Mount St Helens blew off its summit in May 1980 with the energy of 20,000 Hiroshima-size atomic bombs. The resulting rock blast and mudslides killed 57 people and around 7,000 large animals, engulfed 200 houses, choked rivers, buried highways and flattened trees like matchsticks. Fine-grained ash closed nearby airports for up to two weeks, grounding thousands of flights. The damage cost $1.1 billion to repair.
The volcano remains active and America’s second-most dangerous. It sits on the Ring of Fire – a 40,000-kilometre (25,000-mile) horseshoe of volcanoes circling the Pacific Ocean. Beneath Mount St Helens, two of the massive rock plates that form the Earth’s crust are colliding; the oceanic Juan de Fuca Plate is sliding beneath the continental North American Plate.
As the ocean plate grinds down into the Earth’s crust, water is released. The water helps to melt the overlying hot rock into magma, which erupts through the brittle crust. The old North American crust contains lots of silica, which makes the magma sticky. Gas builds up in this thick magma until it violently erupts with gas, rock and steam. This debris piles up into steep-sided volcanoes.
Before the 1980 eruption, Mount St Helens was 3,000 metres (1,000 feet) tall and had been dormant since 1857. The volcano reawakened in March 1980 with a series of tremors and a growing bulge on its north side. A week before the volcano, the bulge grew two metres (6.6 feet) daily. After the eruption, Mount St Helens had shrunk by about 400 metres (1,300 feet).