1,721,022 research outputs found
Structures and grain size distribution in surge deposits as a tool for modelling the dynamics of dilute pyroclastic density currents at La Fossa di Vulcano (Aeolian Islands, Italy)
No full textTurbulent, dilute pyroclastic density currents (surges) represent the most common and hazardous type of eruptive event at La Fossa di Vulcano. During the volcanic history these currents have frequently passed over the topographic barrier of La Fossa caldera, maintaining a high velocity.
We interpret lateral facies Variation in terms of fluid-dynamic behaviour by analyses of structures, thickness and grain size of correlated surge layers from the Palizzi succession. Using these data we validate a sedimentation model of traction fed by a low concentration, turbulent and stratified current. By the use of a sedimentological model, we have then reconstructed the fluid-dynamic parameters of the current and calculated the maximum horizontal distance that solid particles travelled before deposition. Results have been checked against actual deposit features. Consistency of data indicates that the method used in this study, even if it is approximate, and needs further refinement by specific experimental approaches and numerical simulations, gives a reasonable indication of surge characters such as velocity and density, which are relevant for hazard calculation and risk assessment
Unità stratigrafiche a limiti inconformi e storia evolutiva del vulcano medio-pleistocenico di Monte Vulture (Appennino meridionale, Italia)
No full textUnconformity-bounded stratigraphic units and evolutionary history of the middle Pleistocene Monte Vulture volcano, southern Apennines, Italy
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Interaction between particles transported by fallout and surge in the deposits of the Agnano-Monte Spina eruption (Campi Flegrei, Southern Italy)
No full textThe Agnano-Monte Spina Tephra (AMST) is a complex sequence of beds generated by contrasting fragmentation and transportation dynamics. The 4.1 ka eruption was accompanied by a volcano-tectonic collapse, part of which is the present Agnano plain. The pyroclastic sequence is subdivided into members and submembers, each characterized by different lithological and sedimentological features. Plinian/subplinian fallout deposits frequently alternate with base-surge beds of phreatomagmatic origin. Analysis of lateral facies variations and vertical facies associations of correlated layers of submembers B2, D2 and E2 reveals that during some eruption phases the contrasting eruptive dynamics were almost contemporaneous. Base-surge deposits of submember B2 formed during the declining stage of a plinian column. They resulted from highly energetic and steady pyroclastic density currents that traveled long distances from the vent area and surmounted topographic obstacles such as the Posillipo hill (similar to 150 m) and the northern sector of the Camaldoli hill (similar to 250 m). Submember D2 pyroclastic density currents formed when contemporaneous fallout from an eruptive column was present. Coarse particles from the column settled throughout the pyroclastic density current, determining a significant increase of the solid load of the base surge. The consequent increase of supply rate from the transportation to the depositional zone of the base surge led to the formation of unsteady flows that could not efficiently transport the solid load and did not have the ability to travel long distances. Base-surge deposits of submember E2 were fed by a pulsating phreatomagmatic activity, which was punctuated by a short-lived fallout phase. Fallout material, which was incorporated as an additional load to the base surges, was partially transported by low-energy, steady pyroclastic density currents that traveled over the Agnano plain but did not surmount either the Camaldoli or the Posillipo hills. Only the very fine material, in continuous suspension in the upper, no-shearing part of the base-surge cloud, was dispersed higher in the atmosphere and quietly settled over a large area outside the caldera rim. The phreatomagmatic origin of base surges, which contrasts with the magmatic origin of fallout activity, suggests that the pyroclastic density currents of the Agnano-Monte Spina eruption did not result from eruption column collapses. They were most likely related to radially spreading clouds which were contemporaneous with fallout activity but issued from distinct zones in the vent area. The turbulent nature and the high expansion of base surges made them capable, under certain conditions, of passing over high topographic obstacles, with hazardous effects in distal areas
The evolution of the Monte Vulture volcano (southern Italy): Inferences from volcanological, geological and deep dipole electrical soundings data
No full textDeposits and physical properties of pyroclastic density currents during complex Subplinian eruptions: the AD 472 (Pollena) eruption of Somma-Vesuvius, Italy
No full textSmall-scale pyroclastic density currents (PDCs) associated with the AD 472 (Pollena) eruption of Somma-Vesuvius, Italy, were generated by both magmatic and phreatomagmatic explosive fragmentation. The resulting deposits were emplaced under flow boundary conditions dominated by varying combinations of grain interaction, fluid escape and traction processes. Stratigraphic and lithofacies analysis of these PDCs offers a new perspective on the en masse versus progressive aggradation debate for PDC deposition. In particular, the analyses indicate that PDCs were density stratified with a basal underflow dominated by grain interactions. The underflows comprised trains of self-organized granular pulses of variable thickness and magnitude, depending on the overall particle concentration and fluid turbulence. A change in gradient between the upper and lower slopes of the volcano promoted deposition and the different pulses aggraded sequentially (stepwise). In this model each pulse stops en masse and the whole deposit aggrades progressively. Particle concentration, density, mean velocity, and flow height were assessed for the studied PDCs using differaent methods for massive and stratified deposits. The calculated mobility of the flows was 0.2 to 0.3, in the expected range for small-scale PDCs
Generation of pyroclastic density currents from pyroclastic fountaining or transient explosions: insights from large scale experiments
No full textStatistical analysis of textural data from complex pyroclastic sequences: implications for fragmentation processes of the Agnano-Monte Spina Tephra (4.1 ka), Phlegraean Fields, southern Italy
No full textThe Agnano-Monte Spina eruption (4.1 ka) generated pyroclastic deposits with complex internal stratigraphy that reflects variations in eruptive style. To obtain constraints on the fragmentation processes, we analysed grain-size and scanning electron microscopy (SEM) data by means of multivariate statistical techniques of the cluster and factor analysis types. The results allow the identification of end-member deposits related to either pure magmatic activity, or pure phreato-magmatic activity. In addition, some deposits show evidence of the contemporaneous operation of both end-member fragmentation modes. The presence of the two contrasting fragmentation processes during the various phases of explosive activity is tentatively interpreted to be a function in inhomogeneities in gas exsolution processes within the magma column that feeds the eruption
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