How Volcanoes Work

 Image Described in Caption
 Lahar from Mt. St. Helens -- On March 19, 1982, an explosive eruption on Mt. St. Helens generated a 14-kilometer-high eruption plume. Melted snow from the eruption produced the dark-colored lahar seen in this photo. Part of the lahar flowed into Spirit Lake (lower left). However, most of the lahar flowed into the North Fork of the Toutle River valley (right), eventually reaching the Cowlitz River, 80 kilometers downstream. Courtesy of Thomas J. Casadevall, USGS.