Download Examples of Rock Families in the San Francisco Bay Area Yilin Lu

Examples
of
Rock Families
in the San Francisco Bay Area
Yilin Lu
GEOL 350
05/14/2015
Table of Contents
Introduction ----------------------------------------------------------------------------------------------Pg. 3
Location Map --------------------------------------------------------------------------------------------Pg. 5
Site #1: Point Bonita Lighthouse, Marin Headlands, Igneous Volcanic, Pillow Basalt -------Pg. 7
Site #2: Bunker Road, Mill Valley (Marin Headlands), Sedimentary, Chert and Shale ------Pg. 9
Site #3: Marshall Beach, Presidio, Metamorphic, Serpentinite ---------------------------------Pg. 13
Bibliography --------------------------------------------------------------------------------------------Pg. 16
The make and model of camera: iSight Camera of iPhone 5s - 8 megapixels with 1.5µ pixels
Reviewed by
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Introduction:
The San Francisco Bay Area is known for its famous view. Yet, not so many people know
that it is also an ideal place to study geology since it contains all four major rock families:
igneous rock, sedimentary rock, metamorphic rock and hydrothermal rock. In this project, I
chose three sites as three examples of the rock families to show how amazing the Bay Area was
and how easy people actually could approach these million-year records.
All three sites (two in Marin County and one in the city of San Francisco) are located on the
east of the San Andreas Fault, where the basement complex is Franciscan Complex. In contrast,
on the west of the San Andreas Fault, the basement complex is Salinian Complex. Franciscan
Complex is comprised of five rocks: pillow basalt (igneous volcanic rock), serpentinite
(metamorphic rock), radiolarian chert, shale and greywacke (all latter three are belong to the
sedimentary rock family) while Salinian Complex is mainly comprised of granodiorite (igneous
plutonic rock). In this project, we will find pillow basalt at Site 1 - Point Bonita Lighthouse,
radiolarian chert and shale at Site 2 – near Bunker Road and, finally, serpentinite at Site 3 Marshall Beach.
Over 100 million years ago, in the Mesozoic Era, the North America Plate converged with the
Farallon Plate. The oceanic Farallon Plate was made of basalt, having a higher density than the
continental North America Plate made of granodiorite. As a result, the Farallon Plate was
subducted, creating a trench off the west coast of North America. Meanwhile, a continental
volcanic arc formed as subducted materials partially melted, which provides Andesite magma to
build stratovolcanoes. This volcanic arc was the ancestry of Sierra, Cascades and Sierra Madre
volcanic arcs. During the process of subduction, some rocks from the Farallon Plate were added
to the North American Plate, forming the Francisco Basement Complex. After slow geology
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processes, they are exposed in San Francisco today. The boundary between the Farallon Plate
and the North America Plate was convergent boundary.
30 million years ago, in the Cenozoic Era, the Pacific Plate began to meet the North America
Plate as parts of the Farallon Plate were subducted under the North America Plate completely.
Instead of compression forces between the Farallon Plate and the North America Plate, there
were shear forces between the Pacific Plate and the North America Plate. As a result, the San
Andreas Fault, a right lateral strike-slip fault, was born. The boundary between the Pacific Plate
and the North America Plate is transform boundary.
Today, most Farallon Plate has been subducted. There is no trench near the coast of the San
Francisco. Yet, the remaining of the Farallon Plate, for example, the Juan de Fuca Plate, is still in
process of subduction, providing magma to active volcanoes on land. Only five of six marine
environments exist off the San Francisco coast– beach, shelf, slop, abyssal floor and ridge.
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Location Map:
5
*
Map 2:
Site 3 - Marshall Beach
Golden Gate Bridge
Bus #28
Battery to Bluffs Trail
Ÿ
Site 3
Presidio
✪
To Baker Beach
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Site #1, Igneous Volcanic: Pillow Basalt, Point Bonita Lighthouse, Marin Headlands
Photos:
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Site Description:
Photo #1-a shows a rock composed of pillow basalt, which is located beside the Point Bonita
Lighthouse in the Marin Headlands, Marin County, California. The rock is black as basalt, an
igneous volcanic rock, is high in iron. A part of the rock is exposed above the sea surface – 16
meters (50 feet) above sea level during low tide (National Park Service1).
Photo #1-b shows a closer look at the other side of this fine-grained rock. Green color is the
mineral chlorite brought by seawater. In the photo, we can see some pillow basalts having noses
overturned due to plate movement. The noses were originally upward. After trying to use a knife
to cut some pillow basalts found near the site, we found that the rock is hard to cut. No vesicular
texture is observed.
Site Interpretation:
Pillow shapes indicate that basalts are formed underwater while the basalts formed on land
will have a shape of column. In addition, grain size is associated with energy of environment.
Fine grains are related to low environmental energy. Among all six marine environments, ridge
is the one that has the lowest energy. While molten rock was emitted from basalt magma beneath
oceanic ridge, it was cooled immediately by water, rotating and forming pillow-shaped crust
while the inside was still melting. This process also trapped gases inside the basalt so there is no
vesicular texture observed. The basalt magma is formed by decompression melting (partial
melting) of mantle rock, peridotite. During the slow process of the subduction of the Farallon
Plate under the North America Plate over 100 million years, the pillow basalts have traveled a
great distance from the East Pacific Rise where they formed to the current location (Stoffer, n.d.
Pg. 21-222).
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Site #2, Sedimentary: Chert and Shale, Bunker Road, Mill Valley (Marin Headlands)
Photos:
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Site Description:
Photo #2-a shows inclined layers of radiolarian chert and shale in Marin Headlands, Marin
County, California. Both of them are lithified sedimentary rocks. Moreover, both syncline fold
(concave upward) and anticline fold (convex upward) can be observed. The folds are associated
with compression forces applying before layers were inclined.
Photo #2-b shows a closer look at both rocks. Both of them are reddish-blown and finegrained. The thinner layers are composed of softer rock – shale, and the wider layers are
composed of hard rock – chert. Chert contains microfossils of radiolarians, which have silica
shells instead of calcite shells. So, chert won’t respond to acid test. Radiolarians are responsible
for the reddish-blown color of chert. Shale is composed of clay, which is much softer than silica.
In the photo, joints can be observed, as there is no evidence of movement. A white hydrothermal
vein is also observed. It also does not respond to acid test, and it is hard to cut. So, it is a milky
quartz (silica) hydrothermal vein and younger than the host rock chert (principle of cross-cutting
relations).
Photo #2-c shows a fault across the layers, showing evidence of movement – layers are not
matched. Before tilting, hanging wall is moved down and footwall moved up. It is a normal fault.
Site Interpretation:
Lithified sedimentary rocks formed in a marine environment, and fine-grained texture
indicates that they formed in an environment with low energy. The sediments were originally
deposited at a ridge. The interbedded layers suggest climate change has happened during the past
million years. During 80,000-year Glacial Period in Ice Ages, seawater was cold and dense,
which could hold more nutrients for marine creatures including radiolarians. So, wide layers of
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radiolarian cherts formed. On the other hand, during 20,000-year Interglacial Period, seawater
was warm and less dense, providing fewer nutrients for creatures. There was no microfossil
sediments deposited in the thin layers of shales as a result (Stoffer, n.d. Pg. 383). Each pair of
chert and shale layers represents 100,000 years. The sedimentary rocks were also brought and
uplifted to the current position by plate tectonic motion occurring in the past 100 million years.
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Site #3, Metamorphic: Serpentinite, Marshall Beach, Presidio
Photos:
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Site Description:
Photo #3-a shows serpentinite found at Marshall Beach, San Francisco, California.
Serpentinite is a metamorphic rock and the California state rock.
Photo #3-b shows uplifting joints cutting serpentinites. Uplifting joints have formed during
uplifting process that brought underground serpentinite to current location over millions of years.
Between the serpentinites, there is a shear zone with big tectonic mélange, caused by tectonic
plate motion and a strike-slip fault.
Photo #3-c shows white inclusion and intrusion in green serpentinites. The minerals in the
white vein reacts with acid and fizzes while the ones in the intrusion doesn’t. The vein is calcite
hydrothermal vein. Meanwhile, in order to identify the minerals composited of the white
intrusion and the serpentinite, respectively, we need to do more tests on them.
Photo #3-d shows shiny surfaces – slicken sides of serpentinites. Each one of them is a
separate fault plane. Serpentinite is soft and easy to cut.
Site Interpretation:
Serpentinite forms from recrystallization of mantle rock peridotite, which is ultramafic,
containing only 40% silica. In a water-rich environment, peridotite goes through pressure change
and temperature change slowly and forms serpentinite gradually (Stoffer, n.d. Pg. 544).
Serpentinite is at the bottom of Ophiolite Sequence (from top to bottom: chert, basalt, gabbro,
and serpentinite). It is squeezed up by faults and brought to the San Francisco coast from the
oceanic ridge where it formed by the subduction of the Farallon Plate over the past 100 million
years.
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Bibliography:
1
National Park Service. (2015). Rocks on the Move Investigation Sites. Retrieved from:
http://www.nps.gov/goga/learn/education/rocks-on-the-move-investigation-sites.htm
2
Stoffer, P. (n.d). USGS. Rocks and Geology in the San Francisco Bay Region. Pg. 21-22.
Retrieved from: http://pubs.usgs.gov/bul/2195/
3
Stoffer, P. (n.d). USGS. Rocks and Geology in the San Francisco Bay Region. Pg. 38
Retrieved from: http://pubs.usgs.gov/bul/2195/
4
Stoffer, P. (n.d). USGS. Rocks and Geology in the San Francisco Bay Region. Pg. 54
Retrieved from: http://pubs.usgs.gov/bul/2195/
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