The main Aucanquilcha volcano formed in four stages. Between 1.04-0.92 mya the bulk of the volcano formed in the Azufrera stage. One lava flow toward the southwest is unusually long at . A vent at altitude fed most of this edifice; a second vent on the northern flank at the summit generated three lava flows, two shorter ones and a long one to the northwest. The Azufrera stage lavas are blocky dacites with large clasts and flow fronts up to high. These flows are moderately altered and have oxidation rinds. There is little evidence of explosive activity, but it may have been obscured by glacial erosion. The total volume is , indicating a flux rate of . This volcano was probably an isolated cone, but the existence of a previous stage cannot be excluded.