Former Oki Peninsula
Oki has not always been a group of isolated islands. There were periods in which the islands were connected to the mainland of Japan. The most recent occurrence was 20,000 years ago during the last glacial maximum. It is thought that they became isolated again 10,000 years ago.
This cycle of becoming a peninsula to becoming isolated islands again was not caused by plate activities but rather changes in climate that caused fluctuations in the sea-level.
During the glacial age, water that would have flowed into the sea became glaciers, and the drop in the sea temperature caused the volume of seawater to decline resulting in a decline in sea-level. The changes in sea level have become more frequent and dramatic in the last 5 million years. The most dramatic among these was the most recent glacial age. At this time the sea level declined around 100 meters over a period of 10,000 years. The depth of the Oki Strait between the islands and the mainland is around 70-80 meters today, so the Oki Islands became connected to the mainland repeatedly. Many different species of plants and animals crossed over from the mainland to Oki during this time.