On March 27, 1980, a series of volcanic explosions and pyroclastic flows began at Mount St. Helens in Skamania County, Washington, United States. A series of phreatic blasts occurred from the summit and escalated until a major explosive eruption took place on May 18, 1980, at 8:32 AM. The eruption, which had a Volcanic Explosivity Index of 5, was the most significant to occur in the contiguous United States since the much smaller 1915 eruption of Lassen Peak in California.[2] It has often been declared the most disastrous volcanic eruption in U.S. history.

Quick facts: 1980 eruption of Mt. St. Helens, Volcano, Sta...
1980 eruption of Mt. St. Helens
Photograph of the eruption column, May 18, 1980, taken by Austin Post
VolcanoMount St. Helens
Start dateMarch 27, 1980; 42 years ago (1980-03-27)[1]
Start time8:32 a.m. PDT
TypePhreatic, Plinian, Peléan
LocationSkamania County, Washington, U.S.
46°12′1″N 122°11′12″W
ImpactApproximately 57 deaths, about $1.1 billion in property damage (or $3.6 billion today, adjusted for inflation); caused a collapse of the volcano's northern flank, deposited ash in 11 U.S. states and five Canadian provinces
Map of eruption deposits

The eruption was preceded by a two-month series of earthquakes and steam-venting episodes caused by an injection of magma at shallow depth below the volcano that created a large bulge and a fracture system on the mountain's north slope. An earthquake at 8:32:11 am PDT (UTC−7) on Sunday, May 18, 1980,[3] caused the entire weakened north face to slide away, a sector collapse which was the largest subaerial landslide in recorded history.[4] This allowed the partly molten rock, rich in high-pressure gas and steam, to suddenly explode northward toward Spirit Lake in a hot mix of lava and pulverized older rock, overtaking the landslide. An eruption column rose 80,000 feet (24 km; 15 mi) into the atmosphere and deposited ash in 11 U.S. states[5] and various Canadian provinces.[6] At the same time, snow, ice, and several entire glaciers on the volcano melted, forming a series of large lahars (volcanic mudslides) that reached as far as the Columbia River, nearly 50 miles (80 km) to the southwest. Less severe outbursts continued into the next day, only to be followed by other large, but not as destructive, eruptions later that year. Thermal energy released during the eruption was equal to 26 megatons of TNT.[7]

About 57 people were killed, including innkeeper and World War I veteran Harry R. Truman, photographers Reid Blackburn and Robert Landsburg, and geologist David A. Johnston.[8] Hundreds of square miles were reduced to wasteland, causing over $1 billion in damage (equivalent to $3.6 billion in 2021), thousands of animals were killed, and Mount St. Helens was left with a crater on its north side. At the time of the eruption, the summit of the volcano was owned by the Burlington Northern Railroad, but afterward, the railroad donated the land to the United States Forest Service.[9][10] The area was later preserved in the Mount St. Helens National Volcanic Monument.

Mount St. Helens from Monitor Ridge, this image shows the cone of devastation, the huge crater open to the north, the posteruption lava dome inside, and Crater Glacier surrounding the lava dome. The small photo on the left was taken from Spirit Lake before the eruption and the small photo on the right was taken after the eruption from roughly the same place. Spirit Lake can also be seen in the larger image, as well as two other Cascade volcanoes.

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