Mount St. Helens Erupted 44 Years Ago Today: Here’s How It Unfolded

After the eruption, ash poured into the atmosphere for nine hours

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.

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That changed on May 18 when the volcano erupted for the first time in more than a century. Instead, there was a horseshoe-shaped crater in its place. The crater’s highest point located on the southwestern side of the mountain is 8,365 feet. The eruption came almost exactly two months after seismic activity began on the long-dormant volcano. 

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—it’s first since 1857. 

In that eruption, steam explosions blasted a 200- to 250-foot wide crater through the volcano’s summit ice cap and covered the snow-clad southeast sector with dark ash. 

These eruptions continued through April 22. 

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

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By that time, more than 10,000 earthquakes had shaken the volcano and the north flank had grown outward about 450 feet to form a prominent bulge. 

This bulge was strong evidence that molten rock (magma) had rose 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. 

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. 

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 650 feet high. 

The volume of the avalanche was the equivalent of one million Olympic-sized swimming pools. 

Following the landslide, the destruction continued. 

Mount St. Helens © Rex Vogel, all rights reserved

The landslide had removed part of the cryptodome which was a very hot and highly pressurized body of magma. 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. 

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.

The eruption then became a Plinian eruption defined as as one that produces a sustained convecting plume of pyroclasts and gas rising more than 15 miles above sea level.

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

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The ash was so thick that the city of Spokane, Washington, located 250 miles from Mount Saint Helens was plunged into complete darkness. Over four inches of ash covered Yakima, Washington. 

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 930 miles away. 

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.

Mount St. Helens © Rex Vogel, all rights reserved

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.

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. 

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. 

Mount St. Helens © Rex Vogel, all rights reserved

Mt. St. Helens eruption 44th anniversary

Mount St. Helens erupted on May 18, 1980 at 8:32 am

The volcano, located in southwestern Washington used to be a beautiful symmetrical cone about 9,600 feet above sea level.

Vancouver! Vancouver! This is it!

On March 1, 1980, a new system of seismographs at the University of Washington went into operation to monitor earthquake activity in the Cascades. On March 20, it recorded a magnitude-4.2 earthquake deep beneath Mount St. Helens inaugurating a round-the-clock watch that was to save many lives. From March 25 to March 27, quakes of magnitude 4.0 rocked the mountain as many as three times a day and smaller quakes occurred several times every hour.

At 8 a.m. PST on March 27, the U.S. Geological Survey issued an official Hazard Watch for Mount St. Helens; around noon, the first eruption of steam from the summit sent a column of ash and steam 6,000 feet into the air. Twin fissures opened on the mountain’s north face.

On the morning of May 18, USGS volcanologist David A. Johnston camped on the ridge with his lasers, radioed in his regular 7 a.m. report. The changes to the bulging mountain were consistent with what had been reported several times daily since the watch began. At 8:32, a magnitude-5.1 earthquake registered on the seismographic equipment. His excited radio message, “This is it!” was followed by a stream of data. It was his last transmission; the ridge he camped on was within the direct blast zone.

Mount St. Helens © Rex Vogel, all rights reserved

By the numbers

  • 57: Lives lost
  • $1.1 billion: Damage costs
  • 9,600 feet: Height before eruption
  • 8,300 feet: Height after eruption
  • 200: Homes destroyed
  • 90 mph: Mudflows speed
  • 5,400,000 tons: Estimated ash
  • 2,200 square miles: Ash covered
  • 185 miles: Roads destroyed
  • 15 miles: Railways damaged
Mount St. Helens © Rex Vogel, all rights reserved

Go deeper

Worth Pondering…

Each volcano is an independent machine—nay, each vent and monticule is for the time being engaged in its own peculiar business, cooking as it were its special dish, which in due time is to be separately served.

—Clarence Edward Dutton, American geologist (1841-1912)