The violent eruption of Mount St. Helens on May 18, 1980 was not a one day event. For two months magma had been accumulating below the volcano, creating a huge ominous bulge on the mountain’s north slope, and generating thousands of small earthquakes.
A USGS geologist at Coldwater II observation post watching Mount St. Helens. (USGS)
On March 27, steam blasted through the volcano’s summit ice cap, creating a crater over 200 feet wide, covering the snow on the southeast side with dark ash. Within a week, the crater had grown to twice the size and two giant cracks emerged across the summit.
In March, eruptions were occurring approximately once per hour, then slowed to once per day by April 22, and then the activity subsided.
But on May 7, the eruptions resumed again, shaking the volcano and the bulge expanded at a rate of 6.5 feet (2 m) per day.
The crater area dropped in relation to the summit, and the growing bulge (right) shows pronounced fracturing because of its increased expansion, prior to the eruption of Mount St. Helens. (USGS)
Then, on May 18, at 8:32 a.m. local time, with no immediate precursor, a series of cataclysmic events began.
First came the earthquake, then the bulge and summit slid away in the largest debris avalanche in recorded history.
The landslide removed the part of the cryptodome, a very hot, highly pressurized body of magma inside the volcano. The removal of the cryptodome triggered powerful eruptions that unexpectedly, blasted laterally through the side of the mountain, sending a wave of fire-hot debris sliding down, overtaking the avalanche debris. The eruption removed nearly 1,000 feet (300 m) of the cone.
A few minutes later, the plume of ash rose up to the sky, reaching a height of 15 miles (24 km) within 15 minutes. For nine hours ash continued to spew, with the eruption reaching its peak between 3:00 and 5:00 p.m.
Ash column from the eruption of Mount St. Helens on May 18, 1980. (USGS)
Winds carried 520 million tons of ash east across the country, covering Spokane, Washington in complete darkness, and reaching as far as as the Great Plains of the Central United States, over 930 miles (1,500 km) away.
The largest concentration of ash was nearest the Mt St. Helens eruption site. The ash spread to Washington, Wyoming, North Dakota, South Dakota, Colorado, Utah, Oklahoma, and Idaho. (USGS)
Within 15 days, the ash cloud has circled the entire Earth.
Vegetation on Mount St. Helens (red) in 1979. (USGS)
Vegetation on Mount St. Helens (red) after the May 18, 1980 eruption. (USGS)
Mount St. Helens Facts
During the past 4,000 years, Mount St. Helens has erupted more frequently than any other volcano in the Cascade Range.
3,600 years ago, Native Americans abandoned hunting grounds devastated by an enormous eruption four times larger than the May 18, 1980 eruption.
In 1975, USGS geologists forecasted that Mount St. Helens would erupt again, “possibly before the end of the century.”
There was a lodge built on top of Mount St. Helens made by president Eisenhower.
On March 20, 1980, a magnitude 4.2 earthquake signaled the reawakening of the volcano after 123 years.
The blast and landslide removed the upper 1,306.8 ft (396 m) of the volcano.
The blast traveled at speeds of up to 670 mph (1,072 km/hr).
Within 15 minutes, a vertical plume of volcanic ash rose over 80,000 feet.
The volcanic ash cloud drifted east across the United States in 3 days and encircled Earth in 15 days.
Mount St. Helens destroyed over 500 homes.
The eruption killed many animals and people.
A two-month series of earthquakes followed the eruption.
The eruption was the most economically destructive volcanic event in U.S. history.
In September 2004, Mount St. Helens reawakened, and it erupted continuously until January 2008.