The Ogasawara Islands is an island group formed on an oceanic crust, with Minamitorishima on the Pacific Plate and the other islands on the Philippine Sea Plate. The Ogasawara Island Group comprised of Mukojima, Chichijima and Hahajima Island Groups, and Kazan (Iwo) Island Group are located on the Izu-Ogasawara Arc, which formed by the subduction of the Pacific Plate beneath the Philippine Sea Plate. The Izu-Ogasawara Arc is an island arc-trench system extending approximately 1,500 km and 400 km wide, and is an oceanic island arc comparable in size to the Honshu Arc. Consisting of three ridges (underwater mountain ranges), namely the Ogasawara Ridge, the Shichito-Iwoto Ridge, and the Nishi-Shichito Ridge, the Ogasawara Island Group are located on the Ogasawara Ridge, and the Kazan Island Group is located on the Shichito-Iwojima Ridge. Of these, the Ogasawara Ridge is a submarine volcano that was active during the Paleogene era and rose to the surface, while the Shichito-Iwoto Ridge is the present volcanic front where active volcanoes are aligned north to south. (Ref. 1)


Elucidating the formation mechanism of the continental crust is extremely important for understanding the history of the earth and is therefore attracting attention from around the world. (Ref. 2) 

Ref. 1: Umino Susumu, “Lecture Guide to the Nature of Ogasawara: Geology of the Ogasawara Islands,” Geology of the Ogasawara Islands 2010 edition, Kanazawa University Graduate School of Natural Science and Technology.

Ref. 2: Takahashi, N., S. Kodaira, Y. Tatsumi, M. Yamashita, T. Sato, Y. Kaiho, S. Miura, T. No, K. Takizawa and Y. Kaneda, Structural variations of arc crusts and rifted margins in the　southern Izu-Ogasawara arc-back arc system. Geochem. Geophys. Geosyst., 10（2009）Q09X08, doi: 10.1029/2008GC002146.