Abstract
In association with the September 2014 phreatic eruption (VEI 1–2) at Ontake Volcano, a syn-eruptive and two post-eruptive lahars occurred in the Akagawa–Nigorigawa River, southern flank of the volcano. The present contribution describes and discusses the contrasting features of the two post-eruptive lahars, which caused a major impact on downstream river morphology, and re-examines the description of the syn-eruptive lahar in the previous study. The first post-eruptive lahar occurred 8 days after the eruption by the rainstorm (October 5, 2014, before the snowy season), and the second lahar was associated with the rain-on-snow (ROS) event on April 20, 2015, in the early spring of the snowmelt season. The October rain-triggered lahar, which can be interpreted as a cohesive debris flow, reached at least ~ 11 km downstream and left muddy matrix-rich sediments with high clay content (10–20 wt% of clay in matrix). The lahar deposits contain hydrothermally altered rock fragments, sulfide/sulfate minerals, and clay minerals and show extremely high total sulfur content (10–14 wt%) in matrix part, indicating source material from the September phreatic eruption deposits. The presence of “rain-triggered” clay-rich lahar and deposits originating from a single small phreatic eruption is important because usually such clay-rich lahars are known to occur in association with large-scale sector collapse and debris avalanches. The April ROS-triggered lahar was caused by the heavy rain and accompanying snow melting. The lahar was dilute and partly erosional and evolved into hyperconcentrated flow, which left fines-depleted sandy and gravelly deposits. Despite these lahars that originated from the same volcanic source and occurring within a 7-month period, the flow and resulting depositional characteristics are totally different. These different types of lahars after a single eruptive event need different simulations and mitigation of lahar hazards with timing (season) of the lahar onset. In comparison with rainfall intensity, snow-melting rate, and the contrasting lahars occurred in 2014/2015, it is postulated that the generation, size, and types of lahars can vary with the timing of eruption, whether it happens during the pre-snow season, snow season, or rainy season.
Highlights
On September 27, 2014, Ontake Volcano in central Japan confronted mountaineers with an unpredicted smallscale phreatic eruption (VEI 1–2: Maeno et al 2016) and caused 63 fatalities at the summit
Further concerns and interests immediately after the eruption were about future eruptions and their prediction: whether the eruption would evolve into magmatic explosive phase. There was another concern for lahar hazards with snow/ice meltwater (Major and Newhall 1989; Pierson et al 1990; Manville et al 2000; Waythomas 2014) such at the seasonally snow-clad Ontake Volcano
This paper focuses on three lahars generated at Ontake Volcano within a 7-month period, each triggered by a different mechanism: a phreatic eruption, heavy rainfall, and rain-on-snow event
Summary
On September 27, 2014, Ontake Volcano in central Japan confronted mountaineers with an unpredicted smallscale phreatic eruption (VEI 1–2: Maeno et al 2016) and caused 63 fatalities (including 5 missing) at the summit. Further concerns and interests immediately after the eruption were about future eruptions and their prediction: whether the eruption would evolve into magmatic explosive phase There was another concern for lahar hazards with snow/ice meltwater (Major and Newhall 1989; Pierson et al 1990; Manville et al 2000; Waythomas 2014) such at the seasonally snow-clad Ontake Volcano. The first recorded (witnessed) historical eruption at Ontake was a phreatic eruption which occurred on October 28, 1979, from vents in Jigokudani and Haccho-tarumi (e.g., Soya et al 1980). Since the traces of smallscale eruptions including phreatic eruptions in geological records are limited, the eruptive records and associated lahar history of Ontake Volcano still need to be examined in detail
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