Mount Rainier, proving that they, too, were not merely picked up local-
ly by wind and then redeposited.

The tephra layers studied are separated by beds of mostly darker
sand- and silt-size, predominantly lithic particles (frontispiece). Some
of these dark beds probably are also tephra, and, if so, they represent
additional eruptions of the voleano. They were not studied in detail,
however, because they could not be readily distinguished from sand
and dust that actually was picked up from the volcano’s slopes and
redeposited by wind. A recent example of such a windblown deposit is
the mantle of dust, locally as much as 2 cm thick, that fell on the east
side of Mount Rainier in 1964 (Crandell and Fahnestock, 1965, p. 25).
The dust resulted from rockfalls and avalanches from Little Tahoma
Peak, and it was spread over the east flank of the volcano by winds.

The tephra layers selected for study have been designated by letters
of the alphabet. These letters, however, do not fall in alphabetical order
when the deposits are listed according to age. (See table 1.) When the
most conspicuous tephra units were first studied (Crandell and others,

1962), they were known to be only a few of the many layers that are
present. No orderly alphabetical sequence for all the deposits could be
set up at that time, so the layers described were arbitrarily named by

letters derived from some word used in field descriptions. Thus, the
letter Y was used for a yellow pumice, and W for a white one.

The tephra layers from Mount Rainier and two of those from other
voleanos are described individually and are named by single letters.
The others, all from Mount St. Helens, are grouped into two sets that
also are designated by single letters. Each set contains several different
beds which are similar in composition and which in many places cannot
be identified one from another. Where specific beds within a set can be
recognized, they are designated by adding a second letter; layer Yn, for
example, is a thick distinctive bed within set Y.

The outcrop pictured in the frontispiece, on the southeast flank of
Mount Rainier, shows the general appearance of almost all the layers
described in the report. It also illustrates some common differences
between tephra deposits from Mount Rainier and foreign tephra
deposits in the park. Those from Mount Rainier are relatively dark, and
locally they are thick. They are also coarse grained as compared with
their thickness; most Mount Rainier tephras are no more than a few
times as thick as the diameter of their large fragments. In contrast,
most non-Rainier tephra units are conspicuously lighter in color, and
they are fine grained as compared with their thickness. The foreign
tephra beds commonly are from 10 to 100 times as thick as the
diameter of the largest grains.

Comparison of the tephra beds at several sites around the volcano
(fig. 4) shows the changes in thickness within short distances that are

characteristic of many Rainier deposits. Units such as layers L, D, and

12



In some places these beds have an admixture of fine pyroclastic or
windblown material derived from Mount Rainier, but elsewhere Mount.
Rainier detritus is absent, showing that the deposits must have come
from a distant volcano. Their source is regarded to be Mount St. Helens
because they are lithologically similar to pumice beds in set Y at that
locality, and because layer Yn can be traced to that volcano. The age of
set Y has been determined at Mount St. Helens to be between about
4,000 and 3,000 years (Mullineaux, Hyde, and Rubin, 1972).

TEPHRA LAYER Yn

Layer Yn is a yellow to brown deposit of pumice ash and small lapilli
(figs. 6, 7) that is about 2 cm (1 in.) to more than 30 cm thick within the

sists of loose, nearly uniformly sized grains that are small as compared
with the thickness of the bed. These grains roll, slide, and wash down
the slopes-and cutbanks so that the layer appears to be thicker than it
is. Although the pumice is white to colorless on fresh surfaces, the par-
ticles commonly have a yellowish-brown surface stain or alteration that
colors the entire deposit. Lithic fragments are sparse, and crystals are
abundant only in the ash-size fraction. The only iron-magnesium
minerals in the pumice fragments are hornblende and cummingtonite;
hypersthene, which is _present_in_almost_all other tephra deposits at.
Mount Rainier (table 3), is absent.

—__________DISTRIBUTION, THICKNESS, AND GRAIN SIZE ___

Layer Yn pumice forms a long narrow tongue of tephra that extends
from Mount St. Helens north-northeastward across Mount Rainier
National Park into Canada (fig. 15). It is the most voluminous tephra
within the park boundaries. The thickest and coarsest axial part of its
long lobe lies west of the park boundary; as a result, the pumice
becomes progressively thinner and finer grained toward the northeast
(fig. 16). From an average thickness of slightly more than 30 cm (12 in.)
in the southwest corner, it decreases to about 20 cm in the northwest
corner and to about 2 cm along the eastern boundary. Lapilli as large as
2 cm across are common in the southwest corner, where the median
grain size is about 1 mm. The median decreases to about 0.7 mm near
the northwest corner and to 0.6 mm along the eastern boundary.

Abrupt local variations in thickness are common. This pumice fall
produced a deep loose deposit of small lightweight particles, which
probably blanketed and destroyed much vegetation, So the grains were
easily reworked by wind and water. Accumulations of pumice several
times the estimated original airfall thickness are common. Deposits as
much as 1 m (3 ft) thick have been found within the park, a layer at

33



DISTRIBUTION, THICKNESS, AND GRAIN SIZE

The tephra of layer O fell throughout the park and far beyond its
boundaries. No consistent trends in thickness were noted within the
park, and layer O is 1-5 em (4-2 in.) thick nearly everywhere. The
deposit consists mostly of silt-size fine ash, with lesser amounts of
coarser grains as much as 0.5 mm across. The median grain size of
samples of this layer at 10 sites within the park ranged from ap-
proximately 0.015 mm to 0.035 mm. No trends in grain size were evi-
dent in the field, but laboratory analyses show that the deposit is
slightly coarser in the southern part of the park than in the north.

Considerable local thinning and thickening can be seen in this layer,
and locally its original thickness appears to have been as much as 8-10
em (8-4 in.), The layer is. much thicker where it was washed into ponds
that existed along streams when the tephra fell. Thicknesses of 25 cm
at such sites are not unusual, and the thickest deposit of layer O seen is
slightly more than 50 cm thick. The ash has been segregated into con-
spicuously coarse and fine fractions in some of the pond deposits. The
median grain size of one such coarse zone in a deposit near Mystic Lake
is about 0.2 mm, about 10 times the average median diameter of the 10
airlaid samples analyzed.

SOURCE AND AGE

The relatively uniform thickness and grain size of layer O across the
park imply that it originated at some volcano other than Mount
Rainier. Its age, thickness, and grain size are all consistent with what
would be expected in this region for the Mazama ash, which covers
much of the Pacific Northwest (Powers and Wilcox, 1964), and Wilcox
(oral commun., 1962) has confirmed that it is petrographically similar

to the Mazama ash. Thus, layer O is regarded to be a part of the
Mazama tephra blanket that is near the western limit of distribution
now recognized for that tephra (fig. 12).

Organic material from just above layer O in the park has been dated
as 6,730+250 years old (table 5). Radiocarbon ages for the Mazama ash
elsewhere vary by several hundred years; a commonly used age of 6,600
years is based largely on a date of 6,640+250 years for charred wood
from within the layer near Crater Lake and on dates of 6,600+400 and
6,630+400 for peat below and above the ash, respectively, near Seattle,
Wash. (Rubin and Alexander, 1960, p. 161, 164).

DISTINCTIVE FEATURES

This ash bed can generally be distinguished from other tephras in the
park by its color and nearly uniform grain size and thickness (table 3).

29


that forms a distinct bed, and it locally makes a conspicuous band at or
near the ground surface (frontispiece). In some other places, it occurs
as a white sand spread over the surface.

Pumice and crystals compose almost the entire deposit. The pumice
generally is white and is not noticeably weathered, but in some places
the entire bed is stained brown. Dark minerals and a few lithic
fragments are conspicuous enough in a few places to give the bed a
“salt-and-pepper” appearance.

DISTRIBUTION, THICKNESS, AND GRAIN SIZE

\ orm alton ra) ha no heas om Moun

Helens; its thickest and coarsest axial zone passes through the
southeast corner of the park, where its average thickness is 5-8 cm (2-3
in.). It becomes thinner and finer to the west_and north and is absent in
the northwestern part of the park (fig. 18). The deposit is generally well
preserved, and reworked accumulations seldom are more than twice
the average thickness. The largest particles are lapilli about 8 mm
across, and the median grain size in the southeast corner of the park is
0.4-0.5 mm. The median size decreases to about 0.3 mm at the north

boundary of the park and at the western limit of the well-defined bed of
ash.

SOURCE AND AGE

Like layer Yn, this deposit increases in thickness and grain size
southwestward from Mount Rainier and can be traced to Mount St.
Helens. It forms a thick deposit of pumice lapilli and blocks on the
flanks of that voleano, but thins and abruptly becomes absent farther
southwest.

Near Mount Rainier, the layer is underlain and overlain by wood
fragments, whose radiocarbon content indicates ages of about 320 and

290 years, respectively (Crandell and others, 1962). The calendar age of
Ww

(Stuiver and Suess, 1966). Because trees as much as 435 years old are
growing on terraces near Mount Rainier that are younger than layer W

, » DP. A

DISTINCTIVE FEATURES

Layer W is distinguished from most other ash layers in the park by
its white color and sand-size pumice grains (table 3) and by its position
at_or not far below the surface. Where necessary, it can be dis-
tinguished from set-Y and set-P tephra by laboratory examination to
determine identity and refractive index of its iron-magnesium
minerals. (See table 8; p. 69.)

38


Preliminary results of the present study have been used for an evalua-
tion of volcanic hazards at Mount Rainier (Crandell and Mullineaux,
1967) and in reports on other aspects of geology in the park (Crandell,
1969a, 1969b, 1971).

Study of the tephra deposits was made concurrently with, and has
benefited substantially from, other U.S. Geological Survey in-
vestigations in the park that included research on lahars and glacial
deposits by D. R. Crandell, R. D. Miller, and H. H. Waldron and on
vegetation recovery by R. S. Sigafoos and E. L. Hendricks. R. E. Wilcox
provided advice concerning petrographic criteria likely to distinguish
one tephra deposit from another and methods especially suitable for
their examination (Wilcox, 1962), and the minerals and their optical
properties were identified on a spindle stage developed by Wilcox

(1959b). Radiocarbon dates cited in the report were determined in the
laboratory of the U.S. Geological Survey under the supervision of
Meyer Rubin. Chief Park Naturalist N. A. Bishop and many other
National Park Service people provided countless courtesies and abun-
dant information about the park that also significantly helped the in-
vestigation. ‘

TERMINOLOGY

Familiarity with a few technical terms is necessary for the non-
geologist to understand the descriptions or to use the report to identify
tephra deposits in the field. The term pyroclastic will be used to refer
to an explosive eruption that ejects fragments of rock from a volcano.
Tephra refers to fragments that have been thrown through the air by a
pyroclastic eruption or to deposits of such particles. These fragments
may be erupted either as pieces of rock that had previously cooled and
solidified or as clots of fluid or plastic molten material. Magma refers
to molten rock within the earth’s crust; it may move upward into a
volcano and subsequently be erupted, either quietly or explosively, as

lava at the surface.

The term vesicular refers to rocks that have a large amount of visi-
ble pore space. Vesicular tephra is formed by explosive eruption of gas-
rich magma that is stiff enough to hold the gas. The gas expands and
creates bubbles in the molten rock, which then cools rapidly and
hardens to a rock “froth.” Pumice is highly vesicular tephra and is
light in color. Scoria is somewhat less vesicular tephra and is darker in
color; the darker color usually indicates that the rock is richer in iron
and magnesium but poorer in silica than the pumice. Pumice and scoria
fragments at Mount Rainier consist largely of volcanic glass because
the lava cooled too rapidly to permit mineral crystals to form after its
eruption. However, they do contain crystals that formed in the magma
before it was erupted.



proposed for the clayey component of the Osceola Mudflow, a large

clay-rich Holocene mudflow that originated on the northeast flank of

Mount Rainier (Crandell and Waldron, 1956). The “airlaid facies” of

the Osceola described by Crandell and Waldron is the clay-rich layer F

northeast of the summit. The eruptions envisioned by Crandell and

Waldron as the cause of the Osceola Mudflow could easily have thrown

enough altered material into the air to form the clayey component of

layer F.

Layer F, then, is regarded as having resulted from three or more
pyroclastic eruptions. The sequence of events that formed layer F is
visualized as follows:

1. A violent steam eruption threw lapilli- and ash-size fragments of
previously solid, partly altered rock onto areas mostly northeast of a
vent that presumably was at or near the summit of Mount Rainier.

2. A subsequent eruption ejected crystals and molten material along
with a small proportion of solid rock fragments, depositing a crystal
and pumice ash over a broad area east of the summit.

3. Eruption of some molten material continued or was repeated but was
accompanied by ejection of a much larger proportion of fragments of
partly altered rock so that a second shower of clay-rich ash was

deposited north and east of the summit.
No evidence of a time interval separating the three units of layer F was
found, and all three could have been deposited within a short time,
perhaps within a few hours or days.

AGE

The age assigned to layer F (table 1) depends in part on its correla-
tion with the Osceola Mudflow, which has been dated as slightly less
than 5,000 radiocarbon years old (Crandell, 1963a). Layer F is younger
than a bed of peat in Cowlitz Park that has been dated as 5,0204+300
years old. (See table 5.) The ash has not been found above the Osceola
Mudflow, despite careful searches in areas where the ash is thick and

well preserved on ridges next to the mudflow; thus, the ash evidently is
at least as old as the mudflow. The close correspondence of age as well
as the clay content of layer F and the Osceola indicates that they are
correlative.

ORIGIN OF THE OSCEOLA MUDFLOW

The composition of layer F seems to bear on two aspects of the origin
of the Osceola Mudflow: the place of formation of the clay minerals in
the mudflow, and the manner in which the mudflow was initiated.
Crandell and Waldron (1956) and Crandell (1963a; 1963b; 1971) inferred
that the montmorillonitic clay minerals in the mudflow came from
altered rock within Mount Rainier, and that the mudflow probably was

66


LAYER C

Layer C is a thick coarse brown bed of lapilli, blocks, and bombs that
commonly lies at or near the ground surface. It is the largest and most
widespread of the Mount Rainier tephra deposits and contains brown
to_nearly white pumice, dark-gray scoria, and lighter gray lithic
fragments. Some large bombs of pumice and scoria enclose white
pumice lapilli and also angular lithic fragments.

Layer C varies somewhat in proportion of rock types, both vertically
within the layer and horizontally from place to place. Dark scoria, for
example, is concentrated at the base and on the north flank of the
volcano. Gray lithic fragments are most abundant near the base but
above the scoria and on the northeast and east flanks of the mountain.
Brown pumice occurs vertically throughout the layer but constitutes

_______ most. of the middle and upper parts, and it is also most commononthe
northeast and east flanks. These vertical and horizontal variations
seem to record either more than one eruptive shower or, possibly,
changes in type of material erupted and in wind direction during a con-
tinuous eruption.

Two thin beds of brown pumice lapilli in the stratigraphic position of
layer C are separated by a few centimeters of ash in several outcrops on
the southeast flank of the volcano. No other evidence of a time interval
between the two pumice beds was seen, however, and a separation of
layer C into two beds is not evident in most outcrops in that area.

Fragments in layer C are not strongly weathered; most pumice lapilli
are firm, and weathering is limited to a thin zone at the surface of the
lapilli. The deposit appears unweathered and unstained where it lies at
the ground surface though it commonly is stained brown where it is
buried (fig. 6).

Dist ibutio a thick ess, a d voru ie

Layer C occurs in a wide northeasterly trending lobe that extends
over much of the park and beyond its boundaries to the north and east
(fig. 24). The thickest airfall deposit seen is at Summer Land, where it
is about 30 cm (12 in.) thick. The layer is at least 15 cm thick over a
wide area that includes Ohanapecosh Park and that reaches nearly to
Yakima Park (fig. 24). At the northeast boundary of the park, it is still
8 cm thick at a distance of 25 km (15 miles) from the summit. Layer C is
equally thick over a wide arc near the voleano—the isopach for a

"thickness of 10 cm, for example, outlines a very broad lobe. Farther
away, however, the thick part narrows markedly and the isopachs for
lesser thicknesses outline more elongate lobes to the northeast (fig. 24).

Variations from average thicknesses are relatively unimportant and
indicate only minor reworking of this layer. Layer C rarely is absent
within the distribution shown in figure 24 where other tephra layers

~——~are preserved. Thicker accumulations from wind drift and stream

48


121°45"

oO 10 MILES

T
JOKILOMETERS

e4

DISTRIBUTION AND AVERAGE THICKNESS. in centimeters, of layer D (Fig. 23)

rapidly, to less than 8 cm thick in a distance of only about 5 km (3
miles). Variations from the average thicknesses, however, are common.
Although the deposit seldom is completely eroded away at sites where
other tephra deposits are well preserved, reworking of small lapilli into
thick wind-drift or fluvial accumulations is fairly common. At one site
southeast of the summit, an accumulation 60 cm thick was found
where the average thickness is less than 10 em.

The volume has been calculated roughly for the part of layer D that
lies within the 2-em isopach and the total present and freshly fallen
volumes have been estimated (table 7) in the same manner as for layer
L. (See p. 42.)

Grain size

Layer D consists chiefly of lapilli throughout its extent in the park.
Overall, it is coarsest grained directly east of the voleano’s summit,
where it is thickest. In Ohanapecosh Park and at Summer Land the

46


re

10 MILES

T
JOKILOMETERS,

o4

MINIMUM EXTENT of layer N. Limit is highly approximate. (Fig. 26)

Layer N is distinct from Yakima Park south to Cowlitz Park (fig. 26).
Scattered lapilli in the same stratigraphic position that probably are
part of this deposit also occur a few kilometers north of Yakima Park
and as far east as the park boundary. The layer is thickest and best
preserved in Ohanapecosh Park, but even there it is typically less than
2m thick. The largest lapilli, most no more than 1 cm in diameter, are
in the same area. Lapilli, however, make up only a small part of the
layer, which is chiefly ash.

Only a very rough estimate of the volume of the layer can be made
because so little is known of its thickness and extent. An estimated
volume of tephra as freshly fallen, based on an average thickness of 2
cm in a lobe that reaches Ohanapecosh and Cowlitz Parks, is 2-3 million
m*.

The limited distribution of layer N east of the volcano and its
lithologie similarity to other tephra from Mount Rainier indicate that
it was erupted by Mount Rainier. Its pumice and scoria lapilli lack

52


a few millimeters inward from their surfaces, and many particles in
the deposit-are covered witha _yellowish-brown coating.

Lapilli or bombs in layer B have been found only in or near
Ohanapecosh and Cowlitz Parks on the east and southeast flanks of the
volcano (fig. 28). Bombs-are-abundant-only-in the southern part of
Ohanapecosh Park, where they reach 8 cm (3 in.) in diameter, about
twice the size of the largest fragment seen in Cowlitz Park. In most
places, the associated-ash is no-more than 2-cm thick, and the max-
imum thickness seen is about 5 cm. Although the lapilli have been
found only in a very limited area, dark ash that is at the same horizon
in the ash sequence and that may have resulted from the same eruptive
episode. is present from the northeast flank clockwise around to the
south flank of the volcano.

Although the distribution_of layer B is not-well enough _known_to
permit calculation of its volume, a rough estimate of its freshly fallen
volume is about 5 million m*.

The presence and location of bombs in layer B leave little doubt of its
origin at Mount Rainier. The restriction of the bombs and lapilli to the
east and southeast flanks of the voleano suggests a significant west to
northwest_wind during their eruption. On the other hand, the wide ex-
tent of ash that apparently is associated with the bombs suggests that
wind directions changed during eruption of the finer material.

This deposit is distinguished from most other tephra layers by its
content of scoria bombs and lapilli. Only layers D and C contain similar
fragments, and both are thicker and coarser overall than layer B in
Ohanapecosh and Cowlitz Parks. Under the microscope, layer B is dis-

tinguished from layers D and C chiefly by the absence of hornblende,
and by the abundance and refractive index of olivine (fig. 33).

LAYER X

Layer X consists of sparse pumice and scoria lapilli that have been

a continuous layer; they 2 are found ‘scattered 0 on n the surface of barren
moraines in alpine areas, or nearly hidden in surficial duff on lower,
forested moraines. The deposit is inconspicuous even on some of the
barren moraines where only a few lapilli per square meter might be
found. In addition, the  lapill are unevenly distributed and locally may

Pumice and scoria are the “only materials recognized; any Hithi¢
material that might have fallen has not been distinguished from the
lithic fragments in the underlying moraines. Though most of the lapilli
are brown, some are white or various shades of gray.

Layer X has been found on moraines in three places on the northeast
and east flanks of the voleano—in Glacier Basin below Inter Glacier, in

56


southerly and perhaps variable winds blew during ejection of a small
part of the tephra, perhaps during one or more pulses of the waning
stage of the eruption.

Distinctive features

On the southeast flank of the volcano, thickness and coarse grain size
distinguish layer L from all tephras except layers D and C. Layer L
consists of lighter colored fragments than layer D, it is noticeably
lower in the tephra sequence than layer C, and it contains fewer lithic
fragments than either of the other two coarse tephras. It also differs in
mineral content from the other two. Where layer L is thin and fine
grained, its stratigraphic position between the prominent layers O and
D is the most helpful field criterion for identification. Layer L usually
is underlain and overlain by thin beds of dark-gray ash, whereas layer
A, the only other pumiceous tephra in that stratigraphic position, is
not. Laboratory examination of the refractive index of olivine also
helps to distinguish layer L from layer A.

LAYER D

Layer D is a conspicuous thick yellowish- to reddish-brown deposit
(fig. 22) that consists mostly of scoria and pumice lapilli. Its striking
features are (1) a relatively strong color, (2) abundant dark-gray scoria,
(3) scattered bombs that are much larger than the average fragment
size, and (4) abundant hornblende in both scoria and pumice. The
pumice, usually pale brown, is not nearly as abundant as the darker
scoria. Lithic fragments are common though they usually are masked
by reddish-brown weathering products. These products, probably
derived largely from the scoria, generally stain the entire layer. Large
scoria lapilli have visible weathered rinds, and some small lapilli are
weathered enough to be soft all the way through. The weathered rinds
are soft, and smear easily to a brown gritty clay. The alteration
products have formed chiefly from volcanic glass, for the crystals are
firm. Layer D is more weathered than most other tephra deposits,
probably because of its more basic composition (table 10).

Distribution, thi

Layer D forms a well-defined lobe that extends eastward from
Mount Rainier to far beyond the park boundary (fig. 23). It forms a
somewhat thinner but broader main lobe than does layer L (fig. 19).

The average thickness of the deposit is greatest in the northern part
of Ohanapecosh Park and at Summer Land (fig. 23). The maximum
thickness of undisturbed tephra in those areas is about 15 cm (6 in.).
Thicknesses of 10 em are common elsewhere in and near Ohanapecosh
Park, and at the eastern park boundary the layer still is as much as 8
em thick (fig. 23). To the north and south across the lobe, it thins more

44



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fational Part

~\ Mount St Heleris
—_——

‘ -
OREGON \ IDAHO |

\
Crater Lake National Park | \
(site of Mount Mazama) ! \

ALBERTA

Edmonton;

1
SASKATCHEWAN

oCalgary

\

\

—Leanapa | _ =
UNITED STATES

. MONTANA

——

an

500 MILES
500 KILOMETERS

MINIMUM EXTENT OF LAYER Yn (patterned). Dashed line marks thick coarse axial zone. From
Crandell, Mullineaux, Miller, and Rubin (1962); Fulton and Armstrong (1965); Nasmith,
Mathews, and Rouse (1967); and Westgate, Smith. and Nichols (1970). (Fig. 15)

least 1.5 m thick is known northeast of the park, and a bed that is as
much as 8 m thick lies just outside the northwest corner of the park.

SOURCE AND AGE

Layer Yn increases in grain size and thickness progressively
southwestward from Mount Rainier to the base of Mount St. Helens,
where it consists of more than 1 meter of lapilli and bombs of pumice;
farther southwest, however, it is absent. On Mount St. Helens itself the
layer is buried by younger deposits. Layer Yn is bracketed by

34


SOURCE AND ORIGIN
The composition and extent of layer F show that it came from Mount
Rainier. Its broad sheetlike distribution demonstrates that it was air-
laid, whereas the lapillisize fragments and the montmorillonite clay

wind. The abundance of pumice and glass-encrusted crystals, especial.
ly in the middle unit, verifies that molten material was erupted during
formation of this layer.

The origin of the abundant clay is of special interest, inasmuch as no
more than a trace of crystalline clay mineral was found in any of the
other postglacial tephra layers. The clay minerals in layer F must have
been formed elsewhere before their deposition as part of that tephra
layer rather than in place by weathering afterward. Their restriction to
certain stratigraphically well defined beds in layer F and the oc-
currence of both fresh pumice and partly altered lithic fragments in the
same beds provide strong evidence that the clay minerals formed
before they were emplaced in the tephra, Clay minerals are abundant
in the lower and upper units of layer F, yet absent in the middle unit
and in other ash layers and volcanic sands below and above layer F. At
the McNeeley site near Yakima Park (fig. 32), for example, the propor-
tion of montmorillonitic clay in the clay-size fraction of samples from
the upper and lower units is at least 80 percent (table 10). No mont-
morillonitic clay was detected by the same technique in samples from
the middle unit of layer F or from the other beds immediately below
and above layer F. Restriction of clay minerals to the two separate un-
its in layer F indicates that the clay minerals did not form as a part of a
clay-rich surficial-weathering profile.

In addition, delicate pumiceous glass exists along with lithic
fragments that are altered partly to clay in the upper unit of layer F
and elsewhere in the undivided layer. The glass would be expected to be
altered more easily by weathering than the lithic fragments. Thus, sur-
vival of the glass implies that surficial weathering did not form the
clay minerals that occur with it.

Iron-stained crusts within the tephra sequence were cited by Fiske,
Hopson and Waters (1963, p. 85) as evidence of formation of clays by
weathering of tephra. Iron-stained crusts that probably result from
leaching occur under layer F at several localities, but they also occur
under ash deposits that do not contain clay. In addition, they are absent
in many places where layer F is rich in clay minerals. Thus, the iron-
oxide crusts probably demonstrate the occurrence of leaching, but they
do not record the formation of clay minerals.

The most probable origin of the clay seems to be by hydrothermal
alteration of solid rock within the voleano, and the ejection of that
altered material by later pyroclastic eruptions. Such an origin was

65


Contents

Page.

Abstract .... 0... cece cece cece cee nett teen cece teen eee 1
Introduction 0.0.0.0... 0. ce cece eee ee eee 2
Previous work and acknowledgments ... 4
Terminology ..............0eeee eee 6
General setting . 8
Brief description of tephra deposits .. 8
Postglacial activity of Mount Rainier .. 15
Eruptive history ...............6- 15
Volume oo... . cece cece cece cece eens 18
Kinds of eruptions ................----+ 18
Hazards from future eruptions of tephra . 21
Kinds of hazards ................00. 21
Location... 22
Warning 0... 2... cece cece eee eee eect eee e eee 23
Ages of tephra layers and their use as marker beds .. . 23
Tephra from distant volcanoes .............. cee cee e eect e ee 27
Tephra layer O (Mazama ash) .............2.-0-e0ee eee 27
Distribution, thickness, and grain size .. 29

Source and age ..........-...20eeeeeee 29
Distinctive features ... 29

Tephra set Y ..........- 30
Tephra layer Yn... 0. ee eevee eevee eevee . 83
Distribution, thickness, and grain size .............. - 33

Source and age .......... 0. sce e eee e cece eee ene 34
Distinctive features 36
Tephra set P............ 36
Tephra layer W .................00 ee eeeee 37
Distribution, thickness, and-grain-size—. 38

Source and age ....6..... 0. cece cece eee . 88
Distinctive features ........... 0. cece cece eee eens 38

Mount Rainier tephra deposits ..............0..e scenes eens 39
Tephra layers L, D, andC.. 40
Layer Lo... c een eee eee 41
Distribution, thickness, and volume... 42

Grain size 20.0.0... 0. cece eee eee eee . 42

Source 2.0.0.0... cece cece eee cece eee 43

Distinctive features 44



source or age of a layer. For example, the presence of cummingtonite in
several thin ash beds invites comparison with layer Yn and suggests
the possibility that Mount St. Helens was their source, and a high silica
content for a tephra layer in the park at least suggests a source volcano
other than Mount Rainier.

pumice or scoria fragments whose abundant constituents are volcanic
glass, plagioclase, opaque minerals, and iron-magnesium minerals; the
same_constituents_make up the ash-size particles. _In_addition, the
tephra deposits include very small amounts of other minerals, such as
apatite and zircon. Mineral content, refractive index of iron-
magnesium minerals, refractive index of glass, clay-mineral analyses,
and a few whole-rock chemical analyses were tried as aids to identifica-
tion. Of these properties, the mineral content and refractive indices of
iron-magnesium minerals proved to be the most useful.

IRON-MAGNESIUM-MINERAL CONTENT

The iron-magnesium minerals that occur in the tephra layers are
listed in table 9; they are the predominant nonopaque grains that were
obtained by crushing lapilli or coarse-ash-size pumice or scoria. The
minerals listed for deposits that lack coarse debris are ones that occur
as discrete crystals that are at least partly mantled by vesicular glass.
Three mineral associations are evident: (1) hypersthene and augite,
with or without olivine or hornblende, (2) hypersthene and hornblende,
and (3) cummingtonite and hornblende. The hypersthene-augite com-
bination is found in all Mount Rainier tephra deposits; absence of
olivine seems to distinguish two of the Rainier layers, presence of
minor amounts of hornblende marks two more, and abundance of
hornblende marks only one. The cummingtonite-hornblende suite

marks the older Mount St. Helens tephras, and the hypersthene-
hornblende suite marks the younger ones.

Some differences in mineral content, such as the absence or abun-
dance of hornblende or the contrast between hypersthene and cum-
mingtonite as principal constituents, can be used to help distinguish
tephra layers in the field. For example, the abundant hornblende in
crushed lapilli of layer D commonly is distinct enough to be identified
with a hand lens. With practice, one can also differentiate enough
between the color of cummingtonite in layer Yn and that of hypersthene
in layer W to help differentiate those two deposits. Concentrates of
heavy minerals can be prepared conveniently in the field by crushing
and panning pumice or scoria lapilli with a small mortar and pestle.

Most Mount Rainier tephra layers are similar in mineral content to
Mount Rainier lava flows, which repeatedly have been described as
remarkably uniform in composition (Hague and Iddings, 1883; Coombs,

68


caused in some way by an eruption. Crandell (1963b, p. B139) suggested
that a phreatic explosion had caused a large part of a former summit to
avalanche and form the mudflow. These hypotheses require that the
clay minerals in the “airlaid facies of the Osceola” (layer F of this
report) were formed before they were deposited.

Fiske, Hopson, and Waters (1963), however, held that the clay com-
ponent of the Osceola originated by postglacial weathering of
F si ° . : : 1
deposits like the “airlaid facies.” The brown iron-stained crusts within
the tephra sequence were regarded as evidence of long-continued oxida-
tion and leaching of ash after its deposition (Fiske and others, 1963, p.
85). They proposed that the mudflow was formed by mobilization of a
thick valley fill of partly altered pumiceous sediments. They, too,
suggested that a volcanic eruption was a_possible—though not

necessary—cause of the mudflow.

Evidence in the tephra sequence adequately demonstrates that the
clay minerals in layer F (the “airlaid facies” of the Osceola) were
formed before deposition of the tephra rather than after. (See p. 65.)
The fact that montmorillonitic clays have not been identified in other
postglacial tephra deposits suggests that no-large amount of clay was
formed in pumiceous materials in valley fills after the last glaciation.
Thus, surficial weathering seems to have been inadequate to provide
the large amount of clay that is present in the mudflow. A more likely
source of the clay was the altered rocks within the volcano, where the
clay formed before both layer F and the mudflow.

The evidence provided by layer F also supports the proposal that the

Osceola Mudflow originated during an eruption. There seems to be no
question that’ layer F and the Osceola are related and that layer F
records multiple pyroclastic eruptions that included ejection of molten

material during that episode. The Osceola Mudflow thus probably was
initiated during a complex eruptive episode which included pyroclastic
eruptions. It cannot be proved that the Osceola was caused directly by
an eruptive event, yét the timing suggests strongly that it was
triggered by one. Either of the explosions that produced the lower and
upper units of layer F might have forcefully ejected rock particles and
clay into the air to form layer F and might have also dislodged the
massive amounts of material that formed the Osceola Mudflow.

Petrography And Chemistry

Laboratory determinations of the composition of tephra have proved
to be useful supplements to field criteria for identification of tephra
layers. Mineral and element content also may suggest correlations or
comparisons with other known units, and thereby point to a possible

67



TEPHRA UNIT

R A L D N F H

|
AUGITE | 4

| .

|

r

|

|

|

|

|

F ;
veo Pope bel yp phy,

HYPERSTHENE

2
|
T

a
]
T

10 OLIVINE | 4

169+ | \, | i

saa! | \ . | on | 4

1.67 ly 1

REFRACTIVE INDEX (ng

HORNBLENDE

168 4

167- rl fl

se inl in
sk | | ;

Ri .
or range of index measured for a single fragment. (Fig. 33, above and facing page)

layer A are different from those in layer L. Similarly, the high R.I. of

hypersthene distinguishes layer W from the otherwise similar layer P.
_____ Refractive-index measurements of glass have proved to be less useful ____

for identification (fig. 34). A few measurements showed a very wide

range in R.I. values for glass in most tephra deposits from Mount

Rainier. The indexes seem to correlate closely with (and a:

useful than) the more easily observed color of the fragments. R.I.

10



subdivided by the presence of layer W into those that are older and
younger than about 450 years (Crandell, 1969a, p. 30).

Lahars and associated alluvium of various ages are highly similar
but can be subdivided into units younger and older than various tephra
deposits. Layers O, Yn, and W are particularly valuable for such work
because they are noticeably different in composition from the
materials derived from Mount Rainier volcano. The use of these
tephras to subdivide valley-floor deposits is illustrated by the sequence
in the Nisqually River valley (Crandell, 1971). The sequence there in-
cludes at least 12 lahars and 8 fluvial deposits, and those deposits are
subdivided into lahar and alluvium assemblages of 4 ages that are in-
terbedded with the tephra units, as follows (Crandell, 1971, p. 32):

Lahar assemblage D :
Pyroclastic (tephra) layer W
Lahar assemblage C
Pyroclastic layer Yn

Lahar assemblage B
Pyroclastic layer O

Lahar assemblage A

These particular tephra layers generally can be identified in the
field, and the various lahar assemblages could be identified fairly
readily wherever the tephra deposits are interbedded with them.
Similarly, deposits that are about the same age can be identified from
one valley to another by their relation to those tephra layers (Crandell,
1971, p. 11).

_ Fephra From Distant Volcanoes

The non-Rainier or exotic tephra layers are generally fine grained
and_light-colored_and-are_rarely_more_than-a_few-centimeters-thick.
They stand out in outcrops as thin white or light-colored bands that
contrast with the darker and duller colors of the Mount Rainier tephra.
Layers O, Yn, and W are the most widely preserved and readily
recognized and are therefore the most important marker beds. All the
foreign tephra beds described here except layer O are from Mount St.
Helens voleano, which is 80 km (50 miles) south-southwest of Mount

Rainier. Layer O was erupted by prehistoric Mount Mazama at the pre-
sent site of Crater Lake, Oreg., about 440 km south of Mount Rainier.

TEPHRA LAYER O (MAZAMA ASH)

Layer O in the park is a thin yellowish-orange to pale-brown fine ash
that occurs near the base of the tephra sequence (fig. 11). It is
widespread and probably the most visually striking ash in the park
(frontispiece, figs. 5, 7). Locally it is interbedded with peaty material

27


C that are 10 cm (4 in.) or more thick in some exposures are so thin as to
be nearly unrecognizable, or are absent, only a few kilometers away. In
contrast, non-Rainier tephra units, such as layers O, Yn, and W, are
relatively uniform in thickness over much of the park (fig. 4). Figures
5, 6, and 7 illustrate the tephra sequence at three of the localities shown
in figure 4.

Figure 8 is a diagram of a sequence of tephra that might be present
in alpine meadows directly east of the summit if all the Mount Rainier
tephras had been blown directly eastward, rather than in several

TEPHRA LAYERS IN AN ALPINE MEADOW (McNeeley site), 0.7
km (0.4 mi) north-northwest of Ranger Station at Yakima Park.
The middle unit of layer F forms a thin gray bed that is both un-
derlain and overlain by thicker yellowish-brown ash of the lower
and upper units. Layer O (“Mazama ash”) is iron stained to a
yellow or brown in most places in Mount Rainier National Park.
(Fig. 7)

13


previously solidified rocks in the cone, for those rocks are solid or only
slightly vesicular. Instead, the particles prove that the erupted
material was still fluid or plastic enough to allow the contained gas to
expand and form bubbles. Because the highly vesicular particles were
molten when ejected, they record the presence of magma in the volcano
before the eruption.

Eruption of molten lava, especially of a large amount, implies a kind
of eruptive episode in which magma rises under and into the volcano
from a considerable depth. Generally, it causes earthquakes as it rises,
as well as increased heat flow and gas emission from the volcano. Small
amounts of magma might possibly be formed within a volcanic cone by
melting of previously formed rocks, but that would require at least the
rise of abundant heat into the cone, probably accompanied by gases.
Significantly, the earthquakes and increased escape of heat and gas
that accompany the rise of magma under a volcano probably would be
observed before a major eruption actually occurred.

An obvious property of molten lava is its heat. A small amount of
fine pumice or scoria ejected high into the air might cool before
reaching the ground surface, but a large volume of coarse fragments
could still be very hot when it fell. Falling pumice and scoria can be hot
enough to start forest fires.

Most tephra deposits from Mount Rainier contain lithic fragments as
well as pumice or scoria. A high proportion of lithic particles suggests
eruption of abundant volcanic gas, carrying with it fragments of
previously solidified rock. These fragments may be torn from the
crusts formed on the surface of magma exposed in a vent or from the
vent walls, or they may be particles of rocks that have slumped down
into the vent from the crater walls. Fragments that are all nearly alike,
such as the abundant lithic fragments in the lower part of layer C,
probably were derived from a crust formed on the magma column.
Lithic particles in most other tephra deposits, however, consist of a
variety of rock materials and probably represent debris derived from
the walls of the vent and crater.

Tephra layer S contains no pumice or other evidence of eruption of
molten lava and may have originated from an eruption of only gases,
probably mostly steam. The lithic lower member of layer F may also
have been ejected by a steam blast. These eruptions might represent
merely one kind of event that occurred during an eruptive episode dur-
ing which magma was also ejected. Such events could also occur
without any rise of magma into the volcano and, therefore, without the
associated events that usually precede an eruption of magma.

The great extent and good sorting of several tephra units indicate
that the units resulted from strong high-angle eruptions of gases and
tephra. Layers D and-C, for example, occur chiefly as long tongues of
well-sorted lapilli and ash that extend many kilometers beyond Mount

19


a distance of about 7 km (4 mi) from the summit. It has the disadvan-
tage, however, of overemphasizing layer L, which is conspicuous in ex-
posures only in a narrow lobe.

Table 3 summarizes identification characteristics of the tephra units.
Few if any of these units can be recognized by any one feature. For
some, color is fairly distinctive, but even in those the color does vary,
and no color is unique to any one layer. Color combined with thickness
and grain size are enough to identify some deposits, however, when
those characteristics are compared with the values expected at a given
locality. Identification is most reliable if all the layers present are con-

also the position of that layer relative to ‘all the other tephra units.

Postglacial Activity Of Mount Rainier
ERUPTIVE HISTORY

The tephra layers constitute proof of repeated eruptions of Mount
Rainier at irregular intervals during the last 10,000 years. Overall, the
11 pyroclastic eruptions represented had an average frequency of about ©
1 every 1,000 years. However, the average does not portray the actual
timing of the eruptions (table 4). For example, 8 of the 11 occurred dur-
ing the period between about 6,500 and 4,000 years ago, at the relative-
ly high rate of nearly 1 every 300 years. In contrast, the eruptions
centration of eruptions near the middle of postglacial time is a striking
feature of Mount Rainier’s recent eruptive history.

Each of thé average frequencies stated above is a minimum because
presumably there were some pyroclastic eruptions in postglacial time
that have not been recognized in the tephra sequence. For example,
any-eruption that ejected only_lithic ash is not included-in the figures.
Similarly, any eruption that threw tephra only as far as the upper
slopes of the volcano would not be represented. Even somewhat more
extensive tephra -deposits, of the size of layers A and H {table 4), might
not be recognized if they were ejected in early or very late postglacial
time; preservation is so poor in the deposits older than about 7,000
years and younger than about 2,000 years that thin tephra layers would
be very difficult to identify.

The eruptive episodes represented by the 11 recognized tephra

them, Furthermore, the ‘eruption of each of the pumice or scoria
deposits probably represents a period of only a few hours or a few days
during an eruptive episode. Such an episode may have extended over a

15


TEPHRA

LAYER
| W (450 yr}
|
Yn (3,400 yr)
30-12
2)
oi
no
| 0 (6,600 yr) ri
2/6
E|2z2
Ez
w
oO
bo o-Lo

REPRESENTATIVE STRATIGRAPHIC POSITIONS,
thicknesses, and ages, in radiocarbon years, of
layers O, Yn, and W, relative to the postglacial
tephra sequence as a whole. (Fig. 11)

and is white (fig. 6), but at most places it is strongly colored. Although
most of it is no more than 5 cm (2 in.) thick, it is remarkably well
preserved. It is found not only in alpine meadows, but also on steep
valley walls, and on valley floors, where it is commonly interbedded
with river sediments and mudflow deposits.

Individual ash particles, most of which are too small to be seen with
the naked eye, are chiefly crystal fragments and highly vesicular
pumice. The larger pumice grains appear silky under a microscope
because their vesicles are drawn out into tiny elongate tubes. The
pumice is not strongly weathered, even though individual particles
have rusty coatings that give the layer its color. These coatings may
consist largely of noncrystalline clay material and iron oxide; no more
than a trace of crystalline clay minerals was detected by X-ray-
diffraction analyses of six samples of the ash (p. 72).


ol

zS ° 10 KILOMETERS
3* a)
SUMMIt bn,

McNeeley

$6," Ohanapecosh Park Ohanapecosh Park
Millliwakas

M

Ridge

Mazama Ridge Williwakas
—
C poe ccc
P (upper 2 beds)
Yn pases:
5 = —
D ie a
= =

Mystic Lake

w
c

P (upper 2 beds)

30-712
che
g
g/g
= &
Eg z
&
co
o-Lo

CORRELATION DIAGRAM from south, counterclockwise, around to north flank of Mount Rainier, showing thicknesses of major tephra units. (Fig. 4)


Mullineaux, D. R., Sigafoos, R. S., and Hendricks, E. L., 1969, A

historic eruption of Mount Rainier, Washington, in Geological
Survey research 1969: U.S. Geol. Survey Prof. Paper 650-B, p. B15-
B18.

Nasmith, H., Mathews, W. H., and Rouse, G. E., 1967, Bridge River ash
and some other Recent ash beds in British Columbia: Canadian
Jour. Earth Sci., v. 4, no. 2, p. 163-170.

Powers, H. A., and Wilcox, R. E., 1964, Voleanic ash from Mount
Mazama (Crater Lake) and from Glacier Peak: Science, v. 144, no.
3624, p. 1334-1336.

Rubin, Meyer, and Alexander, Corrinne, 1960, U.S. Geological Survey
radiocarbon dates V: Radiocarbon Supp., v. 2, p. 129-185.

Sigafoos, R. S., and Hendricks, E. L., 1961, Botanical evidence of the
modern history of Nisqually Glacier, Washington: U.S. Geol.
Survey Prof. Paper 387-A, p. A1-A20.

1969, The time interval between stabilization of alpine glacial
deposits-and establishment of tree seedlings, in-Geological Survey
research 1969: U.S. Geol. Survey Prof. Paper 650-B, p. B89-B93.

Smith, G. O., 1900, The geology of Mount Rainier: Mazama, v. 2, no. 1,

p—18-24

p-18-24.

Stuiver, Minze, and Suess, H. E., 1966, On the relationship between
radiocarbon dates and true sample ages: Radiocarbon, v. 8, p. 534-
540.

Suess, H. E., 1970, Bristlecone-pine calibration of the radiocarbon
time-scale 5200 B.C. to the present, in Olsson, I. U., ed., Radiocar-
bon variations and absolute chronology: New York, John Wiley &
Sons, Inc., p. 303-309.

Thorarinsson, Sigurdur, 1954, The tephra-fall from Hekla on March
29th, 1947, v. 2, no. 8, of The eruption of Hekla 1947-1948: Visin-
dafélag Islendinga Rit, 68 p.

1967, The eruptions of Hekla in historical times—A
tephrochronological study, with Appendix—Mineralogical and
petrographical characteristics of Icelandic tephra, by Jens
Témasson, v. 1, of The eruption of Hekla, 1947-1948: Visindafélag
Islendinga Rit, 183 p.

Westgate, J. A., Smith, D. G. W., and Nichols, H., 1970, Late Quater-
nary pyroclastic layers in the Edmonton area, Alberta, in Pawluk,
S., ed., Pedology and Quarternary Research—Symposium, Ed-

monton, Alberta, 1969: Natl. Research Council Canada and Alber-
ta Univ., p. 179-186.

Wilcox, R. E., 1954, Petrology of Paricutin volcano, Mexico: U.S. Geol.
Survey Bull. 965-C, p. 281-353.

1959a, Some effects of recent volcanic ash falls, with especial

reference to Alaska: U.S. Geol. Survey Bull. 1028-N, p. 409-476.

82


Taste 9. —Content and proportions of iron-magnesium minerals in heavy-mineral

fractions
[Queried minerals are those that may have been derived from previouly solidified rocks rather than from pumice or
scoria
Tephra Principal constituents Secondary constituents Minor
layers (>30 percent) (2-30 percent) constituents
(<2 percent)
x Hornblende,
olivine.
Cc Do.
B. _N -
H. . Olivine.
F. . None.
N. - None.
D. . Olivine.
L. . Do.
A. . Do.
R. Do.
Volcanoes other than Mount Rainier
W...... Hypersthene .........-.--+- Hornblende......... | None.
Set P:
Upper _Hypersthene, hornblende.... None............... None.
two.
Lower ..do ....... cee cece eee None ...........-00- Cummingtonite (?),
two. augite (?).
Set Y:
Upper Hypersthene (?),
two. augite (?).
Yn.. None.
Lower we ...  Hypersthene (7),
two. | augite (?).
oO. Hypersthene ... None.

1936; Fiske and others, 1963). Only in layer D does a tephra mineral
suite differ strikingly from that of the flows. Because hornblende is

considered to be stable under high water pressure, its presence may in-
dicate that the magma of layer D was brought up relatively rapidly
from depths at which hornblende was stable and was ejected before the
hornblende was altered or resorbed.

REFRACTIVE-INDEX MEASUREMENTS

Refractive index (R.I.) values of iron-magnesium minerals provide
additional help in identifying some tephra layers that are similar in
mineral content (fig. 33). For example, the R.I. values of olivines in

69


The rubble of layer S has been found interbedded with other tephra
deposits only on the ridge from Yakima Park east to Sunrise Ridge.

Island Mountain may also be part of the deposit, but their relation to
other ash layers was not seen there. Thin lithic ash that probably is
part of layer S has been found in Huckleberry Park north of Yakima
Park.

The thickest and coarsest bed of layer S that lies between other
tephra deposits can be seen in roadcuts at the switchback of the
highway to Yakima Park that is directly south of Dege Peak. There, the
deposit is 50-125 em (20-50 in.) thick and contains blocks as much as 45
em across. From that locality the layer thins abruptly and becomes
finer grained to both the west and the east along the ridge. It is only 2-5
cm (1-2 in.) thick near the Ranger Station at Yakima Park, and the
largest fragments are about 2 cm across. The ash in Huckleberry Park,
to the north, is less than 2 em thick. The deposit is absent at Summer
Land, south of Goat Island Mountain.

Too little of layer S is seen to caculate its volume even roughly, but it
may have been larger than all but a few other Mount Rainier tephra
deposits. If it originally was a minimum of 50 em thick between out-
crops of that thickness and either-a summit or-a flank vent, for exam-
ple, the volume probably was at least 20 million m*.

bayer 8 seems to have been erupted from Mount Rainier inasmuch

tribution. The "decreases | in thickness and | grain 5 size along the ridge
from Rene? Peak westward toward Yakima Park, however, suggest that

pattern of grain size e and thickness could have resulted from ejection of
a very narrow lobe of coarse debris from a summit vent or of a
somewhat broader lobe froma point lower on the east flank of the
volcano.

Layer S is described as tephra because of (1) the size and extent of
its large blocks and lapilli, (2) its rock types, and (3) its position
between other tephra layers. Its blocks and lapilli of Mount Rainier
lava mantle the area from Yakima Park to Sunrise Ridge and they
were laid down over easily eroded ash layers without significantly dis-
turbing them. The rubble is not avalanche debris or talus, for there is
no adequate source of Mount Rainier lava higher on that slope. They
are not frost-heaved blocks, for the underlying ash beds are not dis-
turbed; and they cannot be glacier deposits because they lie far beyond
the limits reached by glaciers of Holocene age (D. R. Crandell, oral
commun., 1971). They must have been airlaid, but they obviously were
not windblown. Thus, the evidence seems to require that the rubble of
layer S was distributed by a pyroclastic eruption, and the lack of



Color, size, thickness, gross composition, and stratigraphic position permit field iden-
tification of most of these layers where the sequence is fairly complete. The content and
refractive index of iron-magnesium minerals, however, are particularly useful for initially
establishing the sequence of beds present and for identifying beds that occur alone.
Three mineral pairs are especially common: (1) hypersthene and augite, generally

______ occurring with _olivine_and_less commonly with hornblende; (2) hypersthene_and
hornblende; and (3) cummingtonite and hornblende. The hypersthene-augite suite is
found in all tephra from Mount Rainier; olivine is absent in only two of those layers, and
hornblende occurs in three. The other two pairs of minerals commonly occur without
other iron-magnesium minerals and are characteristic of Mount St. Helens ash layers.

The tephra record shows that Mount Rainier has erupted at very irregular intervals
throughout the last 10,000 years. The earliest postglacial eruption was more than 8,750
years ago, then the volcano was relatively quiet until about 6,500 years ago. It was
especially active between about 6,500 and 4,000 years ago, when 8 of the 11 known tephra
layers from Mount Rainier were ejected, including 4 of the 6 most voluminous deposits. No
eruption of Mount Rainier is recorded in the tephra sequence between 4,000 and 2,500
years ago. The most voluminous postglacial tephra deposit was erupted between 2,500
and 2,000 years ago. Since that time, the 19th-century eruption is the only one known to
have produced a tephra layer.

Large tephra deposits from Mount Rainier that consist chiefly of pumice and scoria

volcano from a considerable depth. Such an upward movement of magma expectably
would be accompanied by earthquakes and by increased flow of heat and gases from the
voleano. Thus, an initial explosive eruption probably would be preceded by recognizable
signs. A pyroclastic eruption that ejected only fragments of previously solidified rock,
however, might occur with no preliminary movement of magma, and perhaps without
even significantly increased heat or gas emission. Such an eruption might occur without
any kind of recognizable preliminary events.

Hazards from pyroclastic eruptions include the effects of impact and accumulation of
tephra fragments, of volcanic gases, and of secondary landslides, mudflows, and floods
set off by the eruptions. Eruptions of molten material, because they probably would be
preceded by warning signs, are potentially less dangerous to persons than nonmagmatic
eruptions that might occur with no warning.

Introduction

Many volcanic eruptions within the last 10,000 years—roughly the
time since the last major glaciation—are recorded by thin widespread
layers of volcanic airfall debris, called tephra, that blanket much of
Mount Rainier National Park in western Washington (fig. 1). Twenty-
two of these layers were studied in detail to document the recent erup-
tive history of Mount Rainier volcano and to identify “marker beds”
that could be used to date other kinds of geologic deposits. The
appearance of some of the tephra deposits is shown in the frontispiece,
and their sources and ages are listed in table 1.

Eleven of these layers were erupted by Mount Rainier. Their source
is shown by coarse fragment size on the flanks of that volcano, by rapid
decreases in thickness and grain size away from it, or by distribution
limited to the area near Mount Rainier. Because prevailing winds in



limiting dates. For example, the closest limiting dates for layer L are
6,440+250 and 6,380+250 years (table 5); these are also the only dates

ilable from sedimen at are just under and over that tephra

Taste 5.—Limiting dates and ages assigned to tephra layers

Bracketing radio-

pproxima
Layer Approximate ______ Bristlecone- _tree-ring calendar
H i 2 ts te®
“Be Sample Radiocarbon pine age as of 1970 date
No. years before
1950
120
x 125. A.D_ 1825
150
W1120 290200 500-300 435

WwW 450 A.D. 1500
W1119 3204200 500-400 .
W13932,040£200 2,100-1,900 ~~
Cc 2,200 300 B.C.
W1396 2,340 +200 2,400 . .
W1394 2,460+200 2,800-2,500

SetP __3,000-2,500 : ‘1200-700 B.C
W2675 2,9604250 3,300-3,200 - 7
Upper 3,300-3,000 1600-1300 B.C.
eds
wt yd y 4oo 2549 9:8508250 8,600, nes 00Re.

W1115 3,500#250 —4,000-3,700
W1752 35104250 4;000-3,700. - .
Lower 3,900-3,500 2400-2000 B.C.

beds
W2677 3,9004250  4,500-4,400
B 4,500 3400 B.C.
H 4,700 : +. 3600B.C.
F “5,000 3800 B.C.
s 5,200 W2053 5,020 300 5,800 0 wees eee 4200 B.C.
N 5,500 4400 B.C.
W2437 5,7704250 6,600 sss
D 6,000 4900 B.C.
W2424 6.3804 250 7,300 vse
L 6,400 5300 B.C.
W2423 6440250 7,800 eevee
A 6,500 5400 B.C.
W2422 6,730.4 250. 7,600. see ess
fe) 6,600
W951 8,750+280 «) 5500 B.C.
R >8,750

‘Years before 1950. Ages of layers X and W are based on tree-ring counts; all others are radiocarbon ages. Radiocar-
bon ages are used herein for discussion because they are in wider general use than bristlecone-pine ages.

Based on comparison of radiocarbon dates to bristlecone-pine tree-ring dates (Suess, 1970; and H. Suess, oral com-
mun. to Meyer Rubin, 1972) in years before 1950, Fluctuation in atmospheric radiocarbon concentrations in the past
can cause one radiocarbon age to be equivalent to a range of bristlecone- pine ages.

Calendar dates are derived from bristlecone-pine ages and differ from r: arbon ages in this table by as much as
900 years, because of fluctuation of atmospheric radiocarbon concentrations in the past.

‘Based on several radiocarbon dates. (See Crandell, 1971, p. 24.)

SBased_on_several radiocarbon dates._( 29.)

No date available.

25



from its deposition largely on ice, snow, and barren ground that were
present at the time, which may have been during the waning stages of
the last major glaciation.

Layer R is not significantly different from several other tephra
deposits from Mount Rainier, but it is the only pumiceous layer that is
below the highly distinctive layer O and, so, is not likely to be confused
with other tephra deposits.

TEPHRA LAYER A

Layer A (fig. 29) is a thin inconspicuous bed of brown ash that is
marked by scattered white and brown pumice lapilli. The largest of
these lapilli are white, and their diameters are locally much greater
than the thickness of the associated ash. Typically, they form a discon-
tinuous line of white lapilli in an outcrop that is more noticeable than
the ash bed (fig. 22). Brown pumice lapilli in the layer generally are
smaller. Pumice is abundant in the ash, but less so than mineral

erystals and lithic particles. The brown lapilli seem somewhat
weathered, and the white ones are stained brown, but weathering of
the layer is not enough to color the bed strongly; consequently, it is not
conspicuous in outcrops.

This deposit crops out widely from the north flank of Mount Rainier
clockwise around to the southeastern slopes, and locally along the east
boundary of the park (fig. 31). Lapilliare common only east of the sum-
mit, where the layer is also thickest. The thickest deposit seen,
however, is less than 5 cm (2 in.) thick, and the largest lapilli are slight-
ly less than 5 cm across. In most places, the ash is much less than 2 cm
thick. The layer apparently has been completely stripped from many
sites, but no thick fluvial or wind-drift accumulations of it have been
seen.

The volume of layer A is calculated roughly as at least 5 million m*.

The inference that layer A was erupted from Mount Rainier is based
on the presence of large lapilli on the flanks of the voleano and on the
similarity of the tephra to that in other deposits which clearly were
erupted from Mount Rainier. The distribution of layer A suggests that

was gentle 0 or variable enough to allow ‘wide ‘dispersal of ash.
_Layer A ash i is identified d chiefly by its sparse content of large white

conspicuous in n color or size, but its occurrence in a group ‘of grayish-
brown voleanic sand beds just above layer O and below noticeably
darker sands (fig. 22) seems to be typical. The pumice of layer A is not
different in mineral content from that in several other tephra beds. The
refractive index of its olivine, however, seems to separate it from layers
R and L that are also in the older part of the tephra sequence (fig. 33).

61


re

10 MILES
4

T
10 KILOMETERS

o4

INFERRED DISTRIBUTION of three units that make up layer F. Northwestern boundanes of
the three units are approximately the same. (Fig. 32)

mation of clay minerals occurred before the materials were ejected to
become part of layer F, and they do not represent weathering of this
deposit since its formation.

COMPOSITION

The three stratigraphic units recognized differ markedly in propor-
tions of the various constituents. The lower one consists chiefly of
lithic fragments, about half of which are partly altered to clay, in a
matrix of clay minerals. Mineral crystals, scoria, and pumice grains are
minor components. The middle unit of ash, in contrast, consists almost
entirely of mineral crystal and pumice grains and contains only a few
lithic fragments and no clay. The upper limit is like a mixture of the
lower two: it contains abundant clay and altered and fresh lithic
fragments, and its coarse ash fraction includes abundant crystal and
pumice grains.


The layer may consist of more than one of the three units just
deseribed,-even—where-no-subdivisions-can-be-seen. In-some-places
where no subdivisions are evident at Berkeley and Yakima Parks, the
layer consists predominantly of altered and unaltered lithic fragments
and-clay_like_those_in the lower-and_upper-units._ The -proportion-of
those constituents decreases toward the east and south with the in-
crease in the proportion of mineral and pumice grains that are typical
of the middle unit. These changes in composition are attributed _to the
offset distribution pattern of the three units (fig. 32).

The content of clay and altered-rock particles in this deposit is unique
among tephra layers at Mount Rainier. The clay minerals are chief-
ly montmorillonite, but minor amounts of illite and kaolinite have been
identified in the matrix and also in altered zones within discrete rock
fragments._Altered_parts_of many lithic fragments—also_contain
cristobalite, iron oxide, and probably zeolite and opal. Other fragments,
however, appear to be unaltered. Both altered and unaltered fragments
are from rocks of the type that make up Mount Rainier rather than
from the older rocks in the surrounding mountains.

DISTRIBUTION, THICKNESS, AND VOLUME

Layer F is spread thinly over most of the eastern part of the park,
and it extends well beyond the park boundaries (fig. 32). The layer is 1-
2cm ('-in.) thick over most of its area of distribution. It is substantially
thicker only on the northeast flank of the volcano; near Yakima Park
and Berkeley Park, the thickness probably averages as much as 10 cm.
It varies markedly in thickness over short distances, however, and is

absent at many outcrops where other tephra layers are well preserved.
It also locally has been reworked into thick accumulations; the thickest
such deposit found, along Granite Creek, is as much as 40 em thick.

The volume of the deposit probably was originally at least 25 million
m*, :

GRAIN SIZE

Layer F is predominantly a silt- and sand-size ash that contains from
about 5 to 25 percent clay-size particles. The lowest of the three
stratigraphic units within layer F consists of lapilli in ash that includes
only about 5-10 percent of clay. At one locality in Yakima Park, lapilli
make up as much as 30 percent of that unit and are as large as 2m (1
n
clay- or lapilli-size material. The uppermost unit seems to have fewer
lapilli and more clay than the lowest one.

As a whole, the deposit shows a decrease in proportions of both clay
and lapilli from Yakima Park southward. The largest lapilli seen,
however, are at Ohanapecosh Park, where pumice fragments are as
much as 5 cm in diameter.

64


reach the lower flanks of Mount Rainier at virtually any point. Layers
L, D, 8, and C, for example, all contain bombs or blocks large enough to
be dangerous at the distance of the Paradise and Yakima Park visitor
areas. Similar fragments have been thrown by eruptions of voleano
Hekla to distances equal to the distance from Mount Rainier’s summit
to the park boundaries (Thorarinsson, 1954). A substantial risk of in-
jury by inhalation of dust-size tephra also exists, especially if the
material is hot. A wide variety of mechanical equipment that ingests
air or water might also be damaged by air or water contaminated by
voleanic ash.

Even if not seriously affected by impact, structures can be damaged

by an accumulated load of tephra if its volume is great. Tephra ac-
cumulations can also disrupt water-supply systems, make roads im-
passable or dangerous, and impose severe clean-up problems. In addi-
tion, forest fires, which occasionally are started by tephra falls, could
be a special problem in the park.

Other serious consequences might result from the secondary effects
of a tephra eruption. Thick accumulations of tephra on steep slopes
could slide and evolve into mudflows as they moved into stream
channels. Mudflows or floods might also be formed if the fall of hot
tephra melted snow or ice; melt water mixed with hot rock debris can
form highly mobile and dangerous mudflows. Even relatively small
voleanie mudflows and floods can damage or destroy bridges,
highways, and other structures and can kill persons caught in their
paths.

Overall, hazards mount as quantities of erupted tephra increase. If

the wind direction were to change during an eruption, tephra would be
spread over a wider area, and the amount deposited at a given locality
might be reduced. Strong winds during a tephra eruption have a
similar effect in that they spread the same amount of debris over a
greater distance downwind. The hazard would also be lessened if
tephra were to be erupted in small increments over a substantial period
of time.

LOCATION

The areas of greatest direct hazard from an eruption of tephra are
close to the vent and directly downwind from it. Risk is high close the
the vent regardless of wind because large bombs and blocks can be
thrown out of the crater at high velocity in any direction. As shown by
Minakami (1942a), the distribution of large bombs and blocks is in-

of the crater rim than it is by the wind. Other hazards are concentrated

downwind. Figure 23 shows the distribution pattern of a tephra layer
controlled chiefly by wind. The hazard is highest where the tephra is

22


The variation of R.I.'values implies a wide range of silica contents for
glass in various tephra deposits. Silica contents of glass alone seem to
range considerably above and below those recorded by the bulk
chemical analyses of whole-rock samples of either tephra or flow rocks.

dicate ‘that : some of the ‘glass i in layers A, F, C, and xX whose RL is near
1.50 probably have silica contents of 70 Percent, or even more.

contains as little as about 50 percent silica.
The refractive-index values also suggest that most of the glass within
fi nae ldt 1 ili 1
rock as a whole. A silica content of about 70 percent in glass of the
predominant brown pumice of layer C, indicated by its R.I. value of

whole-rock samples of layer C. Yet, other glass in scoria lapilli from
layer C (R.I. as high as 1.54) probably has a silica content of slightly
less than 60 percent (fig. 35), virtually the same as that in whole-rock
samples of layer C. Glass in scoria in layer D (R.L., 1.52-1.55) probably
ranges in silica from 65 percent down to about 55 percent; the latter is

Taste 11.—Chemical analyses, in percent, of tephra and of lava flows in Mount Rainier
National Park
[Whole-sample analyses of layers L, D, C, Y, and W done in U.S. Geological Survey laboratories under direction of

Leonard Shapiro, L. C, Peck, and W. W. Brannock by methods described in U.S, Geol. Survey Bulls, 1144-A and 1036-
C, supplemented by atomic absorption analyses]

MOUNT RAINIER ROCKS
Layer L Layer D Layer C
Lava flows
Field No .......0...0008 9423-65-22  9-28-65-21 7-11-62-1 cot LT-4 (4 andesite flows)’

SiO. .. 573 «547 = 89.4. 59.77.—59.B(60.53-63.57
AlOs 176 «©1938 «= 172,s«17.86=—188——«17.01-17.25
Fe:0.~ 26 39 1.71.59 17 1.17-2.12
FeO. 3.6 3.0 40 3.74 35 3.32-3.85
Mgo . 34 3.1 36 © 3.51 25 2.78-3.47
CaO. 35 63 585.71 5.6 5.11-5.80
Na,O 3.7 36 40 3.97 43 4.01-4.21
K.0. 14 11 1b oT 16 1.58-1.84
H,0- 14 MZ 32082) 44 f.09-.25
HO+ ... 14 LT 53 (0.7%S (A O41
1.0 10 «089.84 18-95
92 269 20-098 Pa
10 100.09 .09 08-10
<.05  <05 0.02 05
100 99 100 100

74


Ficure 2. Photograph of Mount Rainier volcano, showing the Page

young summit cone and the barren upper slopes 5
8. Index map of Mount Rainier National Park ....... 9
4. Correlation diagram showing thickness of major
tephra units ........ 6c cece ee eee eee eee eee 10
5-7. Photographs showing:
5. Tephra layers on Mazama Ridge ........ 11
6. Tephra layers in the southeastern part of
Ohanapecosh Park .................-- 11
7. Tephra layers in an alpine meadow ..... 13
8. Diagram of a sequence that might be present inalpine
meadows east of the summit if all the Mount
Rainier tephra units (except layer S) extended
directly east from the summit—......... eee 14
9. Sketch showing example of dating tephra from tree-

ring counts been cette eee e eens 24

radiocarbon samples from sediments between

layers Cc and 0 26

positions, thicknesses, and ages of layers O, Yn,

E23 06 Ok) 28

Western United States and Canada............ 30
18. Section showing stratigraphic position of thin ash

beds of set Y at Mount Rainier................ 31
14. Photograph showing appearance of set-Y and set-P

thin ash beds at Mount Rainier ............... 32
15. Map showing minimum extent of layer Yn........ 84

16. Map showing average thickness and median grain size
trends of layer Yn within Mount Rainier National

Park . 0... eck eee eee ene 35
17. Section showing stratigraphic positions of set-P ash
beds at Mount Rainier...................0000. 37
18. Map showing distribution and average thickness of
layer W within Mount Rainier National Park... 39
19. Section showing representative stratigraphic position,
thickness, and age of layers L, D, andC....... 40
20. Map showing generalized distribution patterns of
layers L, D, and C ...... ee cee eee eee eee es 41
21. Map showing distribution and average thickness of
ayer Li. cece cece eee eee eens 43

22. Photograph of tephra sequence showing the color,
grain size, and thickness of layers D,N,F,andB 45

23. Map showing distribution and average thickness of
layer Do... ccc cence ete nee 46

VIL


Mount Rainier tephra deposits—Continued
Tephra layers L, D, and C—Continued

Layer Doce cece cece cceeeeetseseceeeenees 44
Distribution, thickness, and volume. a. 44
Grain Size 0.66... eee ccc e eee eee w. 46
Source and origin ...............00000e ... «AT
Distinctive features ..

Layer Co... cece cece eee e eee

Grain size ........
Source and origin .. 50
Distinctive features ...... 50
Tephra layers N, S, H, B, and X .. 50
Layer N....... ce eee ee eee 51
Layer S... 53
Layer H... 55
Layer B... 55
Layer X.... 56
Tephra layer R ... ... 58
Tephra layer Ao... . ee cece cece nee e een eee 61
Tephra layer F..... 2... eee ence eee eee eee eens 62
Composition ..............ee0eseee 63
Distribution, thickness, and volume . ... 64
Grain size e -++ 64

Source and origin .

Petrography and chemistry ............. seceeeeees 67
Iron-magnesium-mineral content .. 68
Refractive-index measurements .... ves 69
Clay minerals ..........0 0.0 e ccc cc cece eee teen eee eens 72
Chemical composition ............ 0.0 e ec ec seen reece ee eee 73

Measured sections a TT

References cited 0.0.0... ccc cece eee eee cece e eben eee ene 80

e
Illustrations
Frontispiece. Photograph showing tephra layers in Mount
Rainier National Park
Ficure 1. Map showing location of Mount Rainier National Park me
and Mount St. Helens ................0.. cece 3

VI



W (at pick pantie

7
Set Pa

TEPHRA LAYERS IN SOUTHEASTERN PART OF OHANAPECOSH PARK, approximately 1 km (0.6
mi) northeast of Indian Bar shelter cabin. Tephra layers in the lower part of this sequence
are little stained by iron oxide, and, so, layers O and F are white, rather than brown. Beds in the

upper part of the sequence, especially layers C and W, are stained to a stronger brown than in

most outcrops. (Fig. 6)
ll


PUMICE AND OTHER
PYROCLASTIC DEPOSITS IN
MOUNT RAINIER NATIONAL PARK,

WASHINGTON

By DONAL R. MULLINEAUX

ABSTRACT

At least 22 layers of tephra—volcanic ash and coarser airfall pyroclastic debris—can
be recognized among the postglacial deposits in Mount Rainier National Park. Each of
these tephra layers records a separate eruptive event. Eleven of the layers were erupted
from Mount Rainier; 10 originated at Mount St. Helens volcano 80 km (50 miles) south-
southwest of Mount Rainier; and the other came from prehistoric Mount Mazama at the
present site of Crater Lake, Oreg., 440 km (275 miles) south of Mount Rainier. Two tephra
beds, layer O from Mount Mazama (6,600 years old), and layer Yn from Mount St. Helens
(3,400 years old) covered the entire park. These two form highly conspicuous light-colored
layers among the darker tephra erupted from Mount Rainier. They are especially valuable
marker beds that delineate the boundaries of three nearly equal segments of the ap-
proximately 10,000 years of postglacial time. Layer Yn, locally more than 30 cm (12 in.)
thick, is the most voluminous tephra deposit in the park. Only one other tephra bed from

Mount St. Helens is conspicuous, layer W, about 450 years old; the others are thin and
obscure.
i but 0 one of the tephra layers crupted from Mount Rainier contain abundant

and coarsest t three layers from Mount Rainier (layers L, D, and C) form wwell- defined
lobes that extend east of the summit vent; layers L, D, and c are nearly absent in the
western part of the park. All three, where thick, are conspicuous deposits that can be
recognized in-a variety of environments. Seven smaller, generally thinner deposits form
less well defined lobes east of the summit and are recognized only where exposures are
relatively good. One of these smaller deposits is the only layer composed wholly of solid
rock fragments and, so, does not record eruption of molten lava. Another one, which was
erupted early in the 19th century, represents the most recent eruption of Mount Rainier
that resulted in a recognizable deposit.

One other layer from Mount Rainier (layer F) is a widespread, generally thin ash that
consists of three overlapping ash beds. In contrast to most of the tephra layers, which
contain little or no clay minerals, the lower and upper beds of layer F are rich in mont-
morillonitic clay. The middle bed, however, consists of mineral crystals and pumice and
contains little or no clay. Layer F was laid down at the time the Osceola Mudflow

mudflow.


1959b, Use of the spindle stage for determination of principal in-
dices of refraction of crystal fragments: Am. Mineralogist, v. 44,
nos. 11-12, p. 1272-1293. -

1962, Cherkasov’s “focal screening” for determination of refrac-
tive index by the immersion method, in McCrone, W. C., ed., Inter-

nat. Microscopy Symposium, Chicago, 1960, Proc.: Chicago,
McCrone Associates, p. 160-165.
1965, Volcanic-ash chronology, in Wright, H. E., Jr., and Frey,

D. G., ed., The Quaternary of the United States: Princeton, N.J.,
Princeton Univ. Press, p. 807-816.

Williams, Howel, 1942, The geology of Crater Lake National Park,
Oregon, with a reconnaissance of the Cascade Range southward to
Mount Shasta: Carnegie Inst. Washington Pub. 540, 162 p.

Williams, Howel, and Goles, Gordon, 1968, Volume of the Mazama ash-
fall and the origin of Crater Lake caldera, in Andesite Conference
Guidebook—Internat. Upper Mantle Proj., Sci. Rept. 16-S: Oregon
Dept. Geology and Mineral Industries Bull. 62, p. 37-41.

‘fr U.S. GOVERNMENT PRINTING OFFICE : 1974 O—554-574

83



80

T T T

70 4
F
Zz
w
oO
G
z
¢
an

50 4

40 | i _t

1.48 7.52 1.56 7.60 7.64

REFRACTIVE INDEX

RELATION OF SiO, content to refractive index of volcanic glass. Curve A is for glasses produced

;_curve B is for natural volcanic
glasses (George, 1924). (Fig, 35)

no-in Me

voleano Hekla in Iceland by Thorarinsson (1967).

The light and dark tephra deposits from Mount Rainier are more
Coombs (1936) and by Fiske, Hopson, and Waters (1963). The tephra
layers are uniform in the sense that they are mostly andesitic, but they
exhibit both mineral and chemical differences in detail. The lava flows
might show a comparable range of mineralogical and chemical
character on closer examination, but it seems likely at present that the
tephra layers actually are marked by greater variety.

Chemical analyses of pumice lumps from exotic layers O, Yn, and W
show that they generally are more silicic than tephra from Mount

16


Section C.—Bank of intermittent stream, southeast part of Ohanapecosh Park, 1 km
north of Wauhaukaupauken Falls— Continued

Sand, dark-gray . .
Sand, brownish-gray .

Sand, -dark-gray
Sand and silt, brown ...

Layer F; ash, yellowish white
Sand and silt, disturbed (?) ..

Layer D; lapilli and ash, grayish brown....
Sand, dark-gray, pale-brown bed in middle .
Layer L; coarse ash, brown
Sand, dark-gray

Layer A; ash, brown; contains scattered lapilli
Sand, grayish-brown .
Layer O; ash, pale yellowish gray
Sand and silt, gray and brown, interbedded .-

iS ~
DEE NWHaAdwor

Section D,—Bank of creek 0.7 km north-northwest of Ranger Station at Yakima Park
(“McNeeley site”)
Thickness (em)

Mixed sand, silt, and ash; includes grains from layer W .. . 10-15
Layer C; lapilli and coarse pumiceous ash, brown... 13
Sand, gray to brown ... 5-10
Set P; pumiceous and crystal ash, very.pale brown 1

Sand, fine, silty, dark-reddish-gray to brown ......-- + 15

Set P; pumiceous and crystal ash, white . 0.5
Silt, gray to brown .......... 15
Set P; silt-size ash, light gray 0.5
Sand and silt, brown; iron stained to reddish brown in upper 0.5-1 cm .. 6

Reworked ash (?), silt-size, light-gray .
Silt, brown .
Set Y; ash, silt size,

Silt, brown ..............0..

Layer Yn; ash, coarse, pumiceous, brown

Sand, brown 1
Set_¥;-fin : gray wepeeeers 0.25-.5
Bedded sand and silt, brown to gray; contains granules near top . 8
Sand, silty, dark-gray 1

Layer F, upper unit; ash, clayey, lithic, pale brown; darker in upper part .
Middle unit; ash, crystal and pumiceous, grayish brown .
Lower units; ash, clayey, lithic, yellowish brown; contains lithic lapilli as

much as 7 MM ACIOSS .... 1... eect eee eee nee cette eet e ee 6

Sand and silt, bedded, pale-brown to reddish-gray; contains lapilli of layer D . 11

Sand, bedded, dark-gray to pale-brown; zone of pumiceous coarse ash in middle 4

Sand, interbedded, pale-brown and gray; scattered pumice lapilli in upper part 2
Layer O; ash, pumiceous and crystal; grades finer from bottom to top; very pale

10 5-6

Sand, silty, brownish-gray to brown; includes carbonaceous zones (source of
radiocarbon sample W-951) and thin granule zones .
Layer R; ash and small lapilli, reddish brown

Sand-silty—b ish
Sand, silty, brownish-gray—.. >see ee a eee eee eee PEE EE EET



TaBLe 4.—Tephra layers that record eruptions of Mount Rainier

Approximate Probable minimum
Layer age Predominant materials volume
(years)* (million m’)
xX Pumice 1
c.. Pumice, scoria, lithic fragments 300
B.. Scoria, lithic fragments ..... 5
H.. Pumice, lithic fragments 1
F Lithic fragments, pumice, crystals,
and clay 7 an 25
Ss Lithic fragments .. 20
N Lithic fragments, pu 2
D Scoria, lithic fragments . 15
L Pumice . 50
A Pumice, lithic fragments 5
R.. . do 25

1 Age of layer X was determined by tree-ring counts. All other ages are in radiocarbon years. (See p. 24-25).

period of months or years, during which time there were intermittent
ash eruptions on too small a scale to be recognized in the tephra se-
quence.

The tephra layers suggest an overall pattern of rather short eruptive

episodes separated by longer periods of relative quiet. If eruptions did
occur during the relatively quiet times, they were too mild to produce
recognizable tephra layers. An early postglacial eruption is recorded by
layer R, after which the volcano was relatively quiet until about 6,500
years ago when the major postglacial eruptive episode began. Intermit-
tent strong pyroclastic eruptions threw out large volumes of molten
lava as pumice and scoria. These eruptions were interspersed with
steam blasts that ejected fragments of previously solidified lava from
the vent and crater walls that had been partly altered to clay.
During the same eruptive episode, rock avalanches and volcanic
mudflows repeatedly swept down valleys heading on the volcano
(Crandell, 1971). By about 4,000 years ago, the volcano had again
become relatively quiet, and no other eruptions were recorded until
after 2,500 years ago. Then, strong pyroclastic eruptions of large
volumes of magma produced showers of pumice and scoria that form
layer C. At about the same time, floods and volcanic mudflows again
moved down valley floors (Crandell, 1971, p. 43). After that major
episode, the voleano again returned to a relatively quiet state.

and Waters (Hopson and others, 1962; Fiske and others, 1963) and to
have originated from a major eruption of Mount Rainier is much older
and was erupted by Mount St.Helens. (Seep. 34.) Thus, so far-as the
tephra record shows, the recent quiescence of Mount Rainier has been
interrupted only by the ejection of layer X some time between 1820 and
1855.

17


(200) Cefag R

no-13ab y

PUMICE AND
OTHER PYROCLASTIC DEPOSITS IN

Mount Rainier National Park,
WASHINGTON

By
Donal R. Mullineaux

254924

GEOLOGICAL SURVEY BULLETIN 1326


Secrion E.—Composite section along bank of stream 0.2 km northwest of Mystic Lake

Thickness (cm)
Silt, sand, roots and other organic matter 20
Layer W; ash, gray 1-2

Sand and silt ...... -_ ar)
Layer-C;-small_lap .

Sand and silt, brown ......
Sand, coarse, dark-gray......
Silt and sand, ash (?), light-gray

Set P; sand- and silt-size ash, gray
Sand and silt, purplish-brown ...

3

8

1

: 1

Sand, brown .........-...00see eee . 3
1

2

1

1

Set P; ash, gray...... . <
Sand, brown .......

Set P; silt-size ash ... <1
Sand, brown, apparently disturbed, pumiceous 16

Set Y; silt-size ash ...... . <1

Sand and silt, brown . 2
Layer Yn; brown 5-10
Peat; minor amount of silt and sand. 5
Set Y; ash, ent gray <t
Peat and silt . 16
Sand, gray . . 5
Layer O; locally in thicl pond deposit 3-25
Sand and angular rubble >5

References Cited

Carithers, Ward, 1946, Pumice and pumicite occurrences of
Washington: Washington Div. Mines and Geology Rept. Inv. 15, 78 p.

Coombs, H. A., 1936, The geology of Mount Rainier, National Park:
Washington Univ. Pubs. Geology, v. 3, no. 2, p. 181-212.

1939, Mt. Baker, a Cascade volcano: Geol. Soc. America Bull., v.

50, no. 10, p. 1498-1510.

Crandell, D. R., 1963a, Surficial geology and geomorphology of the
Lake Tapps quadrangle, Washington: U.S. Geol. Survey Prof.
Paper 388-A, 84 p.

1963b, Paradise debris flow at Mount Rainier, Washington, in

Short papers in geology and hydrology: U.S. Geol. Survey Prof.

——___.1969a, Surfieial geology of Mount Rainier National Park,
Washington: U.S. Geol. Survey Bull. 1288, 41 p. [1970].

1969b, The geologic story of Mount Rainier: U.S. Geol. Survey

Bull. 1292, 46 p.

1971, Postglacial lahars from Mount Rainier volcano,

Crandell, D. R., and Fahnestock, R. K., 1965, Rockfalls and avalanches
from Little Tahoma Peak on Mount Rainier, Washington: U.S.
Geol. Survey Bull. 1221-A, 30 p.

80


tephra in elongate lobes downwind from the vent. Grain size and
thickness decrease rapidly downwind, and also from the axis to the
sides of the lobes. These rapid changes in grain size and thickness from
place to place are typical of Mount Rainier tephra layers, in contrast
with the exotic layers.

in the following descriptions, Mount Rainier tephra deposits are
grouped according to their volume, grain size, and the kinds of distribu-
tion patterns they form. Three large coarse-grained deposits (layers L,
D, C), form well-defined lobate patterns. Five smaller, finer grained
deposits (layers N, S, H, B, X) seem to form lobes that are confined to
the northeast, east, and southeast flanks of the voleano. Two additional
deposits (layers R, A) are more widespread and form indistinct lobes.
Layer F also covers a broad area, is very fine grained, and apparently
consists of three overlapping ash beds.

TEPHRA LAYERS L, D, AND C

Tephra layers L, D, and C (fig. 19) all consist predominantly of
pumice and scoria and form especially well defined lobes that lead

TEPHRA TEPHRA
LAYER LAYER
W (450 yr) FE
C (2,200 yr)
ser P<
Yn (3,400 yr) ff
D (6,000 yr)
L (6,400 yr) ome
n
Fr
—a bo
O (6,600 yr) [ee wily
2/6
Elz
SE
w
oO
—— oto

REPRESENTATIVE STRATIGRAPHIC POSITION, thickness, and
age of layers L, D, and C. (Fig. 19)

40


this region are from the west, most of the Mount Rainier tephra was
earried-east-of the summit, into the eastern part-of the park.

Each of these tephra layers represents one or more explosive erup-
tions, and the extent, thickness and content of each tell something about

gives evidence of many eruptions, those eruptions did not “significantly
change the form of the volcano, which was mostly built long before 10,-
000-years ago. Moreover, the explosive eruptions are not the only erup-
tions, or even the only kinds of eruptions, that have occurred within the
last 10,000 years. Other outbursts caused huge avalanches to cascade
down the volcano’s slopes, and quieter eruptions of flowing lava built a
young cone at the top of the volcano less than 5,000 years ago (fig. 2).

The deposits of tephra, however, are the only record of many erup-
tions of Mount Rainier, and they add to our knowledge of the eruptive
history of the volcano. This historical record has been used, in turn, to
evaluate the likelihood of future eruptions of Mount Rainier and how

h i . :
and Mullineaux, 1967).

The other 11 tephra layers came from volcanoes other than Mount Rain-
ier, as is shown by their broad distribution and fairly uniform thickness
and grain size. In contrast to the rather limited extent of Mount
Rainier tephra, some layers from distant volcanoes cover the entire
park. The ages of most of these foreign tephra deposits have been deter-
mined by measuring the content of radioactive carbon in wood or peat
associated with them. Because they are relatively conspicuous and dis-
tinctive as well as widespread, they are especially useful for es-
tablishing the ages of a wide variety of geologic deposits with which
they are interbedded.

The possibility of future eruptions and of their consequences is of in-
terest and significance to residents of the region near Mount Rainier.
Furthermore, visitors to Mount Rainier National Park may share this
interest and may wish to examine the tephra deposits there. Conse-

quently, this report is intended to serve park visitors who are not earth
scientists, as well as professional geologists. Toward that end, the: se-
quence and general character of all the tephra deposits are briefly
described in mostly nontechnical terms in the first part of the report.
That description is followed by sections that discuss some conclusions
of the study. Detailed descriptions of the tephra units and petrographic
and chemical data that are of interest mostly to professional geologists
are presented in later sections of the report (p. 27-80).

PREVIOUS WORK AND ACKNOWLEDGEMENTS

Tephra on the slopes of Mount Rainier was mentioned but not



a Rainier National Park

Mount St. Helens

tC) 100 MILES

i) s00KILOMETERS

LOCATION OF MOUNT RAINIER NATIONAL PARK and Mount St. Helens, western Washington.

(Fig. 1)

Tapie 1.—Tephra units, their source volcanoes, and approximate ages
[Boldface letter indicates conspicuous unit. Ages of layers X and W were determined by tree-ring counts; all other ages

are in radiocarbon years, (See text p. 24-26.)]

‘Tephra unit Source volcano

Approximate age

Mount Rainier
Mount St. Helens .
Mount Rainier ............. s
Mount St. Helens .

Set P (4 layers)

2 layers . 2 oy re
Set Y< Yn ...... cee ee doe.
2 layers tases cveies + dO..., wud gatat avenge
Mount Rainier
+ OO ...--peegve

1d.

150

450

2,200
2,500-3,000
3,000-8,300
3,400



RUBBLE OF LAYER S above undisturbed continuous layer O. Roadcut is 3 km (2 miles) east of
Ranger Station at Yakima Park.(Fig. 27)

hornblende, showing that it was not derived from the underlying layer
D but represents a different, younger eruption.

The variety of rock types from place to place and the broad extent of
such a thin deposit suggest that layer N may have originated from
more than one eruptive pulse. The kind of ash ejected and also the
direction of the wind, though generally westerly, probably changed
during the eruptive episode.

Layer N is not distinctive in the field, and it is identified chiefly as a
yellowish-brown layer between the more conspicuous layers D and F.
Its principal minerals are the same as those in several other tephra
deposits from Mount Rainier, but the lack of even minor amounts of
olivine seems to distinguish it from all but layer F (table 8). It differs
from layer F in its generally duller color, its coarser ash fraction on the
east flank of the volcano, and its lack of clay.

LAYER S

Layer S is a reddish-gray to reddish-brown deposit of large angular
blocks and lapilli in ash that is as much as 1.25 m (4 ft) thick (fig. 27).
This coarse rubble consists almost entirely of red and gray lithic
fragments; pumice and scoria are absent. The lithic fragments are
pieces of Mount Rainier lava, and they seem unweathered.

53


The date of 500-600 years old obtained by Hopson and his colleagues
for the pumice at the Kautz Creek locality was derived from tree-ring
counts that showed a minimum age of 550 years and a radiocarbon
sample whose maximum age was interpreted to be about 500 years
(Hopson and others, 1962, p. 641, 643). The radiocarbon sample was
taken from a tree stump on the streambed of Kautz Creek; the bouldery
deposit in which the stump grew was held to be the same deposit as
that which underlies the sand- and granule-size pumice in the nearby
streambank (Hopson and others, 1962, p. 642). The stump was,
therefore, judged to be older than the pumice. It seems certain that the

two bouldery deposits are not the same, however, inasmuch as other
~~ limiting dates for the pumice show that it is much older than 500-600
years.

DISTINCTIVE FEATURES

Layer Yn is the only tephra in Mount Rainier National Park that
forms a thick yellow to brown pumice deposit of coarse ash and small
lapilli size (table 3). Where it is thin, microscopic identification of its
mineral content can be used to distinguish layer Yn, because no other
coarse granular pumice in the park contains the same group of
minerals (table 3).

TEPHRA SET P

At least four thin ash beds compose tephra set P (Mullineaux, Hyde,
and Rubin, 1972) in Mount Rainier National Park. All are white to light
gray except the lowest, which commonly is brown. All are less than 1
em (4 in.) thick, and all consist of crystals and milky, largely non- _
vesicular glass particles. Some of these beds are not distinguishable in
the field from thin beds in set Y, but the two sets can be separated by
laboratory examination of their iron-magnesium mineral suites (table
38). Locally, the two lower beds of this set contain fragments of cum-
mingtonite and rock derived from set-Y tephra and Mount Rainier
lavas, whereas the upper two are nearly free of such particles.

The four beds, which are seen only in relatively complete tephra se-
quences, include the brown sand-size ash at the base, which is overlain

ry in silt-size ash and then by two li -size al
top (fig. 14, 17). The lowest bed has been found only in the southeastern
part of the park, but the silt-size ash is common over most of the
eastern part. The upper two beds have been found everywhere except in
the northwest corner of the park; there, only one, probably the upper-
most, has been seen.

The uppermost of these beds is widely preserved and is a useful
marker bed. It is nearly identical to the set-P ash bed just below it,
though somewhat browner and slightly coarser, but apparently very

36


Rainier (table 11). Layer Yn, for example, which has the lowest
measured silica content of the three, is higher in silica than any of the
Mount Rainier tephra samples analyzed. An overlap in range of silica
content might be found if more Rainier tephra were analyzed, for the
silica content of layer Yn does overlap the range reported for Mount
Rainier lava flows. However, the silica content reported for layer Yn
may be relatively low mainly because the analysis was made of pumice
lapilli that included abundant iron-magnesium minerals. Comparison
of layer Yn and layer O analyses indicates that whole samples of layer
Yn are less siliceous than those from layer O, but that glass from layer
Yn is at least as siliceous as glass from layer O (table 11).

Exotic tephra at Mount Rainier would be expected to have a generally
higher silica content even if their source volcanoes were not, overall,
any more silicic than Mount Rainier. Because most eruptions of
siliceous magma are more explosive than eruptions of mafic magma,
siliceous tephra is spread farther from the source voleano. Only the
more siliceous tephra from the source voleano would reach a second,
distant voleano. At the distant voleano, the siliceous tephra becomes in-
terbedded with locally derived tephra deposits that include beds of

Jatively low sili

Measured Sections

Section A.—Streambank on Mazama Ridge about 1.2 km north of the park highway

Thickness (em)
Sand, dark-gray to brown, and organic material ................00 000s eee ee
Layer W; ash, pumiceous, gray to brown...........-...0 cc eeeeeee een eeeeees
Sand, brown; contains zone of layer-C fragments
Sand, dark-brownish gray..............2..000 ee
Set P; 3 gray ash beds, separated by thin sand and silt beds .
Sand, brown, strongly iron-stained ...
Set Y; two ash beds, gray, and interlayer:
Layer Yn; ash, coarse, brown
Sand, dark-brownish-gray ....
Set Y; single ash bed, gray to white ....
Sand, dark-brownish-gray; contains roots .
Layer F; ash, crystals and glass, pale brown
Sand, grayish-brown; contains scattered lapilli of layer D
Avalanche deposit at Paradise (Crandell, 1969b, p. 36); gray, angular pebble
granule-, and sand-size fragments in silt and clay matrix
Sand, fine, grayish-brown .
Layer O; ash, grayish yellow . .
Sand, purplish-brown; contains organic material .
Layer R; ash and scattered lapilli, reddish brown
Sand and silt, layered brown and gray

RMN HAH RNoNa

oo
roa a a

on

7



settles over the entire landscape, it can be preserved on, and buried by,
many other kinds of deposits. Even if no date is determined for such a
layer, other deposits associated with it can be correlated from place to
place by the knowledge that they are either older or younger than the
tephra. But usually a tephra layer can be dated at one place or another,
and the date obtained is valid for that layer wherever it can be
recognized. Consequently, an identifiable tephra bed marks a single
time-horizon of known age in widely varied kinds of geologic deposits
over a broad region.

Only rarely can the age of a tephra bed be obtained by dating
material in that deposit. Instead, the age is generally obtained by
determining a date that is older than the bed and another that is
younger. Most of those “bracketing” or “limiting” dates at Mount
Rainier have been determined from the proportion of radioactive car-
bon in wood samples from sediments that lie below, between, and
above the various tephra layers. Other dates have been obtained by
counting annual growth rings to learn the ages of trees; the tree ages
can then be used to determine approximately the age of the youngest
surface on which the tephra is present, and the age of the oldest surface
on which it is absent (fig. 9). Both surfaces, of course, must be within
the area covered by the tephra fall. The dates obtained by these
methods, however, are only approximate. Radiocarbon methods give
ages that probably are accurate only to within a few hundred years,
and tree-ring-counting methods can give ages that are accurate to
within a few tens of years.

The age assigned to a tephra layer for discussion in this report is
some arbitrarily chosen even number between the closest available

EXAMPLE OF DATING TEPHRA by tree-ring counts. Sequence of moraines from older (A) to
younger (D) was left by Emmons Glacier retreating from left to right. Dates are the earliest year
recorded by an annual ring reached by a corer, in the oldest tree found on the moraine; the ring
is close to the first-year growth ring of the tree. Layer X fell after formation of moraines A and
B but before deposition of moraines C and D. It clearly is older than 1854. Studies of ger-
mination and growth of trees in Mount Rainier National Park by Sigafoos and Hendricks (1969)
indicate that a maximum of 10 years should be allowed for germination of trees on moraine B
after its formation, and a maximum of 20 years for growth to the height at which it was cored,
which was approximately 60 cm (2 ft). Thus, moraine B probably formed in about 1820 or later,
and layer X fell between 1820 and 1854. (Fig. 9)


southeast, east, and northeast away from Mount Rainier, respectively
(fig. 20). All locally are at least 15 cm (6 in.) in maximum thickness,
contain bombs or blocks, and make conspicuous brown, coarse deposits
where they are more than about 2 cm thick. These deposits have
resisted erosion well, are among the best preserved of all tephra units,
and are the easiest of the Rainier tephra layers to find and identify.
They are the most commonly preserved Mount Rainier tephras
between alluvial and mudflow deposits on valley floors.

LAYER L
Layer L is a locally thick yellowish-brown layer of pumice lapilli,
bombs, and ash (frontispiece) that is conspicuous only on the southeast
flank of the volcano in a little-visited area. The pumice fragments are
more uniform in color and vesicularity than those in layers D and C,
and lithic lapilli are sparse, although both lithic and crystal fragments

—
<7 a,

Sy

128°30"

“ae

a
&

he ee come one

1OMILES

T
10 KILOMETERS

GENERALIZED DISTRIBUTION PATTERNS of layers L, D, and C where they are more than 5 cm
thick. Limits are highly approximate. (Fig. 20)

41


thickest near the vent, and it diminishes with decreasing thickness
both downwind from the vent and across the pattern from its axis to
both margins.

Most winds in this region blow generally eastward, so that the
danger is substantially higher east of the vent. Figure 20, which shows
the overlapping distribution patterns of the three major tephra layers
from Mount Rainier, also outlines a sector of relatively high risk in the
park that extends from approximately northeast to southeast of the
summit. The wind does blow from other directions, however, and it
should be possible to distribute tephra in a pattern similar to that of
figure 23 in any direction from the volcano.

The secondary hazard of landslides is restricted to areas of heavy
tephra fall, but the hazard from mudflows and floods extends down all
valleys that head in those areas. The distance downvalley to which the
risk extends would depend mainly on the volume and mobility of a
mudflow or on the volume of floodwaters.

WARNING

A key factor in determining the severity of the hazard from an erup-
tion—especially the hazard to life—is the probability of warning. The
recognition of warning signs soon enough before an eruption could, for
example, prevent any loss of human life if people in the endangered
areas were evacuated. Any substantial eruption of pumice would likely
be preceded by the rise of molten magma into the volcano. The move-
ment of the magma should cause a variety of phenomena that would
provide warning, including volcanic earthquakes, increase of volcanic-
heat flow that would, in turn, cause an increase in steaming and
melting of snow and ice, changes in the magnetic field around the
volcano, and swelling of the voleano accompanied by tilting of its flanks
and perhaps an increase in the number of rockfalls and landslides.
Property damage and loss of life would then depend largely on the
measures taken to reduce the effects of the eruption.

The eruption of only gases and previously solidified lithic tephra, in
contrast, might occur with no such warning signs. The tephra record
suggests that the probability of a steam blast of large proportions is
low, but such an eruption could be highly dangerous because of the lack
of a warning.

Ages Of Tephra Layers And Their Use
As Marker Beds

Tephra beds extend over wide areas and, in terms of geologic history,
the tephra falls everywhere at the same time. Because the tephra

23


10 MILES

1OKILOMETERS

MINIMUM EXTENT of layer R, and area in which deposit averages more than 8 cm thick. (Fig. 30)

however, seem to consist chiefly of lapilli that have been eroded from
slopes and redeposited along stream channels. One such fluvial ac-
cumulation, along Huckleberry Creek in Huckleberry Park, is 25 cm (10
in.) thick. Another, in Berkeley Park, west of Yakima Park, includes
four separate lapilli beds.

A rough calculation, based on an estimated average thickness of 1 em
over most of the eastern part of the park and an average thickness of 10
cm in the lobe extending from the summit of Mount Rainier to Yakima
Park, indicates a minimum volume of 25 million m’ of layer R tephra as
freshly fallen.

The large fragments northeast of the volcano and their petrographic
character strongly indicate that layer R came from Mount Rainier. The
limited distribution of large fragments also suggests that a
southwesterly wind blew during the major part of the eruption.
Conversely, the widely dispersed smaller lapilli and ash indicate chang-
ing wind directions during other, less voluminous episodes of the erup-
tion. The very spotty preservation of the deposit may have resulted

60


Ficure_24. Map showing distribution and average thickness of Pate

layer Ci... cece eect eee en ene ene

25. Section showing stratigraphic positions of layers N, S,

H, B, and X oo. eee eee ee eee eee ete eee
26. Map showing minimum extent of layer N.........
27. Photograph showing rubble of layer 8 above un-
disturbed continuous layer O......2...........
28. Map showing minimum extent of lapilli-bearing parts
of layers H, B, and X ....... 0. cece eee eee
29. Section showing stratigraphic position of layers R, A,
E00 <n
30. Map showing minimum extent of layer R and area in
which deposit averages more than 8 cm thick ..

. Pp
32. Map showing inferred distribution of three units that

60

make up layer F .. 63
+83. Chart showing refractive indices of iron-magnesium
minerals. ....... eee cece eee eect e nee eee 70
34, Chart showing refractive indices of glass ......... 712
35. Graph showing relations of SiO2 content to refractive
index of volcanic glass ............00. eee ee eee 16
Tables
Page
Taste 1. Tephra units, their source volcanoes and approximate
AGES eee eee eee teen aes 3
2. Terminology applied to tephra particles 7
3. Characteristics useful for identification of tephra
UNItS oe eee eee eee eee eeeteees 16
4. Tephra layers that record eruptions of Mount Rainier 17
5. Limiting dates and ages assigned to tephra layers. 25
6. Volume of layer L 43
7. Volume of layer D . 47
8. Volume of layer C ... 49
9. Content and proportions of i iron- -magnesium minerals
in heavy-mineral fractions ...................- 69
10. Crystalline clay minerals in tephra layers ........ 13

Vill

11. Chemical analyses of tephra and of lava flows in



TEPHRA EXPLANATION
UNIT

ee)

oa. | Pumice and scoria
S

Lithic fragments

Sand-size tephra

Silt-size tephra

Lithic sand and silt

30--12
n
oi
618
2) =z
2/6

L -|[2
2
a
oO

A o+o

fe)

R

DIAGRAM OF A SEQUENCE that might be present in alpine meadows
about 7 km (4 mi) east of the summit if all the Mount Rainier tephra
units (except layer S) extended directly east from the summit. (Fig. 8)

different directions. The thicknesses of non-Rainier tephras and in-
terbedded lithic sands are shown as they actually occur in those
meadows. This hypothetical sequence allows a comparison of max-

_________ imum thicknesses and grain sizes of the Mount Rainier tephra units at

14


Sscrion B.—Bank of small creek in an alpine meadow between Williwakas Creek and
Cowlitz River valley, 0.6 km north of the Williwakas Creek falls noted on topographic
map, at altitude of about 5,900 feet (1,800 m) on the southeast flank of Mount Rainier

Thickness (em)

Layer W; ash, gray.
Sand and silt, brown .
Layer C; lapilli and ash, brown ..
Sand and silt, brown .
Sand and silt, gray .

Set P; three ash beds and intervening thin silt beds
Set P; lowest ash, brown, sand size .

Sand, gray...... 00... eee eee eee
Set Y; two gray ash beds and intervening sand beds ..

Layer Yn; ash, brown; swells and pinches out locally
Silt, gray.
BE

Set Y; ash, very light gray .
Sand, medium- to dark gray .
Sand, grayish-brown ....
Set Y; ash, very light gray .
Sand, medium- to dark-gray .
Sand and silt, gray and brown, laminated .
Layer F; ash.........

A

A

Silt and sand, brown .
Layer D; lapilli and ash, brown ..
Sand, dark-gray
Silt and sand, pale-grayish-brown
Sand, gray, contains scattered lapilli..
Layer L; lapilli and ash, yellow brown
Sand and silt, dark-gray .......

NAOH RoOH OHH NAOH EN oOWwnND

Layer O; ash, reddish yellow; 1-7 cm thick, average .
Sand and silt, reddish-gray; contains organic material .
Layer R; stream accumulation ..............0.. 000000
Sand, gray, contains lithic fragments ...........6.-. 02000 e cere rete

Bw Ao

Secrion C.—Bank of intermittent stream, southeast part of Ohanapecosh Park, 1 km

north of Wauhaukaupauken Falls
Thickness (em)

Silt, sand, and organic material .. 3
Layer W; ash, pale brown. 2
Sand, silt, and organic material .. 3
Layer C; lapilli and ash, reddish brown . 12
Sand, silty, gray, brownish at top 7

Set P; lowest ash bed, brown a

<1
Sand, silty, gray .............. 8
Set Y; ash, gray, and interbedded silt and sand... 2
Layer Yn; brown... 5-10
Sand, gray-brown <1

Sand, gray...

78



TEPHRA
LAYER

W (450 yr)

C (2,200 yr)

Set PS]

Yn (3,400 yr)

D (6,000 yr)

n

L (6,400 yr)

O (6,600 yr) foe

© CENTIMETERS 8
INCHES

0
STRATIGRAPHIC POSITIONS of layers N, S, H, B, and X. (Fig. 25)

of the volcano. Three layers (N, H, B) consist of lithic and pumice or
scoria lapilli in thin ash; one layer (X), where found, consists only of
pumice and scoria lapilli, and the other (S) consists of lithic blocks and
lapilli in lithic ash. All but layer S are too thin to be readily recognized
anywhere but in the alpine meadows, or, as for layer X, on glacial
deposits that are so young that no other tephra deposits lie on them.

LAYER N

Layer N is a thin inconspicuous gray to reddish-brown layer of small
lapilli and relatively coarse ash (fig. 22). It consists chiefly of lithic
fragments but locally includes abundant brown pumice and brown to
gray scoria. The proportions of these constituents vary considerably
from place to place; most of the larger particles are brown pumice in
Yakima Park, scoria or lithic fragments in Ohanapecosh Park, and
lithic fragments in Cowlitz Park. In many places the layer is stained
yellowish or reddish brown, yet individual lapilli are not strongly
weathered.

51


TABLE 11.—Chemical Analyses, in percent, of tephra and of lava flows in Mount Rainier
National Park— Continued

EXOTIC TEPHRA DEPOSITS

Layer W

Layer O(Mount Mt Layer ¥ (Mount StH me

Helens)

Whole Glass Whole Glass Whole

sample? ore. 3 sample (V8: 9 sample? sample

Field No ............08 . 8-27-61-14 8-15-64-2
sio 68.56. 72.59 63.4 75.02 67.5
AlOs.. 14.22 14.42 172 14.51 16.2
FeO; . 1.42 2.08 16 {13
FeO. 1a), ° 23 4 21
MgO .. 0.83 0.54 13 0.48 99
CaO 2.35 LT Al 1.85 3.6
Na,O 5.18 5.15 43 4.46 4.8
K0. . BAT 2.70 12 1.98 1.6
H,0- 16 “14
H,0+ . } re 2 ae 1.0
TiO» .. 0.58 0.48 51 0.16 44
P.05 0.10 0.06 AT 0.04 12
MnO 0.03 0.04 08 0.08 06
Ops ivveveveevees eneee epee 05s 05
101 99 100

‘Reported by Coumbs (1939) and cited in Fiske and others (1963).
*Mount_Mazama whole-sample analysis from Moore (1937, p.159). Analyst, JJ. Fahey.
‘From Westgate, Smith, and Nichols (1970).

approximately the same proportion as was measured for the whole-
rock sample from that layer.
In several Mount Rainier tephra deposits, dark and light fragments
of different chemical composition seem to occur together vertically
throughout the layers rather than to be concentrated in distinct lenses
or beds. Evidence of compositional stratification was seen only in layer
C. That evidence consists of the stratigraphic zones in which certain
rock types are concentrated:(1) Dark scoria is concentrated in, though
not restricted to, the lower third of the deposit, and (2) brown pumice is
predominant in the middle and upper parts of the layer, although some
~———~also occurs in the lower part.
Thus, several tephra deposits show evidence of eruption of magma of
slightly different chemical composition at the same time, and layer C
shows evidence of a distinct shift in chemical makeup of ejecta during
an eruption. These compositional differences lead to the inference that
the magmas that were available for eruption were not homogeneous.

6


Crandell, D. R., and Miller, R. D., 1964, Post-hypsithermal glacier ad-
vances at Mount Rainier, Washington, in Geological Survey
research 1964: U.S. Geol. Survey Prof. Paper 501-D, p. D110-D114.

Crandell, D. R., and Mullineaux, D. R., 1967, Volcanic hazards at Mount
Rainier, Washington: U.S. Geol. Survey Bull. 1238, 26 p.

Crandell, D. R., Mullineaux, D. R., Miller, R. D., and Rubin, Meyer,
1962, Pyroclastic deposits of Recent age at Mount Rainier,
Washington, in Short papers in geology, hydrology, and
topography: U.S. Geol. Survey Prof. Paper 450-D, p. D64-D68.

Crandell, D. R., and Waldron, H. H., 1956, A Recent volcanic mudflow
of exceptional dimensions from Mt. Rainier, Washington: Am.
Jour. Sci., v. 254, no. 6, p. 349-362.

Fiske, R. S., Hopson, C. A., and Waters, A. C., 1963, Geology of Mount
Rainier National Park, Washington: U.S. Geol. Survey Prof. Paper
444, 93 p.

Fulton, R. J., and Armstrong, J. E., 1965, Day 11—September 16, in
Guidebook for Field Conference J, Pacific Northwest—Internat.
Assoc. Quaternary Research, 7th Cong., U.S.A., 1965: Lincoln,
Nebr., Nebraska Acad. Sci., p. 87-98.

George, W. O., 1924, The relation of the physical properties of natural
glasses to their chemical composition: Jour. Geology, v. 32, no. 5, p.
353-372.

Hague, Arnold, and Iddings, J. P., 1883, Notes on the volcanoes of
northern California, Oregon, and Washington Territory: Am. Jour.
Sci., 3d ser., v. 26, no. 158, p. 222-235.

Hopson, C. A., Waters, A. C., Bender, V. R., and Rubin, Meyer, 1962,
The latest eruptions from Mount Rainier volcano: Jour. Geology, v.
70, no. 6, p. 635-647.

_____ Huber, N. K., and Rinehart, C. D., 1966, Some relationships between

the refractive index of fused glass beads and the petrologic affinity
of voleanic rock suites: Geol. Soc. America Bull., v, 77, no. 1, p. 101-
110.

Minakami, Takeshi, 1942a, The distributions of voleanic bombs ejected
by the recent explosions of Asama, Pt. 1 of On the distribution of
volcanic ejecta: Tokyo Univ. Earthquake Research Inst. Bull., v.
20, p. 65-92.

1942b, The distribution of Mt. Asama pumice in 1783, Pt. 20fOn
the distribution of volcanic ejecta: Tokyo Univ. Earthquake
Research Inst. Bull., v. 20, p. 93-105.

Moore, B. N., 1937, Nonmetallic mineral resources of eastern Oregon:
U.S. Geol. Survey Bull. 875, 180 p.

Mullineaux, D. R., Hyde, J. H., and Rubin, Meyer, 1972, Preliminary
assessment of upper Pleistocene and Holocene pumiceous tephra
from Mount St. Helens, southern Washington: Geol. Soc. America
Abs. with Programs, v. 4, no. 8, p. 204-205.

81



eruption that is recorded by a recognized tephra deposit. In the White
River valley, they occur on moraines that, as indicated by the age of
trees growing on them, probably formed as late as 1820, and they are
absent on moraines formed before 1854 (Mullineaux, Sigafoos, and
Hendricks, 1969). Thus, layer X tephra apparently was erupted in the

first half of the last century.

Layer X cannot be distinguished lithologically from layer C at this
time. Where layer C underlies a surface at shallow depth, fragments
from it work to the surface, where they mingle with any layer X lapilli
that are present. Consequently, layer X has been recognized only on
deposits, such as moraines, where layer C is absent.

Tephra lapilli described here as layer X were initially regarded as
part of a deposit called layer G by Crandell, Mullineaux, Miller, and
Rubin (1962), Finer grained materials that were described as parts of
layer G are so variable in composition from place to place that they are
no longer regarded as parts of a single definable tephra deposit.

TEPHRA LAYER R

Layer R, the oldest postglacial ash recognized (frontispiece, fig. 29),
is a reddish-brown deposit of ash and lapilli that occurs widely in the
eastern part of the park in small lenses or in a thin, discontinuous
layer. Northeast of the volcano, however, it is several inches thick and

contains abundant large lapilli. These lapilli are mostly brown pumice
or dark-gray scoria. Pumice and scoria are also common as ash-size
grains, but the ash includes abundant lithic particles, and, on the
southeast side of the volcano, consists mostly of such particles. Layer R
lapilli do not contain hornblende as was previously reported (Crandell
and others, 1962). The earlier description resulted from my misiden-

tification of a bed of layer D lapilli in an outcrop as layer R, before
layer D was recognized as a separate deposit.

Layer R is relatively strongly weathered, and the entire deposit
typically is stained. Weathered rinds several millimeters thick have
formed in most pumice and scoria lapilli, and some small lapilli are
weathered and soft all the way through. Lithic fragments are coated

with weathering products but are not otherwise obviously affected. The
weathering seems to be limited mostly to the glassy parts of the
fragments, for the mineral crystals are firm. Examination of the
weathered material by X-ray diffraction methods showed the presence
of only amorphous clayey material rather than crystalline clay
minerals.

More than one bed of pumice lapilli in this deposit, interbedded with
stream deposits, has been seen in several places. These multiple beds
suggest that products of more than one eruption might be included in

58



the parts of tephra set Y (Mullineaux, Hyde, and Rubin, 1972) that
reached Mount Rainier. Only the one thick bed (layer Yn), however, is
individually traceable across the park, for the others are not only thin
but discontinuous. Nearly complete tephra sequences commonly show
one and locally two beds below and two more above layer Yn (figs. 13,
14). Above these upper two set-Y beds are two other thin beds which
are similar in appearance but which contain abundant particles
probably derived from Mount Rainier. It is not clear whether these last
two are primary ash layers of set Y that were contaminated by dust-
size Mount Rainier debris or are merely layers of dust derived from set-
Y tephra and local rocks on the flanks of Mount Rainier.

The thin ash beds of set Y are white or light gray, less than 1 cm (4
in.) thick, and composed of silt and sand-size particles. All consist
chiefly of crystals and particles of milky nonvesicular to vesicular
glass. They generally are not distinguishable from each other and are
not readily separable from some tephra beds in the overlying set P ex-
cept by laboratory examination of mineral content. Although locally

TEPHRA

LAYER

W (450 yr)

Yn (3,400 yr)
30-12
o
oi
5 | a
fo}

(6,600 yr) 2135
-E|2
blz
a
oO
0

°

STRATIGRAPHIC POSITION of thin ash beds in set Y at Mount
Rainier. (Fig. 13)

31


Set Y Set P.
° Below Yn Yn Above Yn |Lowest bed Top bed w
AUGITE
1.70b : 4
1.69 | 4
1.68 4
i
1,.72/- HYPERSTHENE 4
I
171 It
” ade ’ i i
= =
=
x
w 4
a
w =
$1.67 4
rE —
q 3—
& = CUMMINGTONITE
cles 4
+p rn .
164,- > mu iF 4
a}
2
163-6 7
2
Ps
&
é HORNBLENDE
1.68b +
! - | 4
af" ' . l
| q
t : Ke
' aye i !
Wi . " | i}
1,65,- 4

values of glass in the exotic layers show smaller ranges and are
generally lower than R.I. values for glass in Rainier tephras. The
values for most exotic layers, however, overlap the values measured for

71



APPEARANCE OF THIN ASH BEDS in set Y and set P at Mount Raimer. (Fig. 14)

set-Y tephra beds have been seen both above and below layer Yn on the
north and west flanks of Mount Rainier, they are common only in the
southern and eastern parts of the park.

32


Taste 7.— Volume of layer D, in millions of cubic meters

‘Total -volume within Estimated minimum
2-em isopach total volume
In present state .......... 23 45
As freshly fallen.......+ 38. 15.

largest bombs are about 15 cm (6 in.) across, and others more than 5 cm
across are abundant. At the eastern boundary of the park, a few bombs
are as large as 5 cm across, and lapilli as large as 2 cm in diameter are
common. Even at the mapped margins of this bed, the bed consists
~~ chiefly of lapilli rather than ash.

The largest bombs, however, do not lie in the thickest part of the
layer. The largest ones found, about 50 cm (20 in.) across, are on
Mazama Ridge southeast of the voleano summit where layer D is less
than 2 em thick. Other bombs as much as 15 em across have been found
on Stevens Ridge, as well as at Ohanapecosh Park and Summer Land,
but none that big was seen in Cowlitz Park. The large bombs seem to lie
-in two different sectors, one approximately east and the other
southeast of the summit.

Source and origin

The marked decrease in grain size and thickness away from Mount
Rainier points to a summit vent as the source of layer D. The eastward
distribution of lapilli and small bombs probably was controlled by a
steady wind from the west at the time of eruption. Because there is no
evidence of change in wind direction during that time, it seems likely
that the eruption was of relatively short duration.

Some factor other than wind, however, must be responsible for the
projection of large bombs southeastward from the summit. It seems
likely that they were ejected at high velocities with such a strong
horizontal component that their flight was little affected by the wind.
Studies by Minakami (1942a) of the 1935 and 1941 eruptions of Asama
volcano in Japan show that large bombs fell over a much wider arc than
small ejecta. There, the bombs that traveled farthest from the vent
were those that had the lowest possible angle of emission that per-
mitted escape over the crater wall. A low saddle in the southeastern
part of the crater wall of Mount Rainier during eruption of layer D
could have allowed the projection of large bombs onto Mazama Ridge.

Distinctive features

Layer D can be recognized readily i in most Places east of the volcano

dant scoria * generally distinguishes it ‘from all other tephra deposits.
Where field criteria are not adequate, layer D can be identified under
the microscope by its abundant hornblende.

47


Rainier. Most debris in these layers probably was carried high above
the voleano by powerful continuous ejection-of columns of gases-and
fragments. The fragments were then carried downwind as they fell,
and the smaller particles were carried progressively farther from the
vent. However, some large bombs_in_layer_D_probably_were thrown
directly to their present sites by the force of an explosion, for the
largest of them extend in directions different from the direction the
wind-blown fragments were carried. The flight paths of similar bombs
ejected during the 1935-41 eruptions of Asama voleano in Japan were
calculated by Minakami (1942a). The formulas Minakami derived,

these ‘bombs could have reached their present sites by being ejected
with an initial velocity of about 250 meters per second at an angle of
near 40° to the horizontal

Layer S appears to represent a highly explosive, but laterally
directed eruption that also threw large fragments directly to their pre-
_______ sent locations. The limited overall distribution of that deposit and the —___
presence of large blocks in ash several kilometers away from any likely
vent site suggest that the mass was ejected at a high velocity and at an
______ angle low enough that very little of it was carried and sorted by wind.

Several tephra layers that are thin, fine grained, poorly sorted, and
limited chiefly to the volcano’s flanks are thought to have resulted
from fairly mild eruptions. They lack large fragments that might in-
dicate a powerful explosion, and they do not show either good sorting or
a strongly wind controlled distribution pattern that would suggest that
they were carried high above the voleano.

Virtually all the pumice and scoria of layers L and D probably were
thown from the vent during continuous eruptions. Both deposits are
nearly uniform in composition and texture from bottom to top and
show no evidence of significant changes in conditions of either eruption
or deposition during their formation. In contrast, the changes in com-
position and texture from bottom to top of layer C record intermittent
eruptions. A pause in the eruption that was long enough for a non-
vesicular crust to form on the magma may be recorded by a concentra-
tion of lithic fragments just above the pumice and scoria in the lower
part of the layer. The crust may then have been disrupted and thrown
out as lithic fragments during the next eruptive pulse. The predomi-
nant pumice in the upper part of layer C differs slightly in color and
texture from that in the lower part and probably represents a change in
composition, though not necessarily a pause in the activity. Layer F
also shows evidence of a discontinuous eruptive episode that included
an initial explosion of voleanic gases, a later eruption of a small volume
of crystal-rich molten material, and a final eruption of both molten and
solid-rock particles.

Repeated pulses of activity are a common feature of many observed

20


total volume is at least twice the amount within the 2-cm isopach and
that the present volume is 60 percent of the freshly fallen volume. (See
p. 42.)

Grain size

Layer C is overall the coarsest of the tephra layers, and it is coarsest
where it is thickest. It contains some large bombs, but they are not as
large as those in layer D. The largest seen, 25-30 cm (10-12 in.) across,
are at Summer Land and at Cowlitz Park, at distances of about 8 km (5
miles) from the summit. The grain size diminishes rapidly farther from
M Rainier: at O Park, 9-10 km fr it, tI
largest bombs are about 15 cm across. The large bombs are spread over
a wide arc on the mountain flanks, whereas at greater distances the
largest sizes form a progressively narrower northeast-trending lobe.
Contours that show maximum grain size form nearly the same pattern
as the thickness pattern (fig. 24). Layer C decreases in predominant
particle size to small lapilli rather than ash along the lateral margins.

Source and origin

The changes in grain size and thickness within the park suggest that
layer C was erupted from the central vent at Mount Rainier’s summit.
Horizontal and vertical variations of rock types, however, indicate that
wind conditions. Dark scoria from an explosion early in the eruptive
episode was blown chiefly to the north, then lithic fragments from a
subsequent explosion were carried chiefly to the northeast and east.
Later, a much larger volume of pumice buried the scoria and lithic
fragments and spread far to the northeast, implying a southwesterly
wind. No evidence has been seen of a long period of time between the
successive eruptions.

Distinctive features

Layer C is so thick and coarse that it is not likely to be confused with
any layer other than L or D, and with them only south or east of the
volcano. Its prominent brown pumice distinguishes it from layer D,
and its dark scoria and lithic fragments should separate it from layer
L. In the laboratory, the presence of hornblende helps distinguish layer
C from most other tephra deposits from Mount Rainier, and the
sparseness of that mineral shows that it is not layer D.

TEPHRA LAYERS N, S, H, B, AND X

Five deposits that seem to form small lobes east of Mount Rainier’s
summit are designated, from oldest to youngest, layers N, S, H, B, and
X (fig. 25). Each of these has been recognized only in a small area east

50



TEPHRA TEPHRA
LAYER LAYER

W (450 yr) [EE]

C (2,200 yr)

Yn (3,400 yr)

B-
Be --F (5,000 yr}
D (6,000 yr) |
L (6,400 yr) 304-12
n
c
w
|”
O (6,600 yr) w e
2/6
Ee Zz
zion
3
‘aa R (> 8,750 yr)
a oto

STRATIGRAPHIC POSITION of layers R, A. and F. (Fig. 29)

layer R as described here, but no multiple beds were seen in sequences
of only airlaid material.

Layer R probably covered nearly all the park east of Mount Rainier’s
summit and extended well beyond the park boundaries (fig. 30). It is
preserved so sparsely, however, that its extent and thickness are not
well defined. It seems to be no more than 2 cm (1 in.) thick over most of
the area where it has been recognized. Lapilli are common in the
deposit everywhere except southeast of the volcano, but they generally
are not more than 1 cm in diameter. The deposit is significantly thicker
and coarser only in the vicinity of Yakima Park and Grand Park
northeast of the summit. A northeast-trending lobe that extends to
Yakima Park is consistently at least 8 cm thick (fig. 30). Large lapilli
are common in this vicinity, and the largest lumps are nearly 5 cm in
diameter.

Layer R has been completely eroded away in many localities where it
would be expected and where other tephra layers are preserved, and ac-
cumulations several times the average thickness of the layer, locally in-
cluding multiple beds of lapilli, are common. Those accumulations,

59


UNITED STATES
DEPARTMENT OF THE INTERIOR

ROGERS C. B. MORTON, Secretary

GEOLOGICAL SURVEY
V. E. McKelvey, Director

ibrary 0 . 110

Ve

JAN 08 1975 n
Lippe k=

U.S. GOVERNMENT PRINTING OFFICE - 1974.
For sale by the Superintendent of Documents, U.S. Government
Printing Office, Washington, D.C. 20402 - Price $1.35 (paper cover)
Stock Number 2401-02550


121°58° 121° 45° 121°30°

'
| ,
1
|
Ve Lake
|
H
|
'

ro

TO MILES

T
0 10 KILOMETERS

DISTRIBUTION AND AVERAGE THICKNESS of layer W within Mount Rainer National Park. (Fig. 18)

Mount Rainier Tephra Deposits

Certain features are common to nearly all the tephra layers from
Mount Rainier and locally distinguish those layers in the field from the
exotic tephra beds and interbedded sands. Coarse grain size is the most
distinct feature: all the Rainier tephra beds described include lapilli-
size fragments; and some layers, especially on the volcano’s flanks, con-
sist mostly of lapilli and bombs or blocks. In contrast, lapilli are absent
or sparse in the exotic tephras. Mount Rainier tephra also is commonly
moderate brown to grayish brown, and the brown color is emphasized
by reddish-brown weathering products. Their color generally is darker
than that of the exotic tephra beds, yet is lighter or browner than the
gray to brownish-gray of many of the interbedded sands. (See fron-
tispiece.)

Mount Rainier tephra layers also are mostly limited to areas east of
the volcano. Westerly winds during most eruptions concentrated the

39


TEPHRA
LAYER

W (450 yr)

SetP

Yn (3,400 yr)

®
}
8

O (6,600 yr)

0

© CENTIMETERS
INCHES

STRATIGRAPHIC POSITION of set-P ash beds at Mount Rainier
(Fig. 17)
close to the same age. It is somewhat similar in appearance to layer W,
but in most places is separated from that deposit by coarse brown
pumice of layer C from Mount Rainier. Where necessary, set-P tephra
can be distinguished from layer-W tephra by the refractive index of the
mineral hypersthene. (See p. 69.)

These four tephra beds are fairly uniform in thickness and grain size
wherever they are found in the park and must have come from a
volcano other than Mount Rainier. Although they have not been traced
to Mount St. Helens, they are similar lithologically to set-P beds that
are known to originate at, and extend far northeast from, that volcano;
for the present, they are regarded as part of the Mount St. Helens set-P
tephra. At Mount St. Helens, the oldest set-P tephra is about 3,000
years old (Mullineaux, Hyde, and Rubin, 1972), and all set-P tephra at
Mount Rainier are older than about 2,460 years (table 5).

TEPHRA LAYER W

Layer W is a thin white, locally light-brown bed of loose sand-size
pumiceous and crystal ash. It is the youngest tephra at Mount Rainier

37


Minimum extent
iN

ro

JO MILES

T
10 KILOMETERS

o4

DISTRIBUTION of layer A. (Fig. 31)

TEPHRA LAYER F

Layer F is a thin, locally brown but more commonly bright-reddish-
yellow to yellow clay-rich ash (figs. 7, 22), that is especially conspicuous
in the northern and eastern parts of the park. It consists of widely
varying proportions of altered and unaltered lithic fragments, pumice,
crystals, and clay; the variations seem to be related to the distribution
of three stratigraphic units in the layer that are distinctly visible in
only a few exposures in and near Yakima Park and Berkeley Park (fig.
32). The lower and upper of these units extend north and east from the
volcano, and they consist mostly of partly altered lithic fragments and
clay; almost all particles in the deposit in those areas are coated with
clay and iron oxides that give the layer its conspicuous color. The mid-
dle unit extends from northeast to south of the mountain, and it con-
sists chiefly of pumice and crystal grains.

Layer F appears weathered because of its color and content of partly
altered particles and of clay minerals. However, the alteration and for-

62


Tephra fragments that are nonvesicular are called lithic in this report.
They | can form if lava i is low in gas content when it is 5 erupted, or if the

formed mostly by explosions that break and eject ‘pieces of older,
previously solidified lava. Most lithic Fragments i in the park contain

fragments of “crystals are also common in tephra layers but are ¢ abun-
dant only i in fine-grained deposits (table 2).

particle size eand commonly i is used with an adjective that indicates the
Kinds of particles (table 2). Blocks and bombs (table 2) fall close to the

Smaller lapilli and ash particles (table 2) tend to be ‘carried higher into
the air and are blown farther horizontally by the wind. The wind-

extend downwind from the source vent. The , grain s size and thickness of
the tephra particles decrease progressively away from the vent and

“ ”

Fine volcanic ash can be carried for hundreds or even 1 thousands of
miles by wind; if a volcanic explosion carries ash high enough into the

D ; D
tephra layer indicate that the source vent was nearby, and a rapid in-
crease in fragment size within a short distance can reveal the direction

thickness and grain size over a broad area suggests that the source was
distant.

for discussion into two sets. Bach set includes several tephra beds, each
of which contains the same principal iron-magnesium minerals and

above them at Mount St. Helens (Mullineaux, Hyde, and Rubin, 1972).

Taste 2.—Terminology applied to tephra particles

{Diameter of particles (see scale below) is sbown in parentheses]

Particle terms

Character

of particle Ash Lapilli Bombs-and-block
pil ani

(<0.4 em) (0.4-3.2 em) (232 em)
Vesicular .......... Pumice or scoria Pumice or scoria Pumice or scoria
ash lapilli- blocks and bombs.
Nonvesicular ....... Lithieash.......... Lithic lapilli ....... Lithic bombs or
: lithic blocks.
Crystalline......... Crystalash ........ () (0)
‘Crystals are generally less than 0.4 em in diameter
Seale: 0 0.4 3.2CM
0 T-INCH



TO MILES

T
10 KILOMETERS

e+

DISTRIBUTION AND AVERAGE THICKNESS, in centimeters, of layer C. (Fig. 24)

deposition also generally are less than twice the airfall thicknesses in
the same area. The thickest reworked accumulation seen is at one site
in Yakima Park, where the deposit is nearly 30 cm thick, about twice
the airfall thickness at that locality.

Layer C is the largest of the Mount Rainier tephra layers, and most
of it lies outside the park boundaries. Calculation of even the volume
lying within the 2-cm (1 in.) isopach (table 8) requires projection of
isopachs considerably beyond the park. The estimates of the total and
freshly fallen volumes (table 8) are based on the assumptions that the

Taste 8.—Volume of layer C, in millions of cubic meters

Total volume within Minimum total
2-cm isopach volume
In present compacted state 90 180
As freshly fallen ......... 150 300



pumice or scoria suggests that the eruption was one or more steam ex-
plosions (Crandell and Mullineaux, 1967, p. 7).

This deposit is similar only to some rock-avalanche deposits that
locally are interbedded with tephra. It differs from other tephra layers
in its content of large angular lithic blocks and in the absence of pumice
or scoria.

LAYER H

Layer H is a thin obscure gray to brown bedded ash that contains
scattered pumice and scoria lapilli whose diameters locally are greater
than the thickness of the ash. White pumice lapilli generally occur at
the base of the ash, whereas brown scoria lie slightly higher. Both the
pumice and scoria contain hypersthene, augite, and olivine, but the
refractive index of olivine differs from one to the other (fig. 33). Ash-
size particles are lithic fragments and crystals and smaller amounts of
the pumice and scoria.

Lapilli of layer H have been identified only in alpine meadows from
Summer Land south to Cowlitz Park (fig. 28), in ash that typically is
less than 2 em (1 in.) thick. The largest lapilli, about 2 cm in diameter,
were found in Cowlitz Park. The largest seen in Ohanapecosh Park and
at Summer Land are about half that size. Layer H is one of the
smallest and least known tephra deposits; the meager evidence
available suggests that its original volume was only on the order of 1
million m‘,

The limited distribution, content of large lapilli, and similarity to
other tephra deposits from Mount Rainier indicate that layer H was
derived from that volcano. Lithologice differences indicate that the
tephra was not derived from underlying layers F, N, and D; thus, layer
H represents an eruption of the volcano. Minor differences in
stratigraphic position and character of the white and brown lapilli may
indicate that the deposit represents more than one small eruption.

Neither the lapilli nor the ash is conspicuous or distinctive in the
field. The deposit is identified by its scattered lapilli and their position
relative to the more distinctive layers F and B (fig. 22). Under the
microscope, the lapilli differ in mineral content from those in the un-
derlying layers F, N, and D (table 3).

LAYER B

Layer B is a medium- to dark-brownish-gray deposit of scattered
lapilli and small bombs in thin, dark-gray ash (fig. 22). Almost all
lapilli and small bombs are brown to gray scoria, though a few lapilli
are lithic. These scoria contain a larger proportion of olivine crystals
than the other tephras. Ash-size particles consist mostly of the same
rock types, but some particles are crystal fragments and grains of
black scoria. Scoria lapilli and bombs commonly appear weathered for

55


9 10 MILES

0 10 KILOMETERS,

INDEX MAP of Mount Rainier National Park. (Fig. 3)

for study because these deposits are abundantly exposed and are
preserved well enough to permit recognition of the sequence of layers.
Outcrops of older pumice, in contrast, are too sparse to permit the se-
quence of older tephra to be identified in the time available. Several old
pumice deposits were noted, some older and others younger than the
lava flows and breccias that form the bulk of Mount Rainier volcano.

The eruptive origin of the layers selected is indicated by their dis-
tribution, in combination with their grain size, thickness, and composi-
tion. The layers form thin blankets of locally uniform size and
thickness over ridges and swales alike; changes in grain size and
thickness are not related to shape of the land surface, indicating that
the particles fell from the air. All the tephra units erupted by Mount
Rainier contain bombs or lapilli that are too large to have been picked
up and carried to their present sites by wind; these show that some
other agent, such as a volcanic explosion, must have thrown them into
the air. Tephra deposits from other volcanoes are finer grained, but
they consist of rock types that are different from those that make up

9


TEPHRA kAYERS in Mount Rainier National Park, Pumice and scoria layers from Mount Rainier
volcano (note layers R, L, D, and C) typically are stained to fairly strong brown or reddish br
interbedded lithic ash deposits have relatively neutral but somewhat darker brownish-gray
colors. Ash beds from other volcanoes (note beds marked O, set Y, set P, and W)
characteristically are lighter in color than the locally derived deposits that enclose them. Site 1s
in an alpine meadow near Williwakas Glacier on the southeast flank of Mount Rainier.



500 MILES
a

T
0 500 KILOMETERS

MINIMUM EXTENT OF MAZAMA ASH (patterned) in the Western United States and Canada.
Circles mark sites where ash layer has been identified. From Powers and Wilcox (1964),
Westgate, Smith, and Nichols (1970), and Wilcox, R. E. (oral commun., 1971). (Fig. 12)

At some places, it is similar in appearance to layer F (figs. 6, 7), but the
latter contains a different suite of iron-magnesium minerals (table 3)
and is limited to the eastern part of the park. Layer O also differs in
that it contains no more than traces of clay minerals, whereas layer F
locally is rich in montmorillonite and kaolinite (p. 72).

TEPHRA SET Y

One thick and several thin tephra layers that are characterized by a
cummingtonite-hornblende iron-magnesium mineral suite constitute

30


TEPHRA UNIT

R ry t D Ww F w B c x

REFRACTIVE INDEX
8
4
4
+
+
+
4
+t
}

1.520)

1.480

a
8

_

‘rom Wilcox (1965)] O

| =>
+

—_>
A

+ +

REFRACTIVE INDICES of glass. (Fig. 34)

Rainier tephra (fig. 34), so use of this property for identification would
require measurement of a large number of grains.

CLAY MINERALS

Layers 0, N, and F and weathered rinds from pumice and scoria

fraction methods. No crystalline "clay minerals were detected i in the
weathered rinds or in layer N. Neither were any crystalline clay
minerals detected in six samples of layer O from several sites around
the volcano, except for a trace of possible mixed-layer montmorillonite-
mica in one sample (table 10). In contrast, crystalline clay minerals
were found in each of five samples from the upper and lower units of
layer F, and in two whole-layer samples of that deposit from the
northern and northeastern parts of the park. In these samples the clay-
mineral proportion ranged from approximately 50 to 90 percent of the

clay-size fraction and from about 1 to 25 percent of whole samples.
Montmorillonite is by far the most abundant clay mineral detected; it
occurs as a relatively pure mineral and in mixed-layer assocation with
mica. Discrete mica and kaolinite were detected in four of the seven
samples from layer F. Clay minerals were not detected in whole-layer
samples of layer F from Ohanapecosh and Cowlitz Parks nor in a single
sample of the middle unit of layer F from northeast of the volcano.

72



The tephra sequence sheds some light on the time of formation of the
postglacial history of the peak. No thick coarse layer C has been seen on
the snow-free parts of that young cone, and the cone probably is
younger_than-layer-C-In valleys below, numerous-voleanie-mudflows
and flood deposits, perhaps started by eruptions that produced the
young cone, also are younger than layer C but older than layer W
(Crandell, 1971). No other major post-C eruptive episode is recorded by
mudflow and alluvial deposits (Crandell, 1971) or by the tephra se-
quence. Consequently, it seems likely that the young summit cone
formed during an eruptive episode of which layer C represents an carly
part and that it is on the order of 2,000 years old.

VOLUME

Another notable feature of the tephra record is the considerable

difference in volume of material ejected during different eruptions.
Rough calculations based on areas covered and thicknesses within the
park suggest that the volume of freshly fallen tephra in layer C may
have been as much as 300 million cubic meters, which is approximately
one-twentieth of a cubic mile. Similar calculations suggest that layer D
is the next largest Mount Rainier deposit, followed in volume by
layers, L, F, and R (table 4). A few thick remnants near Dege Peak
suggest that layer S conceivably might be as large as any of the last
three named; so little is known of its thickness and distribution,
however, that many more assumptions must be made to calculate its
volume than for the other tephra deposits. The sparseness of layer X
everywhere it has been found indicates that it is the smallest of the

tephra layers recognized.

Even the largest Mount Rainier tephra, however, is small in volume
compared with tephra deposits from Mount Mazama and Mount St.
Helens. The present volume of the Mazama airfallayer of which layer
O is a part has recently been calculated as 29-37 km’ (Williams and
Goles, 1968), which is roughly 100 times the volume of layer C. The
volume of layer Yn indicated by data now available probably is more
than 10 times the volume of layer C.

KINDS OF ERUPTIONS

The kinds of particles in the tephra deposits and their distribution

and texture give some evidence of the character of the eruptions they
record. The coarse highly vesicular pumice or scoria contained in most
tephra units, for example, must have been molten when it was thrown

from the volcano. Such particles could not have been derived from

18


PUMICE AND OTHER
PYROCLASTIC DEPOSITS IN

Mount Rainier National Park,
WASHINGTON



are abundant in the ash-size fraction. Pumice lapilli are commonly
somewhat weathered on their outer surfaces but the weathering rinds
are thin.

Distribution, thickness, and volume

Layer L occurs widely in the eastern half of the park, but it is thick
only in a narrow southeast-trending tongue (fig. 21). It is thickest in
Cowlitz Park, where locally its undisturbed thickness is 25 cm (10 in.).
It is at least -15-em thick in many other exposures in Cowlitz Park and
on the next ridge to the southwest near Fan Lake. Although exposures
are poor farther southeastward, thicknesses of 8cm are common 10 km
(6—miles)—to—the—southeastin—the-Ohanapecosh—River—valley_The
thickness diminishes more rapidly across the lobe to the northeast and
southwest; in Ohanapecosh Park and on Stevens Ridge, at the same dis-

thick. Although thickness changes are rapid, they seem to be
gradational. Layer L occurs s discontinuously but widely i in n the northern

of its thick lobe (fig. D1).

A volume has been calcuated roughly for the part of layer L that is more
than-2-em(1 in.)-thick {table 6). The original -volume-of the entire-deposit
may have been two or three times that amount; such a ratio for volume
within the 2-cm isopach to the total volume has been calculated for
historic eruptions of voleano Hekla in Iceland (Thorarinsson, 1967, p.
48, 94). Several authors, including Minakami (1942b, p. 95) and
Thorarinsson, 1967, p. 34, 48) have noted that with time the tephra
becomes compacted to slightly more than half the volume of freshly
fallen material. The volume of freshly fallen layer L is estimated on the
assumption that the present volume equals 60 percent of the original
volume.

Grain size

Sorting in layer L is better than in most other tephra layers from
Mount Rainier—large bombs are not common in lapilli deposits, and
lapilli are not abundant where the layer is chiefly ash. The deposit is
coarsest in Cowlitz Park and near Fan Lake in the areas where it is also
thickest (fig. 21). The deposit consists chiefly of lapilli in those areas
but contains small bombs as much as 8 5 em 1 (2 m) across. Ten kan 6

still | is chiefly lapilli and commonly contains lumps as s much as 3 cm
across. The grain size decreases more ¢ rapidly across the lobe to the

only about 3 km from ‘Cowlita Park 3 most of ‘the largest fragments a are
less than 2 cm across.

42


Tephra layer W
Sample W-1119 (320 + 200 years)

Sample W-1397 (1500 + 200 years)
Sample W-1393 (2040 + 200 years)
Tephra layer C

STRATIGRAPHIC POSITIONS of selected radiocarbon samples from sediments
between layers C and W. (Fig. 10)

layer. The assigned age of 6,400 years is an obvious choice of an even
number within that range. Table 5 lists the dates that have been used
to assign ages to all the tephra deposits. Only the closest limiting dates
available for each tephra unit, however, are listed in table 5. For exam-
ple, more than two dates have been obtained from sediments in various
stratigraphic positions between tephra layers C and W (fig. 10). The
oldest of these (sample W-1393) is most useful for dating layer C, and
the youngest (W-1119) is most useful for dating layer W; other
available dates do not need to be listed in the table.

The tephra beds most useful as marker beds for dating or correlation
of other deposits are those which are widespread, well preserved, con-
spicuous, and easy to identify. In Mount Rainier National Park, the
tephras from volcanoes other than Mount Rainier meet those re-
quirements best. Those from Mount Rainier that are thick are highly
useful, however, and even thin beds can be valuable tools where con-
ditions are suitable.

In the park, tephra beds have been used to date other deposits, but
have been used more to subdivide and correlate deposits from one place
to another. They have been especially valuable for use in correlation of
glacial deposits on the uplands, and of sequences of voleanic mudflows
(lahars) and river deposits (alluvium) on the valley floors (Crandell,
1971). For example, layer O, the most conspicuous and widespread of
the tephras, is so widely preserved that it is almost invariably present
on moraines of the last major glaciation and, so, distinguishes them
from the younger Neoglacial moraines. The presence of layers Yn, L, D,
or F, each of which is widespread and distinctive, can also be used to
distinguish the older moraines from the Neoglacial ones. Layer C,
however, was erupted after the earlier of two Neoglacial episodes but
before the younger (Crandell and Miller, 1964). The presence of the
widespread and distinctive layer C thus differentiates the moraines of
the older Neoglacial episode from those of the younger, on which no
layer C is present. The younger Neoglacial moraines can be further

26


° 10 MILES

TOKILOMETE! RS
AVERAGE THICKNESS (solid lines) ny median grain size (dashed lines) trends of layer Yn

within Mount Rainier National Park. Thicknesses estimated from field observations; grain size
determined by mechanical analysis in the laboratory. (Fig. 16)

radiocarbon-dated samples from near Mount Rainier that are about 3,-
500 and 2,900 years old and by other samples from near Mount St.
Helens that are about 3,500 and 3,350 years old. It is assigned an ar-
bitrary age of 3,400 years (table 5).

Layer Yn is the layer Y of Crandell, Mullineaux, Miller, and Rubin
(1962). It is also the deposit described by Hopson and his coworkers as
the characteristic layer of sand- to granule-size pumice that is the
“main ash fall” that “constitutes the major part of the youngest ash
blanket from Mount Rainier” (Hopson and others, 1962, p. 641). The
cummingtonite content, thickness, and grain size of that sand and
granule-size pumice layer, including the deposit at the critical Kautz
Creek locality (Crandell and others, 1962, p. D67; Hopson and others,
1962, p. 641; fig. 16, this report), however, demonstrate that the sand-
and granule-size pumice is part of layer Yn, rather than a pumice
erupted by Mount Rainier.


eruptions of modern volcanoes. Voluminous pumice deposits erupted

represented only the first phase of recurring activity that, with minor
interruptions, lasted months or years (Thorarinsson, 1967). Possibly,
therefore, an eruptive episode that produced a tephra deposit like layer
L consisted of more than the single pulse represented by the pumice.
The dark lithic ash that occurs with the pumice may have resulted
from_minor eruptions during the same episode.

Hazards From Future Eruptions
Of Tephra

The ejection of tephra described in this report is one of several ways
in which voleanic eruptions can threaten lives and property. Some
other kinds of hazards associated with eruptions are lava flows, ex-

and rock debris to form hot avalanches, and various kinds of eruptions
that initiate volcanic mudflows. The expectable danger at Mount
Rainier from future tephra eruptions is greater than the direct danger
from lava flows, but it is substantially less than that from voleanic
mudflows and associated floods (Crandell and Mullineaux, 1967;
Crandell, 1971).

KINDS OF HAZARDS

A tephra eruption can endanger lives and property by the effects of
the associated volcanic gases or the tephra fragments. Various
probable effects of a tephra eruption have been discussed at some
length by Wilcox (1959a), who also noted that psychological factors,
such as fear and possible resulting panic, might be more dangerous
than physical factors in such an eruption.

Volcanic gases are hazardous largely because of the harmful effects
of their acid-forming constituents, including sulfur, chlorine, fluorine,
and water vapor. The chemical compounds that result are likely to
irritate or damage eyes and respiratory systems if “inhaled in suf-
ficient concentration for a sufficient length of time” (Wilcox, 1959a, p.
442). They can also severely attack cloth and metal objects, and
fluorine has been regarded as responsible for the deaths of many
animals after eruptions of the voleano Hekla in Iceland (Thorarinsson,
1967). The hazard from the gases decreases rapidly away from the vent,
however, and probably would be relatively minor in a tephra eruption.

The tephra itself poses a danger from the direct effects of impact and
inhalation, and simply by its presence where it is thick. Bombs and
blocks large enough to kill or seriously injure people might conceivably

21



MOUNT RAINIER VOLCANO viewed from the east, showing the young summit cone (arrow) and
the barren upper slopes of rock, snow, and ice. (Fig. 2)

Coombs (1936). Carithers (1946) studied pumice deposits erupted at
Mount St. Helens and noted that some of them extended into Mount
Rainier National Park. Crandell and Waldron (1956) described the se-
quence of tephra deposits in the park relative to the Osceola Mudflow.
Crandell’s later use of several tephra layers as marker beds led to the
study to determine the diagnostic features, sources, and ages of the
most conspicuous layers by Crandell, Mullineaux, Miller, and Rubin
(1962). Concurrently, Waters investigated the young tephra deposits in
the park, and Hopson and Bender evaluated published accounts of
volcanic activity in historic time to determine the age of the most re-
cent eruptions of Mount Rainier (Hopson and others, 1962; Fiske and
others, 1963). Those studies led to some conclusions substantially
different from those of our 1962 report—and of this one—regarding the
age and source of the most recent tephra layers. (See p. 36.)


|
|
!
a

a
&

1O MILES
4

TO KILOMETERS

MINIMUM EXTENT of lapilli-bearing parts of layers H. B. and X. (Fig, 28)

the White River valley below Emmons Glacier, and in the Ohanapecosh
River valley below Ohanapecosh Glacier (fig. 28). The lapilli do not
seem to be significantly more abundant in one place than another.
Fragments are somewhat larger on the moraines of Emmons Glacier,
however, where they reach 5 cm (2 in.) in diameter. In contrast, the
general maximum diameter on other moraines is about 3 cm. Ash-size
pumice or scoria is not abundant; ash-size particles might be sparse
because they have filtered down into the loose moraine, or because very
little ash-size material was produced by the eruption.

The sparseness of layer X lapilli suggest that the layer is the smallest
of the tephra deposits, and probably is considerably less than 1 million
m? in volume.

The size and distribution of layer X lapilli, along with the similarity
of the lapilli to those in other Mount Rainier tephra, show that Mount
Rainier is the source of the tephra. Their relation to glacial moraines
proves that they are very young; they resulted from the most recent

57


Tape 10.—Crystalline cluy minerals in tephra layers

Layer Locality Field No. Clay minerals’

F (whole layer) Ohanapecosh Park 9-12-67-23 None detected.

Chinook Pass ..... 8-27-61-6 Montmorillonite, 1; mixed layer, 1.

North of Mount 40-6 Montmorillonite, 4; mica, 2;
Fremont kaolinite 2.

Cowlitz Park ..... 8-12-67-25 None detected.

F (upper unit) . “McNeeley” site .. 7-5-63-16 80 percent mixed layer
(montmorillonitic)
F (middle unit) “McNeeley” site .. 7-5-63-17 None detected.
F (lower unit) . “McNeeley” site .. 7-5-63-18 80-90 percent montmorillonite.
Yakima Park 7-6-63-3 Montmorillonite, 2; mixed layer, 5;
kaolinite, less than 1.

do 9-23-65-9 Montmorillonite, 6.

OL. ee Van Trump Park . 17-6-28a-3 None detected.
oe: 17-6-27 Mixed-layer montmorillonite-mica (?),
trace.
Mowich Lake ..... 8-22-61-4 None detected.
Ohanapecosh Park 8-14-62-6 Do.
“MeNeeley” site .. 7-5-63-21 Do.
Longmire 8-18-63-3 Do.

‘Proportion in clay-size fraction, relative abundance or parts in 10.

CHEMICAL COMPOSITION

Chemical analyses of pumice and scoria have been made of whole
samples from three of the major Mount Rainier tephra layers (table
11). All the fragments analyzed are andesitic, with SiO, values of ap-
proximately 55-60 percent. The fragments included a single piece of
brown pumice from layer L, a dark scoria fragment from layer D, and
three lapilli from layer C. One of the lapilli from layer C was a sample
of the predominant brown pumice; the others were not identified as to
type, but they are very similar chemically to the one identified. The
lowest silica content reported in these samples was for the hornblende-
rich scoria of layer D; thus, at Mount Rainier, as at Mount Mazama
(Williams, 1942, p. 145-146), abundant hornblende is associated with
scoria rather than with the more silicic pumice. Even the highest silica
content measured in the tephra, for layer-C lapilli, is lower than the
silica contents reported previously for Mount Rainier lava flows (table
11). The tephra samples analyzed, however, probably do not represent
either the most-mafic or-most silicic rocks-in- the tephra beds from
Mount Rainier. The very pale color and low refractive index of glass in
some pumice lapilli within layer C and in a few other layers suggest
that these fragments are as silicic as the lava flows.

3



°
t

1O MILES

T
10 KILDMETERS

e+

DISTRIBUTION AND AVERAGE THICKNESS, in centimeters, of layer L. Checks mark sites
where layer has been found but is less than 1 cm thick. (Fig. 21)

Taste 6.—Volume of layer L, in millions of cubic meters

Total volume within Minimum total
2-cm isopach volume
In present state ......... 15 30
As freshly fallen ......... 25 50

Source

Progressive increases in thickness and grain size in the narrow
tongue of layer L point to a vent at the summit of Mount Rainier as its
specific source. The narrow band of thick and coarse tephra also in-
dicates that a consistent northwest wind blew while most of the
material was being erupted. The spotty, thin deposits of layer L tephra
in the northern part of the park, on the other hand, suggest that

43


TEPHRA SEQUENCE in the southwestern part of Ohanapecosh Park, showing the color, grain size,
and thickness of layers D, N, F, and B (at pick point). Most pumice and scoria layers from Mount
Rainier here show strong brown colors that distinguish them from the interbedded gray to
brownish-gray lithic ash beds. Layer B, however, typically is gray; and lapilli of layer C that are
exposed at the ground surface, as they are here, are typically light brown or gray. Compare with
figure 7. (Fig. 22)

45


GENERAL SETTING

The tephra deposits can be studied best in the park along stream-
banks in moist, gently sloping alpine meadows, which are common on
ridges and in shallow basins between altitudes of about 5,000 and 7,000
feet (1,500 and 2,000 m). In these meadows, other postglacial deposits
rarely are thick enough to hide the tephra. Moreover, even thin beds
often are well preserved because they have not been disturbed by
burrowing animals or-tree roots. _In addition, the colors of the tephra
are more intense there, because the deposits usually are wet. Several
layers make very conspicuous bands along both natural and artificial

crops are ‘scraped with a , piek ¢ or knife blade; the outerops ‘pictured i in
figures 5, 6, and 7 have been smoothed to show details of the sequence.
The exposures in the alpine meadows have provided most of the
evidence used to determine the sequence and features of tephra
deposits in the park.

In contrast, tephra is poorly preserved on or is absent on the higher,
barren slopes of rock, snow, and ice (fig. 2). Tephra that fell high on the
volcano near the summit vent has been largely eroded away or is buried
by snow and ice. Thus, it is likely that the thickest deposits of Mount
Rainier tephra—those that fell closest to the vent—are not seen; the
nearest well-preserved deposits, at altitudes of about 7,000 feet, are all

Many exposures at altitudes lower than the alpine meadows also dis-
play tephra deposits, but the sequence of layers is usually incomplete.
In most exposures, one or more layers have been eroded away, have
been mixed with other sediments, or are hidden by thick mudflow or
stream deposits. Tephra layers that can be identified, however, have
been used to determine the ages of associated deposits, especially in the
accumulations of mudflow and river deposits on valley floors.

The index map (fig. 3) shows the approximate location of several
alpine meadow areas called “parks” in which tephra layers are well dis-
played; these are the parks mentioned most often in the report.
Descriptions of the location of other sites and references to altitudes
and distances refer to the Mount Rainier National Park quadrangle
topographic map, scale 1:62,500, of the U.S. Geological Survey.
Although most measurements in this report are given first or only in
metric units, altitude is stated primarily in feet because the
topographic map shows altitudes in feet only.

Brief Description Of Tephra Deposits

The 22 layers studied in detail are all postglacial in age, and each
clearly represents an eruption. The postglacial deposits were selected

8