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Kusatsu-Shirane volcanoKusatsu-Shirane volcano is a complex of overlapping cones and 3 lake-filled craters (Karagama, Yugama, Mizugama) at the summit. It is located 150 km NW of Tokyo.
All historical eruptions have consisted of phreatic explosions from the acidic crater lakes or their margins. There are fumaroles and hot springs around the flanks of the volcano, and many rivers draining from the volcano are acid. Similar to Ijen volcano, the crater was the site of active sulfur mining for many years during the 19th and 20th centuries.
Background:The andesitic-to-dacitic volcano was formed in 3 eruptive stages beginning in the early to mid Pleistocene. The Pleistocene Oshi pyroclastic flow produced extensive welded tuffs and non-welded pumice that covers much of the east, south and SW flanks. The latest eruptive stage began about 14,000 years ago.
Yugama crater ("hot-water cauldron") contains the most active vent. Its lake is 270 m wide and up to 27 m deep and contains highly acidic (pH = 1.2 ) and salty (app. 1% salt) water.
The surface temperature of the lake is heated by vents at the lake bottom. In the interval 1910 to 1918, water temperatures reached a maximum of 100°C. Subaqueous fumaroles with molten sulfur pools exist in the center of the lake.
- Smithsonian / GVP
- B. Takano, K. Watanuki (1990) "Monitoring of volcanic eruptions at Yugama crater lake by aqueous sulfur oxyanions", Journal of Volcanology and Geothermal Research, Volume 40, Issue 1, January 1990, Pages 71-87
A small hydrothermal explosion likely occurred at Kusatsu-Shirane volcano on 7 February 1996.
Broken pieces of the ice sheet 20-30 cm in diameter were found washed ashore and discolored water was observed at the NW part of the lake. Probably, a sudden sudden discharge of fluid or a minor hydrothermal explosion was the cause.
A small eruption occurred from Yugama crater on 6 January 1989. It produced small amounts of volcanic ash on the frozen surface of the crater lake.
Kusatsu-Shirane volcano had 5 phreatic eruptions during 1982-83. Eruptions occurred on 26 October 1982, 29 December 1982, 26 July 1983, 13 November 1983, and on 10 December 1983. The water temperature at Yukama crater lake increased strongly and reached to 55.5 deg C after the first eruption, but did not increase further during the later eruptions.
(Source: GVP/monthly reports)
A phreatic eruption occurred at Mizugama Crater on 2 March 1976. A new crater 50 m in diameter and 10 m deep was formed in the NE part of Mizugama Crater.
The only event interrupting 34 years of quiet at Kusatsu-Shirane volcano between the eruptions in 1942 and 1976 was a small phreatic explosion in 1958.
An eruption occurred in 1942 and was unusual as it came from a different location than most activity at the volcano. A cluster of small craters was formed on the north flank of Mizugama Crater. A second group of craters formed on the south flank of Yugama Crater.
The largest historical eruption at Kusatsu-Shirane volcano was in 1932. It producing a 20 cm ash deposit reaching up to 2 km east of Yugama Crater.