Search

The rising velocity of an air bubble in a non‐Newtonian shear‐thinning fluid at low Reynolds numbers is generally similar to the Newtonian case given by Stokes' law. However, when the shear‐thinning fluid is subject to mechanical oscillations, the rising velocity could significantly increase. Here, we present a series of experiments quantifying the rising velocity of single bubbles during shaking in very high‐viscosity (2,000–30,000 Pa·s) shear‐thinning silicone oils. Air bubbles (18–30 mm diameter) were injected in a tank mounted on a shaking table. The tank was horizontally oscillated, at accelerations between 0.4 and 2 g. We observed a small increase in the rising velocity of the shaking cases at our experimental conditions. The increase was larger when bubbles were large and accelerations were high. Larger accelerations experienced the largest observational errors and we emphasize the exploratory nature of our results. We also measured the change in bubble diameter during the oscillations and computed the shear rate at the bubble surface. Maximum shear rates were in the range of 0.04–0.08 s−1. At these shear rates, our analysis indicates that shear thinning behavior of our analog fluids is expected to be small and compete with elastic behavior. This transitional viscous/elastic regime helps explain the small and variable results of our experiments. Our results are relevant to the study of earthquake‐volcano interactions. Most crystal‐free silicate melts would exhibit a purely viscous, shear‐thinning behavior in a natural scenario. Seismically enhanced bubble rise could offer an explanation for the observed increased degassing and unrest following large earthquakes. Plain Language Summary: The speed at which air bubbles rise in a liquid is generally well‐known. However, in non‐Newtonian fluids (liquids with non‐constant viscosity), previous experiments have shown that this speed could be significantly increased by shaking the fluid container. In this study, we present experiments where we measure the rising speed of an air bubble in a viscous non‐Newtonian fluid while shaking it. We found that bubbles rose slightly faster during shaking, especially for strong shaking and/or large bubbles. However, the shaking of the table reduced the precision of our measurements, and our results need to be further validated. We were also able to measure how quickly the bubbles deformed during shaking. When the bubbles deform quickly, our fluids may behave like solids rather than liquids. This helps explain why our results are less striking than previous experiments. Our results can be applied to the study of volcanoes. Magma inside a volcano contains vapor bubbles. When an earthquake occurs close to a volcano, these bubbles may accelerate toward the surface and lead to more gas coming out of the volcano. Key Points: We measured the rising rate of bubbles in shear‐thinning fluids subject to mechanical shakingThe bubble rising velocity increases slightly during shaking versus non‐shaking experimentsNatural magmas are likely to exhibit shear‐thinning behavior and increased bubble rise during large earthquakes [ABSTRACT FROM AUTHOR]

Mt. Ruapehu, in the central North Island of New Zealand, hosts a hot acidic Crater Lake over the active volcanic vent with a surface elevation of c. 2530 m.a.s.l. Volcanic activity and other montane processes have previously resulted in catastrophic releases of some or all of the c. 10 Mm 3 of water retained in the lake, creating serious hazards downstream. A major lahar in March 2007 exposed a 10 m high face representative of the rock units impounding the lake, providing an opportunity to conduct both field and laboratory analysis to characterise the rock mass conditions at the outlet to assess the stability of the outlet area. This paper presents an engineering geology model of Crater Lake outlet. Our model shows three andesitic geological units at the outlet, each with different geological histories and physical and mechanical properties, which impact its stability. Geotechnical methods used to characterise the outlet include laboratory testing of the strength, stiffness, porosity and unit weight, and field-based rock mass characterisation using the geological strength index (GSI) and rock mass rating (RMR). Field observations, geomorphology mapping, historic and contemporary photographs, and laboratory results are combined to create cross sections that provide key information for establishing the engineering geology model. The units are recognised in what is informally termed the Crater Lake Formation: i) The upper surface layer is a c. 2 m thick sub-horizontal dark grey lava unit (Armoured Lava Ledge) with sub-horizontal cooling joints spaced at 0.2 m to 2.0 m intervals. The intact rock has a porosity range of 15–27%, density range of 1723–2101 kg/m 3 , GSI range of 45–75, and unconfined compressive strength (UCS) range of 19–48 MPa. ii) Below this, and outcropping down the majority of the outlet waterfall is a poorly sorted breccia unit composed of block and matrix material (Lava Breccia). The blocks range from 0.1 m to 0.8 m in diameter with an average porosity of 21%, a density of 1956 kg/m3 and strength of 85 MPa. The matrix has soil-like properties with an estimated UCS of 1.5 MPa. iii) At the base of the waterfall, the material sharply transitions into a light grey, slightly weathered unit (Lower Grey Member). This lower unit has an irregular surface expression with sub-vertical discontinuities. Porosity is 6%, density is 2569 kg/m 3 , the GSI range is 65–75, and the UCS is 98 MPa. The engineering geology model portrays the relationships between the units in three dimensions, highlights key structures and takes into consideration the material source, transportation and depositional processes. Historical outlet photographs suggest past eruptive and glacial activities are both significant factors controlling the deposition and erosion of material at the outlet. The Lower Grey Member appears to be a sound material for the outlet and water fall to be founded on. The upper aa Armoured Lava Ledge currently has moderate strength and GSI, and is resistive, providing protection for the underlying weaker block and matrix unit, however, continued incision by the outlet stream will eventually expose the weaker block and matrix material of the Lava Breccia. Once exposed, the Lava Breccia could rapidly erode or fail down to the Lower Grey Member and could potentially release 1 Mm 3 of hot, acidic Crater Lake water. We recommend that erosion rates for the upper Armoured Lava Ledge be established to aid in preparation for eventual rim breakout. [ABSTRACT FROM AUTHOR]

Extension fractures in volcanic systems exist on all scales, from microscopic fractures to large fissures. They play a fundamental role in the movement of fluids and distribution of pore pressure, and therefore exert considerable influence over volcanic eruption recurrence. We present here laboratory permeability measurements for porous (porosity = 0.03–0.6) andesites before (i.e., intact) and after failure in tension (i.e., the samples host a throughgoing tensile fracture). The permeability of the intact andesites increases with increasing porosity, from 2 × 10 − 17 to 5 × 10 − 11 m 2 . Following fracture formation, the permeability of the samples (the equivalent permeability) falls within a narrow range, 2 – 6 × 10 − 11 m 2 , regardless of their initial porosity. However, laboratory measurements on fractured samples likely overestimate the equivalent permeability due to the inherent scale-dependence of permeability. To explore this scale-dependence, we first determined the permeability of the tensile fractures using a two-dimensional model that considers flow in parallel layers. Our calculations highlight that tensile fractures in low-porosity samples are more permeable (as high as 3.5 × 10 − 9 m 2 ) than those in high-porosity samples (as low as 4.1 × 10 − 10 m 2 ), a difference that can be explained by an increase in fracture tortuosity with porosity. We then use our fracture permeability data to model the equivalent permeability of fractured rock (with different host rock permeabilities, from 10 − 17 to 10 − 11 m 2 ) with increasing lengthscale. We highlight that our modelling approach can be used to estimate the equivalent permeability of numerous scenarios at andesitic stratovolcanoes in which the fracture density and width and host rock porosity or permeability are known. The model shows that the equivalent permeability of fractured andesite depends heavily on the initial host rock permeability and the scale of interest. At a given lengthscale, the equivalent permeability of high-permeability rock ( 10 − 12 to 10 − 11 m 2 ) is essentially unaffected by the presence of numerous tensile fractures. By contrast, a single tensile fracture increases the equivalent permeability of low-permeability rock ( < 10 − 15 m 2 ) by many orders of magnitude. We also find that fractured, low-permeability rock (e.g., 10 − 17 m 2 ) can have an equivalent permeability higher than that of similarly fractured rock with higher host rock permeability (e.g., 10 − 15 m 2 ) due to the low-tortuosity of fractures in low-porosity andesite. Our modelling therefore outlines the importance of fractures in low-porosity, low-permeability volcanic systems. While our laboratory measurements show that, regardless of the initial porosity, the equivalent permeability of fractured rock on the laboratory scale is 2 – 6 × 10 − 11 m 2 , the equivalent permeability of low-permeability rock is significantly reduced as the scale of interest is increased. Therefore, due to the scale-dependence of permeability, laboratory measurements on pristine, low-permeability rocks significantly underestimate the equivalent permeability of fractured volcanic rock. Further, measurements on fractured rock samples can significantly overestimate the equivalent permeability. As a result, care must be taken when selecting samples in the field and when using laboratory data in volcano outgassing models. The data and modelling presented herein provide insight into the scale-dependence of the permeability of fractured volcanic rock, a prerequisite for understanding outgassing at active volcanoes. [ABSTRACT FROM AUTHOR]

We offer new insights into how an explosive eruption can transition into an effusive eruption. Magma containing >0.2 wt% dissolved water has the potential to vesiculate to a porosity in excess of 80 vol.% at atmospheric pressure. Thus all magmas contain volatiles at depth sufficient to form foams and explosively fragment. Yet gas is often lost passively and effusive eruptions ensue. Magmatic foams are permeable and understanding permeability in magma is crucial for models that predict eruptive style. Permeability also governs magma compaction models. Those models generally imply that a reduction in magma porosity and permeability generates an increased propensity for explosivity. Here, our experimental results show that surface tension stresses drive densification without creating an impermeable ‘plug’, offering an additional explanation of why dense magmas can avoid explosive eruption. In both an open furnace and a closed autoclave, we subject pumice samples with initial porosity of ∼70 vol.% to a range of isostatic pressures (0.1–11 MPa) and temperatures (350–950 °C) relevant to shallow volcanic environments. Our experimental data and models constrain the viscosity, permeability, timescales, and length scales over which densification by pore-scale surface tension stresses competes with density-driven compaction. Where surface tension dominates the dynamics, densification halts at a plateau connected porosity of ∼25 vol.% for our samples. SEM, pycnometry and micro-tomography show that in this process (1) microporous networks are destroyed, (2) the relative pore network surface area decreases, and (3) a remaining crystal framework enhances the longevity of macro-pore connectivity and permeability critical for sustained outgassing. We propose that these observations are a consequence of a surface tension-driven retraction of viscous pore walls at areas of high bubble curvature (micro-vesicular network terminations), and that this process drives bulk densification and permits continued outgassing. We propose a regime diagram of the relative dominance of surface tension and gravitational compaction that illustrates the interplay between viscosity, permeability, lengthscale and timescale. We contend that surface tension-driven magma densification is an as-yet overlooked phenomenon that extends our volcanological, geothermal and hydrothermal knowledge of how gas can escape densifying volcanic plugs and why dense lavas remain permeable. [ABSTRACT FROM AUTHOR]

Ballistic projectiles are the most frequently lethal volcanic hazard close to the vent. Recent eruptions of Ontake in 2014 and Kusatsu-Shirane in 2018 showed that un-reinforced, timber-framed buildings - those typically considered highly vulnerable to the dangerous penetration of ballistics - provided life-saving shelter from ballistic impact. Modelled kinetic energies of some non-penetrating impacts were an order of magnitude above expected penetration thresholds. It has been hypothesised that a pre-existing layer of tephra on the roofs cushioned impacts. To quantitatively test this, and improve our understanding of how buildings respond to projectile impacts, we used a pneumatic cannon to simulate block impacts to clay tiles and reinforced concrete roof slabs covered with tephra layers 0–20 cm thick. Substantially higher impact energies were resisted when tephra was present with 5 cm of tephra approximately tripling the penetration threshold of both building materials. Fragility curves, which relate ballistic hazard intensity with the probability of building damage, were developed from our experimental data following three curve fitting approaches: generalised link models, cumulative link models and data binning. A key benefit of these approaches is that confidence in these curves can be robustly quantified from the data – the first time that this has been attempted for volcanic fragility curves. This study shows how the extent of building damage can be strongly influenced by the sequence of volcanic hazards and provides an example of proactive risk management through testing of physical mitigation strategies in a laboratory environment. • Ballistics can cause less damage to roofs when impacts are cushioned by tephra. • 5 cm thick tephra deposits tripled penetration thresholds for both materials tested. • Specific multi-hazard sequences can produce less damage than a single hazard. • Volcanic fragility curves with uncertainty calculated solely from experimental data. • Laboratory simulation of hazards can proactively inform risk mitigation measures. [ABSTRACT FROM AUTHOR]

The thermal conductivity of volcanic rock is an essential input parameter in a wide range of models designed to better understand volcanic and geothermal processes. However, although volcanoes and geothermal reservoirs are often characterised by temperatures above ambient, laboratory thermal conductivity measurements are often performed at ambient temperature. In addition, there are currently few data on the temperature dependence of thermal conductivity for andesite, a common volcanic rock. Here, we provide elevated-temperature (up to 120 °C) laboratory measurements of thermal conductivity for variably porous (∼0.05 to ∼0.6) and variably glassy andesites from Mt. Ruapheu (New Zealand) using the transient hot-strip method. Our data show that (1) the thermal conductivity of these andesites has little to no temperature dependence and, therefore, (2) there is also no influence of porosity on the temperature dependence of thermal conductivity. We compare our new data with compiled published data to show that the thermal conductivity of volcanic rocks may decrease, remain constant, or increase as a function of temperature. We show that the thermal conductivity of amorphous glass and crystalline material increase and decrease, respectively, as temperature increases. We therefore interpret the temperature dependence of the thermal conductivity of volcanic rock to be dependent on glass content. The thermal conductivity of the studied andesites, the microstructure of which can be characterised by phenocrysts within a variably glassy groundmass, has little to no temperature dependence because the decrease in the thermal conductivity of the crystalline materials, due to decreases in lattice thermal conductivity, is offset by the increase in the thermal conductivity of the amorphous glass. A simple modelling approach, using the temperature dependence of the thermal conductivity of glass and crystalline material, provides a crystal content of 0.26 for a thermal conductivity independent of temperature, a common crystal content for andesite dome rock. Our findings imply that calculations of heat transfer through partially glassy volcanic rocks need not consider a temperature-dependent thermal conductivity, but that decreases and increases in thermal conductivity with temperature should be expected for fully crystallised or devitrified volcanic rocks and completely glassy volcanic rocks, respectively. We highlight that more experimental studies are now required to assess the evolution of thermal conductivity as a function of temperature in a wide range of volcanic rocks with different crystallinities. • Thermal conductivity of porous andesites has little to no temperature dependence. • Temperature-dependence of thermal conductivity of volcanic rocks depends on glass content. • Modelling shows that crystal content of 0.26 is required for temperature-independent thermal conductivity. • Heat transfer calculations for partially glassy volcanic rocks need not use a temperature-dependent thermal conductivity. [ABSTRACT FROM AUTHOR]