By Sid Perkins
On May 18, 1980, Mount St. Helens blew its top. After the northern face of the mountain slumped away in the most massive landslide ever recorded, molten rock gushed up from reservoirs several kilometers below the exploding peak. Within minutes, a plume of volcanic ash surged more than 20 km into the sky. Immense flows of mud and ash covered 62 square kilometers, devastated forests, rivers, and lakes, and killed 57 people.
Contrast that event with the latest activity of Hawaii’s Kilauea volcano, an event that began early in 1983 and is still under way. Although the mountain occasionally spews fountains of lava hundreds of meters high or burps small clouds of ash, the molten rock it exudes usually just flows down the slopes. Sometimes, the magma races along in torrents, but at other times the flow is so slow that scientists can safely walk up to the searing trickle and pry away fresh samples of congealing rock.