The Hakkōda Mountains are two clusters of stratovolcanoes in the Northeastern Japan Arc that consists of lava flows, debris flows, and pyroclastic ejecta and flows arranged into southern and northern groups. The volcanic peaks are made of non-alkali mafic rock; mostly andesite, dacite, and basalt. Although both groups of mountains formed in the Pleistocene, the southern group is older than the northern group. The southern group is made from rock that is 700,000 to 1,700,000 years old, while the northern group is made from rock that is 13,000 to 700,000 years old along the southwestern rim of an older caldera.