Naruko

The Naruko volcano was formed through two pyroclastic flows, which occurred 73,000 and 45,000 years ago. The first lava domes were formed approximately 20,000 years ago. The rocks produced by the pyroclastic flows and lava domes vary in composition, but are generally dacitic. Subsequent volcanic activity continued to create more lava domes, while also resulting in explosive eruptions that destroyed such domes. Tephra created by an explosive eruption approximately 18,000 years ago is said to be related to the formation of Katanuma lake. The lake is located in the center of the volcano, and is known as one of the most acidic lakes in Japan, with a pH around 1.6.