The fluid mechanics inside a volcano.

Abstract The style and evolution of volcanic eruptions are dictated by the fluid mechanics governing magma ascent. Decompression during ascent causes dissolved volatile species, such as water and carbon dioxide, to exsolve from the melt to form bubbles, thus providing a driving force for the eruption. Ascent is influenced not only by the nucleation and growth of gas bubbles, but also magma rheology and brittle deformation (fragmentation). In fact, all processes and magma properties within the conduit interact and are coupled. Ultimately, it is the ability of gas trapped within growing bubbles to expand or to be lost by permeable gas flow, which determines whether ascending magmas can erupt nonexplosively. We review and integrate models of the primary conduit processes to show when each process or property dominates and how these interact within a conduit. In particular, we illustrate how and why ascent rate may control eruptive behavior: slowly ascending magmas erupt effusively and rapidly ascending magmas erupt explosively

Relating vesicle shapes in pyroclasts to eruption styles

Vesicles in pyroclasts provide a direct record of conduit conditions during explosive volcanic eruptions. Although their numbers and sizes are used routinely to infer aspects of eruption dynamics, vesicle shape remains an underutilized parameter. We have quantified vesicle shapes in pyroclasts from fall deposits of seven explosive eruptions of different styles, using the dimensionless shape factor , a measure of the degree of complexity of the bounding surface of an object. For each of the seven eruptions, we have also estimated the capillary number, Ca, from the magma expansion velocity through coupled diffusive bubble growth and conduit flow modeling. We find that Ω is smaller for eruptions with Ca 1 than for eruptions with Ca 1. Consistent with previous studies, we interpret these results as an expression of the relative importance of structural changes during magma decompression and bubble growth, such as coalescence and shape relaxation of bubbles by capillary stresses. Among the samples analyzed, Strombolian and Hawaiian fire-fountain eruptions have Ca 1, in contrast to Vulcanian, Plinian, and ultraplinian eruptions. Interestingly, the basaltic Plinian eruptions of Tarawera volcano, New Zealand in 1886 and Mt. Etna, Italy in 122 BC, for which the cause of intense explosive activity has been controversial, are also characterized by Ca 1 and larger values of Ω than Strombolian and Hawaiian style (fire fountain) eruptions. We interpret this to be the consequence of syn-eruptive magma crystallization, resulting in high magma viscosity and reduced rates of bubble growth. Our model results indicate that during these basaltic Plinian eruptions, buildup of bubble overpressure resulted in brittle magma fragmentation.National Science Foundation EAR-1019872National Science Foundation EAR-081033

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 (5–10 μm) 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 100–200 μm, 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 220 MPa to the surface, H2O, CO2 and S profiles for all embayments can be fit with a relatively narrow range in decompression rates of 0.3–0.5 MPa/s, equivalent to 11–17 m/s ascent velocity and an 8 to 12 minute duration of magma ascent from ~ 10 km 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 (0.05–0.3 MPa/s) at high pressure (< 145 MPa), 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

Quantifying the Water-to-Melt Mass Ratio and Its Impact on Eruption Plumes During Explosive Hydromagmatic Eruptions

The interaction of magma with external water commonly enhances magma fragmentation through the conversion of thermal to mechanical energy and results in an increased production of fine-grained volcanic tephra. Magma-water interaction is thus of importance for hazard mitigation on both a local and a regional scales. The relative proportion of water that interacts with magma, quantified as the water-to-melt mass ratio, is thought to determine the efficiency of thermal to mechanical energy conversion, termed the fragmentation efficiency. Here, we analyze the pyroclast size distributions from the 10th century Eldgjá fissure eruption in Iceland, where parts of the fissure erupted subglacially and other erupted subaerially. The subglacially erupted magma passed through a column of glacial meltwater, resulting in a larger proportion of finer pyroclast sizes relative to the subaerially erupted, purely magmatic tephra. This finer grain size distribution has been attributed to quench-granulation induced by enhanced cooling upon interaction with external water. We hypothesize that the additional fragmentation (surface) energy required to produce the finer grained hydromagmatic deposits is due to the conversion of thermal to mechanical energy associated with the entrainment of water into the volcanic jet, as it passed through a column of subglacial melt water. Based on field and granulometry data, we estimate that the interaction of the volcanic jet with the meltwater provided an additional fragmentation energy of approximately 3–14 kJ per kg of pyroclasts. We numerically model the hydrofragmentation energy within a jet that passes through a layer of meltwater. We find that the water-to-melt mass ratio of entrained water required to produce the additional fragmentation energy is in the range of 1–2, which requires a minimum ice melting rate of 104 m3 s−1. Our simulation results show that the water-to-melt ratio is an important parameter that controls the ascent of plume in the atmosphere

Reconciling bubble nucleation in explosive eruptions with geospeedometers

The authors simulate bubble nucleation in silica-rich magma with conditions appropriate for Plinian eruptions. They demonstrate that the gap between decompression rate estimates from bubble number density and independent geospeedometers can be largely closed if nucleation is heterogenous facilitated by magnetite crystals and decompression rate is calculated as time-averaged values

Fragmentation and Plinian eruption of crystallizing basaltic magma

Basalt is the most ubiquitous magma on Earth, erupting typically at intensities ranging from quiescently effusive to mildly explosive. The discovery of highly explosive Plinian eruptions of basaltic magma has therefore spurred debate about their cause. Silicic eruptions of similar style are a consequence of brittle fragmentation, as magma deformation becomes progressively more viscoelastic. Magma eventually crosses the glass transition and fragments due to a positive feedback between water exsolution, viscosity and decompression rate. In contrast to silicic eruptions, the viscosity of basaltic magmas is thought to be too low to reach conditions for brittle fragmentation. Pyroclasts from several basaltic Plinian eruptions, however, contain abundant micron-size crystals that can increase magma viscosity substantially. We therefore hypothesize that magma crystallization led to brittle fragmentation during these eruptions. Using combined oscillatory and extensional rheometry of concentrated particle-liquid suspensions that are dynamically similar to microcrystalline basaltic magma, we show that high volume fractions of particles and extension rates of about 1 s?1or greater result in viscoelastic deformation and brittle fracture. We further show that for experimentally observed crystallization rate, basaltic magma can reach the empirical failure conditions when erupting at high discharge rates.by Pranabendu Moitra, Helge M. Gonnermann, Bruce F. Houghton and Chandra S. Tiwar

Identifying rheological regimes within pyroclastic density currents

Abstract Pyroclastic density currents (PDCs) are the most lethal of all volcanic hazards. An ongoing challenge is to accurately forecast their run-out distance such that effective mitigation strategies can be implemented. Central to this goal is an understanding of the flow mobility—a quantitative rheological model detailing how the high temperature gas-pyroclast mixtures propagate. This is currently unknown, yet critical to accurately forecast the run-out distance. Here, we use a laboratory apparatus to perform rheological measurements on real gas-pyroclast mixtures at dynamic conditions found in concentrated to intermediate pumice-rich PDCs. We find their rheology to be non-Newtonian featuring (i) a yield stress where deposition occurs; (ii) shear-thinning behavior that promotes channel formation and local increases in velocity and (iii) shear-thickening behavior that promotes decoupling and potential co-PDC plume formation. We provide a universal regime diagram delineating these behaviors and illustrating how flow can transition between them during transport

Identifying rheological regimes within pyroclastic density currents

Pyroclastic density currents (PDCs) are the most lethal of all volcanic hazards. An ongoing challenge is to accurately forecast their run-out distance such that effective mitigation strategies can be implemented. Central to this goal is an understanding of the flow mobility—a quantitative rheological model detailing how the high temperature gas-pyroclast mixtures propagate. This is currently unknown, yet critical to accurately forecast the run-out distance. Here, we use a laboratory apparatus to perform rheological measurements on real gas-pyroclast mixtures at dynamic conditions found in concentrated to intermediate pumice-rich PDCs. We find their rheology to be non-Newtonian featuring (i) a yield stress where deposition occurs; (ii) shear-thinning behavior that promotes channel formation and local increases in velocity and (iii) shear-thickening behavior that promotes decoupling and potential co-PDC plume formation. We provide a universal regime diagram delineating these behaviors and illustrating how flow can transition between them during transport

Sitúate : revista digital de situaciones de aprendizaje

Para el Día de Canarias se realiza como tarea y producto de la misma “El Baile del Vivo”. Se comienza visualizando distintos archivos multimedia del baile “Valentina la de Sabinosa”, escribiendo y aprendiendo varias de las estrofas de la canción a través de su audición. Para ser acompañadas, se interpreta silábicamente y con percusión corporal el ritmo antes de interpretarlo con los instrumentos del aula a modo de tambor herreño. Se mueven por el espacio al compás antes de comenzar a bailar en parejas. Se trabaja la imitación, la gesticulación y la expresión gestual como lo hacen los bailarines herreños. Se toca la melodía en la flauta una vez sea cantada, todo esto para actuar en el festival que organiza el centro como producto de la tarea. Para esta situación de aprendizaje se toma como referencia el Plan de Convivencia, que recoge instrucciones acerca de las normas durante las actividades grupales y el trabajo colaborativo.ES

Biochar particle size, shape, and porosity act together to influence soil water properties

<div><p>Many studies report that, under some circumstances, amending soil with biochar can improve field capacity and plant-available water. However, little is known about the mechanisms that control these improvements, making it challenging to predict when biochar will improve soil water properties. To develop a conceptual model explaining biochar’s effects on soil hydrologic processes, we conducted a series of well constrained laboratory experiments using a sand matrix to test the effects of biochar particle size and porosity on soil water retention curves. We showed that biochar particle size affects soil water storage through changing pore space between particles (interpores) and by adding pores that are part of the biochar (intrapores). We used these experimental results to better understand how biochar intrapores and biochar particle shape control the observed changes in water retention when capillary pressure is the main component of soil water potential. We propose that biochar’s intrapores increase water content of biochar-sand mixtures when soils are drier. When biochar-sand mixtures are wetter, biochar particles’ elongated shape disrupts the packing of grains in the sandy matrix, increasing the volume between grains (interpores) available for water storage. These results imply that biochars with a high intraporosity and irregular shapes will most effectively increase water storage in coarse soils.</p></div