The volcanic complex sits on top of ignimbrite layers, the Miocene Ujina and Pleistocene Pastillos Ignimbrites. These ignimbrites are c. and thick, the former is a welded ignimbrite that was erupted 9.3 ± 0.4 mya and the latter in two stages 0.79 ± 0.2 - 0.73 ± 0.16 mya and 0.32 ± 0.25 mya. In terms of composition, the Ujina is pink-grey crystals and pumice and the Pastillos a gray-white pumice forming the lower member and the upper member of the Pastillos contains cinerites with accessory claystones, siltstones and diatomites. Further volcanic rocks beneath Irruputuncu are hydrothermally altered dacites that may be part of an older now deeply eroded edifice.