Copahue sits on a basement of sedimentary and volcanic rocks ranging in age from Eocene to Pliocene. The modern volcano sits in a volcanically active area, with a caldera from the Pliocene, measuring 20 km by 15 km, lying to the east of Copahue. The modern volcano became active roughly 1.2 million years ago (Ma). The modern caldera formed 0.6 to 0.4 Ma, and produced large pyroclastic flows, extending up to 37 km from the volcano.