The basement beneath Ampato is formed by sedimentary and volcanic rocks of the Western Cordillera of Peru, and the rocks are of Mesozoic to Cenozoic age. A high plateau formed by ignimbrites and lavas of Pliocene to Miocene age rises above this basement. The terrain is cut by several different fault systems; one of these, the northeastward striking Sepina fault has been seismically active in the 20th and 21st centuries and seems to have controlled the development of the Ampato and Sabancaya volcanoes.