Geological Survey Bulletin 1292 The Geologic Story of Mount Rainier

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, (fig. 21; see p. 38) Burroughs Mountain, Grand Park, and Klapatche Ridge (fig. 5). 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.

COLUMNS OF DARK-GRAY ANDESITE at the east end of an old lava flow from Mount Rainier. This outcrop is near the point at which the highway to Yakima Park crosses Yakima Creek. (Fig. 5)

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.