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.
[Illustration: 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.
[Illustration: 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 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.