Geohistory 1. Continental Period
250 million years ago (Mesozoic–Triassic)
About 250 million years ago, there was one large continent called Pangea. At that time the Japanese archipelago, including the Oki Islands, was part of this continent, located at the edge of what is now the Eurasian Continent.
Later, the continent gradually split apart due to changes in the Earth’s crust.
Oki gneiss: proof that the Oki Islands were once a part of the continent
Oki gneiss provides us with evidence that the Japanese archipelago, including the Oki Islands, was once part of the continent.
Oki gneiss is a metamorphic rock about 250 million years old, formed by pressure and heat deep underground.
Since these rocks are formed deep underground, they cannot normally be seen on the surface of the earth. The Oki Islands are one of the few islands where you can see them on the surface. This is due to their geological history of being uplifted by tectonic plate activity and volcanic activity about 6 million years ago.