Complex facies architecture of unusual pyroclastic density current deposits at the Rincón de la Vieja Volcano, Costa Rica

Abstract

Pyroclastic Density Currents (PDCs) constitute a complex, multiphase, and multiscale volcanic process generated in a wide spectrum of eruptive scenarios and being among the most deadly and destructive volcanic hazards. Over the north flank of the Rincón de la Vieja Volcano, Costa Rica, previous works identified PDC deposits related to the plinian Río Blanco eruption, which is the largest Holocene explosive event recorded at this volcano. However, so far, there have been no in-depth studies aimed at understanding the eruptive style, and dynamics of the parental PDCs. This work presents, for the first time, a detailed facies analysis and architecture reconstruction (plus a geochemical characterization and novel C14 age determinations) of these outstanding deposits. The presence of cauliflower bombs, juvenile mega-blocks, scorias, and pumices, coupled with the scarcity of accidental and/or accessory lithics, is interpreted here as evidence of an unusual eruptive style. We propose that the Río Blanco eruption was a single complex eruption consisting of two successive phases, each involving the drainage of distinct magma batches. The PDCs were related to the sudden spill out of a lava-pool (Arenal-type PDC), during the collapse of an ultraproximal pyroclastic cone that was built in an initial plinian phase. The collapse of this ultraproximal cone produced a leading-edge pumice-rich PDC, which increased its mobility through fragmentation-induced fluidization. This leading edge PDC was followed by successive high concentrated, mafic, modified granular flow pulses derived from the lava-pool spill out. These modified granular flows were highly channelized in proximal zones, but when they reached the main break in slope, the basal underflow froze en masse while its upper, more diluted part started to decouple and move in a more unconfined manner. After the decoupling, the material feeding towards the basal zone of the surviving current produced a new underflow by rapid suspension sedimentation ("surge-derived pyroclastic flows"). Finally, our results indicate that the generation of Arenal-type PDCs is an underestimated scenario that should be considered in future short-term hazard and risk assessments at the Rincón de la Vieja Volcano, as well as in many other stratovolcanoes worldwide.