Ascent Velocity Research Articles

Diffusive fractionation of H2O and CO2 during magma degassing

H2O–CO2 systematics of melt inclusions and obsidian pyroclasts has been used widely to explore the behaviour of fluids in magmatic systems under the assumption that a fluid–melt equilibrium is attained. However, fluid exsolution is a process involving the diffusive transfer of volatiles, and kinetic effects may control the composition of volatiles. In this study, single-step decompression experiments were carried out on H2O–CO2-bearing basaltic and rhyolitic melts to investigate the evolution of H2O–CO2 compositions during vesiculation. Microanalysis of the volatiles using Raman and infrared spectroscopy showed that CO2 content decreased towards bubbles, while H2O content was almost constant throughout the quenched glass samples. This resulted in higher CO2/H2O ratios than estimated from the equilibrium degassing, which is interpreted as a kinetic effect: the diffusivity of H2O is higher than that of CO2. A simple model for the diffusive degassing of ascending magma was developed to investigate the disequilibrium evolution of its H2O–CO2 content. CO2/H2O ratios were strongly dependent on the magma ascent velocity. The model was applied to melt inclusions with high CO2/H2O ratios from Etna and Stromboli volcanoes, and it was shown that the high ratios could not be explained by diffusive fractionation under a typical magma ascent velocity. Rather, these ratios are affected by other processes, such as CO2 fluxing.

An Analysis of Ascent And Descent Velocity of The Lifter And Barbell During A Clean

Research has shown that barbell velocity plays a critical role in successful lifting, and is highest during the second pull phase of a clean. PURPOSE:The purpose of this study was to determine the ascent and descent velocity of both the barbell and lifter during a clean. METHODS: Eight competitive weightlifters (7 men, 1 woman; mean ± SD: age, 24.88 ± 1.36 years; body mass, 86.58 ± 14.75 kg; height, 178.28 ± 9.75 cm) participated in this two-day study. On day one, participants performed a brief warm-up, then established a 1-repetition maximum (1RM) in the clean (mean ± SD: 103.38 ± 23.59 kg). On the second day, one week later, participants did the same warm-up as day one. They then performed 3 cleans at 85% of their 1RM, resting 1 minute between repetitions. Three velocity transducers were used (1 on either end of the barbell, and 1 attached to the lifter at hip level with a weight belt). RESULTS: Descriptive results (mean ± SD) revealed peak bar ascent velocity, 1.88 ± .06 m/s; peak bar descent velocity, -1.22 ± .26 m/s; peak human ascent velocity, .76 ± .24 m/s; and peak human descent velocity, -1.40 ± .13 m/s. CONCLUSIONS: These results show that the barbell reached its maximum velocity during the ascent but was slower during the descent. The lifter, however, had a much slower ascent during the initial pull from the floor, and then rapidly increased velocity as they descended under the barbell. Interestingly, the lifter traveled at a higher velocity under the barbell then the barbell itself during the descent. Establishing data on these velocities may be useful to coaches who are working with athletes to develop maximal speed during different phases of the clean.

Correlation of size, velocity, and autonomous phase of a plasmoid in atmosphere with the dissipated energy

The visual properties of a large plasmoid rising from a water container into the air for up to 450 ms are brought into correlation with the total energy dissipated into the system, and, in particular, with the energy used for plasma generation. The latter parameters are deduced from the time-resolved discharge current and voltage of the capacitor bank which is used as energy supply. By varying the experimental parameters, the energy dissipated to the system varies between 5 kJ and 30 kJ from which 10% to 30% is transferred to the plasma. Clear correlations are obtained for the size of the plasmoid changing from 15 cm to 35 cm in width, the ascent velocity ranging from 1 m/s to 2 m/s, and the rising height for which up to 85 cm is measured. For the relation of the autonomous phase with the energy transferred to the plasma, two trends are observed: 450 ms duration is achieved in maximum with the present setup being almost independent on the electrode gap, the voltage-on time, the water conductivity, or the type of salt dissolved in the water. On the other hand, an almost linear dependence is obtained by changing the capacitance.

Rates of water exsolution and magma ascent inferred from microstructures and chemical analyses of the Tokachi–Ishizawa obsidian lava, Shirataki, northern Hokkaido, Japan

Very few quantitative textural data exist for viscous obsidian lava eruptions, and it is still unclear from the mechanical behavior of ascending magmas if outgassing is controlled dominantly by brittle or ductile deformation. In order to obtain insights into how degassing and ascent proceed in such highly viscous magmas, we conducted textural and chemical analyses of the Tokachi–Ishizawa (TI) obsidian lava, in the Shirataki rhyolite volcanic area, northern Hokkaido, Japan, and estimated the water exsolution rate and ascent rate. The storage conditions of the TI lava are estimated from the Rhyolite-MELTS program as T=840–860°C and P=50MPa using the mineral assemblages and the chemical compositions of plagioclase phenocrysts and glass. To estimate the magma ascent rate, we measured the length, width, and number of oxide microlites using three-dimensional techniques. Textural analysis indicates that the microlite number densities (Nv [number/m3]) of oxide microlites in TI lava samples are 2.1×1013 to 1.4×1014, which correspond to water exsolution rates of 3.5×10−9 to 1.7×10−8wt.%/s and ascent rates of 1.7×10−6 to 1.1×10−5m/s. Together with an estimate of viscosity, the inferred ascent velocities allow us to examine the mechanical behavior of the magma in the conduit. We conclude that the development of permeability leading to outgassing is controlled by ductile deformation rather than brittle fracturing.

Volcano-structural analysis of La Garrotxa Volcanic Field (NE Iberia): Implications for the plumbing system

The Garrotxa Volcanic Field is related to the Neogene–Quaternary European Rift system and is the youngest representation of monogenetic volcanism in the Iberian Peninsula. It encompasses over 50 eruptive vents, most of them well-preserved cones, in an area of about 600km2 lying between the cities of Olot and Girona (NE Spain). In this paper we investigate the relationship between the Neogene extensional tectonics and the spatial distribution of the volcanoes in the area. The analysis includes the distribution of faults, fissures and vents, as well as morphostructural lineaments, and the morphometrical analysis of volcanic cones and craters. In addition, we use the location of the regional seismicity recorded since 1978 and the sites of freshwater springs and mantle-derived gases as indicators of active faults and open fractures. Finally, we consider the location of ultramafic xenoliths within volcanic deposits as a way of identifying the deepest fractures in the zone and estimating magma ascent velocities. The results obtained show that this volcanic area consists of an extensional basin delimited by two principal NW–SE faults that favoured the ascent of magma from either the source region or from shallower reservoirs located at the base of the crust. Towards the upper part of the crust, magma transport was captured by shallow secondary extensional NNW–SSE striking faults whose formation is linked to the slight transtensional movement of the main bounding faults. Our study provides evidence of how the local stress field and contrasts in substrate stratigraphy could have controlled magma migration, which suggests that precise knowledge of the stress configuration, substrate geology and structural discontinuities in such regions is crucial in the forecasting of monogenetic volcanism.

Impact of left ventricular diastolic dysfunction on outcome of catheter ablation for atrial fibrillation in patients with hypertrophic cardiomyopathy.

The relationship between outcome of radiofrequency catheter ablation (RFCA) for atrial fibrillation (AF) and the severity of left ventricular (LV) diastolic dysfunction in patients with hypertrophic cardiomyopathy (HCM) remains unknown. Twenty-two HCM patients (12 female, aged 65 ± 11 years) with paroxysmal (n=5; 23%) or persistent (n=17; 77%) AF were enrolled. LV diastolic function was evaluated according to the ratio of the mitral inflow early filling velocity to the velocity of the early medial mitral annular ascent (E/e') measured on pulsed wave and tissue Doppler assessments in all patients. Pulmonary vein isolation was performed in all patients. A second procedure was performed in 3 patients. During a follow-up of 21 ± 12 months, sinus rhythm was maintained in 13 of 22 patients (59%). E/e' was significantly higher in the patients with AF recurrence than in those without (18 ± 7 vs. 11 ± 3; P<0.01). On Kaplan-Meier analysis the prevalence of AF recurrence was significantly higher in patients with E/e' ≥ 15 (n=6) than in those with E/e' <15 (n=16; P<0.01). On multivariate Cox regression analysis the only significant and independent predictor for AF recurrence was E/e' (hazard ratio, 1.16; 95% confidence interval: 1.01-1.37, P=0.03). LV diastolic dysfunction evaluated using E/e' was associated with difficulty of rhythm control after RFCA in patients with HCM and AF.

Types and mechanisms of strombolian explosions: characterization of a gas-dominated explosion at Stromboli

Textural and chemical analyses of bombs quenched directly from a normal explosion at Stromboli volcano (Italy) were integrated with coincident seismic, acoustic, and thermal data. The data set defines a new gas-dominated type of “strombolian” eruption, named type 0. These events are characterized by high-velocity emission (150–250 m s−1) of a few relatively small, juvenile particles, with entrained non-juvenile clasts that previously fell back into the vent to be re-erupted. For the studied event, the explosion depth was more than 250 m deep, and the particles showed little residence time in the shallow system. Slug ascent velocities over the final 20–35 m of magma-filled conduit, and the low viscosity of the resident magma, are all consistent with simple bubble burst in a “clean” conduit. This conduit type and eruption style likely fit popular slug ascent and burst models used to explain “strombolian” eruptions. In contrast, the ballistic-dominated type of explosions (type 1) are associated with larger proportions of stagnant material in the shallow system magma mix. We argue that the additional volume of this stagnant material pushes the free surface upward. Because of the larger volume of material available for entrainment into a type 1 slug burst, which has to fragment through a thick cap of degassed material, type 1 events tend to be rich in particles. In contrast, the less spectacular, gas-rich (type 0) events have little material to entrain, thus being poor in lapilli and bombs.

Timescales of mingling in shallow magmatic reservoirs

Abstract Arrival of magma from depth into shallow reservoirs has been documented as one of the possible processes leading to eruption. Magma intruding and rising to the surface interacts with the already emplaced, degassed magmas residing at shallower depths, leaving chemical signatures in the erupted products. We performed two-dimensional numerical simulations of the arrival of gas-rich magmas into shallow reservoirs. We solve the fluid dynamics for the two interacting magmas, evaluating the space–time evolution of the physical properties of the mixture. Convection and mingling develop quickly into the chamber and feeding conduit/dyke, leading on longer timescales to a density stratification with the lighter, gas-richer magma, mixed with different proportions of the resident magma, rising to the top of the chamber due to buoyancy. Over timescales of hours, the magmas in the reservoir appear to have mingled throughout, and convective patterns become harder to identify. Our simulations have been performed changing the geometry of the shallow reservoir and the gas content of the initial end-member magmas. Horizontally elongated magma chambers, as well as higher density contrasts between the two magmas, cause faster ascent velocities and also increase the mixing efficiency. Supplementary material: Videos showing the evolution of magma composition with time in the shallow chamber for simulations 1–4 are available at www.geolsoc.org.uk/SUP18819

Complex dynamics of small-moderate volcanic events: the example of the 2011 rhyolitic Cordón Caulle eruption, Chile

After decades of repose, Puyehue-Cordón Caulle Volcano (Chile) erupted in June 2011 following a month of continuously increasing seismic activity. The eruption dispersed a large volume of rhyolitic tephra over a wide area and was characterized by complex dynamics. During the initial climactic phase of the eruption (24–30 h on 4–5 June), 11–14-km-high plumes dispersed most of the erupted tephra eastward towards Argentina, reaching as far as the Atlantic Ocean. This first eruptive phase was followed by activity of lower intensity, leading to the development of a complex stratigraphic sequence, mainly due to rapid shifts in wind direction and eruptive style. The resulting tephra deposits consist of 13 main layers grouped into four units. Each layer was characterized based on its dispersal direction, sedimentological features, and on the main characteristics of the juvenile fraction (texture, density, petrography, chemistry). The lowest part of the eruptive sequence (Unit I), corresponding to the tephra emitted between 4 and 5 June, is composed of alternating lapilli layers with a total estimated volume of ca. 0.75 km3; these layers record the highest intensity phase, during which a bent-over plume dispersed tephra towards the southeast-east, with negligible up-wind sedimentation. Products emitted during 5–6 June (Unit II) signaled an abrupt shift in wind direction towards the north, leading to the deposition of a coarse ash deposit in the northern sector (ca. 0.21 km3 in volume), followed by a resumption of easterly directed winds. A third phase (Unit III) began on 7 June and resulted in tephra deposits in the eastern sector and ballistic bombs around the vent area. A final phase (Unit IV) started after 15 June and was characterized by the emission of fine-grained white tephra from ash-charged plumes during low-level activity and the extrusion of a viscous lava flow. Timing and duration of the first eruptive phases were constrained based on comparison of the dispersal of the main tephra layers with satellite images, showing that most of the tephra was emitted during the first 72 h of the event. The analyzed juvenile material tightly clusters within the rhyolitic field, with negligible chemical variations through the eruptive sequence. Textural observations reveal that changes in eruption intensity (and consequently in magma ascent velocity within the conduit) and complex interactions between gas-rich and gas-depleted magma portions during ascent resulted in vesicular clasts with variable degrees of shear localization, and possibly in the large heterogeneity of the juvenile material.

Melting phase relations of the Udachnaya-East Group-I kimberlite at 3.0–6.5 GPa: Experimental evidence for alkali-carbonatite composition of primary kimberlite melts and implications for mantle plumes

Experiments on the origin of the Udachnaya-East kimberlite (UEK) have been performed using a Kawai-type multianvil apparatus at 3–6.5GPa and 900–1500°C. The studied composition represents exceptionally fresh Group-I kimberlite containing (wt.%): SiO2=25.9, TiO2=1.8, Al2O3=2.8, FeO=9.0, MgO=30.1, CaO=12.7, Na2O=3.4, K2O=1.3, P2O5=1.0, Cl=0.9, CO2=9.9, and H2O=0.5. The super-solidus assemblage consists of melt, olivine (Ol), Ca-rich (26.0–30.2wt.% CaO) garnet (Gt), Al-spinel (Sp), perovskite (Pv), a CaCO3 phase (calcite or aragonite), and apatite. The low pressure assemblage (3–4GPa) also includes clinopyroxene. The apparent solidus was established between 900 and 1000°C at 6.5GPa. At 6.5GPa and 900°C Na–Ca carbonate with molar ratio of (Na+K)/Ca≈0.44 was observed. The UEK did not achieve complete melting even at 1500°C and 6.5GPa, due to excess xenogenic Ol in the starting material. In the studied P–T range, the melt has a Ca-carbonatite composition (Ca#=molar Ca/(Ca+Mg) ratio=0.62–0.84) with high alkali and Cl contents (7.3–11.4wt.% Na2O, 2.8–6.7wt.% K2O, 1.6–3.4wt.% Cl). The K, Na and Cl contents and Ca# decrease with temperature. It is argued that the primary kimberlite melt at depths>200km was an essentially carbonatitic (<5wt.% SiO2), but evolved toward a carbonate–silicate composition (up to 15–20wt.% SiO2) during ascent. The absence of orthopyroxene among the run products indicates that xenogenic orthopyroxene was preferentially dissolved into the kimberlite melt. The obtained subliquidus phase assemblage (Ol+Sp+Pv+Ca-rich Gt) at P–T conditions of the UEK source region, i.e. where melt was in the last equilibrium with source rock before magma ascent, differs from the Opx-bearing peridotitic mineral assemblage of the UEK source region. This difference can be ascribed to the loss of substantial amounts of CO2 from the kimberlite magma at shallow depths, as indicated by both petrological and experimental data. Our study implies that alkali-carbonatite melt would be a liquid phase within mantle plumes generated at the core–mantle boundary or shallower levels of the mantle, enhancing the ascent velocity of the plumes. We conclude that the long-term activity of a rising hot mantle plume and associated carbonatite melt (i.e. kimberlite melt) causes thermo-mechanical erosion of the subcontinental lithosphere mantle (SCLM) roots and creates hot and deformed metasomatic regions in the lower parts of the SCLM, which corresponds to depths constrained by P–T estimates of sheared Gt-peridotite xenoliths. The sheared Gt-peridotites undoubtedly represent samples of these regions.

Understanding which parameters control shallow ascent of silicic effusive magma

AbstractThe estimation of the magma ascent rate is key to predicting volcanic activity and relies on the understanding of how strongly the ascent rate is controlled by different magmatic parameters. Linking potential changes of such parameters to monitoring data is an essential step to be able to use these data as a predictive tool. We present the results of a suite of conduit flow models Soufrière that assess the influence of individual model parameters such as the magmatic water content, temperature or bulk magma composition on the magma flow in the conduit during an extrusive dome eruption. By systematically varying these parameters we assess their relative importance to changes in ascent rate. We show that variability in the rate of low frequency seismicity, assumed to correlate directly with the rate of magma movement, can be used as an indicator for changes in ascent rate and, therefore, eruptive activity. The results indicate that conduit diameter and excess pressure in the magma chamber are amongst the dominant controlling variables, but the single most important parameter is the volatile content (assumed as only water). Modeling this parameter in the range of reported values causes changes in the calculated ascent velocities of up to 800%.

Vertical distribution of Microcystis colony size in Lake Taihu: Its role in algal blooms

This study investigated the vertical distributions of Microcystis cell density and colony size in Lake Taihu where algal blooms occur frequently. Measurements were made from April 2011 to January 2012 to gain a seasonal outlook on the role of such distributions in the blooms. It was found that large colonies tended to accumulate on the water surface, but the cell density fluctuated widely. The cell density in the water column increased continuously from spring to summer (i.e., April to October) and decreased after late autumn, showing apparent seasonal variations. The abrupt occurrence and disappearance of Microcystis blooms over short periods of time were not caused by the rapid growth of Microcystis but by the rise and accumulation of large Microcystis colonies at the water surface, both of which are affected by colony size. The ascent velocity of large colonies was higher than that of small ones, which enables large colonies to more readily overcome the stirring effects of water flows, waves and perturbations to rise to the surface. The results of canonical correspondence analysis (CCA) of Microcystis vertical distribution in relation to environmental factors suggested that nutrient concentrations and temperature were the main influencing factors related to bloom formation by Microcystis in Lake Taihu during our investigation.

Insight into monogenetic eruption processes at Pelagatos volcano, Sierra Chichinautzin, Mexico: a combined melt inclusion and physical volcanology study

Abstract Eruptions and magma evolution of monogenetic volcanoes are thought to be controlled by rapid ascent of magmas over a short period of time. Volatiles degassing from magmas control the ascent velocity and therefore eruption intensity. Complex feedbacks exist between the rate and extent of volatile exsolution at shallow levels and groundmass crystallization, affecting the magma rheology, extent of fragmentation, resulting eruptive style and related hazards. Melt inclusions record the volatile contents and compositions of melts at various stages during their evolution, providing insights into degassing–crystallization processes at shallow crustal levels. Here we present new volatile and major element data from olivine-hosted melt inclusions from Pelagatos scoria cone, Mexico. These new data, combined with recent geochemical and textural data on Pelagatos eruptive products, allow us to propose a model for shallow magmatic processes at this volcano. Discharge of volatile-poor, low-viscosity magma drives early effusive activity along the fissure. Decreasing magma fluxes lead to the clogging of the fissure and the formation of a shallow magma reservoir where degassing and fractional crystallization take place. Subsequent explosive cone-forming activity is triggered by influx of deeper ( c. 5 km), less evolved, more volatile-rich magma into this shallow ( c. 1 km) reservoir. Supplementary material: new whole rock and matrix glass date are available at www.geolsoc.org.uk/SUP18782 .

NanoSIMS results from olivine-hosted melt embayments: Magma ascent rate during explosive basaltic eruptions

The explosivity of volcanic eruptions is governed in part by the rate at which magma ascends and degasses. Because the time scales of eruptive processes can be exceptionally fast relative to standard geochronometers, magma ascent rate remains difficult to quantify. Here we use as a chronometer concentration gradients of volatile species along open melt embayments within olivine crystals. Continuous degassing of the external melt during magma ascent results in diffusion of volatile species from embayment interiors to the bubble located at their outlets. The novel aspect of this study is the measurement of concentration gradients in five volatile elements (CO2, H2O, S, Cl, F) at fine-scale (5–10μm) using the NanoSIMS. The wide range in diffusivity and solubility of these different volatiles provides multiple constraints on ascent timescales over a range of depths. We focus on four 100–200μm, olivine-hosted embayments erupted on October 17, 1974 during the sub-Plinian eruption of Volcán de Fuego. H2O, CO2, and S all decrease toward the embayment outlet bubble, while F and Cl increase or remain roughly constant. Compared to an extensive melt inclusion suite from the same day of the eruption, the embayments have lost both H2O and CO2 throughout the entire length of the embayment. We fit the profiles with a 1-D numerical diffusion model that allows varying diffusivities and external melt concentrations as a function of pressure. Assuming a constant decompression rate from the magma storage region at approximately 220MPa to the surface, H2O, CO2 and S profiles for all embayments can be fit with a relatively narrow range in decompression rates of 0.3–0.5MPa/s, equivalent to 11–17m/s ascent velocity and an 8 to 12minute duration of magma ascent from ~10km depth. A two stage decompression model takes advantage of the different depth ranges over which CO2 and H2O degas, and produces good fits given an initial stage of slow decompression (0.05–0.3MPa/s) at high pressure (>145MPa), with similar decompression rates to the single-stage model for the shallower stage. The magma ascent rates reported here are among the first for explosive basaltic eruptions and demonstrate the potential of the embayment method for quantifying magmatic timescales associated with eruptions of different vigor.

Increased radon-222 in soil gas because of cumulative seismicity at active faults

This study demonstrates how the radon-222 (222Rn) concentration of soil gas at an active fault is sensitive to cumulative recent seismicity by examining seven active faults in western Japan. The 222Rn concentration was found to correlate well with the total earthquake energy within a 100-km radius of each fault. This phenomenon can probably be ascribed to the increase of pore pressure around the source depth of 222Rn in shallow soil caused by frequently induced strain. This increase in pore pressure can enhance the ascent velocity of 222Rn carrier gas as governed by Darcy's law. Anomalous 222Rn concentrations are likely to originate from high gas velocities, rather than increased accumulations of parent nuclides. The high velocities also can yield unusual young gas under the radioactive nonequilibrium condition of short elapsed time since 222Rn generation. The results suggest that ongoing seismicity in the vicinity of an active fault can cause accumulation of strain in shallow fault soils. Therefore, the 222Rn concentration is a possible gauge for the degree of strain accumulation.

Mechanisms of fluidic microbubble generation Part II: Suppressing the conjunctions

In the first part of this paper was demonstrated fact that microbubbles, because of their low ascent velocity, grow by repeated mutual conjunctions while they are still near their aerator exits. In this second part of the paper is shown that this undesirable growth may be avoided by a fluidic oscillator in the gas supply into the aerator. The oscillation leads to periodic sucking in and then expelling out water from the aerator channel during the two halves of the oscillation cycle—and moves back into the aerator exit during the suction half-cycle also the newly generated microbubble. This return prevents it from conjunction with the previously generated microbubble. In the subsequent half-cycle the inertia of water carries the microbubble (with surrounding water) sufficiently far away, out of the influence of the subsequent suction. As a result of the different motion directions, the two bubbles generated one after the other cannot get into a mutual contact—and thus remain small.

Ta'u and Ofu/Olosega volcanoes: The “Twin Sisters” of Samoa, their P, T, X melting regime, and global implications

AbstractThe Samoan islands of Ta'u and Ofu/Olosega (Ofol hereafter) are single shield volcanoes that have erupted alkali basalt for the past 70 and 440 kyr, respectively. They are 20 km apart, and are the easternmost subaerial expressions of the Samoan plume. The isotopic data for these islands are published; we report here the first major and trace element data for Ofol. The two islands are similar isotopically and in trace elements. Their high 3He/4He marks them as being a FOZO mantle end‐member. By comparing data from both volcanoes, we test the efficacy of melting models in constraining the mantle compositions and their P‐T of melting. We show that the mantle sources are similar, with spidergrams that peak at Ta (4× BSE), and Lu ∼0.5× BSE. Melts and mantle sources both have Lu/Hf ratios that are too low to support measured 176Hf/177Hf ratios, pointing to a young enrichment event in the mantle source. Degrees of melting are 6.5% for Ta'u and 5.2% for Ofol. P‐T of melting show a wide, overlapping range, but define a precise array; average values are 1475°C—77 km for Ta'u and 1550°C—110 km for Ofol. The deepest P‐T estimate is 4.2 GPa and 1550°C. The P‐T array is either a melting adiabat, or a mixing line of melts equilibrated at various depths. Kinetic modeling shows melt re‐equilibration will be likely for ascent velocities (m/yr) less than 40/(conduit radius in cm)2. P‐T estimates for melting may typically be minimum values.

Mechanisms of fluidic microbubble generation Part I: Growth by multiple conjunctions

Microbubbles, less than 1mm in diameter, are currently in the focus of attention due to their potential use in many chemical, biomechanical, and other processes. Their usual generation by percolation through small aerator passages has been often unsuccessful since microbubbles tend to be much larger than the size of aerator exits. The reason was identified in high-speed camera images showing the large size to be a consequence of growth by conjunctions immediately at the aerator exit, because of slow microbubble ascent velocity. The surface energy released in the conjunction causes the resultant larger bubble to oscillate and this further decreases its ascent. The elucidation of this mechanism opens a way towards producing small microbubbles by suppressing the conjunctions using a fluidic oscillator in the gas supply.

Thermal imaging and analysis of short-lived Vulcanian explosions at Volcán de Colima, Mexico

Vulcanian explosions present a major hazard at many active volcanoes, but they also provide useful insights into the underlying behaviour of the volcanic system and therefore require close monitoring. Thermal infrared cameras are an effective tool for imaging Vulcanian explosion plumes since they capture detailed temperature information, and can reveal the internal dynamics of the plume-forming explosions. High spatial resolution thermal images of 200 small to moderate sized Vulcanian explosions from the summit crater of Volcán de Colima, Mexico, recorded between 2006 and 2011, were analysed to distinguish different event types and develop an explosion classification scheme. Explosions display a broad spectrum of sizes and characteristics, ranging between two typical end-members: “large-impulsive” events producing rapidly ascending explosion plumes up to heights of 600–1600m above the crater rim, and “small-diffusive” events with plumes restricted to heights <600m. Most explosion plumes comprise a steady “gas-thrust” feeder plume below a convecting plume front. Others, that lack sufficient kinetic energy, rise buoyantly throughout the explosion, with steady buoyant ascent velocities ranging from ~1ms−1 to ~29ms−1. A time-series of thermal imagery throughout the period 2006–2011 reveals a weak relationship between apparent plume temperatures and lava dome extrusion, with the highest explosion temperatures coinciding with the onset of dome growth in early 2007. Temporal variations in the source locations of explosions across the summit crater are also identified and appear to show a close relationship to the patterns of lava dome growth and thermal evolution, with explosion source locations associated with the highest temperature thermal features.

Rheology of bimodal crystals suspensions: Results from analogue experiments and implications for magma ascent

The effect of crystal size of bimodal suspensions on rheology of magmas at strain rates between 0 and 1 s−1 is studied. Suspensions consist of silicon oil and two populations of natural crystals with size 63–125 and 250–500 µm mixed in different proportions; the total solid fraction ϕ of the mixtures is between 0.25 and 0.5. At ϕ ≤ 0.4, finer, coarser, and bimodal suspensions display comparable viscosities. At ϕ ≥ 0.4, the viscosity of the bimodal suspensions is larger than that of the unimodal ones. The bimodal suspension, made mainly of finer crystals, shows a stronger increase of viscosity with ϕ. The addition of finer crystals to a suspension of coarser ones produces a more pronounced increase of viscosity with respect to suspensions of coarse or fine crystals alone, and of finer crystals with added coarser ones. The bimodal suspensions of coarser crystals develops yield stress at ϕ ≥ 0.25, the others at ϕ ≥ 0.4. It is modeled the ascent velocity in a 20 m wide dike of magmas with bimodal and unimodal populations of crystals of different size. For ϕ ≤ 0.4, the crystal size has not effects on the ascent velocity of magmas. For ϕ ≥ 0.4, the velocity of a magma with growing phenocrystals decreases as ϕ increases less than that of a magma with forming microlites, and more of a magma with microlites and growing phenocrystals. A magma with phenocrystals and forming microlites has the lowest ascent velocity.