1,721,067 research outputs found
Dike emplacement and related hazard in volcanoes with sector collapse: the 2007 Stromboli (Italy) eruption
No full textUnderstanding dike propagation is fundamental for hazard mitigation. In February 2007, two effusive vents opened (at 650 and 400 m a.s.l.) along the inner flank of Sciara del Fuoco (SdF) sector collapse at Stromboli. The summit craters collapsed, obstructing the central conduit, choking the vents and increasing the deformation within SdF. Here a new vent opened (500 m a.s.l.), releasing the deformation. The eruption continued from the 400 m vent, after a summit explosion, until early April. The vents were fed by laterally propagating feeder dikes, following the maximum gravitational stresses within a cone with sector collapse. Vent location is similar to that of the 2002-2003 eruption, fed by dikes triggering landslides and tsunami. Such a dike propagation represents a constant feature of the recent Stromboli eruptions. However, this eruption did not develop catastrophic landslides, suggesting that their triggering also depends on other factors, such as the magmatic pressure
Unraveling the solidification path of a pahoehoe "cicirara" lava from Mount Etna volcano
No full textThe solidified surface of a lava flow reflects the viscosity of its molten fraction and the crystal content during flow; crystal-poor basaltic lavas produce pahoehoe fields, whereas crystal-rich ones solidify with aa carapaces. At Mount Etna volcano aa morphologies are very common, in turn, pahoehoe lavas are rare. The latter are locally named “cicirara” due to the presence of cm-sized plagioclase phenocrysts much more abundant than in aa lavas. The phenocryst content of “cicirara” lavas contrasts with the low viscosity generally associated with pahoehoe morphology. Therefore, to reconcile the discrepancy between textural and volcanic observations, we have studied the most primitive pahoehoe “cicirara” lava sampled until now. Two samples at 0.5 and 1 m from the bottom of the two-meter thick lava flow were investigated on the basis of their mineral compositional variations and textural features, i.e. size frequency and crystal size distribution (CSD). Results coupled with rheological models indicate that only large phenocrysts of plagioclase (>1 mm) and clinopyroxene have grown before eruption. Thermobarometric models and petrological computations based on the composition of plagioclase and clinopyroxene phenocryst cores highlight that only a small amount (10-15 vol.%) of crystals equilibrated at 12 km of depth. Cumulative size frequency and CSD data also indicate that plagioclase and clinopyroxene phenocryst rims grew heterogeneously and coalesced around their cores at depths <1 km, before eruption. In this view, the “cicirara” lava was erupted with a low crystalline content that favoured the formation of its pahoehoe surface; however, crystals with a size <1 mm (~75 vol.%) solidified at post-eruptive conditions.
Our findings underline that the emplacement of high-viscosity aa or low-viscosity pahoehoe lavas is driven by the degree of undercooling imposed by the volatile exsolution rate in the shallowest portion of the Etnean plumbing system. A slow magma ascent rate promotes significant intratelluric degassing and widespread nucleation; consequently, the viscosity of the suspension significantly increases leading to an aa morphology. In contrast, pahoehoe “cicirara” lavas are associated with a rapid rise to the surface of poorly degassed, undercooled magmas
The volatiles flushing triggers eruptions at open conduit volcanoes: evidences from Mount Etna volcano (Italy)
No full textAt the Mount Etna volcano (Italy) the massive release of magmatic gases (especially H2O, CO2 and
SO2) during explosive eruptions and through the persistent gas plume, raises important issues: i) the
volume of magma erupted at given periods is insufficient to feed the gas plume of the related periods;
ii) gas-dominated, explosive eruptions do not emit differentiated products, but relatively Mg-rich
magmas; iii) H2O measured in melt inclusions (~3.5 wt.% at about 0.4 GPa) exceeds what is expected
from intraplate mantle-derived melts (<1.4 wt%).
Literature data on melt inclusions (MI) along with textural and compositional analyses of plagioclase
and phase stability constrained by MELTs calculations, enabled us to build a model that reconciles the
above mentioned discrepancies with the high variability of the water content along the Etnean feeding
system.
We propose that along an open magma conduit, continuous gas loss from the free surface of magma at
depth promotes an almost steady stream of H2O-rich fluid extending well below the gas saturation
depth. The velocity of volatiles migration might be enhanced by the huge amount of CO2 present at
Mount Etna, because the CO2 exsolved at high-pressure conditions may act as carrier for other volatile
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species.
In this way the H2O-undersaturated primitive magma present in the plumbing system, may undergo a
volatile “flushing” proportional to the residing time. This process is fundamental to increase the
originally low H2O content of primitive magma (<1.4 wt%) allowing it to overcome the saturation
threshold, to exsolve as gas and to promote the eruption of Mg-rich lavas. Such mechanism would also
account for the unexpectedly high amount of magmatic water released during non-eruptive periods
Lateral facies variations in hydromagmatic pyroclastic deposits at Linosa, Italy
No full textThe Linosa volcano, less than a million years old, located in the Sicilian Channel, is formed by a series of coalescing tuff cones, tuff rings, spatter cones, and lava flows among which the Fossa Cappellano produced a succession of surges (the "Fossa Cappellano 2" deposits). Which travelled up to 1225 m. The lapilli and coarse ashes in the deposits are generally mud-coated, accretionary lapilli are common, and the tuffs are frequently vesiculated suggesting that the surges responsible for the deposits were wet. The continuous lateral exposures, from the interior of the Fossa to the extreme edge of the island, show facies evolving in the direction of transport from a massive, in the most proximal section to one dominated by cross-laminated beds in the most distal section, passing by a parallel-laminated (plane bed) facies. In each facies, mean bed thickness, and grain size decrease regularly down current. In the cross-laminated facies, mega-ripple wavelength and amplitude decrease in the direction of transport. In the coarser sizes there is a significant increase in the direction of transport of the lower-density scoriae coupled with a decrease of the non-vesiculated lithics. In the finer sizes, there is a significant increase in relative abundance, of the lower-density feldspar crystals relative to the Fe-Mg crystals. The composition, texture, and primary structure variations of the Fossa deposits are identical to those observed in turbidite sequences, and are similar to the characteristics observed in dry surge deposits recently described from Korea. They are interpreted similarly in terms of decreasing flow power in the direction of transport. © 1992
3D microstructure of magnesium potassium phosphate ceramics from X-ray tomography: new insights into the reaction mechanisms
Full text linkMagnesium potassium phosphate ceramics are chemically bonded ceramics employed as biomaterials, in nuclear waste encapsulation and for concrete repair. The microstructure dictates material performance and depends on the raw mix composition. Synchrotron X-ray computed microtomography was employed to describe the 3D microstructure and its time evolution during hardening and gain insights into the reaction mechanisms. Any excess water with respect to the stoichiometry of the reaction brought about an increase in porosity, but, notably, a reduction in the average pore size. Crystals filled the water ‘pockets’ in the ceramic volume by growing larger, although less densely packed, increasing the complexity of the pore shape. Platelet over elongated crystal habit was favoured. Such a change in shape is likely related to a change in reaction mechanism, as crystallization from a gel-like amorphous precursor is hindered and progressively substituted by a through-solution mechanism. It is proposed that the time evolution of the microstructure is dictated by the balance between crystallization from amorphous precursor, prevailing in relatively ‘dense’ systems (with stoichiometric water or in low excess), and water segregation, prevailing at higher water contents. The former mechanism was shown to produce an increase in porosity with time, because of the density mismatch between the amorphous and the crystalline phase
Structural features of the 2007 Stromboli eruption
No full textOn 27 February 2007, two NE–SW and NNW–SSE dike-fed effusive vents opened to the North (at 650 and
400 m above sea level, asl) of the summit craters at Stromboli, forming a fissure parallel to the inner walls of
the Sciara del Fuoco (SdF) sector collapse depression. The formation of these vents was soon followed by
rapid subsidence of the summit crater area. This partly obstructed the central conduit, temporarily choking
the fissure and increasing the deformation of the upper part of SdF. The reactivation of the NNW–SSE vent
and the opening of a new vent located at 500 m asl, fed by a second dike, released the internal pressure and
surface deformation ceased. The eruption then continued again from the 400 m vent, after a summit
explosion on 15 March, until ending in early April after a progressive decrease of magma output. Repeated
NE–SW dike intrusions have occurred in recent years, close to the upper SE limit of the SdF. In that zone,
named Bastimento, the eruptive fractures traced the discontinuities that borders the SdF, increasing the risk
of triggering new sector collapse. Whereas the NE–SW trending structures lie along the regional volcanostructural
trend of the Aeolian arc through Stromboli, the NNW–SSE vents are oblique to this trend and may
be controlled by the anomalous stress field within the unstable flank of the SdF. Another fundamental aspect
of the 2007 eruption is the collapse of the central conduit, due to the rapid and deep magma drainage linked
to the opening of the 400 m vent. The intrusion of dikes and development of flank vents during the 2007
eruption could possibly have triggered catastrophic landslides and related tsunami or eruptive paroxysms,
but the opening of new effusive vents released the internal pressures, diminishing the hazard.Work funded by INGV and Dipartimento Protezione Civile, Italy.Published137-1441.5. TTC - Sorveglianza dell'attività eruttiva dei vulcani3.2. Tettonica attiva3.5. Geologia e storia dei vulcani ed evoluzione dei magmi3.6. Fisica del vulcanismo4.3. TTC - Scenari di pericolosità vulcanicaJCR Journalreserve
Low compressibility accretion disc formation in close binaries: the role of physical viscosity
No full textAims.Physical viscosity naturally hampers gas dynamics (rarefaction or compression). Such a role should
support accretion disc development inside the primary gravitation potential well in a close binary
system, even for low compressibility modelling. Therefore, from the astrophysical point of view, highly
viscous accretion discs could exist even in the low compressibility regime showing strong thermal
differences to high compressibility ones
Methods.We performed simulations of stationary Smooth Particle Hydrodynamics (SPH) low compressibility
accretion disc models for the same close binary system. Artificial viscosity operates in all models. The
absence of physical viscosity and a supersonic high mass transfer characterize the first model. Physical
viscosity and the same supersonic high mass transfer characterize the second model, whilst physical
viscosity and a subsonic low mass transfer characterize the third model. The same binary system
parameters, such as stellar masses and their separation, have been adopted, as well as the same
polytropic index . Thus we investigated the role of physical viscosity in mass and angular
momentum transport in the two viscid models and compare them to the inviscid model. An initial value of
the parameter has been considered for the physically viscous models, according to the
well-known Shakura and Sunjaev formulation, but simulations were carried out also for and
in the case of a supersonic mass transfer. Physical viscosity is represented by the
viscous force contribution expressed by the divergence of the symmetric viscous stress tensor in the
Navier-Stokes equation, while the viscous energy contribution is given by a symmetric combination of the
symmetric shear tensor times the particle velocity.
Results.The results show that physical viscosity supports
and favours accretion disc formation despite the very low compressibility assumed. On the contrary,
in the inviscid case no evident disc structure appears. In all models neither shock fronts nor
extended clear spirals in the radial flow develop
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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