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The phreatic eruption at Ontake volcano on 27 September 2014, which caused the worst volcanic disaster in the past half-century in Japan, was reconstructed based on observations of the proximal pyroclastic density current (PDC) and fallout deposits. Witness observations were also used to clarify the eruption process. The deposits are divided into three major depositional units (Units A, B, and C) which are characterized by massive, extremely poorly sorted, and multimodal grain-size distribution with 30–50 wt% of fine ash (silt–clay component). The depositional condition was initially dry but eventually changed to wet. Unit A originated from gravity-driven turbulent PDCs in the relatively dry, vent-opening phase. Unit B was then produced mainly by fallout from a vigorous moist plume during vent development. Unit C was derived from wet ash fall in the declining stage. Ballistic ejecta continuously occurred during vent opening and development. As observed in the finest population of the grain-size distribution, aggregate particles were formed throughout the eruption, and the effect of water in the plume on the aggregation increased with time and distance. Based on the deposit thickness, duration, and grain-size data, and by applying a scaling analysis using a depth-averaged model of turbulent gravity currents, the particle concentration and initial flow speed of the PDC at the summit area were estimated as 2 × 10−4–2 × 10−3 and 24–28 m/s, respectively. The tephra thinning trend in the proximal area shows a steeper slope than in similar-sized magmatic eruptions, indicating a large tephra volume deposited over a short distance owing to the wet dispersal conditions. The Ontake eruption provided an opportunity to examine the deposits from a phreatic eruption with a complex eruption sequence that reflects the effect of external water on the eruption dynamics.

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Reconstruction of a phreatic eruption on 27 September 2014 at Ontake volcano, central Japan, based on proximal pyroclastic density current and fallout deposits