The mountain is named after Lord St. Helens, a friend of explorer George Vancouver who surveyed the area in the late 1700's.
The volcano is part of the Pacific Ring of Fire which includes over 160 active volcanoes that sit on the edges of the Pacific tectonic plate.
The eruption on May 18, 1980 was the deadliest and most costly volcanic event in the history of the United States. The disaster killed 57 people. Hundreds of homes and miles of bridges, railways and roads were destroyed.
The 1980 eruption reduced the elevation of the mountain from 9,677 ft. (2,950 m) to 8,365 ft. (2,550 m) and left a mile wide horseshoe-shaped crater.