How Volcanoes Work

Image Described in Caption
 Mt. St. Helens Steam Eruption -- This April 10, 1980 eruption of heated groundwater is typical of many such precursor eruptions that occurred near the summit of Mt. St. Helens from late April to early May. Photo is of the northeast flank of the volcano. The northern bulge of the volcano is also evident. Courtesy of Don Swanson, USGS.