanic rock characterization. Yet conventional methods for textural analyses often pose challenges and ...difficulties in terms of sample preparation and/or time and resources consumption, beside their inherent limitations. Here, we performed Confocal Scanning Laser Microscopy (CSLM) to characterize vesicles in volcanic rocks and their experimental analogs and to compare the results to those of conventional Scanning Electron Microscopy (SEM) investigations. Further, we discuss surface roughness analysis as a possible parameter to analyze different vesicle textures. CSLM obviates the need for sample impregnation (using fluorescent resin) and polishing, it enables fast and automated analyses of samples surfaces at lower costs than conventional techniques. Calculated vesicle size distributions and vesicle number densities of the analyzed samples were compared in order to determine the potential and precision of volume and area information directly obtainable from CSLM measurements in 3D. Monodisperse samples with homogeneous vesicle texture show remarkable similarities in vesicle size and volume distribution, when compared directly with the two methods. For all experimental and natural samples, roughly half of them show VNDs and porosity show the same range, which can be explained by vesicle coalescence. After de-coalescing the images of CSLM for FOAMS analysis, these differences vanish almost entirely for most of the sample types. Combining our textural data and our numerical modeling, we present a universal scaling model to link surface roughness with vesicle number density, porosity, and polydispersivity. These results indicate that CSLM can be a powerful tool to investigate vesicle structures from the millimetric to micrometric scale. The resulting data is both quantitatively and qualitatively equivalent to SEM-derived data for a variety of volcanic rock types. This, combined with the simple sample preparation and handling make CSLM a valid alternative to SEM investigations to characterize vesicles, particles, and any sort of rock property that reflects on topography. Surface roughness can become a new key parameter to interpret porous networks in volcanic rocks and can be applied to other geological fields.
•Confocal Scanning Laser Microscopy (CSLM) is introduced to measure vesicle textures in volcanic rocks.•2D and 3D data is compared with SEM measurements and image analysis software.•Stereological conversion yields highly comparable data, while the effects of vesicle coalescence become visible.•We finally present a model to link surface roughness to vesicle number density.
Mt. Tongariro is a multi-vent volcanic complex in the central North Island of New Zealand, constructed along the axis of an active graben. Its five largest-known explosive eruption episodes occurred ...11,000 years ago during a ~200 year-long paroxysm. During these, stable Plinian and collapsing sub-Plinian columns erupted from multiple sequential and synchronous vents along the graben axis. We examined the spatially varying eruption dynamics during the 200-year paroxysm through the lens of pyroclastic textures. Each eruption episode produced micro-vesicular basaltic-andesitic juvenile lapilli with high vesicle number densities and high groundmass crystallinity. Textural characteristics indicate that pressurisation via second boiling drove eruptions and rhyolite-like melt viscosities promoted brittle failure. Highly variable and mingled textures within individual juvenile lapilli evidence diverse conditions developed across the conduit during the eruption. Foamy magma rose rapidly through the conduit core, driving explosive vigour, and dragged and sheared gas-poor crystallised magma from the conduit margins. Consistent with the northward-widening graben, the most stable Plinian eruptions occurred at northern vents, under highest extensional stresses and possibly widest most stable conduits. By contrast, collapsing, unstable eruption columns occurred at southern vents, where mainly degassed and crystalline magma erupted at lower ejection rates through narrower, unstable and shifting conduits. These data collectively show how spatial variations in structure and tectonics may strongly influence explosive eruption style.
•Spatially varying pyroclast textures influenced by the dynamic structural setting•Internal magma overpressure combined with external tectonic drivers triggered eruptions•Variable juvenile textures evidenced diverse conduit conditions during the eruptions
•The polydispersivity S of the VSD strongly controls degassing in basaltic magmas.•S and ϕc influence the broad range of eruptive style observed at basaltic volcanoes.•We discuss the effect of S and ...ϕc for effusive, explosive and submarine eruptions.•Monodisperse bubble networks favour a low ϕc and the formation of plugs and slugs.•Connectivity definitions have different implications for gas percolation in basalts.
Due to their generally low eruptive melt viscosities and concomitant high diffusivities of volatiles, basaltic magmas degas relatively efficiently. This relative efficiency, combined with variations in style, extent, timing and length scales of degassing govern the range of eruptive styles observed at basaltic volcanoes. The result is a surprising complexity of degassing regimes and products in basaltic volcanism. In particular, the transition between closed- and open-system degassing at low pressure at the percolation threshold may strongly affect the type of eruption. Here we aim to better understand degassing and gas percolation processes in basaltic magmas and their implications for eruptive style. Combining new and literature data, we present a database of vesicle metrics in basaltic rocks including vesicularity, vesicle number density, vesicle size distribution (and its polydispersivity), vesicle connectivity and permeability. We combine these textural and petrophysical data with a numerical model of percolation for systems having polydisperse vesicle size distributions. Using this model, we also evaluate different definitions of vesicle connectivity inherent to different measurement techniques. Our results show that polydispersivity exerts a strong control on the percolation threshold of basaltic magmas and consequently on eruptive style. Intermediate to highly polydisperse bubble networks are more typical of Hawaiian activity and are characterized by higher values of percolation threshold. This results in delayed coalescence and an increase in magma vesicularity hindering the formation of large decoupled and buoyant bubbles, which in turn can promote magma acceleration, fragmentation by inertia below the percolation threshold and sustained fountaining activity. Bubble populations with lower polydispersivity, typical of Strombolian eruptions, promote early coalescence prior to fragmentation, which in turn may lead to the formation of large decoupled slugs or gas pockets and/or plugs at the surface via outgassing. Further, we discuss the implications of our findings for Plinian, violent Strombolian, Surtseyan, deep submarine and effusive basaltic eruptions.
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