1,721,064 research outputs found
Post tectonic growth of late diagenetic greigite
No full textPaleomagnetic, rock magnetic and micro-textural
analyses from a Middle Pleistocene lacustrine sequence
in the southern Apennines (Italy) indicate the presence of
greigite and magnetite as the main magnetic minerals at
different stratigraphic levels. In all cases a normal polarity
characteristic remanent magnetization (ChRM) was observed,
in agreement with an 40Ar–39Ar age of 0.712 ± 0.018 Ma
from volcaniclastic sanidine. After correction for bedding tilt,
ChRM directions carried by greigite do not coincide with the
expected geocentric axial dipole field direction at the site
latitude, whereas the magnetite ChRMs directions do. The
data indicate that the greigite magnetization was acquired
after tilting and after lock-in of the magnetite ChRM.
The estimated delay for the remanence carried by greigite
with respect to deposition is 300 ka. Scanning electron
microscope analyses indicate that alteration of detrital
volcanic minerals has occurred and that authigenic greigite
is generally present in agglomerates and around volcanic
grains. This observation is consistent with a late diagenetic
origin of greigite due to anoxic conditions and availability of
dissolved sulfide associated with decomposition of organic
matter in the paleolake. Documentation of a late diagenetic
magnetization confirms that care should be taken when
using greigite-bearing sediments for magnetostratigraphic
and tectonic studies. Citation: Porreca, M., M. Mattei, and
G. DiVincenzo (2009), Post-deformational growth of late diagenetic
greigite in lacustrine sediments from southern Italy
Field observations on the initial lava flow and the fracture system developed during the early days of the Stromboli 2007 eruption
No full textThe February-April 2007 Stromboli eruption was followed by numerous scientists and
well documented from an integrated geophysical analysis. By contrast, direct visual observations
on the summit craters and Bastimento area were difficult during the first three days of the eruption
because of bad weather conditions.
In this paper we report field observations on the fracture system after the crisis of February 27th
and before the summit explosion on March 15th. The observations were focused on the early lava
flow emplacement and deformation at elevations between 400 and 700 m a.s.l., i.e. between the
summit crater area and the lateral fissure eruption, along the Sciara del Fuoco (SdF)
Magnetic fabric and implications for pyroclastic flow and lahar emplacement, Albano maar, Italy
No full textMagma flow direction of dikes in Eastern Iceland: Insights from field, magnetic fabric and thin section analyses
No full textThe propagation direction of dikes and the relationships between magmatism and tectonics within rift systems are still debated. The extinct rift zones in Eastern Iceland show the deep eroded portions of a rift, largely consisting of dikes. Here we present a multidisciplinary study in the Alftafjordur volcanic system (Eastern Iceland), including a: (1) structural field analysis, to compare, at a paleodepth of 1-2 km, the crustal dilation due to dikes with that due to faulting; (2) Anisotropy of Magnetic Susceptibility (AMS) analysis, to investigate the magnetic fabric and reconstruct the flow direction of 25 dikes, assuming this may be representative of their direction of propagation; (3) Thin section analysis on selected dike samples, in order to investigate the origin of the magnetic fabric and support the AMS results in terms of flow direction. Field data show a main NNE-SSW direction of the distal dikes (away from their dominant volcano) and a radial direction for the proximal ones. The cumulative extension due to diking is nearly one order of magnitude larger than that due to faulting. AMS data show that most of the distal dikes (14 out of 19) have an inverse fabric, conversely to the normal fabric (6 out of 6) of the proximal dikes. The petro-fabric in thin sections suggests that the inverse fabric is related to the orthogonal orientation, with respect to the dike margins, of needle-shaped magnetic minerals. The entire dataset suggests that: (1) the stress variation along the magmatic system controls the direction of the dikes; (2) the crustal dilatation in the subsurface portions of the rift system is mainly due to magmatism; (3) the normal fabric of the proximal dikes suggests both a lateral and vertical magma flow; (4) the inverse fabric is caused by the late crystallization of needle-shaped magnetic minerals, parallel to cooling fractures; (5) the dominant flow direction of the distal dikes may be sub-vertical
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
Meandering flow of a pyroclastic density current documented by the anisotropy of magnetic susceptibility (AMS) in the quartz latite ignimbrite of the Pleistocene Monte Cimino volcanic centre (central Italy)
No full textEvidence of two Holocene phreatomagmatic Eruptions at Stromboli volcano (Aeolian Islands) from paleomagnetic data
No full textWe present detailed stratigraphic-sedimentological and
paleomagnetic analyses of Holocene phreatomagmatic
deposits at Stromboli volcano (Aeolian Islands, Italy). The
investigated deposits belong to the Secche di Lazzaro
succession (SDL) and to the Advanced Operations Center of
the Department of Civil Defense (COA) succession, both
lying on the Neostromboli lavas (ca. 13 – 5 ka). The two
stratigraphic successions have similar stratigraphic position
and show a phreatomagmatic origin, likely related to
catastrophic magma-water interaction processes during
sector collapse events. However the thermal remanent
magnetization data of the lava lithics indicate that deposits
of the SDL succession were emplaced at very low
temperatures (less than 140
the COA deposit was emplaced at temperatures between
300 – 340
investigated deposits may be related to two distinct
eruptive events occurred during the Holocene. Recent
phreatomagmatic activity at Stromboli apparently has
occurred more frequently than previously believed.
Cons equent l y, we mus t r econs i der t he r ecur r ence
i n t e r v a l o f t h i s d a n g e r o u s e r u p t i v e s c e n a r i o f o r
Stromboli
Propagation of dikes at Vesuvio (Italy) and the effect of Mt. Somma
No full textDikes provide crucial information on how magma propagates within volcanoes. Somma-Vesuvio ( Italy) consists of the active Vesuvio cone, partly bordered by the older Mt. Somma edifice. Historical chronicles on the fissure eruptions in 1694 - 1944 are matched with an analytical solution to define the propagation path of the related dikes and to study any control of the Mt. Somma relief. The fissures always consisted of the downslope migration of vents from an open summit conduit, indicating lateral propagation as the predominant mechanism for shallow dike emplacement. No fissure emplaced beyond Mt. Somma, suggesting that its buttressing hinders the propagation of the radial dikes. An analytical solution is defined to describe the mechanism of formation of the laterally propagating dikes and to evaluate the effect of topography. The application to Somma- Vesuvio suggests that, under ordinary excess magmatic pressures, the dikes should not propagate laterally at depths > 240 - 480 m below the surface, as the increased lithostatic pressure requires magmatic pressures higher than average. This implies that, when the conduit is open, the lateral emplacement of dikes is expectable on the S, Wand E slopes. The lack of fissures N of Mt. Somma is explained by its buttressing, which hinders dike propagation
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