Cerro Azul, just south of Descabezado Grande volcano, is part of the Descabezado Grande-Cerro Azul eruptive system, a volcanic field that comprises its two large namesake volcanic edifices and several smaller vents, including 12 Holocene calderas. Both volcanoes lie on top of the Casitas Shield, a plateau built of over 100 lava flows that erupted in at least 12 volcanic episodes during the Quaternary period. The upper lava layers are dated at 340,000 years.