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Settling‐driven gravitational instabilities (SDGIs) can form at the base of buoyant particle‐laden suspensions, modulating particle sedimentation in various settings such as meteorological and volcanic clouds, fluvial plumes, magma chambers, submarine hydrothermal plumes, or industrial emissions. These instabilities result in the formation of rapidly descending currents called 'fingers' within which fine particles settle faster collectively than individually. This study investigates SDGI triggering conditions underneath volcanic ash clouds through analogue experiments considering sedimentation from aqueous particle suspensions. We confirm that the conditions for which SDGIs develop are controlled by two dimensionless numbers: Bf (ratio of the characteristic finger velocity to the individual particle settling velocity); and Bi (ratio of timescale for individual particle settling to that for collective settling controlled by inertial drag). SDGIs are triggered for values of Bf and Bi > 1 for which particles are fully coupled with the flow within fingers. Using these parameters, we produce a regime diagram for the 2010 eruption of Eyjafjallajökull (Iceland) that describes particle settling as a function of particle concentration and size. More studies are needed to produce a general regime diagram accounting for the evolution of SDGIs properties with eruption and atmospheric parameters. Nonetheless, our study confirms that fingers affect sedimentation from volcanic clouds with high ash volume fractions above 10−6 vol.%. Our validation of criteria predicting the onset of fingers due to SDGIs constitutes a step forward toward the incorporation of these collective settling processes in volcanic ash transport and dispersion models. Plain Language Summary: Fine particles can fall more rapidly than individually when settling collectively within vertical currents that are usually termed 'fingers'. Fingers have been observed to form in different natural environments, such as the base of volcanic ash clouds, where they can result from the destabilization of the cloud base and affect the deposition of volcanic ash to the ground. We conducted laboratory experiments simulating particle settling in water to understand the conditions that trigger instabilities and the formation of fingers. We confirm that high particle concentrations of fine particles promote the occurrence of instabilities. In general, instabilities occur when particles remain coupled with the fluid, which is the case when the ratio of the finger velocity to the individual particle velocity is greater than one, as parameterized by two dimensionless numbers. Using these parameters, we created a regime diagram describing particle settling during the 2010 Eyjafjallajökull eruption. More research is needed to create a comprehensive diagram considering other factors like eruption parameters. Key Points: Settling‐driven gravitational instabilities can produce fingers under buoyant particle suspensions, impacting particle sedimentationConditions for the onset of instabilities in laboratory experiments are predicted by ratios of the terminal settling and finger velocitiesRegime diagrams predicting the formation of instabilities below volcanic ash clouds can be obtained [ABSTRACT FROM AUTHOR]

The selected ice nanoparticle accelerator, SELINA, was used to prepare beams of single ice particles with positive or negative charge. Positively charged particles were prepared from deionized water and 0.05–0.2 molar solutions of sodium chloride in water, and negatively charged ice particles were generated from water without salt. Depending on the electrospray source configuration, the measured particles vary from 50 to 1000 nm in diameter. The kinetic energy per charge for all particles was set to 200 eV by the collisional equilibration in quadrupoles, which resulted in primary velocities up to 600 m/s for the lowest m/z particles. The electrospray ionization and thus particle formation from SELINA become less efficient with increasing salt concentration, resulting in a lower detected particle frequency and size. Good instrument operation is achievable for concentrations below 0.2 M. After we have verified and characterized positively and negatively charged ice particles, we have combined SELINA with a target and time-of-flight spectrometer for a 'proof-of-principle' post acceleration of 120 nm particles towards hypervelocity (v ~ 3000 m/s) and detection of fragments from the particle impact (SELINA-HIMS). General conditions are discussed for the acceleration of particles between 50 and 1000 nm to velocities well above 3000 m/s with SELINA-HIMS instrument. This article is part of the theme issue 'Dust in the Solar System and beyond'. [ABSTRACT FROM AUTHOR]

Fissure eruptions initiate with magma ascending and spreading through cracks in the ground that can extend for kilometres at the surface. Eruptions eventually localise to form one or a few persistent conduits and ultimately an array of discrete cones or craters. We built a new experimental apparatus to investigate the influences of fissure shape and wall-rock temperature on localisation within a volcanic fissure, and the thermal feedbacks associated with variability of these parameters. Our artificial fissure, or "Artfish," has a slot geometry with adjustable shape and wall temperature. We can simulate both starting variability in fissure geometry and wall temperature, as well as changes in these parameters during an experiment to replicate, for example, blockage by wall-rock collapse, widening by wall-rock erosion, and warming by adjacent intrusions. We use polyethylene glycol (PEG 600) for our analogue fluid. A variable-speed pump allows for a range of fluid injection and ascent rates. Initial tests showcase the capabilities of the model and the types of data that may be acquired. Additional key features achieved include a stable and planar injection system, fluid recycling, and the use of particle tracers for monitoring flow patterns and velocities. The thermal evolution of the fluid-wall interface is quantitatively measured with thermal sensors, and the change in state of the PEG provides a clear visual indication of flow behaviour and solidification progress recorded on video. The potential experiments that can be conducted with this highly versatile model are numerous and will be used to gain a better understanding of the thermal controls on flow localisation and conduit development. This will assist hazard modellers to assess controls on eruption evolution and potentially to forecast sites where an initial fissure eruption may focus. [ABSTRACT FROM AUTHOR]

We conducted analogue experiments to examine flux‐driven and buoyancy‐driven magma ascent, which included a series of isothermal experiments and thermal, solidification‐prone experiments. We measured the internal flow using 2D particle image velocimetry, which indicates that buoyancy has a strong control on the flow pattern of isothermal dikes. Dikes that are not buoyant (likely driven by source pressure) take on a circulating pattern, while buoyant dikes assume an ascending flow pattern. Solidification modifies the flow field so that flow is confined to the dike's upper head region. The lower tail becomes mostly solidified, with a narrow conduit connecting the source to the head. We interpret that this conduit acts as a high velocity point source to the head, promoting a circulating flow pattern, even as the dike becomes buoyant. We then perform particle tracking velocimetry on several particles to illustrate the complexity of their paths. In a circulating flow pattern, particles rise to the top of the dike, descend near the lateral edge, and then are drawn back into the upward flow. In an ascending pattern, particles ascend slightly faster than the propagation velocity, and therefore are pushed to the side as they approach the upper tip. In erupting dikes, particles simply flow to the vent. In the context of crystal growth in magmatic dikes, these results suggest that crystal growth patterns (e.g., normal or oscillatory zoning) can reflect the magma flow pattern, and potentially the driving forces. Plain Language Summary: Dikes are magma‐filled cracks, which grow through the earth and can feed volcanic eruptions. Depending on the forces acting on the magma, including buoyancy, the magma inside can flow in different ways. We studied the flow inside of dikes using small, laboratory experiments. We injected oil, water, or liquid gelatin, mimicking the magma, into a block of transparent gelatin, mimicking the Earth's crust. We added small tracer particles to the liquids, which could be tracked using cameras, to understand the flow inside of our experiments. We found that when buoyancy is low, for example, when dikes are small, the internal flow circulates up and down. However, as the buoyancy increases, they begin to flow upward. These results can be related to small crystals that form in magma in nature and record chemical information in layers (known as zoning), like rings in a tree. If a crystal circulates up and down, it can look different from one that just rises up. Such crystals may give us information about the forces that control dikes at volcanoes. Key Points: In isothermal, buoyantly propagating dikes, a dike's internal flow ascends, whereas in source‐pressure controlled dikes, it circulatesSolidification generates a conduit in the dike tail, which acts as a high velocity point source to the head, promoting circulationErupting dikes assume a simple ascending flow pattern, as liquid flow directly from the source to surface [ABSTRACT FROM AUTHOR]

Mountain landscapes respond to transient tectonic and climate forcing through a bottom‐up response of enhanced bedrock river incision that undermines adjoining hillslopes, thus propagating the signal from the valley bottom to the valley ridges. As a result, understanding the mechanisms that set the pace and pattern of bedrock river incision is a critical first step for predicting the wider mechanisms of landscape evolution. Typically, the focus has been on the impact of channel bed lowering by the upstream migration of knickpoints on the angle, length and relief of adjoining hillslopes, with limited attention on the role of dynamic channel width. Here, we present a suite of physical model experiments that show the direct impact of knickpoint retreat on the reach‐scale channel width, across a range of flow discharges (8.3 to 50 cm3 s−1) and two sediment discharges (0 and 0.00666 g cm−3). During knickpoint retreat, the channel width narrows to as little as 10% of the equilibrium channel width, while the bed shear stress is >100% higher immediately upstream of a knickpoint compared to equilibrium conditions. We show that only a fraction of the channel narrowing can be explained by existing hydraulic theory. Following the passage of a knickpoint, the channel width returns to equilibrium through lateral erosion and widening. For the tested knickpoint height, we demonstrate that the lateral adjustment process can be more significant for hillslope stability than the bed elevation change, highlighting the importance of considering both vertical and lateral incision in landscape evolution models. It is therefore important to understand the key processes that drive the migration of knickpoints, as the localized channel geometry response has ongoing implications for the stability of adjoining hillslopes and the supply of sediment to the channel network and export from landscapes onto neighbouring depositional plains. [ABSTRACT FROM AUTHOR]

Spaceborne impact ionization mass spectrometers, such as the Cosmic Dust Analyzer on board the past Cassini spacecraft or the SUrface Dust Analyzer being built for NASA's upcoming Europa Clipper mission, are of crucial importance for the exploration of icy moons in the Solar System, such as Saturn's moon Enceladus or Jupiter's moon Europa. For the interpretation of data produced by these instruments, analogue experiments on Earth are essential. To date, thousands of laboratory mass spectra have been recorded with an analogue experiment for impact ionization mass spectrometers. Simulation of mass spectra of ice grains in space is achieved by a Laser Induced Liquid Beam Ion Desorption (LILBID) approach. The desorbed cations or anions are analyzed in a time‐of‐flight mass spectrometer. The amount of unstructured raw data is increasingly challenging to sort, process, interpret and compare with data from space. Thus far this has been achieved manually for individual mass spectra because no database containing the recorded reference spectra was available. Here we describe the development of a comprehensive, extendable database containing cation and anion mass spectra from the laboratory LILBID facility. The database is based on a Relational Database Management System with a web server interface and enables filtering of the laboratory data using a wide range of parameters. The mass spectra can be compared not only with data from past and future space missions but also mass spectral data generated by other, terrestrial, techniques. The validated and approved subset of the database is available for general public (https://lilbid-db.planet.fu-berlin.de). Plain Language Summary: Thousands of laboratory mass spectra, each with an individual set of experimental parameters, have been recorded so far using a facility situated at Freie Universität Berlin, Germany. The mass spectra help analyze and interpret data returned from spacecraft in the vicinity of icy moons in the Solar System. The unstructured laboratory data is increasingly challenging to sort and compare to the data from space. We developed an extendable database containing the laboratory data. The database is available for general public and allows filtering the stored data for a wide range of experimental parameters and, in turn, significantly improves analysis of data not only from past space missions but also future missions in particular. Key Points: We describe the development of a comprehensive spectral database containing laboratory analogue data for spaceborne mass spectrometersThe database is based on a Relational Database Management System with a web interface and accessible for community useFiltering the laboratory data using a wide range of experimental parameters allows a straightforward analysis of returned flight data [ABSTRACT FROM AUTHOR]

Three-phase suspensions, of liquid that suspends dispersed solid particles and gas bubbles, are common in both natural and industrial settings. Their rheology is poorly constrained, particularly for high total suspended fractions (≳0.5). We use a dam-break consistometer to characterize the rheology of suspensions of (Newtonian) corn syrup, plastic particles and CO2 bubbles. The study is motivated by a desire to understand the rheology of magma and lava. Our experiments are scaled to the volcanic system: they are conducted in the non-Brownian, non-inertial regime; bubble capillary number is varied across unity; and bubble and particle fractions are 0 ≤ φgas ≤ 0.82 and 0 ≤ φsolid ≤ 0.37, respectively. We measure flow-front velocity and invert for a Herschel–Bulkley rheology model as a function of ϕgas , ϕsolid , and the capillary number. We find a stronger increase in relative viscosity with increasing ϕgas in the low to intermediate capillary number regime than predicted by existing theory, and find both shear-thinning and shear-thickening effects, depending on the capillary number. We apply our model to the existing community code for lava flow emplacement, PyFLOWGO, and predict increased viscosity and decreased velocity compared with current rheological models, suggesting existing models may not adequately account for the role of bubbles in stiffening lavas. [ABSTRACT FROM AUTHOR]

Settling-driven gravitational instabilities observed at the base of volcanic ash clouds have the potential to play a substantial role in volcanic ash sedimentation. They originate from a narrow, gravitationally unstable region called a Particle Boundary Layer (PBL) that forms at the lower cloud-atmosphere interface and generates downward-moving ash fingers that enhance the ash sedimentation rate. We use scaled laboratory experiments in combination with particle imaging and Planar Laser Induced Fluorescence (PLIF) techniques to investigate the effect of particle concentration on PBL and finger formation. Results show that, as particles settle across an initial density interface and are incorporated within the dense underlying fluid, the PBL grows below the interface as a narrow region of small excess density. This detaches upon reaching a critical thickness, that scales with (ν2/g′)1/3, where ν is the kinematic viscosity and g′ is the reduced gravity of the PBL, leading to the formation of fingers. During this process, the fluid above and below the interface remains poorly mixed, with only small quantities of the upper fluid phase being injected through fingers. In addition, our measurements confirm previous findings over a wider set of initial conditions that show that both the number of fingers and their velocity increase with particle concentration. We also quantify how the vertical particle mass flux below the particle suspension evolves with time and with the particle concentration. Finally, we identify a dimensionless number that depends on the measurable cloud mass-loading and thickness, which can be used to assess the potential for settling-driven gravitational instabilities to form. Our results suggest that fingers from volcanic clouds characterised by high ash concentrations not only are more likely to develop, but they are also expected to form more quickly and propagate at higher velocities than fingers associated with ash-poor clouds.

Dikes feed most eruptions, so understanding their mechanism of propagation is fundamental for volcanic hazard assessment. The variation in geometry of a propagating dike as a function of the injection rate remains poorly studied. Here we use experiments injecting water into gelatin to investigate the variation of the thickness, width and length of a flux-driven dike connected to its source as a function of the injection time and intruded volume. Results show that the thickness of vertically propagating dikes is proportional to the injection rate and remains constant as long as the latter is constant. Neither buoyancy nor injected volume influence the thickness. The along-strike width of the dike is, however, proportional to the injected volume. These results, consistent with the inferred behavior of several dikes observed during emplacement, open new opportunities to better understand how dikes propagate and also to forecast how emplacing dikes may propagate once their geometric features are detected in real-time through monitoring data.

Abstract Numerical simulations of lava flow emplacement are valuable for assessing lava flow hazards, forecasting active flows, designing flow mitigation measures, interpreting past eruptions, and understanding the controls on lava flow behavior. Existing lava flow models vary in simplifying assumptions, physics, dimensionality, and the degree to which they have been validated against analytical solutions, experiments, and natural observations. In order to assess existing models and guide the development of new codes, we conduct a benchmarking study of computational fluid dynamics (CFD) models for lava flow emplacement, including VolcFlow, OpenFOAM, FLOW-3D, COMSOL, and MOLASSES. We model viscous, cooling, and solidifying flows over horizontal planes, sloping surfaces, and into topographic obstacles. We compare model results to physical observations made during well-controlled analogue and molten basalt experiments, and to analytical theory when available. Overall, the models accurately simulate viscous flow with some variability in flow thickness where flows intersect obstacles. OpenFOAM, COMSOL, and FLOW-3D can each reproduce experimental measurements of cooling viscous flows, and OpenFOAM and FLOW-3D simulations with temperature-dependent rheology match results from molten basalt experiments. We assess the goodness-of-fit of the simulation results and the computational cost. Our results guide the selection of numerical simulation codes for different applications, including inferring emplacement conditions of past lava flows, modeling the temporal evolution of ongoing flows during eruption, and probabilistic assessment of lava flow hazard prior to eruption. Finally, we outline potential experiments and desired key observational data from future flows that would extend existing benchmarking data sets.

Mixed‐mode fluid‐filled cracks represent a common means of fluid transport within the Earth's crust. They often show complex propagation paths which may be due to interaction with crustal heterogeneities or heterogeneous crustal stress. Previous experimental and numerical studies focus on the interplay between fluid overpressure and external stress but neglect the effect of other crack parameters. In this study, we address the role of crack length on the propagation paths in the presence of an external heterogeneous stress field. We make use of numerical simulations of magmatic dike and hydrofracture propagation, carried out using a two‐dimensional boundary element model, and analogue experiments of air‐filled crack propagation into a transparent gelatin block. We use a 3‐D finite element model to compute the stress field acting within the gelatin block and perform a quantitative comparison between 2‐D numerical simulations and experiments. We show that, given the same ratio between external stress and fluid pressure, longer fluid‐filled cracks are less sensitive to the background stress, and we quantify this effect on fluid‐filled crack paths. Combining the magnitude of the external stress, the fluid pressure, and the crack length, we define a new parameter, which characterizes two end member scenarios for the propagation path of a fluid‐filled fracture. Our results have important implications for volcanological studies which aim to address the problem of complex trajectories of magmatic dikes (i.e., to forecast scenarios of new vents opening at volcanoes) but also have implications for studies that address the growth and propagation of natural and induced hydrofractures. Plain Language Summary: Fluids move within the Earth by means of different mechanisms. One of the most relevant mechanisms, particularly for magma transport within the lithosphere, is the propagation through fluid‐filled fractures: the fluid (or magma) can create its own path through the crustal rocks by fracturing them. If the density of the fluid is lower than the density of the rocks, the fluid would be pushed upward by buoyancy (similarly to a gas bubble in water). However, the propagation path followed by these fluid‐filled fractures may be complex. This may be due to several factors, including the forces (stresses) acting within the crust because of plate tectonic or because of remarkable topographic features. Here we make use of computer simulations and laboratory experiments to test how fluid‐filled fractures interact with such crustal stresses. We quantify how the competition between (i) crustal stresses, (ii) fluid (or magma) pressure, and (iii) the length of a fluid‐filled fracture may affect its direction of propagation. We define a critical range of values for a parameter which may help identifying the path of a fluid‐filled fracture propagating through the Earth crust. Our results may have important implications for volcanological studies which aim to forecast scenarios of new eruption locations. Key Points: We make use of analogue experiments and numerical simulations to study the propagation path of fluid‐filled cracks in interaction with crustal stressesWe show and quantify how the competition between crustal stresses, fluid pressure, and crack length affect the path of fluid‐filled cracksWe provide a critical range of values for a parameter which may help predicting the propagation path of a fluid‐filled crack [ABSTRACT FROM AUTHOR]

From deformed quartzites in the Singhbhum Shear Zone, eastern India, we report shear fractures of varying surface roughness: very smooth, containing no lineation to strongly rough with prominent slickenlines. We reproduced them in analogue laboratory experiments, which suggest that the modes of shear failure (brittle versus ductile) and the fracture orientation are potential factors to control the fracture roughness. The experiments were conducted on cohesive sand‐talc models with varying sand:talc volume ratio. Pure sand models underwent Coulomb failure in the brittle regime; this failure mode switched to plastic yielding in the ductile regime with increasing talc content. Such a transition in failure behavior resulted in a remarkable variation in the fracture roughness characteristics. Shear fractures produced by Coulomb failure are smooth, and devoid of any slickenlines, whereas those produced by plastic yielding display strongly linear roughness, defined by cylindrical ridges along the slip direction. Such linear irregularities become more prominent with increasing fracture orientation (θ) to the compression direction (θ = 30 to 60°). We develop a new computational technique, based on controlled optical images to map the shear surface geometry from field casts and laboratory samples. Binarization of the irregular surface images (cantor set) provides 1‐D fractal dimension (D), which is used to quantify the roughness variability, and the degree of their anisotropy in terms of ΔD (difference in D across and along the slip direction). From numerical models, we finally show onset of wave instability in the mechanically distinct rupture zone as an alternative mechanism for slickenline formation. Key Points: Three types of shear fracture surfaces observed in field: smoothly polished, linear roughness with small ridges and courser ridgesExperimental data show that fracture roughness is largely controlled by the rheology and orientation of the fracture with principal stressesThe study proposes slip‐induced wave‐instability as an alternative mechanism for slickenline formation in field [ABSTRACT FROM AUTHOR]

Abstract Whereas tectonically erosive convergent margins make up nearly 60% of all present-day convergent margins, processes and amounts of material transfer and recycling remain enigmatic. Removal of material from the frontal forearc leaves no features which could be imaged seismically or probed through drilling. Only a few scaled laboratory analogue experiments have analyzed material transfer at convergent margins characterized by long-term subduction erosion and none has focused on processes and amounts of material transfer and recycling. Therefore, in this study, we attempt to identify potentially relevant parameters controlling material transfer in brittle, tectonically erosive forearcs. This is addressed by a series of sandbox experiments performed with granular materials. We here mainly focus on the amount of sediment that can be transported downdip through the subduction channel (SC). We analyzed our experiments with regard to the evolution of internal and basal material transfer, material transfer mode patterns, wedge geometry and SC evolution. To achieve a truly erosive scenario, we built a large initial wedge representing the brittle forearc, featuring a moderately high-friction basal detachment, and composed of "strong" granular material. There was no incoming material. The slope of the wedge initially had a critical taper. Through opening a subduction gap (SG), the wedge was subject to rear material loss. Our observations point to a close interaction between SC segmentation and wedge deformation. Basal erosion mainly took place beneath the middle-upper slope. Our experiments confirmed the strong influence of the SC on the wedge evolution. A larger amount of material loss led to the development of a SC with an inhomogeneous distribution of velocities along the base of the wedge, segmenting the SC and slope geometry. The latter was comparable to observations in natural forearcs, which are segmented into lower, middle and upper slopes. Highlights • Our experiments with a thin Subduction Gap developed a Subduction Channel with uniform velocities along the base of the wedge. • A thick Subduction Gap led to a segmented Subduction Channel and also a wedge, indicative of basal removal of material. • Basal erosion exceeded frontal erosion when the capacity of the Subduction Gap exceeded that of the Subduction Channel. [ABSTRACT FROM AUTHOR]

Passive degassing – i.e., the open-system degassing of a magmatic gas phase that is decoupled from the melt phase – is common at many basaltic volcanic systems and consistently makes a greater contribution to total volcanic gas emissions than eruptive degassing. However, the mechanism for passive degassing is not fully understood. We investigate the feasibility of permeable gas flow through connected bubbles or pathways using experiments with aqueous solutions of hydroxyethyl cellulose (HEC), a non-Newtonian analogue material for magma. Stable chains of connected bubbles have previously been reported in similar non-Newtonian polymer solutions. We observe a range of bubble chain phenomena and identified five regimes, numbering in order of increasing gas flow rate: 1) small individual bubbles; 2) chains of rounded bubbles; 3) chains of elongate bubbles; 4) pipe-like 'winding flue'; 5) large individual bubbles. The bubble chain phenomena (regimes 2–4) are observed over a restricted interval of gas flow rates. We determine the rheology of the solutions and conclude that the HEC solutions that produced bubble chain phenomena in our experiments are well scaled to shear-thinning and viscoelastic magmas, hence bubble chain phenomena could form in magmas. Our analysis also suggests that the viscoelastic rheology of HEC plays a fundamental role in the observed bubble chain phenomena. • We use analogue experiments to study a potential mechanism for open-system degassing in basaltic systems • We investigate the phenomena of connected chains of bubbles in a viscoelastic magma analogue • We identify five regimes of gas transport, which depend on gas flow rate and magma analogue concentration • We show that the rheology of the analogue solutions scales appropriately to bubbly magma • The rheology of bubbly magma is compatible with chain formation, but more experiments are needed to verify feasibility [ABSTRACT FROM AUTHOR]

Shear fracture surfaces are commonly rough on varied scales, and understanding the roughness properties is crucial to predict their slip behaviour. Using natural rock samples from the Singhbhum Shear Zone, eastern India, this study develops an approach to heterogeneous surface roughness analysis of shear fractures, forming two distinct domains: striated, anisotropic zone and non-striated, isotropic zone. Analogue experiments run on brittle-ductile models under a pure shear condition produce similar heterogeneous roughness characteristics of shear fractures. Using a laser profilometer, we constructed 3D topologies of both natural and experimental shear surfaces, which are presented to show the distinctive roughness characteristics of the striated and non-striated zones in terms of their fractal properties. ΔD , which stands for the difference between across- (D ⊥) and along- (D ∥) the slip direction, is calculated to evaluate the degree of their roughness anisotropy. This fractal parameter indicates strong anisotropy with ΔD = 0.0787–0.2118 in striated zones, which is significantly weak, where ΔD = 0.0024–0.0603 in non-striated zones. Finally, the article presents an in-depth discussion of this roughness study from the perspective of slip-stuck kinematics on shear surfaces from laboratory experiments and their geophysical implications, e.g., interpretation of earthquake event patterns on seismic faults and permeability of fractured rocks. [Display omitted] [ABSTRACT FROM AUTHOR]

The origin of subparallel, regularly-spaced longitudinal ridges often observed at the surface of volcanic and other rock avalanche deposits remains unclear. We addressed this issue through analogue laboratory experiments on flows of bi-disperse granular mixtures, because this type of flow is known to exhibit granular fingering that causes elongated structures resembling the ridges observed in nature. We considered four different mixtures of fine (300–400 μm) glass beads and coarse (600–710 μm to 900–1000 μm) angular crushed fruit stones, with particle size ratios of 1.9–2.7 and mass fractions of the coarse component of 5–50 wt%. The coarse particles segregated at the flow surface and accumulated at the front where flow instabilities with a well-defined wavelength grew. These formed granular fingers made of coarse-rich static margins delimiting fines-rich central channels. Coalescence of adjacent finger margins created regular spaced longitudinal ridges, which became topographic highs as finger channels drained at final emplacement stages. Three distinct deposit morphologies were observed: 1) Joined fingers with ridges were formed at low (≤ 1.9) size ratio and moderate (10–20 wt%) coarse fraction whereas 2) separate fingers or 3) poorly developed fingers, forming series of frontal lobes, were created at larger size ratios and/or higher coarse contents. Similar ridges and lobes are observed at the debris avalanche deposits of Tutupaca volcano, Peru, suggesting that the processes operating in the experiments can also occur in nature. This implies that volcanic (and non-volcanic) debris avalanches can behave as granular flows, which has important implications for interpretation of deposits and for modeling. Such behaviour may be acquired as the collapsing material disaggregates and forms a granular mixture composed by a right grain size distribution in which particle segregation can occur. Limited fragmentation and block sliding, or grain size distributions inappropriate for promoting granular fingering can explain why ridges are absent in many deposits. [ABSTRACT FROM AUTHOR]

Arrays of real mangrove pneumatophores (i.e. aboveground pencil roots) and artificial dowel mimics were constructed in a laboratory flume to examine differences in canopy flow dynamics. Compared to the uniform-height dowel canopy, the non-uniform height of the pneumatophores significantly reduced the intensity of the canopy shear, and shifted the turbulence maxima observed directly above the dowels upwards by approximately the standard deviation of the pneumatophore heights. Consequently, bed shear stresses were up to two times greater in the uniform-height dowel canopy than in a pneumatophore canopy of similar density. At the same time, ratios of the within-canopy velocity to the free-stream velocity above the canopies were not significantly altered by the heterogeneous height, shape and spatial distribution of the pneumatophores. Our results emphasize that uniform dowels are poor proxies of real pneumatophore canopies and may lead to underestimations of sediment-trapping efficiency. [ABSTRACT FROM AUTHOR]

The interpretation of geophysical measurements at active volcanoes is vital for hazard assessment and for understanding fundamental processes such as magma degassing. For Strombolian activity, interpretations are currently underpinned by first-order fluid dynamic models which give relatively straightforward relationships between geophysical signals and gas and magma flow. However, recent petrological and high-speed video evidence has indicated the importance of rheological stratification within the conduit and, here, we show that under these conditions, the straightforward relationships break down. Using laboratory analogue experiments to represent a rheologically-stratified conduit we characterise the distinct variations in the shear stress exerted on the upper sections of the flow tube and in the gas pressures measured above the liquid surface, during different degassing flow configurations. These signals, generated by varying styles of gas ascent, expansion and burst, can reflect field infrasonic measurements and ground motion proximal to a vent. The shear stress signals exhibit timescales and trends in qualitative agreement with the near-vent inflation–deflation cycles identified at Stromboli. Therefore, shear stress along the uppermost conduit may represent a plausible source of near-vent tilt, and conduit shear contributions should be considered in the interpretation of ground deformation, which is usually attributed to pressure sources only. The same range of flow processes can produce different experimental infrasonic waveforms, even for similar masses of gas escape. The experimental data resembled infrasonic waveforms acquired from different vents at Stromboli associated with different eruptive styles. Accurate interpretation of near-vent ground deformation, infrasonic signal and eruptive style therefore requires detailed understanding of: a) spatiotemporal magma rheology in the shallow conduit, and b) shallow conduit geometry, as well as bubble overpressure and volume. [ABSTRACT FROM AUTHOR]

The behaviour of a granular material is mainly dependent on its frictional properties, angle of internal friction, and cohesion, which, together with material density, are the key factors to be considered during the scaling procedure of analogue models. The frictional properties of a granular material are usually investigated by means of technical instruments such as a Hubbert-type apparatus and ring shear testers, which allow for investigating the response of the tested material to a wide range of applied stresses. Here we explore the possibility to determine material properties by means of different empirical methods applied to mixtures of quartz and K-feldspar sand. Empirical methods exhibit the great advantage of measuring the properties of a certain analogue material under the experimental conditions, which are strongly sensitive to the handling techniques. Finally, the results obtained from the empirical methods have been compared with ring shear tests carried out on the same materials, which show a satisfactory agreement with those determined empirically. [ABSTRACT FROM AUTHOR]

Numerical simulations of lava flow emplacement are valuable for assessing lava flow hazards, forecasting active flows, designing flow mitigation measures, interpreting past eruptions, and understanding the controls on lava flow behavior. Existing lava flow models vary in simplifying assumptions, physics, dimensionality, and the degree to which they have been validated against analytical solutions, experiments, and natural observations. In order to assess existing models and guide the development of new codes, we conduct a benchmarking study of computational fluid dynamics (CFD) models for lava flow emplacement, including VolcFlow, OpenFOAM, FLOW-3D, COMSOL, and MOLASSES. We model viscous, cooling, and solidifying flows over horizontal planes, sloping surfaces, and into topographic obstacles. We compare model results to physical observations made during well-controlled analogue and molten basalt experiments, and to analytical theory when available. Overall, the models accurately simulate viscous flow with some variability in flow thickness where flows intersect obstacles. OpenFOAM, COMSOL, and FLOW-3D can each reproduce experimental measurements of cooling viscous flows, and OpenFOAM and FLOW-3D simulations with temperature-dependent rheology match results from molten basalt experiments. We assess the goodness-of-fit of the simulation results and the computational cost. Our results guide the selection of numerical simulation codes for different applications, including inferring emplacement conditions of past lava flows, modeling the temporal evolution of ongoing flows during eruption, and probabilistic assessment of lava flow hazard prior to eruption. Finally, we outline potential experiments and desired key observational data from future flows that would extend existing benchmarking data sets. [ABSTRACT FROM AUTHOR]

The canonical Strombolian paradigm of a gas slug ascending and bursting in a homogeneous low-viscosity magma cannot explain the complex details in eruptive dynamics recently revealed by field measurements and textural and geochemical analyses. Evidence points to the existence of high-viscosity magma at the top of the conduit of Strombolian-type volcanoes, acting as a plug. Here, new experiments detail the range of flow configurations that develop during the ascent and burst of a slug through rheologically stratified magma within a conduit. End-member scenarios of a tube fully filled with either high- or low-viscosity liquid bracket three main flow configurations: (1) a plug sufficiently large to fully accommodate an ascending gas slug; (2) A plug that can accommodate the intrusion of low-viscosity liquid driven by the gas expansion, but not all the slug volume, so the slug bursts with the nose in the plug whilst the base is still in the low-viscosity liquid; (3) Gas expansion is sufficient to drive the intrusion of low-viscosity liquid through the plug, with the slug bursting in the low-viscosity layer emplaced dynamically above the plug. We show that the same flow configurations are viable at volcanic-scale through a new experimentally-validated 1D model and 3D computational fluid dynamic simulations. Applied to Stromboli, our results demonstrate that the key parameters controlling the transition between each configuration are gas volume, plug thickness and plug viscosity. The flow processes identified include effective dynamic narrowing and widening of the conduit, instabilities within the falling magma film, transient partial and complete blockage of the conduit, and slug disruption. These complexities influence eruption dynamics and vigour, promoting magma mingling and resulting in pulsatory release of gas. [ABSTRACT FROM AUTHOR]

Many intrusion-hosted Ni-Cu magmatic sulfide deposits, such as Voisey's Bay (Labrador, Canada) and Jinchuan (China), show evidence for late-stage mobilization of high-density, low-viscosity sulfide- rich melts, which range from sulfide-rich silicate magma pulses to coherent pulses of pure sulfide liquid. Here we investigate the possibility that such sulfide-rich pulses can be generated by the upward entrainment of pooled sulfide liquid by immiscible silicate magma within a two-phase, density-stratified staging chamber. The potential degree of entrainment is measured by the critical draw-up height d, defined as the height of the overlying silicate magma at which the lower sulfide liquid will be mobilized and entrained during withdrawal of silicate magma from the chamber. Previous analytical solutions for axisymmetric withdrawal from a density-stratified reservoir have focused on the volcanology of bimodal, felsic-mafic silicate magma systems. We present new laboratory experiments of axisymmetric withdrawal that are tuned to the physical properties of mafic-sulfide systems, and find that the draw-up scales previously applied to felsic-mafic systems can be applied to both viscous and inertial entrainment of sulfide liquid into silicate magma. Within viscous (Reynolds number, Re<10) to transitional regimes (Re~10-1000) d is dependent on the viscosity of the mafic layer, but independent of the viscosity of the sulfide layer; at a given volumetric magma flow rate (Q), higher viscosity mafic magmas will be more efficient sulfide entrainers than lower viscosity magmas. Entrainment is independent of viscosity within inertial regimes (Re>1000). In all dynamic regimes d is proportional to Q and inversely proportional to the density difference between the fluids. Significant mobilization and entrainment of sulfide liquid, as indicated by predicted draw-up cones with d between 1 and 10m, can occur under inertial conditions at high (Q~1-1000 m³ s-1) magma volumetric flow rates that are plausible for mid- to upper-crustal magma chambers. Although entrainment can potentially generate sulfide-rich magma pulses, it probably represents only an initial step in the mobilization of sulfide liquid and the genesis of magmatic sulfide deposits. Entrained sulfide liquids must be transported upwards and/or laterally from their site of mobilization towards their site of deposition, which may occur as coherent masses or as droplets depending on the dynamics of the system. Although entrainment may be viable at the scale of sills, dykes, chonoliths and lava flows, it may not be significant in scenarios where significant (>100m) thicknesses of molten mafic magma must be expelled prior to reaching 1-10m scale critical draw-up heights; in such scenarios gravitational backflow of sulfide may be a more viable mechanism for producing economic concentrations of Ni-Cu sulfide mineralization. [ABSTRACT FROM AUTHOR]

Strombolian activity is common in low-viscosity volcanism. It is characterised by quasi-periodic, short-lived explosions, which, whilst typically weak, may vary greatly in magnitude. The current paradigm for a strombolian volcanic eruption postulates a large gas bubble (slug) bursting explosively after ascending a conduit filled with low-viscosity magma. However, recent studies of pyroclast textures suggest the formation of a region of cooler, degassed, more-viscous magma at the top of the conduit is a common feature of strombolian eruptions. Following the hypothesis that such a rheological impedance could act as a ‘viscous plug’, which modifies and complicates gas escape processes, we conduct the first experimental investigation of this scenario. We find that: 1) the presence of a viscous plug enhances slug burst vigour; 2) experiments that include a viscous plug reproduce, and offer an explanation for, key phenomena observed in natural strombolian eruptions; 3) the presence and extent of the plug must be considered for the interpretation of infrasonic measurements of strombolian eruptions. Our scaled analogue experiments show that, as the gas slug expands on ascent, it forces the underlying low-viscosity liquid into the plug, creating a low-viscosity channel within a high-viscosity annulus. The slug's diameter and ascent rate change as it enters the channel, generating instabilities and increasing slug overpressure. When the slug reaches the surface, a more energetic burst process is observed than would be the case for a slug rising through the low-viscosity liquid alone. Fluid-dynamic instabilities cause low and high viscosity magma analogues to intermingle, and cause the burst to become pulsatory. The observed phenomena are reproduced by numerical fluid dynamic simulations at the volcanic scale, and provide a plausible explanation for pulsations, and the ejection of mingled pyroclasts, observed at Stromboli and elsewhere. [ABSTRACT FROM AUTHOR]

Traditional strain analysis assumes that a particle deforms affinely with its matrix and that the magnitude and geometry of particle deformation are representative of the bulk strain. However, there are forces, lumped together under the terms of competency or viscosity contrast, which oppose particle deformation. For small weak particles, surface tension may also be important in resisting particle deformation. We performed analogue model experiments to investigate the effect of surface tension on the deformation of a weak viscous particle (bubble or droplet) enclosed in a viscous matrix. The experiments were performed using a ring-shear apparatus allowing large magnitudes of plane strain, simple shear. The results demonstrate the controlling influence of capillary number on particle deformation. The particles did not continuously elongate with increasing shear strain but achieved a steady-state value dependent on capillary number. Because capillary number represents the ratio of shear stresses, which deform particles, to the surface (interfacial) stresses restraining elongation, there should be a strong dependence of particle strain on the deformed particle size and bulk strain rate. The most important geological implications of this study are in the field of melt segregation in deforming anatectic systems. • Deforming viscous particles attain steady strain. • The strain magnitude of deformed viscous particles is representative of the strain rate. • Capillary number of the deformation causes dependence of strain on the deformed particle size. • Melt extraction may be more efficient in faster deforming migmatites. [ABSTRACT FROM AUTHOR]

We develop a model for the rheology of a threephase suspension of bubbles and particles in a Newtonian liquid undergoing steady flow. We adopt an 'effective-medium' approach in which the bubbly liquid is treated as a continuous medium which suspends the particles. The resulting three-phase model combines separate two-phase models for bubble suspension rheology and particle suspension rheology, which are taken from the literature. The model is validated against new experimental data for three-phase suspensions of bubbles and spherical particles, collected in the low bubble capillary number regime. Good agreement is found across the experimental range of particle volume fraction (0 = ɸp ≲ 0.5) and bubble volume fraction (0 = ɸb ≲ 0.3). Consistent with model predictions, experimental results demonstrate that adding bubbles to a dilute particle suspension at low capillarity increases its viscosity, while adding bubbles to a concentrated particle suspension decreases its viscosity. The model accounts for particle anisometry and is easily extended to account for variable capillarity, but has not been experimentally validated for these cases. [ABSTRACT FROM AUTHOR]

Abstract: The prediction of lava flow advance and velocity is crucial during an effusive volcanic crisis. The effusion rate is a key control of lava dynamics, and proxies have been developed to estimate it in near real-time. The thermal proxy in predominant use links the satellite-measured thermal radiated power to the effusion rate. It lacks however a robust physical basis to allow time-dependent modeling. We investigate here through analogue experiments the coupling between the spreading of a solidifying flow and its surface thermal signal. We extract a first order behavior from experimental results obtained using polyethylene glycol (PEG) wax, that solidifies abruptly during cooling. We find that the flow advance is discontinuous, with relatively low supply rates yielding long stagnation phases and compound flows. Flows with higher supply rates are less sensitive to solidification and display a spreading behavior closer to that of purely viscous currents. The total power radiated from the upper surface also grows by stages, but the signal radiated by the hottest and liquid part of the flow reaches a quasi-steady state after some time. This plateau value scales around half of the theoretical prediction of a model developed previously for the spreading and cooling of isoviscous gravity currents. The corrected scaling yields satisfying estimates of the effusion rate from the total radiated power measured on a range of basaltic lava flows. We conclude that a gross estimate of the supply rate of solidifying flows can be retrieved from thermal remote-sensing, but the predictions of lava advance as a function of effusion rate appears a more difficult task due to chaotic emplacement of solidifying flows. [Copyright &y& Elsevier]

Abstract: We demonstrate the rise and expansion of a gas slug as a fluid dynamic source mechanism for infrasonic signals generated by gas puffing and impulsive explosions at Stromboli. The fluid dynamics behind the rise, expansion and burst of gas slugs in the confines of an experimental tube can be characterised into different regimes. Passive expansion occurs for small gas masses, where negligible dynamic gas over-pressure develops during bubble ascent and, prior to burst, meniscus oscillation forms an important infrasonic source. With increasing gas mass, a transition regime emerges where dynamic gas over-pressure is significant. For larger gas masses, this regime transforms to fully explosive behaviour, where gas over-pressure dominates as an infrasonic source and bubble bursting is not a critical factor. The rate of change of excess pressure in the experimental tube was used to generate synthetic infrasonic waveforms. Qualitatively, the waveforms compare well to infrasonic waveforms measured from a range of eruptions at Stromboli. Assuming pressure continuity during flow through the vent, and applying dimensionless arguments from the first-order experiments, allows estimation of eruption metrics from infrasonic signals measured at Stromboli. Values of bubble length, gas mass and over-pressure calculated from infrasonic signals are in excellent agreement with those derived by independent means for eruptions at Stromboli, therefore providing a method of estimating eruption metrics from infrasonic measurement. [Copyright &y& Elsevier]

To improve knowledge of the quartz sand used in analogue modeling, two laboratory tests were carried out to investigate the frictional properties of colored sand. The first one estimated the coefficient of internal friction from the reverse fault dip simulated in a pack of different colored dry sand layers. The second test consisted of measuring stress-strain curves separately for different colored sand using a ring-shear tester. The coefficient of internal friction determined from fault dips is significantly lower and less precise than that determined on the same material with a ring-shear tester. Comparisons with previous granular material tests reveal that although our ring-shear test results are high, they are within the range of values reported in literature. Our results also show that natural and colored sands have different angles of internal friction, and that the Acrilex dye produces a distinct mechanical behaviour for different colored sand. The variation is related to the chemical composition of sand dyes and should be carefully evaluated in future sandbox experiments [ABSTRACT FROM AUTHOR]

Abstract: Core–mantle differentiation of small planetesimals has been suggested to be initiated by deformation-assisted segregation of molten Fe–S through a crystalline peridotite matrix. In this study we used two different experimental approaches to investigate the effect of varying the strain rate and of varying the dihedral angle of the liquid phase on liquid interconnectivity. (i) High pressure experiments were performed on simplified systems consisting of olivine plus a small percentage of liquid FeS (dihedral angle ~75°) or gold (dihedral angle ~150°) at a temperature above the FeS/Au solidus but below the silicate solidus. Samples were deformed at high pressure in a deformation-DIA multianvil press at strain rates between 10−3 and 10−6 s−1. (ii) Optical in situ analogue experiments on norcamphor plus H2O (dihedral angle ~86°) were performed at room temperature to complement the d-DIA experiments. The crystalline matrix in both the d-DIA and analogue experiments deforms by grain boundary migration assisted dislocation creep under the experimental conditions. Furthermore, the liquid pockets display a similar strain rate dependent behaviour in both sets of experiments: At high strain rates deformation is strongly localised by the liquid phase, and liquid interconnection and segregation occurs. At low strain rates the geometry of liquid pockets is controlled predominantly by surface tension and little elongation and interconnection occurs. We propose a new empirical scaling approach using the d-DIA results, which indicates that some degree of high dihedral angle liquid interconnection and segregation may be possible down to a few percent residual liquid under natural conditions for moderate dihedral angles (60°<Θ<90°). This, however, would not suffice for efficient core–mantle segregation and an additional mechanism, such as a magma ocean, would be required. [Copyright &y& Elsevier]

Abstract: We investigate the structural evolution of fault-propagation folds and fold-thrust systems with scaled analog modeling carried out using the 5.5 m radius geotechnical centrifuge at C-CORE, St. John’s NL. The experiments presented here are the first of their kind, scaled ten times larger than predecessors and deformed using a custom rig with load monitoring and displacement control. Plane-layered models approximately 1 m long and representing 50 km sections are shortened horizontally under an enhanced gravity field of 160 g. The large model scale allows for a proportionally large number of bedding laminations that act as strain markers. This allows detailed analysis of strain partitioning and interplay, both at the scale of a fold-thrust system and the individual fold-thrust structure. Layer-parallel shortening (“LPS”) and rotation of fault-bounded blocks are revealed by mapping contraction fault populations and bedding-contraction fault intersection angles. Low-angle contraction faulting and LPS are found to be dominant at early stages of development and rotation of fault-bounded blocks occurs during progressive folding of the hanging-wall panel during fault-propagation folding. Displacement-distance data obtained from major thrusts in the model show relative stretch values, and consequently fault slip/propagation ratios, that are similar to natural structures. [ABSTRACT FROM AUTHOR]

Abstract: The Sudbury Basin is a non-cylindrical fold basin occupying the central portion of the Sudbury Impact Structure. The impact structure lends itself excellently to explore the structural evolution of continental crust containing a circular region of long-term weakness. In a series of scaled analogue experiments various model crustal configurations were shortened horizontally at a constant rate. In mechanically weakened crust, model basins formed that mimic several first-order structural characteristics of the Sudbury Basin: (1) asymmetric, non-cylindrical folding of the Basin, (2) structures indicating concentric shortening around lateral basin termini and (3) the presence of a zone of strain concentration near the hinge zones of model basins. Geometrically and kinematically this zone corresponds to the South Range Shear Zone of the Sudbury Basin. According to our experiments, this shear zone is a direct mechanical consequence of basin formation, rather than the result of thrusting following folding. Overall, the models highlight the structurally anomalous character of the Sudbury Basin within the Paleoproterozoic Eastern Penokean Orogen. In particular, our models suggest that the Basin formed by pure shear thickening of crust, whereas transpressive deformation prevailed elsewhere in the orogen. The model basin is deformed by thickening and non-cylindrical synformal buckling, while conjugate transpressive shear zones propagated away from its lateral tips. This is consistent with pure shear deformation of a weak circular inclusion in a strong matrix. The models suggest that the Sudbury Basin formed as a consequence of long-term weakening of the upper crust by meteorite impact. [ABSTRACT FROM AUTHOR]

Abstract: Gas mobility plays an important role in driving volcanic eruptions and controlling eruption style. The explosivity of an eruption depends, among other factors, on how easily gas can escape from the magma. Many magmatic systems have high concentrations of suspended crystals that inhibit gas migration through the melt. We use suspensions of plastic beads in corn syrup to investigate interactions between rising bubbles and particles. We observe different interaction styles as the ratio ψ of bubble to particle size is varied. Large bubbles (ψ>1) deform and sometimes break up as they move around particles. Small bubbles (ψ<1) are frequently trapped within the suspension, increasing the concentration of gas held within the system. We compare our experiments to bubble populations in tephra from Stromboli volcano, Italy. We show that these samples typically have bubbles and crystals of similar sizes and suggest that crystals might play a role in controlling bubble size in this natural system as well as in our experiments. Because small bubbles (ψ<1) get trapped within the suspension, and can be formed by breakup of larger bubbles, we expect that an increase in gas flux will result in an increase in the population of small bubbles. Changes in bubble number density and vesicularity in tephra erupted during periods of different eruptive intensity may thus provide a way of tracking changes in gas flux through the magma prior to eruption. [ABSTRACT FROM AUTHOR]

Abstract: During an explosive volcanic eruption, tephra fall out from the umbrella region of the eruption cloud to the ground surface. We investigated the effect of the intensity of turbulence in the umbrella cloud on dispersion and sedimentation of tephra by performing a series of laboratory experiments and three dimensional (3-D) numerical simulations. In the laboratory experiments, spherical glass-bead particles are mixed in stirred water with various intensities of turbulence, and the spatial distribution and the temporal evolution of the particle concentration are measured. The experimental results show that, when the root-mean-square of velocity fluctuation in the fluid (W rms) is much greater than the particle terminal velocity (v t), the particles are homogeneously distributed in the fluid, and settle at their terminal velocities at the base of the fluid where turbulence diminishes. On the other hand, when W rms is as small as or smaller than v t, the particle concentration increases toward the base of the fluid during settling, which substantially increases the rate of particle settling. The results of the 3-D simulations of eruption cloud indicate that W rms is up to 40 m/s in most of the umbrella cloud even during a large scale plinian eruption with a magma discharge rate of 109 kg/s. These results suggest that relatively coarse pyroclasts (more than a few mm in diameter) tend to concentrate around the base of the umbrella cloud, whereas fine pyroclasts (less than 1/8 mm in diameter) may be distributed homogeneously throughout the umbrella cloud during tephra dispersion. The effect of the gradient of particle concentration in the umbrella cloud explains the granulometric data of the Pinatubo 1991 plinian deposits. [Copyright &y& Elsevier]

Abstract: Discrete explosive bursts are known from many volcanic eruptions. In maar–diatreme eruptions, they have occurred in debris-filled volcanic vents when magma interacted with groundwater, implying that material mobilized by such explosions passed through the overlying and enclosing debris to reach the surface. Although other studies have addressed the form and characteristics of craters formed by discrete explosions in unconsolidated material, no details are available regarding the structure of the disturbed debris between the explosion site and the surface. Field studies of diatreme deposits reveal cross-cutting, steep-sided zones of non-bedded volcaniclastic material that have been inferred to result from sedimentation of material transported by “debris jets” driven by explosions. In order to determine the general processes and deposit geometry resulting from discrete, explosive injections of entrained particles through a particulate host, we ran a series of analogue experiments. Specific volumes of compressed (0.5–2.5 MPa) air were released in bursts that drove gas-particle dispersions through a granular host. The air expanded into and entrained coloured particles in a small crucible before moving upward into the host (white particles). Each burst drove into the host an expanding cavity containing air and coloured particles. Total duration of each run, recorded with high-speed video, was approximately 0.5–1 s. The coloured beads sedimented into the transient cavity. This same behaviour was observed even in runs where there was no breaching of the surface, and no coloured beads ejected. A steep-sided body of coloured beads was left that is similar to the cross-cutting pipes observed in deposits filling real volcanic vents, in which cavity collapse can result not only from gas escape through a granular host as in the experiments, but also through condensation of water vapour. A key conclusion from these experiments is that the geometry of cross-cutting volcaniclastic deposits in volcanic vents is not directly informative of the geometry of the “intrusions” that formed them. An additional conclusion is that complex structures can form quickly from discrete events. [Copyright &y& Elsevier]

Abstract: The evolution of a high-dihedral-angle liquid in a crystalline matrix, such as Fe–S melt in an olivine matrix, is of vital importance for a range of questions including planetary core–mantle differentiation. We performed hydrostatic in situ analogue experiments on norcamphor–H2O liquid, and high-temperature experiments on olivine–FeS melt to investigate the evolution of the liquid and the attainment of textural equilibrium in the liquid and the solid. The liquid distribution was dominated by large unconnected liquid pools, smaller melt lenses, and the regular occurrence of dry three-grain triple junctions. Compared to low-dihedral-angle systems, the liquid pockets had a very low mobility during grain growth and disequilibrium features, such as elongated liquid pools, only reequilibrated slowly. The results have several consequences for the evolving microstructure: (1) Immobile liquid pockets hinder grain boundary migration and promote abnormal grain growth; (2) The attainment of textural equilibrium in solid–liquid regions is delayed with respect to liquid-free regions; (3) Grain growth does not promote liquid pocket growth, as in low-dihedral-angle systems, and liquid pockets do not directly interact with each other during annealing; (4) An existing liquid network is unstable and pinches off in the long-term, even if the liquid-fraction is above the theoretical percolation threshold. With regard to this final point, percolative core–mantle differentiation through an interconnected melt network may not be a viable mechanism, even above the suggested percolation threshold of ∼5 vol.%. [Copyright &y& Elsevier]

Typical pull-apart structures were created in scaled clay experiments with a pure strike-slip geometry (Riedel type experiments). A clay slab represents the sedimentary cover above a strike-slip fault in the rigid basement. At an early stage of the development of the deformation zone, synthetic shear fractures (Riedel shears) within the clay slab display dilatational behaviour. With increasing basal displacement the Riedel shears rotate and open further, developing into long, narrow and deep troughs. The shear displacement and the low angle with the prescribed principal basal fault set them apart from tension gashes. At a more evolved stage, synthetic segments (Y-shears) parallel to the basal principal fault develop and accommodate progressive strike-slip deformation. The Y-shears connect the tips of adjacent troughs developed from the earlier Riedel shears, resulting in the typical rhomb-shaped structures characteristic for pull-apart basins. The Strait of Sicily rift zone, with major strike-slip systems being active from the Miocene to the Present, comprises pull-apart basins at different length scales, for which the structural record suggests development by a mechanism similar to that observed in our experiments. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, we discuss the possibility that the North Anatolian fault (NAF) results from the deep deformation of the slab beneath the Bitlis–Hellenic subduction zone. We described the tectonic evolution of the Anatolia–Aegean area in three main steps, before, during and after the formation of the NAF. We remark that the tectonic conditions that are assumed to have triggered the formation of the NAF, i.e. collision to the east and extension to the west, was already achieved before the onset of that strike-slip fault system. We also highlight that the formation of the NAF was accompanied by the uplift of the Turkish–Iranian plateau and by a surge of volcanism in the eastern Anatolia collisional area and probably by the acceleration of the Aegean trench retreat. We show tomographic images from global P-wave model of Piromallo and Morelli [C. Piromallo, A. Morelli, P wave tomography of the mantle under the Alpine–Mediterranean area, J. Geophys. Res. 108 (2003) doi: 10.1029/2002JB001757.] showing that the slab beneath the Bitlis collisional belt is not continuous and that its possible rupture pursues to the west at least up to Cyprus and possibly up to the eastern end of the Hellenic trench. All these observations suggest that the plate tectonic re-organization occurred in the Late Miocene–Early Pliocene in the region results from slab break-off in the Bitlis area and from its lateral propagation to the West. This idea is tested in analogue laboratory experiments, which confirm that the break of the slab under the collisional belt may trigger, (1) the acceleration of slab retreat to the west due to the increase in slab pull force, (2) the indentation of the continent in the collisional area and (3) produce the conditions that permit the lateral escape of material towards the west and the formation of the NAF. [Copyright &y& Elsevier]

We model using analogue experiments the subduction of buoyant ridges and plateaus to study their effect on slab dynamics. Experiments show that simple local (1-D) isostatic considerations are not appropriate to predict slab behaviour during the subduction of a buoyant ridge perpendicular to the trench, because the rigidity of the plate forces the ridge to subduct with the dense oceanic lithosphere. Oceanic ridges parallel to the trench have a stronger effect on the process of subduction because they simultaneously affect a longer trench segment. Large buoyant slab segments sink more slowly into the asthenosphere, and their subduction result in a diminution of the velocity of subduction of the plate. We observe a steeping of the slab below those buoyant anomalies, resulting in smaller radius of curvature of the slab that augments the energy dissipated in folding the plate and further diminishes the velocity of subduction. When the 3-D geometry of a buoyant plateau is modelled, the dip of the slab above the plateau decreases, as a result of the larger velocity of subduction of the dense ‘normal’ oceanic plate on both sides of the plateau. Such a perturbation of the dip of the slab maintains long time after the plateau has been entirely incorporated into the subduction zone. We compare experiments with the present-day subduction zone below South America. Experiments suggest that a modest ridge perpendicular to the trench such as the present-day Juan Fernandez ridge is not buoyant enough to modify the slab geometry. Already subducted buoyant anomalies within the oceanic plate, in contrast, may be responsible for some aspects of the present-day geometry of the Nazca slab at depth. [ABSTRACT FROM AUTHOR]

Abstract: Experiments were carried out on granular flows generated by instantaneous release of gas-fluidised, bidisperse mixtures and propagating into a horizontal channel. The mixture consists of fine (<100 μm) and coarse (>100 μm) particles of same density, with corresponding grain size ratios of ∼2 to 9. Initial fluidisation of the mixture destroys the interparticle frictional contacts, and the flow behaviour then depends on the initial bed packing and on the timescale required to re-establish strong frictional contacts. At a fines mass fraction (α) below that of optimal packing (∼40%), the initial mixtures consist of a continuous network of coarse particles with fines in interstitial voids. Strong frictional contacts between the coarse particles are probably rapidly re-established and the flows steadily decelerate. Some internal friction reduction appears to occur as α and the grain size ratio increases, possibly due to particle rolling and the lower roughness of internal shear surfaces. Segregation only occurs at large grain size ratio due to dynamical sieving with fines concentrated at the flow base. In contrast, at α above that for optimal packing, the initial mixtures consist of coarse particles embedded in a matrix of fines. Flow velocities and run-outs are similar to that of the monodisperse fine end-member, thus showing that the coarse particles are transported passively within the matrix whatever their amount and grain size are. These flows propagate at constant height and velocity as inviscid fluid gravity currents, thus suggesting negligible interparticle friction. We have determined a Froude number of 2.61±0.08 consistent with the dam-break model for fluid flows, and with no significant variation as a function of α, the grain size ratio, and the initial bed expansion. Very little segregation occurs, which suggests low intensity particle interactions during flow propagation and that active fluidisation is not taking place. Strong frictional contacts are only re-established in the final stages of emplacement and stop the flow motion. We infer that fines-rich (i.e. matrix-supported) pyroclastic flows propagate as inviscid fluid gravity currents for most of their emplacement, and this is consistent with some field data. [Copyright &y& Elsevier]

Abstract: The deformation behaviour of partially molten rocks was investigated using in situ analogue experiments with norcamphor+ethanol, as well as partially molten KNO3+LiNO3. Three general deformation regimes could be distinguished during bulk pure shear deformation. In regime I, above ca. 8–10vol.% liquid (melt) fraction (ϕ bulk), deformation is by compaction, distributed granular flow, and grain boundary sliding (GBS). At ϕ bulk&#60;8–10vol.% (regime II), GBS localizes in conjugate shear zones. Liquid segregation is inefficient or even reversed as the dilatant shear zones draw in melt to locally exceed the 8–10vol.% threshold. At even lower ϕ bulk (regime III), grains form a coherent framework that deforms by grain boundary migration accommodated dislocation creep, associated with efficient segregation of remaining liquid. The transition liquid fraction between regimes I and II (ϕ LT) depends mainly on the grain geometry and is therefore comparable in both analogue systems. The transition liquid fraction between regimes II and III (ϕ GBS-L) varies between 4–7vol.% for norcamphor–ethanol and ca. 1vol.% for KNO3+LiNO3 and depends on system specific parameters. Regime II behaviour in our experiments can explain the frequently observed small melt-bearing shear zones in partially molten rocks and in HT experiments. [Copyright &y& Elsevier]

This study presented a method of laboratory analogue experiments based on a geographical scaling model and a waveguide model to investigate the characteristics of the propagation of seismic electromagnetic signals in the crust and the atmosphere. Some controlled experiments were done to evaluate the possible influence on the experimental results from the background electromagnetic field, geographical conditions, boundary effects, the source of electromagnetic signals (position, magnitude, and frequency), and media conductivity. The reliability and the extensibility of the above analogue experimental method were also investigated. This study indicated that such kind of analogue experimental method provided an intuitionistic way of studying the propagation of seismic electromagnetic signals, which is one of the most difficult research topics in seismo-electromagnetism. [ABSTRACT FROM AUTHOR]

Abstract: Vertical stratification of particle concentration is a common if not ubiquitous feature of submarine particulate gravity flows. To investigate the control of stratification on current behaviour, analogue stratified flows were studied using laboratory experiments. Stratified density currents were generated by releasing two-layer glycerol solutions into a tank of water. Flows were sustained for periods of tens of seconds and their velocity and concentration measured. In a set of experiments the strength of the initial density and viscosity stratification was increased by progressively varying the lower-layer concentration, C L. Two types of current were observed indicating two regimes of behaviour. Currents with a faster-moving high-concentration basal region that outran the upper layer were produced if C L &#60;75%. Above this critical value of C L, currents were formed with a relatively slow, high-concentration base that lagged behind the flow front. The observed transition in behaviour is interpreted to indicate a change from inertia- to viscosity-dominated flow with increasing concentration. The reduction in lower-layer velocity at high concentrations is explained by enhanced drag at low Reynolds numbers. Results show that vertical stratification produces longitudinal stratification in the currents. Furthermore, different vertical and temporal velocity and concentration profiles characterise the observed flow types. Implications for the deposit character of particle-laden currents are discussed and illustrated using examples from ancient turbidite systems. [Copyright &y& Elsevier]

Abstract: Laboratory experiments on air-filled fracture propagation in solidified homogeneous and layered gelatine have been carried out, providing an analogue model for magma-filled dikes ascending in the crust. The effects of layering on fracture velocity and shape have been analyzed in detail. The free surface is found to accelerate approaching fractures. Layering accelerates or decelerates fractures approaching discontinuities of the elastic parameters, depending on the value of the rigidity contrast. The shape of fractures are strongly influenced as they pass from one layer to another. The observed cross-sectional shape when crossing a layer interface and the acceleration with decreasing rigidity can be explained with theoretical models. Our experiments also reproduce the arrest of fractures in proximity of joints and the formation of sills in the layer below the interface. These findings could help in the interpretation of accelerated seismicity and deformation rates observed in volcanic areas. [Copyright &y& Elsevier]

We report the results of experiments on the initiation of subduction, using stratified analogue models in a large centrifuge, where the experiments are driven only by the enhanced gravity of the centrifuge without the effect of external lateral stresses. The scaled density of the stratified components resembles that of the asthenosphere and the continental and oceanic lithospheres. The experiments demonstrate that under the effect of enhanced gravity, the layers simulating the oceanic lithosphere detach from the front of the‘continental lithosphere’ and plunge under it, pushing the more pliable asthenosphere downwards. Simultaneously, the‘continental lithosphere’ is thrust over the downgoing slab; where friction is low, the‘continental lithosphere’ extends considerably so that the‘ductile continental lithosphere’ is exposed in some places. The rate of thrusting of the experimental continental slab over the oceanic one, as well as the amount of extension of the overriding slab and the extent of the rollback of the subduction zone that follows the initial lithospheric detachment, are controlled by friction and density differences between the subducting and the overthrust slabs. The analogue experiments emphasize the role of lateral density variations in incipient subduction and the effect of differential seismic friction along the subduction plane on the evolution of subduction zones, their shape and the evolution of their backarc basins. The morphological resemblance of the experimental results to various subduction systems seems to support their applicability to real subduction systems. [ABSTRACT FROM AUTHOR]

Abstract: Brittle faults reflect a complex strain history that emerges from contrasting modes of distributed and localised deformation, and their interaction on various spatial and temporal scales. To better understand this process, we monitor the displacement field in scaled tectonic model experiments using high-resolution optical image correlation techniques (particle imaging velocimetry, PIV). 2D and 3D surface displacement data of extensional and contractional sandbox experiments show that the mode, pattern, and temporal variation of strain accumulation are strongly dependent on the non-linear strain-dependent frictional strength of granular model materials similar to natural deformation processes in brittle rocks. Strain hardening and softening control the shear zone formation. The pattern of localised deformation is established much earlier than is visible from visual inspection of the experiment. Evolution of distributed strain in the surrounding material of the shear zones and discontinuous shear re-localisation control later stages irrespective of the kinematic boundary conditions. High-resolution optical strain monitoring quantifies the spatial and temporal patterns of strain accumulation in our model experiments with unprecedented detail. Together with detailed characterisation of the deformation behaviour of the model materials, our experiments will help to re-evaluate important scaling issues, and allow accurate comparison of analogue experiments with numerical simulations. [Copyright &y& Elsevier]

Intrusive rocks may be arranged in the form of strings of lenses or beads, as found on the Cap de Creus Peninsula, NE Spain, and in the South Finland Migmatite–Granite Belt. These structures first appear to be the result of stretching and boudinage of intrusive sheets or dykes. However, closer examination reveals that they are not boudins, but are instead primary intrusive structures. A detailed study was performed on a swarm of pegmatite intrusions at Cap de Creus. Layering is often continuous between beads, and, in some cases, individual beads exhibit a very irregular shape. These observations are shown to be incompatible with an origin by boudinage. Analogue experiments were used to test the effect of the two models (boudinage and emplacement) on the structures around beads, and show that an emplacement model of local expansion and collapse of magmatic sheets is the most compatible with field observations. Such structures can only form when dykes intrude hot rocks, when magma solidifies slowly enough to allow enough ductile flow of the wall rock to accommodate the formation of the beads. The fact that the pegmatite bead strings are not boudins has repercussions for the interpretation of the deformation at Cap de Creus and necessitates caution in the interpretation of deformation based on apparent boudins in intrusive rocks in other areas. [Copyright &y& Elsevier]

A series of analogue laboratory experiments were carried out to model melting and mixing processes occurring at the roof and sides of a magma chamber when hot, dense mantle-derived magmas are emplaced in the deep crust. The experiments investigated the influence of a viscosity contrast between crustal melts and input magma, the presence of crystals in the input magma, and roof geometry. The crust was modelled by aqueous ice or a cold, low-melting point wax, arranged as a flat roof or in an upside-down U. The input magma was modelled by a hot, dense aqueous solution of NaNO3 or FeSO4, which was injected into the cavity between the walls. The experiments revealed the potentially important role that side-wall processes may have in crustal melting and mantle magma contamination in this environment. The walls melted back much faster than the roof and the melts from the opposite walls ponded next to each other under the roof and mixed with each other only in the central region. When the input solution was able to crystallise, it did so preferentially in the active side-wall boundary layers. The tiny, dense crystals, which were dragged up in the melt boundary layer and then dropped into the interior fluid, enhanced hybridisation of the melt and contamination of the input fluid and, through their influence on the boundary layer structure, reduced the melting rate. [Copyright &y& Elsevier]