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Sulpizio R.,CNR Institute for the Dynamics of Environmental Processes | Sarocchi D.,Institute Geologia
Journal of Volcanology and Geothermal Research | Year: 2014

Pyroclastic density currents (PDCs) are mixtures of two components, namely solid particles and fluid (gas) phase. They macroscopically behave as dense, multiphase gravity currents (flowing pyroclastic mixtures of particles and gas) immersed in a less dense, almost isotropic fluid (the atmosphere). As for other natural phenomena, their study needs a multidisciplinary approach consisting of direct observations, analysis of the associated deposits, replication through laboratory experiments, and numerical simulations. This review deals with the description of the current state of the art of PDC physics, and combines analysis of data from various methodologies. All of the above-mentioned approaches have provided significant contributions to advancing the state of the art; in particular, laboratory experiments and numerical simulations deserve a special mention here for their tumultuous growth in recent years.A paragraph of the review is dedicated to the puzzling behaviour of large-scale ignimbrites, which are (fortunately) too rare to be directly observed; they cannot be easily reproduced through laboratory experiments, or investigated by means of numerical simulations.The final part is dedicated to a summary of the whole discussion, and to a comment on some perspectives for future developments of PDC studies. © 2014 . Source

Garzanti E.,University of Milan Bicocca | Ando S.,University of Milan Bicocca | Censi P.,University of Palermo | Vignola P.,CNR Institute for the Dynamics of Environmental Processes
Earth and Planetary Science Letters | Year: 2011

Sediments carried in suspension represent a fundamental part of fluvial transport. Nonetheless, largely because of technical problems, they have been hitherto widely neglected in provenance studies. In order to determine with maximum possible precision the mineralogy of suspended load collected in vertical profiles from water surface to channel bottom of Rivers Ganga and Brahmaputra, we combined Raman spectroscopy with traditional heavy-mineral and X-ray diffraction analyses, carried out separately on low-density and dense fractions of all significant size classes in each sample (multiple-window approach). Suspended load resulted to be a ternary mixture of dominant silt enriched in phyllosilicates, subordinate clay largely derived from weathered floodplains, and sand mainly produced by physical erosion and mechanical grinding during transport in Himalayan streams. Sediment concentration and grain size increase steadily with water depth. Whereas absolute concentration of clay associated with Fe-oxyhydroxides and organic matter is almost depth-invariant, regular mineralogical and consequently chemical changes from shallow to deep load result from marked increase of faster-settling, coarser, denser, or more spherical grains toward the bed. Such steady intersample compositional variability can be modeled as a mixture of clay, silt and sand modes with distinct mineralogical and chemical composition. With classical formulas describing sediment transport by turbulent diffusion, absolute and relative concentrations can be predicted at any depth for each textural mode and each detrital component. Based on assumptions on average chemistry of detrital minerals and empirical formulas to calculate their settling velocities, the suspension-sorting model successfully reproduces mineralogy and chemistry of suspended load at different depths. Principal outputs include assessment of contributions by each detrital mineral to the chemical budget, and calibration of dense minerals too rare to be precisely estimated by optical or Raman analysis but crucial in both detrital-geochronology and settling-equivalence studies. Hydrodynamic conditions during monsoonal discharge could also be evaluated. Understanding compositional variability of suspended load is a fundamental pre-requisite to correctly interpret mineralogical and geochemical data in provenance analysis of modern and ancient sedimentary deposits, to accurately assess weathering processes, sediment fluxes and erosion patterns, and to unambiguously evaluate the effects of anthropogenic modifications on the natural environment. © 2010 Elsevier B.V. Source

Branca S.,Italian National Institute of Geophysics and Volcanology | Coltelli M.,Italian National Institute of Geophysics and Volcanology | Groppelli G.,CNR Institute for the Dynamics of Environmental Processes
Italian Journal of Geosciences | Year: 2011

An updated geological evolution model is presented for the composite basaltic stratovolcano of Mount Etna. It was developed on the basis of the stratigraphic setting proposed in the new geological map that was constrained by 40Ar/ 39Ar age determinations. Unconformitybounded stratigraphy allows highlighting four main evolutionary phases of eruptive activity in the Etna region. The Basal Tholeiitic Supersynthem corresponds to a period, from about 500 to 330 ka, of scattered fissure-type eruptions occurring initially in the foredeep basin and then in a subaerial environment. From about 220 ka, an increase in the eruptive activity built a lava-shield during the Timpe Supersynthem. The central-type activity occurred at least 110 ka ago through the Valle del Bove Supersynthem. The earliest volcanic centres recognized are Tarderia, Rocche and Trifoglietto and later Monte Cerasa, Giannicola, Salifizio and Cuvigghiuni. During the Stratovolcano Supersynthem, from about 57 ka ago, the intense eruptive activity of Ellittico volcano formed a roughly 3600 m-high stratocone that expanded laterally, filling the Alcantara and Simeto paleovalleys. Finally, effusive activity of the last 15 ka built the Mongibello volcano. Its eruptive activity is mainly concentrated in three weakness zones in which the recurrent magma intrusion generates flank eruptions down to low altitude. The four main evolutionary phases may furnish constraints to future models on the origin of Etna volcano and help unravel the geodynamic puzzle of eastern Sicily. © Società Geologica Italiana, Roma 2011. Source

Zucali M.,University of Milan | Spalla M.I.,University of Milan | Spalla M.I.,CNR Institute for the Dynamics of Environmental Processes
Journal of Structural Geology | Year: 2011

Detailed mapping of superposed fabrics and their mineral support allows for reconstruction of the tectonometamorphic evolution of the Ivozio Complex, within the inner portion of the Sesia-Lanzo Zone (Western Italian Alps). The resulting evolution is characterized by a multi-stage structural and metamorphic re-equilibration during Alpine subduction, starting from the pre-Alpine igneous association (Amp0 + Cpx0). The prograde associations begin with S1a marked by AmpI + ZoI which pre-date the growth of GrtI (S1b); successive increase in pressure stabilizes a second generation of Amp + Grt (S1c AmpII + ZoI + GrtII). The growth of prograde lawsonite and omphacite occur during S1d (OmpI + Lws + GrtII + AmpII) within lawsonite-bearing eclogites, while S1e is associated with the break-down of lawsonite, producing the association OmpI + Ky + ZoII + GrtII + AmpII (lws-bearing eclogites); S1d-e stages are associated with AmpII + ZoI + GrtII + OmpI in eclogites. The second generation of penetrative foliation (S2), describing the retrograde evolution, is divided into S2a (AmpII + GrtII + Pg + ZoII) and S2b (Chl + AmpIII + Pg + Ab). The comparison between the reconstructed evolution of the Ivozio Complex and P-T paths inferred in the Southern Sesia-Lanzo Zone suggests a non-uniqueness of the Sesia-Lanzo Zone continental crust, during the Alpine subduction. © 2010 Elsevier Ltd. Source

Rebay G.,University of Pavia | Spalla M.I.,CNR Institute for the Dynamics of Environmental Processes | Zanoni D.,University of Pavia
Journal of Metamorphic Geology | Year: 2012

Petrological investigations supported by multi-scale structural analysis of eclogitized serpentinite in the Zermatt-Saas Zone of the Western Alps allows for the determination of mineral assemblages related to successive fabrics, upon which the P-T-d-t path of these hydrated mantle rocks can be inferred. Serpentinites of the upper Valtournanche, with lenses and dykes of metagabbro and meta-rodingite, display an Alpine polyphase metamorphic evolution from eclogite to epidote-amphibolite facies conditions associated with three successive foliations having different parageneses in these rocks. Serpentinite mainly consists of serpentine with minor magnetite; however, where S1 and S2 foliations are pervasive, metamorphic olivine, together with Ti-clinohumite and clinopyroxene, are also found. The mineral assemblage associated with D1 includes serpentine1, clinopyroxene1, opaque minerals, titanite±olivine1, Ti-clinohumite1 and ilmenite; the D2 assemblage is the same (±chlorite) but minerals have different compositions. The assemblage associated with D3 comprises serpentine3, opaque minerals, ±chlorite3, ilmenite and amphibole3. Ti-clinohumite is associated with veins that are older than D2 and pre-date D3. Veins that post-date D3 are characterized by amphibole + chlorite or by serpentine. P-T conditions for S2 parageneses evaluated using two pseudosections for different bulk compositions suggest that these rocks experienced pressures >2.5±0.3GPa at temperatures slightly higher than 600°C. The late epidote-amphibolite facies re-equilibration associated with D3 and D4 developed during late syn-exhumation deformation related to folding and testifies to a small temperature decrease. These results, which were integrated in the regional framework, suggest that different portions of the Zermatt-Saas Zone registered different P-T peak conditions and underwent different exhumation paths. In addition, the inferred P-T-d-t path suggests that the Valtournanche serpentinites re-equilibrated close to the UHP conditions registered by the Cignana meta-cherts. These results imply that tectonic slices exhumed after UHP metamorphism might be wider than previously reported or that small-size UHP units, tectonically sampled during the Alpine convergence, are more abundant than those that have been detected to date. © 2012 Blackwell Publishing Ltd. Source

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