1,721,036 research outputs found
Eruption History and Depositional Processes of the Campanian Ignimbrite Based on its Lithofacies Architecture
No full textLarge ignimbrites are the product of high-temperature pyroclastic density currents (PDCs) spreading over vast regions and representing one of the most devastating natural catastrophes in human history. We present a detailed examination of the ignimbrite sequence, emplaced from 10 to 80 km from the source area, of the Campanian Ignimbrite (CI) eruption, a caldera-forming Plinian event, occurred 39 ka ago, whose PDC spread over a huge area from Campi Flegrei (Italy). The CI sequence comprises seven lithofacies based upon the different sedimentary structures: 1) massive tuff or lapilli tuff; 2) diffuse-stratified tuff; 3) fines-poor lithic lapilli tuff; 4) pumice-rich lapilli tuff; 5) lenses of pumice lapilli; 6) cross-stratified tuff; 7) accretionary lapilli-bearing. Three main vertical facies association have been identified: a) fines-poor lithic lapilli to diffuse stratified or massive or inverse graded; b) cross-stratified or diffuse stratified to massive or normal- to inverse-graded; c) massive to inverse graded. These vertical variations define a dominant trend during which the concentration into the boundary layer was progressively increasing, at a fixed point, and records the increasing importance of fluid-escape condition with time. Distribution of lithofacies in the different units shows a remarkable lack of lateral variation at regional scale. This suggests an overall uniform behavior of the PDC. A temporal evolution, from traction- to granular- or fluid-escape- dominated mechanisms, records unsteadiness conditions and contrast with persistent lateral facies reflecting an overall uniform behavior of the current. Stratigraphical and chemical evidences indicate the presence of a bypass zone around the caldera
Dynamics of large pyroclastic currents inferred by the internal architecture of the Campanian Ignimbrite
No full textLarge ignimbrites are the product of devastating explosive eruptions that have repeatedly impacted climate and life on global scale. The assemblage of vertical and lateral lithofacies variations within an ignimbrite sheet, its internal architecture, may help to determine how the parental pyroclastic current evolves in time and space. The 39 ka Campanian Ignimbrite eruption, vented from Campi Flegrei caldera, laid down a thick ignimbrite over an area of thousands of km2. A detailed reconstruction of the vertical and lateral variation of the seven lithofacies recognised in the ignimbrite medial sequence constrains the behaviour of this event. The pyroclastic current flowed over a wide area around Campi Flegrei without depositing (bypass zone), and inundated a huge area during most of the paroxysmal, waxing phase, emplacing a mainly incipiently- to strongly- welded ignimbrite. Following this waxing phase, the leading edge of the current retreated back towards the source as the current waned, impacting a progressively smaller area and leaving an unconsolidated ash and lapilli deposit, later lithified. Our study illustrates how large pyroclastic currents can evolve in time and space and the importance of both internal (eruptive and transport mechanisms) and external (topography, surficial water and rain) factors in governing their behaviour
Components Evolution During the Sustained Phases of the AD 79 Vesuvius Eruption
No full textPlinian eruptions are short-lived events that can have long-lasting consequences. They produce high eruptive columns by injecting huge volumes of pyroclastic fragments and gases into the stratosphere. Pyroclastic clasts fall back to the ground forming widespread tephra sheets in which three different components are distinguished: juvenile clasts, lithic clasts and crystals. Frothy, well-vesiculated (pumice) juvenile clasts, produced during magma fragmentation in the volcanic conduit, are the main component of these deposits. This ‘classical’ behavior is well represented by the AD 79 Vesuvius eruption that produced a sustained column phase, which deposited a thick pumice lapilli deposit consisting of a white, phonolitic layer A overlain by a gray, tephriphonolitic layer B.
New field and laboratory data define the reprise of the sustained column during the second phase of the eruption, characterized by a prevalence of collapsing column phases, that produced numerous pyroclastic density currents. Five lithic-rich fall layers have been recognized in the pyroclastic density currents post-Plinian sequences of the Vesuvius AD 79 eruption cropping out at various locations from the Vesuvius slopes to the edge of the Campanian Plain, 20 km from the source. These lithic-rich horizons, named D, G1, G3, I and X2 from base to top, exhibit mantling structures, are massive and generally well sorted (sf = 1.05- 1.55). Lithic-rich layers show the same dispersal direction as the basal lapilli Plinian deposit, with their axes trending between N130°E and N140°E. The relative abundance of the components for all samples has been plotted on a triangular diagram (Fig. 1). Plotted fields for each layer are only partially overlapping. Layers A and B are rich in juvenile clasts (average 77-82 and 64-66 wt%, respectively), while layer D has 63-70 wt% of lithics and 24 wt% of juvenile clasts. The upper layers show 82-94 wt% of lithics and less than 5 wt% of juvenile material. The main difference with the basal pumice lapilli deposit is the strong enrichment in lithic clasts, possibly associated with an instability in the conduit-vent system
Small-Scale Lateral Variations in the Post-Plinian AD 79 Pyroclastic Density Currents Deposits
No full textPlinian eruptions form a sequence of pyroclastic fall deposits commonly overlain by ignimbrites. Partial column collapses often form intraplinian pyroclastic density currents (PDCs), and sustained column phases punctuate late, prevalent column collapse phases. In both cases, PDC deposits show longitudinal and lateral variations on a regional scale. Considerable facies variations have been identified in the AD 79 PDC deposits on a regional scale. We investigate small-scale lateral variations of these Vesuvian deposits to document factors governing PDC emplacement mechanisms. Here we present data related to post-Plinian deposits which exhibit the most widespread distribution and are traceable on mountain slopes about 20 km from the vent. We used a stratigraphic approach to identify and correlate marker planes. We found many localities where, up to five, thin post-Plinian fall layers and ignimbrite deposits are interbedded. Detailed documentation of these fall deposits enabled a consistent stratigraphy to be recognized and correlated over a wide sector S of the caldera. The detailed and systematic study of the AD 79 post-Plinian PDC units showed that many of these lateral variations occur in a few meters or tens of meters and can be caused by small obstacles. Several lateral facies variations are observed at different distances from the vent. In proximal locations, on the volcano slopes, the most recurrent variation consists of thick, up to 4 m, massive or faintly stratified, coarse, lithic-rich deposits that pass laterally to few centimeters thin, well-stratified ash layers. This lateral variation coincides with paleovalley – ridge topography. In medial locations (<10 km from the source) we observe both plane-parallel, reverse graded layers passing to bedforms up to 4 m in wavelength and 45 cm in amplitude, and massive, poorly sorted deposits passing to stratified layers. In distal areas, 20 km from the source, thin, massive fine ash layers pass to few decimeters thick beds with well-rounded pumice clasts dispersed in an ashy matrix. The currents are very sensitive even to small variations of the substrate that induce changes in thickness and facies both in proximal and distal areas. This confirms that the facies completely depends on the depositional rather than the transport mechanism
Resumption of the sustained column during the post-Plinian phase of the 79 AD Vesuvius eruption
No full textPlinian eruptions are particularly dangerous eruptions that often have longlasting consequences. They produce high eruptive columns that inject considerable volumes of pyroclastic fragments and volcanic gases into the stratosphere. Pyroclastic clasts fall back to the ground forming widespread tephra sheets in which different components are distinguished: juvenile clasts, lithic clasts and crystals. This ‘classical’ behaviour is well represented by the main sustained phase of the 79 AD Vesuvius eruption, which deposited a thick white to gray pumice lapilli fallout deposit. This phase was followed by a column collapse phase, which mainly emplaced several pyroclastic density currents (PDCs). While tephra deposits of Plinian eruptions are wellconstrained in their sedimentological features and transport processes, little attention has been paid on the occasional fallout from postPlinian activity. One outstanding reason for this is the impermanent nature of these fall deposits, which are generally small in volume and eroded by successive PDCs in proximal and medial sectors and swept away especially in their distal portions, due to the combined effect of rill and wind erosion, soil creep and other forms of mass wasting. Because of this, hazard assessment is inherently incomplete. Here, we present stratigraphic and volcanological evidences for the existence of five lithicrich lapilli fallout layers interstratified with the pyroclastic density current deposits emplaced after the collapse of the 79 AD Plinian column. These late fall products are distributed south of the Vesuvius, at distances between 5 and 20 km from the vent (from Mt. Somma slopes to the Mt. Lattari). These lithicrich horizons, named D, G1, G3, I and X2 from base to top, exhibit mantling structures, are massive and generally well sorted (sorting = 1.051.55). The Md Φ ranges from 8 mm (unit G1) to almost 0.5 mm (unit G3). Component analysis shows that layer D has 61 wt.% of lithics and 23 wt.% of juvenile clasts, while the upper layers have about 7083 wt.% of lithics to less than 0.50 wt.% of juvenile material. The remaining part is made up of crystals and not separated fine ash. A number of outcrops were identified to reconstruct the distribution for the thicker units D and G1. We estimated the plume height by apply the method of Wilson and Walker [1987] to the lithics isopleth maps. We prefer to use lithic isopleths rather than pumice isopleths due to fragile behaviour of the pumice clasts, which could lead to a less reliable result. When more than one isopleth is used, Ht is the average value, and the error represents the difference between the maximum and minimum. These data highlight the oscillating behaviour of the 79 AD eruption plume during the postPlinian fall phase. During the emplacement of layer D, the column rose about 17 km. A second sustained column, which rose to 19 km, occurred during the emplacement of layer G1. Both distributions show a SE trend even if the azimuth of the dispersal axis of layer G1 seems to be rotated by 1020° relative to that of D. Erosion acting preferentially on the upper part of the succession prevented the isopleths of G3, I and X2 from being defined. The dispersion of the lithicrich layers is similar to that of the basal lapilli Plinian deposit, including rotation of the dispersal axis. Our study suggests that the resumption of a sustained column was repeatedly established during the postPlinian phase of the 79 AD Vesuvius eruption. The main difference with the basal pumice lapilli deposit is the strong enrichment in lithic clasts, possibly associated with a recurrent instability in the conduitvent system
The assembly and discharge of the Vesuvius 79 AD eruption magma chamber: new insights from petrological and chemostratigraphic evidence
No full textFollowing the recent revision of the stratigraphic framework, a detailed petrological and chemostratigraphic investigation of the Vesuvius 79 AD eruption juvenile clasts is presented. This has resulted in an excellent case-study for unravelling the processes that lead to the genesis of zoned pyroclastic sequences, allowing a reassessment of how pre- and syn-eruptive processes, as well as emplacement dynamics, influenced the geochemical variations recorded by the juvenile clasts. The opening pyroclastic density current (PDC) unit C1 and the white pumice lapilli Plinian fall A are dominated by white pumice clasts, much rarer in the following grey pumice lapilli Plinian fall B, intra-Plinian PDC (i-PDC), post-Plinian PDC (p-PDC) deposits. White pumice clasts are strongly vesicular, nearly aphyric, with sanidine, green Al-rich clinopyroxene, garnet, leucite and amphibole, and display a strongly evolved phonolitic composition. Grey pumice clasts, prevailing in the deposits following the fall A, are less vesiculated and slightly richer in crystals, with sanidine, green Al-rich clinopyroxene, biotite, colourless diopsidic clinopyroxene and amphibole, ranging in composition from tephriphonolitic to phonolitic. The late-stage post-Plinian fall (p-f) layers are characterised only by grey pumice clasts, which frequently include “dark patches” with MgO-rich clinopyroxene, olivine and biotite phenocrysts.
Chemostratigraphic trends of generally decreasing degree of evolution in the fallout units, from fall A to fall B up to p-f, are thought to reflect compositional trends in the plumbing system. This is interpreted as consisting of an upper vertically stratified “white magma” cap (as indicated by the decreasing degree of evolution with increasing stratigraphic height in fall A), and a lower “grey magma”. The first derives from the differentiation of tephriphonolitic/phonolitic magmas through the removal of alkali feldspar syenite assemblages. The tephriphonolitic/phonolitic magmas, in turn, derive from the prolonged differentiation of tephritic magmas. The grey magma results from mixing tephriphonolitic/phonolitic magmas with i) new arrivals of near-primitive tephritic melts - previously only hypothesised, but here directly evidenced by the dark patches - and ii) melts from alkali feldspar syenite cumulates. This confirmed the common role of cumulate melting processes in the genesis of zoned evolved pyroclastic deposit, although comparisons with literature case studies also highlighted that the factors governing the extent to their involvement, as well as that of the recharge magma, need to be further investigated.
As for the PDC deposits, the occurrence of rarer white and more abundant grey pumice clasts at the same stratigraphic levels, both showing large chemostratigraphic oscillations, was observed to be rather common, and not episodic as previously reported. This association is never observed in p-f layers interstratified with p-PDC deposits, suggesting that it is not determined by eruptive mechanisms and/or withdrawal dynamics. The white pumice fragments of the i-PDC and p-PDC are interpreted as clasts eroded from the underlying fall A deposits, then redeposited by the pyroclastic currents. The presence of the two pumice types in the C1 deposit is instead a primary feature, reflecting simultaneous tapping of the white and grey magmas, likely related with a triggering event of magma rejuvenation
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
Nuovi dati stratigrafici, tafonomici e vulcanologici dalla Regio V: il calco mancato dell’ “ultimo fuggiasco”.
No full text- …
