Geologic Background:
The andesitic Fukutoku-okanaba (Shin-Iwo-jima)
volcano is one of the most active submarine volcanoes in
Japan. The volcano is located 5 km NE of the pyramidal
island of Minami-Iwo-Jima, a Quaternary stratovolcano,
and 55 km SSE of Iwo-Jima.
Historic Activity:
- At least 20 submarine eruptions between 1904 and
1985.
- Water discoloration is frequently observed, and
20th-century eruptions resulted in the formation of
ephemeral islands (Shin-Iwo-Jima) in 1904, 1914, and
1974.
Recent Activity:
- Discolored water at the site was reported 18 times
from December 1983 to October 1985.
- Eruption in mid January 1986 built an island about
700 meters long, 500 meters wide, and 15 meter high. By
mid-March 1986, wave action had eroded the island below
sea level.
- Submarine eruptions were marked by discolored water
and spouting seawater during July to September 1987, and
again in August and September 1988.
- Discolored water from January to May and in September
and November of 1990.
- Discolored water in February, September, and
October1991.
- On Nov. 10, 1992, a dark plume was observed by a
fishing boat. Later overflights observed no activity but
pumice and water discoloration were seen. Smaller areas
of discoloration were seen on Nov. 12 and 18.
- Discolored seawater over an area a few hundred meters
wide was seen during monthly aerial observation in
February, March, June, and September 1993.
- Discolored seawater was reported on Nov. 25, 27, and
28, 1995.
- Discolored seawater was reported on Jan. 12, 22, and
23, 1996. Again on April 4, 6, 7, 12, and 30. On April 12
the plume was about 4 km long and 200 m wide. More
reports in first half of May and again on Sept. 6 and 18.
Another report on Dec. 12
Data Sources:
- Smithsonian's SEAN Bulletin (V. 10, No. 10 to V. 14,
No. 7)
- Global Volcanism Network (V. 15, No. 12 to V. 22, No.
1).
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