Geohistory 3. Volcanic Islands Period

26–6 million years ago (Paleogene–Neogene of the Cenozoic era)

Over time, the land of the Oki Islands, which was once located deep beneath the sea, was uplifted due to tectonic plate activity. Approximately 6 million years ago, two sets of powerful alkaline volcanic rock activity took place where the current-day Dōzen and Dōgo are located, shaping the islands into their original form.

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The land of the Oki Islands created by volcanic activity

Dōzen Caldera

The Dōzen Islands comprise a prominent mountain in the center, encircled by the sea and then the islands on the outside. This geographical formation is known as a caldera, which is a volcanic depression formed by a significant volcanic eruption. Mt. Aso, located in Japan's southern region Kyūshū, is a renowned example of such a caldera.


Dōgo Island's representative rock composition is whitish rhyolite, which formed from lava approximately 5.5 million years ago. This rock constitutes a major portion of the mountains visible along the southern, western, and northern coastlines of the islands. Notably, this rock composition is similar to those found on the continent, a characteristic uncommon in Japan.

Mantle Xenoliths

Basalt rocks found along the east coast of Dōgo Island, ranging from approximately 4 to 2 million years old, contain mantle xenoliths including black pyroxene and yellow-green olivine, minerals which are found in the upper mantle. Mantle xenoliths are fragments of the Earth's interior that have been brought to the surface. These xenoliths can be considered precious geological treasures, often referred to as a "gift from the Earth," as they provide valuable insights into the composition and dynamics of the deep Earth.