Distribution and characteristics of the eruption vents
Distribution and characteristics of the pyroclastic flows
Pyroclastic flow-like units occurred during this eruption. Although no examples of pyroclastic flow typical of magmatic eruptions were observed, flows comprising a mixture of ash and gas moved down the slope by gravity (Yamamoto 2014). We referred to these as “pyroclastic flows.”
The pyroclastic flow moved away from the vent in a southwesterly direction along the Jigokudani valley for approximately 2.5 km, as this area was lower than the surrounding areas (Fig. 5a, b). Areas affected by the pyroclastic flow appeared whitish because of the presence of ash on vegetation (Fig. 5d). The absence of burned or fallen trees implied that the temperatures and forces associated with the pyroclastic flow events were both low.
Camera footage recorded by the Ministry of Land, Infrastructure, Transport and Tourism, taken from near Takigoshi village on the southern foot of the volcano (http://www.cbr.mlit.go.jp/tajimi/sabo/ontake/, https://www.youtube.com/watch?v=jt36uloZ3oI), revealed that the pyroclastic flow reached a distance of 2.5 km away from the vent at 11:57 about 5 min after the beginning of the eruption. The front of the pyroclastic flow passed through the point of an altitude of 2500 m of the Jigokudani valley around 11:53; thus, it traveled 2.1 km in 4 min. This means that the pyroclastic flow in this area moved at an average speed of 32 km/h (8.8 m/s), which is considered slow for pyroclastic flow (see Cas and Wright 1987). Indeed, the pyroclastic flows observed in this study could be considered a kind of pyroclastic surge that is characterized by low speed and low temperature. Similar pyroclastic flows were also observed in the phreatic eruption of Miyakejima on August 29, 2000 (Nakada et al. 2005), indicating that this kind of pyroclastic flow might be typical of low-temperature, phreatic eruptions with no magmatic material in the ejecta.
Distribution and characteristics of the air-fall deposits
The height of the eruption plume increased over time and, based on metrological radar observations, was inferred to have reached an altitude of approximately 11 km around 12:10 (Sato et al. 2015). Although the plume, precipitating ash over an extensive area, tilted toward the northeast at low altitudes, it moved east-northeast at high altitudes. A mixture of air-fall and pyroclastic flow deposits appears to have settled in the vicinity of the summit.
The depositional axis of the air-fall ash, indicated by a whitish color on the ground, extended to the east-northeast (Fig. 5a, c). The arrow labeled “e” in Fig. 5c indicates air-fall ash deposits comprising fine ash particles aggregated with accretionary lapilli (Fig. 5e); the thickness of this layer was 2–3 mm, and the size of the particles was 1–2 mm.
At the summit of Ichinoike, which was covered by a thick layer of ashy deposits, sun cracks developed on the surface (Fig. 5f), and water collected in the bottom of the impact craters on the day after eruption, even in those on part of the inner wall of the Ichinoike cone (Fig. 5g). These findings reveal that the ash (air-fall/pyroclastic flow) was enriched with water components, corroborating the observation of accretionary lapilli. According to a hiker at the summit, although the ash was initially dry, it became wetter, taking on the form of “mud rain” in the final stages (Kaito 2014). In video footage taken immediately after the eruption (https://www.youtube.com/watch?v=ODiqlpUwcVM), the top 1–2 cm of the few tens-of-centimeters of ash that was deposited near the summit appeared wet and semisolid, looking dark in color. This water is considered to have been derived from the precipitation of water vapor contained in the eruption plume.
Distribution of ballistic ejecta
The phreatic eruption of Mount Ontake generated large amounts of ballistic ejecta. According to a hiker who sought shelter in the cottage at the summit, the generation of ejecta continued intermittently for about 1 h (Kato 2014). This report is concordant with the morphological variation observed in the impact craters produced by the ballistic ejecta, which included craters with both well-defined and indistinct outlines (Fig. 5f).
The diameters of impact craters ranged between several tens-of-centimeters to 1 m, while those of ballistic ejecta ranged from 10 cm to several tens-of-centimeters (maximum c. 1 m). The distribution density was classified into four zones, based on the number of craters per unit area (5 × 5 m); these zones were called Zones A, B, C, and D, with Zone A having the highest density of impact craters and Zone D having no impact craters. Because the distribution density of craters decreased with increasing distance from the vents, the distribution density was very high between Kengamine and Ichinoike (Fig. 7, Zone A ➀–➂). The furthest impact crater was located at Ninoike pond, 950 m from the vents in the Jigokudani valley (Fig. 7, Zone C ➈). We were unable to survey the distribution of impact craters in the area to the south of the vents because they were obscured by the volcanic plume at the time the observations were made, and the deposition of ash layer was too thin to leave clear crater structures by impact.
The distribution of craters was not isotropic but slightly extended in a north-northeasterly direction (dotted line in Fig. 6). Because the Jigokudani valley extends along a north-northeast to south-southwest axis, and because the vents are located on the valley floor, it is possible that the valley walls acted as barriers to ejecta, preventing ballistics from being ejected far beyond the valley walls along both sides of the valley.
A volcanic eruption of Mount Ontake (御嶽山,Ontake-san) took place on September 27, 2014, killing 63 people. Mount Ontake is a volcano located on the Japanese island of Honshu around 100 kilometres (62 mi) northeast of Nagoya and around 200 km (120 mi) West of Tokyo. It was the first fatal volcanic eruption in Japan since the 1991 collapse of a lava dome at Mount Unzen, and the deadliest volcanic eruption in Japan since Torishima killed an estimated 150 people in 1902.
The volcanic eruption happened at 11:52 Japan Standard Time (UTC+9). There were no significant earthquakes that might have warned authorities in the lead up to the phreatic eruption—caused by ground water flashing to steam in a hydrothermal explosion. The mountain is a popular tourist attraction for hikers, being considered good for beginner climbers and relatively safe, and the weather was also good, so there were several hundred people on its slopes at the time. The police said that they were searching for people remaining on the mountain. By 17:00 the police reported that three people were missing and were believed to be under ash. Another person was rescued from under the volcanic ash, but remained unconscious. Six people were injured, one by flying rocks.
By 19:30, the number of people believed to remain buried in ash rose to six. Nine people had been reported to be injured, five of whom had fractured bones. Later, at least 40 people were reported to be injured, and another 32 were believed to be missing. The JSDF began carrying out helicopter searches for missing people. One woman was reported to have died from the eruption.
On September 28, the police reported that over 30 people had been found in "cardiac arrest" near the summit. Japanese emergency services often refer to people who show no vital signs, and are apparently dead, as being in cardiac arrest, as legally, only an authorised physician can pronounce a person dead. By September 29, a total of 36 bodies had been found, and 12 people had been pronounced dead; the search was suspended due to dangerous conditions, including hydrogen sulfide gas spewing from the mountain. On September 30, fears of escalating volcanic activity on Mount Ontake continued to hinder rescue efforts.
On October 1, 2014, eleven new bodies were discovered by rescuers on the slopes of Mount Ontake after searching in previously unexplored areas of the ash-covered peak, bringing the total body count from 36 to 47; a revision after an erroneous initial count of 48.
On October 4, 2014, four new bodies were discovered by rescuers on the slopes of Mount Ontake after searching in previously unexplored areas away from trekking roads. Those four were confirmed to have died.
Typhoon Phanfone prevented searching activities from October 5 till 6. On October 7, three more bodies were discovered, bringing the total of confirmed deaths to 54. As of October 11, the death toll is at 56. The victims of the Mount Ontake eruption were mourned on October 27, as authorities and residents marked a month since the volcano killed 57 people and left six others missing.
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