How Volcanoes Work

 Image Described in Caption
 Pumice flow, Mt. St. Helens -- The photo of this small ignimbrite was taken one day after its deposition on October 22, 1980. Although the surface of the flow is dominated by whitish blocks of pumice, ash dominates the interior matrix beneath the surface. The darker surface beneath the ignimbrite is the top of the ignimbrites deposited five months earlier, during the climactic eruption of Mt. St. Helens on May 18, 1980. Courtesy of USGS.