Download Geologic Overview of Mount Mazama and the Crater Lake Caldera

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Hirt – Mount Mazama and Crater Lake
INTRODUCTION
Prior to 7,700 years ago, Mount Mazama
was a broad stratovolcano whose glacier clad
slopes rose to a summit about 3,700 m (12,000
feet) above sea level (Fig. 1a). Suddenly, over
a period of perhaps only hours to days, a massive eruption drained nearly 50 km3 (12 mi3) of
volatile-rich magma from a shallow reservoir
that had grown beneath the mountain. Mount
Mazama’s summit foundered as the reservoir
emptied and this subsidence created a steepwalled caldera (Williams, 1942) that is 8 km (5
mi) across and about 1.6 km (1 mi) deep (Fig.
1b). Most volcanic activity on the caldera floor
had waned within a few hundred years after
the climactic eruption and rain and snow have
since accumulated to form a lake that is about
600 m (1,900 feet) deep. Today, Crater Lake is
renowned for its clarity and beauty and is the
centerpiece of one of the West’s best known
National Parks.
This paper presents a brief summary of the
geology of Mount Mazama and Crater Lake
that will serve as an introduction to the features
we will visit during our upcoming field trip. The
research presented here has been drawn from
many sources, especially papers by: Williams
(1942); Bacon and his co-workers (1983; 1989;
1997; 2002; and 2006) and Nelson and others
(1999). The complete list of the references cited
in this work is given at the end of the paper.
Definitions of words that are italicized in the
text will be found in a glossary that follows the
references.
GEOLOGIC SETTING
Cascade subduction
Eruptive activity at Mount Mazama and the
other High Cascade volcanoes is the result of
subduction along the Pacific Northwest coast.
The North American lithospheric plate is overriding three small oceanic plates that lie to the
west (Fig. 2). As the largest of these, the Juan
de Fuca plate, sinks beneath southern Oregon
it carries water bound into its surface deep into
the mantle. Heat from the surrounding mantle
warms the sinking plate and causes the waterbearing minerals it contains decompose. The water-rich fluid they release rises into the “wedge”
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Figure 3. Schematic cross-section of a continental
margin subduction zone showing the regions of
partial melting in the mantle wedge where mafic
magmas are formed and the lower crustal MASH
zone where intermediate and perhaps some felsic
magmas originate.
Figure 1. (a-top) Mount Mazama during the single
vent phase of the climactic eruption, immediately
preceding development of the ring vent and caldera collapse; (b-bottom) Mount Mazama shortly
after caldera collapse but prior to the formation of
Crater Lake. Paintings by Paul Rockwood, NPS.
of hot peridotite above the plate and causes the
rock there to partially melt (Fig. 3). The resulting basalt and basaltic andesite magmas are less
dense than the surrounding peridotite and rise
slowly until they either cool and solidify underground or reach the surface as lavas.
The magmas that sustain Mount Mazama’s
activity are rising from a narrow zone where the
top of the Juan de Fuca plate is about 100 km
(60 mi) deep. Some geologists believe this is the
depth at which the mineral amphibole breaks
down and triggers partial melting of the mantle
(Stern, 1998). Others point out that many different minerals break down to release water from
a subducting plate, and suggest that 100 km
is simply the depth at which the mantle is hot
enough to produce a separable amount of melt
(Schmidt and Poli, 1998).
Regional faulting and volcanism Geologic relations suggest that Mount
Figure 2. Map of the Pacific Northwest showing
the locations of plate boundaries, major stratovolcanoes and rear-arc volcanoes in the Cascade arc
(Crater Lake = 15), and outcrops of Quaternary lavas associated with the arc. From Hildreth (2007).
Mazama is a large volcanic center because it
lies at the intersection of two fault systems that
serve as conduits for rising magmas (Fig. 4).
Steep north-trending faults strike parallel to the
axis of the Oregon High Cascades and indicate
that the range is undergoing east-west extension in this region (Bacon, 1983). A second set
of steep north-northwest trending faults strikes
into the mountain from the Klamath Lake area
to the southeast. These faults, which define the
East and West Klamath Lake Fault Zones, are
related to large-scale “stretching” of the lithosphere in the Basin and Range province to the
east. Basin and Range extension began about
17 million years ago after the North American
plate overrode a spreading center to the west and
came into contact with the Pacific plate along
the San Andreas fault (Atwater, 1970). Shearing
along the fault has detached and rotated blocks
of western North America and caused the crust
to stretch and break along steep faults farther
east. Upwelling of hot mantle rock along the
trace of the old spreading center or through a
“slab window” that is opening behind the sinking
Farallon plate may also be contributing to Basin
and Range extension. Regardless of how the
deep extensional faults in this region have been
formed, however, where they cut across axis of
the Cascades they create pathways for magmas
from the underlying subduction zone to traverse
the crust beneath Mount Mazama.
GEOLOGIC HISTORY
Growth of Mount Mazama
Mount Mazama is a complex of overlapping
shield volcanoes and stratovolcanoes that has
been active for more than 400,000 years. Mapping and dating of rocks exposed on and around
the mountain indicate that eruptions began
at Mount Scott, just east of the caldera, about
420,000 years ago. This activity subsequently
migrated westward as eruptions built the main
body of Mount Mazama from four overlapping
stratocones: Phantom, Danger Bay, Dutton Cliff,
and Sentinel Rock. These cones grew atop one
another prior to a major episode of Pleistocene
glaciation that occurred about 110,000 years ago.
A large andesitic shield volcano subsequently grew on the northern side of this
composite stratovolcano at Llao Bay, and was, in
turn, “capped” by a complex of dacite lava flows
and domes called the Merriam Point sequence.
Today, the remains of all of these ancient cones
can be seen in the caldera walls and at Phantom
Ship (Fig. 5) where part of the Phantom Cone
still stands above lake level.
Following an episode of early Wisconsin
glaciation 60,000 to 70,000 years ago, andesite
cones grew at Cloudcap Bay and Hillman Peak.
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Hirt – Mount Mazama and Crater Lake
Figure 5. Phantom Ship consists of material from
the Phantom Cone, including a dike, that has been
exposed by erosion and projects above the surface
of the southern side of the lake.
Figure 4. Map showing the locations of major
faults and earthquake epicenters in the Crater Lake
region. Note that Crater Lake (the former Mount
Mazama) lies near the intersection of north-trending faults that parallel the axis of the High Cascade
volcanic arc and north-northwest trending faults
that bound the Klamath Graben. The intersection
of these two extensional fault systems is likely to
have provided conduits for mantle-derived magmas to rise through the crust to build the Mount
Mazama volcanic center.
The growth of these cones was followed by
dacite flows at Steel Bay and Scott Bluffs. The
Tephra of Pumice Castle, which forms a series of
welded and non-welded layers that are exposed
around much of the caldera, was erupted from a
vent near the “Pumice Castle” (Fig. 6) on eastern
lank of the volcano between 50,000 and 60,000
years ago. This tephra may be related to the eruption of the dacite lava at Scott Bluffs. Several
additional andesite and dacite flows, including
those that built the Watchman, subsequently
erupted prior to a period of late Wisconsin glaciation that occurred between 30,000 and 50,000
years ago.
During late Pleistocene time, dacite lavas
were erupted as flows and domes from five sites
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Figure 7. Locations of vents and eruptive products
formed before and during first phase of Mount
Mazama’s climactic eruption. Open circles mark
dacite vents, filled circles mark basalt vents, gray
areas indicate dacite lava flows, and arrows indicate pyroclastic flow directions. (A) Late Pleistocene vents on Mount Mazama. Note that Williams
Crater is labeled “Forgotten Crater” on this map.
(B) Dacite domes and flows formed shortly before
the climactic eruption. (C) Pyroclastic flow directions during the single-vent phase of the climactic
eruption. Diagrams modified from Bacon (1983).
years ago, and their lavas contain hornblende
which is also found in the younger units from
the climactic eruption. This indicates that the
reservoir that produced the climactic eruption
had begun to develop by this time. Mingled
andesite-dacite pumice blocks that were erupted
in basalts from Williams Crater (see Fig. 17)
also imply that the climactic reservoir was well
developed when this vent was active. In fact, the
Williams Crater eruption occurred when a basalt
dike broke into the margin of the climactic reservoir and entrained some of the dacite magma
it contained.
Figure 6. Pumice Castle (ribbed structure on the
right, just above the trees) is exposed on the
southeastern wall of the caldera. It is composed of
welded and non-welded ash-flow tuff layers that
were erupted 50,000 to 60,000 years ago during
the growth of Mount Mazama.
on the flanks of Mount Mazama: Munson,
Sharp Peak, Hill 7352’, Williams Crater, and
Palisade (Fig. 7a). At least three of these eruptive
centers were active between 22,000 and 30,000
Climactic eruption and caldera formation
Between 7,900 and 7,700 years ago four
dacite domes and lava flows: Llao Rock, Grouse
Hill, Redcloud, and Cleetwood, erupted on the
northern and eastern flanks of Mount Mazama
(Fig. 7b). The compositions of these dacites indicate that they came from the climactic reservoir. The youngest of these units, the Cleetwood
dacite, was erupted only a short time before the
climactic eruption began. After this flow was
Figure 8. Cleetwood backflow, where the molten
interior of the Cleetwood dacite flow spilled back
into the caldera after the flow was “beheaded”
during caldera collapse. This relationship indicates
that the flow was erupted only a short time before
collapse occurred.
“beheaded” by caldera collapse, its partially molten interior actually oozed back into the caldera
to form the Cleetwood backflow (Fig. 8) .
The first phase of the climactic eruption
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Hirt – Mount Mazama and Crater Lake
Figure 9. Outcrop of the Wineglass Tuff on the
eastern margin of the caldera. Locally, the tuff
consists of four separate cooling units that record
deposition from multiple pyroclastic flows moving
down the same valley.
began at a single vent on the northeastern flank
of Mount Mazama (circles labeled 1 and 2 in
Fig. 7c). This phase initially produced large
volumes of rhyodacite pumice that blanketed a
wide area of the western United States from an
eruption column that rose to a height of perhaps
50 km (30 mi). As the eruption continued, the
rate of pumice discharge gradually increased and
the eruption column became denser. When the
column finally collapsed under its own weight it
generated ground-hugging pyroclastic flows that
swept down valleys on the northern and eastern
sides of the mountain (Fig. 7c). These flows deposited the Wineglass Tuff, in which pumice and
tephra were so hot that they welded together to
form a dense black glass that resembles obsidian
(Fig. 9). Some welded sections of the Wineglass
Tuff were beheaded by caldera collapse, and the
partially molten pumice clasts they contained
oozed out. This indicates that caldera collapse
occurred only a short time after the single vent
phase of the eruption.
As the single vent phase continued, Mount
Mazama’s summit began to founder into the
partially emptied top of the underlying reservoir
along a series of steep faults that merged to form
an oval ring fracture (Fig.10). The eruption of
magma at multiple points along this fracture
marked the onset of the ring vent phase of the
climactic eruption (Fig. 11). Eruption rates were
much higher during this phase so that most of
the erupted pumice and gas surged out as large
pyroclastic flows that swept down the flanks of
Hirt – Mount Mazama and Crater Lake
Figure 11. Schematic map of the ring-vent phase
of the climactic eruption with arrows indicating
the directions of pyroclastic flows. Modified from
Bacon (1983).
Figure 10. Contour map of the caldera floor showing the locations of the ring fracture (oval bounded
by dashed lines), hydrothermal vents, areas of high
heat flow, phreatic (steam explosion) craters, and
the depth to “rocky” basement. From Nelson and
others (1999).
formed several hundred years after the climactic
eruption.
the volcano, burning and burying valleys up to
70 km (40 mi) from the summit. These flows
stripped away light pumice and small rocks from
the upper slopes of the volcano and left a lag
breccia of coarse rock fragments near the caldera
rim.
The climactic eruption ended when the
water-rich rhyodacite magma that occupied the
upper part of the reservoir was exhausted and
more mafic crystal-rich magma was drawn up
from beneath it (Fig. 12). Tapping the compositionally layered reservoir from the top down produced a zoned pyroclastic flow deposit in which
light-colored rhyodacite tephra from the upper
part of reservoir are overlain by darker more
mafic tephra from the deeper part (Fig. 13).
The present caldera is much larger than the
ring fracture that formed during the climactic eruption (see Figs. 10 and 11). During and
after the subsidence of Mount Mazama’s summit, blocks of rock along the margins of the
caldera slumped into the growing basin. Some
were pulverized and ejected during the climactic eruption, whereas others slid down into the
caldera later. A recent survey of the lake bottom
by Bacon et al. (2002) has shown, for example,
that the partially submerged Chaski Bay slide
block on southern side of the caldera actually
Post-collapse volcanism and Crater Lake
Following caldera collapse, continuing
volcanic activity and the accumulation of water
from rain and snow interacted to shape the floor
of the caldera. Mapping and sampling of the
caldera floor have shown that steam explosions
excavated pits around the base of the caldera
walls where surface waters percolated down to
reach hot rock along the ring fracture (see Fig.
10).
Eruptions of andesite lavas also began to
build Wizard Island and the Central Platform
(Fig. 14) shortly after the collapse. Terraced
shorelines show that eruptions at Wizard Island
and the Central Plateau initially kept these vents
above the surface of the rising lake. The Central
Plateau was eventually submerged, however, and
the last eruption at Wizard Island took place
when lake level was 80 m (260 ft) lower than it
is today.
The lake continued to rise until it reached
the level of a permeable horizon in northeastern
wall of the caldera that serves as a natural drain.
Merriam Cone, a third major site of underwater
eruptions, probably never rose above lake level.
Eruptions at all three of these vents had subsided within 750 years of caldera collapse. A small
dacite dome that formed below lake level on the
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Figure 12. Samples of the earliest and latest
products of the climactic eruption. Light-colored
rhyodacite pumice on the left is typical of magma
from the top of the reservoir, and was erupted first.
Dark-colored hornblende-rich cumulate on the
right is typical of magma from the bottom of the
reservoir, and was erupted last. The bubble holes
in the sample on the left indicate the pumice was
formed from magma with a high volatile content.
It was the expansion of these volatiles that drove
the explosive climactic eruption.
Figure 13. Pyroclastic flow deposits from the
climactic eruption exposed in Annie Creek canyon.
Note that the lower part of the deposit is lighter in
color than the upper part reflecting, in part, a compositional difference between magmas at different
levels in the reservoir.
eastern margin of the Wizard Island platform
about 4800 years ago marks the site of the last
eruption at Crater Lake.
PETROLOGY OF MOUNT MAZAMA
Geologists classify volcanic rocks primarily
according to the amounts of silica (SiO2) they
contain (Fig. 15) for two reasons. First, silicon
and oxygen are the most abundant elements
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Figure 15. Classification of igneous rocks according
to their silica contents. The minerals typically found
as coarser crystals (phenocrysts) in each rock type
are shown by the gray bars.
Figure 14. Geologic map showing major features
on the floor of the Crater Lake caldera.
in Earth’s crust and mantle and make up the
majority of all common volcanic rocks. Second,
silica content determines what type of eruption a lava will tend to produce. Silica-rich lavas
(dacites and rhyodacites) are “pasty” and tend to
trap and “hold in” dissolved volatiles more effectively than runny, silica-poor ones (basalts and
basaltic andesites). The expansion of dissolved
volatiles is what drives explosive eruptions, so
volatile-rich silicic magmas tend to erupt more
violently than their silica-poor counterparts.
Volcanic rocks from the Crater Lake region span
a wide range of silica contents, from basalts with
about 47 weight percent SiO2 to rhyodacites
with about 72 percent (Bacon and Druitt, 1988).
Regional mafic volcanism
Mafic volcanic rocks in the Crater Lake
region include basalts and basaltic andesites.
Studies suggest that the basalts result from small
degrees of “dry” partial melting of the astheno-
sphere as it wells up during “corner flow” behind
the sinking Juan de Fuca plate. The basaltic
andesites, on the other hand, are apparently the
products of more extensive “wet” partial melting of asthenosphere that has been fluxed by
fluids or melts released from the sinking plate
(Bacon et al., 1997b). Because mafic magmas
are denser than felsic ones they typically cannot
rise through bodies of felsic magma that have
accumulated in the crust. This may explain why
mafic magmas were erupted from vents on the
flanks of Mount Mazama but not from near its
summit when the climactic reservior was present
(Fig. 16).
Andesite and dacite lavas at Mount Mazama
Volcanic rocks of intermediate and felsic
composition at Crater Lake include andesites
(intermediate) as well as dacites and rhyodacites
(felsic). The magmas that form these rocks are
derived from rising basalts and basaltic andesites
by three interrelated processes: crystal fractionation, assimilation and magma mixing. As
magma cools, crystals of minerals richer in iron,
magnesium and calcium than the original melt
grow and are removed by accumulation onto the
floor or walls of the reservoir. Removal of these
crystals depletes the magma in these elements
and enriches it in complementary ones such as
silicon, sodium and potassium. Enrichment of
these latter elements may change one type of
magma to another (e.g., an andesite to a dacite)
by raising its silica content.
Assimilation occurs where a rising magma
engulfs and melts pieces of the crustal rocks that
surround it. Material from these rocks is then
incorporated into the magma and may change its
Figure 16. Generalized geologic map of Mount Mazama that shows the distributions of rock units of different
ages and compositions. Asterisks mark the locations of vents. From Bacon and Lanphere (2006).
composition. Partially-melted blocks of granitic
rocks occur in the lavas of the climactic eruption (Druitt and Bacon, 1989), for example, and
suggest that assimilation of upper crustal wallrock may have played a role in determining the
composition of the climactic rhyodacite.
Finally, separate batches of magma may be
present beneath a volcano at the same time. If
these batches encounter one another they may
mingle or mix to produce a new magma of intermediate composition. Mingled magmas are easy
to recognize because the separate components
have not completely combined so that swirls or
quenched blobs of one can be seen in the other
(Fig. 17). Mixed magmas are more difficult to
detect, however, because the two components are
completely hybridized. Commonly, only detailed
studies of chemistry or mineral composition
can confirm that a magma is truly a mixture.
Interestingly, the climactic rhyodacite appears to
be a mixture of two melts—each of which was
formed by fractional crystallization of a separate
andesite parent. The origin of this magma is
outlined below.
Climactic magma chamber development
Dating of dacitic and mixed lavas erupted
from vents on the upper part of Mount Mazama
indicates that a distinctive felsic magma had
begun to develop in a reservoir under the summit between 25,000 and 30,000 years ago. The
reservoir was initially filled with an andesite
magma that had a relatively low strontium (Sr)
content. Crystal fractionation of this magma
produced cumulates along the base and sides of
the reservoir as well as a complementary rhyodacite melt that rose to fill its top (Fig. 18a). Some
of this low-Sr rhyodacite magma erupted to
form the domes and flows at Steel Bay, Grouse
Hill, and Redcloud Cliff.
Sometime later, between about 10,000 and
25,000 years ago, a second batch of andesite
magma that was rich in strontium rose into the
reservoir and “ponded” between the early low-Sr
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Figure 17. Mingled lavas from Mount Mazama.
Andesite-dacite pumice from Williams Crater (left);
and quenched blobs of andesite in a glassy dacite
from the Llao Rock flow (right).
cumulates and the overlying low-Sr rhyodacite
melt. This new magma also underwent crystal
fractionation to produce a layer of Sr-rich cumulates and a separate rhyodacite melt that mixed
with the low-Sr rhyodacite already in the reservoir (Fig. 18b). Between about 10,000 and 7,900
years ago, eruptions from the partially mixed,
stratified reservoir produced both a hybrid
rhyodacite (Sharp Peak) and a low-Sr rhyodacite
(Llao Rock). By the time the Cleetwood flow
and the climactic eruption took place 7,700 years
ago, however, mixing had eliminated the small
amount of low-Sr rhyodacite that remained in
the reservoir and only hybrid rhyodacite was
produced (Fig. 18c).
CRATER LAKE GEOLOGIC HAZARDS
In light of its more than 400,000 year
eruptive history and the ongoing subduction of
oceanic lithosphere beneath the High Cascades,
it is very likely that Mount Mazama will erupt
again. Perhaps the best guide to what the volcano is likely to do in the future is a knowledge
of what it has done in the past, and this information comes from mapping and dating a volcano’s
ancient deposits. Bacon et al. (1997a) have
combined information on Mount Mazama’s
past activity with insights gained from studies of
similar eruptions at other volcanoes to estimate
Figure 18. Schematic cross-sections of the developing climactic magma reservoir at (a) 25-30 ka; (b)
25-7.9 ka; and (c) 7.7 ka. Homogeneous hybrid
rhyodacite magma from the 7.7 ka reservoir fed
both the Cleetwood flow and the climactic eruption. Fragments of high-Sr andesite cumulates
found in the uppermost part of the climactic
pyroclastic flow deposits suggest that the eruption
stopped when magma had been drawn down to
the level of the cumulates. From Druitt and Bacon
(1989).
the likely frequencies and magnitudes of various
hazards in and near Crater Lake National Park.
The key findings from their study are summarized below.
Hazards related to intracaldera eruptions
Eruptions beneath Crater Lake are likely
to trigger steam explosions when rising magmas come into contact with lake water. Except
near the shore, water pressure is likely to inhibit
explosive fragmentation of the lava. Nearshore
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Figure 19. Eruption of basaltic lava in shallow water. Heat from the lava flashes the water to steam
and triggers an explosive eruption. The chilled lava
is fragmented into tephra and thrown from the
vent by the force of the explosions. Here, the eruption column is small, but collapse of a large column
could produce laterally directed surges that would
travel several kilometers from the vent. From Chernicoff and Whitney (2002).
Figure 20. Volcanic debris flows (lahars) may form
on the flanks of Mount Mazama if hot tephra or
pyroclastic flows melt snow and are transformed
into dense slurries of volcanic rock and water.
Because such slurries are commonly much denser
than pure water they have the ability to pick up
and carry large pieces of debris. From Chernicoff
and Whitney (2002).
eruptions (Fig. 19), on the other hand, may
produce pyroclastic surges—blasts of steam,
lava, and rock fragments—that are less dense
than pyroclastic flows and so less likely to be
restricted to valleys. These surges are expected to
travel several kilometers down the flanks of the
volcano, and up to 5 km (3 mi) if they are channeled along valleys. If such a surge is formed by
column collapse rather than a smaller explosive
eruption it is expected to travel up to 30 km (19
mi).
Near shore eruptions may also throw blocks
(“ballistics”) tens of centimeters in diameter for
distances of 1 to 4 km (0.6 to 2.5 mi) from their
vents. Such blocks are expected to travel up to
1.5 km (0.9 mi) outside the caldera. In addition,
seiches up to a few meters high may also be generated on the lake, and it is possible that a near
shore steam explosion could expel enough water
or melt enough snow to produce debris flows on
the flanks of the volcano.
Another caldera-forming eruption is unlikely because virtually all of the volatile-rich
rhyodacite that had accumulated in the summit
reservoir prior to the climactic eruption has been
expelled. Only less explosive andesites and their
derivatives have been erupted within the caldera
during the past 7,700 years, and these magmas
are unlikely to have reached the surface if a
reservoir of rhyodacite magma was still present beneath the summit. It is possible that felsic
magma is still forming beneath Crater Lake, but
if it is doing so at the same rate it did before the
climactic eruption, only about 10 km3 (2.4 mi3)
would now be present—enough for an eruption
but not another caldera collapse.
Spring water rising into the bottom of
Crater Lake contains magmatic CO2 dissolved
as bicarbonate. This gas poses a threat because
it is denser than air and can “pond” in low areas
and suffocate anyone trapped there. Carbon
dioxide can accumulate in the bottom water of
a lake and then be released suddenly when the
lake overturns. At Crater Lake, however, carbon
dioxide is unlikely to accumulate in large enough
quantities to cause a sudden release because: (1)
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the upper 200 m of Crater Lake overturns and
releases its accumulated CO2 twice per year; and
(2) the deeper water in the lake mixes with the
upper water every 2.5 to 3.5 years and thereby
releases its CO2 gradually. This mixing and overturn precludes the buildup of CO2 at depth.
Breaching of the lake would release 17 km3
(4.1 mi3) of water and cause severe flooding in
valleys leading away from the volcano. Because
the lowest part of the caldera wall is 165 m (540
ft) above lake level, however, breaching of the
caldera wall by a future eruption or landslide is
considered unlikely.
Hazards related to flank eruptions
Eruptions of felsic lava outside the caldera
are likely to produce tall, gas-driven columns
that could blanket the surrounding terrain with
tephra up to several hundred kilometers from
Mount Mazama.
Volcanic debris flows may be formed by the
rapid entrainment of sediment in large volumes of water that are either expelled from the
caldera or formed by the melting of snow on the
mountain’s flanks. Such flows may travel tens of
kilometers at speeds of up to 20 m/s (45 mph) in
valleys on the steep slopes near the volcano. They
are likely to deposit large amounts of sediment
in valleys close to the mountain and change into
floods farther downstream.
Small eruptions of basalt and andesite lavas
on the flanks of Mount Mazama are likely to
be rare, with an estimated chance of about a 1
in 10,000 of a new vent opening during a given
year. Eruptions from flank vents are likely to
produce slow-moving lava flows that will not
travel more than a few tens of kilometers. If
these eruptions are explosive they are likely to
produce tephra that will blanket a few square
kilometers.
Interestingly, eruptions are relatively infrequent at Crater Lake and throughout the
Oregon High Cascades compared to the parts
of the arc just to the north (Mount Saint Helens–Mount Rainier) and south (Mount Shasta–
Medicine Lake volcano). Recent high-resolution
GPS studies indicate that the western part of the
North American plate is fragmented into several
small “blocks” that are rotating slightly relative to
Hirt – Mount Mazama and Crater Lake
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one another (see Lisowski et al., 2000), and that
eruptive frequencies are higher at the margins
of these blocks, where faulting enables magmas
to more easily reach the surface, than within the
blocks themselves (Fig. 21).
Hazards related to seismicity
Seismic hazards may be as severe a threat
at Crater Lake as volcanic hazards. In addition
to shaking, quakes could trigger landslides and
rockfalls that might close roads, block trails, and
cause destructive waves on the lake. The potential magnitudes of future tectonic quakes in the
Crater Lake area are estimated to be as large
as M = 7 for those that occur on the Western
Klamath Lake Fault Zone, and as large as M = 8
to 9 for those that occur on the Cascadia Subduction Zone. Earthquakes related to volcanic
activity at Mount Mazama are likely to be
smaller, with mangitudes up to M = 5.
Information on the locations and magnitudes of recent earthquakes in the Crater Lake
area can be found online at: http://www.pnsn.
org/CRATER/welcome.html.
REFERENCES CITED
Atwater, T., 1970, Implications of plate tectonics
for the Cenozoic tectonic evolution of western
North America: Geological Society of America
Bulletin, v. 81, p. 3513-3536.
Bacon, C.R., 1983, Eruptive history of Mount Mazama and Crater Lake Caldera, Cascade Range,
U.S.A.: Journal of Volcanology and Geothermal
Research, v. 18, p. 57-115.
Bacon, C.R., 1989, Mount Mazama and Crater Lake
caldera, Oregon, in Muffler, L.J.P., Bacon, C.R.,
Christiansen, R.L., Clynne, M.L., DonnellyNolan, J.M., Miller, C.D., Sherrod, D.R., and
Smith, J.G., Excursion 12B: South Cascades arc
volcanism, California and southern Oregon, in
Chapin, C.E., and Zidek, J., eds., Field excursions to volcanic terranes in the western United
States, Volume II: Cascades and Intermountain
West: New Mexico Bureau of Mines and Mineral Resources Memoir 47, p. 203-211.
Bacon, C.R., Bruggman, P.E., Christiansen, R.L.,
Clynne, M.A., Donnelly-Nolan, J.M., and
Hildreth, W., 1997b, Primitive magmas at five
Cascade volcanic fields: Melts from hot, heterogeneous sub-arc mantle: Canadian Mineralogist,
v. 35, p. 397-423.
From Lisowski et al. (2000).
Figure 21. Diagram illustrating the correspondence between lithospheric block boundaries and eruptive frequencies in the High Cascades. Tectonic map (left) shows the locations and rotation vectors of the lithospheric blocks identified using GPS. Eruptive frequency diagram (right) shows the number of dated eruptions at
that have occurred at each of the major High Cascade volcanoes during the past 4,000 years. Note that the
largest numbers of eruptions have occurred near the northern and southern ends of the Oregon Coast block.
Bacon, C.R., and Druitt, T.H., 1988, Compositional
evolution of the zoned calcalkaline magma
chamber of Mount Mazama, Crater Lake, Oregon: Contributions to Mineralogy and Petrology, v. 98, p. 224-256.
Bacon, C.R., Gunn, S.H., Lanphere, M.A., and
Wooden, J.L., 1994, Multiple isotopic components in Quaternary volcanic rocks of the
Cascade Arc near Crater Lake, Oregon: Journal
of Petrology, v. 35, no. 6, p. 1521-1556.
Bacon, C.R., and Lanphere, M.A., 2006, Eruptive
history and geochronology of Mount Mazama
and the Crater Lake region, Oregon: Geological
Society of America Bulletin, v. 118, no. 11/12, p.
1331-1359.
Bacon, C.R., Mastlin, L.G., Scott, K.M., and
Nathenson, M., 1997, Volcano and earthquake
hazards in the Crater Lake region, Oregon: U.S.
Geological Survey Open-File Report 97-487, 32
p.
Chernicoff, S., and Venkatakrishnan, R., 1995, Geology: New York, Worth Publishers, 593 p.
Chernicoff, S., and Whitney, D., 2002, Geology, 3rd
ed.: Upper Saddle River, New Jersey, Pearson-
Prentice Hall, 679 p.
Druitt, T.H., and Bacon, C.R., 1989, Petrology of the
zoned calcalkaline magma chamber of Mount
Mazama, Crater Lake, Oregon: Contributions to
Mineralogy and Petrology, v. 101, p. 245-259.
Hoblitt, R.P., Miller, C.D., and Scott, W.E., 1987,
Volcanic hazards with regard to siting nuclearpower plants in the Pacific Northwest: U.S. Geological Survey Open-File Report 87-297, xx p.
Lisowski, M., Dzurisin, D., and Roeloffs, E., 2000,
Cascades volcano PBO instrument clusters:
Menlo Park, U.S. Geological Survey proposal
summary (http://www.scec.org/news/00news/
images/pbominiproposals/Lisowskipbo13.pdf ).
Nelson, C.H., Bacon, C.R., Robinson, S.W., Adam,
D.P., Bradbury, J.P., Barber, J.H., Jr., Schwartz,
D., and Vagenas, G., 1994, The volcanic, sedimentologic, and paleolimnologic history of the
Crater Lake caldera floor, Oregon: Evidence for
small caldera evolution: Geological Society of
America Bulletin, v. 106, p. 684-704.
Williams, H., 1942, The Geology of Crater Lake
National Park, Oregon: Carnegie Institution of
Washington Publication, no. 540, 162 p.
14
Hirt – Mount Mazama and Crater Lake
GLOSSARY
Andesite: Volcanic rock with an intermediate silica content (about 57 to 63 wt. %)
that typically has a fine gray groundmass
and contains coarser crystals of plagioclase,
augite, and hypersthene.
Asthenosphere: Layer of Earth’s upper mantle
that lies between depths of about 100 and
350 km and is relatively “soft” or weak
because of the presence of a small amount of
melt along mineral grain boundaries within
the peridotite.
Basalt: Volcanic rock with a low silica content
(about 47 to 52 wt. %) that typically has a
fine black groundmass and contains coarser
crystals of olivine, plagioclase, and augite.
Basaltic andesite: Volcanic rock with a low
silica content (52 to 57 wt. %) that typically
has a fine black groundmass and contains
crystals of olivine, hypersthene, augite, and
plagioclase.
Caldera: Circular or elliptical depression formed
when the block of crust that overlies a
shallow magma reservoir subsides after the
reservoir has been partially emptied by an
eruption.
Cumulates: Igneous rocks formed by the accumulation of early-formed crystals in a
magma. Cumulates are formed by settling
of dense crystals to the bottom of a magma
reservoir and by explusion of melt from
a crystal “mush” undergoing gravitational
compaction.
Dacite: Volcanic rock with a high silica content
(about 63 to 68 wt. %) that typically has a
fine gray groundmass and contains coarser
crystals of plagioclase, hornblende, and hypersthene, and quartz.
Debris flow: Dense suspension of rock fragments in water that moves down slope under
the influence of gravity. The density of these
flows enables them to easily carry large
blocks of rock at speeds up to 50 kph.
Dike: A sheet-like body of igneous rock that
cuts across older rock bodies and is formed
from magma that solidified within a fracture.
Dome: Volcano formed where a batch of viscous
magma (typically dacite or rhyolite) rises to
the surface and piles up in a mound on top
of the vent. Domes are typically 1 to 5 km in
diameter.
Hydrothermal: Literally, “hot water”. Hydrothermal systems in volcanic areas are typically fed by rain or snow melt that percolates
down into the Earth, is heated by hot rock
or magma at a shallow depth, and rises back
to the surface.
Lithospheric plate: Slab of Earth’s outer surface
that consists of the crust (continental or
oceanic) and the cool, rigid upper mantle
that underlies it. Plates are typically 100
to 150 km thick and move about relative to
one another on a warmer, softer layer of the
mantle beneath them.
Magma: Partially-molten rock; typically a
mixture of melt, mineral crystals, and gas
bubbles.
Peridotite: Coarse-grained igneous rock that
forms Earth’s mantle and consists mostly of
peridotite, augite, and hypersthene.
Pyroclastic flow: Hot, dense suspension of lava
fragments, volcanic gases, and entrained air
that may travel at speeds of up to 100 kph
down the slopes of a volcano.
Pleistocene: Interval of time between 1.8 Ma
and approximately 10 ka during which
landmasses at high elevations and latitudes
were subjected repeated glacial advances and
retreats (the “Ice Ages”).
Rhyodacite: Volcanic rock with a high silica
content (68 to 72 wt. %) that typically has a
fine, light gray to pink groundmass and contains coarser crystals of plagioclase, quartz,
and biotite.
Seiche: A wave formed in an enclosed or semienclosed body of water that has a period
which depends on the dimensions of the
basin holding the water.
Shield volcano: Volcano with low slopes that is
composed of hundreds of thin flows of low
viscosity basaltic or basaltic andesite lava
erupted from a central vent or fissure. The
shield volcanoes in the southern Cascades
typically have diameters of 5 to 15 km.
Stratovolcano: Volcanic cone, typically on the
order of 20 to 30 km in diameter, that is
composed of alternating layers of lava and
pyroclastic debris.
Hirt – Mount Mazama and Crater Lake
Subduction: Process in which a plate of oceanic
lithosphere is overridden by another plate
at a convergent boundary and sinks into the
mantle.
Tephra: Pyroclastic (“fire broken”) material of a
wide range of sizes—from fine dust to large
blocks— that is ejected explosively from a
volcano and flies through the air before falling to Earth.
Volatiles: Chemical elements and compounds,
such as H2O, CO2, Cl and SO2, that occur as
gases at relatively low temperatures.
FIELD TRIP ROAD LOG
Site descriptions in this log are mostly modified
from those of Bacon (1989).
Mileage:
0.0
Junction of U.S. Highway 97 and Oregon Highway 62. 20.4
20.4 Boundary of Crater Lake National Park.
Remember, collecting or disturbing rocks or
other natural features in the park is prohibited.
As we drive northward towards the caldera the
road is climbing the gently sloping surface of
the ring-vent phase ash-flow tuff. To your right,
through the trees, note the steep canyon Annie
Creek has cut into the gray, columnar jointed
tuff. 8.6
29.0 STOP 1: Godfrey Glenn turnout. The
rock exposed here in Annie Creek Canyon is the
medial facies of the ash-flow tuff produced by
the climactic eruption (see Fig. 13). The lower
part of the deposit consists of rhyodacite pumice with 70.4% SiO2; most of the upper part
consists of a mixture of this pumice (20 to 80%)
with andesite to basalt scoria (mafic equivalent
of pumice). Most of this scoria is apparently
cumulate material, and has a range of compositions from 61 to 48% SiO2. The percentage of
scoria increases upward in the deposit (inverted
zonation of the climactic reservoir; see Fig. 18)
but the color change from buff to gray reflects
the increased emplacement temperature of the
upper part of the deposit as much as its composition. (Higher emplacement temperatures
welded more of the pumice to a dark, obsidianlike glass.) Note the bleached zone beneath
15
the uppermost 1 m of fine ash and the erosionresistant pinnacles in the canyon wall. These are
both products of alteration near fumaroles that
formed as volatiles streamed out of the deposit
36.8 STOP 2: Rim Village overlook. This
site affords a panoramic view of the northern,
southern, and eastern caldera walls. Referring
to Fig. 22, take a moment to locate the following landmarks starting from your left: Wizard
Island, The Watchman, Hillman Peak, Devils
Backbone, Llao Rock, Cleetwood Backflow,
Cloudcap, Sentinel Rock, Kerr Notch, and Garfield Peak.
The following descriptions are excerpted
from Bacon (1989). The imposing cliff on the
north wall of the caldera is Llao Rock, a ventfilling rhyodacite flow that is about 100-200
years older than the climactic eruption. The walls
below Llao Rock consist mostly of andesite and
dacite flows; as many as five erosional surfaces
are present in this sequence and the lava at lake
level is dated at 190 ka. The thin sheet-like flows
consist of agglutinated spatter topped by rubble
and were apparently fountain-fed.
The eye-shaped cliff at the caldera rim east
of Llao Rock is the Rhyodacite of Steel Bay. It
is one of the earliest products from the climactic
reservoir and was emplaced about 30 ka. East of
Llao Rock are Pumice Point (approximately in
line with the flat-topped cone of Timber Crater
north of the caldera) and the Cleetwood backflow (see Fig. 8).
West of Llao Rock are the Devils Backbone dike, Hillman Peak, and The Watchman.
Hillman Peak consists of three pyroxene- and
hornblende-bearing andesite flows that are dated
at about 70 ka. The Watchman flow is about
50 ka, and the dike that fed it can be seen on
the caldera wall below the saddle between The
Watchman and Hillman Peak. In the southwest
wall are lavas older than Hillman Peak, an erosional surface overlain by an ash-flow tuff that
weathers orange (Pumice Castle), and andesite
flows as young as about 50 ka. Wizard Island is
a tephra cone that stands atop a pile of postcaldera andesite that was last erupted when the
lake level was about 90 m lower.
16
Hirt – Mount Mazama and Crater Lake
Hirt – Mount Mazama and Crater Lake
17
Figure 23. Sketch of the outcrop at STOP 6 where airfall tephra and pyroclastic flow deposits from the
climactic eruption overlie the Cleetwood Cove rhyodacite flow. Note the reddening and induration of airfall
pumice deposits immediately above the flow. This oxidation and welding was caused by heat and volatiles
from the lava flow which was still hot when the climactic deposits were laid down on top of it. Modified from
Bacon (1989).
miceous rhyodacite that grades downward into
obsidian. Lithophysal cavities and spherulites
(devitrification structures) become more abundant as you walk southward along the cut and so
move deeper into the flow. Abundant inclusions
of quenched andesite are also mingled with the
rhyodacite vitrophyre (see Fig. 17).
Figure 22. Geologic sketch maps of the caldera walls from Bacon and Lanphere (2006). Sample ages determined by K-Ar or 40Ar/39Ar are given in thousands of years with their estimated errors (e.g. 354 ± 4).
41.2 STOP 3: Pumice Desert overlook.
From this large turnout on the northwest side of
the road you can see Red Cone (north) and Bald
Mountain (north-northwest), both of which are
basaltic andesite tephra cones on the flank of
Mount Mazama, as well as the poorly-forested
Pumice Desert. In the middle distance are, from
left to right, Mount Bailey, Diamond Peak, and
Mount Thielsen (the “lightning-rod of the Cascades”). To our left are the vents of the Williams
Crater complex. It consists of a small dacite
dome, a basalt flow, a tephra cone, and three
more flows of mingled andesite and dacite with
basalt inclusions. The entire complex was formed
rapidly when a basalt dike intruded the margin
of the climactic reservoir during late Pleistocene
time
42.4
STOP 4: Glacial striations. The
outcrop just below the second paved turnout exposes glacially-striated and polished hornblende
andesite from Hillman Peak. This flow has been
modified by the same alpine glacial processes
that cut the U-shaped valleys marked by Kerr
Notch (southeast) and Sun Notch (southsoutheast) on the opposite side of the caldera.
These features highlight the complimentary roles
played by both erosional and volcanic processes
during Mount Mazama’s development.
43.0 STOP 5: Llao Rock vitrophyre and
proximal ash-flow deposit. Park in the large
turnout on the left side of the road just past the
big cut in the obsidian, and be careful crossing
the road. A 20 cm-thick layer of pink, glassy airfall tephra overlies ash-flow tuff which, in turn,
overlies lag breccia at the north end of the cut.
The top of the Llao Rock flow is marked by pu-
47.2 Cleetwood Cove trailhead and boat tour.
The hike down to the lake takes about 20-30
minutes, the tour itself takes 2 hours, and the
hike back up takes 30-40 minutes. Please do not
get off the boat at Wizard Island because of the
long delay it would cause.
47.5 STOP 6: Cleetwood Cove. Park in the
paved turnout on the right, across from the brick
red pumice, and be careful crossing the road.
Below the parking area are cliffs in the Cleetwood rhyodacite flow and the tongue of lava
that oozed back into the caldera (Cleetwood
backflow; see Fig. 8). The roadcut exposes air-fall
tephra from the climactic eruption lying on the
Cleetwood flow (Fig. 23). Near their contact
these units are highly oxidized (reddened) and
the pumice is sintered (partially fused together).
The pumice, which was hot when it landed,
blanketed the cooling flow and trapped and
heated air. The trapped air, perhaps accompanied
by degassing from the lava, caused the oxidation.
Fumarolic alteration cuts the air-fall tephra and
overlying lag breccia, showing that the entire climactic eruption took place before the Cleetwood
flow had completely cooled.
49.0 STOP 7: Wineglass Welded Tuff. Park
at the west end of the paved turnout on the right
and walk to the caldera rim. This is an outcrop of
the Wineglass Welded Tuff which was deposited
by pyroclastic flows produced as a result of column collapse during the single-vent phase of the
climactic eruption. Note that the Wineglass Tuff
is thinner at higher elevations to the west, and
is thickest to the east in the low area between
here and Roundtop. The tuff was produced by
ground-hugging flows, and was only deposited
in valleys on the northeastern flank of Mount
Mazama. The top of the welded tuff has gash
fractures (Fig. 24) that strike parallel to the caldera rim. These fractures opened when the tuff
slumped towards the caldera while still hot and
plastic, and provide compelling evidence of how
quickly caldera collapse followed the single-vent
phase of the climactic eruption.
Turning your attention to the southern
and eastern walls of the caldera, the Redcloud
Cliff rhyodacite flow forms the prominent cliff
on the east wall just south of Skell Head. Like
the rhyodacites of Grouse Hill and Steel Bay,
it is a late Pleistocene lava that leaked from
the climactic reservoir. Immediately south and
stratigraphically below Redcloud Cliff are the
18
Hirt – Mount Mazama and Crater Lake
Hirt – Mount Mazama and Crater Lake
Figure 24. Tension gashes in the Wineglass Welded
Tuff at STOP 7. These curved fractures, which are
highlighted by shadows, formed in the brittle crust
of the tuff as its plastic interior flowed back into
the caldera. Pen points towards caldera and is 14
cm long.
Figure 25. South wall of the Crater Lake caldera
showing, from left to right, Dutton Cliff, Sun Notch,
Applegate Peak, the Chaski slide, and Garfield
Peak. The top of the Chaski slide block is highlighted by snow.
Pumice Castle and related dacite flows that we
will discuss at STOP 9. Andesites below these
dacites fall into at least two groups with ages
between 220 and 340 ka (see Fig. 22). Between
Pumice Castle and Kerr Notch is Sentinel Rock
where thick intracanyon dacite flows (about 300
ka) lie on an older, glaciated andesite (about 340
ka). Between Kerr and Sun Notches is Dutton
Cliff, and at its base the oldest rocks exposed in
the caldera (about 400 ka) are found at water
level. These agglutinated andesite flows comprise
the Phantom cone, and their altered tops appear
as “stripes” on the caldera wall.
Applegate Peak and Garfield Peak form
summits on the south wall west of Sun Notch,
and are underlain by andesite and low-silica
dacite flows. The altered flows seen near water
level below the talus slopes between Applegate
and Garfield Peaks comprise the Chaski slide,
a block of the caldera wall that failed to slip
completely beneath lake level (Fig. 25). West
of Garfield Peak is the head of Munson Valley,
where Crater Lake Lodge and Rim Village are
located.
57.7 Road to The Pinnacles on the left. Note
that lag breccia from the climactic eruption is
exposed on the right side of the Rim Drive here.
53.6 Parking area for the Mount Scott trail
on the left. Mount Scott consists of the oldest
dated lavas of Mount Mazama (about 420 ka)
which are sheets of agglutinated low silica dacite
with abundant andesite inclusions. Glaciation
has exposed the core of Mount Scott, and its
rocks are variably hydrothermally altered.
63.6 STOP 8: The Pinnacles. Well-known
exposure of the compositionally-zoned ash-flow
tuff from the ring-vent phase of the climactic
eruption. The “pinnacles” are the roots of fumaroles in which the tuff has been indurated
by vapor-phase alteration (Fig. 26). Note the
reddened of this pyroclastic flow deposit which
indicates it was emplaced at a high temperature.
69.6
Return to Rim Drive.
xx.x STOP 9: Pumice Castle overlook. Park
in the large paved turnout where the road bends
left and walk to a few feet west of the stone wall.
Features you can see from here on the eastern wall of the caldera include the Cleetwood,
Palisade, and Roundtop flows, the Wineglass (a
scree chute containing exposures of the Wineglass Welded Tuff ), and Redcloud Cliff. South
of Redcloud Cliff is another cliff formed by a
Pleistocene dacite flow that, in turn, overlies
the dacite tephra of Pumice Castle (about 70
ka). This widespread unit becomes progressively
better welded to the north, and several vitrophyric layers and a stubby lava flow just south of
Redcloud Cliff includes. Pumice Castle itself is a
prominent set of orange to brick-red towers with
resistant welded layers (see Fig. 6).
Figure 26. The Pinnacles, towers eroded into the
pyroclastic flow deposit from the ring-vent phase
of the climactic eruption where it was well indurated by gases that rose to feed fumaroles.
Below the pumice are sheets of basaltic
andesite (about 220 ka) lying on altered andesite
lavas (about 340 ka). Numerous dikes cut all of
the pre-Pumice Castle units. Immediately east
of the turnout is the southermost outcrop of the
Wineglass Welded Tuff. Confinement of the
tuff to depressions from just south of Llao Rock
clockwise to here indicates the single-vent phase
of the climactic eruption was centered northeast
of Mount Mazama’s summit.
73.5 STOP 10: Parking area for Sun Notch.
A 400 m walk to the caldera rim affords fine
views of Phantom Ship and Dutton Cliff. Phantom Ship (see Fig. 5) is a small island that is
partly composed of dikes related to the 400 ka
Phantom Cone.
77.9
Turn left to exit Rim Drive.
82.2 Turn left at the junction with Highway
62 to exit the park. Log ends. 
Last updated 3-Aug-2016.
19