Eruption of the Ontakesan, 27th September, 2014
launched:30th September, 2014

Volcanic ash of the 27 September 2014 eruption at Ontake Volcano
(Volcano Research Center: Setsuya Nakada)
Field inspection on volcanic ash from the eruption which started at 11:52, 27 September was carried out and the ash collected was investigated under the microscope to understand the characteristics of this eruption. No juvenile component was confirmed. The eruption on 27 September is considered to be a phreatic event.
On site investigation
During 5 to 9 PM, 27 September, and after 9 AM on the next day, inspection on ash fall deposits was conducted at Kiso town in Nagano prefecture. A volcanic ash layer which covered cars parked in the Ropeway-station became hard like cement by being dried out. Wet, fine ash particles covered tree leaves stickily along the road. This indicates that the ash was wet when it started falling and gypsum that was crystalized when dried, had increased the consolidation of the accumulated ash particles, which is often observed in phreatic eruption. Ash had been deposited partly as aggregates on the road. According to the interview to the climbers who returned to the parked cars in the evening, these ash is considered to be accumulated within about 1hour from the first eruption.

(Research group: ERI, U-Tokyo, Mount Fuji Research Inst., Tokyo Inst. Technology, Hokkaido Research Organization, Izu-Peninsula Geopark)

Volcanic ash analysis

Volcanic ash which was collected in the ropeway station on the 27 September evening was inspected under the microscope, after cleaning with the super-sonic washer. The ash consists almost of altered lava (andesite-dacite) particles and crystal fragments (photos below). Non-altered lava particles are rare. Tiny crystals of pyrite are attached on the altered particles. No juvenile particles (fresh foamy, glassy particles) were confirmed.

Considering the testimony of climbers and the above result, the possible eruption sequence is as follows. A reservoir of hydrothermal fluid which located beneath southwest of the summit was heated (or depressurized), leading to expansion and burst of the reservoir. The eruption started such that ash cloud boiled over from newly-formed craters, being accompanied by ash cloud surges which moved along the valley to the south. Soon after, ash cloud column rose upward. This phenomenon is similar to that observed at Miyakejima volcano on 29 August 2000; where ash cloud boiled over from the summit caldera and moved slowly on the northern slope of the mountain to the sea
(Nakada et al., 2005 Bull. Volcanol. 67:205-218).
ontake_0929 ontake_0929_b-300x225
Microphotographs of volcanic ash from the 27 September eruption at Ontake Volcano. Ash particles are dipped in water after cleaning. Many small blight spots on the particles are pyrite. The photo widths are about 2.5 mm (above) and about 2 mm (below)

Ash fall distribution and amount of the Ontakesan eruption on 27th September 2014
(Volcano Research Center: Fukashi Maeno)
The eruptive volume on 27th September is estimated to be 60-110 million ton (23-44 ×104 m3 in DRE, dense rock equivalence). As from the eruptive volume and analysis of volcanic ash component, the eruption on 27th September is considered to be a phreatic event similar to that of in 1979. The eruptive volume is estimated by using the relation between ash fall volume or mass per unit area and the area covered by ash (ex: Rose et al. 1974; Takarada et al, 2001; Maeno et al. 2014), based on the field survey. For thickness/weight conversion, deposit density of 1000 kg/m3was applied.
(Research Organs: University of Tokyo, Mount Fuji research Institute, Tokyo Institute of Technology, Hokkaido Research Organization, Izu Peninsula Geopark)
Fig.1: Isomass map of ash fall of the eruption on 27th September 2014
fig.2: Relation between ash fall volume or mass per unit area and the area covered by ash