Unravelling the Dynamics and Hazards of the June 3rd, 2018, Pyroclastic Density Currents at Fuego Volcano (Guatemala)

The June 3rd, 2018, eruption of Fuego volcano (Guatemala) produced a complex sequence of small-volume pyroclastic density currents (PDCs) that inundated all sectors around the volcano and propagated >12 km on the southeastern flank, deposited similar to 50 million m(3) of pyroclastic material, and killed at least 430 people in the village of San Miguel de Los Lotes and along the RN-14 road. This eruption has illustrated once again that the behavior of small-volume PDCs remains difficult to predict, demonstrating the need for an improved physical understanding of their generation, transport and depositional processes to ultimately improve assessment of their local hazard potential. In this work, we present preliminary results of an interdisciplinary study of the 2018 PDC deposits of Fuego, integrating: (1) field data of the pristine deposit characteristics recorded immediately after the eruption and after one rainy season, (2) sedimentological data (including grain size distributions and compo-nentry) from samples collected at each lobe front and several exposed stratigraphic sections inside three principal ravines draining the volcano and affected by the eruption (Seca, Ceniza and Las Lajas), (3) preliminary quan-tification of the observed range of different clast surface textures, shape and vesicularities, as determined by SEM analyses, (4) surface morphological data from high-resolution satellite and aerial photography, and (5) advanced digital terrain model analysis correlated with sedimentological data to reconstruct the sequence of events on June 3rd and propose a conceptual model of PDC generation and emplacement on the SE flank of Fuego. In this model, a sequence of packages of material involved in discrete failure events of a perched mass of pyroclastic material accumulated within an old collapse structure on the upper SE flank, corresponds to the emplacement of a series of pulses of valley-confined PDCs down the Las Lajas channel. Headwall failures and gravitational col-lapses correlate with specific stratigraphic, sedimentological and textural characteristics of the valley-confined PDC units. The duration of individual granular collapse events serves as a proxy for the mobility of the trig-gered series of PDCs. Also, our detailed stratigraphic work shows evidence of stacked, massive flow units deposited by rapid stepwise aggradation of successive block-and-ash flow (BAF) pulses and their distribution along the southeastern flank supports the interpretation that the June 3rd PDCs, including the dilute parts of the currents, were largely controlled by the topography. The lack of strong changes in the grain size distributions of the fine sub-populations inside the seven valley-confined BAF lobate front units, together with their high content of fines, imply the presence of a self-limiting attrition process, decreasing the bulk porosity of such long-runout BAFs and lowering their effective friction coefficient during transport to reach longer runout distances. This multi-faceted approach completes previous studies already performed on small-volume PDCs at other volcanoes (Merapi, Colima, Unzen) and opens a unique perspective that will provide a significant step forward in our understanding of how such currents are emplaced.