The oldest volcanic rocks in the Pasto Ventura region are of Miocene age. Among the old volcanism are basaltic andesite lava flows in the eastern part of the field; they are partially buried by sediments and have lost their surface features. Other volcanic centres are well preserved despite their age, as erosion rates are low on the Puna plateau. The faults include both faults that began to form in the Quaternary and older faults which were later reactivated.