Massive Chokaisan volcano is composed of two overlapping stratovolcanoes with a conical profile that inspired the local names of Akita-Fuji or Dewa-Fuji. The summit of the gently sloping western edifice (Nishi-Chokaisan) is cut by a large scarp open to the south that contains lava domes. The younger eastern volcano (Higashi-Chokaisan) began forming about 20,000 years ago, and also has a large scarp, breached to the north. During an eruption about 2,600 years ago it was the source of the voluminous Kisakata debris avalanche, which reached the Pacific coast. Two post-caldera lava domes have been constructed at the upper SE end of the caldera. Intermittent reports of eruptions date back to the 6th century CE.