How did the Oki Islands come to be? Unravel the geohistory of the land that has continued for hundreds of millions of years.

How the Present-Day Oki Islands Were Created

The Oki Islands underwent various changes through time. We will explain the geohistory of this land—the origins of the unique ecosystem that is rare even from a global perspective, and of the local history and culture.


250 million years ago (Permian–Triassic)

Continental Period

Oki Islands were once part of the continent


26–6 million years ago (Neogene)

Sea of Japan Formation Period

From the "bottom of a lake" to the "bottom of a sea"


6 million–400 thousand years ago (Neogene–Quaternary )

Volcanic Islands Period

Powerful eruptions and the birth of the Oki Islands


400 thousand years ago–present

From a Peninsula to Remote Islands

Separation from Japan's main Honshu island and the present Oki Islands

Geology of the Oki Islands

The land of the Oki Islands was formed by volcanic activity that began about 6 million years ago. On the surface we can see even older layers that formed at the bottom of the sea, and layers formed deep underground when the islands were part of the continent; these provide us with information about the Earth from 250 million years ago until the present.

  • Quaternary, alluvial stratum
  • Neogene, basalt
  • Neogene, quartz syenite
  • Quaternary, basalt
  • Neogene, rhyolite and trachyte
  • Paleogene, granite
  • Neogene, trachyte
  • Neogene, sedimentary rock
  • Permian to Triassic, gneiss