I realize everyone will be featuring this, but I love volcanoes, so here goes anyway...
At 8:32 AM on May 18, 1980, Mount St. Helens (Loo-Wit), in southern Washington decided she was going to blow her top. After all, the venerable lady was sick of being fought over by Mounts Hood (Wy'east) and Adams (Pahto) [First Nations' legend]. The volcano had been dormant since 1857. She started showing signs of activity in March 1980 with earthquakes, steam vents and a small eruption on March 27th. A bulge began forming under the crust of the north slope as magma and gas pooled underneath it.
Vulcanologist David Johnston, manning a watchpost ten miles away, was able to trigger a warning just before the pyroclastic cloud took over his position. A 5.1 earthquake had triggered a landslide that swept away the north face of the mountain. One of the largest landslides ever recorded, it deposited more than half a cubic mile of debris a distance of 13 miles down the Toutle River.
The landslide uncovered the steam and magma vents under the mountain's surface, causing an eruption that overtook the landslide itself. The first of many pyroclastic flows (composed of super-heated gas, rock and ash) moved laterally as it followed the landslide. The blast was calculated at about 24 megatons. Within fifteen minutes a tower of ash and smoke had reached about 80,000 feet high.
The eruption lasted nine hours. By the time it was over 57 people had died, including David Johnston, whose body was never found. Countless animals were killed and 230 square miles of land were damaged and enough trees were blown down to build 300,000 two-bedroom houses. The plume of smoke and ash blew as far as 22,000 square miles away - in Colorado and Oklahoma. Ash circled the globe in two weeks. She was reduced in height from 9677 feet to 8364 feet almost instantly.
Sites about the eruption:
USGS: Mount St. Helens - 1980
Mount St. Helens National Volcanic Monument
1980 Eruption - Wikipedia
1980 Eruption - The History Channel
First Nations' Legends regarding Mount St. Helens and the Cascades:
The Bridge of the Gods
The Gardner School description of area names
USGS Mount St. Helens site