Download Lassen Volcanic National Park

Geologic Overview of
Lassen Volcanic National Park
William Hirt
Department of Biological and Physical Sciences
College of the Siskiyous
Weed, California
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Hirt – Lassen Volcanic National Park
INTRODUCTION
Lassen Peak is the southernmost of the
fifteen or so major volcanoes that dominate
the High Cascade Range—a chain of volcanic
peaks that stretches from northern California to
southern British Columbia. Prior to the eruption of Mount St. Helens in 1980 Lassen Peak
was the most recently active volcano in the High
Cascades. Between 1914 and 1917 its eruptions
(Fig. 1) focused public attention on the volcanic
character of northeastern California and led
to the designation of the area surrounding the
peak as a national park.
Lassen Peak is a prominent part of the
600,000-year old Lassen Volcanic Center (LVC)
where ongoing thermal and seismic activity
indicate the presence of a modern magmatic
system. Because another eruption is possible
at any time and is likely to produce fast-moving
pyroclastic flows and volcanic debris flows that
could devastate low-lying areas tens of kilometers from the volcano (Hoblitt et al., 1987),
the LVC continues to be closely monitored by
geologists.
This paper presents a brief summary of the
geology of Lassen Volcanic National Park that
will serve as an introduction to the features we
will be visiting during our upcoming field trip.
The research presented here has been drawn
from many sources, especially works by Clynne
and Muffler (2010), Clynne et al. (2000) and
Kane (1980). 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 in Lassen Volcanic National
Park (LVNP) is the result of plate interactions
along the western margin of North America.
Along the Cascadia subduction zone, which
lies just offshore along the Pacific Northwest
coast, the North American lithosphere is overriding three small oceanic plates that lie to the
west (Fig. 2). As the southermost of these, the
Gorda plate, sinks beneath northern California
it carries water bound into its surface deep into
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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).
Figure 1. Two views of Lassen Peak’s eruption on
22-May-1915. (top) View of the tephra cloud rising
from the summit; (bottom) view of the Devastated
Area that was swept by pyroclastic and debris
flows. Photos by B.F. Loomis.
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”
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 eruptive activity at
the Lassen Volcanic Center in the western part
of the park are rising from a narrow zone where
the top of the Gorda plate is about 100 km (60
mi) deep (Fig. 4). 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 extension and volcanism In the eastern part of the park, on the other
hand, the North American lithosphere is being pulled apart by Basin and Range extension.
This 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 normal faults farther east. As the North American plate has been
thinned by extension, the underlying asthenosphere has welled up and partially melted due
to decompression. Basalt magmas produced by
this partial melting have risen along the steep
faults that cut across the eastern part of the
park and erupted to build many small shield
volcanoes and tephra cones.
GEOLOGIC HISTORY
Cascade Volcanism in the Lassen Peak Area
The rise of magmas from the Cascadia subduction zone began to build the High Cascade
volcanoes several million years ago. In the Lassen region at least five volcanic centers (Fig. 4)
that each consisted of a central stratovolcano
flanked by smaller domes have developed during the past 3.5 million years (Clynne, 1990a).
The development of each volcanic center followed a similar pattern. First, silica-poor lavas
called andesites and basaltic andesites erupted
from a central vent and built up a cone of
alternating lava flows and layers of pyroclastic
materials. Next, thick lava flows of more silicarich andesite spilled down the sides of this early
cone and completed its construction. Finally, silica-rich lavas called dacites and rhyolites erupted
from vents around the flanks of the cone and
formed domes and short, thick lava flows on its
lower slopes.
Beneath each volcanic center, however, the
magma that fed the final phase of its activity
continued to release a tremendous amount
of heat as it cooled and crystallized (Fig. 5).
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Figure 4. Tectonic map of the Lassen Park region. Heavy blue lines are depth contours on the underlying
Gorda Plate. Dashed black lines indicate the southern limits of Cascade volcanism 3 million years ago (3 Ma)
and today (0 Ma). Large black letters indicate the locations of volcanic centers active in this region during the
past 3.5 Ma: S = Snow Mountain; Lt = Latour; Ln = Lassen; C = Caribou; D = Dittmar; and M = Maidu. Green
background indicates outcrops of old Sierra Nevada and Klamath basement rocks, whereas yellow indicates
outcrops of young Cascade and Basin and Range volcanic rocks. Light gray lines are the traces of normal
faults, and brown dots mark the locations of volcanic vents younger than 7 Ma. From Clynne et al. (2000).
This heat warmed the groundwater below the
central cone and formed a hydrothermal system
beneath the old vent. The rising water carried
S and Cl compounds that had been expelled
from the magma, and oxidation of hydrogen
sulfide to sulfate rendered the water acidic as
it reached the surface. The reaction of this hot,
acidic water with the fresh lavas of the cone
produced soft clays and opal. Once the center
of the cone had been “softened-up” by hydrothermal alteration it was preferentially removed
by later stream and glacial erosion. This process
left only segments of the unaltered flank lavas
to mark the original extent of each cone.
The Lassen Volcanic Center is the youngest of the five centers in the vicinity of the park
(Fig. 6) and its history has followed the model
outlined above fairly closely. It is also the only
regional volcanic center in which the hydro-
thermal system is still active. The growth of the
stratovolcano that marked the center’s first two
phases of activity (I and II on Fig. 7) began about
0.7 to 0.8 Ma, and ended about 0.61 Ma. The
cone, called the Brokeoff Volcano (or Mount
Tehama), is estimated to have been about
12 km in diameter and to have had a summit
elevation of about 3,350 m (11,000 ft) by Clynne
(1990b). Its vent appears to have been centered
above the Sulfur Works geothermal area in the
southwestern part of the park. Hydrothermal
activity, which continues today at sites such as
the Sulfur Works and Bumpass Hell, extensively
altered the core of the old cone. Streams and
Pleistocene glaciers have removed most of this
altered rock, leaving only the relatively unaltered masses of flank lavas such as Brokeoff
Mountain, Mount Diller and Mount Conard to
outline its original extent.
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Figure 5. Schematic west-to-east cross-section
through Lassen Volcanic National Park showing
the vertically extensive magmatic system that has
developed beneath the western part of the park
(Lassen Volcanic Center) and the much smaller
regional flux of mafic magmas that are rising
beneath the eastern part of the park in response
to Basin and Range extension. From Clynne et al.
(2000).
complexes that have been built during this pulse
of activity (IIIL on Figure 7).
A decrease in the strength of Earth’s gravitational field within a 25 km-wide oval area encompassing Lassen Peak and the Central Plateau
to the east may reflect the presence of a body
of low-density magma at depth. Clynne (1989)
has argued that such a body would probably
consist of partially molten dacitic magma, be 5
to 8 km across, and lie at a depth of 10 to 20 km
Fig. 5). The presence of quenched basalt and
andesite inclusions in recent eruptive products
indicates that magmas from the mantle are still
adding heat and mass to this dacitic magma
chamber. Thusfar, however, efforts to image the
body using seismic waves have been unsuccessful. Because rock is a poor conductor of heat
it is likely that the cooling and crystallization of
such a large mass of magma will sustain activity
at the Lassen volcanic center for several hundred thousand more years.
The third phase of activity at the Lassen
volcanic center has continued sporadically
during the past 600,000 years, with eruptions
having occurred mostly in three pulses. The first
pulse, which began about 614,000 years ago
(Lanphere et al., 1999), formed a small caldera
on the northern flank of the cone and produced
rhyolite pyroclastic deposits (the Rockland
Tephra) as well as several domes and flows (IIIR
in Fig. 7). Little trace of this caldera remains
because younger third-phase lavas have apparently filled it.
The second pulse of activity occurred between 250,000 and 200,000 years ago and built
a series of dacite domes and lava flows on the
northern flank of the old cone. Bumpass Mountain, Ski Heil Peak and Reading Peak are some of
the domes that grew during this episode (IIIB on
Fig. 7).
The third pulse of activity has occurred during the past 100,000 years and has produced
a distinctive suite of dacite lavas that contain
quenched inclusions of more mafic (andesite
and basalt) magmas (Fig. 8). Eagle Peak (57,000
years old), Lassen Peak (27,000 years old; Fig. 9)
and the Chaos Crags (about 1,100 years old) are
three of the more prominent domes or dome
Origin of the Lavas in the Lassen Region
Magmas entering the crust from the underlying subduction zone are basalts and basaltic
andesites, and have continued to erupt around
the margins of the center throughout its history
(Clynne and Muffler, 1989). The Hat Creek Basalt (Fig. 10), which forms the Subway Cave lava
tube has formed just north of the park, is typical
of this suite of magmas. The initial compositional variability of these magmas is probably due to
differences in the degrees of partial melting or
water contents of their mantle source regions,
as at nearby Mount Shasta (Baker and others,
1994). Early in history of the Lassen volcanic
center these magmas ascended through relatively “cool” crust and underwent relatively little
interaction with it.
As the cone-building phase progressed,
however, rising magmas warmed the crust and
lowered its density and viscosity. These changes
slowed the ascent of later batches of magma
and led to the development of small magma
reservoirs in the crust. Within these reservoirs
the compositions of the rising magmas were
modified by assimilation of the surrounding
crustal rocks and by the fractionation of earlycrystallized minerals. These processes led to the
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Hirt – Lassen Volcanic National Park
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Figure 7. Simplified geologic map of Lassen Volcanic National Park and vicinity. From Clynne et al. (2000).
to rhyodacites with about 73 percent (Clynne et
al., 2000).
Figure 5. Simplified geologic map of the Lassen region showing the outcrop patterns of volcanic rocks produced by five of the volcanic centers that have developed in this part of the High Cascades during the past
3.5 m.y. From Clynne et al. (2000). Note the park boundary shown in red.
development of the progressively more silicarich andesites and dacites that have dominated
the second and third eruptive phases. The present chamber is envisioned as a body that is vertically stratified from a dacitic cap, through an
andesitic dominant volume, to a basaltic base
(Fig. 5). Basaltic magmas cannot rise directly
through the chamber because of the lower densities of the magmas that overlie them. They do
occasionally intrude into the dacite, however,
and mix or mingle with it to form the distinctive
mafic inclusions characteristic of the lavas that
have been erupted from the center during the
past 100,000 years (Fig. 8).
PETROLOGY OF THE LASSEN PARK REGION
Geologists classify volcanic rocks primarily
according to the amounts of silica (SiO2) they
contain (Fig. 11) 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 Lassen Volcanic National Park span a wide range of silica contents, from basalts with <50 weight percent SiO2
GEOLOGIC HAZARDS AT LASSEN PARK
During its recent history the Lassen Volcanic
Center has typically produced dacitic and rhyolitic lavas. Because of their high silica contents
these lavas are very viscous and may retain
volatiles such as water, carbon dioxide, and
hydrogen sulfide until high vapor pressures are
reached. When the magmas approach Earth’s
surface these volatiles form rapidly expanding
bubbles that tear the lava apart in explosive
eruptions. Such eruptions are expected to
produce towering ash clouds that will spread
airfall tephra tens of kilometers downwind from
the volcano (Fig. 1a). These clouds may also
“collapse” to form ground-hugging pyroclastic
flows that will devastate lowland areas over
similar distances. Smaller pyroclastic flows are
also likely to be formed when the steep sides of
silicic domes or lava flows collapse (Hoblitt and
others, 1987).
If an eruption were to occur in the winter
or spring, hot tephra or pyroclastic material
could melt the thick snow pack that covers the
area and produce floods and volcanic debris
flows (lahars) that would devastate river valleys
well downstream from the volcano. Finally, the
presence of a relatively shallow dacitic magma
chamber and the formation of at least one
caldera during the past several hundred thousand years suggest that another caldera-forming
eruption is a distinct possibility (Christiansen,
1982).
REFERENCES CITED
Atwater, T., 1970, Implications of plate tectonics for the Cenozoic tectonic evolution of
western North America: Geological Society
of America Bulletin, v. 81, p. 3513-3536.
Bacon, C.R., 1983, Eruptive history of Mount
Mazama and Crater Lake Caldera, Cascade
Range, U.S.A.: Journal of Volcanology and
Geothermal Research, v. 18, p. 57-115.
Bacon, C.R., 1989, Mount Mazama and Crater
Lake caldera, Oregon, in Muffler, L.J.P.,
Bacon, C.R., Christiansen, R.L., Clynne, M.L.,
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Hirt – Lassen Volcanic National Park
Figure 8. Fragment of the dacite lava from Lassen
Peak’s 1915 eruption showing an mingled glob of
andesite magma (bottom).
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 Na-
Figure 9. Lassen Peak (viewed from the south
across Lake Helen) is actually a large dacite dome
that grew about 27,000 years ago on the northern
flank of the Brokeoff Volcano.
thenson, 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
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,
Hirt – Lassen Volcanic National Park
9
Figure 11. Classification of igneous rocks according to their silica contents. The minerals typically
found as coarser crystals (phenocrysts) in each
rock type are shown by the gray bars.
Figure 10. Hat Creek Basalt collected from a road
cut just north of the park. The abundant vesicles in
this sample suggest it was volatile rich at the time
of eruption.
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
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Hirt – Lassen Volcanic National Park
or magma at a shallow depth, and rises back
to the surface.
Lithospheric plate: Slab of Earth’s outer surface
that consists of the crust (continental or
oceanic) and the cool, rigid upper mantle
that underlies it. Plates are typically 100
to 150 km thick and move about relative to
one another on a warmer, softer layer of the
mantle beneath them.
Magma: Partially-molten rock; typically a
mixture of melt, mineral crystals, and gas
bubbles.
Peridotite: Coarse-grained igneous rock that
forms Earth’s mantle and consists mostly of
peridotite, augite, and hypersthene.
Pyroclastic flow: Hot, dense suspension of lava
fragments, volcanic gases, and entrained air
that may travel at speeds of up to 100 kph
down the slopes of a volcano.
Pleistocene: Interval of time between 1.8 Ma
and approximately 10 ka during which
landmasses at high elevations and latitudes
were subjected repeated glacial advances
and retreats (the “Ice Ages”).
Rhyodacite: Volcanic rock with a high silica
content (68 to 72 wt. %) that typically has a
fine, light gray to pink groundmass and contains coarser crystals of plagioclase, quartz,
and biotite.
Seiche: A wave formed in an enclosed or semienclosed body of water that has a period
which depends on the dimensions of the
basin holding the water.
Shield volcano: Volcano with low slopes that is
composed of hundreds of thin flows of low
viscosity basaltic or basaltic andesite lava
erupted from a central vent or fissure. The
shield volcanoes in the southern Cascades
typically have diameters of 5 to 15 km.
Stratovolcano: Volcanic cone, typically on the
order of 20 to 30 km in diameter, that is
composed of alternating layers of lava and
pyroclastic debris.
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
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miles to the Bumpass Hell trail parking lot. 3.3
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
We will plan to visit five stops today as well as
allow some time for a visit to the Park Service’s
interpretive displays at the visitors center near
the south entrance. Please wear hiking boots
and bring a lunch, water, a hat, and sunscreen.
Site descriptions in this log are mostly modified
from those of Clynne et al. (2000).
Mileage:
102.4 Junction of U.S. Highway 44 and road
into Lassen Volcanic National Park. 0.6
103.0 Boundary of Lassen Volcanic National
Park. Remember, collecting or disturbing rocks
or other natural features in the park is prohibited. As we drive eastward towards the Loomis
museum (103.6) we’ll pass Manzanita Lake
which was impounded by by the Chaos Jumbles
rockfall deposit only a few hundred years ago.
27.6
131.2 STOP 1: Visitors center. From our
vantage point at the park’s visitor center note
the stratified lavas and pyroclastics in Brokeoff
Mountain (Fig. 12) that preserves a remnant
of the flank of the Brokeoff Volcano. Proceed
north on the main road. At about 0.5 miles we
pass the Sulfur Works, a small thermal area with
fumaroles and boiling springs that is thought to
mark the approximate location of the vent of
the Brokeoff Volcano. Proceed 1.4 miles further
north on the road to a turnout on right-hand
side. 2.3
133.5 STOP 2: Diamond Peak overlook. Diamond Peak is a relatively unaltered sequence
of andesitic lava flows and pyroclastic rocks
that were deposited just east of the vent of the
Brokeoff Volcano. The panoramic view to the
south (Fig. 13) shows nearly the entire stratigraphy of the volcano as well as its contact with the
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Figure 12. Brokeoff Mountain, here viewed from
the north, is an erosional remnant of the silicic
andesites erupted during stage two of the growth
of the Brokeoff Volcano.
underlying rocks of the deeply eroded Maidu
Volcanic Center. Volcanic bedding can be correlated from one side of the cone to the other,
and shows no evidence of offset along a caldera-forming fault. The wall of Little Hot Springs
Valley to the east (Fig. 14) also exposes thinlybedded stage-one lavas (brown) and altered
pyroclastics (yellow), and is capped by a thicker
stage-two flow.
Proceed 3.1 miles north to the south end of Emerald Lake where the road crosses the contact
between the uppermost lavas of the Brokeoff
Volcano (dark andesite) and the lighter gray,
stage-three dacite of Ski Heil Peak. Continue 0.4
137.0 STOP 3: Bumpass Hell geothermal area.
The first part of the 4 km trail to Bumpass Hell
passes through the stage-three dacite of the
Bumpass Mountain dome. Note the well-developed glacial striations and polish on the surface
of the dome near the beginning of the trail (Fig.
15). At the viewpoint 0.85 km from the parking lot, the trail crosses a contact between the
Bumpass Mountain dacite and an underlying
stage-two andesite. Continue down the slope to
the east to the Bumpass Hell thermal area (Fig.
16). For your safety, be sure to stay on the established trails and boardwalks while exploring
the thermal area. Temperatures in the vapordominated part of the geothermal reservoir
that underlies Bumpass Hell are 235˚C, and the
water discharged by the springs and fumaroles
is rich in sulfates and quite acidic (Clynne and
Muffler, 1989). Intense alteration of the host
rocks is apparent in the development of the
clay minerals seen in the boiling mudpots and
the white outcrops just south of the boardwalk
entrance. As the thermal waters reach the surface and cool, they deposit pyrite (iron sulfide)
as a black “mud” in the streams that drain from
several of the thermal pools.
Figure 13. Panorama looking south from the Diamond Peak overlook. From Clynne et al. (2000).
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Figure 14. Altered stage I lavas from the Brokeoff
Volcano exposed in the northern wall of Little Hot
Springs Creek Canyon.
Return to the parking lot and continue north on
the road past Helen Lake, Lassen Peak and the
Reading Peak domes. 7.7
144.7 STOP 4: Overview to the East. From the
crest of the road you can look to the east across
the Central Plateau, a region covered by young
hybrid andesite lavas that have erupted during the past 0.30Ma, to the Prospect Peak and
Mount Harkness basaltic shield volcanoes in the
distance. Turn into the Devastated Area parking
lot. 5.3
150.0 STOP 5: Devastated Area. The “Devastated Area” is a swath of land that was swept
by repeated debris avalanches, mudflows, and
pyroclastic flows during Lassen Peak’s 1915
eruptions (Fig. 1b). Beginning on the peak’s
northeastern slope, this area extends across
parking and at least a kilometer into the forest
beyond. In mid-May of 1915 a small dome of
glassy dacite rose into a summit crater that had
been opened by earlier steam explosions. This
dome was blown apart by an explosion on the
night of May 19-20th, and its hot fragments
melted snow on the peak and produced a large
debris flow that travelled 15 km down the canyon of Lost Creek. Some of the dome fragments
cooled and fractured after they had come to
rest, creating the distinctive “prismatically-jointed blocks” seen near the parking lot. (Notice the
abundant inclusions of andesitic lava in these
dacite blocks.) A small tongue of dacite lava
welled up out of the vent after the destruction
Hirt – Lassen Volcanic National Park
Figure 15. Glacially polished and striated outcrop
of the Bumpass Hell dacite near the start of the
Bumpass Hell trail.
of the dome and flowed several hundred meters
down the western and northeastern sides of the
summit. On May 22nd, a great eruption cloud
rose from the summit and collapsed, sending
a pyroclastic flow and a second debris flow
down Lost Creek. Airfall tephra from this cloud
was reported to have fallen as far east as Elko,
Nevada. Harris (1988) gives a detailed account
of Lassen Peak’s entire 20th century eruptive
episode.
As you return to the park road and continue
north, note Raker Peak on the right. It is an
early stage-three rhyolite dome that is similar in
composition to the Rockland Tephra. Its steep
southeastern face may mark part of the margin
of the 0.61 Ma caldera, but has been eroded
by subsequent glaciation (Clynne and Muffler,
1989). Continue north and west on the road
into the Chaos Jumbles and pull into the turnout
on the right. 8.0
158.0 STOP 5: Chaos Crags and Chaos Jumbles. The Chaos Crags are a suite of five dacite
domes that were emplaced over a period of
about 100 years, beginning 1,100 years ago
(Clynne and Muffler, 1989). The growth of these
domes was preceded by explosive eruptions
that produced an air-fall tephra and several
pyroclastic flows. During the emplacement process, the outer parts of several of the hot domes
collapsed and produced additional pyroclastic
flows. 300 to 400 years ago the cold outer part
of dome number 2 collapsed (Fig. 17) in a series
13
Figure 17. Chaos Jumbles rock avalanche deposit
in the foreground and the partially-collapsed face
of Chaos Crags dome 2 in the background.
of three rockfall avalanches that travelled up to
4.5 km and formed the Chaos Jumbles on which
we are now standing (Fig. 18). Note that most of
the dacite blocks in the Jumbles are extensively
oxidized (reddened). This oxidation probably occurred while the hot lava was exposed to the air
on the surface of the young dome.
Figure 16. Boiling pools and fumaroles at the
Bumpass Hell geothermal area. Acidic vapors have
bleached the dacite in the background white, and
the deposition of tiny pyrite crystals colors vents in
the foreground black.
Continue west on the highway to the park headquarters at Manzanita Lake, where we’ll stop
briefly at the Loomis Museum (158.9). Our field
trip will end at the museum. Log ends. 
Last updated 16-Aug-2011.
Figure 18. Geologic map of
the Chaos Jumbles showing its
source in the Chaos Crage, the
extent of the deposit and how
it has dammed Manzanita Lake.
Map by H. Williams.