On this day in history, May 18, 1980, Mount St. Helens erupts, triggers largest landslide in recorded history


Washington’s Mount St. Helens erupted for nine hours on this day in history, May 18, 1980 — killing 57 people and triggering the largest landslide in recorded history. 

Prior to the eruption, Mount St. Helens stood at 9,677 feet, says the website for the United States Geological Survey (USGS). It was the fifth-tallest mountain in the state of Washington.

“It stood out handsomely, however, from surrounding hills because it rose thousands of feet above them and had a perennial cover of ice and snow,” said the site.

That changed on May 18, when the volcano erupted for the first time in more than a century, says the USGS. 

Once the dust literally and figuratively settled after the eruption, it was discovered that Mount St. Helens had lost roughly 1,300 feet from its cone. 

Instead, there was a horseshoe-shaped crater in its place, says the USGS. 

The crater’s highest point, located on the southwestern side of the mountain, is 8,365 feet, said the website. 

The eruption came almost exactly two months after seismic activity began on the long-dormant volcano, said USGS. 

On March 16, 1980, a “series of small earthquakes” began to shake the area. Eleven days later, on March 27, following hundreds of small earthquakes, Mount St. Helens had a relatively small eruption — its first since 1857. 

In that eruption, “steam explosions blasted a 60- to 75-m (200- to 250-ft) wide crater through the volcano’s summit ice cap and covered the snow-clad southeast sector with dark ash,” said the USGS. 

These eruptions continued through April 22, said the USGS. 

After about a two-week stop in volcanic activity, smaller eruptions and earthquakes continued from May 7 through the 17th. 

“By that time, more than 10,000 earthquakes had shaken the volcano and the north flank had grown outward about 140 m (450 ft) to form a prominent bulge,” said the USGS. 

This bulge was “strong evidence that molten rock (magma) had risen high into the volcano,” and was growing at a rate of up to five feet per day.  

At 8:32 a.m. on May 18, a 5.1 earthquake “with no immediate precursors” struck Mount Saint Helens, triggering “a rapid series of events,” said the USGS. 

“At the same time as the earthquake, the volcano’s northern bulge and summit slid away as a huge landslide — the largest debris avalanche on Earth in recorded history,” they said. 

“A small, dark, ash-rich eruption plume rose directly from the base of the debris avalanche scarp, and another from the summit crater rose to about 200 m (650 ft) high,” they said. 

The volume of the avalanche is the equivalent of one million Olympic-sized swimming pools, the USGS added. 

Following the landslide, the destruction continued. 

The landslide had removed part of the “cryptodome,” which was “a very hot and highly pressurized body of magma,” said the USGS. 

With the cyptodome removed, Mount St. Helens’s magmatic system depressurized, triggering “powerful eruptions that blasted laterally through the sliding debris,” knocking 1,000 feet off the height of the mountain. 

The cloud of tephra, or rock fragments, reached 15 miles within 15 minutes, noted the USGS. 

The aftermath of the initial eruption was devastating. 

“Virtually no trees remained of what was once a dense forest” in the six-mile radius of the former summit, and other trees were knocked to the ground and seared, said the USGS.

“The 600 km2 (230 mi2) devastated area was blanketed by a deposit of hot debris carried by the blast,” the site said. 

The eruption then became a “Plinian eruption,” defined as “as one that produces a sustained convecting plume of pyroclasts and gas rising >25km above sea level,” says the website Science Direct.

The Plinian eruption lasted for nine hours, sending 520 million tons of ash into the air.

The ash was so thick that the city of Spokane, Washington, located 250 miles from Mount Saint Helens, was plunged into complete darkness, said the USGS. 

Over four inches of ash covered Yakima, Washington, reported Getty at the time. 

“Major ash falls occurred as far away as central Montana, and ash fell visibly as far eastward as the Great Plains of the Central United States, more than 1,500 km (930 mi) away,” said the USGS. 

“The ash cloud spread across the U.S. in three days and circled the Earth in 15 days.”

The blast also triggered something called a “lahar,” which is an “Indonesian term that describes a hot or cold mixture of water and rock fragments that flows down the slopes of a volcano and typically enters a river valley,” said the USGS.

In Mount Saint Helens’ case, the snowy peak melted in the initial eruption.

That rush of water, combined with the rock flow, created a lahar. 

In the weeks leading up to May 18, people who lived near Mount St. Helens were evacuated, according to an article in American Scientist.

The area immediately surrounding Mount St. Helens was divided into a “red zone” and a “blue zone.” 

Of the 57 people who died in the eruption, only one — Harry Randall Truman — did not have express permission to be near the mountain the day it erupted, and most of the deaths actually occurred outside the boundaries of the blue zone, said American Scientist. 

Truman, an 83-year-old man who had lived near Mount St. Helens for 54 years, refused to comply with evacuation orders and leave the red zone. 

In a colorful interview with National Geographic prior to the eruption, Truman said, “I’m going to stay right here because, I’ll tell you why, my home and my (expletive) life’s here.” 

“My wife and I, we both vowed years and years ago that we’d never leave Spirit Lake. We loved it. It’s part of me, and I’m part of that (expletive) mountain,” he said.

Truman’s remains were never found.