Volcanoes are usually considered to be active if they have had eruptions during the Holocene. However, as demonstrated by the unexpected eruption of Chaiten volcano in Chile in May 2008, even long-inactive volcanoes can become active again. Such volcanoes can constitute a threat to regions with seemingly quiet volcanism. Ciomadul has had repose periods of over 10,000 years and possibly even longer. Zircon crystallization data imply that the magma chambers of Ciomadul were active over time spans of over 300,000 years.