U.S. Geological Survey Open-File Report 95–642
Mount Rainier is an active volcano that first erupted about half a million years ago. Because of Rainier's great height (14,410 feet above sea level) and northerly location, glaciers have cut deeply into its lavas, making it appear deceptively older than it actually is. Mount Rainier is known to have erupted as recently as in the 1840s, and large eruptions took place as recently as about 1,000 and 2,300 years ago.
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Sisson, T.W., 1995, History and hazards of Mount Rainier, Washington: U.S. Geological Survey Open-File Report 95–642, 2 p. (Available at https://pubs.usgs.gov/of/1995/0642/.)