Mount St. Helens is an active Composite volcano located in Skamania County, Washington, in the Pacific Northwest region of the United States. (Composite volcanoes are also known as strato-volcanoes.)

Before its eruption on 10 May 1980

On 18 May 1980, Mount St. Helens' eruption was the deadliest and most economically destructive volcanic event in the history of the United States. Fifty-seven people were killed; 250 homes, 47 bridges, 15 miles (24 km) of railways, and 185 miles (298 km) of highway were destroyed.

Why was Mount St. Helens' eruption so devastating?

Like other Composite volcanoes, Mount Helen had steep-sides composed of many layers of volcanic rocks, made from thick sticky lava, ash and rock debris (broken pieces). The magma inside was thick and sticky (like honey), meaning that the gas could not escape, so instead the gases inside built up and up until it exploded sending out huge clouds of burning rock and gas.

The explosion caused a massive debris avalanche, reducing the elevation of the mountain's summit from 9,677 feet (2,950 m) to 8,365 feet (2,550 m) and replacing it with a 1 mile (1.6 km) wide horseshoe-shaped crater.

3,000 ft (1 km) steam plume on 19 May1982

See what Mt. St. Helens is doing right now
Check out the Live VolcanoCam from Johnston Ridge

Also on this day ...

18 May 1991: Britain's first astronaut, 27-year-old Helen Sharman from Sheffield, blasted in to orbit