The life span of a volcano can be compared to that of an
individual—after his birth and a brief youth, he matures and grows old.
The birth date of Mount Rainier is not known for sure, but it must have
been at least several hundred thousand years ago. We cannot tell much
about the volcano’s complex youth because most of its earliest deposits
are now buried under later ones. At an early age, well before the cone
grew to its present size, thick lava, like hot tar, flowed repeatedly
5-15 miles down the deep canyons of the surrounding mountains. Because
these lava flows resisted later erosion by rivers and glaciers, most of
them now form ridgetops, as at Rampart Ridge, Burroughs Mountain, Grand
Park, and Klapatche Ridge (figs. 5 and 6). Violent explosions
occasionally threw pumice onto the slopes of the growing volcano and the
surrounding mountains. As the volcano matured, the long thick flows were
succeeded by thinner and shorter ones which, piled on top of one
another, built the giant cone that now dominates the region. Even though
Mount Rainier has grown old now, it has revived briefly at many times
during the last 10,000 years or so and may erupt again in the future.
The events of the last 10,000 years, because they are so recent, in
terms of geologic time, are better known than those of any earlier time,
and we can examine this part of the volcano’s history in some detail. We
will study three principal subjects: eruptions—because they have had
widespread effects; glaciers—because they are such conspicuous features
on the mountain; and landslides—because they have drastically changed
the volcano’s shape.