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Alvarado, Guillermo E.
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- PublicationRestrictedLike a cannonball: origin of dense spherical basaltic ejecta(2017)
; ; ; ; ; ; ; ; ; ; ;Cannonballs are rare spherical to sub-spherical eruptive products associated with basaltic explosive activity. The origin of cannonballs is still debated and subjected to a wide spectrum of different interpretations. In order to better understand the physicochemical mechanisms controlling the formation of these explosive products, we investigated the textural and chemical features of cannonballs from the Cerro Chopo monogenetic volcano (Costa Rica). These explosive products ubiquitously show a core domain with coalesced bubbles (30–36% porosity) wrapped in a dense rim domain with small, isolated bubbles (20–27% porosity). Both domains are identical in terms of bulk rock composition and mineral chemistry and are portions of the same magma batch. Results from combined petrological and thermodynamic modeling indicate that a low-viscosity (~20 Pa s) melt containing early-formed olivine phenocrysts (~9 vol.%) ascended from storage at a decompression rate of 0.5 MPa s−1 until it reached a depth of 4.5 km (equivalent to a pressure of ~150 MPa). While rising from depth to 4.5 km, the melt underwent rapid decompression (0.5–2.6 MPa s−1) and H2O exsolution, driving late-stage crystallization of the groundmass. The fast ascent velocity (21–110 m s−1) while rising between 4.5 km and the surface induced turbulent (Re >103), annular flow development in the uppermost region of the conduit. We propose that cannonballs represent blebs of fluid magmas that underwent shear-driven detachment from the annulus of magma lining the conduit walls at depths lower than 4.5 km. The formation of such cannonballs is dictated by magma transport dynamics of low-viscosity, phenocryst-poor, and volatile-rich melts that rapidly accelerate within the shallow conduit.167 2 - PublicationOpen AccessLA ACTIVIDAD EXPLOSIVA DEL VOLCÁN TURRIALBA (COSTA RICA) EN EL PERÍODO 2010-2016 - THE EXPLOSIVE ACTIVITY OF TURRIALBA VOLCANO (COSTA RICA) IN 2010 - 2016(2016)
; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; The most recent eruptive activity of Turrialba volcano began on the 5th of January 2010, after more than a century of dormancy. The fragmentation process and aerodynamic behavior of the ash from Turrialba’s vulcanian eruptions were investigated by combining grain-size, petrography, mineralogy, Scanning Electron Microscopy (SEM) and Energy Dispersive System (EDS) analyses. The ash components include variable percentages of accessory fresh (no necessary juvenile) to hydrothermally altered lithics (15 - 50 % vol.) with hydrothermal minerals (≤ 12 % vol.: anhydrite, gypsum, bassanite, alunite, hexahydrite, pyrite, heulandite, native sulfur), clay minerals (8 - 17 % vol.: montmorillonite, halloysite, allophane), and a smaller quantity of fresh glassy ashes (5 - 49 % vol.) as fragments and shards (3 - 20 % vol. tackylite and 2 - 26 % vol. sideromelane), as well as primary and eroded/recycled phenocrysts (3 - 13 % vol.: 1 - 5% vol. plagioclase, 1 - 7 % vol. pyroxene, 0 - 1 % vol. olivine, 0 - 6 % vol. opaques, cristobalite and tridimite), and xenocrysts (≤ 1 % vol.: riebeckite and biotite). The secondary minerals were sourced from the deeper to surficial hydrothermal system. Textural features identified in ash particles (90 - 350 μm) suggest that they were formed by brittle fragmentation of vesicle-poor magma/water interaction; molten structures seems to be related to the ductile behavior of some fragments, probably due to the high temperature (> 600 °C) of the fumarolic/magmatic system. The percentage of juvenile components was low (1 - 2 % vol.) at the first opening eruptive phase in January 2010, and it increased steadily until the end of 2016 (ca. 12 - 25 % vol.) . The ash eruptions in the Western Crater from 2014 to 2016 were related to one and later two or three simultaneously active vents fed by distinct conduit branches. The alternation of volcanic explosions (VEI: 0 - 2), from closed conduit and the formation of new craters, to open system with phreatomagmatic events, and the repose intervals (inter-eruptive exhalative degassing), were controlled by the rate at which magma ascended and remained in the volcanic edifice. The recent tephra sequence consists of a complex succession of layers generated by contrasting fragmentation and transportation dynamics. They resulted from fully diluted, low temperature (< 300 °C), pyroclastic density currents (wet surge deposits), originated by short-lived, singlepulse, column collapse of phreatomagmatic columns, which traveled short distances (< 1000 m) from the vent area and surmounted topographic obstacles, and simultaneous fallout and ballistic ejecta. The fine material, in continuous suspension within the uppermost part of the convective plume, was dispersed into the atmosphere and finally settled down on the Valle Central. The quiescent phases could be related to a temporal cooling of the magmatic dike system or to a waning of magmatic activity at depth. Sequential fragmentation/transport theory (SFT) was used to decompose grain-size distributions into five different sub-populations. A new way of using the resultant fragmentation coefficient to assign sub-populations to different fragmentation mechanisms, even in cases when modes overlapped, is presented. For the first time the corresponding results are consistent with the phreatomagmatic eruptions, as well as with those derived from ab initio fractal model.113 49 - PublicationOpen AccessEruptive activity at Turrialba volcano (Costa Rica): Inferences from 3He/4He in fumarole gases and chemistry of the products ejected during 2014 and 2015(2016-10-17)
; ; ; ; ; ; ; ;Rizzo, Andrea Luca; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Palermo, Palermo, Italia ;Di Piazza, Andrea; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;de Moor, J. Maarten; Observatorio Vulcanol ogico y Sismol ogico de Costa Rica (OVSICORI) ;Alvarado, Guillermo E.; Instituto Costarricense de Electricidad, Apdo ;Avard, Geoffroy; Observatorio Vulcanol ogico y Sismol ogico de Costa Rica (OVSICORI) ;Carapezza, Maria Luisa; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Mora, Mauricio M.; Escuela Centroamericana de Geolog ıa, Universidad de Costa Rica; ; ; ; ; ; A new period of eruptive activity started at Turrialba volcano, Costa Rica, in 2010 after almost 150 years of quiescence. This activity has been characterized by sporadic explosions whose frequency clearly increased since October 2014. This study aimed to identify the mechanisms that triggered the resumption of this eruptive activity and characterize the evolution of the phenomena over the past 2 years. We integrate 3He/4He data available on fumarole gases collected in the summit area of Turrialba between 1999 and 2011 with new measurements made on samples collected between September 2014 and February 2016. The results of a petrological investigation of the products that erupted between October 2014 and May 2015 are also presented. We infer that the resumption of eruptive activity in 2010 was triggered by a replenishment of the plumbing system of Turrialba by a new batch of magma. This is supported by the increase in 3He/4He values observed since 2005 at the crater fumaroles and by comparable high values in September 2014, just before the onset of the new eruptive phase. The presence of a number of fresh and juvenile glassy shards in the erupted products increased between October 2014 and May 2015, suggesting the involvement of new magma with a composition similar to that erupted in 1864–1866. We conclude that the increase in 3He/4He at the summit fumaroles since October 2015 represents strong evidence of a new phase of magma replenishment, which implies that the level of activity remains high at the volcano.270 130