From April 12 until April 22 2017 Poás volcano experienced a new explosive phase with ballistic fragments reaching up to ~2,2 km. At its final phase non juvenile bocks predominated, product both of a 1953 lava body destruction and a lacustrine floor, followed by an increment of juvenile fragments (glassy bread crust blocks, scoriaceous bombs and, rarely, fusiform bombs). The largest ones measured up to 20 m in diameter and 1,5 m in thickness, reaching up to 200 m from the crater. The juvenile 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 increased from approximated 9% to 85% at the end, consisting of a strombolian activity. When hitting the floor, the bombs generated perpendicular cooling joints and shear stress conjugate pairs. While cooling, the joints were parallel to impact surface generating a symmetric 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, and a terminal velocity (defined in the text) in the 124-250 ms-1 range. Flight time from bottom of Main crater to the visitor area (View point) was estimated in the 8-15 secs range. At the end, a set of recommendations for future volcanic ballistic studies is included based on a totally new physics model using actual range and impact angle. It is here recommended that more shelters be built on the path between the active and Botos craters.
