Latest Issue

2023 Vol.132 No.6

Minami-Ioto's Summit Covered with a Cloud

Minami-Ioto is an uninhabited volcanic island located at the southern end of the Izu-Bonin Arc. Although the island is only about 2 km in diameter, its summit rises 916 m, making it the highest peak in the Izu-Bonin Islands. The island is steeply conical, with sea cliffs extending along the coast.
 Geologically, it is a stratovolcano composed mainly of basaltic lava and agglutinate, with a composition intermediate between tholeiite and alkaline series. Although there is no record of historical eruptions, Fukuyama (1983) estimated the age of the volcano to be several hundred thousand years or younger, based on the normal magnetic polarization of the lava flows and dikes. Furthermore, a K-Ar age of ca. 30,000 years ago has also been reported from basaltic rocks (Nakano et al., 2009).
 Minami-Ioto is located on the southern margin of one of the largest submarine calderas in the Izu-Bonin Arc, called the Kita-Fukutoku Caldera (16 × 10 km). Based on compilation of the bathymetric data, it is now evident that the Minami-Ioto is part of a giant submarine volcanic complex measuring approximately 50 km north-south and 25 km east-west (Minami and Tani, 2023). Fukutoku-Oka-no-Ba volcano, which erupted in August 2021, was also formed by post-caldera volcanism within the Kita-Fukutoku Caldera.
 This photograph was taken during a rapid response survey cruise of the 2021 eruption of Fukutoku-Oka-no-Ba Volcano using the R/V Shinsei Maru.

(Photograph & Explanation: Kenichiro TANI;
Photographed on September 29, 2022)

References

[Fukuyama, H. (1983): Geology of Minami-Iwo volcano, Izu-Bonin Islands, Japan. Journal of Geography (Chigaku Zasshi), 92, 55-67. (in Japanese with English abstract)]

Minami, H. and Tani, K. (2023): The Fukutoku Volcanic complex: Implications for the northward extension of Mariana rifting and its tectonic controls on arc volcanism. Marine Geology, doi:10.1016/j.margeo.2023.106996.

Nakano, S., Matsumoto, A., Ohta, Y., Nakamura, H. and Furukawa, R. (2009): K-Ar ages of volcanic rocks from Kita-Iwo-To and Minami-Iwo-To Islands. Abstract of the Japan Geoscience Union Annual Meeting, Chiba, Japan 2009 May.