‘Green Tuff’ is a sedimentary rock that provides us with evidence about the formation process of the Japanese Archipelago.
This green-hued rock was formed between 20 million and 15 million years ago in rivers and lakes. It is now widely distributed along the Sea of Japan side of Honshu Island. This tells us that at the time of the rock’s formation, the Sea of Japan coast was actually located in a wide area (such as a lake or river) in which sand and dirt became sediment.
In addition, the greenish colour of the rock is a result of volcanic ash contained within it. This tells us that during the time in which the rock formed, volcanic activity occurred. Moreover, fossils of animals that lived in river or lake environments have been discovered in this rock, providing information about the type of environment that existed at the time.
This rock tells us the process of formation of the Japanese Archipelago, which broke off from the Eastern coast of the Eurasian Continent.
- Plate movement caused the land to move toward the Pacific Ocean and pull apart the land around what is today the Sea of Japan.
- This land became a large lake.
- There are many volcanoes where this plate movement occurred.
- The environment at the time was warm, much like Southeast Asia today.
In Oki, Green Tuff sediments contain fossils of crocodiles, soft-shelled turtle, carp, warm-temperate species of bivalves, and relatives of pond snails.