Potassium-argon dating of Lastarria has yielded ages of 600,000 ± 300,000 and less than 300,000 years ago. The older date refers to the Negriales lava field, which has also been dated at 400,000 - 116,000 ± 26,000 years ago. The South Spur is dated at 150,000 ± 50,000 years ago. The main edifice started forming 260,000 ± 20,000 years ago. One andesite lava flow was dated at 51,000 ± 13,000 years ago by argon-argon dating.