The first phase of activity resulted in the creation of Armadillo Peak seven million years ago, today represented by an eroded remnant of a small caldera flanked by steep-sided light-coloured secondary lava domes, including Cartoona Peak, Tadeda Peak, IGC Centre, and Sezill Volcano, and a thick pile of interlayered light-coloured lava flows, pyroclastic flows, air-fall pumice, and epiclastic deposits. It is the most central of the four central volcanoes and its summit is capped by of fine-grained silica-rich trachyte lava flows which were ponded inside the caldera to produce a lava lake six million years ago during its final stage of activity.