Mount Aniakchak was previously a glacially eroded stratovolcano of andestic composition, with a pre-caldera volume of . Andesitic material in the volcano included basalt and dacite. The mountain collapsed, forming the present day caldera, during a major eruption of VEI=6, which left evidence in ice cores dated to 1645 BC. which was later moved to 1641 BC in the Greenland Ice Core Chronology 2005 (GICC05) time scale. Recently, McAneney and Baillie have suggested that the GICC05 ice core dates in the 17th century BC may be around 14 years too old, meaning that Aniakchak actually erupted in 1628 or 1627 BC.