Chapter I: Front Matter

The Geologic Story
of Mount Rainier

By
Dwight R. Crandell

_A look at the geologic past of one of America’s most scenic volcanoes_

GEOLOGICAL SURVEY BULLETIN 1292

UNITED STATES
DEPARTMENT OF THE INTERIOR

WALTER J. HICKEL, Secretary

GEOLOGICAL SURVEY

William T. Pecora, Director

Library of Congress Catalog-Card No. 79-601704

U.S. GOVERNMENT PRINTING OFFICE: 1969

For sale by the Superintendent of Documents, U.S. Government Printing
Office
Washington, D.C. 20402—Price 65 cents (paper cover)

contents

Page
The changing landscape of 12-60 million years ago 3
Thumbnail biography of Mount Rainier 11
Results of recent eruptions 12
Why glaciers? 23
Work habits of glaciers 25
Yesterday’s glaciers 29
Landslides and mudflows—past, present, and future 35
The volcano’s future? 42
Further reading in geology 43

Frontispiece. Eunice Lake, northwest of Mount Rainier.
Figure Page
1. Outcrop of sandstone and shale in the Puget Group 6
2. Outcrop of welded tuff in the Stevens Ridge Formation 8
3. Granodiorite looks like granite 9
4. Geological cross section of Mount Rainier 10
5. An old lava flow which forms Rampart Ridge 13
6. Columns of andesite at the end of an old lava flow 13
7. Layers of pumice on the floor of a cirque 14
8. Generalized distribution of some pumice layers 16
9. Breadcrust bomb enclosed in a mudflow deposit 19
10. Pumice layer C, which consists of light-brown fragments 20
11. The recent lava cone lies in a depression 21
12. Two ice streams meet to form Cowlitz Glacier 23
13. Glacier-smoothed and grooved rock 26
14. A lake lies behind an end moraine of Flett Glacier 27
15. Recessional moraines on the valley floor of Fryingpan Creek 28
16. Extent of glaciers between 15,000 and 25,000 years ago 30
17. Lateral moraine at Ricksecker Point 31
18. Rock-glacier deposit at The Palisades 33
19. Hummocky end moraine in front of Emmons Glacier 34
20. Avalanche deposits in the White River valley 37
21. The northeast flank of Mount Rainier 39

Table
1. Characteristics, sources, and ages of pumice layers, Mount
Rainier National Park 17
2. Summary of important geologic events in the history of Mount
Rainier National Park 41

The Geologic Story of
Mount Rainier
By Dwight R. Crandell

WASHINGTON
Seattle
Tacoma
CASCADE RANGE
Mount Rainier
Mount Adams
Mount St Helens
OREGON
Portland
Mount Hood
_Crater Lake_

Ice-clad Mount Rainier, towering over the landscape of western Washington, ranks with Fuji-yama in Japan, Popocatepetl in Mexico, and Vesuvius in Italy among the great volcanoes of the world. At Mount Rainier, as at other inactive volcanoes, the ever-present possibility of renewed eruptions gives viewers a sense of anticipation, excitement, and apprehension not equaled by most other mountains. Even so, many of us cannot imagine the cataclysmic scale of the eruptions that were responsible for building the giant cone which now stands in silence. We accept the volcano as if it had always been there, and we appreciate only the beauty of its stark expanses of rock and ice, its flower-strewn alpine meadows, and its bordering evergreen forests.

Mount Rainier owes its scenic beauty to many features. The broad cone spreads out on top of a major mountain range—the Cascades. The volcano rises about 7,000 feet above its 7,000-foot foundation, and stands in solitary splendor—the highest peak in the entire Cascade Range. Its rocky ice-mantled slopes above timberline contrast with the dense green forests and give Mount Rainier the appearance of an arctic island in a temperate sea, an island so large that you can see its full size and shape only from the air. The mountain is highly photogenic because of the contrasts it offers among bare rock, snowfields, blue sky, and the incomparable flower fields that color its lower slopes. Shadows cast by the multitude of cliffs, ridges, canyons, and pinnacles change constantly from sunrise to sunset, endlessly varying the texture and mood of the mountain. The face of the mountain also varies from day to day as its broad snowfields melt during the summer. The melting of these frozen reservoirs makes Mount Rainier a natural resource in a practical as well as in an esthetic sense, for it ensures steady flows of water for hydroelectric power in the region, regardless of season.

Seen from the Puget Sound country to the west, Mount Rainier has an unreal quality—its white summit, nearly 3 miles high, seems to float among the clouds. We share with the populace of the entire lowland a thrill as we watch skyward the evening’s setting sun redden the volcano’s western snowfields. When you approach the mountain in its lovely setting, you may find something that appeals especially to you—the scenery, the wildlife, the glaciers, or the wildflowers. Or you may feel challenged to climb to the summit. Mount Rainier and its neighboring mountains have a special allure for a geologist because he visualizes the events—some ordinary, some truly spectacular—that made the present landscape. Such is the fascination of geology. A geologist becomes trained to see “in his mind’s eye” geologic events of thousands or even millions of years ago. And, most remarkable, he can “see” these events by studying rocks in a cliff or roadcut, or perhaps by examining earthy material that looks like common soil beneath pastureland many miles away from the volcano.

Our key to understanding the geology of Mount Rainier is that each geologic event can be reconstructed—or imagined—from the rocks formed at the time of the event. With this principle as our guide, we will review the geologic ancestry of this majestic volcano and learn what is behind its scenery.

The Changing Landscape of 12-60 Million Years Ago

The rocks of the Cascade Range provide a record of earth history that started nearly 60 million years ago. Even then, as today, waves pounded on beaches and rivers ran to the sea, molding and distributing material that formed some of the rocks we now see in the park.

You may find it difficult to imagine the different landscape of that far distant time. There was no Mount Rainier nor Cascade mountain range. In fact, there was very little dry land in the area we call western Washington. Instead, this was a broad lowland of swamps, deltas, and inlets that bordered the Pacific Ocean. Rivers draining into this lowland from the east spread sand and clay on the lush swamp growth. Other plants grew on the deposits, and they were covered, in turn, by more sand and clay. In this way, thousands of feet of sand and clay and peat accumulated and were compacted into sandstone, shale, and coal. We can see some of the rocks formed at that time in cuts along the Mowich Lake Road west of the park (fig. 1). Seams of coal were mined at Carbonado and Wilkeson, 10 miles northwest of the park, during the late 19th and the early 20th centuries.

These beds of sandstone, shale, and coal make up a sequence of rocks called the Puget Group, which is 10,000 feet thick. Wave-ripple marks and remains of plants show that the rocks were formed in shallow water fairly close to sea level. How could the rocks have piled up to this great thickness? The coastal plain and adjacent basin must have been slowly sinking, and the influx of sand and clay must have just barely kept pace with the downward movements.

A little less than 40 million years ago, the western Washington landscape changed dramatically. Geologists R. S. Fiske, C. A. Hopson, and A. C. Waters have discovered that volcanoes then rose on the former coastal plain at the site of Mount Rainier National Park and became islands as the area sank beneath the sea. When molten rock was erupted underwater from the submerged flanks of these volcanoes, steam explosions shattered the lava into countless fragments. The resulting debris, mixed with water, flowed as mud across great areas of the submerged basin floor.

You can see rocks formed from these layers of volcanic mud and sand in cuts along the highway on the east side of Backbone Ridge and between Cayuse Pass and Tipsoo Lake. Look there for alternating beds of grayish-green sandstone and breccia, a concretelike rock in which the pebbles have sharp corners. These rocks are known as the Ohanapecosh Formation. Like the Puget Group, the Ohanapecosh Formation is at least 10,000 feet thick. Yet, nearly all of it accumulated in shallow water as western Washington continued to sink slowly during the volcanic eruptions.

The long-continued sinking finally ended after the Ohanapecosh volcanic activity ceased. Western Washington was then lifted several thousand feet above sea level, and the Puget and Ohanapecosh rocks were slowly compressed into a series of broad shallow folds. Before eruptions began again, rivers cut valleys hundreds of feet deep, and weathering of the rocks produced thick red clayey soils similar to those that are forming in some areas of high rainfall and high temperature today. Look for the red rocks formed from these old soils in roadcuts as you drive along the Stevens Canyon road about 2 miles southeast of Box Canyon.

The next volcanic eruptions, which may have begun between 25 and 30 million years ago, differed from those of Ohanapecosh time. These volcanoes, somewhere beyond the boundaries of the park, erupted great flows of hot pumice that, being highly mobile, rushed down the flanks of the volcanoes and spread over many square miles of the adjacent regions. The pumice flows were “lubricated” by hot volcanic gas emitted from inside each pumice particle, which buffered it from other particles. Some hot pumice flows were 350 feet deep. The heat still remaining in the pumice after it stopped flowing partly melted the particles to form a hard rock known as welded tuff. Repeated pumice flows buried the hilly landscape and eventually formed a vast volcanic plain. The rocks, which are mostly welded tuffs, are now the Stevens Ridge Formation, which you can see along the highway in Stevens Canyon 1-2 miles west of Box Canyon. You can recognize the welded tuff by its light-gray to white color and its many flattened and sharp-edged inclusions of darker gray pumice (fig. 2).

Another period of volcanism followed, of still a different kind, when lava flowed outward from broad low volcanoes. The flows were of two kinds: basalt, the kind now erupted by Hawaiian volcanoes, and andesite, the type erupted by Mount Rainier. Individual flows 50-500 feet thick were stacked on top of one another to a total depth of fully 2,500 feet. We know these rocks as the Fifes Peak Formation. They form many of the cliffs and peaks in the northwestern part of the park. You can examine them in cuts along the Mowich Lake Road between Mountain Meadows and Mowich Lake. The time of the eruption of the Fifes Peak lavas may have been between 20 and 30 million years ago.

When the Fifes Peak volcanoes finally became extinct, this part of western Washington changed again. The rocks once more were uplifted and compressed into broad folds parallel to those formed at the end of Ohanapecosh time. The rocks buckled and, in places, broke and shifted thousands of feet along great fractures, or faults.

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 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.

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).

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.