The Geologic Story of Mount Rainier / A look at the geologic past of one of America's most scenic volcanoes

Glacier-smoothed and grooved rock along the Wonderland Trail between Indian Bar and Panhandle Gap. (Fig. 13)

A muddy grayish-blue lake several hundred feet long lies behind a small horseshoe-shaped end moraine of Flett Glacier, on the northwest side of Mount Rainier. The glacier is mostly out of view to the left. (Fig. 14)

Glaciers erode, transport, and deposit huge quantities of rock debris. So do their coworkers, the melt-water streams. These turbulent streams flow from tunnels beneath every glacier, and their degree of muddiness roughly shows how active the glacier is. Glaciers that move very slowly, or that are stagnant, produce relatively clear melt water because they are not actively eroding bedrock. In contrast, streams of muddy water that look like chocolate milk often come from very active or “live” glaciers. These streams carry rock debris ranging from flour-size particles to large boulders. You can sense the carrying power of this swiftly moving water on warm summer days, when large cobbles and boulders are bumping along in a stream swollen by rapid glacier melting. Although you can rarely see these boulders, you can hear their constant low thunder. Their repeated impacts on other boulders in the streambed will vibrate the nearby streambanks beneath your feet. Hikers often find that a melt-water stream safe to cross in the early morning of a warm summer day is an impassable torrent at the same spot by early afternoon.

Four curved recessional moraines are spread over a distance of 2,000 feet on the valley floor of Fryingpan Creek. They were formed within the last few hundred years as Fryingpan Glacier lost volume and shrank back toward its present position above a line of cliffs. (Fig. 15)

A melt-water stream generally deposits coarse material wherever the slope of the valley floor decreases and the stream loses some of its velocity and carrying power. Only a flood may move the boulders farther downstream. However, the current carries fine material far downstream to deposit it in lakes, in Puget Sound, or in the Pacific Ocean. The Puyallup River, for example, is still very muddy where it enters Puget Sound at Tacoma, more than 40 miles from its source in the glaciers on Mount Rainier.

During the last glaciation, when glaciers were much larger than they are now, melt-water streams carrying great quantities of sand and gravel built valley floors up to levels tens of feet higher than they are today. Later, as the glaciers grew smaller, the rivers cut down into their valley floors and remnants of the sand and gravel deposits were left standing in benches or terraces along the sides of the valleys. You can see a good example of such a terrace in the Nisqually River valley beyond Ashford, which is 5 miles west of the park. You cross it on the highway that leads to the park. Cuts beneath the terrace reveal deposits of sand, cobbles, and boulders that look the same as those deposits being formed today by melt-water streams. The terrace west of Ashford was formed a little more than 15,000 years ago, when a glacier extended down the Nisqually River valley to the vicinity of Ashford.

Yesterday’s Glaciers

Mount Rainier’s great sprawling cone would seem incomplete without the glistening sheets of ice that descend its flanks. We have reason to believe that the volcano has borne glaciers ever since its origin—sometimes smaller than now, at other times vastly larger. Mount Rainier probably started to grow during the middle part of the Pleistocene Epoch, or Ice Age, which began more than 1 million years ago, but glaciers had covered this part of the mountains even before the volcano appeared. Masses of rock debris formed by ancient glaciers occur beneath lava flows from Mount Rainier on the west side of Mazama Ridge just upslope from Narada Falls, on the north side of Glacier Basin, and at a few other places in the park.

Mount Rainier may have reached its present size by about 75,000 years ago. Since that time great icefields and glaciers have formed at least three times on the slopes of the volcano and in the nearby mountains. During the first two glaciations, ice completely buried the flanks of the volcano and the surrounding mountains, except for the very highest ridges and peaks. These great ice masses slowly flowed down all the valleys that head at Mount Rainier. The glacier in the Cowlitz River valley, for example, extended 65 miles from the volcano and reached a point about 33 miles west of the community of Randle. Deposits of the younger of these two glacial episodes can be seen in cuts along the Mowich Lake Road for a distance of about 1½ miles inside the park boundary. The glacial deposits were originally more widespread, but in most of the park they have been removed by erosion or covered by the deposits of yet younger glaciers.

Extent of glaciers in the Cascade Range near Mount Rainier between about 15,000 and 25,000 years ago. Arrows show the direction of ice movement; solid black represents modern glaciers on Mount Rainier. (Fig. 16)

During the most recent major glaciation of the park, which lasted from roughly 25,000 to 10,000 years ago, ice again sheathed the slopes of the volcano, but glaciers in the nearby mountains were smaller than before. Most of the glaciers originated at the valley heads, where they gouged out countless bowl-shaped bedrock basins called cirques. Many of the basins held lakes after the glaciers disappeared. (See frontispiece.) Hikers on the trail to the Paradise ice caves cross the floor of a typical cirque at the head of Paradise Valley. From the Sunrise Visitor Center at Yakima Park you can walk a short distance to a point along the crest of the Sourdough Mountains and stand at the rim of a deep north-facing cirque. Ice originating in this cirque and in the cirques adjacent to it moved northward down the valley of Huckleberry Creek at least as far as the park’s north boundary (fig. 16).

These glaciers left most valley walls in the park covered with rock debris. Lateral moraines can be seen along the highway at and just east of Ricksecker Point (fig. 17). Other glacial deposits are especially well displayed in roadcuts along the north wall of the White River valley.

Lateral moraine of rock debris at Ricksecker Point. It was formed by Nisqually Glacier when the glacier was at least 1,000 feet thick and about 15 miles longer than it is today. (Fig. 17)

A little more than 15,000 years ago the long glaciers began to shrink and recede. By 11,000 years ago there was only about as much ice on Mount Rainier as there has been within the last century. Then, during a short period of renewed glacier growth, most glaciers expanded short distances and new ones appeared in cirques from which ice had disappeared only a short time before. In some of these cirques so much rock debris was being dislodged from surrounding cliffs by repeated freeze and thaw that a rock glacier, consisting mostly of rock fragments bound together by ice, was formed. A trail to the Huckleberry Creek valley crosses hummocky rock debris left by such a rock glacier in a cirque on the southeast side of Mount Fremont. A larger rock-glacier deposit lies about 2 miles north of Sunrise Point in an east-facing cirque between The Palisades and Hidden Lake (fig. 18).

In other cirques, where the proportion of ice to rock debris was greater, the glacier transported the debris a short distance forward and built a terminal moraine. You can see particularly good examples of moraines formed about 11,000 years ago near Tipsoo Lake, where the pond southeast of the lake is dammed by a moraine, and at Mystic Lake. The ice that formed the terminal moraine at Mystic Lake was a tongue of Carbon Glacier.

In some places the orientation or altitude of the cirque did not permit enough snow to accumulate to form a glacier but just enough to create a permanent snowbank. Rock debris that fell from the surrounding cliffs rolled down these snowbanks and formed low ridges at their toes. Such a ridge is called a protalus rampart because it is found just in front of the apron of rock fragments, called talus, that lies beneath cliffs. A trail at Sunrise Point leads to protalus ramparts along the north side of Sunrise Ridge.

During the last 10,000 years, glaciers have been very small by comparison with the great ice mantles that overwhelmed the park earlier. However, glaciers have grown larger at least twice just within the last 3,000 years. During both of these periods most glaciers were slightly larger than they are today, and ice occupied most cirques at altitudes above 6,500 feet—even some that are now free of ice. The most recent time of extensive glacier growth began at least 800 years ago, and various glaciers in the park reached their maximum size between the mid-14th century and the mid-19th century. Oddly enough, even though all the glaciers headed on Mount Rainier, they did not all attain their maximum size simultaneously. The largest terminal moraine of this most recent glacial period was built by Emmons Glacier in the White River valley (fig. 19). It is now largely forested, and cores taken from the trees with a special boring tool that does not harm the tree show ages indicating that the moraine was stable enough to permit seedlings to survive on it by the mid-17th century. A similar but smaller terminal moraine built by Cowlitz Glacier has trees on it that started to grow in the mid-14th century.

Rock-glacier deposit (light-gray rubble beyond the brown slopes in the foreground) at The Palisades, which was formed about 11,000 years ago when the climate was colder than it is today. Rocks fell from the cliffs in such great quantity that a small glacier in front of the cliffs consisted of more rock debris than ice. The melting of the ice left a mass of broken rock several hundred feet thick which covers about 80 acres. (Fig. 18)

The hummocky end moraine at the left still had blocks of ice buried in it when this picture was taken in 1954. The front of Emmons Glacier was near the left edge of the bare moraine in about 1900. Now the glacier ends 1 mile farther upvalley at the upper right. The valley floor and moraine were buried by an avalanche of rock debris from Little Tahoma Peak in 1963. (Fig. 19)

Nearly all the glaciers gradually decreased in size after the mid-18th century. Although the shrinkage was sometimes interrupted by short periods of renewed glacier growth, by 1950 the glaciers at Mount Rainier covered only about two-thirds of the area that had been buried by ice only a century before. The overall loss of volume by Rainier’s glaciers, as well as those elsewhere in the Pacific Northwest, was slowed or halted by slightly cooler temperatures and higher precipitation starting in the mid-1940’s. Volume increases in their upper reaches caused the larger glaciers to grow from year to year, and since the early 1950’s the terminuses of many glaciers have been advancing. This renewed growth of glaciers is not unique at Mount Rainier—similar changes have been observed at other glaciers in the Cascade Range and elsewhere.

Landslides and Mudflows—Past, Present, and Future

Some of the most effective means of erosion at Mount Rainier are landslides and mudflows. Erosion of this kind is sometimes spectacular. Within an interval of only minutes or a few hours huge masses of rock may fall, slide, or flow off the volcano and move far downvalley.

Large landslides have occurred at many other places in the park—one in the area northeast of Mount Rainier is so conspicuous that its source has been named Slide Mountain. The ragged scar left by another slide near Grand Park is aptly called Scarface. You cross a slide on the Mather Memorial Parkway (U.S. Highway 410) just north of Cayuse Pass. Broken and jumbled rock debris of many sizes borders both sides of the highway there. This landslide broke loose in rocks of the Ohanapecosh Formation, slid downslope to the bottom of the valley, and dammed Klickitat Creek to form Ghost Lake. Rocks have also slid downslope on the west side of Backbone Ridge and on the east side of the Ohanapecosh River valley a short distance north of Ohanapecosh campground. The slide on Backbone Ridge is still slowly moving today. Another slide moves a few inches each year on the west side of the Nisqually River valley about 1 mile northwest of the visitor center at Paradise Park. You can recognize the slide by a jagged horizontal crack 1,000 feet long at its top.

A far more spectacular variety of landslide occurs when a mass of rock drops from a cliff to form a rockfall. The largest rockfalls on Mount Rainier in historic time occurred in December 1963 on the east flank. Masses of rock hundreds of feet across fell repeatedly from the steep north face of Little Tahoma Peak onto Emmons Glacier. The rock masses shattered into dust and countless fragments, fanned out across the glacier, then avalanched down the steep ice surface at tremendous speed. When the avalanches reached the end of the glacier they shot out into space as sheets of rock debris. As these hurtling sheets settled toward the valley floor, a cushion of compressed air formed beneath them, comparable to the air cushion that momentarily buoys up a sheet of plywood that is dropped onto a flat surface. Air that was trapped beneath these speeding avalanches reduced friction and permitted one of the avalanches to move almost 2 miles beyond the end of the glacier. This avalanche passed completely over a small wooden gage house about 5 feet high on the valley floor without damaging it, then ran headlong into the north base of Goat Island Mountain where it scraped away trees and bushes. A later avalanche stopped just short of the gage house, and wind that was expelled from beneath the rock debris blasted the still-undamaged house several hundred feet forward. It now rests in the scar left by the earlier avalanche on the side of Goat Island Mountain.

At least seven rockfalls and avalanches descended from Little Tahoma Peak, separated perhaps by only minutes or hours. The reddish-gray masses of broken and pulverized rock—some spread helter-skelter, some piled in long sharp-crested ridges—now lie on the valley floor between the White River campground and Emmons Glacier (fig. 20).

The rockfalls might have been caused by a steam explosion near the base of Little Tahoma Peak. Steam jets and small explosions are not unusual phenomena at Mount Rainier, although they never have had such dramatic effects in historic time.

Incredibly larger avalanches of rock fell repeatedly from the sides of Mount Rainier during prehistoric time. One such avalanche originated near the summit of the volcano and blanketed Paradise Park and Paradise Valley with a yellowish-orange mixture of clay and rocks sometime between 5,800 and 6,600 years ago. You can see this avalanche deposit in shallow cuts along trails and roads in the Paradise area. Huge blocks of rock that came down with the avalanche are scattered in the meadows of Paradise Park between the visitor center and Panorama Point. Although the deposit is now less than 15 feet thick in most places, the mass that flowed down Paradise Valley must have been 600 feet thick, because we can find remnants of it on top of Mazama Ridge. A tongue of the wet mass flowed through a low saddle near the south end of Mazama Ridge and extended into the basin now occupied by the Reflection Lakes. You can see the yellowish-orange deposit in the first roadcut west of the lakes, where it lies on top of gray glacial deposits.

Avalanche deposits in the White River valley. The rockfalls and avalanches from Little Tahoma Peak formed a mass of reddish-gray rock debris that contrasts with the darker gray glacial debris deposited by Emmons Glacier within the last century. The avalanche deposits are about 1,500 feet across at their widest point. (Fig. 20)

The avalanche probably was wet when it crossed the Paradise area, and the moisture in it may have already been present in the rocks in which the avalanche originated. The mass was fluid enough to move down the Paradise and Nisqually River valleys as a mudflow hundreds of feet thick, and the resulting deposits extend at least 18 miles downstream from the volcano. The volume of rock that slid off to produce the mudflow may have been as much as 100 million cubic yards—or roughly enough to cover a 1-mile-square area to a depth of 100 feet.

At about the same time as the Paradise avalanche and mudflow occurred, a tremendous rock mass also slid off the east side of the volcano in the area between Steamboat Prow and Little Tahoma Peak. This slide formed a mudflow on the floor of the White River valley that was several hundred feet deep at the north boundary of the park and that extended at least 30 miles beyond the base of the volcano. The most remarkable feature of the deposit left by this mudflow is its surface, which is dotted with scores of mounds 5-35 feet high and as much as several hundred feet in diameter. These mounds have cores of huge rocks which are similar in size to those scattered on the surface of the avalanche deposits from Little Tahoma Peak. You can see the mounds best in an area which is a few miles north of the park boundary, west of the White River, and which can be reached by the Huckleberry Creek Forest Road. The total volume of this mudflow deposit may be as much as one-fifth of a cubic mile.

These great landslides and mudflows were followed shortly by another whose size surpassed that of anything before or since. This was the remarkable Osceola Mudflow, which streamed down the valleys of the White River and West Fork about 5,800 years ago. When these great rivers of mud joined in the White River valley, they formed an even larger mudflow which swiftly flowed downvalley for a distance of 15 miles and then spread beyond the Cascade mountain front into the Puget Sound lowland. There the mudflow submerged a total area of more than 100 square miles to depths as great as 70 feet and buried the sites of the present towns of Enumclaw and Buckley. One tongue of it even flowed into an arm of Puget Sound, south of Seattle, that has since been filled with river deposits to form the fertile valley occupied by the towns of Kent, Auburn, Sumner, and Puyallup.

The Osceola Mudflow is remarkable in that it affected areas so far from its place of origin. This long distance of travel was due to its great volume, which we estimate to have been more than half a cubic mile, and to the abundance of slippery clay in it. The clay had been formed by the alteration of rocks in the volcano by hot gases and solutions over many centuries.

The northeast flank of Mount Rainier. A remnant of the Osceola Mudflow lies at the summit of Steamboat Prow in the center. Two and one-half miles to the left is Little Tahoma Peak, from whose steep north face at least seven large masses of rock fell in 1963. Mount Adams volcano can be seen at the left, and Mount Hood, Oregon, in the far distance. (Fig. 21)

Where did the Osceola Mudflow originate on the volcano? This we must deduce from several lines of evidence. The mudflow occurred so long ago that there is no historical record, and volcanic events since that time have covered up part of the scar it left on the volcano. Remnants of the Osceola Mudflow veneer the sides and ridges of Glacier Basin, and a small amount of it is even preserved at the top of Steamboat Prow, at an altitude of 9,700 feet (fig. 21). This distribution tells us that the slides responsible for the mudflow originated somewhere on the volcano above Steamboat Prow. But now there is no great chasm in the side of the volcano large enough to have provided a source of the mudflow; so we must consider a former summit of the volcano itself as a possible source.

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 time scale	Years ago	Geologic events in the area of the park
“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 (Ice Age)		Last major glaciation.
	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.

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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