Resumo

BRENES-ANDRE, José et al. Analysis of the dynamic and the ballistic products of the 2017 eruption of Poas Volcano (Costa Rica). Rev. Geol. Amér. Central [online]. 2020, n.62, pp.74-101. ISSN 0256-7024.  http://dx.doi.org/10.15517/rgac.v62i0.40640.

From April 12 until April 22 2017 Poás volcano experienced a new explosive phase with ballistic fragments reachingup to ~2,2 km. At its final phase non juvenile bocks predominated, product both of a 1953 lava body destruction and a lacustrinefloor, followed by an increment of juvenile fragments (glassy bread crust blocks, scoriaceous bombs and, rarely, fusiformbombs). The largest ones measured up to 20 m in diameter and 1,5 m in thickness, reaching up to 200 m from the crater. Thejuvenile fragments showed a varied vesiculation (15-50% vesicles) with a basaltic andesitic composition (phenocrysts of plag 20-22%, cpx 6-8%, opx 3-5%, lo 1-3%, mt 0-1%) within an intersertal matrix. The juvenile phreatomagmatic ash content increasedfrom approximated 9% to 85% at the end, consisting of a strombolian activity. When hitting the floor, the bombs generated perpendicularcooling joints and shear stress conjugate pairs. While cooling, the joints were parallel to impact surface generating asymmetric conjugated rectangular pattern. When fallon surfaces with a slope the pattern was more complex due to viscous flow.The ballistic initial velocity was estimated in the 100-300 ms-1 range, the final velocity was estimated in the 36-120 ms-1 range, anda terminal velocity (defined in the text) in the 124-250 ms-1 range. Flight time from bottom of Main crater to the visitor area (Viewpoint) was estimated in the 8-15 secs range. At the end, a set of recommendations for future volcanic ballistic studies is includedbased on a totally new physics model using actual range and impact angle. It is here recommended that more shelters be built onthe path between the active and Botos craters.

Palavras-chave : Ballistic; Velocities; Phreatomagmatic; Bomb jointing;; Newtonian regime.

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