On 18 May 1980 Mount St Helens in Washington state, USA, triggered the largest landslide in history, killing 57 people and causing over $1 billion in damage.

Mount St. Helens takes its English name from that of the British diplomat Alleyne FitzHerbert, 1st Baron St Helens. Its known as Lawetlat’la to the local Cowlitz people, and Loowit or Louwala-Clough to the Klickitat.

It is an active volcano.

On May 18 1980, at 8:32 am, a second earthquake, of magnitude 5.1, triggered a massive collapse of the north face of the mountain. It was the largest known debris avalanche in recorded history. The magma in St. Helens burst forth into a large-scale pyroclastic flow that flattened vegetation and buildings over an area of 230 square miles (600 km²). More than 1.5 million metric tons of sulfur dioxide were released into the atmosphere. On the Volcanic Explosivity Index scale, the eruption was rated a 5, and categorized as a Plinian eruption.

For more than nine hours, a vigorous plume of ash erupted, eventually reaching 12 to 16 miles (19 to 26 km) above sea level. The plume moved eastward at an average speed of 60 miles per hour (100 km/h) with ash reaching Idaho by noon. Ashes from the eruption were found on top of cars and roofs the next morning as far away as Edmonton, Alberta, Canada.