Volcanoes may be active, intermittent, dormant or extinct depending on how often they erupt. Active volcanoes erupt often. Intermittent volcanoes erupt at regular intervals. Dormant volcanoes have been inactive for a long time. They are the most dangerous because they are merely ‘sleeping’ and can erupt without warning. Extinct volcanoes have not erupted for thousands of years. It is difficult to distinguish between dormant and extinct volcanoes because some volcanoes may remain quiet for a long time before suddenly becoming active again.