2014 Mount Ontake eruption: characteristics of the phreatic eruption as inferred from aerial observations
© Kaneko et al. 2016
Received: 20 November 2015
Accepted: 20 April 2016
Published: 3 May 2016
The sudden eruption of Mount Ontake on September 27, 2014, led to a tragedy that caused more than 60 fatalities including missing persons. In order to mitigate the potential risks posed by similar volcano-related disasters, it is vital to have a clear understanding of the activity status and progression of eruptions. Because the erupted material was largely disturbed while access was strictly prohibited for a month, we analyzed the aerial photographs taken on September 28. The results showed that there were three large vents in the bottom of the Jigokudani valley on September 28. The vent in the center was considered to have been the main vent involved in the eruption, and the vents on either side were considered to have been formed by non-explosive processes. The pyroclastic flows extended approximately 2.5 km along the valley at an average speed of 32 km/h. The absence of burned or fallen trees in this area indicated that the temperatures and destructive forces associated with the pyroclastic flow were both low. The distribution of ballistics was categorized into four zones based on the number of impact craters per unit area, and the furthest impact crater was located 950 m from the vents. Based on ballistic models, the maximum initial velocity of the ejecta was estimated to be 111 m/s. Just after the beginning of the eruption, very few ballistic ejecta had arrived at the summit, even though the eruption plume had risen above the summit, which suggested that a large amount of ballistic ejecta was expelled from the volcano several tens-of-seconds after the beginning of the eruption. This initial period was characterized by the escape of a vapor phase from the vents, which then caused the explosive eruption phase that generated large amounts of ballistic ejecta via sudden decompression of a hydrothermal reservoir.
Mount Ontake erupted for the first time in recorded history in 1979 (Kobayashi 1980), and since then, small phreatic eruptions occurred in 1991 and 2007 (Oikawa 2008; Oikawa et al. 2014). On September 10 and 11, 2014, the frequency of tremors under the volcano increased before subsiding again on September 12 (Japan Metrological Agency (JMA) 2014). Then, at 11:52 on September 27, the volcano erupted suddenly from a vent in the Jigokudani valley on the southern side of the summit (JMA 2014). The volcanic tremors that began 11 min before the eruption were followed by rapid inflation of the edifice 4 min later (JMA 2014). The eruption plume, which resembled cumulonimbus clouds, finally reached a height of approximately 11 km above sea level (ASL; Sato et al. 2015), and pyroclastic flows flowed down the slopes. The ballistic ejecta that were generated were considered to be the main cause of the many casualties, according to comments from the local hospital (Nagano Prefectural Kiso Hospital). Metrological radar observations indicated that the explosive phase ceased around 12:40 (Sato et al. 2015). As with previous eruptions, the 2014 eruption was considered to be a phreatic eruption, as no fresh magma was contained (Oikawa et al. 2015).
This paper attempted to interpret the status and progression of the volcanic activity associated with the 2014 Mount Ontake eruption through aerial observations and to analyze the eruptive processes and mechanisms associated with the phreatic eruption that resulted in the disaster on Mount Ontake.
In this study, analyses of the eruption processes based on erupted materials were complicated by the extensive disturbance and disruption of the original structure of the deposits that were due to the extensive rescue operations and rainfall following the eruption. Specifically, rescue efforts involved extensive excavation of the deposits near the summit, and the rains disrupted large amounts of tephra and ejected materials during the one-month period when access to the site by volcanologists was strictly prohibited. We therefore focused our analysis on aerial photographs taken immediately after the eruption, as these preserved the original situation and structure of the erupted materials and topography, both of which are important for elucidating eruption processes. On September 28, the day following the eruption, between 13:54 and 15:15, one of us (TK) surveyed the volcano from a media helicopter (Chunichi/Tokyo Shimbun Newspaper Co.) and took more than 350 images of the areas affected by the eruption with a digital camera (SONY DSC-RX100M2). In addition to these photographs, we also used many images and videos of the eruption that were captured by hikers and members of the mass media. Together with the eyewitness testimonies, the image information was useful for inferring and refining the temporal relationships among the events associated with the eruption.
Results: characteristics of vents and depositional areas inferred from aerial observations
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.
Discussion: occurrence of ballistic ejecta and mechanism of phreatic eruption
Estimation of the initial velocity of ballistic ejecta
Relationship between the occurrence of the eruption plume and impacts of ballistic ejecta
The volcanic tremor and inflation of the edifice that occurred 11 and 7 min before the eruption, respectively, are considered to have been caused by the sudden migration of the vapor phase, comprising water vapor and other gasses, to shallower depths (Kato et al. 2015). The initial phase of the eruption may therefore have been caused by the ejection of this vapor phase to the surface at 11:52 on September 27; however, the ejection of this gas phase did not generate large amounts of ballistic ejecta, probably because the intensity of the resulting explosion was too low. The removal of a large amount of the vapor phase from a hydrothermal reservoir in which the water–vapor system is in equilibrium can cause rapid boiling (bumping) of water due to a decrease in the boiling temperature via sudden decompression of the system (Taniguchi and Ueki 2014). This likely caused the explosive phase of the eruption, which then generated a cluster of ballistic ejecta, several tens-of-seconds after the initial phase of the eruption. The occurrence of rapid boiling and the accompanying explosions may have gradually propagated to the deeper parts of the hydrothermal system, resulting in the intermittent generation of ballistic ejecta during the explosive phase of the eruption, which continued for approximately 1 h.
The vents involved in the eruption on the southwestern side of the summit are arranged along a west-northwest to east-southeast axis, which is sub-parallel to the axis of the vents involved in the 1979 eruption.
Among the three major vents on the floor of the Jigokudani valley, the vent at the center (J4) was the main vent involved in the eruption on September 27, with the other vents likely forming as a result of non-explosive processes on the following day.
The pyroclastic flow travelled about 2.5 km along the Jigokudani valley at an average speed of 32 km/h. Since no burned or fallen trees were observed in this region, it appears that the temperature and destructive force associated with the pyroclastic flow were low.
The distribution of ballistic ejecta was inferred from impact craters, and the furthest impact crater was located 950 m from the vents, although the furthest ballistic was found at the mountain cottage, located 1000 m from the summit. Based on ballistic models, the maximum initial velocity of the ejecta was estimated to be 111 m/s.
Immediately after the beginning of the eruption, very few ballistic ejecta were observed around the summit, even though the eruption plume had risen above the summit. Based on this observation and the relationship between the speed of upward expansion of the eruption plume and the minimum flight duration of ballistic ejecta that landed on the summit, the generation of large amounts of ballistic ejecta is considered to have occurred several tens-of-seconds after the beginning of the eruption, probably in relation to the explosion mechanism of the hydrothermal reservoir deep under the volcano.
The results of this study show that, in the case of phreatic eruptions, there is a window of several tens-of-seconds before the first cluster of ballistic ejecta arrives. In the Mount Ontake 2014 eruption, ballistic analysis revealed that the final velocity of ejecta at the time of landing ranged between 58 and 79 m/s (209 and 284 km/h) near the summit. Anyone struck by such ejecta could be seriously injured. Thus, when hiking on volcanoes that have undergone repeated phreatic eruptions, it is important to minimize the amount of time spent near vents and to be aware of structures that can be used for protection, such as mountain cottages, large rocks, or shelters. Furthermore, in the event of an eruption, it is important to avoid centers of volcanic activity and to seek shelter during the time window before ballistic ejecta are generated.
TK took aerial photographs and analyzed them with FM. SN helped draft the manuscript. All authors read and approved the final manuscript.
We thank the Chunichi/Tokyo Shimbun Newspaper Co. and Usami A for the opportunity to observe the volcano from a helicopter and Noguchi H for permitting us to use photographs of the eruption that were taken by her husband, the late Noguchi I. We are also very grateful to the reviewers, Yoshimoto M, and an anonymous reviewer, whose comments were useful for improving the manuscript. This work was supported in part by a Grant-in-Aid for Scientific Research (A) from the Japan Society for the Promotion of Science KAKENHI (Grant No. 23241055 to TK).
The authors declare that they have no competing interests.
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