How Volcanoes Work

 Image Described in Caption
 Aniachak Caldera, Alaska -- This Crater-Lake type caldera formed by an eruption ~3500 years ago, during which time pyroclastic sheet flows spread 50 km from the caldera. The scoria cone in the right-center of the caldera is analagous to the Wizard Island scoria cone at Crater Lake. Courtesy of M. Williams, Natural Park Service.