The Chichima Island Group was formed by submarine volcanic activity that began approximately 48 million years ago. In addition to a unique volcanic rock called boninite, it is composed of island arc tholeiitic volcanic rocks, which demonstrate characteristics of magma that was extruded in the early stages of subduction of the Pacific Plate. (Ref.1)

It is believed that submarine volcanic activity around Chichijima subsided approximately 44 million years ago and that the island subsequently reached the surface during the Oligocene era (37.5 million to 22.5 million years ago), after which limestone was deposited by reef-building organisms. Analysis of planktonic foraminiferal fossils in the limestone layers distributed in the area from Minamijima, located southwest of Chichijima, to the Minamisaki cape of Chichijima, suggests that the limestone was deposited between 34 and 27 million years ago. The Ogasawara Islands are the largest exposed area where boninite can be observed on land. (Ref.1, 2)

Ref. 1: Umino Susumu, Nakano Shun, “Geological Map of the Chichijima Retto District,” Regional Geological Research Report (1:50,000 scale map), Geological Survey of Japan, 2007, p. 71. 

Ref. 2: Umino Susumu, “Field Guide to the Chichijima Island Group,” Earth Science Course, Kanazawa University.