Here in the Geohistory Zone, you can learn how the Oki Islands were formed. You can also find out how the Japanese archipelago formed, as well as discover about global environmental changes and the interior of the Earth itself.

Take a look at the explanation panels in front of you. The Japanese archipelago moved to its present location about 15 million years ago. Originally part of the Eurasian continent, Japan was separated from the continent by tectonic plate movement and volcanic activity about 28 million years ago.
The land continues to move even today. In fact, the islands of Hawaii are moving closer to Japan by 12 centimeters every year; it is thought that 100 million years from now, they will collide with the Japanese archipelago.

As the land moved and Japan began to separate from the continent, the land stretched and a depression or dip-like area formed in the middle. Fresh water accumulated in this depression, and over time it formed a large lake. At this time, the land that would one day become the Oki Islands was at the bottom of the lake. Then, the land was further stretched, and the Japanese archipelago completely separated from the continent. About 17 million years ago, sea water flowed in to the space between the new islands and the continent, forming the Sea of Japan. At this time, the land of Oki was still at the bottom of the deep sea. From there, it slowly rose over a long period of time, and about 6 million years ago, two major sets of volcanic activity created the Dōzen and Dōgo Islands.

As you travel around the Oki Islands, evidence of these various stages and time periods is all around you. Even the rocks you see by the roadside are important geological features, providing evidence of the continental period, the lake period, and the Sea of Japan formation period.

Oki Gneiss

Take for example the gray-colored, flat rock sample on display. This rock is called “Oki gneiss,” and serves as important evidence that this land was once part of the Eurasian continent. Oki gneiss was formed about 250 million years ago. Gneiss that formed in the same time period is found not only in Japan, but in China as well, showing us that the Japanese archipelago was once part of the Eurasian continent.

At the time Oki gneiss formed, about 250 million years ago, all of the current continents were combined in one large supercontinent called Pangaea. The sand and mud from which the gneiss originally formed are about 2 billion years old. This accumulated material was carried to a depth of 15 kilometers underground by the movement of tectonic plates, where it underwent great pressure and high temperatures of approximately 800 degrees Celsius. This caused it to change into gneiss, which is a type of metamorphic rock: a rock that has changed from one form into another.

The fact that this Oki gneiss now sits high above ground suggests that it rose from the bottom of the deep sea due to the movement of tectonic plates, and was lifted up by volcanic activity. In the Oki Islands, this gneiss is used as gravel, so as you walk around the islands, most of the rocks under your feet are actually Oki gneiss.

Green Tuff

Next notice the bluish-green rock sample on display. This is called “green tuff”; it is about 20 million years old, and it serves as evidence that the land of Oki was once at the bottom of a lake. Crocodilian footprints and other fossils have been found within green tuff rock layers along the Sea of Japan. The fossilized bone of a crocodilian has also been found in the green tuff in the Oki Islands; from its size, it is estimated that the animal was about 7 meters long. The fact that there was a crocodilian here indicates that this area was not a sea but a lake at that time, and also that the climate of the time was subtropical, like that of Southeast Asia.

Yesso Scallop Fossil

Also on display, you can see a 10-million-year-old yesso scallop fossil. From the discovery of this fossil, we know that this area was not a lake but a sea at that time, and that its environment was cold like that of Hokkaido. Through fossils like these, we can see evidence of the Earth’s changes in temperature, repeatedly getting colder and warmer every 100,000 years.

Mantle Xenoliths

In addition, the Oki Islands provide us with knowledge about the Earth’s interior, through rocks called “mantle xenoliths.” In order to examine the interior of the Earth, one must take samples through a drilling method called “boring.” But even with this method, the Earth’s crust is about 30 kilometers thick, and human technology has only been able to drill down to 13 kilometers below the surface.

However, in the Oki Islands, we can see samples from the Earth’s mantle deep underground, in rocks lying right on the coast. These islands were formed by intense volcanic activity. During these volcanic eruptions long ago, the lava carried parts of the Earth’s mantle with it, bringing it up to the surface. Take a look at the sample on display and notice the green minerals inside the black rock. This is called a “xenolith.” These green minerals came from the Earth’s mantle.

Please proceed to Zone 2: Unique Ecosystem.