rock formations in the region include the mesozoic yura group sediments, the toquepala intrusive rocks, the 30-24 million years old tacaza volcanic rocks, the 24-10 million years old huaylillas volcanic rocks and the 10-3 million years old barroso volcanic rocks; purupuruni is considered to be part of this volcanic formation. purupuruni and neighbouring volcanic centres have formed on the cretaceous sedimentary basement.