How did the Oki Islands come to be? Unravel the geohistory of the land that has continued for hundreds of millions of years.
How the Present-Day Oki Islands Were Created
The Oki Islands underwent various changes through time. We will explain the geohistory of this land—the origins of the unique ecosystem that is rare even from a global perspective, and of the local history and culture.
Revealing Oki Islands' History: The Geological Map
The land of the Oki Islands was formed by volcanic activity that began about 6 million years ago. On the surface we can see even older layers that formed at the bottom of the sea, and layers formed deep underground when the islands were part of the continent; these provide us with information about the Earth from 250 million years ago until the present.
When you look at a geological map, you can see that the Oki Islands went through various geological phases, such as the Continental Period, Lake and Sea Period, and Volcanic Islands Period.
Continental Period
The Oki Islands were once part of a continent. We know this because of a type of rock called gneiss, which can be found on the islands. The gneiss on the Oki Islands formed around 250 million years ago, when much older rocks (dating back to approximately 2 billion years ago) were buried deep within the continental crust, where high temperatures transformed them into gneiss.
Gneiss from the same period is widely found on the Chinese mainland, suggesting that the Oki Islands were once connected to the continent.
Lake and Sea Period
As Japan began to separate from the continent, water gradually covered the land that would later become the Oki Islands, and it turned into the floor of a lake and later a shallow sea. We know this from rocks such as conglomerates, mudstones, and tuff found here. Fossils within these rocks include teeth from crocodiles and sharks, shells of clams that lived in shallow seas, and small organisms from slightly deeper marine environments. These findings indicate that the Oki Islands were once submerged under a lake and shallow sea.
Volcanic Islands Period
After the Sea of Japan was formed, the Oki Islands were part of an elevated area on the seabed. This elevated area experienced volcanic activity, leading to the creation of the Oki Islands as volcanic islands. We know this from the types of volcanic rocks that cover most of the islands, such as alkali basalt, trachyte, and rhyolite, that are representative of lava flows, intrusive rocks, and pyroclastic rocks. All of these are products of volcanic eruptions. The pyroclastic rocks reveal the structure of the area near where the eruption occurred. From this, we know that the Oki Islands went through a volcanic islands period.
Please enjoy this video* for a visual journey through the Oki Islands’ geological history.
*English subtitles available
Age
Geology
From a Peninsula to Remote Islands
400 thousand years ago to today
Terrace and current river deposits (gravel, sand, mud)
Volcanic Islands Period
4.7 million to 400 thousand years ago
Alkali basalt (lava, pyroclastic rocks)
Inner bay to river deposits (gravel, mud)
7 to 5 million years ago
Trachyte to rhyolite (lava, intrusive rocks, pyroclastic rocks)
