Geological Survey 18th Annual Report (Part II) Glaciers of Mount Rainier Rocks of Mount Rainier

Mount Rainier is an extinct volcano. The residual heat of its once molten rocks still gives origin to steam jets, slightly impregnated with sulphurous gases in a few instances, which escape from crevices in the now partially snow-filled craters at the summit.

As has been determined by Bailey Willis, the mountain stands on a slanting peneplain, which consists of granites, schists, and coal-bearing Tertiary rocks; that is, the region where Mount Rainier is situated was eroded during late Tertiary times until it was reduced practically to a plain at sea level. Such a plain is known among geographers as a peneplain. This peneplain was then upraised and tilted so as to slope gently westward. Since the plain was elevated it has been deeply dissected by erosion, and the land masses between the sunken stream channels have been worn into mountain forms. The general level of the summits which mark approximately the position of the tilted peneplain, in the region adjacent to Mount Rainier on the north, is about 6,500 feet. The streams that have roughened the plain by excavating deep channels in the rocks composing it were guided westward by the slope produced by the uplifting; that is, their courses were determined by the slope of the land, due to tilting, and for this reason they are classed as consequent streams. Several rivers have their sources in the glaciers of Mount Rainier, and flow away from the mountain in all directions, and have deeply eroded its sides. These, too, are consequent streams; that is, their courses were determined by the original slopes of the mountain.

The latest marked eruption of Mount Rainier ceased before the work of erosion, now evidenced by such conspicuous results on its lower slopes and in the platform on which it stands, was far advanced. The rocks all about the base of the volcano are mainly Tertiary sediments and the products of ancient volcanic eruptions. Of older date than the Tertiary, and apparently rising through the rocks of that age at a few localities, are isolated areas of light-colored granite. Outcrops of granite were noted on the south side of Mount Rainier, near the extremity of the Nisqually Glacier, by Kautz in 1857, and also by Emmons in 1870. Similar granite, forming bold, rounded knobs and mountain-like masses on the north side of the mountain, between Carbon and Winthrop glaciers, was discovered in 1896 during the reconnaissance which formed the basis of this paper. A line connecting the granite outcrops on the two sides of the mountain bears slightly east of north, and possibly indicates the direction of the line of fracture that admitted of the extrusion of the lavas of which Mount Rainier is built. An hypothesis suggested by the observations made by myself last summer is that the granite was raised by faulting after the adjacent Tertiary rocks were deposited but previous to their being worn down to a peneplain. No opportunity was found, however, for testing this suggestion.

The main mass of Mount Rainier is composed of andesite and basalt, which were ejected to a considerable extent in a fragmental condition as scoria, pumice, lapilli, bombs, etc. Lava flows were not abundant during the later stages of eruption. The mountain ranks as a composite cone, but so far as its structure is revealed in the canyons and amphitheaters sculptured in its sides, and as is indicated also by the profiles of the great cone, it was built largely of material thrown out by explosions from a summit crater. The profiles of the mountain and the character of its summit show that at the time of its greatest perfection and beauty it rose as a tapering cone, with gently concave sides, to a height about 2,000 feet greater than its present elevation. At a later date it was truncated, probably by an explosion, which removed the upper 2,000 feet and left a summit crater from 2 to 3 miles in diameter. Remnants of the rim of this immense crater now form Peak Success and Liberty Cap. Subsequently explosive eruptions partially filled the great crater and formed two smaller craters within it. The rims of the smaller craters are still clearly traceable, although at present the depressions they encircle are nearly filled with snow. A moderately prominent point between the two younger craters, known as Crater Peak, is now the actual summit of the mountain.

As Mount Rainier stands to-day it has lost much of its youthful grace and symmetry. Its rocks have yielded to frost and storms and have been deeply sculptured by glaciers. The characteristic features produced by the decay, dissection, and erosion of the rocks forming the mountain, and the future topographic changes that may be expected to take place if the present destructive agencies continue their work, will be considered after the nature of the glaciers, to which the changes in the physiography of the mountain are mainly due, are briefly discussed.