669 research outputs found

    Maars to calderas. End-members on a spectrum of explosive volcanic depressions

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    We discuss maar-diatremes and calderas as end-members on a spectrum of negative volcanic landforms (depressions) produced by explosive eruptions (note—we focus on calderas formed during explosive eruptions, recognizing that some caldera types are not related to such activity). The former are dominated by ejection of material during numerous discrete phreatomagmatic explosions, brecciation, and subsidence of diatreme fill, while the latter are dominated by subsidence over a partly evacuated magma chamber during sustained, magmatic volatile-driven discharge. Many examples share characteristics of both, including landforms that are identified as maars but preserve deposits from non-phreatomagmatic explosive activity, and ambiguous structures that appear to be coalesced maars but that also produced sustained explosive eruptions with likely magma reservoir subsidence. A convergence of research directions on issues related to magma-water interaction and shallow reservoir mechanics is an important avenue toward developing a unified picture of the maar-diatreme-caldera spectrum

    Corrigendum. Maars to calderas: end-members on a spectrum of explosive volcanic depressions

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    A corrigendum on Maars to calderas: end-members on a spectrum of explosive volcanic depressions by Palladino, D. M., Valentine, G. A., Sottili, G., and Taddeucci, J. (2015). Front. Earth Sci. 3:36. doi: 10.3389/feart.2015.00036 Reason for Corrigendum: In the original article (Palladino et al., 2015), there was an error in Figure 1. The vertical axis of the qualitative plot reported erroneously “ratio of juvenile to lithic materials in deposits outside of depression”. The correct wording is as follows: “ratio of juvenile to total (i.e., juvenile+lithic) materials in deposits outside of depression”. In fact, as it was reported correctly in the text, the amount of juvenilematerial (i.e., scoria or pumice) deposited ouside the different types of explosive volcanic depressions increases from zero (i.e., no juvenile, all lithic products), as is the case of hydrothermal (phreatic) explosion craters, to become largely dominant over the lithic component in the case of ash flow deposits associated with large overpressure collapse calderas. The corrected Figure 1 appears below. The authors apologize for this error and state that this does not change the scientific conclusions of the article in any way

    An analytical model for gas overpressure in slug-driven explosions:insights into Strombolian volcanic eruptions

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    Strombolian eruptions, common at basaltic volcanoes, are mildly explosive events that are driven by a large bubble of magmatic gas (a slug) rising up the conduit and bursting at the surface. Gas overpressure within the bursting slug governs explosion dynamics and vigor and is the main factor controlling associated acoustic and seismic signals. We present a theoretical investigation of slug overpressure based on magma-static and geometric considerations and develop a set of equations that can be used to calculate the overpressure in a slug when it bursts, slug length at burst, and the depth at which the burst process begins. We find that burst overpressure is controlled by two dimensionless parameters: V', which represents the amount of gas in the slug, and A', which represents the thickness of the film of magma that falls around the rising slug. Burst overpressure increases nonlinearly as V' and A' increase. We consider two eruptive scenarios: (1) the "standard model," in which magma remains confined to the vent during slug expansion, and (2) the " overflow model," in which slug expansion is associated with lava effusion, as occasionally observed in the field. We find that slug overpressure is higher for the overflow model by a factor of 1.2-2.4. Applying our model to typical Strombolian eruptions at Stromboli, we find that the transition from passive degassing to explosive bursting occurs for slugs with volume >24-230 m(3), depending on magma viscosity and conduit diameter, and that at burst, a typical Strombolian slug (with a volume of 100-1000 m(3)) has an internal gas pressure of 1-5 bars and a length of 13-120 m. We compare model predictions with field data from Stromboli for low-energy " puffers," mildly explosive Strombolian eruptions, and the violently explosive 5 April 2003 paroxysm. We find that model predictions are consistent with field observations across this broad spectrum of eruptive styles, suggesting a common slug-driven mechanism; we propose that paroxysms are driven by unusually large slugs (large V')

    Performance of the neutron polarimeter NPOL3 for high resolution measurements

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    We describe the neutron polarimeter NPOL3 for the measurement of polarization transfer observables DijD_{ij} with a typical high resolution of \sim300 keV at TnT_n \simeq 200 MeV. The NPOL3 system consists of three planes of neutron detectors. The first two planes for neutron polarization analysis are made of 20 sets of one-dimensional position-sensitive plastic scintillation counters with a size of 100 cm ×\times 10 cm ×\times 5 cm, and they cover the area of 100 ×\times 100 cm2\mathrm{cm}^2. The last plane for detecting doubly scattered neutrons or recoiled protons is made of the two-dimensional position-sensitive liquid scintillation counter with a size of 100 cm ×\times 100 cm ×\times 10 cm. The effective analyzing powers Ay;effA_{y;\mathrm{eff}} and double scattering efficiencies ϵD.S.\epsilon_{\mathrm{D.S.}} were measured by using the three kinds of polarized neutrons from the 2H(p,n)pp{}^{2}{\rm H}(\vec{p},\vec{n})pp, 6Li(p,n)6Be(g.s.){}^{6}{\rm Li}(\vec{p},\vec{n}){}^{6}{\rm Be}(\mathrm{g.s.}), and 12C(p,n)12N(g.s.){}^{12}{\rm C}(\vec{p},\vec{n}){}^{12}{\rm N}(\mathrm{g.s.}) reactions at TpT_p = 198 MeV. The performance of NPOL3 defined as ϵD.S.(Ay;eff)2\epsilon_{\mathrm{D.S.}}(A_{y;\mathrm{eff}})^2 are similar to that of the Indiana Neutron POLarimeter (INPOL) by taking into account for the counter configuration difference between these two neutron polarimeters.Comment: 28 pages, 18 figures, submitted to Nucl. Instrum. Methods Phys. Res.

    Production of Radioactive Nuclides in Inverse Reaction Kinematics

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    Efficient production of short-lived radioactive isotopes in inverse reaction kinematics is an important technique for various applications. It is particularly interesting when the isotope of interest is only a few nucleons away from a stable isotope. In this article production via charge exchange and stripping reactions in combination with a magnetic separator is explored. The relation between the separator transmission efficiency, the production yield, and the choice of beam energy is discussed. The results of some exploratory experiments will be presented.Comment: 10 pages, 4 figures, to be submitted to Nucl. Instr. and Met

    Le guerre illiriche (229-219 a.C.)

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    Questo lavoro mira a ricercare le cause dei due interventi militari romani in Illiria del 229 e del 219 a.C. Dopo avere preliminarmente analizzato la situazione politica dell’Illiria alla vigilia di tali avvenimenti, passeremo in rassegna le varie teorie sulle ragioni che spinsero Roma ad intraprendere per la prima volta nella sua storia un’azione militare ad oriente del mare Adriatico. Punto centrale del lavoro è comprendere se il movente della spedizione del 229 a.C fosse legato a velleità di tipo imperialistico come sostenuto in passato da alcuni studiosi. A questo scopo analizzeremo per iniziare l’entità degli interessi di tipo economico e commerciale che Roma aveva nella regione illirica nel III secolo a.C. In seguito osserveremo il fenomeno della pirateria illirica cercando di comprendere se la minaccia da questa costituita possa aver contribuito alla decisione del Senato di intervenire con le armi in Illiria. Cercheremo inoltre di comprendere quali fossero le responsabilità della regina illirica Teuta nell’esplosione del conflitto. Analizzeremo infine le cause del secondo intervento romano nell’area nel 219 a.C, osservando in particolare l’ambigua condotta del condottiero Demetrio di Faro e dei suoi ripetuti voltafaccia

    IL CONCORDATO PREVENTIVO CON CESSIONE DEI BENI: ASPETTI TEORICO-PRATICI

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    L'elaborato ha lo scopo di illustrare la fase esecutiva del concordato con cessione dei beni. Nel primo capitolo si individua la fattispecie oggetto delle disposizioni di cui all'art. 182 l.f. Nel secondo capitolo si descrivono gli organi della procedura, mentre nel terzo si affronta l'argomento chiave dell'elaborato ovvero l'analisi della liquidazione attuata in fase esecutiva di un concordato con cessione dei beni. La problematica è stata affrontata con un taglio teorico-pratico, in modo da evidenziare anche gli aspetti procedurali che caratterizzano l'intera fase esecutiva

    Sequential Fragmentation / Transport Theory, Pyroclast Size-Density Relationships, and the Emplacement Dynamics of Pyroclastic Density Currents – A Case Study on the Mt. St. Helens (USA) 1980 Eruption

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    Pyroclastic density currents (PDCs) are the most dangerous hazard associated with explosive volcanic eruptions. Despite recent advancements in the general understanding of PDC dynamics, limited direct observation and/or outcrop scarcity often hinder the interpretation of specific transport and depositional processes at many volcanoes. This study explores the potential of sequential fragmentation / transport theory (SFT; cf. Wohletz et al. 1989), a modeling method capable of predicting particle mass distributions based on the physical principles of fragmentation and transport, to retrieve the transport and depositional dynamics of well-characterized PDCs from the size and density distributions of individual components within the deposits. The extensive vertical and lateral exposures through the May 18th, 1980 PDC deposits at Mt. St. Helens (MSH) provide constraints on PDC regimes and flow boundary conditions at specific locations across the depositional area. Application to MSH deposits suggests that SFT parameter distributions can be effectively used to characterize flow boundary conditions and emplacement processes for a variety of PDC lithofacies and deposit locations. Results demonstrate that (1) the SFT approach reflects particle fragmentation and transport mechanisms regardless of variations in initial component distributions, consistent with results from previous studies; (2) SFT analysis reveals changes in particle characteristics that are not directly observable in grain size and fabric data; (3) SFT parameters are more sensitive to regional transport conditions than local (outcrop-scale) depositional processes. The particle processing trends produced using SFT analysis are consistent with the degree of particle processing inferred from lithofacies architectures: for all lithofacies examined in this study, suspension sedimentation products exhibit much better processing than concentrated current deposits. Integrated field observations and SFT results provide evidence for increasing density segregation within the depositional region of the currents away from source, as well as for comparable density-segregation processes acting on lithic concentrations and pumice lenses within the current. These findings further define and reinforce the capability of SFT analysis to complement more conventional PDC study methods, significantly expanding the information gained regarding flow dynamics. Finally, this case study demonstrates that the SFT methodology has the potential to constrain regional flow conditions at volcanoes where outcrop exposures are limited

    Spatially Resolved Chiroptical Spectroscopies Emphasizing Recent Applications to Thin Films of Chiral Organic Dyes

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    Instrumental techniques able to identify and structurally characterize the aggregation states in thin films of chiral organic π-conjugated materials, from the first-order supramolecular arrangement up to the microscopic and meso-scopic scale, are very helpful for clarifying structure-property relationships. Chiroptical imaging is currently gaining a central role, for its ability of mapping local supramolecular structures in thin films. The present review gives an overview of electronic circular dichroism imaging (ECDi), circularly polarized luminescence imaging (CPLi), and vibrational circular dichroism imaging (VCDi), with a focus on their applications on thin films of chiral organic dyes as case studies
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