1,721,003 research outputs found
The basal ash deposit of the Sovana Eruption (Vulsini Volcanoes, central Italy): the product of a dilute pyroclastic density current
No full textA low aspect ratio, decimeter-thick ash deposit, axisymmetrically distributed around the Latera Caldera (Western Vulsini Volcanoes, central Italy) has been studied by means of field and laboratory investigations. Field studies comprise facies analysis at centimeter scale and maximum clast size and deposit thickness measurements. Grain size and component distribution, chemical composition and particle morphoscopic features have been determined on selected samples. We discuss the co-ignimbrite ash fall vs, pyroclastic surge origin of the deposit and the hydrovolcanic vs. magmatic eruption nature. Complex facies association, textural features and grain size data rule out an ash fall origin for the whole deposit. The hydrovolcanic nature of the eruption has been discarded on the grounds of componentry and morphoscopic features of vitric fragments. We propose that the main body of the ash deposit formed from a radially expanding, dilute, turbulent pyroclastic density current, originated by a continuous collapse of a low-altitude (a few kilometers) eruptive column with a possible radial jet component. (C) 1998 Elsevier Science B.V. All rights reserved
Constraints on magma-wall rock thermal interaction during explosive eruptions from textural analysis of cored bombs
No full textCored bombs, a kind of pyroclast consisting of a lithic core surrounded by a chilled shell of juvenile material, record the thermal interaction of magma with wall rocks. We performed textural analysis of cored bombs, solid-melt heat-transfer theoretical modelling, and high-temperature coating experiments to put temporal and intensity constraints on the thermal interaction of potassic magma feeder systems with carbonate wall rocks during explosive eruptions in the Quaternary, Colli Albani Volcanic District (Roman Province). It appears that the degree of thermal alteration of lithic cores records the duration of magma-core heat transfer, whereas the core/shell size ratio records the initial entrainment temperature of the lithic fragment. Both parameters appear to vary significantly with the eruptive style, magnitude and vent location. Specifically, small-scale (similar to 0.1-1 km(3) DRE) hydromagmatic eruptions show magma-core heat-transfer durations of 0.1-10 s and entrainment temperatures in the range of 100-300 degrees C in the case of a monogenetic maar located in the Colli Albani peripheral area, while entrainment temperature is as high as to 800 degrees C for a polygenetic maar in a high-enthalpy geothermal system at the margins of the main Colli Albani magma chamber. A large-scale (similar to 30 km(3) DRE) caldera-forming explosive event shows magma-core heat-transfer duration in the order of 10(2)-10(3)s and temperature of 100-500 degrees C at the initial magma-wall rock contact. On these grounds, we derived the cooling rate of magmas as a function of the initial temperature, mass and size distribution of lithic clasts entrained. Magma cooling by lithic entrainment may have occurred on the same time-scale as that of eruptive pulses (seconds to hours), implying that lithic entrainment may effect changes in magma physico-chemical properties on a short time-scale and, consequently, affect eruptive conduit dynamics. (C) 2010 Elsevier B.V. All rights reserved
Strombolian explosions: relationships between the conduit system and the resulting explosive activity at the vents
Full text linkI vulcani in stato di attività persistente a condotto aperto spesso ospitano un’attività definita stromboliana. Questo stile di attività è caratterizzato da rilasci frequenti (intervalli da secondi a minuti) e impulsivi (della durata di alcuni secondi) di piroclasti e gas, dovuti alla risalita e allo scoppio di grosse bolle di gas (dette slug) vicino alla superficie della colonna di magma. Nel tempo, tali vulcani possono mostrare continui cambiamenti nell’evoluzione e nella migrazione delle proprie bocche attive, dalle quali si può osservare anche un comportamento altamente variabile dell’attività. Le origini di tale variabilità devono essere studiate a scale spaziali e temporali variabili. In una scala di anni/centinaia di metri, l’osservazione diretta della migrazione delle bocche nello spazio e nel tempo nei vulcani contraddistinti da più bocche è ancora limitata, e resta da capire la relazione tra questa attività variabile e il sistema di condotti superficiale. Su una scala di secondi/metri, molti autori si sono focalizzati sulla dinamica del rilascio di gas e sulle modalità di formazione dei piroclasti, finora trascurando ampiamente la dinamica di risalita dei piroclasti dalla profondità di rilascio, dove gli slug scoppiano, sino alla loro espulsione dalla bocca. Lo scopo del mio studio è la definizione delle relazioni tra i parametri fisici all’interno del condotto vulcanico e la loro influenza sulla modalità di espulsione dei piroclasti nelle eruzioni stromboliane, tenendo conto delle due scale sopramenzionate. Per raggiungere tale scopo ho adottato due metodologie separate ma complementari.
La prima richiede la caratterizzazione delle eruzioni stromboliane in natura, indagando i cambiamenti temporali nella posizione delle bocche sulla terrazza craterica di Stromboli e i parametri di esplosione (durata e geometria del getto) usando i video delle telecamere di sorveglianza a infrarossi raccolti tra il 2005 e il 2009. I risultati di questa prima metodologia forniscono un database dettagliato dell’attività stromboliana normale a diverse scale temporali, oltre a consentire di delineare una gerarchia di profondità a cui il sistema di condotti superficiale controlla l’attività esplosiva entro le tre principali aree (cioè la nord-est, la centrale e la sud-ovest) che raggruppano più bocche vicine a Stromboli. Alla profondità più bassa, dove gli slug scoppiano, la forma della bocca e le dimensioni degli slug controllano i parametri di esplosione locali, mentre la ramificazione più in superficie dei condotti determina l’evoluzione delle bocche che esplodono simultaneamente o alternativamente. Al di sotto della profondità di scoppio degli slug, il sistema di condotti che alimenta ciascuna area di bocche controlla quale specifica bocca ospiterà le esplosioni e anche alcune caratteristiche generali di esplosione all’interno di queste aree. A questa profondità si suppone ci sia un collegamento tra l’area centrale e quella sudovest, come supportato da diverse osservazioni. Al livello più profondo, il sistema di condotti è comune a tutte le aree e imposta il tasso eruttivo globale del vulcano, bilanciandolo tra l’area nord-est e quella congiunta sud-ovest e centrale. Questo tipo di analisi può essere eseguito anche in altri sistemi persistenti contraddistinti da più bocche in tutto il mondo, fornendo deduzioni di base sulla geometria e la dinamica del loro sistema di condotti e sulla valutazione della pericolosità connessa.
La seconda metodologia è dedicata alle simulazioni di getti gas-particelle per mezzo di esperimenti analogici in scala utilizzando uno shock-tube trasparente. Questo approccio è incentrato sull’effetto delle condizioni iniziali (cioè, pressione e volume del gas, posizione del campione, dimensione e quantità di particelle, geometria della bocca) sia sull’accelerazione delle particelle all’interno del condotto che sull’espulsione risultante dalla bocca. I risultati mostrano che la velocità massima delle particelle ha una correlazione positiva con l’energia iniziale e una correlazione negativa sia con la profondità del campione rispetto alla bocca che con la dimensione delle particelle. Inoltre, gli esperimenti mostrano tendenze di accelerazione e decelerazione delle particelle all’interno dello shock-tube che dipendono in modo variabile da alcune condizioni iniziali (cioè energia iniziale, profondità del campione e dimensione delle particelle) e influiscono sulla velocità massima delle particelle registrata all’uscita. Rispetto ai processi che si verificano durante le eruzioni esplosive a piccola scala, questi risultati aprono la strada a nuove potenziali deduzioni riguanti i processi che controllano la dinamica dei piroclasti. Le tendenze di accelerazione e decelerazione all’interno dei condotti influenzano i modelli correnti che correlano la velocità di espulsione dei piroclasti con la loro profondità di origine nel condotto vulcanico. Dimostro che le assunzioni del modello non sono più valide nei casi di bassa energia iniziale e un campione più profondo. Pertanto, è necessaria una revisione del modello che tenga conto anche di queste tendenze prima di applicarlo alle eruzioni reali. Questi risultati aprono la strada a numerosi scenari futuri. Ad esempio, ulteriori esperimenti possono chiarire meglio gli effetti dell’accoppiamento gas- particella, analizzare il ruolo di altri parametri sull’espulsione dei piroclasti (ad esempio il diametro del condotto) o studiare altri aspetti (ad esempio, i pennacchi vulcanici utilizzando particelle molto fini, o collisioni particella-particella e particelle-parete nel condotto).Persistent, open-vent volcanoes frequently host Strombolian explosions. This style of activity is characterized by frequent (intervals of seconds to minutes) and impulsive (seconds- long) releases of pyroclasts and gases, due to the rise and burst of large gas bubbles (i.e., slugs) near the surface of the magma column. Over time, such volcanoes can show continuous changes in the evolution and migration of their active vents, from which also a highly variable behavior of the activity can be observed. The sources of such variability need being investigated at variable spatial and temporal scales. On a scale of years/hundred of meters, direct observation of space-time vent migration at multi-vent volcanoes is still limited, and the relationship between this variable activity and the shallow conduit system remains to be understood. On a scale of seconds/meters, many authors are focused on the dynamics of gas release and modes of pyroclasts formation, so far largely neglecting the ascent dynamics of pyroclasts from their release depth, where slugs burst, to their ejection from the vent. The aim of my study is the definition of the relationships between the physical parameters inside the volcanic conduit and their influence on the modes of pyroclast ejection in Strombolian explosions, accounting for the two scales abovementioned. To achieve this aim, I considered two separate yet complementary methodologies.
The first one requires the characterization of Strombolian eruptions in nature investigating temporal changes in vent position at the crater terrace of Stromboli and explosion parameters (jet duration and geometry) using infrared surveillance camera videos collected between 2005 and 2009. Results by this first methodology provide a detailed database of normal Strombolian activity at different time-scales, as well as allowing one to outline a hierarchy of depths at which the shallow conduit system controls the explosive activity within the three main vent areas (i.e., south-west, central, and north-east) at Stromboli. At the shallowest depth, where slugs burst, vent shape and slug size control local explosion parameters, while shallower conduit branching determines the evolution of simultaneous or alternating twin vents. Below the depth of the slug burst, the conduit system feeding each vent area controls which specific vent will host the explosions and also some more general explosion features within a vent area. A link between the central and south-west vent areas is supposed at this depth, as supported by several observations. At the deepest level, the conduit system is common to all vent areas and sets the overall explosion rate of the volcano, balancing it between the north-east and the joint south-west and central vent areas. This kind of analysis may be performed also in other persistent multi-vent systems worldwide, providing basic inferences on geometry and dynamics of their conduit systems and on the hazard assessment.
The second methodology is addressed on gas-particle jet simulations by means of scaled analogue experiments using a transparent shock-tube. This approach focuses on the effect of the initial source conditions (i.e., gas pressure and volume, sample position, size and amount of particles, vent geometry) both on the acceleration of the particles within the conduit and on the resulting ejection from the vent. Results show that maximum particle velocity has a strong positive correlation with initial energy and a weaker, negative correlation with both sample depth from the vent and particle size. Moreover, the experiments show trends of particle acceleration-deceleration in the shock-tube that variably depend on some initial conditions (i.e., initial energy, sample depth, and particle size) and influence particle maximum velocity recorded at the exit. When compared to processes occurring during low-scale explosive eruptions, these results open the way for potential, new inferences on the processes controlling the dynamics of pyroclasts. The acceleration-deceleration trends inside the conduits impact current models relating pyroclast ejection velocity with their source depth in the volcanic conduit. I show that, for a lower initial energy and a deeper sample, the model assumptions do not hold true anymore. Therefore, a model revision taking into account these trends is necessary before applying it to real eruptions. These results open the way to numerous future scenarios. Further experiments may, for instance, clarify, the effects of particle-gas coupling, analyze the role of other source parameters on pyroclast ejection (e.g., conduit diameter) and study other aspects (e.g., the volcanic plumes using very fine particles, or particle-particle and particle-wall collisions in the conduit)
Volcanologic and Geo-petrographic map of the South-western sector of the Vulsini Volcanic District, central Italy
No full textextended abstract FIST-Geoitalia 1999, Bellaria (RN
Magma-water interaction laboratory experiments: first results on the Onano Eruption products (Vulsini Volcanoes).
No full textParameterizing the Dynamics of Strombolian Eruptions through High-Speed Video Analysis
No full textStrombolian explosive activity is generally characterized by a series of short, mild explosions caused by the bursting of large individual gas pockets at the magma free surface; however individual explosions at Stromboli Volcano (Italy) show how these events can be complex and heterogeneous. A new method to study the dynamics of Strombolian eruptions is proposed here, based on the acquisition and elaboration of high-speed videos at Stromboli Volcano. The analysis of high-speed videos of Strombolian explosions reveals previously unreported processes, including pre- and post- explosion crater floor deformation and multiple jet pulses within a single explosion, different explosive styles at individual vents, deformation and fragmentation of in-flight scoria clasts. Video frame elaboration allows us to measure the mass flux, velocity and exit angle of ejecta. In addition, particle velocity trends, in the light of shock tube fragmentation/ejection experiments, may provide information on the source depth of single explosion pulses. These results should be useful for future researches and may help describe quantitatively the dynamics and energetics of Strombolian eruptions, with implications on related hazard assessment
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
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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