Articles de revues : « Emplacement temperature of pyroclastic deposits »

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Adair, R. N., et R. A. Burwash. « Evidence for pyroclastic emplacement of the Crowsnest Formation, Alberta ». Canadian Journal of Earth Sciences 33, n^o 5 (1 mai 1996) : 715–28. http://dx.doi.org/10.1139/e96-055.

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The middle Cretaceous Crowsnest Formation west of Coleman, Alberta, is composed of bedded alkaline volcanic deposits containing heterolithic volcanic rock fragments and crystal clasts. Comparison with modern examples of subaerial pyroclastic rocks suggests that pyroclastic flows, surges, fallout of material from vertical eruption columns, and minor mud flows emplaced the deposits. Textural evidence in the form of plastically deformed volcanic fragments, chilled deposit margins, baked rock fragment margins, recrystallization, and the presence of charred wood and charred wood molds indicate emplacement at elevated temperature. Massive deposits containing a fine-grained basal zone are interpreted as the product of pyroclastic flows, whereas deposits characterized by a block-rich base overlain by a thin layer of block-depleted stratified material are interpreted as the product of density-stratified surges. Deposits exhibiting pronounced stratification were emplaced by ash-cloud surges. Thickly bedded breccias exhibiting rheomorphic textures were emplaced as vent-proximal pyroclastic flows. Deposits characterized by parallel beds and graded structures are interpreted as fallout tephra deposits, and deposition by lahars is indicated by coarse-grained beds that lack evidence for emplacement at elevated temperatures. The eruptions of the Crowsnest Formation were cyclical. An initial explosive phase generated deposits by pyroclastic flows, surges, fallout, and lahars. As an eruption progressed, it evolved into a poorly gas-charged effusive stage that emplaced coarsely porphyritic domes, plugs, spines, and vent-proximal lava flows. Subsequent eruptions destroyed the effusive vent facies deposits and produced abundant heterolithic clasts typical of the formation.

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Schito, Andrea, Alessandra Pensa, Claudia Romano, Sveva Corrado, Alessandro Vona, Matteo Trolese, Daniele Morgavi et Guido Giordano. « Calibrating Carbonization Temperatures of Wood Fragments Embedded within Pyroclastic Density Currents through Raman Spectroscopy ». Minerals 12, n^o 2 (5 février 2022) : 203. http://dx.doi.org/10.3390/min12020203.

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The study of the structural order of charcoals embedded in pyroclastic density currents provides information on their emplacement temperature during volcanic eruptions. In the present work, a set of charcoals from three distinct pyroclastic density currents deposits whose temperatures have been previously estimated by charcoal reflectance analyses to lie between 250 °C and 550 °C, was studied by means of Raman spectroscopy. The analyses reveal a very disordered structural ordering of the charcoals, similar to kerogen matured under diagenetic conditions. Changes in Raman spectra at increasing temperatures reflect depolymerization and an increase of aromaticity and can be expressed by parameters derived from a simplified fitting method. Based on this approach, a second order polynomial regression with a high degree of correlation and a minimum error was derived to predict paleotemperatures of pyroclastic deposits. Our results show that Raman spectroscopy can provide a reliable and powerful tool for volcanological studies and volcanic hazard assessment given its advantage of minimum samples preparation, rapid acquisition processes and high precision.

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Wibowo, Haryo Edi, Anggun Purnama Edra, Agung Harijoko et Ferian Anggara. « Emplacement Temperature of the Overbank and Dilute-Detached Pyroclastic Density Currents of Merapi 5 November 2010 Events using Reflectance Analysis of Associated Charcoal ». Journal of Applied Geology 3, n^o 1 (7 décembre 2018) : 41. http://dx.doi.org/10.22146/jag.42445.

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Merapi eruption in 2010 produced 17 km high column of ash and southward pyroclastic density current (PDC). Based on the deposits characteristics and distributions, the PDC is divided into channel and overbank facies (pyroclastic flow), and associated diluted PDC (pyroclastic surge). The hot overbank PDCs and the associated dilute-detached PDCs are the main cause of high casualty (367 fatalities) in medial-distal area (5–16 km), especially near main valley of Kali Gendol. We reported the emplacement temperature of these two deposits using reflectance analysis of charcoal. We used both entombed charcoals in the overbank PDC and charcoals in singed house nearby. Samples were collected on 6–13 km distance southward from summit. Charcoalification temperatures of the entombed charcoals represent deposition temperature of the overbank PDCs, whereas those of charcoals in the singed house resembles temperature of the associated dilute-detached PDCs. Results show mean random reflectance (Ro%) values of entombed charcoal mainly range 1.1–1.9 correspond to temperature range 328–444 °C, whereas charcoal in singed house range 0.61–1.12 with estimated temperature range 304–358 °C. The new temperature data of the dilute-detached PDCs in the medial-distal area is crucial for assessing impact scenarios for exposed populations as it affects them lethally and destructively

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Scott, Andrew C., et Ian J. Glasspool. « Charcoal reflectance as a proxy for the emplacement temperature of pyroclastic flow deposits ». Geology 33, n^o 7 (2005) : 589. http://dx.doi.org/10.1130/g21474.1.

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Bardot, Leon. « Emplacement temperature determinations of proximal pyroclastic deposits on Santorini, Greece, and their implications ». Bulletin of Volcanology 61, n^o 7 (13 janvier 2000) : 450–67. http://dx.doi.org/10.1007/pl00008911.

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McClelland, Elizabeth A., et Timothy H. Druitt. « Palaeomagnetic estimates of emplacement temperatures of pyroclastic deposits on Santorini, Greece ». Bulletin of Volcanology 51, n^o 1 (janvier 1989) : 16–27. http://dx.doi.org/10.1007/bf01086758.

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MATSU'URA, Tabito, et Takeyuki UEKI. « Emplacement Temperature and Cooling Process of the AD915 Pyroclastic Flow Deposits of Towada Volcano ». Chigaku Zasshi (Jounal of Geography) 117, n^o 5 (2008) : 889–93. http://dx.doi.org/10.5026/jgeography.117.889.

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Paterson, Greig A., Andrew P. Roberts, Conall Mac Niocaill, Adrian R. Muxworthy, Lucia Gurioli, José G. Viramonté, Carlos Navarro et Shoshana Weider. « Paleomagnetic determination of emplacement temperatures of pyroclastic deposits : an under-utilized tool ». Bulletin of Volcanology 72, n^o 3 (19 novembre 2009) : 309–30. http://dx.doi.org/10.1007/s00445-009-0324-4.

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Fontana, Giovanni, Conall Mac Niocaill, Richard J. Brown, R. Stephen J. Sparks et Matthew Field. « Emplacement temperatures of pyroclastic and volcaniclastic deposits in kimberlite pipes in southern Africa ». Bulletin of Volcanology 73, n^o 8 (15 juin 2011) : 1063–83. http://dx.doi.org/10.1007/s00445-011-0493-9.

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Bardot, Leon, Rick Thomas et Elizabeth McClelland. « Emplacement temperatures of pyroclastic deposits on Santorini deduced from palaeomagnetic measurements : constraints on eruption mechanisms ». Geological Society, London, Special Publications 105, n^o 1 (1996) : 345–57. http://dx.doi.org/10.1144/gsl.sp.1996.105.01.30.

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van Otterloo, Jozua, et Ray A. F. Cas. « Low-temperature emplacement of phreatomagmatic pyroclastic flow deposits at the monogenetic Mt Gambier Volcanic Complex, South Australia, and their relevance for understanding some deposits in diatremes ». Journal of the Geological Society 173, n^o 4 (9 mars 2016) : 701–10. http://dx.doi.org/10.1144/jgs2015-122.

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Iurino, Dawid A., Luca Bellucci, Danielle Schreve et Raffaele Sardella. « Exceptional soft tissue fossilization of a Pleistocene vulture (Gyps fulvus) : new evidence for emplacement temperatures of pyroclastic flow deposits ». Quaternary Science Reviews 96 (juillet 2014) : 180–87. http://dx.doi.org/10.1016/j.quascirev.2014.04.024.

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Parés, J. M., J. Martí et M. Garcés. « Thermoremanence in red sandstone clasts and emplacement temperature of a quaternary pyroclastic deposit (Catalan Volcanic Zone, ne Spain) ». Studia Geophysica & ; Geodætica 37, n^o 4 (décembre 1993) : 401–14. http://dx.doi.org/10.1007/bf01613585.

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SOMMER, CARLOS AUGUSTO, EVANDRO FERNANDES DE LIMA, LAURO VALENTIM STOLL NARDI, JOAQUIM DANIEL DE LIZ et RONALDO PIEROSAN. « Depósitos de Fluxo Piroclástico Primários : Caracterização e Estudo de um Caso no Vulcanismo Ácido Neoproterozóico do Escudo Sul-rio-grandense ». Pesquisas em Geociências 30, n^o 1 (30 juin 2003) : 3. http://dx.doi.org/10.22456/1807-9806.19576.

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Volcanic deposits can be included in two main groups: coherent and volcaniclastic. The former results from volcanic and sub-volcanic (syn-volcanic intrusions) effusive events, excluding autoclastic portions, and the second group, which is related to deposits constituted by volcanic fragments, encompassing primary deposits (pyroclastic) generated from fragment dispersion through gases and hot vapour, syn-eruptive resedimented deposits, besides volcanogenic sedimentary deposits. Clast transport processes are separated in three broad categories: mass-flow, traction and suspension. Basic concepts that are used in the study of volcanic rocks, classification and characterization of the main subaerial pyroclastic deposits are discussed in this paper, considering lithological and genetic aspects. Lithological aspects are mainly related to composition, components and grain-size of the deposits, while genetic aspects and interpretations are based on clast-forming and depositional processes, allowing understanding about eruption and emplacement conditions. Emphasis is given to pumiceous pyroclastic flow deposits, describing their main textural features and the post-depositional modifications associated to them. The discussed concepts are applied in the reconstruction of the Neoproterozoic pyroclastic flow deposits of two plateaus in the Sul-rio-grandense Shield, southernmost Brazil. The main characteristic of the Taquarembo Plateau is the occurrence of stratified/partially welded ignimbrites and high-grade welded deposits, while massive and crystal-rich ignimbrites are more common in the Ramada Plateau. The facies association of both plateaus suggests a fissural volcanic regime in a subaerial setting, associated to the post-collisional stages of the Brasiliano-Pan African Orogenic Cicle.

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Karstens, J., G. J. Crutchley, C. Berndt, P. J. Talling, S. F. L. Watt, V. Hühnerbach, A. Le Friant, E. Lebas et J. Trofimovs. « Emplacement of pyroclastic deposits offshore Montserrat : Insights from 3D seismic data ». Journal of Volcanology and Geothermal Research 257 (mai 2013) : 1–11. http://dx.doi.org/10.1016/j.jvolgeores.2013.03.004.

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Taddeucci, Jacopo, et Danilo Palladino. « Particle size-density relationships in pyroclastic deposits : inferences for emplacement processes ». Bulletin of Volcanology 64, n^o 3-4 (1 mai 2002) : 273–84. http://dx.doi.org/10.1007/s00445-002-0205-6.

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van Straaten, Bram I., et M. G. Kopylova. « Pyroclastic kimberlite deposits from the Victor Northwest pipe (Ontario, Canada) : the transition from phreatomagmatic to magmatic explosivity ». Canadian Journal of Earth Sciences 50, n^o 10 (octobre 2013) : 1059–68. http://dx.doi.org/10.1139/cjes-2013-0028.

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Magmas of all compositions, including kimberlites, may undergo both magmatic and phreatomagmatic fragmentation during emplacement. In this contribution we assess the extent of phreatomagmatism at different stages of the pipe formation for the Victor Northwest kimberlite (northern Ontario, Canada). Detailed drill core and petrographic observations of all volcanic facies within the pipe suggest emplacement in two volcanic cycles, each consisting of three repeated eruptive phases. Pyroclastic kimberlite formed at the start of both cycles is characterized by the presence of fine-grained, poorly sorted deposits containing broken olivine crystals, angular country rock fragments, accretionary lapilli, and variably vesicular irregular-shaped juvenile pyroclasts. All observations indicate these deposits formed by phreatomagmatism. Subsequent clastogenic coherent kimberlite deposits were formed as a result of Hawaiian fire-fountaining during the second phase of both cycles. These phases were followed by mass wasting into the partly filled crater. The evolution from phreatomagmatic to magmatic fragmentation is commonplace in volcanic systems, and our study provides the first indication that the evolution from a phreatomagmatic to magmatic eruption style is also present in kimberlite volcanoes, suggesting kimberlite volcanism is not significantly different from more common basaltic to rhyolitic systems. In addition, this research indicates that high fragmentation intensities recorded by the presence of broken olivines, accretionary lapilli, abundant (angular) country rock clasts, and retention of ash are important textural criteria for recognition of phreatomagmatic kimberlite deposits.

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Huguet, David, Jean-Claude Thouret, Pierre Nehlig, Jeannine Raffy et Pierre Rochette. « Les lahars du strato-volcan du Cantal (Massif central, France) ; stratigraphie, modes de mise en place et implications paleo-geomorphologiques ». Bulletin de la Société Géologique de France 172, n^o 5 (1 septembre 2001) : 573–85. http://dx.doi.org/10.2113/172.5.573.

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Abstract Introduction: The study of lahar (Lh) deposits (a) describes sedimentary facies associations in a volcano-sedimentary system, (b) establishes the identification of criteria to recognize epiclastic deposits in fossil volcanic successions [Thouret, 1999] and (c) reconstructs paleo-landforms (stratocones, paleovalleys, and volcaniclastic fans) in an old volcanic massif. Lh deposits form the "complexe conglomeratique superieur" ("upper conglomeratic complex") [Brousse et al., 1972, 1975, 1977, 1980, 1989] associated with pyroclastic deposits and streamflow deposits above the "breche inferieure" ("lower breccia"), reinterpreted as debris-avalanche (DAv) deposits. From 9.5 to 6.5 Ma, a trachyandesitic stratovolcano has been built up. Several sector collapses generated DAv and an explosive activity produced pyroclastic-flow deposits. Pyroclastic deposits and both Lh and DAv deposits built up volcaniclastic fans. The study aims (a) to determine Lh deposit generations associated with paleo-landforms and (2) to use Lh deposits as landmarks to recognize some geomorphologic stages in the history of Cantal volcano (45 degrees N-2.5 degrees E; 2500 km 2 , approximately 380 km 3 , 1 855 m). Lahar generations: Lh deposits (9 km 3 ) cover 280 km 2 (fig. 1). They show two facies, clast-supported and matrix-supported debris-flow deposits (fig. 2 and 3), located as far as 20 km from the geographic centre (Puy Griou). Firstly, field observations and geochronological data enable us to distinguish as much as five Lh deposit generations. Secondly, geometric and stratigraphic relations, between Lh deposits and both pyroclastic and DAv deposits, allow us to decipher the genetic relations between distinct volcaniclastic formations. The Cere valley shows three Lh generations. The "Faillitoux" generation is interbedded with the schistose basement and the lava and pyroclastic deposits of the Elanceze massif (1571 m) (fig. 4). An ankaramitic lava (9.53+ or -0,5 Ma, K/Ar) [Nehlig et al., 1999], fitting into Lh deposits of the Elanceze massif post-dates the apparition of the first lahar generation. The "Curebourse" generation was emplaced above DAv deposits (fig. 2A and 5). Both DAv and Lh deposits of the "Curebourse" generation filled the paleo-Cere valley about 7.1 Ma. The "Thiezac" generation (>6.7 Ma, K/Ar) [Nehlig et al., 1999] (fig. 2C) overlies a thick pyroclastic deposit (fig. 4) and is not related to the DAv and the "Curebourse" generation. The fourth generation (Impradine valley) is stratigraphically and genetically associated with pyroclastic deposits located in the upper Impradine valley (fig. 5). These pyroclastic deposits are older than 7.96 Ma (K/Ar age on a trachyandesitic lava overlying Lh deposits) and result from pyroclastic deposits removed as Lh deposits downvalley. The fifth identified lahar generation is located in the Petite-Rhue valley, to the north of the volcano, where a 5-m-thick pumiceous pyroclastite (7.6+ or -0.03 Ma; 40 Ar/ 39 Ar) [Platevoet, 2000] is interstratified with Lh deposits in Cheylade. Genetic relations with pyroclastic deposits: To determine the nature of the relationships between Lh deposits and DAv deposits, we observed geometric relationships between both formations. Some Lh deposits of "Curebourse" generation filled paleothalwegs (fig. 6) cut into DAv deposits suggesting a remission stage after emplacement of DAv deposits. We did not identify sedimentologic features such as dewatering structures indicating that lahars evolved from the top or the front of DAv deposits. Thus, no obvious genetic link was clearly determined between Lh and DAv deposits. In the Impradine valley, we observe the transformation of these pyroclastic deposits in Lh deposits. A proximal pyroclastic facies (upper Impradine) (fig. 5), intruded by numerous dykes and intercalated with trachyandesitic lava, shows the proximity of a stratocone located 1,5 km to the South-East. Field observations indicate a stratigraphic link between pyroclastic and Lh deposits. Debris flows have removed pyroclastic deposits over a 6 km distance. Lh deposits are ungraded or inversely graded and show matrix- or clast-supported facies. About 50% of dense subrounded to rounded clasts were incorporated during the flow. The remaining 50% are dense trachyandesitic juvenile clasts derived from primary pyroclastic-flow deposits. Geomorphological implications: Determinations of five Lh deposit generations and observations of geometrical relations with volcaniclastic deposits (DAv and pyroclastic deposits) enable us to reconstruct paleo-landforms and some stages of the geomorphic evolution of Cantal. In this way, the Impradine volcaniclastic unit is a fragment of a volcaniclastic fan facing north-east (fig. 7). In the Cere valley, the "Faillitoux" generation is the remnant of a proximal section of a volcaniclastic fan facing south-west. These lahars flowed from a trachyandesitic stratocone located close to the Elanceze massif about 9.5 Ma ago (fig. 7). These paleo-stratocones were eroded and are no longer visible in the present geomorphic landscape. Lh deposits allow us to determine geomorphic inheritances, contemporaneous with the activity of the stratovolcano from 9.5 to 6.5 Ma. About 7.1 Ma, the paleo-Cere valley was filled with DAv and Lh deposits of the "Curebourse" generation. The "Curebourse" generation formed a volcaniclastic fan on the top of DAv deposits. DAv and Lh deposits, that are less resistant than the trachyandesitic Elanceze massif and Plomb-du-Cantal range, have been eroded. Accordingly, the Cere valley is being exhumed. The present-day drainage pattern occupies the paleothalweg. However, in distal positions, paleo-landforms are not as well preserved. The current drainage pattern does not use any more paleothalwegs in contrast to what is seen in proximal position (fig. 8).

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Druitt, T. H., R. A. Mellors, D. M. Pyle et R. S. J. Sparks. « Explosive volcanism on Santorini, Greece ». Geological Magazine 126, n^o 2 (mars 1989) : 95–126. http://dx.doi.org/10.1017/s0016756800006270.

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AbstrctSantorini volcanic field has had 12 major (1–10 km3 or more of magma), and numerous minor, explosive eruptions over the last ~ 200 ka. Deposits from these eruptions (Thera Pyroclastic Formation) are well exposed in caldera-wall successions up to 200 m thick. Each of the major eruptions began with a pumice-fall phase, and most culminated with emplacement of pyroclastic flows. Pyroclastic flows of at least six eruptions deposited proximal lag deposits exposed widely in the caldera wall. The lag deposits include coarse-grained lithic breccias (andesitic to rhyodacitic eruptions) and spatter agglomerates (andesitic eruptions only). Facies associations between lithic breccia, spatter agglomerate, and ignimbrite from the same eruption can be very complex. For some eruptions, lag deposits provide the only evidence for pyroclastic flows, because most of the ignimbrite is buried on the lower flanks of Santorini or under the sea. At least eight eruptions tapped compositionally heterogeneous magma chambers, producing deposits with a range of zoning patterns and compositional gaps. Three eruptions display a silicic–silicic + mafic–silicic zoning not previously reported. Four eruptions vented large volumes of dacitic or rhyodacitic pumice, and may account for 90% or more of all silicic magma discharged from Santorini. The Thera Pyroclastic Formation and coeval lavas record two major mafic-to-silicic cycles of Santorini volcanism. Each cycle commenced with explosive eruptions of andesite or dacite, accompanied by construction of composite shields and stratocones, and culminated in a pair of major dacitic or rhyodacitic eruptions. Sequences of scoria and ash deposits occur between most of the twelve major members and record repeated stratocone or shield construction following a large explosive eruption.Volcanism at Santorini has focussed on a deep NE–SW basement fracture, which has acted as a pathway for magma ascent. At least four major explosive eruptions began at a vent complex on this fracture. Composite volcanoes constructed north of the fracture were dissected by at least three caldera-collapse events associated with the pyroclastic eruptions. Southern Santorini consists of pryoclastic ejecta draped over a pre-volcanic island and a ridge of early- to mid-Pleistocene volcanics. The southern half of the present-day caldera basin is a long-lived, essentially non-volcanic, depression, defined by topographic highs to the south and east, but deepened by subsidence associated with the main northern caldera complex, and is probably not a separate caldera.

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De Astis, G., P. Dellino, R. De Rosa et L. La Volpe. « Eruptive and emplacement mechanisms of widespread fine-grained pyroclastic deposits on Vulcano Island (Italy) ». Bulletin of Volcanology 59, n^o 2 (31 octobre 1997) : 87–102. http://dx.doi.org/10.1007/s004450050177.

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Pascual, Emilio, Teodosio Donaire, Manuel Toscano, Gloria Macías, Christian Pin et Michael Hamilton. « Geochemical and Volcanological Criteria in Assessing the Links between Volcanism and VMS Deposits : A Case on the Iberian Pyrite Belt, Spain ». Minerals 11, n^o 8 (30 juillet 2021) : 826. http://dx.doi.org/10.3390/min11080826.

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VMS deposits in the Iberian Pyrite Belt (IPB), Spain and Portugal, constitute the largest accumulation of these deposits on Earth. Although several factors account for their genetic interpretation, a link between volcanism and mineralization is generally accepted. In many VMS districts, research is focused on the geochemical discrimination between barren and fertile volcanic rocks, these latter being a proxy of VMS mineralization. Additionally, the volcanological study of igneous successions sheds light on the environment at which volcanic rocks were emplaced, showing an emplacement depth consistent with that required for VMS formation. We describe a case on the El Almendro–Villanueva de los Castillejos (EAVC) succession, Spanish IPB, where abundant felsic volcanic rocks occur. According to the available evidence, their geochemical features, εNd signature and U–Pb dates suggest a possible link to VMS deposits. However, (paleo)volcanological evidence here indicates pyroclastic emplacement in a shallow water environment. We infer that such a shallow environment precluded VMS generation, a conclusion that is consistent with the absence of massive deposits all along this area. We also show that this interpretation lends additional support to previous models of the whole IPB, suggesting that compartmentalization of the belt had a major role in determining the sites of VMS deposition.

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Wadge, G. « Morne Patates volcano, southern Dominica, Lesser Antilles ». Geological Magazine 122, n^o 3 (mai 1985) : 253–60. http://dx.doi.org/10.1017/s0016756800031460.

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AbstractMornes Patates is the youngest volcano in Dominica occupying a depression on the southwestern flank of a larger stratovolcano, Morne Plat Pays. Within the depression an arcuate ridge, apparently of acid andesite lavas, has been cored and largely buried by the explosive emplacement of two acid andesite domes and their associated pyroclastic deposits, mainly block-and-ash flow deposits. A basaltic scoria horizon, exposed on the northern side of the depression, has a 14C age of 28450 ± 1500 years B.P. and is not seen on the volcano itself, which suggests that the domes and pyroclastic rocks are younger than this. A 14C age of 450 ± 90 years B.P. from a block-and-ash flow deposit from the Morne Patates dome apparently confirms this and the youthful morphology also suggests a young age. The angular shape of the depression and the steep submarine slopes are consistent with an origin as a lateral gravity slide rather than a caldera collapse. Vigorous fumarolic activity and swarms of local earthquakes suggest that magma exists at depth (several kilometres) beneath the Morne Patates–Morne Plat Pays area.

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Grishin, S. Yu. « The main trends in the dynamics of vegetation on the territory affected by the catastrophic eruption of Bezymyanny Volcano on March 30, 1956 (Kamchatka) ». Известия Русского географического общества 151, n^o 5 (5 novembre 2019) : 32–47. http://dx.doi.org/10.31857/s0869-6071151532-47.

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The transformation of the vegetation cover in the impact zone of the 1956 eruption, in territories covered by various deposits, is considered. As a result of a gigantic eruption (VEI 5), vegetation was exposed to a series of different volcanic impacts. Five main categories of events are distinguished: the movement of material of a huge volume of volcano edifice over a large distance as a result of a giant clastic avalanch, the pyroclastic surge of a direct blast, the pyroclastic flows, the formation of a giant eruptive cloud and ashfalls, as well as the lahars. The volume of erupted (initially high-temperature) deposits was, according to various estimates, in the amount of 1.35-1.5 km3, the volume of cold deposits of a clastic avalanche was 0.5-0.8 km3. The volume of lahar was 0.5 km3. The area covered by the pyroclastic wave of the directed explosion was about 500 km2. Within this lesion zone, deposits of pyroclastic flows have occupied 30-40 km2, and clastic avalanche deposits from 35 to 60 km2. Below 900 m above sea level (a.s.l.) these deposits buried cover of subalpine dwarf alder (dominant species is Alnus fruticosa) and mountain meadow vegetation, as well as forest vegetation (dominant species is Betula ermanii) at its upper limit. Forest and partially dwarf alder vegetation was destroyed on a vast territory mainly under the influence of a pyroclastic wave (in the altitude range from 700-800 to 200 m a.s.l.), as well as lahars (in the range of 250-50 m a.s.l.). Primary successions occur in the alpine and partially subalpine zone on avalanche deposits and pyroclastic flows deposits, as well as in the upper part of the zone impacted by pyroclastic surge of the direct blast (40-45 km2). In part of the territories where thick deposits of the lahars were formed, primary successions also probably occurred. In the zone of primary successions, deposits of a clastic avalanche are settled by plants most slowly due to not-favourable edaphic factors. The process is somewhat more efficient on the deposits of pyroclastic flows (the same ratio was noted on the Shiveluch Volcano). The surface overlapped by deposits of the pyroclastic surge is populated relatively quickly. Secondary succession occurs in the zone of damage to the forest and dwarf trees by the influence of a pyroclastic wave, as well as in the zone of passage of the lahars. Restoring of vegetation to its previous state will take from 50 to ~500 years on different deposits and in different parts of an impact zone.

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TATE, M. P., et M. WILSON. « Emplacement mechanism and lateral correlation of pyroclastic flow and surge deposits in northen St Kitts, Lesser Antilles ». Journal of the Geological Society 145, n^o 4 (juillet 1988) : 553–62. http://dx.doi.org/10.1144/gsjgs.145.4.0553.

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Takagi, T., S. M. Koh, M. S. Song, M. Itoh et K. Mogi. « Geology and properties of the Kawasaki and Dobuyama bentonite deposits of Zao region in northeastern Japan ». Clay Minerals 40, n^o 3 (septembre 2005) : 333–50. http://dx.doi.org/10.1180/0009855054030177.

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AbstractThe Kawasaki and Dobuyama bentonite deposits in northeastern Japan show contrasting properties even though they are only 5 km apart in a sequence of Neogene sedimentary and pyroclastic rocks. The Kawasaki deposit consists of stratiform bentonite layers up to >50 m thick, and its wall rocks are unaltered shallow marine sedimentary rocks. In contrast, the Dobuyama deposit consists of a funnel-shaped ore body 200 m across, and its wall rocks are hydrothermally altered terrestrial rhyolitic pyroclastic rocks. The Kawasaki and Dobuyama bentonites mainly consist of Na-Ca smectite and Ca smectite, respectively, with subordinate opal-CT, quartz and zeolite. The geological occurrences of the deposits and wall-rock properties suggest that the Kawasaki and Dobuyama deposits were probably formed by diagenesis and low-temperature hydrothermal alteration, respectively. The difference in exchangeable cation ratios of the smectite between the two deposits is attributable to the difference in their sedimentary environments and/or burial depth.

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Marantos, I., T. Markopoulos et G. E. Christidis. « Zeolitic alteration in the Tertiary Feres volcano-sedimentary basin, Thrace, NE Greece ». Mineralogical Magazine 71, n^o 3 (juin 2007) : 327–45. http://dx.doi.org/10.1180/minmag.2007.071.3.327.

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AbstractThe Tertiary volcano-sedimentary sequence of the Feres basin (Thrace, NE Greece), includes lavas of andesitic–rhyolitic composition as well as volcaniclastic rocks, pyroclastic flows and pyroclastic fall deposits principally of dacitic–rhyodacitic composition. The pyroclastic flow deposits frequently show intense devitrification, vapour-phase crystallization and evidence of fumarolic activity, which involves deposition of scapolite in pore spaces. The Feres basin can be subdivided on the basis of mineral alteration assemblages: (1) the Pefka region; characterized by intense hydrothermal alteration of the volcanic rocks and mineral zoning(silicic, argillic, sericitic and propylitic zones) with polymetallic mineralization, and (2) the remainder of the basin; where the volcaniclastic rocks are characterized by the alteration of volcanic glass to zeolites (clinoptilolite, heulandite, mordenite, analcime), clay minerals (smectite, illite, celadonite, chlorite), SiO2polymorphs (cristobalite, opal-CT, quartz), K-feldspar and calcite. Laumontite is also present as an alteration product of plagioclase, with stilbite sporadically occurringin veinlets. Locally, rhyolites are also altered to zeolites (clinoptilolite and/or mordenite). The zeolitization process has occurred rapidly with the depositional environment, temperature, rate of cooling(of the volcanic rocks), nature and temperature of the mineral-forming fluids and composition of the parent material controllingthe formation of zeolites.

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Moncinhatto, Thiago R., Maurício B. Haag, Gelvam A. Hartmann, Jairo F. Savian, Wilbor Poletti, Carlos A. Sommer, Alberto T. Caselli et Ricardo I. F. Trindade. « Mineralogical control on the magnetic anisotropy of lavas and ignimbrites : a case study in the Caviahue-Copahue field (Argentina) ». Geophysical Journal International 220, n^o 2 (23 octobre 2019) : 821–38. http://dx.doi.org/10.1093/gji/ggz483.

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SUMMARY Anisotropy of magnetic susceptibility is a petrofabric tool used to estimate the alignment of minerals at the site-scale, the imbrication between the magnetic foliation and the emplacement surface being an indicator of flow direction. However, despite numerous studies examining the flow direction in pyroclastic deposits and lava flows, the effect of magnetic mineralogy and the domain state of ferromagnetic phases on the magnetic fabric remains poorly understood. This paper describes the magnetic mineralogy and its influence on the magnetic fabric of Plio-Pleistocene lava flows and ignimbrites of the Caviahue-Copahue Volcanic Complex in the Andean Southern Volcanic Zone, Argentina. Rock magnetism, anisotropy of magnetic susceptibility and anhysteretic remanent magnetization and petrographic observations were performed on 30 sites of the volcanic complex. Results revealed the extrusive and pyroclastic rocks present varied magnetic mineralogy, formed in different stages of the magmatic evolution. Magnetic mineralogy variations strongly affect the anisotropy of magnetic susceptibility data in volcanic rocks and associated ignimbrites, providing ‘scattered’ fabrics when late Ti-rich titanomagnetite phases dominate the fabric, and ‘inverse’ or ‘intermediate’ fabrics when single-domain grains are present. ‘Normal’ fabrics are typically found when early crystallized pure magnetite is present. Our results highlight the complexity in the interpretation of magnetic anisotropy data in volcanic rocks and ignimbrites.

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Cole, Paul D., Annamaria Perrotta et Claudio Scarpati. « The volcanic history of the southwestern part of the city of Naples ». Geological Magazine 131, n^o 6 (novembre 1994) : 785–99. http://dx.doi.org/10.1017/s0016756800012863.

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AbstractThe southwestern part of Naples was the site of small volume volcanic activity prior to 12000 y B.P. Lava domes and possibly a lava flow were erupted during the earliest period. Explosive activity followed and produced pyroclastic sequences that are the proximal deposits of tuff cones within the city and at its southwestern extreme. The explosive activity was complex but predominantly phreatomagmatic. As the volcanic activity more than 12000 y B.P. in western Campi Flegrei shows a similar evolution from effusive to explosive, it is suggested that a large volcanic field ‘Paleoflegrei’, encompassing the western part of the city of Naples, existed prior to emplacement of the Neapolitan Yellow Tuff. The Neapolitan Yellow Tuff eruption about 12000 y B.P., from a vent in Campi Flegrei, produced widespread deposits up to 150 m thick that blanketed the area of the city of Naples, although the present day topography is strongly influenced by the pre-Neapolitan Yellow Tuff centres. Following the Neapolitan Yellow Tuff a small explosive eruption occurred in the bay of Chiaia, immediately south of the city, and results in the possibility of future eruptions within the city of Naples, outside the confines of Campi Flegrei and Vesuvius.

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Mandeville, Charles W., Steven Carey, Haraldur Sigurdsson et John King. « Paleomagnetic evidence for high-temperature emplacement of the 1883 subaqueous pyroclastic flows from Krakatau Volcano, Indonesia ». Journal of Geophysical Research : Solid Earth 99, B5 (10 mai 1994) : 9487–504. http://dx.doi.org/10.1029/94jb00239.

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Bowles, Julie A., Jeffrey S. Gee, Mike J. Jackson et Margaret S. Avery. « Geomagnetic paleointensity in historical pyroclastic density currents : Testing the effects of emplacement temperature and postemplacement alteration ». Geochemistry, Geophysics, Geosystems 16, n^o 10 (octobre 2015) : 3607–25. http://dx.doi.org/10.1002/2015gc005910.

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Christiansen, Robert L., Andrew T. Calvert, Duane E. Champion, Cynthia A. Gardner, Judith E. Fierstein et Jorge A. Vazquez. « The remarkable volcanism of Shastina, a stratocone segment of Mount Shasta, California ». Geosphere 16, n^o 5 (10 août 2020) : 1153–78. http://dx.doi.org/10.1130/ges02080.1.

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Abstract Mount Shasta, a 400 km3 volcano in northern California (United States), is the most voluminous stratocone of the Cascade arc. Most Mount Shasta lavas vented at or near the present summit; relatively smaller volumes erupted from scattered vents on the volcano’s flanks. An apron of pyroclastic and debris flows surrounds it. Shastina, a large and distinct cone on the west side of Mount Shasta, represents a brief but exceptionally vigorous period of eruptive activity. Its volume of ∼13.5 km3 would make Shastina itself one of the larger Holocene Cascade stratovolcanoes. Its andesite-dacite lavas average 63 wt% SiO2 and have little compositional or petrographic variation; they erupted almost entirely from one central vent, although a single vent below Shastina’s north side erupted a flow of the same composition. Eruptions ended with explosive enlargement and breaching of the central crater and successive emplacement of four, more-silicic dacite domes within the crater and pyroclastic flows down its flank. Black Butte, a large volcanic dome and pyroclastic complex below the west flank of Shastina, is petrographically and chemically distinct but only slightly younger than Shastina itself, part of a nearly continuous Shastina–Black Butte eruptive episode. Shastina overlies the widespread pumice of Red Banks, erupted from the Mount Shasta summit area and 14C dated at ca. 10,900 yr B.P. (calibrated). Shastina and Black Butte pyroclastic deposits have calibrated 14C ages indistinguishable from one another at ca. 10,700 cal. yr B.P. A cognate granitic-textured inclusion in a late Shastina lava flow yields a 238U-230Th date on zircons within error of those ages. Our conclusion that the entire, voluminous Shastina–Black Butte episode lasted no more than a few hundred years is confirmed by almost identical remanent magnetic directions of all of the lavas and pyroclastic deposits. Although extremely similar, the remanent magnetic directions do reveal a short path of secular variation through the eruptive sequence. We conclude that the entire Shastina–Black Butte eruptive episode lasted no more than ∼200 yr. The magmas that produced the Shastina and Black Butte eruptions were separate individual bodies at different crustal levels. Each of these eruptive sequences probably represents magma approximating a liquid composition that experienced only minimal differentiation or crustal contamination and remained separated from the main central conduit for most eruptions of Mount Shasta. The probability of another rapidly developing, brief but voluminous eruptive episode at Mount Shasta is low but should not be ignored in evaluating future possible eruptive hazards.

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Marantos, I., Th Markopoulos, G. E. Christidis et V. Perdikatsis. « Geochemical characteristics of the alteration of volcanic and volcaniclastic rocks in the Feres Basin, Thrace, NE Greece ». Clay Minerals 43, n^o 4 (décembre 2008) : 575–95. http://dx.doi.org/10.1180/claymin.2008.043.4.05.

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AbstractThe Tertiary basin of Feres consists of sedimentary rocks, andesitic-rhyolitic volcanic rocks of K-rich calc-alkaline affinities, rocks with calc-alkaline and shoshonitic affinities and volcaniclastic fall and flow deposits. Volcanic and volcaniclastic rocks have variable concentrations of LIL elements (Ba, Sr, Rb, Th) and HFS elements (Zr, V) due to their mode of origin. The pyroclastic flows frequently show more or less intense devitrification, vapour-phase crystallization and, in some cases, evidence of fumarolic activity, as is indicated by the presence of scapolite. The volcanic and volcaniclastic rocks display various types of alteration including formation of zeolites (clinoptilolite, heulandite, mordenite, and laumontite) and smectite, as well as hydrothermal alteration (development of silicic, argillic, sericitic and propylitic zones) associated with polymetallic mineralization. The behaviour of chemical elements during alteration varies. Some are immobile and their distribution is controlled by the conditions prevailing during parent-rock formation and emplacement, but others, such as Ba and Sr, are mobile and selectively fractionate in zeolite extra-framework sites. The formation of zeolite from alteration of volcanic glass is accompanied by an increase in Mg and Al content, and a decrease in Si and Na content, whereas Ca is not affected by alteration. In certain pyroclastic flows, there is a significant difference in K-content between incipient glass and altered rock, due to K-feldspar formation during devitrification and vapour-phase crystallization.

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BRYAN, S. E., J. MARTÍ et R. A. F. CAS. « Stratigraphy of the Bandas del Sur Formation : an extracaldera record of Quaternary phonolitic explosive eruptions from the Las Cañadas edifice, Tenerife (Canary Islands) ». Geological Magazine 135, n^o 5 (septembre 1998) : 605–36. http://dx.doi.org/10.1017/s0016756897001258.

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Explosive volcanism has dominated the large phonolitic shield volcano of Tenerife, the Las Cañadas edifice, for the last 1.5 m.y. Pyroclastic deposits of the Bandas del Sur Formation are exposed along the southern flanks, and record the last two of at least three long-term cycles of caldera-forming explosive eruptions. Each cycle began with flank fissure eruptions of alkali basalt lava, followed by minor eruptions of basanite to phonotephrite lavas. Minor phonotephritic to phonolitic lava effusions also occurred on the flanks of the edifice during the latter stages of the second explosive cycle. Non-welded plinian fall deposits and ignimbrites are the dominant explosive products preserved on the southern flanks. Of these, a significant volume has been dispersed offshore. Many pyroclastic units of the second explosive cycle exhibit compositional zonation. Banded pumice occurs in most units of the third (youngest) explosive cycle, and ignimbrites typically contain mixed phenocryst assemblages, indicating the role of magma mixing/mingling prior to eruption. At least four major eruptions of the third cycle began with phreatomagmatic activity, producing lithic-poor, accretionary lapilli-bearing fallout and/or surge deposits. The repeated, brief phase of phreatomagmatism at the onset of these eruptions is interpreted as reflecting an exhaustive water supply, probably a small caldera lake that was periodically established during the third cycle. Accidental syenite becomes an increasingly important lithic clast type in ignimbrites up-sequence, and is interpreted as recording the progressive development of a plutonic complex beneath the summit caldera.Successive eruptions during each explosive cycle increased in volume, with the largest eruption occurring at the end of the cycle. More than ten major explosive eruptions vented moderately large volumes (1−[ges ]10 km3) of phonolitic magma during the last two cycles. Culminating each explosive cycle was the emplacement of relatively large volume (>5−10 km3) ignimbrites with coarse, vent-derived lithic breccias, interpreted to record a major phase of caldera collapse. In the extracaldera record, explosive cycles are separated by ∼0.2 m.y. periods of non-explosive activity. Repose periods were characterized by erosion, remobilization of pyroclastic deposits by discharge events, and pedogenesis. The current period of non-explosive activity is characterized by the construction of the Teide-Pico Viejo stratovolcanic complex within the summit caldera. This suggests that eruptive hiatuses in the extracaldera record may reflect effusive activity and stratovolcano or shield-building phases within the summit caldera. Alternating effusive and explosive cycles have thus been important in the volcanic evolution of the Las Cañadas edifice.

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Mingari, Leonardo, Arnau Folch, Lucia Dominguez et Costanza Bonadonna. « Volcanic Ash Resuspension in Patagonia : Numerical Simulations and Observations ». Atmosphere 11, n^o 9 (12 septembre 2020) : 977. http://dx.doi.org/10.3390/atmos11090977.

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Resuspension of pyroclastic deposits occurs under specific atmospheric and environmental conditions and typically prolongs and exacerbates the impact associated with the primary emplacement of tephra fallout and pyroclastic density current deposits. An accurate forecasting of the phenomenon, to support Volcanic Ash Advisory Centers (VAACs) and civil aviation management, depends on adapting volcanic ash transport and dispersion models to include specific ash emission schemes. Few studies have attempted to model the mechanisms of emission and transport of windblown volcanic ash, and a systematic study of observed cases has not been carried out yet. This manuscript combines numerical simulations along with a variety of observational data to examine the general features of ash resuspension events in northern Patagonia following the 2011 Cordón Caulle eruption (Chile). The associated outcomes provide new insights into the spatial distribution of sources, frequency of events, transport patterns, seasonal and diurnal variability, and spatio-temporal distribution of airborne ash. A novel modelling approach based on the coupling between Advanced Research core of the Weather Research and Forecasting (WRF-ARW) and FALL3D models is presented, with various model improvements that allow overcoming some limitations in previous ash resuspension studies. Outcomes show the importance of integrating source information based on field measurements (e.g., deposit grain size distribution and particle density). We provide evidence of a strong diurnal and seasonal variability associated with the ash resuspension activity in Patagonia. According to the modelled emission fluxes, ash resuspension activity was found to be significantly more intense during daytime hours. Satellite observations and numerical simulations strongly suggest that major emission sources of resuspended ash were distributed across distal areas (>100 km from the vent) of the Patagonian steppe, covered by a thin layer of fine ash. The importance of realistic soil moisture data to properly model the spatial distribution of emission sources is also highlighted.

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Lerner, Geoffrey A., Shane J. Cronin, Gillian M. Turner et Elisa J. Piispa. « Recognizing long-runout pyroclastic flow deposits using paleomagnetism of ash ». GSA Bulletin 131, n^o 11-12 (9 avril 2019) : 1783–93. http://dx.doi.org/10.1130/b35029.1.

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Abstract Quantifying the spread of >600 °C pyroclastic flows (more broadly termed pyroclastic density currents—PDCs) is important because they regularly cause major volcanic catastrophes. Far from volcanic flanks, non-welded PDC deposits can be difficult to distinguish from cold-emplaced volcano-sedimentary units. A key indicator of high temperature is the coherence of magnetic remanence among different lithic clasts in a deposit. In long-runout PDCs, distal deposits are dominated by ash particles (<2 mm diameter), often lacking clasts large enough for conventional paleomagnetic sampling. Here we demonstrate a method of consolidating and sampling oriented blocks of friable ash material with a strengthening compound. This method was used to show that a >25 km runout mass-flow deposit from the 2518-m-high Mt. Taranaki (New Zealand) was emplaced as a hot PDC, contrary to an earlier cold lahar interpretation. We corroborate the results from ash with data from clast samples at some sites and show that the matrix was emplaced at temperatures of at least 250 °C, while clasts were deposited at up to 410 °C. Our case-study raises concerns for hazard-identification at stratovolcanoes worldwide. In the Mt. Taranaki case we demonstrate that PDCs traveled >9 km farther than previously estimated—also well beyond the “normal” PDC hazard zones at stratovolcanoes (10 or 15 km from source). Thus, attention should be paid to deposits in the 15–25 km range in other volcanic settings, where large populations are potentially unaware of PDC risk.

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Lebedeva, E. V. « Present sedimentation in the volcanic lakes of the Kurile-Kamchatka region (Russia) as a basis for paleoreconstructions ». Limnology and Freshwater Biology, n^o 4 (2022) : 1464–66. http://dx.doi.org/10.31951/2658-3518-2022-a-4-1464.

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Abstract. The results of modern processes observations of the sedimentation in the volcanic lakes of the region are summarized with the use of material from other regions of the world. The available data on the drained volcanic lakes deposits, Uzon-Geysernaya caldera as example, was analysed. The main sources of material (volcanic and post-volcanic activity, gravitational displacements on slopes, and erosion-denudation processes) and the mechanisms of its entry into volcanic lakes, as well as the features of the subsequent deposits transformation as a result of hydrothermal, seismic, and volcano-tectonic activity, are identified. The results of the studies carried out allow us to conclude that the volcanic lakes deposits are complexly constructed polyfacial complexes with alternating fine-grained lacustrine and lacustrine-swamp deposits with pyroclastic horizons and interlayers of untreated or poorly processed coarse clastic material coming as a result of volcano-tectonic activity, gravitational and erosion processes. The irregularity of horizons along strike is typical; and is characterized by the large-scale sediments deformation under the influence of seismic activity, growth of effusive and extrusive domes, phreatic explosions, etc. Hydrothermal activity contributes to the weathering and cementation of the lake sediments. Lava outpourings and the high-temperature pyroclastic flows provokes sintering of contacting horizons sediments.

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Filocamo, Francesca, Natalia Leone, Carmen Rosskopf, Vittoria Scorpio, Santiago Giralt et Pietro Aucelli. « Quaternary Evolution of the Lower Calore and Middle Volturno Valleys (Southern Italy) ». Water 13, n^o 5 (9 mars 2021) : 741. http://dx.doi.org/10.3390/w13050741.

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The lower Calore and middle Volturno valleys preserve stratigraphical and morphological evidence and tephrostratigraphic markers particularly suitable for reconstructing the long-term geomorphological evolution of the central-southern Apennines. Aim of our study is to identify the main steps of the Quaternary landscape evolution of these valley systems and to improve knowledge about the relationships between fluvial processes and tectonics, volcanic activity, climatic and human influences. To this purpose, we carried out an integrated geomorphological and chrono-stratigraphical analysis of identified fluvial landforms and related deposits, integrated by 230Th/234U datings on travertines from the Telese Plain area. The study highlighted in particular: (1) fluvial sedimentation started in the Middle Pleistocene (~650 ka) within valleys that originated in the lower Pleistocene under the control of high-angle faults; (2) extensional tectonics acted during the Middle and Upper Pleistocene, driving the formation of the oldest fluvial terraces and alluvial fans, and persisted beyond the emplacement of the Campanian Ignimbrite pyroclastic deposits (~39 ka); and (3) from the late Upper Pleistocene onwards (<15 ka), the role of tectonics appears negligible, while climatic changes played a key role in the formation of three orders of valley floor terraces and the youngest alluvial fans.

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Nakaoka, Reina, et Keiko Suzuki-Kamata. « Rock-magnetic evidence for the low-temperature emplacement of the Habushiura pyroclastic density current, Niijima Island, Japan ». Geological Society, London, Special Publications 396, n^o 1 (23 janvier 2014) : 51–66. http://dx.doi.org/10.1144/sp396.7.

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Zanella, Elena, Lucia Gurioli, Roberto Lanza, Roberto Sulpizio et Manuela Bontempi. « Deposition temperature of the AD 472 Pollena pyroclastic density current deposits, Somma-Vesuvius, Italy ». Bulletin of Volcanology 70, n^o 10 (23 février 2008) : 1237–48. http://dx.doi.org/10.1007/s00445-008-0199-9.

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Ludden, John, Claude Hubert et Clement Gariépy. « The tectonic evolution of the Abitibi greenstone belt of Canada ». Geological Magazine 123, n^o 2 (mars 1986) : 153–66. http://dx.doi.org/10.1017/s0016756800029800.

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AbstractBased on structural, geochemical, sedimentological and geochronological studies, we have formulated a model for the evolution of the late Archaean Abitibi greenstone belt of the Superior Province of Canada. The southern volcanic zone (SVZ) of the belt is dominated by komatiitic to tholeiitic volcanic plateaux and large, bimodal, mafic-felsic volcanic centres. These volcanic rocks were erupted between approximately 2710 Ma and 2700 Ma in a series of rift basins formed as a result of wrench-fault tectonics.The SVZ superimposes an older volcanic terrane which is characterized in the northern volcanic zone (NVZ) of the Abitibi belt and is approximately 2720 Ma or older. The NVZ comprises basaltic to andesitic and dacitic subaqueous massive volcanics which are cored by comagmatic sill complexes and layered mafic-anorthositic plutonic complexes. These volcanics are overlain by felsic pyroclastic rocks that were comagmatic with the emplacement of tonalitic plutons at 2717 ±2 Ma.The tectonic model envisages the SVZ to have formed in a series of rift basins which dissected an earlier formed volcanic arc (the NVZ). Analogous rift environments have been postulated for the Hokuroko basin of Japan, the Taupo volcanic zone of New Zealand and the Sumatra and Nicaragua arcs. The difference between rift related ‘submergent’ volcanism in the SVZ and ‘emergent’ volcanism in the NVZ resulted in the contrasting metallogenic styles, the former being characterized by syngenetic massive sulphide deposits, whilst the latter was dominated by epigenetic ‘porphyry-type’ Cu(Au) deposits.

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Kirov, G., E. Šamajova, R. Nedialkov et TS Stanimirova. « Alteration processes and products of acid pyroclastic rocks in Bulgaria and Slovakia ». Clay Minerals 46, n^o 2 (juin 2011) : 279–94. http://dx.doi.org/10.1180/claymin.2011.046.2.279.

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AbstractThe genesis of the products of the alteration of acid pyroclastic material is discussed and interpreted on the basis of the distribution, sedimentation conditions, post-sedimentation activity, mineral and chemical compositions of pyroclastic deposits in Bulgaria and Slovakia.It is found that the disordered nonequilibrium nature of the volcanic glass induces a diagenetic devitrification of the tuffs and formation of clinoptilolitic, adularia-cristobalite and bentonite rocks. With increasing temperature, the volcanic glass tends forward a stable state through a series of zeolite mineral associations: clinoptilolite-mordenite-analcime-feldspar. The change in mineral composition of this series of rocks occurs without a change in the chemical composition of the rocks, which could be explained by the closed nature of zeolite systems. The formation of bentonites is associated with the removal of alkaline ions under diagenetic conditions, while the formation of halloysite rocks is caused by hydrothermal activity.

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Caballero, E., C. Jiménez de Cisneros, F. J. Huertas, F. Huertas, A. Pozzuoli et J. Linares. « Bentonites from Cabo de Gata, Almería, Spain : a mineralogical and geochemical overview ». Clay Minerals 40, n^o 4 (décembre 2005) : 463–80. http://dx.doi.org/10.1180/0009855054040184.

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AbstractThe Neogene volcanic region of Cabo de Gata, Almería, SE Spain, is dotted with many outcrops of bentonite, some of them of significant economic interest. The bentonites have their origin in the hydrothermal alteration of pyroclastic rocks (15-7 Ma). The deposits are usually associated with fractures. The major mineral is a dioctahedral Fe- and Mg-smectite (89-75%) and this is accompanied by minor amounts of feldspars, quartz, amphiboles, pyroxenes, biotite, zeolites, disordered tridymite, calcite, etc. This paper describes the geological background, the general characteristics of the bentonites and major aspects of their formation, e.g. type of low-temperature hydrothermal solutions, mass balance, chemical evolutions of the smectites and geochemistry of trace elements. Finally, the characteristics of three of the most important deposits are described.

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Doronzo, Domenico M., et Pierfrancesco Dellino. « Hydraulics of subaqueous ash flows as deduced from their deposits : 2. Water entrainment, sedimentation, and deposition, with implications on pyroclastic density current deposit emplacement ». Journal of Volcanology and Geothermal Research 258 (mai 2013) : 176–86. http://dx.doi.org/10.1016/j.jvolgeores.2013.04.013.

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Li, Gen, Guicong Fang, Zuohai Feng, Cheng Xu, Zhennan Huang et Chunzeng Wang. « Genetic Relationship between Granite and Fluorite Mineralization in the Shuanghuajiang Fluorite Deposit, Northern Guangxi, South China : Evidence from Geochronology, REE, and Fluid Geochemistry ». Minerals 12, n^o 9 (30 août 2022) : 1102. http://dx.doi.org/10.3390/min12091102.

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Hydrothermal vein-type fluorite deposits are the most important metallogenic type of fluorite deposits in South China, most of which are closely related to granitoid in space; however, the genetic relationship between granitoid and fluorite mineralization remains controversial. The Shuanghuajiang fluorite deposit in northern Guangxi of South China is a typical vein-type fluorite deposit hosted in a granite pluton, with the orebodies occurring within brittle faults. Zircon U-Pb dating of the hosting Xiangcaoping granite yields an emplacement age of 228.04 ± 0.76 Ma (MSWD = 0.072). Fluorite Sm-Nd dating yields an isochron age of 185 ± 18 Ma. The new age data indicate that the fluorite deposit was precipitated significantly later than the emplacement of the hosting Xiangcaoping granite pluton. The fluorite and granite exhibit similar rare earth element (REE) patterns with negative Eu anomalies, suggesting that fluorine (F) was derived from the granite. The fluorite fluid inclusions show a homogeneous temperature mainly ranging between 165 °C and 180 °C. Salinity is typically less than 1% NaCl eqv, while the δ18OV-SMOW and δDV-SMOW values are between −5.2‰–−6.1‰ and −17.35‰–−23.9‰, respectively. These indicate that the ore-forming fluids were a NaCl-H2O system with medium-low temperature and low salinity, which is typical for meteoric water. Given the combined evidence of geochronology, REE, and fluid geochemistry, the mineralization of the Shuanghuajiang fluorite deposit is unrelated to magmatic-hydrothermal activity but achieved via hydrothermal circulation and leaching mechanisms. Our study presents a genetic relationship between the fluorite deposit and granitoids based on an example of northern Guangxi, providing a better understanding of the genesis of hydrothermal vein-type fluorite deposits in granitoids outcropping areas.

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Real, Irene del, Farhad Bouzari, Amelia Rainbow, Thomas Bissig, Jacqueline Blackwell, Ross Sherlock, John F. H. Thompson et Craig J. R. Hart. « Spatially and Temporally Associated Porphyry Deposits with Distinct Cu/Au/Mo Ratios, Woodjam District, Central British Columbia ». Economic Geology 112, n^o 7 (1 novembre 2017) : 1673–717. http://dx.doi.org/10.5382/econgeo.2017.4526.

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Abstract The Woodjam district is a cluster of porphyry Cu-Au deposits of Early Jurassic age (~196 Ma) and is located in the Quesnel terrane in central British Columbia. Porphyry centers include the Southeast zone Cu-Mo porphyry, the Deerhorn and Megabuck Au-Cu porphyries, and the Takom and Three Firs Cu-Au porphyries. The Takomkane batholith, which intruded strata of the Nicola Group and is host to the Southeast zone, has characteristics of a calc-alkalic Cu-Mo porphyry. The Deerhorn, Megabuck, and Takom deposits are centered on narrow monzonite bodies with pencil-like geometries that intruded the Nicola Group volcanic sequence. These small volume intrusions have characteristics of high K calc-alkalic intrusions. The Southeast zone, Deerhorn, Megabuck, and Takom deposits have similar ages and their intrusive units can be divided into two groups with distinct geochemical characteristics. Differences between alteration and mineralization of the deposits are attributed to the magmatic evolution of the system as well as differences in the depth of emplacement and preservation. Based on stratigraphic relationships, the Deerhorn and Megabuck deposits are interpreted to be emplaced at the shallowest structural level in the district and have the highest Au/Cu ratios and the lowest temperature alteration assemblages. The Southeast zone is hosted by the felsic units in the Takomkane batholith and represents the deepest parts of the hydrothermal system. Cu-Au mineralization in the Takom deposit shows characteristics intermediate between the Southeast zone and the Deerhorn deposit. Based on stratigraphic interpretations the difference in depth of emplacement between the Southeast zone and Megabuck is about 1,400 m, with Takom being emplaced about 400 m above the Southeast zone. The Woodjam district illustrates the variety of styles of porphyry mineralization that can form over a restricted time interval (<1 m.y.) within a single district.

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Novembre, Daniela, Domingo Gimeno, Piergiulio Cappelletti et Sossio Fabio Graziano. « A case study of zeolitization process : “Tufo Rosso a Scorie Nere” (Vico volcano, Italy) : inferences for a general model ». European Journal of Mineralogy 33, n^o 3 (2 juin 2021) : 315–28. http://dx.doi.org/10.5194/ejm-33-315-2021.

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Abstract. This paper focuses on the authigenic mineralization processes acting on “Tufo Rosso a Scorie Nere” (TRS), i.e. one of the main pyroclastic units of the Vico stratovolcano (Latium, Italy). The pyroclastic deposits appear in general massive and made of “black vitreous vesiculated juvenile scoriae”, immersed in an ashy matrix lithified after zeolitization processes. The main minerals are chabazite and phillipsite, and the zeolitic content is locally variable, reaching 68 % wt. Zeolites grow replacing both amorphous fraction and pre-existing phases, occurring inside both matrix and scoriae. Concerning scoriae, zeolitization moves from the rim to the core of the scoriaceous fragment as a function of (a) temperature of the fluids and (b) permeability (primary or secondary). Composition of parental fresh glass and that of zeolitized rocks is compatible with trachyte chemistry, lightly undersaturated in SiO2, and the alteration processes modified the parental rock chemical features. Zeolites genesis is ascribed to a “geoautoclave-like system”, and zeolites display a Si/Al ratio similar to that of the parental glasses. TRS presents promising mineralogical characteristics as supplementary cementitious material in the production of mixed cements.

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GISBERT, GUILLEM, et DOMINGO GIMENO. « Ignimbrite correlation using whole-rock geochemistry : an example from the Sulcis (SW Sardinia, Italy) ». Geological Magazine 154, n^o 4 (13 mai 2016) : 740–56. http://dx.doi.org/10.1017/s0016756816000327.

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AbstractIgnimbrites are useful chronological markers in the geological record at local and regional scales. They also provide information on the dynamics of the eruption that produced them, making their study of great importance in terms of volcanic hazard assessment. However, their study is usually hampered by their lateral variation and discontinuity. When stratigraphic and lithologic criteria are not sufficient for correlation purposes, the use of multiple complementary correlation tools may be necessary to correctly determine their areal extension, volume and facies variations. Whole-rock geochemistry is considered one of the less reliable correlation techniques due to the pyroclastic nature of these deposits and their emplacement dynamics. These may introduce vertical and horizontal geochemical heterogeneity in the final deposit. In addition, the occurrence of zoned ignimbrites due to magma supply of changing composition is common. In this work we show that, if appropriately used, whole-rock geochemistry can be a valid and highly useful tool for ignimbrite correlation. We provide an example from the study of an ignimbrite sequence containing 18 units (sensu lato) in the Sulcis region (SW Sardinia, Italy). A protocol has been developed for unit recognition based on successive simple binary diagrams where the whole-rock composition of a problem sample can be plotted. Immobile trace elements have been preferentially used to minimize effects of element mobilization associated with alteration and weathering. The diagrams provided here are designed for the Sulcis, but the methodology followed to develop them may be applied to other study areas.

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Hencz, Mátyás, Tamás Biró, István János Kovács, Roland Stalder, Károly Németh, Alexandru Szakács, Zsófia Pálos, Zoltán Pécskay et Dávid Karátson. « Uniform “water” content in quartz phenocrysts from silicic pyroclastic fallout deposits – implications on pre-eruptive conditions ». European Journal of Mineralogy 33, n^o 5 (24 septembre 2021) : 571–89. http://dx.doi.org/10.5194/ejm-33-571-2021.

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Abstract. Structural hydroxyl content of volcanic quartz phenocrysts was investigated with unpolarized Fourier-transform infrared spectroscopy. The phenocrysts originated from five pyroclastic fallout deposits from the Bükk Foreland Volcanic Area (BFVA), Hungary, and two from the AD 1314 Kaharoa eruption (KH eruption), Okataina Volcanic Complex (Taupo Volcanic Zone), New Zealand. All investigated quartz populations contain structural hydroxyl content in a narrow range with an average of 9.3 (±1.7) wt ppm. The earlier correlated horizons in the BFVA had the same average structural hydroxyl content (within uncertainty). Thus, it can be concluded that the structural hydroxyl content does not depend on the geographical distance of outcrops of the same units or the temperature or type of the covering deposit. The rare outlier values and similar structural hydroxyl contents show that the fallout horizons cooled fast enough to retain their original structural hydroxyl content. The similarity of the structural hydroxyl contents may be the result of similar P, T, and x (most importantly H2O and the availability of other monovalent cations) conditions in the magmatic plumbing system just before eruption. Therefore, we envisage common physical–chemical conditions, which set the structural hydroxyl content in the quartz phenocrysts and, consequently, the water content of the host magma (∼ 5.5 wt %–7 wt % H2O) in a relatively narrow range close to water saturation.

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BROWN, R. J., T. L. BARRY, M. J. BRANNEY, M. S. PRINGLE et S. E. BRYAN. « The Quaternary pyroclastic succession of southeast Tenerife, Canary Islands : explosive eruptions, related caldera subsidence, and sector collapse ». Geological Magazine 140, n^o 3 (mai 2003) : 265–88. http://dx.doi.org/10.1017/s0016756802007252.

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A much-revised Quaternary stratigraphy is presented for ignimbrites and pumice fall deposits of the Bandas del Sur, in southern Tenerife. New 40Ar/39Ar data obtained for the Arico, Granadilla, Fasnia, Poris, La Caleta and Abrigo formations are presented, allowing correlation with previously dated offshore marine ashfall layers and volcaniclastic sediments. We also provide a minimum age of 287±7 ka for a major sector collapse event at the Güimar valley. The Bandas del Sur succession includes more than seven widespread ignimbrite sheets that have similar characteristics, including widespread basal Plinian layers, predominantly phonolite composition, ignimbrites with similar extensive geographic distributions, thin condensed veneers with abundant diffuse bedding and complex lateral and vertical grading patterns, lateral gradations into localized massive facies within palaeo-wadis, and widespread lithic breccia layers that probably record caldera-forming eruptions. Each ignimbrite sheet records substantial bypassing of pyroclastic material into the ocean. The succession indicates that Las Cañadas volcano underwent a series of major explosive eruptions, each starting with a Plinian phase followed by emplacement of ignimbrites and thin ash layers, some of co-ignimbrite origin. Several of the ignimbrite sheets are compositionally zoned and contain subordinate mafic pumices and banded pumices indicative of magma mingling immediately prior to eruption. Because passage of each pyroclastic density current was characterized by phases of non-deposition and erosion, the entire course of each eruption is incompletely recorded at any one location, accounting for some previously perceived differences between the units. Because each current passed into the ocean, estimating eruption volumes is virtually impossible. Nevertheless, the consistent widespread distributions and the presence of lithic breccias within most of the ignimbrite sheets suggest that at least seven caldera collapse eruptions are recorded in the Bandas del Sur succession and probably formed a complex, nested collapse structure. Detailed field relationships show that extensive ignimbrite sheets (e.g. the Arico, Poris and La Caleta formations) relate to previously unrecognized caldera collapse events. We envisage that the evolution of the nested Las Cañadas caldera is more complex than previously thought and involved a protracted history of successive ignimbrite-related caldera collapse events, and large sector collapse events, interspersed with edifice-building phases.

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Alberico, I., P. Petrosino et L. Lirer. « Volcanic hazard and risk assessment in a multi-source volcanic area : the example of Napoli city (Southern Italy) ». Natural Hazards and Earth System Sciences 11, n^o 4 (7 avril 2011) : 1057–70. http://dx.doi.org/10.5194/nhess-11-1057-2011.

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Abstract. The possible emplacement of pyroclastic fall and flow products from Campi Flegrei and Somma-Vesuvio represents a threat for the population living in Napoli city. For this area, the volcanic hazard was always partially investigated to define the hazard related to the Campi Flegrei or to the Somma-Vesuvio activity one at a time. A new volcanic hazard and risk assessment, at the municipality scale, as a vital tool for decision-making about territorial management and future planning, is presented here. In order to assess the hazard related to the explosive activity of both sources, we integrated the results of field studies and numerical simulations, to evaluate the future possibility for Napoli to be hit by the products of an explosive eruption. This is defined for the Somma Vesuvio central volcano through the sum of "field frequency" based on the thickness and distribution of past deposits (Lirer et al., 2001), and for the Campi Flegrei volcanic field by suitably processing simulated events based on numerical modelling (Alberico et al., 2002; Costa et al., 2009). Aiming at volcanic risk assessment, the hazard areas were joined with the exposure map, considered for our purposes as the economical value of artefacts exposed to hazard. We defined four risk classes, and argued that the medium and low-very low risk classes have the largest extent in Napoli municipality, whereas only few zones located in the eastern part of the city and in the westernmost coastal area show a high risk, owing to the correspondence of high economical value and high hazard.

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