Mount St. Helens produced the United States’ deadliest volcanic eruption in 1980. On 18 May, a magnitude 5.1 earthquake triggered the largest recorded landslide in history and removed the volcano’s northern flank.
The collapse uncorked the magma system and generated a lateral blast that devastated forests across a wide area. A towering ash column rose into the atmosphere, while lahars travelled down river valleys. Fifty-seven people and thousands of animals died, and major infrastructure was damaged.
Before the eruption, scientists observed earthquakes, steam explosions, and a growing bulge on the northern slope. These warning signs led authorities to restrict access, although some people remained within the danger zone. The event transformed the volcano from a symmetrical peak into a horseshoe-shaped crater.
Mount St. Helens belongs to the Cascade Range, a chain formed above the Cascadia subduction zone. It has erupted several times since 1980, including a period of dome growth and lava extrusion during the 2000s.