Magma Flow Research Articles

Unexpected large eruptions from buoyant magma bodies within viscoelastic crust

Large volume effusive eruptions with relatively minor observed precursory signals are at odds with widely used models to interpret volcano deformation. Here we propose a new modelling framework that resolves this discrepancy by accounting for magma buoyancy, viscoelastic crustal properties, and sustained magma channels. At low magma accumulation rates, the stability of deep magma bodies is governed by the magma-host rock density contrast and the magma body thickness. During eruptions, inelastic processes including magma mush erosion and thermal effects, can form a sustained channel that supports magma flow, driven by the pressure difference between the magma body and surface vents. At failure onset, it may be difficult to forecast the final eruption volume; pressure in a magma body may drop well below the lithostatic load, create under-pressure and initiate a caldera collapse, despite only modest precursors.

Flow Features of Non-Newtonian Fluid Through a Paraboloid of Revolution

Non-Newtonian fluids encountered in several large-scale industrial applications including blood flows in micro-circulatory system, food and polymer processing, magma and ice flows. Non-Newtonian fluids have applications in chemical materials (polymer liquids, drilling mud, pharmaceutical chemicals, detergents, paints, shampoos), biological materials (animal blood, synovial liquid, vacancies), processing of food (condensed milk, magma, sauces, ketchup, chocolates, ice flows) etc. The study of Carreau fluid has received considerable attention in the recent period, since the traditional Newtonian fluid not able to describe precisely the behavior of fluid flow with suspended particles. This work gives brief summary about the flow attributes of magnetic Carreau and Casson fluids through a paraboloid of revolution under considering convective restrictions along with thermal radiation via porous medium. Similarity functions are useful for renovating the flow governing system to non-linear system. The numerical outcomes are evaluated by enrolling R-K fourth order shooting process. The explicit outcomes are obtained to discuss the reaction of flow and temperature fields for various parameters of interest. The outcomes for skin friction coefficients are exemplified through tables. The Nusselt number’s values are shown via tables. Comparative work with previous outcomes is generated. Four important outcomes are: The Prandtl number’s hiking estimates cause reduction in temperature, the magnifying estimates of magnetic parameter make the velocity profile to decrease, the effect of Casson parameter is to reduce the velocity profile and Nusselt number and enhance temperature profile and skin friction coefficient, Weissenberg number cause reduction in skin friction coefficient and uplift the Nusselt number.

Control of Vent Geometry on the Fluid Dynamics of Volcanic Plumes: Insights From Numerical Simulations

AbstractWe present three‐dimensional numerical simulations of eruption clouds from circular to linear fissure vents to investigate the control of vent shape on the height and stability of volcanic plumes during large explosive eruptions. Our results show that clouds ejected from circular or low‐aspect‐ratio (nearly square‐like) fissure vents can be associated with radially suspended flow (RSF) at the top of the jet region, whereas those emitted from narrow‐fissure vents are not. Non‐RSF plumes are more stable than those associated with RSF because the highly concentrated parts of the ejected mixture are easily dissipated and mixed with air near the vent. Plume height in the RSF regime decreases while that in the non‐RSF regime increases with increasing aspect ratio, even for a fixed magma flow rate. These observations suggest that the efficiency of air entrainment is influenced by the vent shape, which in turn controls the dynamics of eruption plumes.

Magma flow pattern of the 1.78 Ga dyke swarm of the North China Craton during the initial assembly of the Supercontinent Nuna/Columbia: Constraints from rock magnetic and anisotropy of magnetic susceptibility studies

The geodynamic origin of the 1.78 Ga dyke swarm in the North China Craton (NCC) is long debated. It is considered either a post-collision process or paleoplume-related. Constraining its origin is essential for the reconstruction of the NCC position within the supercontinent Nuna (also referred to as Columbia). Here, we report on the rock-magnetism and anisotropy of magnetic susceptibility (AMS) of thirty-three 1.78 Ga dykes, to constrain the magma center of the igneous event. The samples were collected from the entire swarm across the NCC. Magnetite appears to be the dominant magnetic mineral in all the dykes, occasionally with minor maghemite, while the magnetite grain size within the dykes is generally increasing from south to north. AMS results of the dykes portray a magma flow-related fabric. Over half of the studied dykes show a predominance of horizontal to subhorizontal magmatic flow, while a few have been formed by vertical intrusion. These observations imply that the 1.78 Ga dyke swarm may have been generated by sustained magma flow from a magmatic source centered at the south margin of the NCC. During dyke propagation, some localized magma subchambers may have been formed; they could be responsible for the less common vertically intruded dykes and the EW-trending dykes. Therefore, a plume-related tectonic setting for the 1.78 Ga dyke swarm is more compelling. With the eruption center along the margin of the NCC, it may serve as an essential criterion to search for possible counterparts in other tectonic units within Nuna/Columbia to evaluate the proximity of the NCC with them.

Mafic schlieren, crystal accumulation and differentiation in granitic magmas: an integrated case study

Observations in the S-type granites (s.l.) of the Wilson’s Promontory batholith demonstrate that one type of schlieren in granitic rocks represent accumulations of mainly mafic magmatic minerals, with internal layering formed through pulsed magma flow. Loss of interstitial magmatic liquid played, at most, a minor role in shaping the preserved compositions of the schlieren; filter pressing was not involved, and simple gravity settling of crystals was also insignificant. Through dissolution-reprecipitation and reactions with residual magmatic liquids in the Wilsons Promontory schlieren, the original accumulated crystals of mafic minerals were largely supplanted by later generations or completely new phases. In the present case, the original accumulating minerals were garnet and orthopyroxene, with minor biotite and accessory minerals. The schlieren retain some of the early, euhedral, compositionally distinct, accumulated biotite, but most biotite formed through reaction of accumulated orthopyroxene and garnet with residual liquid. Some early, accumulated garnet remains, but this is not the peritectic garnet that was originally entrained into the magmas, at source depths. Rather, these are magmatic crystals formed, at mid-crustal depths, through dissolution–reprecipitation of the original peritectic garnet. At emplacement level, another episode of garnet dissolution–reprecipitation occurred, close to the solidus, extensively reorganising the grain-scale igneous textures. Although schlieren preserve structural, chemical and some textural features that can be used to infer their origins, their present microtextures do not fully reflect their initial formation mechanisms. Also, the physical mechanisms of schlieren formation most commonly have little similarity with the processes that were responsible for the main chemical variations in the batholith. Nevertheless, the presence of schlieren indicates that the granitic magmas were flowing in sheet-like laminae, and they provide information on the high-temperature igneous minerals that were suspended in their parent magmas, prior to emplacement.

A novel experimental apparatus for investigating bubbly flows in a slot geometry.

Bubbly flows occur in a wide variety of industrial and environmental settings. While there is a broad literature that describes bubbly flow behavior in pipes and channels, flow in a high aspect ratio slot has received little attention. We describe the design and construction of a new experimental apparatus to investigate the processes associated with bubbly flows in a slot geometry. The apparatus is designed to perform scaled analog experiments to investigate the flow of bubbly magma through the sub-volcanic plumbing system, but it is sufficiently flexible to address many other flow scenarios. The main bubble column, which can be inclined up to 30° from the vertical, comprises a glass-walled slot 3 m wide and 2 m tall, with 3 cm gap width. A modular and flexible gas injection system allows the number, spacing, and diameter of the gas emission points to be varied, as well as gas injection flux, and a pumped recirculation system allows the concurrent liquid flow rate to be controlled and varied. A dedicated data acquisition system synchronizes high-speed videography with temperature and pressure data from different points in the apparatus. Preliminary data are presented to demonstrate the operation of the apparatus and to illustrate the types of fluid dynamic information that can be captured.

О малых колебаниях магмы в питающей системе вулкана

A simple analytical model representing one of the possible mechanisms for the occurrence of low-frequency oscillations in a feeding system of volcano has been developed. The model is presented for a cylindrical chamber filled with magma with Maxwell rheology. It is shown that damped harmonic oscillations in the magma flow velocity can occur in the volcanic chamber. These damped harmonic oscillations can appear as a reaction to remote seismic events or seismic events in the volcanic feeding system. The dependence of the oscillation frequency on the physical characteristics of the magmatic melt and the geometric dimensions of the volcano chamber is shown. The occurrence of magmatic oscillations can be observed near the surface as a volcanic tremor. The model is applied to the measurements result of low-frequency oscillations for the magma chamber of the Elbrus volcanic center.

Relationship between magnetic fabrics and deformation of the Miocene Pohorje intrusions and surrounding sediments (Eastern Alps)

The Miocene deformation history of magmatic and host metamorphic rocks and surrounding sediments was reconstructed by measuring meso- and microscale structures and anisotropy of magnetic susceptibility (AMS) data in order to constrain the structural evolution of the Pohorje pluton during the onset of lithospheric extension at the Eastern Alps–Pannonian Basin transition. Principal AMS axes, lineation and foliation are very similar to mesoscopic lineation and foliation data from the main intrusive body and from some dykes. Although contribution from syn-magmatic texture is possible, these structures were formed during the cooling of the pluton and associated subvolcanic dykes just shortly after the 18.64 Ma pluton intrusion. Dykes emplaced during progressively younger episodes reflect decreasing amount of ductile strain, while firstly mesoscopic foliation and lineation, and then the tectonic AMS signal gradually disappears. In the structurally highest N–S trending dacite dykes, the AMS fabric only reflects the magmatic flow. The Miocene sediments underwent the same, NE–SW to E–W extension as the magmatic and host metamorphic rocks as indicated by both AMS and fault-slip data. All these events occurred prior to ~ 15 Ma, i.e., during the main syn-rift extension of the Pannonian Basin and during the fastest exhumation of the Tauern and Rechnitz windows, both demonstrating considerable extension of diverse crustal segments of the Alpine nappe pile. After a counterclockwise rotation around ~ 15 Ma, the maximum stress axis changed to a SE–NW orientation, but it was only registered by brittle faulting. During this time, the overprinting of a syn-rift extensional AMS texture was not possible in the cooled or cemented magmatic, metamorphic and sedimentary rocks.

Applicability of AMS technique as a flow fabric indicator in dykes: insight from Nandurbar–Dhule Deccan dyke swarm

Determination of the nature of magma flow (vertical or lateral) from rock fabrics in a dyke swarm holds significant importance in deciphering its emplacement mechanism, depth of magma chamber and its association with mantle plumes. Nandurbar–Dhule dyke swarm from Western India consists of ~ 210 mappable, tholeiitic, large, linear dykes intruding the country rocks of Deccan basalt. We discuss here the development of flow-controlled rock fabrics in these dykes as a function of their thicknesses. Magnetic fabric obtained from anisotropy of magnetic susceptibility (AMS) analysis of the samples collected from these dykes was used as a proxy for rock (flow) fabric. Samples from the central parts of the thicker dykes (strike perpendicular thickness > 10 m) show random and inconsistent development of magnetic fabrics than their marginal counterparts. Petrography suggests the development of relatively clear shape-preferred orientations of the elongated mineral grains only at the margin of a thick dyke. Thinner dykes (strike perpendicular thickness < 10 m) show both consistent magnetic fabrics and clear shape-preferred orientations. We propose that the center of a thick dyke fails to provide (due to high flow velocity, slower cooling rate, lack of friction-induced shearing, and draining back of magma) a favorable condition where (a) flow fabric can efficiently develop and (b) magnetic fabric can effectively mimic the flow fabric. Thinner dykes and the margins of the thicker dykes are, hence, better candidates for observing flow fabrics represented by magnetic fabrics. This work is also motivated by the reservations expressed by earlier researchers in using the AMS technique for magma flow characterization, especially for giant dyke swarms. We have critically evaluated their reservations in light of our results and commented on the applicability of the AMS technique on such studies.

Implications of deflation-inflation event models on Kīlauea Volcano, Hawaiʻi

Surface deformation of volcanic areas can reveal information about subsurface magma reservoirs and how magma is transported between them, which is an important part of volcano monitoring for hazard mitigation. One prominent style of deformation observed at Kīlauea volcano, Hawaiʻi, is episodic deflation-inflation (DI) events, which are recorded in surface deformation data and characterized by deflation of the summit region over hours to days followed by rapid re-inflation. The exact cause of DI events is unknown, however, a commonly proposed explanation is that a temporary blockage occurs in the conduit connecting the south caldera magma reservoir to the shallower Halemaʻumaʻu reservoir, thus interrupting the influx of magma. This model is investigated by testing the hypothesis that during the deflationary phase of a DI event, the volume of magma blocked from reaching the Halemaʻumaʻu reservoir is added to the south caldera reservoir. Using a mass balance approach and deformation modeling, the expected deformation (tilt) pattern was predicted for the reservoirs and compared to tilt observations from 16 large (>4 μrad magnitude) DI events between 2010 and 2012. While the predicted tilts due to inflation from the south caldera reservoir are strong enough to be detected, this signal is not seen in the tilt observations. We also explore the possibility that the blockage occurs below the south caldera reservoir, as well as alternative configurations that include a direct connection between the south caldera reservoir and the eruption site. None of the simple two-reservoir models we test are able to adequately satisfy the observations. Selected DI events show that tilt vectors change in both magnitude and azimuthal direction over the course of deflation. We conclude that the portion of the summit storage and plumbing system active during these DI events is more complicated than two connected chambers, and that it is more likely that these summit reservoirs have their own source conduits connecting in some complex way to the deep magma source.

Regional-scale polydiapirism predating the Neoarchean Yilgarn Orogeny

Gneiss domes occur in a wide variety of orogenic and anorogenic domains of the continental crust, and play a major role in lithosphere dynamics by allowing upwards transfer of heat and mass. They commonly contain a core of granites and/or migmatites, overlain by a mantle of lower-grade rocks. Evolutionary models of many gneiss domes are controversial.Here we use new structural and zircon isotopic data to unravel the tectono-magmatic evolution of the Yalgoo dome and surroundings, at the margins of the Neoarchean Yilgarn Orogen, Western Australia. The study area includes at least seven granite–migmatite domes (5–70 km in average diameter), and several subdomes within the larger domes, all showing comparable structural features and age. In each dome, a concentric domal foliation is concordant with granite–greenstone contacts, bearing a radial, outward-plunging lineation. The bulk of the structures in each dome reflect magmatic flow, with pervasive subsolidus fabrics occurring only along the outer dome margins, and reflecting dome-up kinematics. Narrow greenstone keels pinched between domes and subdomes display vertical constriction, in an area regionally dominated by flattening along the steep sides of the deeply-eroded domes, and along the flat-laying dome roofs.The regional structural pattern and the kinematics of the high-strain zones are best explained by sequential emplacement of nested diapirs, which delivered large volumes of granitic magma in a c. 20 Myr time-span. The dome-and-keel regional architecture and the internal structures of each dome resulted therefore from multiscale polydiapirism. These diapirs were later weakly overprinted by a tectonic fabric that developed during the Neoarchean Yilgarn Orogeny. The contrast between the Yalgoo region and the rest of the craton, which was strongly reworked by this orogeny, highlights the dichotomy of tectonic styles in Archean terranes, demonstrating that diapirism dominated in times of tectonic quiescence.

Midcrust deformation regime variations across the Neoproterozoic Araçuaí hot orogen (SE Brazil): Insights from structural and magnetic fabric analyses

The Neoproterozoic Araçuaí belt of eastern Brazil formed during the amalgamation of Western Gondwana and holds characteristics of a hot collisional belt, involving large amounts of magma, partial melting of the middle crust, and slow cooling rates. To improve our understanding of the mechanical behavior of the continental crust under these conditions we combined structural, magnetic, and geochronological studies to access information related to the flow of rocks, deformation history, and structural patterns associated with the behavior of this orogen. Anisotropy of magnetic susceptibility (AMS) coupled with rock magnetism investigations supports that the magnetic fabric is a good proxy of the structural fabric. Field and AMS measurements of syn-to late plutonic bodies hosted in metasedimentary rocks revealed four domains with contrasting fabric patterns. The structural patterns from W to E are characterized by westward thrusting orthogonal to the belt (region 1), orogen-parallel transpression induced strain partitioning (regions 2 and 3), and orogen-parallel flow and subsidiary eastward vergence magmatic flow (region 4). The use of AMS to determine the lineation was vital to unraveling the kinematics. The composite observed fabric results from the interplay of collision-driven and gravity-driven deformations, induced by the convergence between the São Francisco and Congo Cratons.

Effect of post-magmatic processes on magnetic fabric of teschenite association rocks of the Outer Western Carpathians

Volcanic and subvolcanic alkaline rocks of the teschenite association of the Outer Western Carpathians are characterized by intensive late magmatic, post-magmatic, and also hydrothermal changes that modified their composition not only in rock-forming minerals, but also in accessory magnetic minerals (magnetite, titanomagnetite, maghemite) as indicated by magnetic susceptibility ranging from 10−5 to 10−1 SI units.The magnetic fabric of the most sills and dykes investigated is very roughly conformable to the shapes of the bodies corresponding to the most common type of magnetic fabric in dykes and sills, which no doubt originates through magma flow. However, the fabric shows large variation, much larger than expected for volcanic magnetic fabric, which can be ascribed to late magmatic or post-magmatic alterations of magnetic minerals diminishing the magnetic fabric homogeneity.Even though the teschenite association rocks occur in the sole part of the Silesian thrust sheet, their magnetic fabric and that of host sedimentary rocks show only very weak indications of ductile deformation. Consequently, the Silesian thrust sheet responded to the tectonic movements creating thrust sheet structure by faulting and thrusting of the deformed zones and not by ductile deformation of the bulk of the thrust sheet.

Flow‐to‐Sliding Transition in Crystal‐Bearing Magma

AbstractThe presence of crystals in magmatic flows introduces spatial heterogeneity, which can build up to the degree that the flow behavior of the crystal‐bearing magma becomes substantially different from that of a pure melt. One example is the transition from flow to sliding, in which the deformation in the crystalline magma is concentrated almost entirely in a thin interfacial layer as opposed to being distributed in a typical flow profile throughout the domain. In this study, we use numerical reproductions of existing laboratory experiments of Fuji basalt in a rotational viscometer to identify when the flow‐to‐sliding transition is likely to occur in crystal‐bearing magma. We utilize a direct numerical method to resolve the interactions between the crystals and the magmatic melt at the scale of individual interfaces in 2‐D. All phase interactions and their aggregate effects on the flow emerge self‐consistently from the simulation itself. We find that the long‐range hydrodynamic interactions between crystals create nonlinear flow behavior even at low crystal fractions. By performing simulations in different geometries, we show that the formation and stability of force chains in crystal‐bearing magma depend sensitively not only on the crystal shape and size but also on the geometry of the experimental apparatuses. Our simulations are consistent with the observed discrepancies in previous laboratory measurements of shear thinning behavior, highlighting the delicacies of using point measurements in experiments to constrain the bulk rheology of crystal‐bearing magma.

Ferroan alkalic volcanism associated with Calymmian rifting in the Paramirim aulacogen, São Francisco craton, Brazil: New insights from lithofacies analysis and evidence of mantle-derived alkaline H2O-rich metasomatic fluids affecting ancient crustal materials

Rift-related volcanism is widely used in models of supercontinent reconstruction, while the associated metasomatic fluids provide information about the mantle-crust process during intracrustal melting events. The Paleoproterozoic/Mesoproterozoic Paramirim aulacogen corresponds to multiple intracratonic rift cycles that register many basin developing events. Intense volcanic activity is recorded in some rift events, but the volcanic lithofacies and their petrogenesis are poorly constrained. Herein, we present lithofaciological characterization, whole-rock and mineral chemistry, and U-Pb and Lu-Hf isotope data from the 1.58 Ga volcanic rocks. Trachytic and trachy-andesitic lava flows and primary to secondary volcaniclastic lithotypes characterize this volcanism. The primary volcaniclastic rocks resulted from direct volcanic activity and can be divided into pyroclastic and hydroclastic lithofacies associations. The pyroclastic lithotypes correspond to air-fall, density currents and block-and-ash flow deposits formed in explosive eruptions; hydroclastic lithotypes are represented by hyaloclastites and peperites, reflecting effusive eruptions and interactions between magma, water and sediments. The secondary volcaniclastic lithotypes resulted from late-stage reworking processes and consist of debris flows, ephemeral streams associated with high energy and gravitational flow deposits. The lava flows show well-preserved primary volcanic features, such as magma flow textures and euhedral phenocrysts. Based on petrographic and chemical characteristics, associated with negative ɛHf(t) values, we infer that high melt rate of an ancient crust, previously metasomatized by H2O-rich alkaline mantle-derived fluids, generated the alkalic ferroan lavas. These fluids promoted potassic and sodic metasomatism that affected all the volcanic rocks, modifying the chemistry of the feldspars to pure orthoclase (Or96-98) and albite (Ab98-100). In some lavas, a higher concentration of Na-rich fluids synthesized aegirine at a minimum temperature of 200 °C and promoted its peralkaline character. The potassic-sodic metasomatism influenced the volcano-sedimentary sequence of the Pajeú Group and the oldest rocks of the basement but did not affect the overlying sedimentary sequences, indicating that fluids ceased with the end of volcanism. We suggest that the H2O-rich alkaline fluids arose by focused degassing in response to a rising asthenospheric mantle during the Calymmian rift of the Paramirim aulacogen.

PEMODELAN 3D STRUKTUR BAWAH PERMUKAAN GUNUNGAPI AGUNG PROVINSI BALI MENGGUNAKAN METODE GAYA BERAT

Under surface structure, 3d modelling of Agung volcano in Bali had been conducted by using gravity methods. This research aims to identify the magma chamber and make a model below the surface of the Great Volcano of Bali Province by using gravity anomaly data. Data processing methods performed in this research were consists of six steps: (i) arrangement the contour map of Bouguer anomaly, (ii) spectrum analysis, (iii) anomaly regional and residual separation, (iv) Second Vertical Derivative analysis, (v) cross-section of subsurface geology construction by using 3D (inverse modelling), (vi) qualitative and quantitative interpretation. Anomaly Bougeur contour map result shows the value of Bouguer anomaly in research area around 84 mGal spreading around west-south part of research point until 279 mGal on Agung volcano body. Anomaly regional value is around 97 mGal to 253 mGal. The boundary between basement a sedimentary rock of the research area are at the depth of 0,89 km. The magma chamber of Agung volcano is in the depth of 500 to 8000 metres to a form of towing up and polluted in the middle. The density of the magma chamber is around 2,73 until 2,86 gr/cc. The magma flow in the Agung volcano of visible at the depth of 0 metres.

Emplacement of the Anta Granite from Rio de Janeiro State (SE, Brazil); determined by magnetic fabrics

Magnetic fabric and rock magnetism studies were performed on the three facies of the Anta granite (SE of Rio de Janeiro State, southern Brazil). This intrusion is roughly elliptical (∼ 14 km2), NE-SW oriented and composed of granodiorite, monzodiorite, quartz-diorite, syenogranite, and monzogranite. It shows solid-state deformation and tectonic foliation at its borders and is apparently isotropic in the central part where the internal fabrics are impossible or very difficult to measure through classical techniques. Magnetic fabrics were determined by applying both anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). The AMS and AARM fabrics are coaxial for all facies. The parallelism between AMS and AARM tensors excludes the presence of a single domain (SD) effect in the AMS fabric. Several rock-magnetism experiments performed on one specimen from each facies show that, for all units, magnetic susceptibility and fabrics are carried by magnetite grains. Most of the magnetic foliations are steeply dipping in all facies, and are roughly parallel or sub-parallel to the foliation measured in the field and in the country rocks. In contrast, the magnetic lineations present mostly low plunges for almost the whole pluton, except for one site where it is steep (> 60°). Thin section analysis show that rocks from the borders of the Anta pluton are solid-state deformed, indicating that the borders of the pluton were affected by the regional strain during its emplacement. On the other hand, the lack of solid-state deformation at outcrop scale and in thin sections precludes deformation in the central part of the pluton to be determined, where igneous textures are preserved. This evidence allows us to interpret the magnetic fabrics observed in this part of the pluton as primary in origin (magmatic) and acquired when the rocks were solidified as a result of magma flow, in which steeply plunging magnetic lineation suggests that a feeder zone could underlain this area. Our data suggest that the regional deformation partially affected the emplacement of the Anta granite.

Chapter 12 Dolerites (1.27–1.25 Ga) and alkaline ultrabasic dykes ( c. 1.14 Ga) related to intracratonic rifting

Abstract Doleritic sills, lopoliths and dykes were emplaced into the Paleoproterozoic craton in central Sweden at 1271–1264, 1259–1256 and c. 1247 Ma, a complex temporal zonation occurring in a WSW–ENE direction. The dolerites are subalkaline to alkaline and show predominantly gabbroic, with a trend towards monzogabbroic and quartz monzodioritic, compositions. Positive ɛ Nd and ɛ Hf values suggest a significant depleted mantle component in the source volume of the parental magmas. Dyke orientations indicate extension, at least locally, in a northwesterly direction, consistent with a magma flow direction determined using the anisotropy of magnetic susceptibility values. Intracratonic rifting linked to the break-up of the supercontinent Columbia, back-arc extension above a subduction boundary in a westwards-retreating mode or a mantle plume tail above a continental hotspot have all been proposed for the tectonic setting. Renewed intracratonic rifting at c. 1.14 Ga in the coastal area in northeasternmost Sweden resulted in the emplacement of alkaline ultrabasic dykes, including carbonatites (beforsites), silico-carbonatites and lamprophyres, in a north–south direction along an older shear belt. The broader tectonic setting of this extensional event is not known.

Characterization of volcanic structures associated to the silicic magmatism of the Paraná-Etendeka Province, in the Aparados da Serra region, southern Brazil.

The Paraná-Etendeka Magmatic Province is associated with the distensive tectonics that caused the rupture of the Gondwana continent during the Lower Cretaceous and generated an intense volcanism that covers South America and the NW portion of Namibia in Africa. In Brazil, this volcanic sequence is named Serra Geral Group and predominantly consists of basalts and subordinated silicic rocks. The goal of this study is to characterize the geomorphological features observed in the Aparados da Serra region, southern Brazil, and to evaluate the relationship between these structures and the primary silicic volcanic structures. The geomorphological features were first identified using remote sensing and then correlated with flow structures observed in the field, as well as petrographic and geochemical data. AMS data were used to determine magnetic patterns and the direction of magmatic flow of the rocks. Despite the low degree of anisotropy, clear patterns of lineation and foliation were identified in the studied rocks. Our data shows that Units I and II correspond to silicic lava flows linked to effusive fissure eruptions, presenting a dome morphology caused by differential erosion. Unit III rocks may correspond to true volcanic domes, whereas the Unit IV corresponds to the effusive feeder structures.

Triggering of the powerful 14 July 2013 Vulcanian explosion at Tungurahua Volcano, Ecuador

The 14 July 2013 Vulcanian explosion at Tungurahua occurred after two months of quiescence and was extremely powerful, generating some of the highest infrasound energies recorded worldwide. Here we report on how a combination of geophysical data, textural measurements, and physical and mechanical tests on eruptive products allowed us to determine the processes that led to the pressurization of the conduit and triggering of this large Vulcanian event. Two weeks prior to the 14 July event, daily seismic counts and radial tilt began to steadily increase, indicating the probable intrusion of a new batch of magma into the edifice. The 14 July explosion produced three different juvenile products that were each sampled for this study: airfall; juvenile, vesicular, pyroclastic density current material; and dense plug rocks in the form of ballistic ejecta. Feldspar microlite textures and vesicle size distributions were measured, and used jointly with a two-step recompression model to characterize the spatial distribution of gas in the conduit and the decompression histories of the erupted samples. Model results reveal a vertically stratified conduit with regards to porosity, crystallinity and volatile content prior to the explosion. Overall, our data suggests a complex sequence of events that led eventually to this powerful explosion. 1) Remnants of magma from the previous eruptive phase in May stalled in the shallow conduit, triggering crystallization which, coupled with efficient outgassing, formed a dense (< 2% porosity), highly crystalline plug. This plug had a very low matrix permeability (10−17 to 10−18 m2) and high tensile strength (9 to 13 MPa), forming an efficient rigid seal and preventing significant outgassing from the conduit. 2) Just below the plug, a high porosity zone (up to 50%) formed, acting as a gas storage zone, with pressurization occurring partly under closed-system degassing. 3) The shallow conduit became pressurized due to the combination of both gas accumulation and the resistance of the plug to magma column extrusion in response to a new influx of magma. 4) On the 14th of July a critical gas over-pressure was reached, overcoming the strength of the dense plug of magma, trigging extreme decompression and evacuation of the top two kilometres of the conduit. 5) The newly intruded magma was not directly involved in the explosion and continued to ascend during the week following the explosion.