The volcanic group that forms the main part of the Hahajima Island Group became active approximately 45 million years ago, slightly later than the volcanic activity on Chichijima. The islands are predominantly composed of eruptive material from submarine volcanoes that erupted on the shallow sea floor, and partly composed of lava that erupted on land. The igneous rocks of the Hahajima Island Group are mainly andesitic of the island-arc tholeiite and calc-alkaline types, with basalt and dacite. Boninite characteristic of Chichijima and other islands is not seen here, and magma is thought to have formed deeper underground than boninitic magma. Following the subsiding of volcanic activity, reef-building organisms were deposited near the Sekimon area and formed limestone. Sekimon limestone contains foraminiferal fossils from the late Eocene to Oligocene eras (41-34 million years ago). (Ref.1, 2)

Ref. 1: Umino Susumu, Ishizuka Osamu, Kanayama Kyoko, “Geological Map of the Hahajima Retto District,” Regional Geological Research Report (1:50,000 scale map), Geological Survey of Japan, 2016, p. 46.

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