Thumbnail Biography 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, 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.
Results of Recent Eruptions
While hiking, you soon become aware that there is a large amount of pumice along the trails in Mount Rainier National Park. Pumice is a lightweight volcanic rock so full of air spaces that it will float on water. The air spaces, or bubbles, originated when fragments of gas-rich lava were explosively thrown into the air above the volcano, and the molten rock hardened before the gas could escape. If you examine pumice deposits in a trail cut, in a streambank, or in the roots of blown-over trees, you may also note that there is more than one layer ([fig. 7]). If you circle the volcano on the Wonderland Trail, you may notice that the greatest number of pumice layers are on the east side of the park, but the thickest single layer is on the west side. The explanation lies partly in the source of the pumice deposits, because some pumice was erupted not by Mount Rainier but by other volcanoes in the Cascade Range of Washington and Oregon and brought to the park by strong southerly or southwesterly winds. The layers of pumice thrown out by Mount Rainier within the last 10,000 years lie mostly on the east side of the volcano. Strong winds evidently swept eruption clouds to the east during the outbursts and prevented the pumice from falling west of the volcano. This pattern of distribution, coupled with the coarsening and thickening of the pumice toward the volcano, reveals that the layers were erupted by Mount Rainier.
An old lava flow from Mount Rainier which forms Rampart Ridge west of the meadow at Longmire. The thick lava flowed down an old valley floor and cooled and solidified. Rivers then eroded new valleys along both sides of the flow. These new valleys, subsequently glaciated, are today followed by the Nisqually River and Kautz Creek. Thus, the area of a former valley floor is now a ridge. (Fig. 5)
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. 6)
