How Volcanoes Work

 Image Described in Caption
Obsidian flow -- The Long Valley Caldera, California, was created by crustal collapse associated with an explosive eruption about 650,000 years ago. Since that time, felsic eruptions of de-gassed magma have generated viscous rhyolitic domes and short felsic flows. The example shown in the foreground is Obsidian flow, which erupted about 600 years ago. The flow is about 1.5 km wide across the photo. Wilson Butte, left of Obsidian flow, is a felsic lava dome that erupted about 1400 years ago. Courtesy of S.R. Bradley, USGS.