Ciomadul has been active for over half a million years, with the oldest activity between 1,000,000 and 750,000 years ago forming lava domes. Older estimates indicate that activity did not start before 250,000 years ago, while more recent research indicated a start of volcanism over 600,000/ 850,000 years ago. Volcanism at Ciomadul consisted mostly of the extrusion of lava domes, their collapse forming block-and-ash flows and subplinian and Vulcanian eruptions.