Chapter I: C. Russell, one of the first geologists to study Mount Rainier, wrote

in 1896 that the present summit of the volcano consists of a small lava cone. Enclosing this cone is a broad depression whose rim is partly preserved at Gibraltar Rock, Point Success, and Liberty Cap (fig. 11). High points on the rim indicate that the former summit of the volcano above an altitude of about 14,000 feet was removed in some way. The destruction of the old summit, which may have reached a height of 16,000 feet, left a broad east-facing depression in the top of the volcano between Gibraltar Rock and Russell Cliff. The depression has since been mostly filled by the recent lava cone. You can see these features best from high points east of the mountain.

Our best explanation of how the former top of the volcano was removed also solves the problem of finding an adequate source of material for the Osceola Mudflow. Before 5,800 years ago, the topmost part of Mount Rainier probably consisted of rock that had been weakened by hot volcanic fumes and solutions and partly converted to clay. Then, this mass of weak rock was jostled off or pushed off by a volcanic explosion and slid down the northeast side of the volcano. One or more of these mighty avalanches of moist clay and rock resulted in the Osceola Mudflow.

Large avalanches have also occurred many times during the last 3,000 years on the west side of the volcano. Sunset Amphitheater (fig. 11) is part of the large scar left by them. About 2,800 years ago one of these avalanches created a mudflow in the valleys of the South Puyallup River and Tahoma Creek that was temporarily deep enough to submerge Round Pass (on the West Side Road) to a depth of nearly 400 feet. This is especially remarkable when we see that Round Pass itself is 600-700 feet above the nearby valley floors. Another deep mudflow, started by an avalanche at Sunset Amphitheater, moved down the Puyallup River valley about 600 years ago and buried the site of the present town of Orting in the Puget Sound lowland under 15 feet of mud and rock.

Table 2.—_Summary of important geologic events in the history of Mount Rainier National Park_

Geologic Years ago Geologic events in the area of the park
time scale

“Postglacial” Present summit cone of Mount Rainier
probably was built about 2,000 years
ago. The last known pumice eruption
occurred between 1820 and 1854.
Glaciers started to grow and advance
about 3,000 years ago. Maximum
extents were reached about 1850 A.D.
From then until about 1955, glaciers
were receding; now they are in
balance or advancing.
Huge masses of rock have slid from the
volcano repeatedly during the last
10,000 years. One of these destroyed
the summit of Mount Rainier and
formed the Osceola Mudflow about
5,800 years ago.
10,000
Pleistocene Last major glaciation.
(Ice Age)
25,000
Birth and growth of Mount Rainier
volcano, and repeated glaciation.
2-3 million
Pliocene Uplift and erosion of the Cascade Range.
12 million
Miocene Intrusion of granodiorite.
Folding of older rocks.
Deposition of Fifes Peak and Stevens
Ridge Formations.
26 million
Oligocene Deposition of Ohanapecosh Formation.
37-38 million
Eocene Deposition of Puget Group.
53-54 million

Avalanches and mudflows like those described are normal events at Mount Rainier and are expected to happen again in the future. Almost any cliff on the volcano can produce a large rockfall, but which cliff will collapse next, or when, cannot be predicted. Should the volcano again become active, earthquakes and volcanic explosions would trigger avalanches and mudflows that would rush down the mountain. Molten rock would melt snow and ice at the volcano’s summit and send floods of water down the volcano’s flanks. These indirect effects of an eruption would be much more hazardous than lava flows and pumice, if eruptions are on a scale similar to that of the past 10,000 years.

The Volcano’s Future?

An active volcano changes continually. Repeated eruptions build the cone by piling one lava flow on top of others, or on top of other volcanic formations. Simultaneously, the combined processes of erosion wear the volcano down. The relative importance of the two processes—one building, the other destroying—is reflected in the volcano’s shape. The scarred and deeply gouged sides of Rainier’s cone show that erosion has been dominant here for a long time. Is Mount Rainier now doomed to continued piecemeal destruction until the lofty cone is reduced to a featureless mound? Will future eruptions of lava restore some of the volcano’s bulk? Or will the volcano erupt violently some day, and then collapse as did Mount Mazama to form the deep basin of Crater Lake? The answers may not be known for centuries—or they may appear tomorrow.

Further Reading in Geology

Crandell, D. R., 1969, Surficial geology of Mount Rainier National Park,
Washington: U.S. Geological Survey Bulletin 1288. A geologic map
that shows where glacial deposits, landslides, and mudflows are
located in the park is accompanied by an illustrated nontechnical
description of these and other surficial deposits.

Crandell, D. R., and Fahnestock, R. K., 1965, Rockfalls and avalanches
from Little Tahoma Peak on Mount Rainier, Washington: U.S.
Geological Survey Bulletin 1221-A, 30 pages. A description of the
seven successive landslides of December 1963 that buried the upper
White River valley under thick deposits of rock debris.

Crandell, D. R., and Mullineaux, D. R., 1967, Volcanic hazards at Mount
Rainier, Washington: U.S. Geological Survey Bulletin 1238, 26 pages.
A discussion of Mount Rainier’s eruptions during the last 10,000
years and the anticipated effects of similar future eruptions.

Fiske, R. S., Hopson, C. A., and Waters, A. C., 1964, Geologic map and
section of Mount Rainier National Park, Washington: U.S. Geological
Survey Miscellaneous Geologic Investigations Map I-432, with text. A
geological map of the park’s bedrock is accompanied by a brief
nontechnical discussion of the geological evolution of the park as
recorded by the rock formations.

Sigafoos, R. S., and Hendricks, E. L., 1961, Botanical evidence of the
modern history of Nisqually Glacier, Washington: U.S. Geological
Survey Professional Paper 387-A, 20 pages. A description of the
recent moraines of several glaciers and an explanation of how they
are dated by counting the growth rings of trees growing on them.

U.S. GOVERNMENT PRINTING OFFICE: 1968 O—353-560

Footnotes

[1]The X pumice occurs as scattered fragments and does not form a
continuous layer.

[2]Ages of more than 150 and less than 6,000 years cited in this report
are based on radiocarbon determinations which have been corrected by
the use of a C₁₄ half life of 5,730 years and for variations in
atmospheric C₁₄ (H. E. Suess, written communication to Meyer Rubin,
1968).

[3]For more information about glaciers read “Glaciers” by Robert P.
Sharp, published in 1960 by the University of Oregon at Eugene.

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