Volcanic Island Era (10 million years ago - 5 million years ago)
Violent volcanic eruptions lead to the birth of Oki
Oki was lying at the bottom of the sea. What led to this sleeping bit of land appearing once again on the Earth's surface were volcanoes. Tectonics (plate activity) caused Oki to gradually start rising, until around six million years ago, two incidents of violent alkaline volcanic rock activity caused the current formation of the Oki Islands, split into the Dozen and Dogo areas. (Refer to the charts about alkaline rhyolite and the Dozen Caldera.) Volcanic activity continued, until it finally ended around 500,000 years ago.
Volcanic activity didn't just cause the formation of the Oki Islands. It also sent us “A letter from the Earth”, a mantle xenolith. (See the mantle xenolith.)
Oki, which was lying at the bottom of the sea, gradually began to rise with plate movement. Around 6 million years ago, two cases of large volcanic activity formed the Dozen and Dogo island areas of Oki. This volcanic activity continued for about 5.5 million years.