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1. California Rock Stories
Linking tectonics to rock formation
2. Geologic History of California: Part II
Ellen Metzger
BAESI – July 2012
California’s Rocks
• California, including the Bay Area, has a
greater variety of rocks than do other
regions of the United States.
• This reflects the state’s complex
tectonic/geologic history.
Bay Area Rocks
• Young sedimentary and volcanic rocks
• Mesozoic rocks
 Franciscan Complex
 Coast Range Ophiolite
 Great Valley Group
• Salinian basement
The Rock Cycle
Source: USGS
Bay Area Rocks
• Do your students bring you round,
grungy, fine-grained, black and green
rocks?
How do geologists describe rocks?
Rock Description
• Color
• Texture
• Weathering/resistance to erosion
Cartoon of the subduction zone present on the West Coast
100 million years ago showing position of the Franciscan
accretionary complex. Source: National Park Service
California tectonics in the past: a subduction zone
Cross section of western North America at about 100 million years ago
/www.nps.gov/prsf/naturescience/images/Subduction-animation_1.gif
Terranes in the Bay Area
Terrane
A “terrane” is distinguished
from neighboring terranes
by its different history,
either in its formation or in
its subsequent deformation
and/or metamorphism.
Terranes are separated by
faults. An exotic terrane is
one that has been
transported into its present
setting from some
distance.”
USGS
Source: “Rocks and Geology in the SF Bay Region,”
by Philip Stoffer. USGS Bulletin 2195
Unique Bay Area Rocks
• Metamorphic
Glaucophane schist (“blueschist”) formed under
high P-low T in a subduction zone.
Serpentinite - hydrated mantle rocks
Mantle rock = ultramafic (Si02-poor), dense, dark
 Serpentinite = rock (CA State Rock)
 Serpentine = mineral
 Should serpentinite be “demoted” as our state rock?
Formation of Blueschist in a Subduction Zone
http://openlearn.open.ac.uk/file.php/3512/SXR260_1_007i.jpg
Unique conditions: High pressure combined with low temperature
Note depressed isotherms.
Due to slow heating of cold, down-going oceanic plate
Unique Bay Area Rocks
• Igneous
 Sea floor basalt
Pillow lavas
Greenstone (altered basalt)
• Sedimentary
 Graywacke (“dirty” sandstone)
 Radiolarian Chert
Radiolarians: Tiny ocean animals
that make their skeletons of silica
(SiO2)
http://www.mdia.org/images/Radiolaria.jpg
Rocks we’ll see on the field trip
• Chert
• Sandstones
• Pillow basalt
Altered basalt: greenstone
• Volcanic rocks with:
Phenocrysts
Amygdules
California Tectonics: Present
Source: USGS
California Tectonics: Past
Geologic History Video
http://www.youtube.com/watch?v=YpIbbul0eQ0
The following slides are from a presentation by Mark Williams,
University of Colorado at Boulder
http://snobear.colorado.edu/Markw/Mountains/03/week10.html
Cenozoic
• By about 50 million years ago the ancestral
Nevadan Mountains had eroded down to
relatively low-lying hills.
• Large rivers flowed through the region and
deposited gravels rich in gold.
• The southern Sierra Nevada and Mojave region
were elevated enough to allow for the erosion
and deposition of thick layers of marine
sedimentary rocks in what are now the southern
Coast Ranges and western Transverse Ranges.
Cenozoic
• This deposition continued without
interruption until around 40 million years
ago. Over the next 10 million years the
coast shifted back and forth, producing a
patchwork of marine and non-marine
sedimentary rocks in the Coast Ranges
and western Transverse Ranges.
Cenozoic
• 25-29 million years ago, the oceanic plate that had
been subducting beneath the western edge of North
America became completely overridden, starting in
the south, and the North American and Pacific plates
came into direct contact for the first time.
• Tangential motion and expansion replaced
convergent motion as the North American plate
began interacting with the Pacific plate.
• The San Andreas Fault system was formed.
• In far northern California and the Pacific Northwest,
north of the Mendocino triple junction, convergent
motion has continued right up to present times.
Cenozoic
• Volcanic activity, related to extension and thinning of
the crust, became widespread in the Sierra Nevada and
Mojave regions around 20 mya.
• Around 10-15 mya a series of deep marine basins
formed along the coast between Orange County and the
San Francisco region. The appearance of the basins may
have signaled the passage of what is now coastal
southern and central California over the spreading zone.
• The rocks formed in these basins (Monterey Formation)
are composed mainly of material derived from marine
organisms, rather than terrestrial sediments.
Cenozoic
• About 5 million years ago mountain-building activity
rapidly accelerated, and finally most of the modern
mountain ranges were uplifted, including the Sierra
Nevada and the large fault-block ranges to the east,
the Coast Ranges, the Transverse Ranges, and the
Peninsular Ranges.
• Subduction continued in the north forming the major
volcanoes of the Cascades.
• Pleistocene glaciation in the Sierra Nevada and, to a
minor extent, in the San Bernardino Mountains;
recent volcanic eruptions in the Mojave and Great
Basin regions; and the widespread volcanic activity
that created the southern Cascade volcanoes (Mt.
Shasta and Mt. Lassen) and the lava flows of the
Modoc Plateau region.