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.
[Illustration: An old lava flow from Mount Rainier which forms Rampart
Ridge west of the meadow at Longmire. The thick lava flowed down an old
valley floor and cooled and solidified. Rivers then eroded new valleys
along both sides of the flow. These new valleys, subsequently glaciated,
are today followed by the Nisqually River and Kautz Creek. Thus, the
area of a former valley floor is now a ridge. (Fig. 5)]
[Illustration: Columns of dark-gray andesite at the east end of an old
lava flow from Mount Rainier. This outcrop is near the point at which
the highway to Yakima Park crosses Yakima Creek. (Fig. 6)]
[Illustration: Layers of pumice on the floor of a cirque near Paradise
Park. The yellow bed at the bottom is layer O, which was erupted by
Mount Mazama volcano at the site of Crater Lake, Oregon, about 6,600
years ago. The yellowish-brown layer a few inches above layer O is layer
D, a pumice that was erupted by Mount Rainier between 5,800 and 6,600
years ago. The light-yellowish-brown pumice bed at the top of the
outcrop is layer Y, which originated at Mount St. Helens volcano between
3,250 and 4,000 years ago. Photograph by D. R. Mullineaux, U.S.
Geological Survey. (Fig. 7)]
D. R. Mullineaux of the U.S. Geological Survey has studied in detail the
pumice deposits of Mount Rainier National Park. One of his first and
most important discoveries was that even though some pumice layers are
spread widely over the park, they were erupted from other volcanoes.
Strangely enough, one layer is thicker and more widespread than any
recent pumice erupted by Mount Rainier. We can clearly see that these
foreign pumice layers did not come from Mount Rainier, for they thicken
and coarsen southward, away from the park. The oldest was erupted by
Mount Mazama volcano at the site of Crater Lake, Oregon, about 6,600
years ago; this pumice forms a yellowish-orange layer about 2 inches
thick nearly everywhere in the park. The pumice has a texture like that
of sandy flour, and it feels grainy when rubbed between the fingers. It
is so fine grained because of the great distance to its source, 250
miles due south of Mount Rainier. Near Crater Lake this same pumice
consists of large chunks and is many feet thick.
Two other foreign pumice deposits in the park were erupted by Mount St.
Helens, a symmetrical young volcanic cone about 50 miles southwest of
Mount Rainier. The older of the two is between 3,250 and 4,000 years
old; it forms a blanket of yellow sand-sized pumice that is as much as
20 inches thick in the western part of the park. The younger pumice
layer is most conspicuous at the ground surface in the eastern part of
the park, where it is as much as 4 inches thick and resembles a fine
white sand. It is about 450 years old.
[Illustration: Mount St. Helens as it appears from Mount Rainier.]
An inconspicuous bed of pumice records the first eruption of Mount
Rainier that occurred after Ice Age glaciers melted back to the slopes
of the volcano. It can be found on the east side of the mountain from
Grand Park south to Ohanapecosh campground (fig. 8). In roadcuts near
the east end of Yakima Park (Sunrise) the pumice forms a rusty-brown bed
about 4 inches thick which contains fragments as much as 2 inches
across. Wood from a thin layer of peat just above the pumice was dated
by its content of radioactive carbon as about 8,750 years old; thus, the
pumice is even older. We call this pumice layer R for convenience; other
letter symbols have been assigned to the younger layers (table 1).
[Illustration: Generalized distribution of some pumice layers within
Mount Rainier National Park. The pumice of layers W and Y was erupted by
Mount St. Helens; all the other pumice originated at Mount Rainier.
Letters represent the following localities: C, Cougar Rock campground;
I, Ipsut Creek campground; L, Longmire; M, Mowich Lake; O, Ohanapecosh
campground; P, Paradise Park; S, summit crater; T, Tipsoo Lake; W, White
River campground; and Y, Yakima Park. Based on studies by D. R.
Mullineaux. (Fig. 8)]
[Illustration: Layer X (Between 110 and 150 years old)]
[Illustration: Layer C (Between 2,150 and 2,500 years old)]
[Illustration: Layer D (Between 5,800 and 6,600 years old)]
[Illustration: Layer L (Between 5,800 and 6,600 years old)]
[Illustration: Layer R (More than 8,750 years old)]
[Illustration: Layer W (line pattern), and Layer Y (stipple pattern)
(About 450 years old and 3,250 to 4,000 years old, respectively)]
TABLE 1.—_Characteristics, sources, and ages of pumice layers,
Mount Rainier National Park_
[Based on studies by D. R. Mullineaux]
Common range of
thickness in park
Pumice West East Common Color Source Approximate
layer side side range in age in 1968,
(inches) (inches) diameter or limiting
of pumice dates (years
fragments ago)
(inches)
X Absent [1] ¼-2 Light olive Mount 100-150
gray Rainier.
W 0-1 1-3 Medium White Mount St. [2]450
sand Helens.
C Absent 1-8 ¼-8 Brown Mount 2,150-2,500
Rainier.
Y 5-20 1-5 Coarse Yellow Mount St. 3,250-4,000
sand Helens.
D Absent 0-6 ¼-6 Brown Mount 5,800-6,600
Rainier.
L Absent 0-8 ¼-2 Brown Mount 5,800-6,600
Rainier.
O 1-3 1-3 Flourlike Yellowish Mount About 6,600
to fine orange Mazama.
sand
R Absent 0-5 ⅛-1 Reddish Mount 8,750-11,000?
brown Rainier.
The next two eruptions of Mount Rainier occurred between 5,800 and 6,600
years ago. Again, pumice spread over the area east of the volcano. The
older pumice, which we call layer L, covers a band only a few miles wide
that extends to the southeast from the volcano (fig. 8). The younger
pumice, layer D, covers an area at least 10 miles wide directly east of
the volcano. The distribution of both deposits shows that there were
strong directional winds during the eruptions. The long, narrow pattern
of layer L probably was caused by strong northwesterly winds during a
short-lived eruption. The pattern of layer D was caused by winds from
the west.
Some time during these eruptions, hot volcanic bombs and rock fragments
were thrown out of Mount Rainier’s crater and fell onto surrounding
areas of snow and ice. Wholesale melting resulted, and floods descended
the east flank of the volcano carrying millions of tons of ash, newly
erupted rock debris, and breadcrust bombs. Breadcrust bombs seem to be
solid rock, but if you would break one open, you would find that the
inside is hollow or is filled with a spongy mass of black glass. Their
outer surfaces are cracked like the crust of a loaf of hard-crusted
bread (fig. 9), so we call them breadcrust bombs. They originated as
blobs of soft, red-hot lava which were thrown out of the volcano’s
crater. As the masses arched through the air, they quickly chilled on
the outside, and a hardened skin formed around the still hot and plastic
core. As their outsides cooled, gas pressure in their hot interiors
caused the bombs to expand slightly and their solidified outer skin to
crack. When they struck the ground, many of the bombs became flattened
on one side, but they were still plastic and sticky enough to remain
whole.
Bombs can be found in two deposits that form the south bank of the White
River about half a mile downstream from the White River campground. The
deposits are mudflows caused by the mixing of hot rock debris with the
water from melted snow and ice. As the mudflows moved down the valley
floor they must have resembled flowing masses of wet concrete.
Mount Rainier erupted several times between about 2,500 and 2,000 years
ago. During one of the first eruptions, a mass of hot ash, rock
fragments, and breadcrust bombs avalanched down the side of the volcano
and buried the floor of the South Puyallup River valley. Although this
hot mass flowed like a wet mudflow, the temperature of the rock debris
was above 600°F. Thus, if any water had been present, it would have been
in the form of steam. You can see the resulting deposit in cuts along
the West Side Road on both sides of the bridge across the South Puyallup
River. Innumerable bombs have rolled from the cuts into the ditches
beside the road. A charcoal log found in the deposit had a radiocarbon
age of about 2,500 years.
[Illustration: A large breadcrust bomb enclosed in a mudflow deposit
that consists of a mixture of volcanic ash and rock fragments. The
outcrop is on the south bank of the White River about half a mile
downstream from the White River campground. (Fig. 9)]
Large amounts of pumice were thrown out of the volcano at the same time
as the bombs or soon after. The pumice covers most of the eastern half
of the park, and fragments are scattered as far southwest as Pyramid
Peak and as far northwest as Spray Park. This pumice, called layer C, is
especially thick and coarse at Yakima Park and Burroughs Mountain, where
it lies at the ground surface (fig. 10). Here the light-brown layer is
5-6 inches thick and consists of irregularly shaped pumice fragments as
much as several inches across. Mingled with the pumice fragments are
fist-sized chunks of light-gray rock that probably were simultaneously
thrown out of the volcano by violent explosions. Some of these angular
rocks were hurled as far as Shriner Peak, 11 miles east of Mount
Rainier’s summit.
[Illustration: Pumice layer C, which consists of light-brown fragments,
lies at the ground surface over much of the eastern part of the park.
(Fig. 10)]
The eruptive period was climaxed by the building of the volcano’s
present summit cone, which is at least 1,000 feet high and 1 mile across
at its base. Although dwarfed by the tremendous bulk of Mount Rainier,
it is a little larger than the cone of the well-known Mexican volcano
Parícutin that appeared in 1943 and erupted until 1952. Mount Rainier’s
present summit cone was built within a broad depression at the top of
the main volcano that had been formed nearly 4,000 years earlier (fig.
11; see p. 40). The cone consists of a series of thin black lava flows,
and its top is indented by two overlapping craters. Rocks around the
craters are still warm in places, and steam vents melt caves in the
summit icecap. The first climbers who reached the top of the mountain,
in 1870, spent the night in one of these caves, as have many benighted
climbers since.
[Illustration: The snow-covered lava cone lies in a depression 1¼ miles
wide at the summit of the volcano. The cone was probably built about
2,000 years ago. Liberty Cap is to the left and Point Success is to the
right. The cliffs below and to the right of Liberty Cap enclose Sunset
Amphitheater. (Fig. 11)]
Even though the lava flows that formed the summit cone were relatively
short, their eruption greatly affected some valleys at the base of the
volcano. The hot lava melted snow and ice at the volcano’s summit,
causing floods that rushed down the east and south sides. When the
floods reached the valley floors, they picked up great quantities of
loose rock debris and carried it downstream, sometimes forming mudflows.
The resulting flood and mudflow deposits raised the floors of the White
and Nisqually River valleys as much as 80 feet higher than they are
today. These valley floors, as well as several others, then became broad
wastes of bare sand and gravel that extended beyond the park boundaries.
Later, the rivers cut down to their present levels, but they left
remnants of the flood and mudflow deposits as terraces or benches along
the sides of the valleys. You can camp on such a terrace in the
Nisqually River valley at the Cougar Rock campground. The White River
campground occupies a similar terrace in the White River valley.
When did Mount Rainier erupt last? The most recent pumice eruption was
just a little over a century ago. However, between 1820 and 1894,
observers reported at least 14 eruptions. Some of these may have been
just large dust clouds, caused by rockfalls, that were mistaken for
clouds of newly erupted ash. Other clouds may have been from genuine
eruptions that left no recognizable deposits. D. R. Mullineaux has found
that at least one eruption of that era did spread pumice over an area
east of the volcano between Burroughs Mountain and Indian Bar to a
distance of at least 6 miles from the crater. Pieces of the pumice,
layer X, are light brownish gray and as large as 2 inches across. We
find only scattered fragments of the pumice, and nowhere are they in a
continuous layer. Where the X pumice is directly on top of layer C, we
cannot tell them apart. The best areas for us to study the younger
pumice, therefore, are glacial moraines formed within the last 150
years, because no pumice other than layer X is present on the moraines.
Fortunately, R. S. Sigafoos and E. L. Hendricks of the U.S. Geological
Survey have determined the ages of the moraines by counting the growth
rings of trees on them. Their studies show that the pumice was erupted
between about 1820 and 1854.
Captain John Frémont, an early explorer of the Oregon Territory,
recorded that Mount Rainier was erupting in November 1843, but his
journals give no details. Others have reported eruptions in 1820, 1846,
1854, and 1858. Pumice layer X probably was erupted during one or more
of these times, but we do not know exactly when.
And will Mount Rainier erupt again? We think that it will, but we now
have no sure way of predicting the time, the kind, or the scale of
future eruptions.
Why Glaciers?
We frequently hear the question: “Why are there glaciers on Mount
Rainier?” A glacier forms wherever snowfall repeatedly exceeds melting
over a period of years. Above 6,500-7,000 feet on Mount Rainier, more
than 50 feet of snow falls each winter, and not all of it melts before
the next winter. The survival of this snow from one year to the next
depends partly on the cooler temperatures at the higher altitudes, and
perhaps also on the somewhat deeper snowfalls there.
[Illustration: Two ice streams meet to form the half-mile-wide Cowlitz
Glacier. One heads on the flank of the volcano and the other (Ingraham
Glacier) at the summit. The firnline is a short distance above the
junction of the glaciers. The high bare embankment at the extreme right
is a lateral moraine that was formed about 100 years ago when the
glacier was thicker and about 1½ miles longer. (Fig. 12)]
LITTLE TAHOMA PEAK
MOUNT RAINIER
INGRAHAM GLACIER
COWLITZ GLACIER
A line that marks the limit on a mountain above which snow persists from
one winter to the next is called the annual snowline, and this line on a
glacier is called the firnline (fig. 12). Above the firnline, snow that
falls each year packs down and changes into glacier ice as air is slowly
forced out of it. This part of the glacier is its accumulation area,
where more snow falls each year than is lost by melting. Below the
firnline is the ablation area, where melting predominates. The firnline
on Mount Rainier’s glaciers has been well above 6,500 feet in recent
years. But some glaciers extend to altitudes below 5,000 feet—that is,
far down into the ablation area. They do this by slowly flowing
downhill. Solid ice flows by sliding on the hard bedrock under the
glacier and by slipping along the innumerable surfaces within the ice
crystals that make up the glacier.
The rate of flow and the rate of ablation govern the distance a glacier
extends down into the ablation zone. If these rates remain fairly
constant, the glacier will be in balance and its size will be about the
same from year to year. But if changing weather patterns affect the
rates of ablation or accumulation, or both, the glacier will either
become smaller or grow larger. The change you are most likely to notice
is in the position of the glacier’s terminus, which may either recede or
advance, but precise measurements of the upper reaches of a glacier also
show volume changes there, some of which may not affect the glacier’s
terminus for many years, if ever.
Crevasses are a glacier’s most awesome features and are a constant
hazard for climbers. They form where adjacent parts of a glacier are
moving at different speeds. Some of Mount Rainier’s glaciers may be
flowing at a speed of several thousand feet per year along their centers
but at a much slower rate along their margins. This unequal rate of flow
produces stresses in the ice that cause it to break. Groups of crevasses
often form where the glacier flows over a steep place in its bed. The
ice moves faster here, and pulls apart, and a crevasse is formed.
Although a large crevasse may seem to be bottomless to the observer,
most crevasses are less than 100 feet deep because ice pressure tends to
close the open spaces in the ice below that depth.[3]
Ice covers 37 square miles of the park today. Individual glaciers that
make up this ice blanket are placed into three groups, depending on
their place of origin. Those of one group originate at the volcano’s
summit and flow far down the valleys that radiate from the cone. Most of
the snow that nourishes these glaciers probably falls on them at
altitudes well below the summit. The largest examples of the group are
the Emmons, Nisqually, and Tahoma Glaciers.
Glaciers of the second group originate on the flanks of the volcano,
mostly at altitudes between 7,000 and 10,000 feet. This group is
represented by the South Tahoma, Carbon, and Inter Glaciers. Glaciers of
the third group are on north-facing slopes in the mountains around Mount
Rainier. They are mostly at altitudes of about 6,000 feet, and they owe
their existence to locations well protected from solar heat. Glaciers
representing this group are the Unicorn and Pinnacle Glaciers in the
Tatoosh Range, a small unnamed glacier near the west end of Burroughs
Mountain, and the somewhat larger Sarvent Glaciers east of Mount
Rainier.