what formed the archipelago known as japan?
Starting with plate movement that began around 26 million years ago, the Japanese Archipelago, including the Oki Islands, gradually began moving away from the Eurasian supercontinent. A large lake formed in the area between Japan and Eurasia, and what became Oki was at the bottom of this lake.s the separation from the mainland continued, the land split and let in sea water, and the lake metamorphosed into the ocean. This is the formation of the Sea of Japan. What tells us about this process are shark tooth fossils discovered in a strata that formed around 16 million years ago.