Mount St Helens eruption

Mount St Helens in Washington State in the western United States erupted in May 1980 after two months of earthquakes and steam venting. The eruption caused the entire north face of the mountain to collapse, with the pyroclastic flow destroying everything in its path, melting glaciers on the mountain and then causing massive mud slides (lahars). The volcanic cloud spread across the USA in just three days and encircled the earth in two weeks.  This National Geographic video provides a great introduction to the topic, explaining how volcanoes are formed. Then take a look at this Discovery Channel video about the effects and this clip on the devastating effects of the Mount St Helens eruption.