Basalt is the most prominent type of rock brought about by the volcanic activity that created the Oki Islands 6.5 mln years ago. Basaltic volcanism continued intermittently until 2.8 mln years ago in Dozen, and 400 000 years ago in Dogo Island, creating the land of Oki. While many basaltic rocks are black as they are rich in coloured minerals, they feature reddish-brown oxidated strata. With recurring volcanic eruptions this strata created a striped pattern we can see today in the sheer cliffs of Kuniga Coast. A characteristic of the Oki basalt is its strong alkalinity. The rocks with the highest rate of alkaline components are called trachybasalt.
Dozen caldera, flatland in Nakanoshima Island, lava plateau in Dogo Island with the Oki Airport - both basalt and volcanic activity that produced it created geological features of Oki, intertwined with lifestyles of the islanders.