The eruption (a VEI 5 event) was the only significant volcanic eruption to occur in the contiguous 48 U.S. states since the 1915 eruption of Lassen Peak in California. However, it has often been declared as the most disastrous volcanic eruption in United States history. 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.
On March 27, after hundreds of additional earthquakes, the volcano produced its first eruption in over 100 years. 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. Steam-blast (phreatic) eruption from the summit crater of Mount St. Helens on April 6, 1980.
Within a week the crater had grown to about 400 m (1,300 ft) in diameter and two giant crack systems crossed the entire summit area. The eruption column rose 80,000 feet (24 km; 15 mi) into the atmosphere and deposited ash in 11 U.S. states. 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.
Approximately 57 people were killed directly, including innkeeper Harry R. Truman, photographers Reid Blackburn and Robert Landsburg, and geologist David A. Johnston. Hundreds of square miles were reduced to wasteland, causing over a billion U.S. dollars in damage ($3.03 billion in 2017 dollars ), 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 land passed to the United States Forest Service. The area was later preserved, as it was, in the Mount St. Helens National Volcanic Monument.
(Photo credit: USGS Cascades Volcano Observatory / AP).