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M O D E L L I N G O F B U B B L E N U C L E AT I O N I N T R A C H Y- P H O N O L I T I C M A G M A S : I M P L I C AT I O N S F O R THE DYNAMICS OF ASH-RICH ERUPTIONS
Full text linkNucleation of water gas bubbles in trachyphonolitic magmatic melts has been
investigated integrating theory and numerical modelling with decompression
experiments and analysis of natural ash samples of explosive eruptions. Bubble
nucleation, considered the natural response of magmas to decompression, is
strongly dictated by the gas-melt surface tension. Here, I use an integrated
approach to quantify the role of the surface tension in the nucleation process
combining high pressure - high temperature nucleation experiments with a
numerical modelling based on the gradient theory (Cahn and Hilliard, 1959).
This theory, successfully applied in several studies of industrial polymers (Poser
and Sanchez, 1981; Harrison et al., 1999; Kahl and Enders, 2000; Enders et al.,
2005) was never been used before to study systems of volcanological interest.
I show that surface tension is far to be a constant, but it decreases with in-
creasing nucleation pressure (i.e. the confining pressure). Entering the values
of surface tension into the classical theory of nucleation, I obtain a variable
supersaturation pressure triggering nucleation. The decreasing value of the
gas-melt surface tension with increasing pressure, facilitate bubble nucleation at
high pressure, thus enhancing the explosivity of eruptive events from deeper
reservoirs. Instead, the hindered nucleation at relatively low pressure, due to
high bubble surface tension, implies that the generation of explosive eruptions
from shallow reservoirs requires high decompressions. Finally the vesiculation,
in terms of nucleation and growth, of natural samples of ash-rich eruptions
has been studied by applying a novel technique able to take 3D measurements
of bubbles preserved on ash particle’s surface. The Bubble Size Distributions
(BSD), together with the field evidence, suggest that the ash production in these
ash-rich eruptions, rather than to magma-water explosive interaction, is related
to the high decompression necessary to nucleate bubbles in a shallow reservoir
3D reconstruction of volcanic ash particles using Stereo-SEM: two study cases from 200 Ky ash-rich eruptions
No full textMagmatic vs. hydromagmatic fragmentation and its bearing on the origin of widely dispersed ash deposits
No full textBubble Size Distribution of Ash Samples: a Novel 3D Technique Applied to Italian Quaternary Study cases
No full text
A thermodynamical model for the surface tension of silicate melts in contact with H2O gas
No full textSurface tension plays an important role in the nucleation of H2O gas bubbles in magmatic melts and in the time-dependent
rheology of bubble-bearing magmas. Despite several experimental studies, a physics based model of the surface tension of
magmatic melts in contact with H2O is lacking. This paper employs gradient theory to develop a thermodynamical model
of equilibrium surface tension of silicate melts in contact with H2O gas at low to moderate pressures. In the last decades, this
approach has been successfully applied in studies of industrial mixtures but never to magmatic systems. We calibrate and verify the model against literature experimental data, obtained by the pendant drop method, and by inverting bubble nucleation
experiments using the Classical Nucleation Theory (CNT). Our model reproduces the systematic decrease in surface tension
with increased H2O pressure observed in the experiments. On the other hand, the effect of temperature is confirmed by the
experiments only at high pressure. At atmospheric pressure, the model shows a decrease of surface tension with temperature.
This is in contrast with a number of experimental observations and could be related to microstructural effects that cannot be
reproduced by our model. Finally, our analysis indicates that the surface tension measured inverting the CNT may be lower
than the value measured by the pendant drop method, most likely because of changes in surface tension controlled by the
supersaturation
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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