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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.
Mount Rainier and other similar volcanoes in the Cascade Range, such as
Mount Adams and Mount Baker, erupt much less frequently than the more
familiar Hawaiian volcanoes, but their eruptions are vastly more destructive.
Hot lava and rock debris from Rainier's eruptions have melted snow
and glacier ice and triggered debris flows (mudflows) - with a consistency
of churning wet concrete - that have swept down all of the river valleys that
head on the volcano. Debris flows have also formed by collapse of unstable
parts of the volcano without accompanying eruptions. Some debris flows have
traveled as far as the present margin of Puget Sound, and much of the lowland
to the east of Tacoma and the south of Seattle is formed of pre-historic
debris from Mount Rainier.
-- Sisson, 1995
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