The volcanic complex is formed mainly by aa and block lava flows that rarely reach lengths of about ; additionally pyroclastic flows and tephra occur. The volcanic complex covers an area of about and has a present-day volume of about ; this makes Chachani one of the largest volcanoes of the Andes. Glacial erosion and landslides have affected the volcanic complex, forming cirques and U-shaped valleys and removing much of the original shape of the individual volcanoes. Some moraines have been overrun by lava flows.