Geologic Overview of
Mount Mazama and the
Crater Lake Caldera
William Hirt
Department of Biological and Physical Sciences
College of the Siskiyous
Weed, California
2
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
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 vapor they release rises into the “wedge”
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 indicate that Mount
Hirt – Mount Mazama and Crater Lake
3
Figure 3. Schematic cross-section of a continental
margin subduction zone showing the regions of
mantle and crustal melting. Diagram from Chernicoff, Fox, and Venkatakrishnan (1997).
Figure 2. Simplified tectonic map of the Pacific
Northwest showing the Juan de Fuca ridge, Cascadia subduction zone, and High Cascade volcanoes. Base map after Guffanti and Weaver (1988);
outcrop pattern of High Cascade volcanics from
McBirney and White (1982).
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
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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.
cones grew at Cloudcap Bay and Hillman Peak.
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
Figure 6. Pumice Castle (ribbed structure on the
right, just above the trees) is a formation 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.
Hirt – Mount Mazama and Crater Lake
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).
were erupted as flows and domes from five sites
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 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 magma it contained.
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
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.
5
6
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.
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 “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
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
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).
Mazama’s summit began to subside 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 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
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).
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
formed several hundred years after the climactic
eruption.
Post-collapse volcanism and filling 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
7
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.
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 be-
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Hirt – Mount Mazama and Crater Lake
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.
from basalts with about 47 weight percent SiO2
to rhyodacites with about 72 percent (Bacon
and Druitt, 1988).
Figure 14. Geologic map showing features on the
floor of the Crater Lake caldera.
low lake level on the 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
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,
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
asthenosphere 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
Hirt – Mount Mazama and Crater Lake
9
Figure 16. Generalized geologic map of Mount Mazama that shows the distributions of rock units of different
ages and compositions as well as the locations of key faults and landmarks (numbered 1-10).
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 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
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Hirt – Mount Mazama and Crater Lake
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).
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 lowSr 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
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).
that remained in the reservoir and only hybrid
rhyodacite was produced (Fig. 18c).
GEOLOGIC HAZARDS AT CRATER LAKE
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
Hirt – Mount Mazama and Crater Lake
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).
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
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
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
11
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).
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
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Hirt – Mount Mazama and Crater Lake
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) 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 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.
Hirt – Mount Mazama and Crater Lake
13
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., 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.,
Donnelly-Nolan, 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.
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 97487, 32 p.
Chernicoff, S., and Venkatakrishnan, R., 1995,
Geology: New York, Worth Publishers, 593
14
Hirt – Mount Mazama and Crater Lake
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 nuclear-power 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.
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
Hirt – Mount Mazama and Crater Lake
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.
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:
15
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
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 vent-filling rhyodacite flow that is about 100-200 years
16
Hirt – Mount Mazama and Crater Lake
Figure 22. Geologic sketch maps of the caldera walls from Bacon (1989). A top: hp = intrusions, lavas, and
airfall tephra of Hillman Peak; pc = dacite ash-flow tuff of Pumice Castle (?); pwl = pre-Wisconsinian lavas; wd
= dacite dikes and flows of The Watchman; mwl = middle Wisconsinian lavas; df = dacite fragmental deposits
(S2). B top: lb = lavas of Llao Bay; mp = lavas, domes, and fragmental deposits of Merriam Point; ewl = early
Wisconsinian lavas; pc = dacite tephra of Pumice Castle (?); lpf = dacite lithic pyroclastic flow deposits (S2);
lp = Llao Rock airfall tephra; lr = Llao Rock rhyodacite lava; cl = deposits of the climactic eruption. C top: lwg
= late Wisconsinian till; cc = Cleetwood Cove rhyodacite lava; lp = Cleetwood airfall tephra. A bottom: pcn =
lavas and fragmental deposits of Phantom Cone; db = lavas of Danger Bay; ab = lavas of Anderson Bluff; dc =
lavas of Dutton Cliff; sr = intracanyon lavas of Sentinel Rock. B bottom: cb = lavas of Cloudcap Bay; pc = dacite
tephra (heavy stippling) and lava (light stippling) of Pumice Castle; rc = Redcloud Cliff rhyodacite lava; cl =
deposits of the climactic eruption (not shown where less than a few meters thick).
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
Hirt – Mount Mazama and Crater Lake
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.
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 (south-southeast) on the opposite side of the caldera. These
17
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
air-fall 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
pumiceous 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).
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
18
Hirt – Mount Mazama and Crater Lake
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).
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 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.
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.
57.7 Road to The Pinnacles on the left. Note
that lag breccia from the climactic eruption is
Hirt – Mount Mazama and Crater Lake
19
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.
exposed on the right side of the Rim Drive here.
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.
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.
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).
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
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 21-Aug-2013.
20
Hirt – Mount Mazama and Crater Lake
Hirt – Mount Mazama and Crater Lake