![](https://www.oki-geopark.jp/wp/wp-content/uploads/2023/03/Kuroshima-Island-Mantle-Xenoliths-900-1.jpg)
Geohistory - Rocks from Volcanic Islands Period
Geosites / Okinoshima Town
The small island you can see offshore is called Kuroshima Island. It is the remains of a volcanic crater that erupted around 3.5 million years ago. The whole island contains systematic, vertical cracks called columnar joints. Mantle xenoliths are also found here.