The USGS’s Cascade Volcano Observatory has a very detailed account of the eruption, with a list of references.

The climatic eruption began at 08:32 PDT on May 18, 1980.

57 people were killed directly by the eruption. There was also a plane crash, a traffic accident, and shoveling ash which killed a total of 7 more.

Height of Mount St. Helens
The summit elevation was 9,760 feet (2,975 m) before the eruption. After the eruption the elevation of the new summit was about 8,525 feet (2,600 m).

About 0.25 cubic kilometers of new volcanic rock was erupted on May 18, 1980. This would make a cube about 600 meters (~2000 ft) on a side! But much more material was moved by the eruption: the entire northern side of the volcano collapsed and flowed down hill. The volume of this collapse was about 2.5 cubic kilometers – ten times bigger than the new lava.

The eruption began at 8:45 a.m. At noon, the ash plume (in the upper troposphere and lower stratosphere) had reached Moscow, Idaho. By about 3 p.m. it was near Missoula, Montana and starting to spread south. By 6 pm it was eas! of Pocatello, Idaho. At the end of the day, about 16 hours after the eruption started, the ash plume was near central Colorado.

A huge volume of ash was created by the various 1980 eruptions of Mount St. Helens. Every community affected had its own ways of dealing with the ash. Tons of ash was probably washed down storm drains and into sewer systems as people cleaned roofs and sidewalks. Local landfills received ash. Many tons of ash came down rivers and streams into the Columbia River. The river had to be dredged to allow shipping to pass, and the sand dredged from the bottom was deposited in large dikes along the Columbia. These dikes are now covered in grass and trees, but if a person was to dig down a few feet they would find the ash.

Ash can still be found on the floor of forested areas all around the mountain, and the blast zone is still heavily covered by ash. Souvenir “ash” trays and mugs made from Mount St. Helens ash can be purchased at gift shops near the mountain.

Sources of Information:
Christiansen, R.L., and Peterson, D.W., 1981, Chronology of the 1980 eruptive activity, in Lipman, P.W., and Mullineaux, D.R., eds., The 1980 eruptions of Mount St. Helens, Washington: U.S. Geological Survey Professional Paper 1250, p. 17-30.

Danielsen, E.F., 1981, Trajectories of the Mount St. Helens eruption plume: Science, v. 211, p. 819-821.

McClelland, L., Simkin, T., Summers, M., Nielson, E., Stein, T.C., 1989, Global volcanism 1975-1985: Prentice Hall, Englewood Cliffs, New Jersey, and American Geophysical Union, Washington DC, 655 p.

Sarna-Wojcicki, A.M., Shipley, S., Waitt, Jr., R.B., Dzurisin, D., and Wood, S.H., 1981, Areal distribution, thickness, mass, volume, and grain size of air-fall from the six major eruptions of 1980, in Lipman, P.W. and Mullineaux, D.R., (eds.),The 1980 eruptions of Mount St. Helens, Washington: U.S. Geological Survey Professional Paper 1250, p. 577-600.

Simkin, T., and Siebert, L., 1994, Volcanoes of the World: Geoscience Press, P.O. Box 42948, Tucson, AZ 85733-2948)

Mt. Saint Helens