Outcrop of light-gray welded tuff in the Stevens Ridge Formation along the road in Stevens Canyon. The angular dark-gray fragments in the welded tuff are chunks of pumice. (Fig. 2)

About 12 million years ago one or more masses of molten rock, many miles across, pushed upward through the Puget Group and younger rocks. When this molten rock cooled and hardened, it formed granodiorite, a close relative of granite. Although most of the molten rock solidified underground, some of it reached the land surface and formed volcanoes at a few places within the area of Mount Rainier National Park.

Granodiorite looks like granite and has a light-gray speckled appearance. The knife is about 3 inches long. (Fig. 3)

Granodiorite is probably the most attractive rock in the park. It is mostly white, but it contains large dark mineral grains that give it a “salt-and-pepper” appearance ([fig. 3]). The large size of the grains is a result of the molten rock cooling slowly at a considerable depth below the land surface—the individual minerals had a long time to grow before the “melt” solidified into rock. In contrast, the rocks formed from lavas that flowed onto the ground surface are generally fine grained because the lavas cooled too quickly for the mineral grains to grow appreciably.

Granodiorite underlies the White River valley, the Carbon River valley, and parts of the upper Nisqually River valley and the Tatoosh Range. You can see it in roadcuts between Longmire and Christine Falls and at several places along the road between White River Ranger Station and White River campground.

Geological cross section of Mount Rainier and its foundation rocks from Mother Mountain southward to Tatoosh Range. True-scale cross section is nearly 17 miles long. Slightly modified from U.S. Geological Survey Professional Paper 444, Plate 1. (Fig. 4)

[ (left)] High-resolution Diagram [(right)]

After the granodiorite solidified, the foundation of Mount Rainier was complete except for one other landscape change that preceded the birth of the volcano. Not long after the granodiorite was formed, the Cascade mountain range began to rise—not rapidly, but little by little over many thousands of years. As the land rose, rivers cut valleys into the growing mountains so that by the time the new volcano began to erupt, the Cascades had already been carved into a rugged range of high ridges and peaks separated by deep valleys. Deep erosion thus laid bare the rock layers in which we today read the geologic history of the park ([fig. 4]).