- Open Access
Characteristics of the syneruptive-spouted type lahar generated by the September 2014 eruption of Mount Ontake, Japan
© The Author(s) 2016
- Received: 17 December 2015
- Accepted: 28 July 2016
- Published: 9 August 2016
Mount Ontake erupted at 11:52 am on September 27, 2014, which generated pyroclastic density currents, ballistic projectiles, ash falls, and a small-scale lahar that spouted directly from craters formed by the eruption. Because this lahar may have been generated by water released from within these craters, we refer to this lahar as a “syneruptive-spouted type lahar” in this study. The lahar of the 2014 eruption was small relative to the other syneruptive type lahars reported in the past that were snowmelt type or crater lake breakout type lahars. Nevertheless, in the 2014 event, the syneruptive-spouted type lahar extended approximately 5 km downstream from the Jigokudani crater via the Akagawa River, with an estimated total volume of ~1.2 × 105 m3. We have reviewed other representative syneruptive-spouted type lahars that have been reported in Japan. The syneruptive-spouted type lahar attributed to the September 2014 eruption had the longest runout distance and largest volume of all cases studied. The mineral assemblage identified from samples of the lahar deposits is similar to that of ash-fall deposits from the same eruption. Previous workers deduced that the ash was derived mainly from shallow depths (within 2 km of the surface). The syneruptive-spouted type lahar deposits are therefore also considered to have originated from shallow depths. A syneruptive-spouted type lahar is a small-scale phenomenon that causes little direct damage to infrastructure, but has long-term influence on water quality. Increases in turbidity and decreases in pH are expected to occur in the Mount Ontake area downstream of Nigorisawa after heavy rainfall events in the future. Therefore, the potential indirect (but long term) damage of syneruptive-spouted type lahars should be considered for hazard mapping and planning volcanic disaster prevention measures.
- Phreatic eruption
- Syneruptive-spouted type lahar
- Muddy water
- Water quality
- Ontake volcano
For this study, we have assessed the distribution of syneruptive-spouted type lahar deposits using oblique aerial photographs, analyzed their morphology, evaluated the mechanisms of their generation, investigated their indirect impacts, and compared them with other examples of this type of lahar documented in Japan.
Interpretation of oblique aerial photographs
The syneruptive-spouted type lahar flowed into the Nigorisawa River on the south side of the Jigokudani crater (Fig. 3a). The Jigokudani crater could not be seen in the photographs because of smoke, but a gutter of water from a crater wall could be identified (Fig. 3b). Photographs showing that the lahar overflowed from the Jigokudani crater have been presented by Kaneko et al. (2016). The western crater is a new crater that formed during the 2014 eruption. Flowing water was observed around this new crater and the tongue-shaped lahar, and its presence was confirmed at the bottom of the new crater (Fig. 3c). The ejected volume from this part of the eruption was very small and therefore is not essential to the following discussion.
Although the pyroclastic density current deposits were partially eroded by the flow of the Shirakawa River, no large-scale reworking of these deposits has been recognized (Fig. 3d). In the Akagawa River, downstream of the Jigokudani crater, gray lahar deposits were observed on the valley floor (Fig. 3d, e). The lahar overflowed at the confluence of the Akagawa and Shirakawa Rivers. Muddy water mainly flowed downstream from site C (Fig. 3e, f).
Distribution of the syneruptive-spouted type lahar and muddy water
Characteristics of the syneruptive-spouted type lahar deposits
Occurrence of syneruptive-spouted type lahar deposits
Grain size of syneruptive-spouted type lahar deposits
Matrix components in syneruptive-spouted type lahar deposits
Minerals were identified using X-ray diffraction (XRD). We analyzed a sample of an ash-fall deposit from the same eruption to compare it with samples of the syneruptive-spouted type lahar deposits. XRD analysis was carried out using an XRD-6000 device (Shimadzu Corporation) with a Cu tube analytical setup and operating conditions of 30 kV. A bulk sample was dried and crushed to particles of ≤10 µm in diameter, and the powdered samples were placed in aluminum sample holders and analyzed. Sample fractions of <2 µm in diameter were extracted via elutriation for analysis. We prepared an oriented sample and an ethylene glycol-treated sample for the identification of smectite.
Mineral assemblages of the ash-fall and syneruptive-spouted type lahar deposits
Syneruptive-spouted type lahar deposit (site A)
Syneruptive-spouted type lahar deposit (site D)
Indirect influence on downstream conditions
Lahars that occur after volcanic eruptions are often caused by melting snow or heavy rain (Vallance and Iverson 2015); however, neither snow nor a crater lake was documented near to the summit of Mount Ontake prior to the September 2014 eruption, although the groundwater level may have been particularly high because of heavy rain in the 2 days that preceded the eruption. Both a lahar and hot water were reportedly expelled from the Jigokudani cinder cone and the new western crater that developed during the eruption (Kaneko et al. 2016); therefore, we suggest that the lahars directly overflowed from the crater and that one reached as far as 5 km downstream. Therefore, we term the particular type of lahar investigated for this study a “syneruptive-spouted type lahar.” The syneruptive-spouted type lahar is thought to be a form of primary (syneruptive) lahar (Vallance and Iverson 2015). The syneruptive-spouted type lahar caused by the 2014 eruption was much smaller than other recorded syneruptive lahars, however, such as snowmelt type lahars or a crater lake breakout type lahar.
The mineral assemblage identified from samples of the syneruptive-spouted type lahar deposits is similar to that of the ash-fall deposits from the same eruption (Minami et al. 2016). Minami et al. (2016) estimated that ash falls were derived mainly from shallow depths, within 2 km of the surface. The syneruptive-spouted type lahar deposits are therefore inferred to also have originated from shallow depths.
Comparison of characteristics of syneruptive-spouted type lahars recorded in Japan
Lahar volume (m3)
Distance travelled (km)
1.0 × 105b
4.0 × 101c
2.0 × 103f
1.0 × 105g
Destroyed buildings and bridges
4.0 × 102i
1.2 × 105k
As of 2016, volcanic hazard maps for 42 of the 110 active volcanoes in Japan have been published by local governments; however, phreatic eruptions are noted as a potential hazard on only 22 of these maps. Furthermore, only three volcanoes (Meakandake, Hakone, and Garandake) have been noted as associated with syneruptive-spouted type lahars on these hazard maps. Predicting the occurrence of syneruptive-spouted type lahars alongside phreatic eruptions is challenging because the associated preeruptive phenomena, such as ground deformation and volcanic tremors, are very gentle and/or small in scale. In addition, a syneruptive-spouted type lahar is a small-scale phenomenon that causes little direct damage to infrastructure, although it may have long-term influence on water quality. An increase in turbidity and a decrease in pH are expected to occur downstream of Nigorisawa at Mount Ontake after every heavy rainfall event for several years. Therefore, syneruptive-spouted type lahars that could cause indirect, long-term damage should be considered in hazard mapping when volcanic disaster prevention measures are planned, as was done for the hazard map of the Iwaki Volcano after its 2014 eruption (Aomori Prefectural Government and Aomori Office of River and National Highway 2015). For other volcanoes, corrections should be made to their hazard maps to help create more effective hazard prevention plans.
HS carried out the aerial photointerpretation, field research, and hazard map review and drafted the manuscript. TC conducted field research and analysis of samples and helped draft the manuscript. HK carried out field research and the review of similar phenomena. SN performed the analysis of indirect impacts. All authors read and approved the final manuscript.
We thank Drs. W. Hirose and T. Kanamaru for their assistance in the preparation of this manuscript. We are grateful to the Kiso District Forest Office of the Chubu Regional Forest Office, Forestry Agency, for their assistance. We extend our appreciation to all staff members of Asia Air Survey Co., Ltd., for their assistance and support. This study was supported in part by the Research Subcommittee on Volcanic Engineering of the Geotechnical Engineering Committee, Japan Society of Civil Engineers.
The authors declare that they have no competing interests.
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