Mount St. Helens Eruption (1980): America's Deadliest Volcano

The eruption of Mount St. Helens on 18 May 1980 was the deadliest and most economically destructive volcanic event in the history of the United States. A landslide tore away the volcano’s north side, unleashing a sideways blast, an ash cloud that spread across several states and mudflows that swept down river valleys. Fifty-seven people died, and the eruption changed how scientists monitor volcanoes around the world.

Background: a young and restless volcano

Mount St. Helens lies in south-western Washington State, part of the Cascade Range, a chain of volcanoes formed as the small Juan de Fuca Plate sinks beneath North America. Geologically it is young: most of the cone visible before 1980 had been built within the previous few thousand years. Native peoples of the region had long known it as an active mountain, and it last erupted, on a small scale, between 1800 and 1857.

Before 1980 the symmetrical, snow-covered peak was a popular recreation area, often compared to Mount Fuji. Spirit Lake, at its northern foot, was surrounded by lodges, youth camps and holiday cabins. In 1978 US Geological Survey (USGS) geologists Dwight Crandell and Donal Mullineaux published a report warning that St. Helens was the Cascade volcano most likely to erupt, possibly before the end of the century.

Warning signs: the bulge

On 20 March 1980 a magnitude 4.2 earthquake beneath the volcano marked the start of a new period of unrest. On 27 March steam explosions blasted a crater through the summit ice, and over the following weeks hundreds of small eruptions of steam and old ash darkened the snow.

More alarming was a growing bulge on the north flank. Magma was rising into the volcano without reaching the surface, pushing the slope outwards by about 1.5 to 2 metres a day. By mid-May parts of the north face had moved outwards by roughly 140 metres. Scientists realised the bulge might fail as a landslide, but no one could predict when.

Washington’s governor, Dixy Lee Ray, set up a “red zone” and a “blue zone” around the mountain, limiting access. The zones were drawn mainly around public forest land and left out much of the privately owned timber land to the north-west, an issue later debated. Some residents objected to being kept from their homes. The most famous was Harry R. Truman, the 83-year-old owner of a lodge on Spirit Lake, who refused to leave and became a media celebrity.

18 May 1980: what happened

At 8:32 a.m. on Sunday 18 May, a magnitude 5.1 earthquake shook the volcano. Within seconds the entire bulging north flank broke away and slid downhill. It was the largest landslide in recorded history, with a volume of about 2.5 cubic kilometres. Part of it surged into Spirit Lake, raising the lake bed and sending a wave up the surrounding slopes, while most of it poured down the North Fork Toutle River valley, filling it to an average depth of about 45 metres.

The landslide suddenly removed the weight holding down the gas-rich magma. The result was an explosive lateral blast that shot northwards at speeds estimated at more than 480 kilometres per hour, overtaking the landslide. The hot cloud of rock, ash and gas flattened or killed forests over an area of about 600 square kilometres. Near the volcano trees were stripped away entirely; farther out, millions of large trees were knocked down in neat rows pointing away from the crater.

At an observation post on a ridge about 10 kilometres north of the summit, the USGS volcanologist David A. Johnston radioed his colleagues in Vancouver, Washington: “Vancouver! Vancouver! This is it!” He was killed moments later, and his body was never found. The ridge was later named Johnston Ridge in his honour.

The ash column and mudflows

Within about 15 minutes a vertical column of ash rose to around 24 kilometres. For the next nine hours the volcano produced a continuous Plinian eruption, named after the Roman writer Pliny the Younger, who described a similar eruption of Vesuvius. Winds carried the ash eastward. In Yakima and Spokane, hundreds of kilometres away, the sky turned dark at midday, and streetlights came on. Ash fell over 11 states and reached the Atlantic coast within days, eventually circling the globe.

Pyroclastic flows, fast avalanches of hot gas and pumice, poured down the north side. Melting snow and ice mixed with debris to create lahars, or volcanic mudflows, which raced down the Toutle River and into the Cowlitz River. They destroyed bridges, homes and logging camps, and carried so much sediment into the Columbia River that its shipping channel was temporarily reduced from about 12 metres to around 4 metres deep, stranding ships in Portland.

The ash caused enormous disruption across eastern Washington, northern Idaho and western Montana. Towns had to clear hundreds of thousands of tonnes of fine grey ash from streets and roofs, highways and airports were closed for days, and car engines failed after sucking in the abrasive dust. Health officials advised people to wear masks, although later studies found that the ash caused no lasting harm to most people’s health. Farmers feared for their crops, but the damage to wheat and other harvests proved smaller than first expected.

The volcano did not fall silent after 18 May. Five smaller explosive eruptions followed during the rest of 1980, the first on 25 May, which dropped ash on western Washington and Oregon. From the summer of 1980 thick lava began to pile up inside the new crater, building a dome that grew in a series of episodes until 1986.

The victims

Fifty-seven people died. Some were inside the restricted zones; many were outside them, camping, logging or photographing the volcano from places thought to be safe. Victims included Harry Truman, whose lodge was buried under tens of metres of debris, the photographer Reid Blackburn, and the photographer Robert Landsburg, who protected his film with his body; it was later recovered and developed. Most victims died from asphyxiation caused by inhaling hot ash.

Officials and historians agree that the toll would have been far higher if the eruption had come on a weekday, when hundreds of loggers would have been working in the blast zone. Rescue helicopters saved more than 100 people in the days that followed.

By the numbers

Recovery of the landscape

The blast zone at first looked lifeless, but many organisms survived under snow, in the soil or in lakes. Pocket gophers, which spent the eruption underground, helped mix ash with soil. The prairie lupine, a flowering plant that adds nitrogen to soil, was among the first plants to colonise the bare pumice plain. Elk returned within a few years, and thousands of logs from the blast still float on Spirit Lake.

In 1982 Congress created the Mount St. Helens National Volcanic Monument, setting aside about 445 square kilometres for research, recreation and natural recovery. The area has become one of the world’s most important outdoor laboratories for studying how ecosystems recover from disturbance. Outside the monument, the timber company Weyerhaeuser and the US Forest Service replanted millions of trees.

Legacy for volcanology

The eruption transformed the study of volcanoes. Scientists learned to recognise the danger of flank collapse and lateral blasts, which had rarely been observed directly. The USGS established the Cascades Volcano Observatory in Vancouver, Washington, in 1980, and monitoring of US volcanoes was greatly expanded.

Lessons from St. Helens were applied internationally. After the Armero tragedy in Colombia in 1985, the USGS and the US Agency for International Development created the Volcano Disaster Assistance Program, which helped forecast the eruption of Mount Pinatubo in 1991 and enabled evacuations that saved many thousands of lives. The volcano itself remained active: a lava dome grew in the crater from 1980 to 1986, and a new period of dome building lasted from 2004 to 2008.

Myths and facts

• Myth: it was the largest eruption in modern history. In volume of erupted magma, St. Helens was modest, rated 5 on the Volcanic Explosivity Index. Eruptions such as Krakatoa in 1883 and Pinatubo in 1991 were much larger.
• Myth: no one was warned. Scientists warned for weeks that a major eruption was possible, and restricted zones saved many lives. What no one could forecast was the exact timing and the sideways direction of the blast.

Timeline

• 1978: A USGS report identifies Mount St. Helens as the Cascade volcano most likely to erupt.
• 1980, 20 March: A magnitude 4.2 earthquake begins the period of unrest.
• 1980, 27 March: The first steam explosions open a crater at the summit.
• 1980, April–May: A bulge on the north flank grows by up to 2 metres a day.
• 1980, 18 May, 8:32 a.m.: An earthquake triggers the flank collapse, lateral blast and main eruption.
• 1980, 25 May: A further explosive eruption spreads ash over western Washington and Oregon.
• 1982, 26 August: Congress establishes the Mount St. Helens National Volcanic Monument.
• 2004–2008: A new phase of lava-dome growth takes place in the crater.

Why it matters

Mount St. Helens showed in real time how a volcano can destroy a landscape within minutes, and it gave scientists unprecedented data on landslides, lateral blasts and lahars. Its lessons improved volcano monitoring and hazard planning worldwide, and its recovering landscape continues to teach ecologists how life returns after catastrophe.