An explosion above the water surface from the submarine Fukutoku-Oka-no-Ba volcano was observed by the Japanese Coast Guard on 3 February 2010, and eruption alerts continued into early April of that year (BGVN 35:02). Although intermittent plumes of water discoloration after that indicated some level of ongoing activity through April 2014, the last observation of pumice on the sea surface was on 14 April 2010.

This trachyandesitic volcano is located 5 km NE of the Japanese island of Minami-Ioto, about 1,275 km S of Tokyo in the Izu-Bonin arc (figures 11 and 12). The National Catalogue of the Active Volcanoes of Japan, monthly reports by the Japan Meteorological Agency (JMA), and information tables and photographs prepared by the Japan Coast Guard (JCG) and Japan Maritime Self Defense Force (JMSDF) were the data sources for this report. A summary of the observations recorded during site visits made by Japanese authorities between April 2010 and October 2015 is given here, and additional details are provided in the table at the end of the report (table 3).

Figure 11. Submarine topographic map of the Fukutoku-Oka-no-Ba area. Image date not provided but it appears to pre-date the February 2010 explosion. The island above sea level at bottom is Minami-Ioto. Fukutoku-Oka-no-Ba is 5 km N and slightly E. The NW-SE elongate submarine volcano is Kita-Fukutokutai; the Kaijin Knoll is the northernmost edifice. Courtesy of Japan Coast Guard (from National Catalogue of the Active Volcanoes in Japan).

Figure 12. Submarine topographic map of Fukutoku-Oka-no-Ba after the February 2010 eruption (specific date not provided). Courtesy of Japan Coast Guard (from National Catalogue of the Active Volcanoes in Japan).

Observations from February 2010 through April 2011. On 3 February 2010, a 100-m-high ash and steam eruption emerged from the sea. Floating pumice was later observed on 14 April 2010 (referred to as "floating objects" in the translation of JMA and JCG tables). Discolored water continued to be observed around the volcano for the rest of 2010. Activity from multiple vents created blue-white, milky-white, and greenish-brown plumes of discolored seawater a few hundred meters wide that drifted as far as 2,700 m. In January 2011, blue-white and creamy-colored water plumes from multiple vents drifted NE in a fan shape approximately 500 m across (figure 13). Discolored water plumes were also observed on 8 February and 12 April 2011.

Figure 13. Blue-white and greenish brown discolored seawater at Fukutoku-Oka-no-Ba on 19 January 2011. The source appears to be from multiple vents. The fan shape extends about 500 m NE of the vent source. Minami-Ioto is at the upper left. Courtesy of JMSDF (Japan Maritime Self Defense Force).

Observations from January 2012 through August 2012. Inspections of the volcano in May, July, August, and November 2011 revealed no discolored water plumes or floating objects at the site. A 150-m-diameter plume was visible again on 27 January 2012. Observations on 9 March and 19 April 2012 confirmed the continued presence of discolored water. On 11 May 2012, a 50 x 150 m milky-green-colored area was photographed by the JCG (figure 14). Additional observations of discolored water plumes were made on 13 June, 15 June, and 9 August 2012; no visits were made during September-November 2012.

Figure 14. Milky-green discoloration of seawater at Fukutoku-Oka-no-Ba on 11 May 2012. The dimensions of the greenish plume are about 50 x 150 m. Courtesy of TRCGH (Third Regional Coast Guard Headquarters, JCG).

Observations between December 2012 and April 2014. Green water was originating from multiple locations within a 1,000-m-diameter plume on 11 December 2012. Milky-white, bluish-white, and yellow-brown discoloration of seawater from multiple sources was again observed on 22 January 2013 (figure 15).

Figure 15. Milky-white and bluish-white discoloration of seawater from multiple vents on 22 January 2013 at Fukutoku-Oka-no-Ba. Courtesy of TRCGH (JCG); color adjusted.

Plumes were observed twice in March (8 and 26), April (10 and 21), and on 12 May of 2013 but not during visits in June and July. They were seen again on 21 August and 27 September. No visits were made in October and November, but a 1,850-m-long plume drifting E was observed on 16 December 2013 by JMSDF. The last confirmed observations of discolored water plumes were on 29 January and 28 April 2014 by the JMSDF and JCG. They continued to visit the site nearly every month until October 2015, with no further observations of discolored water plumes. No further reports were made after October.

Table 3. Summary of observations made at Fukutoku-Oka-no-Ba between March 2010 and April 2014 by Japanese authorities. The Japan Coast Guard continued to visit the site almost every month through October 2015 with no further reports of water discoloration, and it was not mentioned after that date in monthly JMA reports. Sources: JMA*= National Catalogue of the Active Volcanoes in Japan, JMA= Monthly Summary Report, Japan Meteorological Agency, JMSDF= Japan Maritime Self Defense Force, TRCGH= Third Regional Coast Guard Headquarters, JCG=Japan Coast Guard.

Reference. Japan Meteorological Agency, 2013, National Catalogue of the Active Volcanoes in Japan (Fourth Edition, English Version), Tokyo, Japan (http://www.data.jma.go.jp/svd/vois/data/tokyo/STOCK/souran_eng/menu.htm).

Geological Summary. Fukutoku-Oka-no-ba is a submarine volcano located 5 km NE of the island of Minami-Ioto. Water discoloration is frequently observed, and several ephemeral islands have formed in the 20th century. The first of these formed Shin-Ioto ("New Sulfur Island") in 1904, and the most recent island was formed in 1986. The volcano is part of an elongated edifice with two major topographic highs trending NNW-SSE, and is a trachyandesitic volcano geochemically similar to Ioto.

Information Contacts: Japan Meteorological Agency (JMA), Otemachi, 1-3-4, Chiyoda-ku Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/); Japan Coast Guard (JCG) Volcano Database, Hydrographic and Oceanographic Department, 3-1-1, Kasumigaseki, Chiyoda-ku, Tokyo 100-8932, Japan (URL: http://www1.kaiho.mlit.go.jp/GIJUTSUKOKUSAI/kaiikiDB/kaiyo24-2.htm).