Dōzen Caldera
The Oki Islands were formed gradually over a period of around six million years by two massive stratovolcanoes that erupted in the Sea of Japan. Approximately10 million years before the volcanoes formed, the land that would eventually become the islands was at the bottom of the Sea of Japan. Tectonic activity slowly raised the ocean floor, from a depth of 1,000 meters to only 30 meters from the surface. The tectonic movement created two huge volcanoes, and as they erupted, the hardened lava accumulated, and the volcanoes gradually grew larger. Eventually, the volcanoes’ tips reached past the surface of the water, creating the islands of Dōzen and Dōgo. At some point over the following 500,000 years, the western volcano collapsed in on itself, forming the great caldera of Dōzen.
Often mistakenly called “craters,” calderas are circular depressions that form around the mouths of volcanoes. They often form after an eruption, when the magma chamber within the volcano is emptied, creating a subterranean hollow area that caves in on itself. After the Dōzen caldera formed, subsequent eruptions piled more lava near the caldera’s center, forming a central pyroclastic cone that is known today as Mt. Takuhi (451.7 m). Eventually, seawater rushed in to fill the caldera, submerging everything but Mt. Takuhi and the outer rim.
In the 30,000 years since humans settled on the islands, the Dōzen caldera has been a shelter from high winds and the waves of the outer seas. For many centuries, the sacred fire at the shrine on Mt. Takuhi served as a beacon for ships seeking safe harbor. From this association, the deity of Takuhi Jinja Shrine has come to be revered for protecting sailors and guiding them safely into port.
