America´s most prominent volcanoes-Mount St. Helens in Washington State and Kilauea in Hawaii-might, at first glance, seem similar: Both attract tourists, and both erupted in the past 25 years. But their destructive powers come from quite different volcanic origins.
The erupting vents beneath Kilauea´s surface carve an elaborate lava-tube system into the Hawaiian countryside. The thin basalt lava from the volcano´s vents flows long distances under the cooled surface crust to produce the gradual slopes characteristic of shield volcanoes. The steady lava flow from 22 consecutive years of activity creates an ever expanding network of pathways-and hazards for nearby workers-across the flattened dome of the 4,190-foot volcano. Luckily, the lava from Kilauea´s constant bursts falls almost entirely on its eastern side, descending into the Pacific Ocean rather than on Hawaiian towns.
Set off by a 5.1-magnitude earthquake, Mount St. Helens´s violent eruption on May 18, 1980, launched landslides, lava and smoke. The landslide left an opening through which 540 million tons of bottled-up ash exploded across 22,000 square miles east of the volcano. Flowing lava quickly followed, destroying almost all the wildlife in the 230-square-mile forest surrounding the 8,300-plus-foot summit. Typical of a stratovolcano, Mount St. Helens´s thick magma walls trap expanding volcanic gases within the central vent, creating enormous internal pressure. When the stiff magma is disrupted by seismic activity or can no longer contain the gases, the volcano can suddenly explode. Despite recent rumblings, Mount St. Helens hasn´t celebrated the 25th anniversary of its famed eruption with a reprise of the powerful fireworks-yet.