After the Miocene, the Pastillo volcanic complex developed in the form of half-a-kilometre-thick (0.3 mi) andesitic rocks; the ancestral composite volcano that holds Huaynaputina is part of the Pastillo volcanic complex and appears to be of Miocene to Pleistocene age. It underwent sector collapses and glacial erosion, which altered its appearance and left traces on its flanks. The amphitheatre which contains the Huaynaputina vents formed probably not as a caldera but either a glacial cirque, a sector collapse scar or another kind of structure that was altered by fluvial and glacial erosion. Other extinct volcanoes in the area have similar amphitheatre structures. It is likely that the development of the later Huaynaputina volcano within the composite volcano is coincidental, although a similar tectonic stress field controlled the younger vents.