Volcanic ash and gases spew from the central lava dome of Mount St. Helens in this photo from 1982.
Latest eruptive activity in 2008; massive eruption in 1980!
Along with Mount Lassen in the state of California, Mount St. Helens is one of two volcanoes that erupted in the continental United States during the 20th century. Mount Lassen erupted in 1915.
The deadliest and most economically destructive volcanic event in the history of the USA was an explosion of Mount St. Helens, a stratovolcano in the Cascades Mountain Range, on May 18, 1980. This eruption, of what was Washington’s fifth-highest peak, reduced the mountain’s elevation from 9,677 feet (2,950 meters) to 8,365 feet (2,550 meters). In total, one explosion of this volcano eradicated the top 1,308 feet (400 meters) of the mountain, leaving a crater 1.2 miles (1.9 km) in diameter.
Note: In this article I use a few basic volcanology terms, which are defined in Types of Volcanoes and Volcanology Terms.
Before and after the major eruption of May 1980.
The May 1980 eruption
In March 1980 there was a magnitude 4.2 earthquake originating within the mountain, and a week later steaming vents opened up. During April the north side of the volcano began to bulge. Then suddenly on May 18th, a magnitude 5.1 earthquake caused the north side of the mountain to collapse, creating the largest debris avalanche in recorded human history. The magma within the mountain became exposed and exploded violently, flattening vegetation, houses, and everything over a 230 square mile (596 square km) area.
The explosion produced a tsunami wave across Spirit Lake that was 853 feet (260 meters) high. Mud and debris flows traveled down rivers destroying bridges, and ash spewed from the volcano for nine hours, reaching more than 12 miles (19 km) high in the atmosphere. Ash rained down on cars, roofs, and everywhere over a wide area, as far as Edmonton, Alberta in Canada, which is over 600 miles/965 km from Mount St. Helens.
Over the next several days several more smaller explosions occurred, and the crater that was formed, where the mountain peak used to be, is about 1 mile by 2 miles (1.6 km by 3.2 km) in size, and about half a mile (800 meters) deep. The north end remains opened in a huge breach from the initial massive explosion.
The energy released by the main explosion was the approximate equivalent of 7 megatons of TNT, also comparable to the energy released in a magnitude 7.8 earthquake. This is also about the same as six modern nuclear bombs, or about 400 times the force of the nuclear weapon dropped on Hiroshima, Japan during World War II.
Damages included 57 people killed, many of whom had thought they were a safe distance from the mountain, over 200 homes destroyed, 185 miles (298 km) of highways destroyed, and 15 miles (24 km) of railways destroyed. If the eruption had occurred on a different day (it occurred on a Sunday), likely many more lives would have been lost. Loggers in the area, for example, had the day off.
Since this event occurred, smaller eruptions and lava dome building have occurred repeatedly, the latest being in 2008. The area is now Mount St. Helens National Volcanic Monument, and the forests in the region are returning to normal. However, much of the mountain remains closed to hikers and visitors.