How Volcanoes Work

 Image Described in Caption
Lahar at El Palmar, Guatemala (August 14, 1989) -- This rapidly moving, hot lahar was generated from eruptive activity at the site of the Santiaguito lava dome. The photo is taken 15 kilometers downstream of the volcano's summit. Before the lahar arrived the river valley was nearly dry. The event was described by USGS geologist Jeff Marso, who was standing on a bridge overlying the valley: "The lahar passed below us at what seemed an incredible speed (~50 km/hr) and with an overwhelming roar. The front of the flow was approximately 5 m high and filled the river channel. As the hot lahar became larger, small rocks and mud splatter were thrown onto and over the bridge. We decided that we were not high enough and ran for the safety of the far bank. There, ground vibrations made it difficult to stand and we had to shout to be heard over the roar." Courtesy of Jeff Marso, USGS.