Atacazo, located about 25 km SW of Quito, consists of a large Pleistocene andesitic stratovolcanoes truncated by a late-Pleistocene to Holocene caldera. Growth of the La Carcacha edifice was followed by construction of Atacazo, a stratovolcano with flank lava domes. The 6-km-wide caldera, which is breached to the SW, is partially filled by three dacitic Holocene lava-dome complexes. Two andesitic lava domes are also found on the SE flank. Several plinian eruptions have occurred at the Atacazo-Ninahuilca complex during the Holocene, accompanied by growth of lava domes. The latest dated eruption took place about 2300 years ago, forming the Ninahuilca Chico lava domes within the summit caldera. Dome formation was accompanied by Plinian explosive eruptions and pyroclastic flows that traveled 35 km down valleys to the west.