Mount Mazama, part of the Cascade Range volcanic arc, was built up mostly of andesite, dacite, and rhyodacite over a period of at least 400,000 years. The caldera was created in a massive volcanic eruption between 6,000 and 8,000 years ago that led to the subsidence of Mount Mazama. About of rhyodacite was erupted in this event. Since that time, all eruptions on Mazama have been confined to the caldera.