Llullaillaco is a stratovolcano which rises above the surrounding terrain. The edifice has an elliptical shape with dimensions of and has a volume of about -. It is formed by thick dacitic lava flows erupted during the Quaternary. These flows extend away from the volcano and form its summit. An older unit is formed by ignimbrites and pyroclastic flows. Older lava flows extend west from the volcano and are partly buried by sediments closer to the edifice. These flows reach lengths of and form about 70% of the surface of the volcano.