A lateral blast shaved 1,300 feet off America’s Fujiyama
Mount St. Helens in Skamania County, Washington—Lawetlat’la to the Cowlitz—erupted on 18 May 1980 in the United States’ costliest volcanic disaster. A magnitude 5.1 quake triggered a debris avalanche and lateral blast that dropped the summit from 9,677 to 8,363 feet, killed 57 people, and wrecked 200 homes plus hundreds of miles of road and rail.
Activity simmered until 2008; Congress created the National Volcanic Monument in 1982. Geologically young—within 40,000 years, with the pre-1980 cone rising about 2,200 years ago—it is the Cascades’ most active Holocene volcano. Before the blast it was Washington’s fifth-highest peak, nicknamed America’s Fujiyama for its symmetric ice, including eleven named glaciers of which only Shoestring partly revived. The collapse left a horseshoe-shaped crater a mile wide and sent 0.6 cubic miles of debris downslope. Aquatic life, by contrast, thrived on the ash, and most nearby lakes had recovered within six years.
It stands 52 miles northeast of Portland and 98 south of Seattle, 34 miles west of Mount Adams and about 50 from Rainier. Annual rain near 140 inches and snowpack to 16 feet feed the Toutle, Kalama, and Lewis rivers. Crater Glacier (once Tulutson) grew inside the new amphitheatre at about 14 feet of thickness per year, covering some 0.36 square miles by 2004.
English naming honours diplomat Alleyne FitzHerbert, Baron St Helens. Future eruptions may grow more forceful as dome geometry demands higher pressure, yet hikers still climb year-round. By 2006 that young ice averaged 300 feet thick and reached 650 in places, and in late May 2008 its two lobes joined around the new lava domes.
Magma collects in two chambers, one roughly 3 to 7 miles down and another 7 to 25 miles deep that Mount Adams may share. The volcano's biggest known Holocene outburst came around 1900 BC, dropping 18 inches of tephra 50 miles away in what is now Mount Rainier National Park, with traces reaching Alberta's Banff. Around 100 BC an andesite and basalt flow ran all the way into the Lewis and Kalama valleys.
Source: Mount St. Helens
