Geohistory - Rocks from Volcanic Islands Period
Basalt is the most prominent type of rock brought about by the volcanic activity that created the Oki Islands around 6.5 million years ago. Basaltic volcanism continued intermittently until 2.8 million years ago on the Dōzen Islands and 400 thousand years ago on Dōgo Island, creating the land of Oki. While many basaltic rocks are black as they are rich in colored minerals, they also feature reddish-brown layers where the lava oxidized. With recurring volcanic eruptions, the combination of oxidized and non-oxidized basalt created a striped pattern we can see today in places like the cliffs of Nishinoshima Island's Kuniga Coast. One characteristic of Oki basalt is its strong alkalinity. The rocks with the highest ratio of alkaline components are called "trachybasalt."
The Dōzen Caldera, the flatland in Nakanoshima Island, the lava plateau in Dōgo Island that is home to the Oki Airport—these geological features of the Oki Islands were created by both basalt and the volcanic activity that produced it. These features are deeply intertwined with lifestyles of the islanders.
