Cerro Bayo is a complex volcano of partial Holocene age located along the Chile-Argentina border. An older center to the south contains a poorly preserved, 800-m-wide crater. A younger northern center along the national border has a well-preserved 400-m-wide crater and produced lava flows to the north and NE. The summit of the complex, located west of the border in Chile, is the source of two viscous dacitic lava flows that traveled to the north and represent the most recent activity of the complex.