At 8:32 AM, May 18, 1980, the largest earthquake recorded at Mt. St. Helens crumbled the weak north face of the mountain. Suddenly, an enormous mass of superheated rock and gas was released, and the cataclysm began.
Within ten minutes, the ash column from the mountain reached 12 miles high (63,000 feet); by noon, ash was falling as far away as Idaho.
Closer to the mountain, a low-hanging cloud of superheated gas and pulverized rock (known as a pyroclastic surge) had been launched by the initial landslide across the surrounding ridges, killing everything its path. Mudflows swelled by melted glaciers roared down the surrounding river valleys, eventually reaching the Columbia River and dropping enough sediment to close the river to navigation for more than a month.
Mt. St. Helens had surprised; no one had expected in 1979 what Mt. St. Helens would do before June 1980. Never had a volcano erupted so catastrophically in the United States in historic time. A major reassessment of the volcanic hazards of the United States would be launched, for the risks were dramatically more apparent, and there are other slumbering mountains that rest far closer to major population centers.
And the mountain continues to surprise. Even now, the mountain is regrowing. Mt. St. Helens' story is far from over.
For more information:
Fire Mountains of the West by Stephen L. HarrisWikipedia - Mount St. Helens
Portland's KGW - May 18, 1980 newscast
Boston.com has a great collection of photographs.