Geohistory - Rocks from Continental to Sea Periods
Green tuff, found on the east coast of Dōgo Island and on Nishinoshima Island, is one of the rocks that help us understand how the Sea of Japan and Japanese archipelago were formed. This green rock, formed 20–15 million years ago in the lake and the sea, is found across a wide area of the west coast of Honshu. Its color comes from a mineral called chlorite; during volcanic activity, the minerals in the volcanic ash were altered when they came in contact with hot water. From this rock, we can see that during the formation of the Sea of Japan, the area between the continent and the Japanese archipelago had many active volcanoes.
