Report on Ontakesan (Japan) — 1 July-7 July 2015
Smithsonian / US Geological Survey Weekly Volcanic Activity Report,
1 July-7 July 2015
Managing Editor: Sally Kuhn Sennert
Please cite this report as:
Global Volcanism Program, 2015. Report on Ontakesan (Japan). In: Sennert, S K (ed.), Weekly Volcanic Activity Report, 1 July-7 July 2015. Smithsonian Institution and US Geological Survey.
35.893°N, 137.48°E; summit elev. 3067 m
All times are local (unless otherwise noted)
JMA reported that activity at Ontakesan continued to decline during 26 June-3 July, although the volcano continued to be seismically active and emit white plumes to less than 1 km high. The Alert Level was lowered to 2 (on a scale of 1-5).
Geological Summary. The massive Ontakesan stratovolcano, the second highest volcano in Japan, lies at the southern end of the Northern Japan Alps. Ascending this volcano is one of the major objects of religious pilgrimage in central Japan. It is constructed within a largely buried 4 x 5 km caldera and occupies the southern end of the Norikura volcanic zone, which extends northward to Yakedake volcano. The older volcanic complex consisted of at least four major stratovolcanoes constructed from about 680,000 to about 420,000 years ago, after which Ontakesan was inactive for more than 300,000 years. The broad, elongated summit of the younger edifice is cut by a series of small explosion craters along a NNE-trending line. Several phreatic eruptions post-date the roughly 7300-year-old Akahoya tephra from Kikai caldera. The first historical eruption took place in 1979 from fissures near the summit. A non-eruptive landslide in 1984 produced a debris avalanche and lahar that swept down valleys south and east of the volcano. Very minor phreatic activity caused a dusting of ash near the summit in 1991 and 2007. A significant phreatic explosion in September 2014, when a large number of hikers were at or near the summit, resulted in many fatalities.