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Right: A view to the north of the "two tone" mountain - an appearance produced by prevailing easterly winds during the initial activity of Mount St. Helens. Mount Rainier is visible in background. Photo by C. Dan Miller, USGS
Eruptions began at Mount St. Helens about 40,000 years ago. The deposits are air-fall tephra and pyroclastic flows, the type of material produced by explosive eruptions. One pumiceous tephra deposit produced during this episode had a volume as great as any subsequent tephra eruption at Mount Saint Helens. I think it would qualify as a big eruption.
A recent paper suggests that eruptions at Mount Saint Helens began as long ago as 80,000 years. Berger and Busacca (1995) dated a loess (wind-blown slit-sized material) deposit just below a tephra layer in eastern Washington that is known to be from Mount St. Helens. The loess is about 80,000 years old. The tephra is thought to be slightly younger.
According to Volcanoes of the World, by Simkin and Siebert (1994, Geoscience Press, P.O. Box 42948, Tucson, AZ 85733-2948), Mt. St. Helens erupted 23 times prior to 1831, based on charcoal dates. After that (and before the 1980 eruption) it erupted in 1831, 1835, 1842, 1847, 1848, 1849, 1853, 1854, and 1857.
There is a good summary in Volcanoes of North America: United States and Canada by Charles Wood and Jurgen Kienle (1990) (Cambridge University Press, New York, NY, (800) 221-4512).
Additional Sources of Information:
Berger, G., and Busacca, AJ., 1995, Thermoluminescence dating of late Pleistocene loess and tephra from eastern Washington and southern Oregon and implications for the eruptive history of Mount St. Helens: Journal of Geophysical Research, v. 100, p. 22,361-22,374.
Mullineaux, D.R., and Crandell, D.R., 1981, The eruptive history of Mount St. Helens, 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. 3-15.