Mount St Helens is a 2,950 m stratovolcano in the Cascade Range of southwestern Washington. Before May 1980 it had been considered one of the most picturesque mountains in the United States — symmetric, snow-capped, often called “the American Fuji.”
Seismic activity began on 16 March 1980 with a magnitude 4.2 earthquake. The first phreatic (steam) explosion punched a crater through the summit ice cap on 27 March 1980. Across April-May 1980 the north face of the volcano bulged outward by approximately 140 metres at a rate of roughly 1.5 metres per day under pressure from intruding magma.
The US Geological Survey under chief volcanologist David Crandell established a Red Zone (no entry) and Blue Zone (loggers permitted with passes) around the volcano. Lodge owner Harry Truman, 83, refused to evacuate from his property on Spirit Lake.
18 May 1980
At 08:32:11 PDT on 18 May 1980 a magnitude 5.1 earthquake triggered failure of the unstable north flank. Approximately 2.5 cubic kilometres of rock — the largest landslide in recorded history — slid northward into the Toutle River valley at approximately 250 km/h.
The landslide unblocked the pressurised magma chamber. A lateral blast of approximately 360°C steam, ash, and pumice exploded northward at supersonic speeds for approximately 13 seconds, expanding into a cloud that levelled forest across approximately 600 square kilometres. Trees were stripped of bark by abrasive ash within an inner zone; in an outer zone trees were laid flat in radial patterns pointing away from the mountain.
USGS volcanologist David Johnston, 30, monitoring from an observation post at Coldwater Ridge approximately 9.5 km north of the summit, radioed “Vancouver! Vancouver! This is it!” to the USGS office at 08:32. He was killed by the blast within seconds. His body has never been recovered.
Harry Truman and his lodge were buried under pyroclastic flow that filled Spirit Lake. He is among the 57 confirmed dead.
What followed
The lateral blast was followed by approximately nine hours of vertical plinian eruption that produced an ash column approximately 24 km high. Ash fell across the eastern Washington wheat country. Yakima, 140 km east of the volcano, was in darkness by 09:30 PDT. Ash reached the US East Coast within three days and circled the globe within two weeks.
The eruption removed approximately 400 metres from the summit elevation — from 2,950 m to 2,549 m. A 1.5 km horseshoe-shaped crater opened to the north. The volcano remained intermittently active across 1980-1986 and again 2004-2008, building a new lava dome inside the 1980 crater.
The 57 confirmed dead included Johnston, Truman, lodge owner Joel Harris, photographer Reid Blackburn (whose camera was recovered), photographer Robert Landsburg (who shielded his rewound camera with his body — his photographs survived), and 53 others. The blast zone is now the Mount St Helens National Volcanic Monument.
Mount St Helens remains the deadliest volcanic event in US history.