Type: Composite volcano with lava dome
On May 18, 1980, U.S. Geological Survey (USGS) scientist David Johnston was camping at an observation post near Mount St. Helens. From a base of operations in Vancouver, Washington, the USGS was monitoring the volcano, aware of the danger for a large eruption.
At 8:32 a.m., Johnston radioed the base: "Vancouver, Vancouver, this is it!"
The picturesque Cascade Range peak had erupted. The violent blast killed Johnston and 56 other people and tore off nearly 1,300 feet (400 meters) of the volcano's top. More than 230 square miles (600 square kilometers) of surrounding forest were devastated.
Volcanologists weren't surprised. The volcano, less than about 37,000 years old, had been especially active over the last 4,000 years. Eruptionsusually explosivehad occurred at a rate of about one every hundred years. Before 1980, the last eruption happened 130 years previously, and scientists were alert for the explosion.
A few months before the May eruption, a series of earthquakes began to shake the volcano. Several small steam eruptions occurred, new fissures appeared, and a bulge developed on the north flank of the volcano.
The bulge got larger and larger. A week before the big eruption, it was expanding at a rate of about seven feet (two meters) per day.
When the big blast occurred, the entire northern slope of the volcano above the bulge slid downward. The huge landslide sparked a hydrothermal blast that swept through the surrounding forests, snapping trees like twigs.
In addition, a column of volcanic rock and lava fragments rose from the volcano's summit to a height of 12 miles (20 kilometers). The eruption lasted nine hours and spread debris as far as the Great Plains.
The eruption destroyed the volcano's peak, replacing it with a horseshoe-shaped crater. Since the 1980 blast smaller eruptions have built a lava dome over the vent.