Effects of extreme rainfall on phreatic eruptions: A case study of Mt. Ontake in Japan

Sometimes, natural disasters caused by volcanic eruptions have tragic consequences. Phreatic eruptions are large explosions of steam rocks and hot water caused by the sudden evaporation of water to steam. The September 2014 eruption of Mt. Ontake in Japan was the deadliest in recorded history. Numerous studies have analyzed the occurrence of phreatic eruptions of Mt. Ontake. However, at present, although it is explained that the magma did not move, studies on the cause of the eruption and the elucidation of the process are limited. This study investigates the role of external water of meteoric origin and determines its role in the eruption process. According to a survey of rainfall records by the Japan Meteorological Agency, heavy rain that broke historical records occurred immediately before the phreatic eruption of Mt. Ontake. It was hypothesized that extreme rainfall was the source of the external water supply that caused the phreatic eruption without the magma moving. Various studies on eruptions have confirmed the consistency of this hypothesis. Regarding the eruptive process, extreme rainfall collided with the hot rocks outside the magma chamber, triggering frequent occurrences of vaporization associated with boiling, leading to large explosions in sealed rocks above the zone of water infiltration. This research can contribute to disaster prevention in the future. In order to achieve this, it is necessary to install rainfall measuring instruments on all volcanoes and perform a comparative, multidisciplinary approach to all the monitored parameters.