Oki Gneiss (a kind of metamorphic rock) was formed 250 million years ago, and is the oldest rock in the Oki Islands. However, it isn’t just an old rock. It is a precious memory of the earth that provides us with information about how the Japanese Archipelago was formed.
In Japan, the same type of rock is also found in Hida of Gifu Prefecture (Hida Gneiss). By looking at the age and locations of these two gneisses, we can determine that they are like the ‘backbone’ of the Japanese Archipelago.
Oki Gneiss can be found in a variety of colours, making it rather difficult to identify. It can be black and white striped, white, light-green, or mottled red. However, the northeastern mountains of Dōgo Island were formed by Oki Gneiss, and it can be easily observed in the cliffs surrounding the Choshi Dam. It is also commonly utilized as gravel or landfill in both Dōgo and the Dōzen Islands.
This apparently ordinary rock is actually an important piece of history that records the formation of the Japanese Archipelago. It was formed during the time when the Japanese Archipelago was still part of the Eurasian Continent (at the time Pangaea Supercontinent), from earth and sand that was carried deep underground. High temperature and pressure within the earth’s crust caused the components of the rock to partially melt and harden, eventually becoming gneiss.
This geological history was made evident by components within the rock, including particles of sand of the original rock, and minerals that were formed at high heat and pressure. By examining these two elements we can identify that the original rock layer was formed between 350 million to 250 million years ago. This is evident because the most recently formed minerals within the rock were formed at 350 million years, telling us that the original rock must have been formed after this time. We also know that around 250 million years ago the rock was subject to high temperature (at the hottest 800 degrees) and pressure about 15km underground.
Another piece of history is recorded within this rock. The original rock layer contains particles of zircon, a mineral formed 3 billion years ago (only 150 million years after the Earth was born!). This tells us that Oki gneiss was formed close by to incredibly old continental rocks. Among minerals, zircon melts at particularly high temperatures and did not melt at the temperature and pressure that created gneiss. Thanks to a technique that allows us to determine the year in which mineral crystals formed (fission track technique) we can discover when it was formed.
* Oki gneiss is a type of metamorphic rock. Metamorphic rocks are formed from an original rock (protolith) that is subject to high temperatures (lower than at which magma is formed) and pressure underground, causing some of the components of the rock to melt and later harden, transforming from the original rock. Because of this, we can determine both the temperature at which the rock formed and the original rock it metamorphosed from.