Geologically this region is in the Fossa Magna, a large depressed zone that divides northeastern from southwestern Japan. It consists of sedimentary, volcanic, and pyroclastic rocks, and has numerous volcanos and hot springs. Mount Asama, at the south end of a chain of volcanos, is an active volcano that has erupted repeatedly in modern times. A large lava field on its north side dates from an eruption in 1783.