The Continental Age (250 million years ago - 26 million years ago)
Oki used to be part of the Asian continent
Long ago, what is now the Oki Islands and the rest of the Japanese Archipelago was attached to the edge of the Eurasian supercontinent. It looked nothing like it does now, as it was just one more piece of land that was part of a huge land mass.
Evidence of this is found in the rocks called Oki gneiss. These are metamorphic rocks that formed deep underground around 250 million years ago, created by the intense heat and pressure there. Having been so formed, these are not normally rocks that would be identifiable on the Earth's surface, but the Oki Islands are one of the few places where they are. The history of the Oki Islands' formation includes plate movement, along with volcanic activity from six million years ago, that caused upheavals, making it possible to view these rocks that are normally found deep underground up close.
Around 250 million years ago, the surface of the Earth was covered by the ocean and a single, huge land mass known as Pangaea. The land that became the Japanese Archipelago, including what is now the Oki Islands, was part of it. As movement in the Earth's crust continued, it gradually separated from the supercontinent.