America's Volcanic Past -
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| "Though few people in the United States may actually experience an erupting volcano, the evidence for earlier volcanism is preserved in many rocks of North America. Features seen in volcanic rocks only hours old are also present in ancient volcanic rocks, both at the surface and buried beneath younger deposits." -- Excerpt from: Brantley, 1994 |
Location Map - Southern California National Parks and Monuments
[46K,InlineGIF]
Volcanic Highlights and Features:
[This list is just a sample of
various California volcanic features or events and is by no means inclusive.]
California's remarkable geology is the result of volcanic and tectonic activity. Its majestic mountains were shaped by glaciers during the ice ages as well as by wind and rain. The scenic coastline of California is continually shaped by the pounding waves of the Pacific Ocean. California has a wealth of mineral resources, including the rich soil of the Central Valley, the gold of the Sierra, and oil off the coast and in various locations across the state.
California can be divided up into 11
Geomorphic Provinces,
many which include volcanic features.
The Provinces are:
The Sierra Nevadas, Cascade Range, Coast Ranges,
Transverse Ranges, Peninsular Ranges, Klamath
Mountains, Great Valley, Basin and Range, Modoc
Plateau, Mojave Desert, and Colorado Desert.
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California's Geomorphic Provinces:17
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| California's Volcanic Rocks |
Tertiary and Quaternary Volcanic Rocks:17
Lava flows erupted from volcanoes. These rocks make up
much of the Cascade Range and the Modoc Plateau, and are
widespread in eastern California. They also occur in
coastal regions.
Mesozoic Granitic Rocks:17
A wide variety of coarse-grained igneous rocks formed when
magma that intruded the earth's crust cooled and was later
exposed by erosion. Granitic rocks occur throughout the
state, but are most common in the mountainous areas such as
the Klamath Mountains, the Sierra Nevada, and the
Peninsular Ranges. Some granitic rocks are Cenozoic,
Paleozoic, and Precambrian.
| California's State Rock |
Serpentine - California's State Rock:17
Serpentine rock is apple-green to black and is often mottled
with light and dark colored areas. Its surfaces often have a shiny or
wax-like appearance and a slightly soapy feel.
Serpentine is usually fine-grained and compact
but may be granular, platy, or fibrous
in appearance. The term "serpentine"
is commonly used by the general public to
refer to the rock type that geologists call
"serpentinite."
Serpentine occurs in central and northern California --
in the Coast Ranges, the Klamath Mountains, and in the Sierra
Nevada foothills.
Serpentine is considered by geoscientists to be the metamorphosed
remains of magnesium-rich igneous rocks,
most commonly the
rock peridotite, from the earth's mantle.
| Amboy Crater |
Amboy Volcanic Field:25
The Amboy lava field covers approximately 70 square miles and consists primarily of vesicular
pahoehoe. The field is in an alluvial-filled valley between the Bullion Mountains to the
southwest and the Bristol Mountains to the northeast. Within the valley, it lies between Bagdad
Dry Lake to the west and Bristol Dry Lake to the east; both are playa lakes typical of the
Mojave Desert. Most of the Amboy lava field is composed of undifferentiated flow units of
relatively dense, "degassed" pahoehoe lavas that form a hummocky terrain. The surface relief on
this unit ranges from 2 to 5 meters. The flow is characterized by abundant tumuli and pressure
ridges and, as is typical for this type of flow, a fractured surface. Lava tubes are not
present in any of the flows, nor are blisters or shelly-type pahoehoe; only a few lava channels
are present.
Amboy Crater:25
Amboy Crater is a prominent, undissected cinder cone in the northeaster quadrant of the lava
field. The volcano erupted along the northern border of Bristol Dry Lake and poured lava onto
its surface, dividing it into the two present playas. The cone rises 75 meters above the
surrounding lava flows and is approximately 460 meters in basal diameter. It is composed of a
loose accumulation of volcanic ejecta with secondary amounts of agglutinated ejecta and flows.
Amboy Crater is not a single cone but is composed of at least four nearly coaxial nested cones.
The outer slopes of the main cone are gullied by erosion. Within the main outer cone, there is
a remnant of a second cone on the west side; both cones are breached on the west. In addition
to the two main cones, there are two relatively undisturbed cone walls within the main crater.
These innermost cones are composed almost entirely of angular scoriaceous cinders.
Amboy Crater - National Natural Landmark:29
San Bernardino County - Excellent example of a recent
volcanic cinder cone with an unusually flat crater floor. (May
1973) Owner: Federal, Private.
DESIGNATION DATE:
May 1973
| Anderson Reservoir Dam and Anderson County Park |
Anderson Reservoir Dam and Anderson County Park:27
Pass the bridge over Coyote Creek (on Highway 101). Anderson Reservoir Dam can be seen blocking a former gorge through the
Yerba Buena Ridge (several miles to the left). This long ridge preserves evidence of the complex geologic
history related to the ongoing development of the Calaveras-Hayward fault system and the uplift of the Diablo
Range. The hilltops above the dam preserve evidence of a late Tertiary volcanic lava flow that formed early in
the development of the San Andreas fault system and the opening of the Santa Clara Valley. These volcanic
rocks unconformably overlie highly-deformed Franciscan assemblage rocks, mostly ancient basalt, greenstone
[altered basaltic volcanic rocks], serpentinite, chert, shale, and graywacke sandstone. (Graywacke is a fancy
name for a “dark, poorly sorted, typically fine-grained, dirty rock.”) Some of the rocks are well-exposed near
the reservoir spillway and the boat-ramp areas in Anderson County Park.
| Black Butte |
Black Butte:14
Holocene eruptions occurred at Black Butte, a group of
overlapping dacite domes
about 13 kilometers (8 miles) west of Mount Shasta.
| Black Mountain |
Black Mountain:23
The Black Mountain
Wilderness is a volcanic flow and mesa
with a deposit of fine grained dune sand in the
southeast corner. Elevations range from 2,080 to
3,941 feet at the summit of Black Mountain. Golden eagles and prairie
falcons have been seen foraging in this area, which is also known for its
occasional display of spring flowers.
Location: San Bernardino County; 13 miles
northwest of Barstow, California.
| Bristol Mountains |
Bristol Mountains:23
A portion of the
rolling Bristol Mountains and
a tilted volcanic plain
form the Bristol Mountains Wilderness. The lack of any
springs and extreme distances make travel in
this wilderness a challenge for the most
experienced desert hiker. The wilderness is contiguous with the Granite
Mountains Wilderness, which is part of the Mojave National Preserve
on its eastern boundary at Budweiser Wash.
Location: San Bernardino County; 40 miles
southeast of Baker, California.
| Cascade Range Volcanoes |
California's Cascade Range:17
The Cascade Range, a chain of volcanic cones,
extends through Washington and Oregon into California. It
is dominated by Mount Shasta,
a glacier-mantled volcanic cone, rising 14,162 feet above sea level. The
southern termination is Lassen Peak
which last erupted in the early 1920s. The Cascade Range is transected
by deep canyons of the Pit River.
The river flows through the range between these two major volcanic
cones, after winding across the
interior Modoc Plateau on its way to the Sacramento River.
(See Black Butte, Glass Mountain, Lassen Peak, Lava Beds, Medicine Lake, and
Mount Shasta below)
|
| Channel Islands National Park |
Channel Islands National Park - Overview:20
Comprised of five in a chain of eight southern
California islands near Los Angeles, Channel
Islands National Park is home to a wide variety of
nationally and internationally significant natural and
cultural resources. Over 2,000 species of plants
and animals can be found within the park.
The park consists of 249,354 acres, half of
which are under the ocean, and include the islands
of San Miguel, Santa Rosa, Santa Cruz, Anacapa,
and Santa Barbara. Even though the islands seem
tantalizingly close to the densely populated,
southern California coast, their isolation has left
them relatively undeveloped, making them an
exciting place for visitors to explore.
Channel Islands:24
The Channel Islands were created
as a result of geologic activity millions of years ago.
The four northern Channel Islands are actually the
western terminus of the Santa Monica Mountains,
separated from the mainland by the waters of the
Santa Barbara Channel.
Twenty thousand years ago, when worldwide sea
level was 300 feet lower than at present, the four islands formed one large
island. The four southern Channel Islands are
thought to have once been connected to the mountains of the Peninsular
Ranges, but faulting in the San Pedro Channel
30 million years ago cut them off from the mainland.
Anacapa Island:20
Anacapa Island is located 14 miles off the
coast from Ventura. It is the only Channel Island to retain its
American Indian name, derived from the
Chumash word, "Eneepah", meaning island of deception or
mirage. Ocean waves have eroded the
perimeter of the island,
creating steep sea cliffs towering hundreds
of feet in height and exposing
the
volcanic origins of air pockets,
lava tubes, and sea caves.
At the east end
of Anacapa a natural bridge has formed
in the ocean. Forty-foot high
Arch Rock is a trademark of Anacapa
and Channel Islands National Park.
Santa Barbara Island:20
Santa Barbara Island (639 acres), 38 miles west of San Pedro,
is the smallest of the California Channel
Islands.
Formed by underwater volcanic activity,
Santa Barbara is roughly triangular in outline and emerges
from the ocean as a giant twin-peaked mesa with steep cliffs.
Even though small in size, Santa Barbara
Island boasts diversity in its habitats,
with a few narrow rocky beaches,
six canyons, and badlands area. It
is much like Anacapa Island
in its being a haven for sea birds.
The steep cliffs and isolation from mainland
predators provide safe breeding sites
for thousands of sea birds.
Santa Cruz Island:20
Santa Cruz is the largest island off the
coast of California.
Located between Anacapa and Santa Rosa
Islands, it lies from 19-25 miles off
the adjacent mainland coast between
Ventura and Santa Barbara.
The scenic beauty of Santa Cruz is
reflected in its many landforms -- two rugged mountain ranges, the
highest peaks on the Channel Islands,
deep canyons, a central valley,
year-round springs and streams, plus
77 miles of craggy coastline cliffs,
giant sea caves, pristine tidepools and
expansive beaches. Lying
directly on the boundary between cold
northern and warm southern waters, this island hosts unique plant,
animal, and marine communities representing
nearly 1,000 miles of coastline.
In its vastness and variety of flora,
fauna and geology, Santa Cruz Island resembles a miniature California.
Geologists believe that the island
never has been attached to the mainland.
All three major rock
types -- igneous, sedimentary and
metamorphic --
are found throughout this rugged, mountainous island.
Nestled between two mountain ranges
that rise above 2,000 feet,
is a pastoral Central Valley that was, and
still is, being created by a major earthquake fault.
| Chemehuevi Mountains |
Chemehuevi Mountains:23
The Chemehuevi
Mountains are a horse-shoe shaped range a few
miles from the Colorado River.
These mountains
transform gradually from almost white granite to
dark red and gray volcanic spires.
Within the
arms of the horseshoe is a large central valley,
its rolling hills covered
by dense stands of cholla and other cactus.
Desert tortoise, bighorn
sheep, wild burros and desert mule
deer are among the wildlife making
their homes here. A popular trail is the
12-mile Trampas Wash, which
bisects the wilderness and ends at the Colorado River.
Springs within
the wilderness are seasonal and should be depended upon for water.
Location: San Bernardino County; 12 miles
southeast of Needles, California.
| Chesbro Reservoir |
Chesbro Reservoir:27
Chesbro Reservoir boat dock - examine serpentinite exposed around
the reservoir
Boulders of
many of the different Franciscan assemblage rocks are conveniently on display around the boat dock parking area.
Boulders around the margin of the boat-dock parking area include examples of graywacke sandstone, metasandstone,
greenstone (metabasalt), serpentinite, red chert, and chert breccia. These rocks occur in scattered pod-like masses (a
classic mélange) exposed along the shoreline of Chesbro Reservoir.
A short walk to the dam (0.2 miles) provides opportunity to view exceptional exposures of serpentinite in road cuts
and in excavations along dam path. Don’t stand along the road because of the hazard of speeding cars.
Instead, the safer place to examine the serpentinite is along the dam path. Some of the serpentinite displays slickensides,
a linear fracture pattern caused by fault shearing motion between rock surfaces.
From the opposite end of the dam you can see the back of El Toro, the peak overlooking Morgan Hill. Downstream
from the reservoir, Llagas Creek flows through Paradise Valley and enters the Santa Clara Valley a couple of miles south
of Morgan Hill. On the north side of the spillway is a landslide formed in the weathered serpentinite. Note the bluish-green
to black color of the residual soil; the soil is rich in the clay mineral, montmorillonite.
| Cima Dome and Volcanic Field |
Cima Dome:11
The northern half of Mojave National Preserve is dominated by a
broad sloping desert upland called
Cima Dome. The dome is the
exposed remains of a massive body of
granite that formed deep underground long ago and was slowly
forced to the surface.
Cima Volcanic Field:7
Volcanic eruptions in the
Cima field first began about 7.6 million years ago
and continued until at least as recently as
10,000 years ago
(based on the K-Ar dating method),
near the end of the most recent ice age.
The field is characterized by basalt,
which is a black to dark gray
volcanic rock formed from lava rich in
magnesium and iron. Each of the 40 cinder
cones in the volcanic field represents
one or more sites from which lava erupted.
| Cinder Cone National Area |
Cinder Cone National Area - National Natural Landmark:29
San Bernardino County - 24 miles east of Baker. A complex of
over 20 large cinder cones of recent origin with extensive and
continuous lava flows. Owner: Federal, State.
DESIGNATION DATE:
May 1973
| Coast Ranges |
Coast Ranges:17
The Coast Ranges are mountain ranges
(2,000-4,000, occasionally 6,000 feet elevation above sea level)
and valleys.
The ranges and valleys trend northwest,
subparallel to the San Andreas Fault.
The province
terminates on the east where
strata dip beneath alluvium of the Great Valley;
on the west by the Pacific Ocean with mountains rising
sharply from uplifted and terraced,
wave-cut coast; on the north by South
Fork Mountain, which has the
characteristic trend of the Coast Ranges,
and on the south by the Transverse
Ranges. The Coast Ranges are
composed of thick late Mesozoic and Cenozoic sedimentary strata.
The northern and southern ranges
are separated by a depression containing the San Francisco Bay.
Offshore, the continental shelf is
transected by submarine canyons.
The Monterey submarine canyon, 10,000 feet
deep, is apparently a submerged river canyon.
The northern Coast Ranges are dominated by irregular,
knobby, landslide topography
of the Franciscan Formation. The eastern border is characterized by
strike-ridges and valleys
in Upper Mesozoic strata.
In several areas, Franciscan rocks are overlain by
volcanic cones and flows of the
Quien Sabe, Sonoma, and Clear Lake volcanic fields.
The Coast Ranges are subparallel to the rift
valley of the active San Andreas Fault.
The San Andreas is more than 600 miles
long, extending from
Point Arena to the Gulf of California.
The Salinian block to the west of the San Andreas
has a granitic core,
extending from the southern extremity of the
Coast Ranges to north of the Farallon Islands.
| Coastal Bluffs and Headlands |
Coastal Bluffs and Headlands:24
Coastal bluffs are composed mainly of sedimentary rocks
such as sandstones and shales that are particularly prone to
erosion. Grains of quartz, feldspar, and mica compressed
into layers of sandstone crumble easily; when wet, shales and
siltstones disintegrate, and clays and mudstones,
soften and liquefy. Lying on top of the sedimentary deposits of many bluffs is
alluvial soil, loosely consolidated sand and
gravel deposited by ancient rivers and streams. Examples of sedimentary coastal
bluffs are the sandstone bluffs of Santa Cruz,
the alluvial cliffs at La Jolla, and the shale cliffs of Point Loma in San Diego
County.
Rocky headlands are composed of igneous rocks -- granites and basalts --
that are resistant to wave erosion.
Granitic
formations include the Point Reyes Headlands in Marin County.
Morro Rock in San Luis Obispo County, Point Dume in Los
Angeles County, and Point Sur in Monterey County are outcroppings of basaltic lava.
| Coso Mountains |
Coso Mountains:23
This Coso Range Wilderness
encompasses the northern section of the Coso
Mountain Range, an area of extensive erosion
revealing outstanding volcanic displays
and numerous valleys and washes. From high points within the wilderness,
most notably Joshua Flat, one can obtain outstanding views of the
Owens Valley and the eastern Sierra Nevadas. Creosote, low desert
shrubs, annuals, cactus and large stands of Joshua trees are the
primary vegetation in the area. Vermillion Canyon and Joshua Flat are
two especially scenic areas within this wilderness. Cactus Flat and
McCloud Flat are two areas of historic mining activity.
Location: Inyo County; 12 miles west of
Darwin, California and 6 miles northeast of
Olancha, California.
| Coso Volcanic Field |
Coso Volcanic Field:17
The Coso volcanic field
is located at the west edge of the Basin and Range province.
Initiation of volcanism at Coso preceded
the onset of Basin and Range crustal extension there,
as expressed by normal faulting.
The earlier of the two principal periods of volcanism began
with the emplacement of basalt flows
over a surface of little relief. Then, during the ensuing
period of about 1.5 million years,
eruptive activity included chemically more evolved rocks
erupted upon a faulted terrain of
substantial relief. Following a 1.5-million-year hiatus with
few eruptions, a bimodal field of
basalt lava flows and rhyolite lava domes and flows developed
on Basin and Range terrain of
essentially the same form as today's landscape. Many of the
young basalt flows are intracanyon,
occupying parts of the presentday drainage system.
The Coso volcanic field is best known
for its Pleistocene rhyolite. Thirty-eight rhyolite
domes and flows form an elongate
array atop a north-tending 8 by 20-kilometer horst of Mesozoic
bedrock. Nearly uneroded constructional
forms are exhibited by most domes. Many are nested
within tuff-ring craters, and a
few filled and overflowed their craters to feed flows a kilometer
or two long.
| Darwin Plateau |
Darwin Plateau:23
The Darwin Plateau and Darwin Hills
form the landscape of the Darwin Falls Wilderness. The plateau, which
is cut by numerous shallow depressions and canyons, displays a variety
of volcanic rock faces and exposures.
Location: Inyo County; 3 miles north of
Darwin, California and 25 miles northeast of
Olancha, California.
|
Death Valley -
Death Valley National Park |
Death Valley:12
The oldest rocks exposed in Death Valley
are about 1.8 billion years (Precambrian),
almost half the age of the Earth.
These venerable rocks are the remnants of an
ancient volcanic mountain belt with its
flanking deposits of mud and sand.
|
Devils Postpile -
Devils Postpile National Monument |
Devils Postpile:12
The Devils Postpile is a fine example of columnar basalt.
Until recently, it was thought to have been
formed about one million years ago.
Current studies suggest that the Devils Postpile was formed less
than 100,000 years ago when a cooling
lava flow cracked into multisided columns.
| Eagle Lake |
Eagle Lake:26
The Eagle Lake volcanic field is located at the junction of three geological provinces: Sierra
Nevada, Cascades, and Basin and Range. The field is obviously defined by late Pleistocene flows
of olivine tholeiitic basalt covering 125 square kilometers within the Eagle Lake
volcano-tectonic depression. Fifteen cinder cone-and-flow vents less than 100 meters high, fed
the flows during two eruptive periods, each very brief, probably between 50 and 100 thousand
years ago.
Several low composite volcanoes and small shields of Pliocene age occur just outside
the late Pleistocene field.
| El Paso Mountains |
El Paso Mountains:23
Numerous reddish-colored buttes and dark, uplifted
volcanic mesas dissected by narrow canyons
distinguish this wilderness.
Badlands topography
surrounds Black Mountain, the central feature of
this wilderness. The most spectacular attribute of this area is the
abundance of cultural sites. The southern portion of the wilderness is
included in the Last Chance Archaeological District and is listed on the
National Register of Historic Places. Wildlife includes raptors, Mojave
ground squirrel and the desert tortoise. Vegetation primarily consists of
creosote bush scrub community with Joshua trees on the western side
of the mountain.
Location: Kern County; 6 miles southwest of
Ridgecrest, California.
| Farallon Islands |
Farallon Islands:24
More than half the California population of resident and migratory
seabirds nests on the Farallon Islands, a group of seven
sparsely vegetated granite outcroppings located offshore of
the Golden Gate channel.
The Channel Islands and the Farallon Islands were created
as a result of geologic activity millions of years ago. The Farallons
are composed of 89-million-year-old granite that emerged
as molten rock from below the sea floor; uplifting of the Pacific
Plate during the mid-Pleistocene completed the island-making process.
The four northern Channel Islands are actually the
western terminus of the Santa Monica Mountains,
separated from the mainland by the waters of the
Santa Barbara Channel.
Twenty thousand years ago, when worldwide sea
level was 300 feet lower than at present, the four islands formed one large
island. The four southern Channel Islands are
thought to have once been connected to the mountains of the Peninsular
Ranges, but faulting in the San Pedro Channel
30 million years ago cut them off from the mainland.
| Glass Mountain |
Glass Mountain Obsidian Flow:2
Medicine Lake volcano is a shield volcano of
basaltic through rhyolitic composition
that lies east of the main axis of the Cascade Range,
approximately 50 kilometers east-northeast of Mount Shasta.
The volcano is thought to be younger than approximately 1,000 years, and
consists of calc-alkaline and tholeiitic lavas.
The most recent
eruption occurred around 1,000 years ago when rhyolite and dacite
erupted at Glass Mountain and associated vents
near the caldera's eastern rim.
(See Medicine Lake Volcano below)
| Granite Mountains |
Granite Mountains:7
Some of the more striking rock
formations in the Mojave National Preserve
lie in the Granite Mountains.
These granitic rocks
have eroded into unusual rounded shapes that include spires,
perched boulders, and curved cliff faces.
Granitic rocks represent the roots of
ancient continental-margin volcanic systems.
Most of the granitic rock in the Mojave Desert
is late Mesozoic in age (80 to
180 million years old).
The Mojave National Preserve lies within a belt of
late Mesozoic granites that parallels the
western continental margin from Mexico to Canada.
The granites formed at depth
within a volcanically active mountain range
comparable in geologic setting to the
Andes Mountains chain in South America.
The granitoids formed by the slow cooling and
solidification of molten magma bodies that
developed above sinking slabs of
oceanic crust overridden by the edge of the
continent. At least 55 or 60 million years
elapsed between the crystallization of the
last Mesozoic magma bodies and
deposition of the youngest-preserved overlying strata.
The Mojave National Preserve probably
formed a highlands during much of
this period and erosion gradually stripped off
Paleozoic and Mesozoic sedimentary rocks
overlying the granites.
| Great Valley |
Great Valley Geomorphic Province:17
The Great Valley is an alluvial plain,
about 50 miles wide and 400 miles long,
between the Coast Ranges and
Sierra Nevada.
The Great Valley is drained by the
Sacramento and San Joaquin rivers, which join and enter
San Francisco Bay.
The eastern border is the west-sloping
Sierran bedrock surface, which continues
westward beneath alluvium and older sediments.
The western border is underlain by east-dipping Cretaceous
and Cenozoic strata that form a
deeply buried synclinal trough,
lying beneath the Great Valley along its
western side.
The southern part of the Great Valley
is the San Joaquin Valley. Its great oil fields follow
anticlinal uplifts that
mark the southwestern border of
San Joaquin Valley and its southern basin.
To the north,
the Sacramento Valley plain
is interrupted by the Marysville Buttes,
and isolated Pliocene volcanic plug about
2,000 feet high.
| Joshua Tree National Monument |
Granite Monoliths:9
The monument encompasses some of the most interesting geologic
displays found in California's deserts.
Rugged mountains of
twisted rock and exposed granite monoliths testify to the
tremendous earth forces that shaped and formed this land.
Arroyos, playas, alluvial fans, bajadas,
pediments, desert varnish,
granites, aplite, and gneiss
interact to form a giant desert mosaic of
immense beauty and complexity.
| Klamath Mountains |
Klamath Mountains:17
The Klamath Mountains have rugged
topography with prominent peaks and ridges reaching
6,000-8,000 feet above sea level. In the western Klamath,
an irregular drainage is incised into an uplifted
plateau called the Klamath peneplain.
The Klamath River follows a circuitous course through the
mountains. The uplift has left
successive benches with gold-bearing gravels
on the sides of the canyons.
The province is considered to be a
northern extension of the Sierra Nevada.
Rocks include
pre-Cretaceous metamorphic,
abundant serpentine, and granitic.
Volcanic rocks of the Cascade Range lie to the east,
Cretaceous sediments lie to the southeast,
and Franciscan and younger Coast Range
formations lie to the west.
|
Lassen Peak -
Lassen Volcanic National Park |
Lassen Peak:12
Lassen Peak is the largest of a group of
more than 30 lava domes in the Lassen domefield.
Such domes are near-vent accumulations of highly viscous lava.
About 27,000 years ago, Lassen Peak formed over a short time,
probably no more than a few years. Its 2,000-foot height and
nearly 2-cubic kilometer volume make it one of the largest lava domes in the world.
When it formed, Lassen Peak looked much like the Chaos Crags domes do today --
covered with angular rock talus and with oversteepened flanks.
During the last ice age, 18,000 to 25,000 years ago, its shape was
significantly modified by glacial erosion. A tiny reminder of one of these
glaciers survives on Lassen's eastern flank. The depression on the northeastern
flank of Lassen Peak is a glacial cirque, where ice accumulated to form a glacier
10 kilometers long.
Lassen Volcanic National Park:10
The Lassen area was made a National Park
in 1916 because of its significance as an active volcanic
landscape. The park is a compact laboratory of
volcanic phenomena and associated thermal features
(except true geysers).
It is part of a vast geographic unit -
a great lava plateau with isolated volcanic
peaks -
that also encompasses Lava Beds National Monument,
California, and Crater Lake National
Park, Oregon.
|
Lava Beds -
Lava Beds National Monument |
Lava Beds:3
Lava Beds National Monument is located on the northern
flank of Medicine Lake volcano and
encompasses mostly
basaltic and some andesitic lavas.
| Lava Mountains |
Lava Mountains:23
The Golden Valley Wilderness is surrounded
on either side by two distinct mountain ranges.
The Lava Mountains stretch across the
northwestern portion of the area, crowned by Dome Mountain at nearly
5,000 feet. This range is cut by several steep walled canyons that
reveal bands of multi-colored sedimentary rocks. The Almond
Mountains, rising to an elevation of
4,500 feet, enclose the valley on the
southeast.
Location: San Bernardino County; 10 miles
southeast of Ridgecrest, California
| Lexington Reservoir |
Lexington Reservoir Dam:28
The geology around the dam site is quite complex.
Near the boat dock parking area are massive
exposures of graywacke sandstone,
whereas a short distance down stream along Los Gatos Creek
gorge the bedrock consists of deeply weathered
serpentinite that is prone to landsliding.
Landsliding can be seen on both sides of the
valley where serpentinite crops out. On the north
side of Highway 17 an
active landslide in the serpentinite continuously dumps material onto the
highway.
| Loma Prieta |
Loma Prieta:27
Loma Prieta (Spanish for “black mountain”) stands above the southern end of the Sierra Azul (Spanish for “blue
mountain”). The Sierra Azul is actually a steep-sided ridge that runs along the crest of the Santa Cruz Mountains
(including Loma Prieta and Mt. Umunhum, and two lower peaks to the north, Mt. Thayer and El Sombroso). The high
ridge separates the Santa Clara Valley around San José from the San Andreas Rift valley to the southwest. Loma Prieta
(3,806 feet) is the highest point in the Santa Cruz Mountains. Its summit consists of a serpentinite diapiric
plug (Coast Range Ophiolite) surrounded on the east by middle Tertiary Temblor Sandstone on the east, and by Late
Cretaceous sedimentary rocks (shale, mudstone, graywacke sandstone, and conglomerate - Great Valley Sequence
equivalent rocks) on the west. The peak is host to a privately-owned radio tower operation.
| Long Valley, Mammoth, Mono, Inyo |
Long Valley and Vicinity:4
About 760,000 years ago
a cataclysmic volcanic eruption in the Long Valley,
California area blew out 150 cubic miles of
magma (molten rock) from
a depth of about 4 miles beneath the Earth's surface.
Rapidly moving flows of glowing hot ash
covered much of east-central California, and
airborne ash fell as far east as Nebraska.
The Earth's surface sank more than 1 mile into the
space once occupied by the erupted magma, forming a large
volcanic depression that geologists call a caldera.
The massive Long Valley eruption was followed by hundreds of
smaller eruptions over the next few
hundred thousand years. These eruptions of lava flows, domes, and pyroclastic flows
were concentrated in the central and western parts of the caldera.
Volcanic activity then moved northward to the Mono Lake area about
35,000 years ago to build the Mono Craters. The most recent eruptions in the area
occurred from the Mono and Inyo Craters about 600 years ago, and from Negit Island
in Mono Lake about 250 years ago.
| Los Angeles Vicinity |
Peninsular Ranges:17
The Peninsular Ranges are a series of ranges
separated by longitudinal valleys,
trending NW-SE, subparallel to faults branching
from the San Andreas Fault.
The trend of topography is similar to the
Coast Ranges, but
the geology is
more like the Sierra Nevada, with granitic rock intruding the
older metamorphic rocks.
The Peninsular
Ranges extend into lower California and are bound on the
east by the Colorado Desert. The Los Angeles
Basin, and island group
(Santa Catalina, Santa Barbara, and the
distinctly terraced San Clemente and San
Nicolas islands),
together with the surrounding continental
shelf (cut by deep submarine fault troughs) are
included in this province.
| Malpais Mesa |
Malpais Mesa:23
The landscape of
the Malpais Mesa Wilderness takes on a variety of forms.
Situated at the southern end of the Inyo
Mountains, rugged valleys, deep canyons, sheer
mountain sides and mesas can be found within a short distance of each
other. To the east, gently sloping bajadas rise to meet
the rugged volcanic lava flow of Malpais Mesa.
Vegetation also takes on many
forms: creosote, low desert shrubs and grasses on lower elevations;
Joshua trees on the eastern slopes at mid elevations; and pinyon pines
and juniper on the higher elevations. Wildlife within the wilderness
includes mule deer and foraging and nesting habitat for golden eagles.
Location: Inyo County; 15 miles northeast of
Olancha, California and 10 miles northwest of
Darwin, California.
| Marysville Buttes |
Marysville Buttes:17
The Sacramento Valley plain
is interrupted by the
Marysville Buttes,
and isolated Pliocene volcanic plug about
2,000 feet high.
| Medicine Lake |
Medicine Lake Shield Volcano:3
Medicine Lake volcano is a Pleistocene and Holocene
shield volcano located in northeastern California
about 50 kilometers east of Mount
Shasta, near the western margin of the Basin and Range tectonic province.
Lava Beds National Monument is located on the northern
flank of Medicine Lake volcano and encompasses mostly
basaltic and some andesitic lavas.
Higher on the volcano, basaltic lava is mostly absent, andesite dominates, and
rhyolite and small volumes of dacite
are present, the latter mainly near the 7 x 12 kilometer Medicine Lake
caldera.
| Modoc Plateau |
Modoc Plateau:17
The Modoc Plateau is
volcanic table land (elevation 4,000-6,000 feet above sea level)
consisting of a thick
accumulation of lava flows and tuff beds with many small volcanic cones.
Occasional lakes, marshes, and
sluggishly flowing streams meander across the plateau.
The plateau is cut by many north-south faults. The
province is bound indefinitely by the Cascade Range on the
west and the Basin and Range on the east and
south.
| Mojave National Preserve |
Mojave National Preserve:12
The volcanic eruptions that rocked what is now Mojave National
Preserve coincided with a period (about 20-5 million years ago) of intense
plate tectonic activity.
During this episode, the Earth's crust was literally
ripped apart, radically altering the landscape of the Southwest.
As the crust stretched, fractures formed along
weak zones and mountain range-sized blocks jolted
and slid against each other along faults. Some of
these enormous blocks of crust rose up, forming
rows of high, elongate mountains. Other blocks slid
down, forming the low valleys. Together, the linear
mountain ranges and intervening valleys define the
Basin and Range Province.
The thinned, faulted crust made it easier for
magma to rise up and follow weaknesses in the
rock. Where magma reached the surface volcanoes
grew and great globs of magma solidified to form
plutons beneath them. In some places within the Preserve these geologically
young plutons can be seen right at the surface. These plutons 'froze' at
depth, then were almost immediately lifted up to the surface as mountains
rose along new faults. This is truly a dynamic place!
| Monterey Peninsula |
Monterey Peninsula:22
Granitic rocks
that make up the Monterey Peninsula
are also seen as
acoustically-distinctive outcrops
on the continental shelf
around the Peninsula out to depths
of from 80 to 100 meters.
| Morro Rock |
Morro Rock:24
Rocky headlands are composed of igneous rocks -- granites and basalts --
that are resistant to wave erosion. Granitic
formations include the Point Reyes Headlands in Marin County.
Morro Rock in San Luis Obispo County, Point Dume in Los
Angeles County, and Point Sur in Monterey County are
outcroppings of basaltic lava.
| Mouny Diablo State Park |
Mount Diablo State Park - National Natural Landmark:29
Contra Costa County - 31 miles east of Berkeley. Contains the
best examples of diapiric (igneous intrusion) geologic
processes in the South Pacific Border natural region. One of
the few places in the region where geologic strata of Jurassic,
Cretaceous, and Tertiary age can be seen in an aggregate
thickness of 42,000 feet. The park also possesses a great
diversity of native plant species and associations. Owner:
State.
DESIGNATION DATE:
May 1982.
| Mount Shasta |
Mount Shasta:14
Mount Shasta is located in the Cascade Range in northern California
about 65 kilometers (40 miles) south of the Oregon-California border
and about midway between the Pacific Coast and the Nevada border.
One of the largest and highest of the Cascade volcanoes, snowclad
Mount Shasta is near the southern end of the range that terminates
near Lassen Peak.
Mount Shasta is a massive compound stratovolcano
composed of overlapping cones centered at four or more main vents;
it was constructed during a period of more than 100,000 years.
Mount Shasta - National Natural Landmark:29
Siskiyou County - 60 miles north of Redding. One of the world's
largest and most impressive stratovolcanoes containing five
glaciers and consisting of four distinct but overlapping cones.
Second highest of the 15 main volcanoes in the Cascade
Range; only Mt. Rainier is higher. Owner: Federal.
DESIGNATION DATE:
December 1976.
Shastina and Hotlum Cones:14
Two of the main eruptive centers at Mount Shasta,
the Shastina and Hotlum cones were
constructed during Holocene time, which includes
about the last 10,000 years.
| Mt. Unumhum |
Mt. Unumhum:27
“Umunhum” is the Ohlone Indian word for “hummingbird.” The “blockhouse” that resides at the 3,486 foot summit
of Mt. Umunhum is a relict of the “Cold War” missile complex and until recently was on land owned by the U.S. Air
Force. In 2001, the land was acquired and added to the Sierra Azul Open Space Preserve. Until environmental restoration
is complete, Mt. Umunhum will remains off-limits to the public. A large cliffy exposure on the northeast face of Mt.
Umunhum is one of the best Bay Area exposures of the Coast Range Ophiolite. The Coast Range Ophiolite is
a piece of Middle Jurassic oceanic crust and mantle that is also known to underlie the rocks of the Great Valley
throughout central California. The original oceanic crust has largely been converted to serpentinite and with extensive
faulting and diapiric injection upward, some of it is migrated to its current exposures on the Earth’s surface, including
this exposure on the face of Mt. Umunhum and the extensive outcrop areas throughout the Santa Cruz Mountain foothills
and elsewhere.
Mt. Umunhum:28
Mt. Umunhum
is dominated by Jurassic-age serpentinitized
ultramafic rock on its eastern flank and
by Cretaceous-age conglomerate on its western
flank.
| Newberry Mountains |
Newberry Moutains:23
The Newberry
Mountains Wilderness is noted for its rugged
volcanic mountains and deep, maze-like
canyons.
Topography ranges from 2,200 feet in
the north to 5,100 feet in the south.
The unique
desert features are the result
of ancient volcanic activity.
Desert bighorn
sheep have historically traveled
this area, and prairie falcons and
golden eagles stop here to
forage and rest. Spring wildflower displays
are likely along the west boundary.
Location: San Bernardino County; 15 miles
east of Barstow, California.
| Old Woman Mountains |
Old Woman Mountains:23
Taking its name from
a granite rock monolith that resembles the figure
of an old woman,
the massive Old Woman
Mountain range dominates the wilderness.
Topography ranges from 800 at the lower
elevations and rises more than 5,300 to the summit of Old Woman
Peak. Creosote bush scrub covers the lower elevations, mixing with
desert scrub at the middle elevations, while at the highest elevations
grows a juniper-pinyon woodland. Bighorn sheep and desert tortoise
live here, and the rocky outcrops and rugged peaks of the range provide
homes for numerous raptors. The Old Woman Mountains have long
been used by man. Traces of prehistoric trails and historic mining routes
entice the experienced desert hiker. Be prepared for dry walks across
extensive bajadas and up long, sandy washes. Do not depend on
mapped springs for water, and remember to carry one gallon of water
per person per day.
Location: San Bernardino County; 35 miles
southwest of Needles, California.
| Peninsular Ranges |
Peninsular Ranges:17
A series of ranges is separated by longitudinal valleys,
trending NW-SE, subparallel to faults branching
from the San Andreas Fault.
The trend of topography is similar to the
Coast Ranges, but
the geology is
more like the Sierra Nevada,
with granitic rock intruding the
older metamorphic rocks.
The Peninsular
Ranges extend into lower California and are bound on the
east by the Colorado Desert. The Los Angeles
Basin, and island group
(Santa Catalina, Santa Barbara, and
the distinctly terraced San Clemente and San
Nicolas islands),
together with the surrounding continental
shelf (cut by deep submarine fault troughs) are
included in this province.
| Pinnacles National Monument |
The Ancient Volcano:8
The story begins more than 23 million years ago when
molten rock poured over the surface of the
land through fissures that opened
as the two plates ground past one another.
As the eruptions grew in
intensity the cone being built by the
volcanic activity grew until a high steep-sided volcano had been
formed. Geologists theorize that this
volcanic mountain once stood nearly a mile higher than North
Chalone Peak, the highest point in the park today.
Yet even as this volcano was being formed, its own
destruction was at hand, for the Pacific Plate
upon which it was located began moving off to the
northwest. In time the portion of the
volcano whose eroded remnants we now know as the Pinnacles
reached its present location, 195 miles
beyond its point of origin. All the while, too,
erosion, was at work carving and breaking
down the once mighty peak to about a third of its
original height, sculpting the spires and
crags you see today. Nor is the geologic process at an
end. Millions of years from now what
little will be left of this ancient volcano will have
moved off to the northwest.
Pinnacles:8
These rocks are the remains of an ancient volcano.
Or rather they are part of the remains, for the rest
of this volcano lies 195 miles to the southeast.
Sound intriguing? It is all part of the story of the
San Andreas Rift Zone, which runs just east of the park,
and the geological forces that have shaped the
face of the landscape in this part of California for millions of years.
The San Andreas Rift Zone, a series of faults,
lies just east of the park.
It was created when the
Pacific Plate collided with and
wrenched off a portion of the North American Plate. Rift
zones are likely places for volcanoes to occur,
for here the earth's crust is broken, allowing
the magma from beneath the surface to well up.
The Pinnacles are the result of these two
factors at work -
an ancient volcano and movement along a rift zone.
| Point Reyes |
Point Reyes Peninsula:19
The Point Reyes Peninsula has long
baffled geologists. Why should
the granite bedrock of this craggy
coast match the bedrock in the
southern Sierra Nevada range more
than 310 miles to the south?
The answer lies in plate tectonics
and the continual movement of
the earth's crust. Geologically,
Point Reyes is a land in motion.
The Peninsula rides high on the
eastern edge of the Pacific plate,
which creeps northwestward about
two inches a year. The slower
moving North American plate travels
westward. In Olema Valley,
near Bear Valley Visitor Center,
the North American and Pacific
plates grind together along the
San Andreas Fault Zone. This fault
zone contains many large and small
faults running parallel and at
odd angles to one another. Because
neither plate can move freely,
tremendous pressures build up. From
time to time this pressure
becomes too great, and the surface
actually moves. This is what
happened in the earthquake of 1906
when the Peninsula leaped 20
feet northwestward. As though to accent
the geological separation
along the San Andreas Fault, the weather
may vary quite markedly
on either side of Inverness Ridge.
A succession of summer days on
the east side may well be warm and
sunny while on the ocean side
a chilling fog may hide the sun.
Inverness Ridge:18
Granodiorite and granite are
exposed along Inverness Ridge,
where dikes and masses
of aplite and alaskite
are locally abundant.
Point Reyes Geology:18
The Point Reyes Peninsula is a roughly
triangular projection of the
Marin County coast with an apex at Point Reyes
and a base along Tomales Bay and the rift valley of the
San Andreas fault zone.
This fault zone forms the
active tectonic boundary between the Pacific
and North American lithospheric plates and here
separates Upper Cretaceous granitic and
older metamorphic
rocks of the Salinian block to the west from
Upper Jurassic to Lower Cretaceous
graywacke sandstone and melange of the
Franciscan complex to the east.
Resting nonconformably upon the crystalline
basement of the Point
Reyes Peninsula is a composite section of as
much as 14,340 feet of
moderately deformed Tertiary
marine sedimentary rocks that comprise three
sequences, separated by unconformities.
The oldest sequence consists of as
much as 700 feet of the Point Reyes
Conglomerate of Galloway (1977) of
early Eocene age that rests
depositionally upon the porphyritic granodiorite of
Point Reyes.
A middle and upper Miocene sedimentary
sequence is as much as 5,300 feet thick
and consists of a basal sandstone unit,
the Laird Sandstone,
and an overlying porcelanite unit,
the Monterey Formation. This Neogene
sequence crops out on the southwest
slope of Inverness Ridge, where it rests
nonconformably upon the
granodiorite and granite of Inverness Ridge.
Tomales Point:18
Hornblende-biotite tonalite that contains
dark diorite inclusions forms the
basement of Tomales Point. This rock is
lithologically similar to the tonalite
that crops out at Bodega Head about
10 kilometers to the north, and both are probably
part of the same granitic mass.
| Rodman Mountains |
Rodman Mountains:23
A series of ridges
and valleys climbing from 2,000 feet to almost
5,000 feet are the result of faults which cross this
wilderness. A lava flow slices this area in two
from northwest to southeast, forming a sloping
mesa.
Colorful escarpments, calico-colored mountains, maze-like
canyons and broad, majestic bajadas come together here. Steep
canyons and cliff-like walls form dry falls along deep drainage channels,
creating cascades during heavy rain storms. More than a half dozen
natural water "tanks" sit within the lava flow. Two of the tanks, Hidden
Tank and Deep Tank, hold thousands of gallons of water. One of only
seven core raptor breeding areas in the desert is within this wilderness,
where prairie falcons and golden eagles are known to survive. The
mountains themselves are part of the historic range of the desert
bighorn sheep. While sheep have not been spotted here, this wildlife
species has been seen in the nearby Newberry Mountains.
Location: San Bernardino County; 30 miles
southeast of Barstow, California.
| Salton Buttes |
Salton Buttes:15
The Salton Buttes comprise five small rhyolite domes
extruded onto Quaternary sediments of the Colorado River delta. Rock Hill
and Mullet Island are simple domes;
Mullet Island is notable for its symmetrical
"onion-skin" pattern of foliation,
attributed to
endogenous growth. Obsidian Butte consists of a central dome
surrounded on all sides by a single flow. Red Island is made up of
two domes, each mantled by subaqueous pyroclastic deposits.
All the domes exhibit
wave-cut benches carved during various stands of pre-historic
Lake Cahuilla, and have been partly buried by lacustrine and aeolian
deposits.
| San Gabriel Mountains |
San Gabriel Mountains:30
The San Gabriel Mountains, with elevations of nearly 7,000 feet, are composed of a complex assemblage of
Precambrian through Cretaceous igneous and metamorphic rocks that have been thrust to
the south over the adjacent basins.
| Santa Cruz Mountains in Santa Clara County |
Anderson Reservoir Dam and Anderson County Park:27
Pass the bridge over Coyote Creek (on Highway 101). Anderson Reservoir Dam can be seen blocking a former gorge through the
Yerba Buena Ridge (several miles to the left). This long ridge preserves evidence of the complex geologic
history related to the ongoing development of the Calaveras-Hayward fault system and the uplift of the Diablo
Range. The hilltops above the dam preserve evidence of a late Tertiary volcanic lava flow that formed early in
the development of the San Andreas fault system and the opening of the Santa Clara Valley. These volcanic
rocks unconformably overlie highly-deformed Franciscan assemblage rocks, mostly ancient basalt, greenstone
[altered basaltic volcanic rocks], serpentinite, chert, shale, and graywacke sandstone. (Graywacke is a fancy
name for a “dark, poorly sorted, typically fine-grained, dirty rock.”) Some of the rocks are well-exposed near
the reservoir spillway and the boat-ramp areas in Anderson County Park.
Chesbro Reservoir:27
Chesbro Reservoir boat dock - examine serpentinite exposed around
the reservoir
Boulders of
many of the different Franciscan assemblage rocks are conveniently on display around the boat dock parking area.
Boulders around the margin of the boat-dock parking area include examples of graywacke sandstone, metasandstone,
greenstone (metabasalt), serpentinite, red chert, and chert breccia. These rocks occur in scattered pod-like masses (a
classic mélange) exposed along the shoreline of Chesbro Reservoir.
A short walk to the dam (0.2 miles) provides opportunity to view exceptional exposures of serpentinite in road cuts
and in excavations along dam path. Don’t stand along the road because of the hazard of speeding cars.
Instead, the safer place to examine the serpentinite is along the dam path. Some of the serpentinite displays slickensides,
a linear fracture pattern caused by fault shearing motion between rock surfaces.
From the opposite end of the dam you can see the back of El Toro, the peak overlooking Morgan Hill. Downstream
from the reservoir, Llagas Creek flows through Paradise Valley and enters the Santa Clara Valley a couple of miles south
of Morgan Hill. On the north side of the spillway is a landslide formed in the weathered serpentinite. Note the bluish-green
to black color of the residual soil; the soil is rich in the clay mineral, montmorillonite.
Highway 101 Serpentinite Boulders:27
Pass the major power-grid station for
San José along U.S. Highway 101. Serpentinite boulders are scattered
across the hillsides on the left. This topography is, in-part,
a weathering effect related to soil development in
areas underlain by serpentinite.
Some of the serpentinite has been silicified (chert is very chemically stable and
is more resistant to erosion on the surface environment).
Areas underlain by this variety of “silicified serpentinite”
can be easily recognized on hillsides throughout the South Bay.
Loma Prieta:27
Loma Prieta (Spanish for “black mountain”) stands above the southern end of the Sierra Azul (Spanish for “blue
mountain”). The Sierra Azul is actually a steep-sided ridge that runs along the crest of the Santa Cruz Mountains
(including Loma Prieta and Mt. Umunhum, and two lower peaks to the north, Mt. Thayer and El Sombroso). The high
ridge separates the Santa Clara Valley around San José from the San Andreas Rift valley to the southwest. Loma Prieta
(3,806 feet) is the highest point in the Santa Cruz Mountains. Its summit consists of a serpentinite diapiric
plug (Coast Range Ophiolite) surrounded on the east by middle Tertiary Temblor Sandstone on the east, and by Late
Cretaceous sedimentary rocks (shale, mudstone, graywacke sandstone, and conglomerate - Great Valley Sequence
equivalent rocks) on the west. The peak is host to a privately-owned radio tower operation.
Mt. Unumhum:27
“Umunhum” is the Ohlone Indian word for “hummingbird.” The “blockhouse” that resides at the 3,486 foot summit
of Mt. Umunhum is a relict of the “Cold War” missile complex and until recently was on land owned by the U.S. Air
Force. In 2001, the land was acquired and added to the Sierra Azul Open Space Preserve. Until environmental restoration
is complete, Mt. Umunhum will remains off-limits to the public. A large cliffy exposure on the northeast face of Mt.
Umunhum is one of the best Bay Area exposures of the Coast Range Ophiolite. The Coast Range Ophiolite is
a piece of Middle Jurassic oceanic crust and mantle that is also known to underlie the rocks of the Great Valley
throughout central California. The original oceanic crust has largely been converted to serpentinite and with extensive
faulting and diapiric injection upward, some of it is migrated to its current exposures on the Earth’s surface, including
this exposure on the face of Mt. Umunhum and the extensive outcrop areas throughout the Santa Cruz Mountain foothills
and elsewhere.
Uvas Reservoir Dam:27
Uvas Reservoir Dam - examine pillow basalt
and other rock outcrops exposed in the
dam spillway and along the exposed shore.
In this region the road follows an elevated
terrace roughly 10 meters above the modern stream. The horse
pastures in this region are littered with
scattered boulders and outcrops of serpentinite and chert. Large outcrops
of metachert and serpentinite that rise above
the grassy landscape are called “knockers.”
This stop involves less than a mile walk to the bottom of the spillway
and return to the parking area near the dam.
Note the variety of rock exposures along the shoreline.
Nearly every rock type of the Franciscan assemblage can be found
within a mile radius of the Uvas Reservoir Dam.
On the shore near the south end of the dam blocks and boulders of
graywacke sandstone, volcanic breccia, limestone,
and other rock litter the shoreline. Examine the shoreline and note
that different rock-types that occur in pod-shaped
masses or as a chaotic mix. This is classic mélange typical of the
Franciscan assemblage.
[Read MORE about Uvaz Reservoir Geology below]
| Sequoia and Kings Canyon National Parks |
Moro Rock:21
Moro Rock is a dome-shaped granite monolith.
Common in the Sierra Nevada, these domes
form by exfoliation, the spalling or
casting off in scales, plates,
or sheets of rock layers on otherwise
unjointed granite. Outward expansion
of the granite results in
exfoliations. Expansion results
from load relief; when the overburden
that once capped the granite has eroded away, the source of
compression is removed and the
granite slowly expands. Fractures that
form during exfoliation tend to cut corners. This ultimately
results in rounded dome-like forms.
| Sheephole Mountains |
Sheephole Mountains:23
The Sheephole
Valley separates the Sheephole Mountains and
Calumet Mountains.
The Sheepholes are a
steep, boulder-strewn, granitic mountain mass.
The Calumets take on a
similar appearance, although rising only halfway to the 4,600-foot tall
Sheepholes. Bighorn sheep make their home within the Sheephole
range, while the desert tortoise enjoys the valleys below. The area's
lack of springs and extreme distances make wilderness travel a
challenge for the most experienced desert hiker.
Location: San Bernardino County; 20 miles
east of Twentynine Palms, California.
| Sierra Nevadas |
Sierra Nevadas:6
California's Sierra Nevada ("snow-capped mountain") is a
west-tilting 350-mile (560-kilometer)-long block of granite.
Extending
from 14,494 feet (Mount Whitney, the highest peak in the lower
48 states) in the east to near sea level in the west, it contains
the spectacular Yosemite and Sequoia National Parks.
The massive granite intruded the crust
in Mesozoic time and was uplifted and faulted in the Tertiary
during formation of the Basin and Range province to the east.
Eroded residue from the Sierra Nevada has filled the Central
Valley of California, giving rise to both extensive agriculture
and the 1849 Gold Rush.
| Sutter Buttes |
Sutter Buttes:6
Interpreted by geologists as
the southernmost volcano in
the Cascade Chain,
the now-inactive and highly dissected
Sutter Buttes show up on a map as a small circular bump
that contrasts sharply with its flat surroundings in
California's Central Valley. It erupted in Late Tertiary time -
punching through flat-lying young sediments eroded from
the rising Sierra Nevada to the east.
| Turtle Mountains Wilderness |
Turtle Mountains:23
Broad, open
bajadas, eroded volcanic peaks, spires and cliffs
converge to form the Turtle Mountains
Wilderness. The volcanic formations shade the
range in colors varying from deep reds, browns,
tans and grays to black.
The range lies in a horseshoe arrangement,
parted by a large flat interior valley cut with numerous shallow washes.
Places with names like Mopah Springs, Mohawk Springs, Coffin
Springs and Gary Wash invite the dauntless wanderer, but do not rely
on mapped springs for water. Always carry a gallon of water per person
per day. Desert bighorn sheep, prairie falcon, golden eagle and desert
tortoise make this wilderness area home.
Location: San Bernardino County; 30 miles
south of Needles, California.
Turtle Mountain Natural Area - National Natural Landmark:29
San Bernardino County - 30 miles south-southwest of Needles.
Two mountain sections entirely different in composition which
illustrate past volcanic phenomena with superimposed
sculpturing of mountain landforms by weathering and uplift.
Owner: Federal, State.
DESIGNATION DATE:
May 1973
| Ubehebe Craters |
Ubehebe Craters:16
The Ubehebe Craters include
over a dozen maar volcanoes formed during hydrovolcanic eruptions
of alkali basalt through permeable fanglomerate
deposits on the north side of Tin Mountain.
The craters formed along a range-bounding fault that marks the western margin of Tin Mountain.
The volcanic center is named Ubehebe Crater, the largest tuff ring (0.8 kilometers wide, 235
meters deep) of the volcanic field. Initial eruptive activity at the center developed a scoria
cone south of Ubehebe Crater. Subsequent activity was predominantly hydrovolcanic and produced
two clusters of explosion craters and tuff rings west of, and south of, Ubehebe Crater. The
southern cluster includes Little Hebe Crater, the second youngest crater in the field. The
youngest eruptive events at the center were the episodic hydrovolcanic explosions that formed
Ubehebe Crater. Ejecta from the crater covers all preexisting craters in the area. The
deposits also overlie lake beds of Lake Rogers, approximately 4 kilometers north of the vent.
This stratigraphic position, and the lack of erosional modification of the pyroclastic surge
apron, suggest the youngest activity was Holocene. The Ubehebe Craters are accessible via a
paved road, west of Scotty's Castle, at the north end of the Death Valley National Monument.
The road leads to an overlook at the west side of Ubehebe Crater.
| Uvaz Reservoir |
Uvas Reservoir Dam:27
Uvas Reservoir Dam - examine pillow basalt and other rock outcrops exposed in the
dam spillway and along the exposed shore.
In this region the road follows an elevated terrace roughly 10 meters above the modern stream. The horse
pastures in this region are littered with scattered boulders and outcrops of serpentinite and chert. Large outcrops
of metachert and serpentinite that rise above the grassy landscape are called “knockers.”
This stop involves less than a mile walk to the bottom of the spillway
and return to the parking area near the dam. Note the variety of rock exposures along the shoreline.
Nearly every rock type of the Franciscan assemblage can be found
within a mile radius of the Uvas Reservoir Dam. On the shore near the south end of the dam blocks and boulders of
graywacke sandstone, volcanic breccia, limestone, and other rock litter the shoreline. Examine the shoreline and note
that different rock-types that occur in pod-shaped masses or as a chaotic mix. This is classic mélange typical of the
Franciscan assemblage.
Uvas Reservoir Bedrock:27
The bedrock around Uvas Reservoir are a mélange (a great chaotic mix) of different rocks ranging in age from Early
Jurassic to Eocene, representing a time span over 150 million years ending roughly 50 million years ago. Similar rocks
are exposed throughout the Coast Ranges of California and are collectively named the Franciscan assemblage (or
Franciscan Group, Franciscan Series, or Franciscan Complex, depending on preferences of various geologists through
time). The word “assemblage” is appropriate because rocks that are included in the Franciscan originally formed in a
variety of geologic settings later to be brought together and mixed along a convergent plate boundary (see fig. 6). General
rock types that occur in the Franciscan assemblage include ultramafic gabbro, serpentinite, basaltic volcanic rocks,
limestone (and dolostone), ribbon chert, shale, graywacke (mudstone and sandstone), serpentinite, and other materials.
As the variety of rocks suggests, the origin of the Franciscan assemblage is complex; a short interpretation of its geologic
history follows.
The oldest Franciscan assemblage rocks include gabbro (original mantle and oceanic crust). In most places, this
original rock has been completely altered to serpentinite. These rocks occur in association with pillow basalt that is
inferred to have formed on a mid-ocean ridge or associated undersea volcanoes. On top of some of these volcanos, or
on shallow ocean platforms, calcareous limey sediments accumulated (forming limestone similar to what is found forming
on modern guyots or atolls). Through time, plate tectonic motion transported new oceanic crust into a deeper water
setting where radiolarian ooze was deposited.
Exposures of ribbon chert in the Santa Cruz Mountains are inferred to have
formed from deep-sea ooze deposited between the Early Jurassic to the middle Cretaceous. The youngest and most
common rocks in the Franciscan assemblage are graywacke mudstone, sandstone, and conglomerate. These sedimentary
rocks frequently occur interbedded with shale and are interpreted as
sediments having been derived from a volcanic arc
and deposited via turbidity currents into a trench or deep water marginal basin offshore.
The chaotic mix of rocks of the Franciscan assemblage completes the geologic history, involving plate-tectonics
(crustal formation, migration, subduction, and accretion). Serpentinite forms from the metamorphic alteration and/or
remelting of rocks of ultramafic composition (typical of lower oceanic crust or even Earth’s mantle).
In the process of
subduction and continental accretion, oceanic sediments and crust broken into bits ranging from hand specimens to
mountain-sized blocks. These rocks were jumbled and crushed into one another, especially along major continental
margin fault zones that formed, faded, and reformed through time. Serpentinite is unusual in that it is less dense in
composition that its original host (rock consisting mostly of dunite and pyroxenite). Through time serpentinite migrates
towards the surface along faults or forming plugs (or intrusions)
that inject upward into the surrounding host rock.
Uvaz Reservoir Greenstone:27
In some locations around the spillway exposure, the basaltic lava has also been partially altered to greenstone.
(Greenstone is a field name for a dark-green altered or metamorphosed basic volcanic rock containing the minerals
chlorite, epidote, or actinolite.) The rock is also broken by fractures that are filled primarily by calcium carbonate
(possibly derived from the recrystallization of the original limey ocean sediment). The outcrop is broken by several small
faults, one of which displays apparent offset of the overlying soil profile. This exposure is on the north side of the cut
about 100 feet south of the end of the cement spillway. Be sure to check out the composition of the stream gravel in
patches along the shore. Gravel bars provide a glimpse of the various rock types derived from in the drainage area. Some
of the gravel is reworked from older stream channel and floodplain deposits, now preserved as terraces, and are exposed
along the hillsides below the dam.
Uvaz Reservoir Pillow Lava:27
Cross the dam and turn right. Follow the path down to the spot where the concrete spillway ends.
In the creek are exposures of relatively unaltered “pillow basalt” - pillow-shaped pods of basalt formed from the rapid
cooling of basaltic lava on the sea floor. Each pillow displays a chilled crust from the molten lava coming in contact with
the cold ocean water. Radiometric determinations suggest these rocks formed in the Lower Cretaceous (between 100-130
million years ago). Note that some of the basaltic lava preserves vesicles (gas bubbles) and that in some places, gray,
limey mudrock or limestone (or dolostone) fills in around some of the pillows. Both occurrences suggest that the lava
formed in relatively shallow ocean depths, probably on a submarine volcano. Under greater ocean depths the confining
pressure would prevent gas vesicles from forming in the lava. In addition, calcareous sediments are not preserved in deep
ocean settings; organic pelagic remains containing calcium carbonate dissolve rapidly in cold marine water under
pressure.
| Whipple Mountains Wilderness |
Whipple Mountains:23
The Whipple
Mountains lie east to west across this
wilderness. The western half of the range is
created from pale green formations. A low angle
fault separates the formation on the west from
striking brick red volcanic
formations to the east.
The landscape is
diverse, ranging from valley
floors and washes to steep- walled
canyons, domed peaks and eroded
spires towering to 4,000 feet.
Flowing westward from these
colorful spires and domes are bajadas
with isolated lava rock
masses and red sandstone outcrops. Sonoran
creosote bush scrub and
the Sonoran thorn forest decorate the area. In
addition, dense stands of
palo verde, ironwood, smoke tree and cholla,
saguaro, foxtail and Mojave
prickly pear cacti all grow here. This unique
array of vegetation support
a variety of wildlife.
Location: San Bernardino County; 10 miles
northwest of Parker, Arizona.
| Yosemite National Park |
Yosemite National Park:
For its towering cliffs,
spectacular waterfalls,
granite domes and spires,
glacially-sculpted and
polished rock, and beautiful alpine scenery,
Yosemite National Park is world famous. Nowhere else
are all these exceptional features so well
displayed and so easily accessible.
Excerpts from:
1) Wood, 1990, IN: Wood and Kienle, 1990,
Volcanoes of North America: United States and Canada:
Cambridge University Press, 354p.
2) Donnelly-Nolan, 1990, IN: Wood and Kienle, 1990,
Volcanoes of North America: United States and Canada:
Cambridge University Press, 354p.
3) Dzurisin, 1992, Geodetic Leveling as a Tool for Studying Restless Volcanoes,
IN: Ewert and Swanson (editors), 1992, Monitoring
Volcanoes: Techniques and Strategies Used by the Staff of the
Cascades Volcano Observatory, 1980-1990: U.S. Geological Survey
Bulletin 1966, p.131;
4) Hill, et.al., 1996,
Living With a Restless Caldera -- Long Valley, California:
USGS Fact Sheet 196-96;
5) U.S. Geological Survey,
Long Valley Observatory Website, 2000, 2001
6) USGS A Tapestry of Time and Terrain Website, 2001
7) U.S. National Park Service Website,
Mojave National Preserve, 2000
8) U.S. National Park Service Website,
Pinnacles National Monument, 2000
9) U.S. National Park Service Website,
Joshua Tree National Monument, 2001
10) U.S. National Park Service Website -
Lassen Volcanic National Park, 2000, 2001
11) California Desert Website, 2000
12) USGS/NPS Geology in the Parks Website, 2001
13) U.S. Forest Service Website, Region 5, 2001
14) Miller, 1980, USGS Bulletin 1503
15) Muffler, 1990, IN: Wood and Kienle, 1990, Volcanoes of North America:
United States and Canada:
Cambridge University Press, 354p.
16) Crowe, 1990, IN: Wood and Kienle, 1990, Volcanoes of North America:
United States and Canada:
Cambridge University Press, 354p.
17) California Geological Survey Website, 2001, 2002
18) Clark and Brabb, 1997,
Geology of Point Reyes National Seashore and Vicinity, California:
A Digital Database: USGS Open-File Report 97-456
19) U.S. National Park Service Website,
Point Reyes National Seashore, 2002
20) U.S. National Park Service Website,
Channel Island National Park, 2002
21) U.S. National Park Service Website, Sequoia and Kings Canyon National
Parks, 2002
22) U.S. Geological Survey,
Western Region Coast and Marine
Geology Website, 2002
23) California Bureau of Land Management Website, 2002
24) California Environmental Resources Evaluation System (CERES) Website, 2003,
California Resources Agency, from the California Coastal Commission's
California Coastal Resource Guide
25) Greeley, 1990, IN: Wood and Kienle, 1990, Volcanoes of North America: United States and
Canada: Cambridge University Press, 354p.
26) Grose, 1990, IN: Wood and Kienle, 1990, Volcanoes of North America: United States and
Canada: Cambridge University Press, 354p.
27) Stoffer and Messina, 2002,
Field-Trip Guide to the Southeastern Foothills of the Santa Cruz Mountains in Santa Clara County, California:
U.S. Geological Survey Open-File Report 02-121.
28) Stoffer and Messina, 2002,
Field-Trip Guide to the Geology of the Lexington Reservoir and Loma Prieta Areas in the Santa Cruz Mountains, Santa Clara and Santa Cruz Counties, California:
U.S. Geological Survey Open-File Report 02-221.
29) U.S. National Park Service, National Natural Landmarks Website, 2003
30) California Department of Conservation,
Division of Mines and Geology, Open-File Report 2000-005,
Seismic Hazard Evaluation of the Mt. Baldy 7.5-Minute Quadrangle, Los Angeles County, California.
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