Download Overview of the Geology of Mount Shasta

Overview of the
Geology of Mount Shasta
Geology 60
Fall 2007
William Hirt
College of the Siskiyous
800 College Avenue
Weed, California
Introduction
Mount Shasta is one of the twenty or so large volcanic peaks that dominate the High
Cascade Range of the Pacific Northwest. These isolated peaks and the hundreds of
smaller vents that are scattered between them lie about 200 kilometers east of the coast
and trend southward from Mount Garibaldi in British Columbia to Lassen Peak in
northern California (Figure 1). Mount Shasta stands near the southern end of the
Cascades, about 65 kilometers south of the Oregon border. It is a prominent landmark
not only because its summit stands at an elevation of 4,317 meters (14,162 feet), but also
because its volume of nearly 500 cubic kilometers makes it the largest of the Cascade
STRATOVOLCANOES (Christiansen and Miller, 1989).
Figure 1: Locations of the
major High Cascade
volcanoes and their lavas
shown in relation to plate
boundaries in the Pacific
Northwest. Full arrows
indicate spreading directions
on divergent boundaries, and
half arrows indicate
directions of relative motion
on shear boundaries. The
outcrop pattern of High
Cascade volcanic rocks is
taken from McBirney and
White (1982), and plate
boundary locations are from
Guffanti and Weaver (1988).
Mount Shasta's prominence and obvious volcanic character reflect the recency of its
activity. Although the present stratocone has been active intermittently during the past
quarter of a million years, two of its four major eruptive episodes have occurred since
large glaciers retreated from its slopes at the end of the PLEISTOCENE EPOCH, only
10,000 to 12,000 years ago (Christiansen, 1985). Mount Shasta's most recent eruption
occurred about 200 years ago (Miller, 1980), and low-levels of geothermal and seismic
activity still occur on and around the mountain today. Because of the potential hazards
that Mount Shasta's future eruptions and debris flow events may pose to the surrounding
communities and the thousands of visitors who pass through them each year, it is
important for everyone who spends time around the mountain to know how to respond
safely in the event of renewed activity.
This paper was written to provide a general introduction to the mountain's geology, and
has been adapted from part of a National Association of Geoscience Teachers conference
guidebook (Hirt, 1999). Individual sections describe Mount Shasta's geologic setting, the
processes active in its development, its geologic history, and its potential hazards. A road
log for the field trip we will take on Saturday is included as Appendix 1. More detailed
information on each of the topics discussed here is available from the sources cited in the
references section, and from the many other geologic papers listed in the comprehensive
bibliography on Mount Shasta compiled by Miesse (1993). I want to emphasize that the
research presented in this paper is not my own. My contribution has been to weave
together material from a variety of sources and add explanations that, I hope, will clarify
geological ideas for general readers. Any errors in fact or interpretation are my
responsibility, however, and should not be attributed to the original authors. Also, please
note that throughout this paper that definitions of terms written in SMALL CAPS are
given in the glossary that follows the references.
Geologic Setting of Mount Shasta
The High Cascades is the younger of two volcanic mountain ranges that have risen
parallel to the Pacific Northwest coast during the past 35 to 40 million years. The lofty
stratovolcanoes that dominate the range are less than 2 million years old, but they stand
atop a massive platform of BASALTS that has been built by eruptions from scores of
vents during the past 12 million years. This entire suite of High Cascade rocks, in turn,
overlies the eroded remnants of an older volcanic chain called the Western Cascades that
was active between about 35 and 17 million years ago. In order to understand why lavas
have risen to build these volcanic mountains over tens of millions of years, we need to
review a bit about the concepts of plate tectonics and, in particular, the process of
SUBDUCTION.
Plate Tectonics
Geologists have long recognized that earthquakes and volcanic activity are not spread
uniformly across Earth's surface. Instead, they are largely confined to narrow zones, like
the circum-Pacific "Ring of Fire", that are the boundaries between the great lithospheric
plates that cover the planet's surface (Figure 2). These rigid slabs of rock include the
crust − thin seafloor basalts and the thicker continental granitic rocks − as well as the cold
dense PERIDOTITE of the uppermost mantle. The plates are 100 to 150 kilometers thick,
and move slowly across the hotter, softer ASTHENOSPHERE beneath them in response to
the tug of sinking ocean lithosphere and thermal circulation in the deeper mantle.
The plates interact with one another along three types of boundaries: divergent
boundaries, where they are moving apart; convergent boundaries, where they are coming
together; and shear boundaries, where they are sliding horizontally past one another.
Here in the Pacific Northwest the Juan de Fuca ridge system, the Cascadia subduction
zone, and the Mendocino fault, respectively, exemplify these three types of plate
boundaries (Figure 1).
Figure 2: Map of the boundaries between Earth's lithospheric plates (Simkin and others, 1994).
Divergent boundaries are shown as gray lines; convergent boundaries as gray lines with sawteeth
pointing in the direction of the downgoing plate; shear boundaries as dotted black lines; and broad plate
boundary zones where individual boundaries are not well defined as gray ruled areas. Dots mark the
positions of hotspots, where plumes of hot mantle rock rise to the surface from Earth's interior.
The Juan de Fuca ridge system is a chain of seafloor volcanoes that marks the rift along
which the Gorda, Juan de Fuca, and Explorer plates are pulling away from the Pacific
plate. Beneath the ridge, hot asthenospheric rock flows slowly towards the surface and
partially melts due to a decrease in confining pressure. Basaltic MAGMAS rise from the
zone of partial melting, filling fractures between the plates and solidifying to form new
oceanic crust. In this way the seafloor lithosphere on each side of the ridge grows wider
by about 3 centimeters per year.
The Cascadia subduction zone is a shallowly dipping fault that separates the Gorda, Juan
de Fuca, and Explorer plates from the overriding North American plate (Figure 3). The
subduction zone dips eastward at an angle of 10 to 15° from its surface trace 50 to 100
kilometers offshore. The boundary lies at a depth of about 100 kilometers below the
High Cascades, and continues deep into the mantle beneath North America. Only the
upper part of the zone however, where the down going oceanic plates are rigid and waterrich, is marked by seismic and volcanic activity. Like most faults, the Cascadia
subduction zone is often "locked" so that plate motion creates strain in rocks of the
adjoining lithosphere. When these rocks break, part of the stored strain is released
suddenly as an earthquake. The Cascadia subduction zone has produced an average of
one large quake every 500 years. The last of these, which occurred in 1700, had an
estimated MAGNITUDE of 9 (Satake and others, 1996). In addition to producing
earthquakes, the subduction zone is also the source of the magmas that sustain volcanism
in the Cascades as described below.
Figure 3: Schematic cross section
of a subduction zone similar to the
Cascadia subduction zone beneath
the Pacific Northwest. The angle
of subduction in Cascadia is
shallower than in this illustration,
but the basic process of triggering
partial melting of the mantle by
dewatering of the downgoing
plate is the same. From
Chernicoff and Fox (1997).
The Mendocino fault is a steep boundary that separates the Gorda plate, which is moving
eastward relative to the underlying mantle, from the Pacific plate, which is moving
westward. As in the Cascadia subduction zone, the sudden release of strain accumulated
along this fault can produce large earthquakes (Dengler and others, 1995). Because the
fault offsets relatively thin oceanic lithosphere and accommodates shear rather than
convergent motion, however, its quakes are likely to be smaller than those generated by
the subduction zone. Nonetheless, because the Mendocino fault lies entirely offshore its
quakes also have the potential to create large TSUNAMIS if the faulting offsets the
seafloor vertically or triggers an undersea landslide.
Cascadia Subduction Zone
As the Gorda, Juan de Fuca, and Explorer plates descend along the subduction zone, they
are warmed by heat that flows into them from the surrounding mantle. The upper parts of
the plates carry water in fractures, seafloor sediments, and the altered minerals of the
oceanic lithosphere itself. As the plates heat up, this water is expelled and rises into the
"wedge" of asthenosphere that lies above them (Figure 3). The presence of the water
lowers the melting temperature of the asthenospheric rock, and enables it to partially melt
and produce a variety of basalt and BASALTIC ANDESITE magmas. At depths of about
100 kilometers, the asthenosphere is so hot that enough melt forms that it is able to
separate from the partially molten peridotite and rise buoyantly towards Earth's surface.
Some of subduction zone magmas rise through parts of the lithosphere that are being
stretched. Here, faults and fractures channel the magmas rapidly to the surface so that
they have little opportunity to cool or interact with crustal rocks. Elsewhere, subduction
zone magmas rise more slowly, and many become "trapped" in parts of the crust that are
less dense than they are. These magmas cool, crystallize, and mix with the surrounding
crustal rocks to form new magmas – ANDESITES and DACITES − that are unlike
anything formed initially in the subduction zone. The wide range of volcanic rock types
found on and around Mount Shasta indicates how complex the structure and composition
of the crust are beneath the southern Cascades.
Geologic Processes in the Mount Shasta Region
Mount Shasta and its immediate surroundings are the products of several geological
processes operating in concert. Volcanism has played a major role in shaping this
landscape, and the variety of volcanic features found in the southern Cascades reflects the
diversity of lavas and eruptive styles common to this region. Episodes of volcanism have
alternated with intervals of erosion during which GLACIERS, streams and DEBRIS
AVALANCHES and rockfalls have modified the original volcanic landforms. This
section briefly introduces the major agents that have shaped the Mount Shasta region,
beginning with volcanic activity and the origins of the lavas that sustain it.
Volcanism and Volcanic Landforms
Silicon and oxygen are the two most abundant elements in Earth's crust and mantle. The
characteristics of lavas depend so strongly on the abundances of these elements that
volcanic rocks are classified according to the amounts of silica (SiO2) that they contain.
Lavas erupted on and around Mount Shasta span a wide range of silica contents, from
basalts with about 49 weight percent to dacites with 71 (Baker, 1988). In the field, these
rocks can usually be distinguished by their colors (silica-poor rocks are typically darker
than silica-rich ones) and by the types of early-crystallized minerals (phenocrysts) they
contain (Figure 4).
A recent experimental study (Baker and others, 1994) indicates that some of the
compositional differences among the lavas in the Mount Shasta area result from different
degrees of partial melting in the mantle above the subduction zone. Most of the magmas
that rise from this zone are silica-poor basalts and basaltic andesites and yet, surprisingly,
about 90 percent of the mountain is built from more silica-rich andesites and dacites.
Clearly, major changes are occurring as magmas rise from the subduction zone, cool, and
interact with crustal rocks on their way to the surface. Detailed studies of Mount Shasta's
lavas show that three processes -- crystal fractionation, assimilation, and magma mixing - play important roles in bringing about these changes (Figure 5).
Figure 4: Classification chart for volcanic rocks found in the Mount Shasta area. Silica contents and
typical phenocryst minerals (shown by bars) are given for each rock type.
Figure 5: Schematic diagrams of three magma
chambers showing the processes that can modify the
compositions of lavas erupted from them.
Top: Fractional crystallization is caused by the
growth and removal of crystals that have compositions
different from that of the original magma.
Middle: Assimilation occurs as the rising magma
engulfs and dissolves blocks of compositionally
dissimilar wall rocks.
Bottom: Magma mixing occurs when two magmas of
different compositions blend together to produce a
new magma of intermediate composition.
Basalts and Basaltic Andesites: Because of their low silica contents and high eruptive
temperatures, basalt and basaltic andesite lavas are "runny" compared to more silica-rich
ones. The basaltic lavas that have reached the surface around the flanks of Mount Shasta
have formed long tube-fed flows, broad SHIELD VOLCANOES, and steep loose TEPHRA
CONES (Figure 6). None of these lavas is directly related to any of Mount Shasta's
eruptive episodes, however, and studies by Baker and others (1994) suggest that they are
formed by small degrees (6 to 10 percent) of partial melting of nearly dry peridotite.
The most mafic magmas directly related to Mount Shasta are basaltic andesites that form
small flank vents such as Green Butte. Some of the basaltic andesites in the Mount
Figure 6: Volcanic features produced by silica-poor lavas. Left: Everitt Hill, a small shield volcano,
was built by thin overlapping basalt flows that poured from the southern flank of Ancestral Mount
Shasta 450,000 years ago and flowed at as far as 70 kilometers down the Sacramento River Canyon!
Right: Photo of a quarry wall that exposes the internal structure of a small tephra cone on the flank of
Deer Mountain, just east of Mount Shasta. Note that the layers of bedded tephra dip gently towards the
central vent, where part of the cone's feeder conduit is exposed.
Shasta area are unusually rich in magnesium (up to 11 weight percent MgO at >52 weight
percent SiO2) and cannot be formed from less magnesian basaltic andesites by fractional
crystallization of the minerals they contain as phenocrysts. Rather, experimental
evidence suggests they have been formed by large degrees (20 to 30 percent) of partial
melting of relatively water-rich parts of the mantle. The origins of the less magnesian
basaltic andesites in the Mount Shasta area have not been studied in detail except to note
that they can be related to one another by crystal fractionation. At the nearby Medicine
Lake volcano, however, the development of similar lavas is thought to have resulted from
mixing of primitive and fractionated basalts that have undergone subsequent crustal
contamination (Baker, 1988).
Andesites and Dacites: Because of their higher silica contents and lower eruptive
temperatures, andesite and dacite lavas are "pastier" than basaltic ones. They tend to
form stout flows or to pile up on top of their vents as steep-sided DOMES (Figure 7). The
pastiness of andesite and dacite magmas also prevents them from releasing dissolved
volatiles readily as they rise towards the surface. When they erupt, the rapid expansion
of these volatiles commonly causes explosions that shatter the lavas into PYROCLASTIC
MATERIALS. The eruptive behavior of andesite and dacite lavas depends strongly on
their volatile contents, and commonly changes during the course of an eruption. As a
result, the same vent may alternately produce both the lava flows and deposits of
pyroclastic material that will build a layered stratovolcano.
The origins of the andesites and dacites that have been erupted at Mount Shasta are
apparently quite complex and, according to Baker (1988), require six unique crustal and
mantle sources to account for all of their compositional differences. Complex mixing of
basaltic andesites and more fractionated magmas, followed by additional fractional
crystallization and crustal contamination, has apparently formed some andesites! In some
instances, the mixing of these different magmas was incomplete and yielded "mingled"
rocks such as the banded pumices of the Red Banks (Figure 8). In most cases, however,
the evidence for mixing of magmas beneath Mount Shasta comes from subtle differences
in the mineral compositions and textures of the lavas. Even though it can be difficult to
Figure 7: Volcanic features produced by silica-rich lavas. Left: Oblique aerial view of part of the Lava
Park andesite flow on the northern slope of Mount Shasta. This "pasty" lava formed thick flow lobes
whose margins are about 100 meters high. Right: Photo of a group domes formed on the northern flank
of Mount Shasta during the Misery Hill eruptive episode. Small masses of dacite lava rose to build this
dome complex, which is about 1 kilometer long and whose flanks are mantled by talus.
Figure 8: Banded block of the Red Banks
pumice collected from an outcrop just
above the head of Avalanche Gulch. Note
the contrast between the light dacite and
dark andesite pumice layers. These bands
sample two coexisting magmas that were
mingled, but not mixed, during the
eruption that created the Red Banks about
9,600 years ago.
detect, the importance of mixing is emphasized by compositional modeling which
indicates that fractionation of the observed phenocrysts cannot relate Mount Shasta’s
andesites to basaltic andesite parents, nor its dacites to andesite parents (Baker, 1988).
Glaciers and glacial erosion
A glacier is mass of land ice so large that it flows downhill under its own weight.
Glaciers develop in cool, wet areas where at least some of the snow that falls during one
winter does not melt or evaporate by the next. Masses of snow and ice typically
accumulate at the upper ends of valleys on peaks like Mount Shasta and, when they
become thick enough, flow down these valleys as glaciers (Figure 9).
Glaciers are powerful erosive agents because flowing ice can pluck blocks from the
underlying bedrock and use rock fragments embedded in itself as abrasives to grind away
the valley floor and walls. Glacial erosion produces distinctive features − including
bowl-shaped CIRQUES, ragged ARETES, and broad U-shaped valleys − all of which can
be found on Mount Shasta (Figure 10). At their downslope ends glaciers come to a halt
where warmer temperatures melt and evaporate the ice faster than it can flow downhill.
Where glaciers stop, all of the rock material that they have been carrying is either
deposited as piles of poorly sorted rock fragments called MORAINES, or carried away as
fine suspended sediment in melt water streams (Figure 11).
Seven major glaciers are recognized on Mount Shasta today (Figure 13), and they have a
total volume of about 140 million cubic meters (Driedger and Kennard, 1986). As
Figure 9: Aerial view of the Whitney Glacier,
which flows down the valley between the
Hotlum (left) and Shastina (right) cones. Only
the ice near the base of the glacier flows
plasticly. During flow, brittle ice in the upper
part of the glacier fractures into blocks that are
separated by the crevasses visible in the photo.
Figure 10: Aerial view of Avalanche Gulch, a
prominent glacial valley on Mount Shasta's
southern side. The bowl-shaped depression at
the head of the valley is a cirque, and the sharp
ridges on either side are aretes. Sargents Ridge
(labelled) is also an arete that lies north of the
Old Ski Bowl. The rusty-red cliff at the head of
Avalanche Gulch is the Red Banks, a pumice
deposit that formed after the last major episode
of glaciation on the mountain.
Figure 11: View looking northeastward across
the lower end of the Hotlum Glacier. The
glacier's terminal moraine is visible as the low
ridges of grayish rock debris on either side of
the small lake. The lake's pale blue color is due
to fine glacial sediment ("glacial flour")
suspended in the water. The lake lies at the
head of Gravel Creek, whose incised channel
can be seen in the middle distance.
impressive as this sounds, it is small by comparison to the amount of ice that mantled the
mountain at least twice during Pleistocene time. Today's glaciers are not actually the
remnants of these larger “ice age” glaciers, but developed independently about 700 years
ago during a period of modest global cooling (Guyton, 1998). Mount Shasta's glaciers
are of concern because they hold an enormous volume of water which, if released
suddenly, could pick up large amounts of poorly-consolidated glacial and pyroclastic
deposits lower on the mountain's slopes and produce devastating debris flows.
Streams and Debris Flows
Although Mount Shasta receives an average of about 168 centimeters of precipitation
each year, there are only a few permanent streams on the mountain. Because much of
Mount Shasta is composed of permeable pyroclastic materials, water percolates into its
slopes and finds its way into fractures and other openings in the underlying bedrock.
Much of this water later emerges at springs around the base of the peak. Only during
heavy rains or periods of rapid melting of snow and ice − when water is supplied faster
than it can soak in − do stream discharges rise. Under normal circumstances these
streams carry small parts of their sediment loads as dissolved or suspended materials, and
push or roll heavier rocks along their beds. As stream flow increases, however, the water
rapidly erodes poorly consolidated pyroclastic and glacial deposits and entrains a large
amount of suspended sediment. Under these conditions, the water becomes slurry that
may be twice as dense as pure water (de la Fuente and Bachmann, 1999) and is capable
of buoying up and carrying much larger rocks than normal. Under these conditions the
stream is transformed into a debris flow, as Whitney Creek was in August 1997. Such
flows are one of the greatest hazards Mount Shasta poses to people and property on its
lower slopes.
Figure 12: Topographic map of the summit of Mount Shasta showing the names and locations of the
glaciers mapped by the United States Geological Survey in 1986. Rhodes (1987) recognizes several
additional glaciers, however, and points out that a two of the mapped bodies (Hotlum and Wintun)
consist of physically separate parts.
Mass movements
Mass movements are down slope falls or flows of weathered rock material driven by
gravity. Although the speeds and coherencies of the moving materials differ, all mass
movements are promoted by three factors: steeper slopes, weaker rock materials, and an
abundance of water. As you would expect from these considerations, many of the mass
movements on Mount Shasta have originated from steep parts of the mountain that have
been altered by the discharge of volcanic gases, and have occurred after periods of heavy
rain or the rapid melting of snow and ice.
Figure 13: Two views of the Shasta Valley debris avalanche deposit. Left: Photo looking
southeastward across the deposit towards Mount Shasta. The small hills in the foreground are coherent
blocks of lavas and pyroclastic rocks from ancestral Mount Shasta that were carried northward in the
finer-grained matrix that surrounds them. Right: Photo that shows a cross-section of one of the blocks,
composed of layered pyroclastic and debris flow deposits, exposed in the wall of a quarry at the north
end of the Lake Shastina dam.
Rockfalls are perhaps the most common mass movements in the Mount Shasta region,
and take place when coherent blocks of rock break loose from steep outcrops and cascade
down slopes to accumulate as talus below. Debris flows are also rapid mass movements,
but the moving masses are incoherent sediment-water slurries rather than solid blocks.
These flows, in turn, are overshadowed by faster and potentially more devastating debris
avalanches like the one that brought down the northern flank of ancestral Mount Shasta
about 300,000 to 350,000 years ago. Collapse of a large mass high on the mountain
produced an avalanche that swept 55 kilometers northward and buried the western Shasta
Valley beneath a deposit of large blocks of volcanic rock surrounded by finer matrix of
shattered volcanic and sedimentary materials (Figure 13). The Shasta Valley debris
avalanche is one of the largest such features we know of, but it is still unclear how it
started and how the avalanche traveled so far. There is no evidence that a volcanic
eruption initiated the avalanche, and other possible triggers include an earthquake or the
sudden saturation of the slope by rain or snowmelt. Our current understanding of debris
avalanches suggests that their long runouts may be the result of the formation of a layer
of shattered rock and air at the base of moving rock mass that behaves as a fluid and
enables the avalanche to travel with relatively little friction over a great distance (Bishop,
2001). Studies indicate that many stratovolcanoes, including Mount Shasta, have been
intensely altered by the circulation of hot, acidic groundwater as they age (Crowley and
others, 1999). It is this sort of alteration, which turns hard lavas into softer clay-rich
rocks, that may be setting the stage for future debris avalanches on Mount Shasta and
elsewhere in the High Cascades.
Geologic History of Mount Shasta
Mount Shasta is a compound stratovolcano that has been built by repeated eruptions
during the past 200,000 years. Although the mountain itself is relatively young, it has
been built atop older basalts and andesites whose ages indicate that volcanism has been
taking place at the site of the present cone for at least the past 600,000 years.
Ancestral Mount Shasta
Pre-Shasta basalts form a number of shield volcanoes, such as Everitt Hill and Ash Creek
Butte, which lie just south and east of the mountain (Figure 14). A suite of coeval
andesites that crop out on Mount Shasta’s southwestern flank are the remnants of an
Figure 14: Index map of the Mount Shasta area showing the areas covered by lavas from modern
Mount Shasta and by the exposed deposits (including the Shasta Valley debris avalanche) from
ancestral Mount Shasta. Also shown are the locations of some of the older shield volcanoes that flank
Mount Shasta (triangles) and the stops for Saturday’s field trip (numbered).
earlier stratocone that stood on the site of the present mountain until about 350,000 years
ago. The youngest rocks from this "ancestral Mount Shasta" yield POTASSIUM-ARGON
DATES of about 360,000 years (Chesterman and Saucedo, 1984) and are found as blocks
in the massive debris avalanche that blankets the western Shasta Valley. Mapping of the
avalanche deposit by Crandell and others (1984) has shown that the avalanche flowed at
least 43 kilometers northwestward from the base of the Mount Shasta and contained at
least 26 cubic kilometers of material. Sedimentary rocks incorporated into the avalanche
deposit (Ui and Glicken, 1986), and soft sediment injected into it along fractures, indicate
that marshy lake and stream deposits covered at least part of the Shasta Valley when the
avalanche swept across it. Following the collapse of the northern flank of ancestral
Mount Shasta, olivine basalt lavas flowed from a vent between The Whaleback and Deer
Mountain and spread across the eastern Shasta Valley. These basalts, which are about
160,000 years old (Christiansen and Ernst, 2001), buried the eastern part of the avalanche
deposit and formed several large lava tubes including Pluto and Barnum Caves.
Growth of Modern Mount Shasta
Modern Mount Shasta has been built atop the remains of its collapsed predecessor during
four relatively brief eruptive episodes, each of which was centered at a separate vent.
The locations of these vents are shown in Figure 15, and the chronology of the eruptive
episodes and intervening glaciations is summarized in Table 1. The pattern of volcanic
activity was similar during each episode, and began with the eruption of roughly equal
proportions of two-pyroxene andesite lavas and pyroclastic flows from a central vent.
The absence of erosional features or soil horizons between successive deposits suggests
that each "cone-building" phase lasted no more than a few hundred to a few thousand
years (Christiansen and others, 1977). These brief periods of intense eruptive activity
were separated by longer intervals during which hornblende-bearing andesite and dacite
domes rose to fill the earlier craters. During each of the first three episodes, the end of
activity at the central vent was followed by minor eruptions of dacites or basaltic
andesites on the mountain's flanks.
Sargents Ridge Cone: This oldest cone forms the southeastern part of the mountain, and
a segment of its crater rim still stands above Thumb Rock near the head of Mud Creek.
Although the cone has been deeply dissected during two episodes of glaciation you can
imagine what it once looked like by projecting the northwest-dipping strata exposed
below Thumb Rock towards the southeast-dipping ones exposed on Sargents Ridge
(Figure 16).
Table 1: Chronology of Eruptive Episodes and Glaciations at Mount Shasta
Eruptive1/glacial2 episode
Episode began (yrs ago)
Episode ended (yrs ago)
Hotlum
9,500-9,700
200?
Shastina
9,700
9,500
32,000
13,000
Late Wisconsin
Misery Hill
30,000-50,000
9,600-9,700
194,000
133,000
Early Illinoian
Sargents Ridge
200,000-300,000
100,000-200,000
1 Dates from Christiansen and Miller (1997) and Miller (1980). 2 Dates from Shackelton and Updyke (1973).
Figure 16: View of the southern flank of
Mount Shasta, looking northward into the
glaciated core of the Sargents Ridge cone (SR).
Lines accentuate the northward (left) and
southward (right) dips of layering away from
the cone's crater which was once located near
the head of Mud Creek. The younger Misery
Hill (MH) and Shastina (Sh) cones are also
labeled.
Misery Hill Cone: This second cone grew atop the glaciated northwestern flank of the
first, and makes up a large part of the present mountain. Part of its crater rim stands
between the summit and Shastina (Figure 17), and the dome that fills its crater is a
prominent landmark to climbers ascending from Avalanche Gulch. Although the main
phase of Misery Hill volcanism preceded a late Pleistocene glaciation that ended about
10,000 years ago, eruption of the 9,600 to 9,700-year old Red Banks pumice post-dated
this glaciation (Christiansen and others, 1977). The pumice forms a 350 square kilometer
airfall deposit on the eastern side of the mountain, and a 100-meter thick arc of sintered
fragments that stands as a prominent cliff across the head of Avalanche Gulch. The Red
Banks pumice was erupted from a zoned reservoir, and the yellowish dacite fragments
that form the cliffs at the head of Avalanche Gulch give way to dark-brown andesite
fragments as one climbs stratigraphically higher through the deposit.
Shastina Cone: Shastina forms a separate peak 3 kilometers west of Mount Shasta's
summit. Its early activity produced a small crater in the saddle between Shastina and the
summit as well as several tongues of andesitic lava that flowed from a second, more
westerly vent where the main cone eventually grew (Figure 18). The growth of
Shastina's main cone was followed by the development of four or five small domes on the
floor of its central crater (Figure 19). Explosions related to the emplacement or
destruction of one of these domes caused the western side of the cone to collapse,
forming Diller Canyon and spawning pyroclastic flows that buried the present sites of
Weed and Mount Shasta City.
Figure 17: Mount Shasta viewed from the
northwest. Part of the crater rim of the
Misery Hill cone (M) can be seen standing
between the younger Hotlum (H) and
Shastina (S) cones.
Figure 19: (above) View of the Shastina
crater looking westward from Misery Hill.
Remnants of several small domes and
internal crater walls are visible in the main
crater, and the rim that surrounds the first
Shastina vent is visible as an arc of darker
rock around a snowfield in the lower central
part of the photo.
Figure 18: (left) Aerial view of the southern
side of Shastina showing one of the stout
andesite flows that spilled from the main vent
before the growth of the Shastina cone.
Black Butte, a complex of hornblende dacite domes that stands next to Interstate 5
between Mount Shasta City and Weed, formed during a late phase of the Shastina
episode. It grew in four distinct pulses from a crater that had opened about 12 kilometers
west of Shastina. A detailed study of dacites from Black Butte (Katz, 1997) suggests that
all of the domes were fed from the same reservoir and that the ascent of each batch of
magma required at least 8 days and continued without pause once it had begun.
RADIOCARBON DATING indicates that the entire Shastina eruptive episode lasted no
more than a few hundred years (Miller, 1980).
Hotlum cone: The eruptive products that form this fourth cone crop out mostly on the
northeastern side of the mountain, and the dome that fills its crater forms the present
summit (Figure 20). The earliest eruptions from the Hotlum vent occurred at the same
time as those from Shastina, but most of the cone has grown since the retreat of large
glaciers from the mountain about 6,000 years ago (Christiansen and Miller, 1997). Early
eruptions produced several andesitic lava flows on the northern and eastern sides of the
peak, including the 9-kilometer long Military Pass flow. Growth of the summit dome
was followed, perhaps no more than 200 years ago, by explosions that blasted out its
central part and produced an eruption cloud that spread gray lithic tephra widely across
the northern flank of the mountain.
Figure 20: The Hotlum cone. Left: Photo showing the eastern part of the summit dome as viewed
from Misery Hill to the south. The higest point is Mount Shasta's summit. The summit hot spring (HS)
lies in the patch of bare rock at the western base of the summit mass. Right: Photo showing the
Military Pass andesite flow on the northeastern flank of the mountain. Note the prominent levees along
its margins and flow folds on its surface.
Three of Mount Shasta's major vents, as well as many smaller ones, are aligned along a
north-trending zone that passes through the summit (Figure 15). This linear zone
parallels local faults and the trend of older rock units exposed south and west of the
volcano, suggesting that bedrock structure has partially controlled the geometry of Mount
Shasta's development (Christiansen and others, 1977).
Modern Geothermal and Seismic Activity
Today, a field of sulfur-encrusted FUMAROLES high on the Hotlum-Bolam ridge and a
small group of boiling springs just west of the summit are the main signs of thermal
activity on Mount Shasta (Figure 21). Modeling of data from a magnetic survey
conducted in the mid-1970s has shown, however, that the main body of the mountain is
less magnetic than Shastina. Because rocks from both parts of the volcano have similar
magnetic properties this difference in field strengths may reflect a weakening of rock
magnetism at high temperatures beneath the Hotlum cone due to the presence of a buried
body of hot rock or magma (Christiansen and others, 1977).
Figure 21: One of several boiling pools that form Mount Shasta's
summit hot spring. A temperature of 84°C was measured for this
spring and several fumaroles in the summit area in July 1975
(Christiansen and others, 1977). Ice axe is 85 cm long.
Figure 22: Histogram showing the number of earthquakes with magnitudes ≥ 1 per 2 month period
beneath Mount Shasta from January 1981 through April 2006. Note the prominent earthquake swarm
that occurred in late 1992 and early 1993 (about month 145). Data are from the Northern California
Earthquake Data Center database.
During the past 20 years, seismic activity beneath Mount Shasta has averaged about 5
quakes with magnitudes greater than or equal to 1 each year. From time to time this
background seismicity is punctuated by earthquake swarms, in which many quakes with
similar magnitudes occur during a short span of time (Figure 22). The most seismically
active area beneath the mountain lies about 18 kilometers southeast of Mount Shasta City
at a depth of 10 to 12 kilometers (Figure 23).
Volcanic Hazards of Mount Shasta
Based on the geologic record of its past activity, pyroclastic flows and debris flows from
Mount Shasta are thought to pose the greatest threats to surrounding communities. Lava
flows, the growth of domes, and the blanketing of the surrounding area by tephra
probably pose much lesser threats. The likely nature and scale of each of these hazards is
described briefly below, and the section closes with a note on the probability of future
eruptions.
Figure 23: Block diagram of earthquake foci recorded beneath the Mount Shasta area between August
1979 and June 1999. The block is 0.5° of longitude long by 0.5° of latitude wide by 28 kilometers
deep, and is centered on Mount Shasta (S). Sizes of the points are proportional to the magnitudes of
the earthquakes, with the largest being magnitude 3 and the smallest magnitude 1. Notice the
concentration of foci southeast of the summit at depths between 5 and 15 kilometers. Data are from the
Northern California Earthquake Data Center database.
Pyroclastic Flows
These hot, mobile suspensions of rock fragments in volcanic gases and superheated air
can sweep down steep slopes at speeds in excess of 100 km/hr (Figure 24). Pyroclastic
flows form as tephra-laden eruption columns fall back to earth or as the steep margins of
domes and lava flows collapse and disintegrate. Dome growth and collapse have played
major roles in each of Mount Shasta's eruptive episodes, and most of the pyroclastic
flows that the volcano has produced are the results of this process.
About 9,500 years ago pyroclastic flows swept down from Shastina and its satellite Black
Butte, burning and burying the forests that stood where the towns of Weed and Mount
Shasta City are today. Radiocarbon dating of charcoal from these forests indicates that
the eruptions occurred over a period of no more than a few hundred years (Miller, 1978).
The flow deposits have red (oxidized) tops and contain prismatically-jointed blocks, both
of which indicate that they were emplaced at temperatures of 400 to 700°C (Figure 25).
Similar pyroclastic flows from the Hotlum cone have traveled 10 to 20 kilometers down
valleys on all sides of the mountain during the past several thousand years, and Miller
(1980) estimates that the collapse of a dome high on Mount Shasta during a future
eruption could create pyroclastic flows that would overrun low-lying areas up to 30
Figure 24: Photograph of a pyroclastic flow
descending the northern flank of Mount Saint
Helens during August 1980. The clouds of hot
gases and suspended tephra rising off of it
mostly hide the dense, blocky base of the flow.
Photo credit: U S Geological Survey/Cascades
Volcano Observatory.
kilometers from the volcano (Figure 26).
Debris Flows
Sudden increases in runoff triggered by the rapid melting of snow and glacial ice or by
heavy rains can mobilize large volumes of pyroclastic or glacial debris to produce these
fast moving flows. Some debris flows have been initiated by volcanic eruptions, as hot
lava or tephra melts the mountain's blanket of snow and ice. Others, including the
August 1997 debris flow in Whitney Creek, are simply the results of climatic
fluctuations.
Both hot and cold debris flows have swept down canyons on all sides of Mount Shasta
Figure 25: Pyroclastic flow deposits from
Shastina and Black Butte exposed in a railroad
cut just north of Black Butte. Note that the
Shastina deposit underlies the two Black Butte
deposits and is crudely layered. The Shastina
deposit contains prismatically jointed blocks up
to a meter in diameter (just out of the photo to
the left) that have been carried 12 kilometers
from their source!
Figure 26: Map of hazard
zones for pyroclastic flows
and surges from eruptions
originating at or near the
summit of Mount Shasta
(Crandell and Nichols, 1987).
Sites in zone 1 are most likely
to be overrun by such flows,
whereas those in zone 3 are
only likely to be affected by
pyroclastic flows longer than
any that have occurred during
the past 10,000 years. Sites in
zone 3 may, however, be
affected by pyroclastic surges
sweeping out ahead of
pyroclastic flows from inner
zones.
during the past 10,000 years, and some have traveled more than 30 kilometers from the
summit. Because they need not be associated with volcanic activity, however, debris
flow events are expected to occur more frequently − and perhaps with less warning −
than eruptions. As Miller (1980) states, such flows "... are likely to cover broad areas in
[a zone 20 to 30 kilometers from the summit] several times per century." Mapped hazard
zones (Figure 27) suggest that towns in drainages on all sides of the mountain may be
threatened by future debris flows.
Lava Flows and Domes
Because lavas erupted from Mount Shasta are predominantly "pasty" andesites and
dacites, flows on the mountain tend to move slowly and travel relatively short distances.
These blocky streams and masses of molten rock pose little direct threat to people or
movable property. The longest flow on Mount Shasta, for example, is the Military Pass
andesite flow, which extends 9 km down slope from its vent near the base of the Hotlum
dome. This flow formed about 9,000 years ago, early in the most recent eruptive episode
(Miller, 1980), and its modest length suggests that even flows erupted from vents low on
the mountain's flanks are not likely to reach more than 15 to 20 kilometers from the
summit. Perhaps the greatest hazard posed by both lava flows and domes is from the
pyroclastic flows that may be spawned by their collapse or explosive disintegration.
Tephra
Historically, Mount Shasta has produced relatively little tephra in comparison to other
Cascade volcanoes. Tephras are commonly composed either of pumice (bubbly volcanic
Figure 27: Map of hazard
zones for debris flows from
Mount Shasta (Crandell and
Nichols, 1987). Sites in zone
A are most likely to be
overrun by future flows,
whereas those in zone C are
least likely to be affected.
Within a given zone, debris
flow hazard decreases as one
moves away from stream
channels and onto higher
ground.
glass) or lithic fragments (bits of older, dense lava shattered by explosions). The Red
Banks eruption 9,600 years ago produced one of Mount Shasta's few recent deposits of
pumiceous tephra. This tephra was deposited across an area of at least 350 square
kilometers on the eastern side of the mountain and has a maximum thickness of about 50
centimeters (Miller, 1980). Explosions at or near the Hotlum dome about 200 years ago
formed a smaller deposit of lithic tephra. This tephra, which looks like fine yellowishgray sand, is spread widely over Mount Shasta's northeastern flank (Christiansen and
others, 1977) and has locally accumulated to thicknesses of at least a meter thick where it
has been washed or blown into surface depressions.
Prevailing winds are likely to carry most tephra from future summit eruptions to the east
and northeast so that large accumulations probably will not occur in the most densely
populated areas on the western and southern flanks of the mountain. However, even a
few centimeters of tephra might be enough to close Interstate 5, shut down services in the
nearby communities, and disrupt the air traffic that uses Mount Shasta as a navigational
landmark (Crandell and Nichols, 1987).
Epilogue
In light of the Mount Shasta volcanic system's nearly 600,000-year eruptive history and
the continuing geothermal and seismic activity on and around the mountain today, future
eruptions are considered very likely. Although predicting the exact times and natures of
volcanic eruptions is notoriously difficult, two techniques are used to estimate the timing
of future eruptions. First, the mountain is monitored for physical changes − such as
increased seismicity, uplift, and the emissions of heat and volatiles − that might be
associated with the rise of magma into the shallow crust. Under favorable circumstances
such changes may give months to weeks of warning in advance of an eruption. Second,
for a longer-term perspective, geologists map and date the mountain's ancient deposits in
order to reconstruct its eruptive history. This information can then be used to calculate
the average recurrence intervals for various types of events. Perhaps the best way to
conclude this summary of Mount Shasta's potential hazards is with a quote from Crandell
and Nichols (1987) on the chances of when its next eruption will occur:
Studies by geologists show that Mount Shasta has erupted 10 or 11 times during
the last 3,400 years and at least 3 times in the last 750 years. Mount Shasta does
not erupt at regular intervals, but its history suggests that it erupts at an average
rate of roughly once per 250 to 300 years. If the behavior of the volcano has not
changed, the chance is 1 in 25 to 30 that it will erupt in any one decade and 1 in
3 or 4 that it will erupt within a person's lifetime.
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Glossary
Andesite: A volcanic rock that contains between 56 and 63 weight percent silica.
Andesites are typically gray or black in color and contain visible crystals of plagioclase,
augite, and hypersthene.
Arête: A narrow, commonly knife-edged ridge that has been undercut on both sides by
glacial erosion.
Asthenosphere: Part of the Earth's mantle that lies below the lithosphere, at depths
between about 100 and 350 kilometers. Rock here is relatively soft because its high
temperature and relatively low confining pressure enable a small amount of melt to form
and lubricate its movement.
Basalt: A volcanic rock that contains between 47 and 52 weight percent silica. Basalts
are typically black and commonly contain visible crystals of olivine and plagioclase.
Basaltic andesite: A volcanic rock that contains between 52 and 57 weight percent silica.
Basaltic andesites are typically black in color and contain visible crystals of plagioclase,
olivine, and augite.
Cirque: A bowl-shaped depression or amphitheater carved at the head of a mountain
valley by glacial erosion.
Crystal: A piece of solid material within which all of the atoms are arranged in a regular,
three-dimensional pattern. Some crystals have smooth external surfaces (faces) that
formed as they grew from a melt or vapor.
Dacite: A volcanic rock that contains between 63 and 72 weight percent silica. Dacites
are typically gray to pink in color and contain visible crystals of plagioclase, hypersthene,
and hornblende.
Debris avalanche: A dense, incoherent mixture of water, rock, and soil that flows down
slope at speeds of 40 to 200 km/hr (25 to 125 mi/hr).
Debris flow: A dense, incoherent mixture of water, rock, and soil that flows down slope
at speed of 2 to 40 km/hr (1 to 25 mi/hr).
Dike: A sheet-like body of solidified magma that cuts across layering or other structures
in the surrounding rock.
Dome: A relatively small, steep-sided volcano formed by pasty lava that has piled up
atop its vent. Domes are typically no larger than 2 to 3 km (1 to 2 mi) in diameter and are
composed of silica-rich lavas.
Focus: The point within the Earth at which at which rock initially breaks to initiate an
earthquake.
Glacier: A mass of land ice that is large enough to flow down hill under its own weight.
Holocene Epoch: The period of time from 10,000 years ago to present. On Earth this
corresponds to the time since the last retreat of widespread continental glaciers.
Lava: Partially molten rock that has risen through Earth's crust and been erupted onto the
surface.
Magma: partially molten rock that consists of melt with or without entrained crystals
and vapor bubbles.
Magnitude: A number that is scaled to the amount of energy released by an earthquake.
An increase of one magnitude unit (say from 5 to 6) corresponds to a 10-fold increase in
the amount of ground motion and an approximately 30-fold increase in the amount of
energy released by an earthquake.
Mineral: A solid natural material that has a specific chemical composition and a unique
internal arrangement of its atoms. For example, quartz consists of silicon and oxygen
atoms in a 1:2 ratio (SiO2), and these atoms are bonded together in a hexagonal structure.
Moraine: A mass of poorly consolidated rock debris deposited by a glacier. Moraines
typically form elongate or arcuate ridges and contain rock fragments of a wide range of
sizes, from fine silt and clay to coarse boulders.
Mudflow: A dense suspension of fine rock fragments in water. A mudflow is a type of
debris flow in which most of the sediment particles are sand-sized (2 mm across) or
smaller.
P-wave: A type of earthquake wave that travels through Earth's interior and causes rock
particles to move back and forth parallel to the direction it is traveling. P-waves are
faster than S-waves and can pass through solids, liquids, and gases.
Peridotite: A dense, coarse-grained rock that consists mostly of the magnesium and ironsilicate mineral olivine (”peridot”).
Phenocryst: A relatively large crystal in a volcanic rock that grew from the surrounding
melt at depth.
Pleistocene Epoch: The period of time between 1.8 million and 10,000 years ago. On
Earth this corresponds to the interval during which large continental glaciers repeatedly
advanced and retreated across landmasses at high latitudes. Informally, this epoch is also
called the "ice age".
Potassium-argon dating: A technique for determining the age of rock and mineral
samples by measuring the amounts of radioactive potassium (40K) and its daughter
element, a form of argon (40Ar). Because 40K decays relatively slowly, this technique
usually only yields reliable ages for samples that are more than about 100,000 years old.
Pumice: A porous, glassy volcanic rock formed by the rapid expansion of gas bubbles in
melt that is quenched as it is erupted.
Pyroclastic material: Volcanic rock that has been fragmented by explosions during an
eruption or by the collapse and disintegration of the flanks of domes or lava flows.
Radiocarbon dating: A technique for determining the age of a sample of organic
material (charred wood, plant roots, cloth) by measuring the rate at which the radioactive
carbon (14C) it contains is decaying. Because 14C decays relatively rapidly, this technique
only yields accurate ages for samples that are less than about 60,000 years old.
Recurrence interval: The average period of time between two episodic events such as
earthquakes, floods or volcanic eruptions.
Rockfall: A moving mass of rock fragments that has broken loose from an outcrop and
cascaded down a slope.
S-wave: A type of earthquake wave that travels through Earth's interior and causes rock
particles to move back and forth perpendicular to the direction it is traveling. S-waves
are slower than P-waves, and cannot pass through liquids or gases.
Seismometer: An instrument that measures the movements of the Earth's surface
produced by earthquakes or other disturbances.
Shield volcano: A broad volcano built of many thin, overlapping flows of basalt or
basaltic andesite lava. Shield volcanoes differ widely in size, from several kilometers to
several hundreds of kilometers in diameter and have very gently sloping flanks.
Silicate: A compound formed of silicon and oxygen with or without other elements.
Silicate minerals and the rocks they are part of make up most of Earth's mantle and crust.
Stratovolcano: A volcano composed of alternating lava flows, layers of pyroclastic
material, and debris flow deposits piled up around a central vent. Stratovolcanoes are
typically 10 to 30 km (6 to 20 mi) in diameter and have slopes that steepen gradually
upwards towards their summits.
Subduction: Process in which a plate of dense oceanic lithosphere sinks back into Earth's
interior along a dipping surface that separates it from the overriding lithosphere and
asthenosphere.
Subduction zone: A dipping surface that separates a sinking plate of oceanic lithosphere
from the overriding lithosphere and asthenosphere.
Tephra: Fragmental volcanic rock formed by lava that has been blown explosively from
a vent and solidified as it traveled through the air.
Tephra cone: A volcano composed of layers of tephra piled up around a central vent or
crater. Tephra cones are typically 1 to 2 km (1 mile) in diameter and have steep (35 to
40º) slopes determined by the angle at which the loose tephra begins to slide.
Texture: Description of how the various materials that make up a rock (crystals, glass,
and fragments of other rocks) are arranged with respect to one another.
Tsunami: A wave produced in a large body of water (ocean or lake) by an earthquake or
volcanic eruption. Tsunamis travel rapidly across deep ocean basins (about 480 mph!)
can grow to heights of tens of meters as they approach shore and enter shallow water.
Tuff: A rock made of fine volcanic tephra that has been cemented or partially fused
(welded) together.
Tuff breccia: A rock made of tephra and other pyroclastic materials of a wide range of
sizes that have been cemented or partially fused together. Large fragments are angular.
Volatiles: Chemical compounds and elements (such as water, H2O, and nitrogen, N2) that
occur as gases at high temperatures and atmospheric pressure.
Volcanic arc: A chain of volcanic peaks that stands above a subduction zone. Because
Earth's surface is spherical, subduction zones and the chains of volcanoes that develop
above them are inevitably curved.
Volcaniclastic material: Fragmented volcanic rock deposited by either pyroclastic or
debris flows.
Appendix 1: Field Trip Road Log
The locations of numbered stops in this log are shown on Figure 14. As you leave the
COS parking lot, turn left onto Siskiyou Way and drive eastward for three blocks until
you reach South Weed Boulevard. Turn left, and continue northward under the freeway
and past the Main Street arch until you reach the “Y” where Highway 97 branches off to
the right. Turn onto Highway 97 and continue eastward out of Weed.
Mileage
Total Interval
0.0
0.0
Intersection of Highway 97 and South Weed Boulevard.
0.8
0.8
Roadcut through a megablock of dense lava in the Shasta Valley debris
avalanche.
3.5
2.7
View of the steep front of the Lava Park andesite flow on the right. This
flow poured from a flank vent on the northern side of the Shastina cone
early in the Shastina eruptive episode (see Figure 6).
4.0
0.5
North of the highway you can see the hummocky topography of the Shasta
Valley debris avalanche which was formed by the collapse of ancestral
Mount Shasta. The small hills scattered across the valley floor are
coherent blocks of andesitic lavas and volcaniclastic rocks set in a finergrained "matrix facies" of similar composition (Figure 13a). The
avalanche deposit is at least 76 m thick in parts of the Shasta Valley
(Crandell et al., 1984), and its runout length of 55 km suggests that
ancestral Mount Shasta was similar in height to the present mountain (Ui
and Glicken, 1986).
4.4
0.4
Turn left onto Big Springs Road and drive northeastward across the 9,700
year old pyroclastic flow deposits that overlie the debris avalanche in this
area.
6.4
2.0
The ridge ahead on the right is the margin of an andesitic lava flow from
the Sargent’s Ridge cone.
9.7
3.3
STOP 1: Dwinnell Dam Quarry – Park along the side of the road near
the intersection with Lakeshore Drive. Walk about 200 m up the hill, turn
right, and walk across the top of the dam to the quarry at its northern end.
The quarry's walls expose the interior of an avalanche block (Figure 13b)
composed of interlayered volcaniclastic breccias, tuffs, and sediments
from the slopes of ancestral Mount Shasta. Note that the internal
stratigraphy of this block is generally coherent despite its disruption by a
series of normal faults. Ui and Glicken (1986) observed that blocks of
volcaniclastic rocks in the avalanche deposit are typically larger and less
fractured than those of massive andesite, suggesting that the
volcaniclastics behaved more ductilely during movement of the avalanche.
Return to the vehicles and continue northeastward on Big Springs Road.
13.1
3.4
Notice the difference between the hummocky topography of the western
part of the Shasta Valley and the relatively smooth topography of the
eastern part. Our route has taken us out of the debris avalanche and we are
now crossing the 300,000-year old high-alumina basalt flows that lap onto
it.
16.1 3.0
Big Springs junction. The peak in the distance at 10 o’clock is Owl’s
Head, an outcrop of Tertiary Western Cascade volcanic rocks. Turn right
and proceed southward on county road A-12.
25.5
STOP 2: Barnum Cave – Turn left and park in the small dirt parking area
just off the road. Walk about 100 meters east to the entrance of Barnum
Cave. This is one of the best preserved and most easily accessible of
several nearby lava tubes that transported olivine basalt lavas from their
vents near the northern base of Mount Shasta out into the eastern Shasta
Valley. Once inside the cave, notice the fallen blocks littering the floor
and the “false ceiling” developed as a crust on a second flow that only
partially filled the tube. "Lavacicles" may be seen hanging from the
bottom of this false ceiling and lava gutters line its walls. Just beyond the
end of the sand that has washed in, pahoehoe flow features are well
developed on the cave floor. Barnum Cave can be followed for about 250
meters, although the floor of the last part (~150 m) is littered with large
blocks before it narrows to a small crawlspace. Return to the vehicles and
continue southward on A-12.
9.4
26.4 0.9
To the right is Yellow Butte, a steptoe that consists of metasediments that
have been correlated with the Siluro-Ordovician (?) Duzel Formation and
are intruded by a Cretaceous (138 Ma) diorite (Wagner and Saucedo,
1987).
27.4
1.0
Junction with Highway 97. Mount Shasta dominates the skyline to the
south. The Hotlum dome forms the summit and, to the right (west), the
Misery Hill crater rim and Shastina cone are visible on the skyline. Turn
left and proceed eastward. In the narrow canyon between Shastina and the
summit lies the Whitney Glacier. It is approximately 3.5 km long and is
the only well-defined valley glacier on Mount Shasta (Biles, 1989).
28.5
1.1
Ahead on the left is Sheep Rock, an outcrop of east-dipping andesitic tuff
breccias and lavas that were erupted during Western Cascade volcanism.
Samples from similar outcrops to the north yield K/Ar whole rock ages
between 21 and 33 Ma (Kelley et al., 1987). These volcanic rocks are, in
turn, overlain by the Pliocene and Quaternary basaltic andesites of the
High Cascades that form a series of summits including Herd Peak and
Goosenest. To the right is The Whaleback, one of a group of Quaternary
basaltic shields that flank Mount Shasta. A sample from The Whaleback
has yielded a K/Ar whole rock age of approximately 300,000 years
(Kelley et al., 1987).
31.8
3.3
Turn right onto Forest Service road 42N12 (also, forest road 19), signed
“Deer Mountain Snowmobile Park”, and proceed southeastward.
35.7
3.9
STOP 3: Deer Mountain Tephra Cone – Turn right and park in the
snowmobile park parking lot. Follow the trail from the south end of the
lot a few hundred meters to the flank of a small berm on the right. Walk
up over the berm and down into a cut that exposes the core of a tephra
cone (Figure 6b). Notice how the dense olivine and plagioclase-bearing
andesite that forms the feeder dike at center of this exposure differs in
color and texture from the scoriaceous material that was deposited on the
flanks of the cone. Also notice the angular unconformity that separates
tephra layers formed during two eruptive episodes and the lack of any
evidence of erosion or soil development between these sets of layers. The
lava erupted from this cone is a relatively magnesian andesite (58 wt. %
SiO2 and 9 wt. % MgO) that Baker and others (1994) interpret as the
product of extensive wet partial melting of the underlying mantle. Return
to the vehicles and continue southeastward on road 42N12.
40.7 5.0
View of the eastern flank of Mount Shasta from The Whaleback. Note the
flow folds and levees developed on the Military Pass andesite flow. This is
the longest lava flow on Mount Shasta, and reaches 9 km down slope from
its vent near the base of the Hotlum dome. It formed about 9,000 years
ago, early in the latest eruptive episode (Miller, 1980), and is partially
covered by the Hotlum Glacier. The Hotlum is the largest glacier on
Mount Shasta with a surface area of 1.6 km2, and the only one with a welldeveloped medial moraine.
42.5
1.8
Stay right at this Y intersection. Continue southward on forest road 19.
48.0
5.5
Junction with the Military Pass Road, 43N19. Proceed southward on
forest road 19. The peak ahead to the left is Ash Creek Butte, another of
the basaltic shields that flanks Mount Shasta. Whole rock samples from
this volcano yield a range of K/Ar ages between 170,000 and 247,000
years (Kelley et al., 1987). Pleistocene glaciations have carved a deep
cirque into the northern side of this shield and left the summit ridge a
castellated arête.
48.4 0.4
To the right is Jackrabbit Flat. The surface of this area is littered with
clasts of the 9,700-year old Red Banks pumice from the final Misery Hill
eruption. Just below the left skyline is the Wintun Glacier, which was the
source of the melt water that produced the debris flow we will visit at stop
4.
54.4
6.0
Junction with road 41N16, which is marked by a sign pointing to "Deter
Camp". Turn right, ford Ash Creek, and continue 2.2 miles west to the
intersection with forest road 31.
56.6
2.2
Bear right at this intersection and proceed 0.1 miles to Ash Creek. Cross
the creek, and park off the road to the left on the far side.
56.7
0.1
STOP 4: Ash Creek Debris Flow − The debris flow deposit here was
formed during the summer of 1977 as snow and ice on the Wintun Glacier,
which lies high in the canyon to the west, melted rapidly. The debris flow
started at the foot of the glacier and scoured its way down Ash Creek
canyon before spreading out across the gentler plains to the east. It
covered a distance of about 20 km. Notice the levees at the margins of the
flow, which is about 5 m thick at this locality (Miller, 1980). The debris
flow killed the trees you see standing as snags to the east by burying their
roots and suffocating them! After lunch, return to the intersection and
proceed southward on forest road 31.
60.5
3.8
Junction with road 41N15 (signed Clear Creek Trailhead). Continue
southward on forest road 31.
63.5
3.0
STOP 5: Mud Creek Mudflow and Diversion Structure – Park off the
road on either side and walk a few hundred meters along the trail to the
south to see the diversion structure and stream cuts into the debris flow
deposits. This area was covered by several large debris flows during the
years 1924 through 1931. These flows were triggered by rapid melting of
the Konwakiton Glacier, and threatened the town of McCloud (Hill and
Egenhoff, 1976). Subsequent downcutting by Mud Creek has incised a 10
to 15 m deep canyon into the old debris flow deposits. The creek has been
routed through a structure designed to trap large debris from future flows
in its steel "bearclaws" while diverting the “watery” parts of the flows out
of Mud Creek and into an adjoining drainage.
68.7
5.2
Look upward towards the right (about 1 o’clock) to see the Konwakiton
Glacier at the head of Mud Creek.
72.4 3.7
Intersection with the paved ski park road (forest road 88). Turn left and
drive southward across the flanks of Everitt Hill, an approximately
450,000-year-old shield volcano, to the intersection with Highway 89.
76.0 3.6
Intersection with Highway 89. Turn right and drive westward, past
McCloud, towards Interstate 5.
81.0 5.0
Intersection with Mount Shasta Boulevard (just east of Interstate 5). Turn
right and drive north through Mount Shasta City to the light at Lake Street.
83.4 2.4
Lake Street intersection. Turn right and follow Lake Street eastward for
0.3 miles. It becomes North Washington as it veers left (northward);
continue on North Washington for 0.2 miles to the light at Rockfellow
Drive.
83.9 0.5
Continue straight ahead at this intersection. Notice that the road has
become Everitt Memorial Highway as it begins to climb northward across
pyroclastic flow deposits on the southwestern slope of Mount Shasta.
88.5
Road crosses Cascade Gulch, site of a debris flow that washed out the
highway on New Year's Eve of 1997 after three days of heavy rains.
4.6
91.2 2.7
Roadcut at this curve exposes the oldest known andesites from the
ancestral Mount Shasta. Samples from this outcrop yielded whole rock
K/Ar ages of 593,000 years (Kelley et al., 1987) and are characterized by
platy jointing developed during magmatic flow.
92.9 1.7
Vista of the older cones of Mount Shasta. The 9,500-year old Shastina
cone is on the left, separated from the main mass of the mountain by
Cascade Gulch. The apparent high point is the dome that fills the crater of
the older Misery Hill cone and just below and it are the Red Banks, a cliff
of pumice from the final phase of Misery Hill activity. The broad, glacial
canyon extending down from the Red Banks is Avalanche Gulch. It is
partially filled by a mass of loose rock debris that has been mapped as a
rock glacier (Christiansen et al., 1977). To the right of Avalanche Gulch
are the glaciated remnants of the Sargents Ridge cone. Across the gulch to
the east stand a chain of dacitic and andesitic flank vents (from north to
south: Gray Butte, Douglas Butte, Upper and Lower McKenzie Buttes,
and Signal Butte) that are aligned along the lineament that passes through
Mount Shasta's summit. These vents were formed during the Sargents
Ridge and Misery Hill eruptive episodes.
93.8 0.9
Red Fir Flat, the highest area on mountain at which rocks of ancestral
Mount Shasta are exposed.
94.1 0.3
On the left, note the road cuts that expose the interiors of the lateral
moraines deposited by the glaciers that once flowed down Avalanche
Gulch.
95.2
STOP 6: Bunny Flat Trailhead – Park in one of the available spaces on
either side of the road. To the north you can look directly up Avalanche
Gulch, a popular climbing route on Mount Shasta. The ridge on the
1.1
eastern side of the gulch is composed mostly of andesitic lavas and
pyroclastic breccias from the Sargents Ridge cone. At the top of the ridge,
near Thumb Rock, these deposits can be seen to dip to the northward,
away from a crater that once lay just over the ridge near the head of Mud
Creek. The steep, reddish cliffs at the head of Avalanche Gulch are the
Red Banks, a sintered mass of andesitic, dacitic, and banded pumices that
were erupted during the final phase of Misery Hill activity about 9,700
years ago. On the western side of the gulch is Casaval Ridge, which is
composed of Misery Hill-age deposits. The upper part of this ridge has
been reduced to an arête by a late Pleistocene glaciation. Return to the
vehicles and continue eastward on the Everitt Memorial Highway.
96.8 1.6
In this area, trees on both sides of the road were sheared off or uprooted by
a slab avalanche that broke loose from the north wall of the Old Ski Bowl
and swept down the slope in January 1997.
97.5 0.7
The low banks of yellowish-gray ash on the left side of the road are
reworked lithic ash from vulcanian eruptions of the Hotlum cone.
98.1
STOP 7: The Old Ski Bowl – Park in the lot at the end of the road. To
the north, the bedded lavas and pyroclastic breccias exposed in the walls
of the Ski Bowl form the core of the old Sargents Ridge cone. These beds
dip away in all directions from a “center just east of the sharp pinnacles of
Sargents Ridge” (Christiansen and Miller, 1989). The reddish hill to the
west is Green Butte, a mass of basaltic andesite lava flows and scoria that
is the highest mafic vent on the mountain. Looking to the southwest, you
can see the Castle Crags on the far side of the Sacramento River canyon.
The Crags are erosional features developed in a strongly jointed Jurassic
(~170 Ma) granite that has intruded Siluro-Devonian gabbros and
peridotites of the Trinity Terrane (Wallin and Metcalf, 1998).
Geophysical evidence and the presence of sparse ultramafic xenoliths in
Mount Shasta’s lavas suggest that the Trinity peridotite also at least
partially underlies the mountain. To the south is the Everitt Hill shield −
the source of basaltic lavas that flowed about 80 km down the Sacramento
River canyon. Return to the vehicles and drive carefully back down the
mountain.
0.6
111.7 13.6
Intersection with Ski Village Drive. Turn right and follow this road
westward and through a small jog to the left to North Mount Shasta
Boulevard.
112.6 0.9
Intersection with North Mount Shasta Boulevard. Turn right and proceed
northward along the western base of Spring Hill, an andesitic lava cone of
Sargents Ridge age (Figure 3) to the onramp for northbound Interstate 5.
Proceed northward on the freeway. Ahead on the right is Black Butte, a
hornblende dacite dome that was emplaced as four distinct segments that
nearly fill a flank crater formed at the end of the Shastina eruptive episode.
Drive past Black Butte to the South Weed interchange.
118.4 5.8
South Weed off ramp. Turn right at the bottom of the ramp onto Vista
Drive and proceed eastward 0.2 miles to the “T” intersection with Black
Butte Drive.
118.8 0.4
Turn right and proceed southward 0.4 miles and then eastward (as the
pavement ends) 0.6 miles until you reach a “T” intersection with Black
Butte Road (and the railroad tracks).
119.8 1.0
Turn right, drive 0.3 miles southwestward, and then turn left into a small
spur road next to the tracks and park. Walk a hundred meters west to stop
8.
120.1 0.3
STOP 8: Shastina and Black Butte Pyroclastic Flow Deposits −
Unfortunately, slope wash and the growth of vegetation have obscured
some of the features once visible in outcrops of the three late Pleistocene
and Holocene units exposed along Interstate 5 in the Black Butte area
(Miller, 1978). From oldest to youngest these are (1) three pre-Shastina
diamictons identified as glacial tills on the basis of their clay-rich matrices
and striated cobbles, (2) three pyroclastic flows from Shastina that are
recognized by their pyroxene dacite clasts set in gray ash matrices, and (3)
two pyroclastic flows from Black Butte that are recognized by their
vesicular hornblende dacite clasts set in yellowish ash matrices. This
locality exposes the upper part of one of the Shastina pyroclastic flow
deposits and both of the overlying Black Butte flows (Figure 25). Red,
oxidized tops and the presence of prismatically-jointed blocks indicate that
these three units were emplaced at high temperatures. Both the Shastina
and Black Butte pyroclastic flow deposits contain charcoal fragments that
yield 14C ages of about 9,500 years. Retrace your route back to the South
Weed interchange.
121.6 1.6
Continue straight ahead under the freeway. Follow Vista Drive as it veers
to the right and becomes South Weed Boulevard. Continue northward for
1.5 miles to the intersection with Siskiyou Way.
123.2 0.2
Follow this westward for three blocks and then turn right onto the campus
loop road and follow it into the parking lot.
Figure 15: Geologic map of Mount Shasta and vicinity, Siskiyou County, California. Modified from
Christiansen (1982).