Understanding silicic volcanism: Constraints from elasticity and failure of vesicular magma

Volcanic eruptions are one of the most spectacular and dangerous natural phenomena. Volcanic activity can either be effusive, dominated by quiescent emission of lava or explosive, dominated by the eruption of pyroclastic material. A rapid transition between these two regimes is possible. possible. On a broad scale, a clear distinction of the eruption style can be drawn by magma composition. Large-scale basaltic eruptions are mostly effusive, whereas large-scale silicic eruptions are mostly explosive. Thus silicic volcanism possesses a severe hazard potential and can have devastating effects on human population. This work comprises experimental investigations of the fragmentation behavior of porous magma as well as of the propagation of elastic waves within this material. The analyses were conducted on samples from Unzen Volcano, Japan as well as samples from Soufrière Hills Volcano, Montserrat, West Indies. The elastic wave velocities of differently porous Unzen dacite samples were investigated with the use of a cubic multi-anvil press. The main results of this study show that porosity (density) and texture affect the elastic properties of samples at a given temperature. In particular, it can be stated: (1) Seismic velocities increase with pressure due to compaction and closing of microcracks. The Vp anisotropy decreases with pressure for the same reason. (2) Increasing the temperature also leads to higher elastic wave velocities and lower anisotropies. This must be highlighted as the inverse behavior is documented for the majority of rocks. The effect may be linked to reduction of pore volume and further closing of microcracks due to reduction of cooling tensions. At 600 °C mean Vp values of 4.31 - 5.64 km/s and mean Vs* values of 2.20 - 3. 32 km/s could be determined. (3) The velocity anisotropy can be linked to the texture of the samples: Those with a high anisotropy show a pronounced shape-preferred orientation of phenocrysts and microcrystals, sometimes in addition to layering within the groundmass of the sample. Since the crystals are typically aligned parallel to walls of volcanic conduits, the velocity normal to the walls is likely to be reduced. The data allows better estimates of the properties of silicic volcanic rocks at shallow depths within volcanoes, e.g. at conduit walls. These estimates are vital for computation of conduit models as well as the modelling of volcano seismic data, and may lead to an improved analysis of precursor phenomena in volcanic areas. The physical properties of magma within volcanic conduits and domes are crucial for modelling eruptions. This study comprises a detailed investigation of the fragmentation behavior (threshold and propagation speed) of differently porous sets of dacitic and andesitic samples derived from Unzen Volcano, Japan and Soufrière Hills Volcano, Montserrat, West Indies. The experiments were performed with a shock-tube based fragmentation apparatus and pertain to the brittle fragmentation process. The results show a strong influence of the open porosity and the initial pressure on the fragmentation behavior. The speed of fragmentation follows a logarithmic relationship with the pressure difference, the fragmentation threshold an inversely proportional power-law relationship with increasing porosity. In this study fragmentation speed values ranging from 15 - 150 m/s were observed for applied pressure differences of up to 40 MPa and open porosities from 2.5 - 67.1 %. The expansion of the pressurized gas in the vesicles largely provides the energy driving the fragmentation process. The fragmentation speed results of all analyzed samples show a close relationship to the energy density (fragmentation energy standardized to volume). A logarithmical increase of the propagation speed was observed with the energy density as soon as the energy threshold of 2.0 x 0.5 J/m³ was exceeded. The fragmentation speed is independent from the origin and composition of the samples, proving the governing role of the energy to the initiation a propagation of fragmentation process. Different fragmentation mechanisms were discussed and the layer-by-layer fragmentation due to vesicle bursting is concluded to be the main process responsible for the disintegration of vesicular rocks. The increased importance of fracturing due to the passing of the unloading wave after a rapid decompression could be proved for low porous samples. Further the influence of the sample’s permeability on the fragmentation behavior was evaluated. It could be shown that a high permeability hinders the initiation of a fragmentation and reduces the propagation speed of this process at a certain energy density. The fragmentation results were applied to the dome collapse events and Vulcanian events of the 1990-1995 Unzen eruption and the 1997 Vulcanian events at Montserrat. Large blocks with layers of various porosity were observed at the block-and-ash flow deposits of Unzen Volcano and support the model that a dome and dome lobes consist of areas of differing porosity. In addition, the samples gained from Montserrat, allow to postulate a porosity gradient within a volcanic conduit, with low porous magma close to the conduit walls. A layered composition of a dome and dome lobes, respectively, may lead to the fragmentation of single layers, followed by the collapse of the overlaying sections. These events could catalyze gravitationally induced dome collapse events leading to vigorous pyroclastic flows and / or trigger a sector collapse followed by an Vulcanian event. A porosity gradient at the magma in the conduit leads to a concave shape of the fragmentation surface and facilitates lateral fragmentation of dense magma close to the conduit walls. Conduit implosion may to occur during most explosive eruptions and is likely to influence the cessation or pulsation of the eruption. The slow magma ascent and extrusion rate at Unzen resulted in relatively dense extruded magma, as the magma could almost completely degas during the ascent. The low porosity of this magma causes a high fragmentation threshold of most material, which is too high for unassisted fragmentation. Therefore dome collapse events were the most abundant events of the 1990-1995 activity of Unzen Volcano, leading to numerous block-and-ash flows. Also the fragmentation-amplified collapse of dome lobes or parts of the dome are reasonable. This accounts especially for the long lasting collapse events with vigorous pyroclastic flows at the early stage of the eruption in June 1991, which were followed by minor Vulcanian events. Nevertheless a larger explosive event would have been possible, triggered by a landslide or a sector collapse of the dome. Similarly to Unzen, the first phase of activity of the recent eruption of Montserrat is characterized by numerous dome collapse events leading to violent pyroclastic flows. As the magma extrusion rate was quite high during this phase, the extruded magma was higher vesiculated compared to the Unzen magma and thus a more violent evolution of the eruption activity took place. Large dome collapse events frequently caused Vulcanian events, and even two cycles of Vulcanian activity from August to September 1997 occurred. The calculations of the fragmentation depth, reached by this explosions yielded about 1500 m, based on the laboratory gained fragmentation speeds, which is in good agreement numerical models and observations. In silicic volcanic systems the conduit seems not to be sharply defined. The conduit walls are more to be seen as a kind of transition zone between a hot, ductile and vesicular magma within the conduit and the host rock. The rocks forming this transition zone are assumed to be quite hot, but presumably below glass transition and react therefore solely brittle. Furthermore these rocks should be quite dense, compared to magma in the conduit, and heavily fractured due to the high shear strains this zone is presumably exposed to. The transition zone is less likely affected by a fragmentation event. Their rocks (magma as well as host rocks) are too dense to fragment due to pore pressure. The needed pressure difference is unrealistically high, for example an overpressure of 18 MPa would be needed to initiate the fragmentation of rocks with a porosity of 7.5 %. Nevertheless this material may be found in the deposits of explosive events, due to processes like conduit wall erosion or as remnant of dense lenses within more porous areas. Indeed, also fragmentation may take place, the most likely process fragmenting even this dense material is by lateral fragmentation, which may occur from a certain depth on behind a fast propagating fragmentation of highly vesicular magma at the center of the conduit. The style and progression of an eruption is depending on the properties of this vesicular magma. If its fragmentation threshold can be exceeded, an explosive event may take place. Otherwise the magma is extruded quiescent in a dome forming eruption. The transition zone bears important implications on the one hand for the explosive event as a lateral fragmentation of a certain area may cause cessation or pulsation of the event, on the other hand for the propagation of seismic signals related to the eruption. Within this transition zone as well as the nearby host rock a decisive change of temperature and porosity can be supposed. This leads to a significant shift of the elastic wave velocities within this zone, sometimes resulting in trapped waves within this zone as observed for Montserrat. Especially the abnormal velocity increase with increasing temperatures has to be mentioned. Thus implications for an overall view of a volcanic system are provided in this study, with the transition zone as the common link. The properties of the elastic wave velocities account for the for host rock as well as the transition zone and bear vital constraints for the interpretation and modelling of volcano seismic data. The results of the fragmentation experiments are applicable for dome rocks, the vesicular interior of a conduit as well as the transition zone and contain important implications for the modelling of conduit processes. Together the results of this study may contribute to a refined understanding of processes typical for silicic volcanism. This may allow an improved analysis of precursor phenomena in volcanic areas and consequently provide important constraints to the hazard and risk management

Combined structural analysis and cathodoluminescence investigations of single Pr3+-doped Ca2Nb3O10 nanosheets

Due to the novel properties of both 2D materials and rare-earth elements, developing 2D rare-earth nanomaterials has a growing interest in research. To produce the most efficient rare-earth nanosheets, it is essential to find out the correlation between chemical composition, atomic structure and luminescent properties of individual sheets. In this study, 2D nanosheets exfoliated from Pr3+-doped KCa2Nb3O10 particles with different Pr concentrations were investigated. Energy dispersive X-ray spectroscopy analysis indicates that the nanosheets contain Ca, Nb and O and a varying Pr content between 0.9 and 1.8 at%. K was completely removed after exfoliation. The crystal structure is monoclinic as in the bulk. The thinnest nanosheets are 3 nm corresponding to one triple perovskite-type layer with Nb on the B sites and Ca on the A sites, surrounded by charge compensating TBA+ molecules. Thicker nanosheets of 12 nm thickness (and above) were observed too by transmission electron microscopy with the same chemical composition. This indicates that several perovskite-type triple layers remain stacked similar to the bulk. Luminescent properties of individual 2D nanosheets were studied using a cathodoluminescence spectrometer revealing additional transitions in the visible region in comparison to the spectra of different bulk phases

Stratigraphic reconstruction of the Víti breccia at Krafla volcano (Iceland): insights into pre-eruptive conditions priming explosive eruptions in geothermal areas

Krafla central volcano in Iceland has experienced numerous basaltic fissure eruptions through its history, the most recent examples being the Mývatn (1724‒1729) and Krafla Fires (1975-1984). The Mývatn Fires opened with a steam-driven eruption that produced the Víti crater. A magmatic intrusion has been inferred as the trigger perturbing the geothermal field hosting Víti, but the cause(s) of the explosive response remain uncertain. Here, we present a detailed stratigraphic reconstruction of the breccia erupted from Víti crater, characterize the lithologies involved in the explosions, reconstruct the pre-eruptive setting, fingerprint the eruption trigger and source depth, and reveal the eruption mechanisms. Our results suggest that the Víti eruption can be classified as a magmatic-hydrothermal type and that it was a complex event with three eruption phases. The injection of rhyolite below a pre-existing convecting hydrothermal system likely triggered the Víti eruption. Heating and pressurization of shallow geothermal fluid initiated disruption of a scoria cone \textquotedblcap\textquotedbl via an initial series of small explosions involving a pre-existing altered weak zone, with ejection of fragments from at least 60-m depth. This event was superseded by larger, broader, and dominantly shallow explosions (\~ 200~m depth) driven by decompression of hydrothermal fluids within highly porous, poorly compacted tuffaceous hyaloclastite. This second phase was triggered when pressurized fluids broke through the scoria cone complex \textquotedblcap\textquotedbl. At the same time, deep-rooted explosions (\~ 1-km depth) began to feed the eruption with large inputs of fragmented rhyolitic juvenile and host rock from a deeper zone. Shallow explosions enlarging the crater dominated the final phase. Our results indicate that at Krafla, as in similar geological contexts, shallow and thin hyaloclastite sequences hosting hot geothermal fluids and capped by low-permeability lithologies (e.g. altered scoria cone complex and/or massive, thick lava flow sequence) are susceptible to explosive failure in the case of shallow magmatic intrusion(s). Supplementary Information The online version contains supplementary material available at 10.1007/s00445-021-01502-y

Rational Strain Engineering in Delafossite Oxides for Highly Efficient Hydrogen Evolution Catalysis in Acidic Media

The rational design of hydrogen evolution reaction (HER) electrocatalysts which are competitive with platinum is an outstanding challenge to make power-to-gas technologies economically viable. Here, we introduce the delafossites PdCrO$_2$, PdCoO$_2$ and PtCoO$_2$ as a new family of electrocatalysts for the HER in acidic media. We show that in PdCoO$_2$ the inherently strained Pd metal sublattice acts as a pseudomorphic template for the growth of a strained (by +2.3%) Pd rich capping layer under reductive conditions. The surface modification continuously improves the electrocatalytic activity by simultaneously increasing the exchange current density j$_0$ from 2 to 5 mA/cm$^2_{geo}$ and by reducing the Tafel slope down to 38 mV/decade, leading to overpotentials $\eta_{10}$ < 15 mV for 10 mA/cm$^2_{geo}$, superior to bulk platinum. The greatly improved activity is attributed to the in-situ stabilization of a $\beta$-palladium hydride phase with drastically enhanced surface catalytic properties with respect to pure or nanostructured palladium. These findings illustrate how operando induced electrodissolution can be used as a top-down design concept for rational surface and property engineering through the strain-stabilized formation of catalytically active phases

Can nanolites enhance eruption explosivity?

Degassing dynamics play a crucial role in controlling the explosivity of magma at erupting volcanoes. Degassing of magmatic water typically involves bubble nucleation and growth, which drive magma ascent. Crystals suspended in magma may influence both nucleation and growth of bubbles. Micron- to centimeter-sized crystals can cause heterogeneous bubble nucleation and facilitate bubble coalescence. Nanometer-scale crystalline phases, so-called “nanolites”, are an underreported phenomenon in erupting magma and could exert a primary control on the eruptive style of silicic volcanoes. Yet the influence of nanolites on degassing processes remains wholly uninvestigated. In order to test the influence of nanolites on bubble nucleation and growth dynamics, we use an experimental approach to document how nanolites can increase the bubble number density and affect growth kinetics in a degassing nanolite-bearing silicic magma. We then examine a compilation of these values from natural volcanic rocks from explosive eruptions leading to the inference that some very high naturally occurring bubble number densities could be associated with the presence of magmatic nanolites. Finally, using a numerical magma ascent model, we show that for reasonable starting conditions for silicic eruptions, an increase in the resulting bubble number density associated with nanolites could push an eruption that would otherwise be effusive into the conditions required for explosive behavior

Primary and secondary fragmentation of crystal-bearing intermediate magma

Crystal-rich intermediate magmas are subjected to both primary and secondary fragmentation processes, each of which may produce texturally distinct tephra. Of particular interest for volcanic hazards is the extent to which each process contributes ash to volcanic plumes. One way to address this question is by fragmenting pyroclasts under controlled conditions. We fragmented pumice samples from Soufriere Hills Volcano (SHV), Montserrat, by three methods: rapid decompression in a shock tube-like apparatus, impact by a falling piston, and milling in a ball mill. Grain size distributions of the products reveal that all three mechanisms produce fractal breakage patterns, and that the fractal dimension increases from a minimum of ~ 2.1 for decompression fragmentation (primary fragmentation) to a maximum of ~ 2.7 by repeated impact (secondary fragmentation). To assess the details of the fragmentation process, we quantified the shape, texture and components of constituent ash particles. Ash shape analysis shows that the axial ratio increases during milling and that particle convexity increases with repeated impacts. We also quantify the extent to which the matrix is separated from the crystals, which shows that secondary processes efficiently remove adhering matrix from crystals, particularly during milling (abrasion). Furthermore, measurements of crystal size distributions before (using x-ray computed tomography) and after (by componentry of individual grain size classes) decompression-driven fragmentation show not only that crystals influence particular size fractions across the total grain size distribution, but also that free crystals are smaller in the fragmented material than in the original pumice clast. Taken together, our results confirm previous work showing both the control of initial texture on the primary fragmentation process and the contributions of secondary processes to ash formation. Critically, however, our extension of previous analyses to characterisation of shape, texture and componentry provides new analytical tools that can be used to assess contributions of secondary processes to ash deposits of uncertain or mixed origin. We illustrate this application with examples from SHV deposits

Auto-granulation of particle surfaces during Surtseyan eruptions

[Abstract unavailable

HVEM Signalling Promotes Colitis

Background Tumor necrosis factor super family (TNFSF) members regulate important processes involved in cell proliferation, survival and differentiation and are therefore crucial for the balance between homeostasis and inflammatory responses. Several members of the TNFSF are closely associated with inflammatory bowel disease (IBD). Thus, they represent interesting new targets for therapeutic treatment of IBD. Methodology/Principal Findings We have used mice deficient in TNFSF member HVEM in experimental models of IBD to investigate its role in the disease process. Two models of IBD were employed: i) chemical-induced colitis primarily mediated by innate immune cells; and ii) colitis initiated by CD4+CD45RBhigh T cells following their transfer into immuno-deficient RAG1-/- hosts. In both models of disease the absence of HVEM resulted in a significant reduction in colitis and inflammatory cytokine production. Conclusions These data show that HVEM stimulatory signals promote experimental colitis driven by innate or adaptive immune cells

Multidisciplinary constraints of hydrothermal explosions based on the 2013 Gengissig lake events, Kverkfjöll volcano, Iceland

Highlights • A multidisciplinary approach to unravel the energetics of hydrothermal explosions. • Pressure failure caused by a lake drainage triggered the hydrothermal explosions. • Bedrock nature controlled the explosion dynamics and the way energy was released. • Approx. 30% of the available thermal energy is converted into mechanical energy. • Released seismic energy as proxy to detect past (and future?) hydrothermal explosions. Hydrothermal explosions frequently occur in geothermal areas showing various mechanisms and energies of explosivity. Their deposits, though generally hardly recognised or badly preserved, provide important insights to quantify the dynamics and energy of these poorly understood explosive events. Furthermore the host rock lithology of the geothermal system adds a control on the efficiency in the energy release during an explosion. We present results from a detailed study of recent hydrothermal explosion deposits within an active geothermal area at Kverkfjöll, a central volcano at the northern edge of Vatnajökull. On August 15th 2013, a small jökulhlaup occurred when the Gengissig ice-dammed lake drained at Kverkfjöll. The lake level dropped by approximately 30 m, decreasing pressure on the lake bed and triggering several hydrothermal explosions on the 16th. Here, a multidisciplinary approach combining detailed field work, laboratory studies, and models of the energetics of explosions with information on duration and amplitudes of seismic signals, has been used to analyse the mechanisms and characteristics of these hydrothermal explosions. Field and laboratory studies were also carried out to help constrain the sedimentary sequence involved in the event. The explosions lasted for 40–50 s and involved the surficial part of an unconsolidated and hydrothermally altered glacio-lacustrine deposit composed of pyroclasts, lavas, scoriaceous fragments, and fine-grained welded or loosely consolidated aggregates, interbedded with clay-rich levels. Several small fans of ejecta were formed, reaching a distance of 1 km north of the lake and covering an area of approximately 0.3 km2, with a maximum thickness of 40 cm at the crater walls. The material (volume of approximately 104 m3) has been ejected by the expanding boiling fluid, generated by a pressure failure affecting the surficial geothermal reservoir. The maximum thermal, craterisation and ejection energies, calculated for the explosion areas, are on the order of 1011, 1010 and 109 J, respectively. Comparison of these with those estimated by the volume of the ejecta and the crater sizes, yields good agreement. We estimate that approximately 30% of the available thermal energy was converted into mechanical energy during this event. The residual energy was largely dissipated as heat, while only a small portion was converted into seismic energy. Estimation of the amount of freshly-fragmented clasts in the ejected material obtained from SEM morphological analyses, reveals that a low but significant energy consumption by fragmentation occurred. Decompression experiments were performed in the laboratory mimicking the conditions due to the drainage of the lake. Experimental results confirm that only a minor amount of energy is consumed by the creation of new surfaces in fragmentation, whereas most of the fresh fragments derive from the disaggregation of aggregates. Furthermore, ejection velocities of the particles (40–50 m/s), measured via high-speed videos, are consistent with those estimated from the field. The multidisciplinary approach used here to investigate hydrothermal explosions has proven to be a valuable tool which can provide robust constraints on energy release and partitioning for such small-size yet hazardous, steam-explosion events