Mount St. Helens is an interesting mountain because it is by far the most destructive volcano in the history of the United States. On May 18, 1980 a 5.1 earthquake struck and this set off the largest mud slide the States had recorded. The entire north slope of the mountain slid away exposing the huge core of the volcano. This exposed superheated core gave off enormous explosions of ash and rock with huge amounts of steam which melted glaciers causing devastating mudslides which took out whole forests and sweep homes, roads, railways and people along with it.
Mount St. Helens ArcScene model Video
The Image on the left is before the eruption, the image in the middle is after the eruption and the image on the right is the before and after images merged together so that you get a better view of the part that was blown off (the blue / purple part in the middle).
Data came from the USGS.
Background Information came from Wikipedia.