Llullaillaco formed during two different phases in the Pleistocene-Holocene out of dacitic lava flows. The oldest rocks are about 1.5 million years old. About 150,000 years ago, the volcano's southeastern flank collapsed, generating a debris avalanche that reached as far as from the summit. The youngest dated rocks were erupted 5,600 ± 250 years ago in the summit region, but there are reports of activity from the 19th century.