LA ACTIVIDAD EXPLOSIVA DEL VOLCÁN TURRIALBA (COSTA RICA) EN EL PERÍODO 2010-2016 - THE EXPLOSIVE ACTIVITY OF TURRIALBA VOLCANO (COSTA RICA) IN 2010 - 2016

Abstract

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.