Facts About Mount Saint Helens

Discover how a mountain lost its top in America’s most economically destructive volcanic eruption.

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 USD 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).

Who is Mount St Helens named after?

Mount St Helens is named after Alleyne Fitzherbert (Baron St Helens) – a British ambassador to Spain. The diplomat was a close friend of explorer Captain George Vancouver, who gave this mountain its name during a surveying expedition from 1791-95.

What are lava tubes?

Lava tube – A lava tube forms when treacle-like basaltic lava flows downhill from a volcano along a channel like a river. Over time, a solid rock crust forms on the channel’s surface as the 1,000-degree-Celsius (1,832-degree-Fahrenheit) lava cools when it’s exposed to air. The lava within can remain hot and runny for tens of kilometres even when the tube is completely crusted over._

Pahoehoe – The ropey-looking lava emerging from the tube is called pahoehoe – a Hawaiian word for flows that form bizarre shapes. The tube is only partly filled by lava: the lava’s heat downcuts through the channel bed. Superheated air and gas fill the space above the lava and re-melt the ceiling to create soda straw stalagmites – formations which are only found in lava tubes.

Facts about the May 18, 1980 eruption

Bulge – Up to 30 mini-earthquakes shake the mountain daily and the volcano’s north slope begins to bulge.

Mega-quake strikes – 20 seconds after 8.32am, a 5.1-magnitude earthquake rumbles 1.6km (a mile) beneath the volcano.

Summit collapses – Ten seconds later the volcano’s bulging north flank slides downhill as a gigantic rock avalanche that moves at up to 69m (226ft) per second.

Sideways blast – Pressurised superheated gas and steam explode sideways, like champagne from an uncorked bottle, after the heavy overlying rock slides away.

Ash eruption – A huge mushroom cloud of ash and steam shoots more than 19km (12mi) into the atmosphere.

Mudflows – The rock avalanche mixes with water to form mudflows in the nearby Toutle River, filling the valley up to 180m (600ft) deep with debris.

Pyroclastic flows – Glowing clouds of volcanic rock, ash and gases froth over the crater rim like a pot of oatmeal boiling over.

Aftermath – Streetlights turn on during the afternoon in parts of eastern Washington as the dense ash cloud turns daylight into darkness.