G/M 309
Fall 2005
Dr. Lisa White
Dr. Dave Dempsey
Name ___________________
Plate Tectonics and the Franciscan Complex
(Due in class Tuesday, Oct. 4. Worth 50 points total, or 5% of your course grade.)
Objectives:
- Become better adquainted with the types of rocks common in San Francisco (part of the
Franciscan Complex) and their origins in the context of the plate tectonic history of the
eastern Pacific Ocean.
Materials Needed:
- Samples of several rocks from the Franciscan Complex
- Paper map of the Pacific Ocean about 100 million years
- Paper map of the Pacific Ocean about 20 million years ago
- Color diagram, "How it looked in the Mesozoic Era"
- Three Quicktime movies (on the desktop of computers in TH 604 and TH 607):
GlobPacNoAm.mov
EPacNoAm.mov
MesozoicSubduction.mov
- A Web browser (for access to a list of rocks in the Franciscan Complex).
- “The Theory of Plate Tectonics”, installed on the computers in TH 604 and TH 607
(click on the “Tasa” icon, on the Dock on Mac laptops in TH 604 and TH 607 or on the
desktop on Windows PCs in TH 607).
Background Review:
1.
Chapter 6 of “The Theory of Plate Tectonics” introduces the theory of plate
tectonics, identifies seven major and several intermediate-sized lithospheric plates (also
called tectonic plates), and describes the three basic types of plate boundaries. If
necessary, you should review this material. (Use your copy of the CD, or click on the
“Tasa” icon on the Dock of the Mac laptops in TH 607 or on the Windows PCs in TH
607 next door.) Make particular note of:
a. the plates that lie beneath the Pacific Ocean;
b. the three types of plate boundaries, defined in terms of the motions of the
plates relative to each and, in one case, the nature of the lithosphere
(continental or oceanic) on each side of the boundary; and
c. the type of plate tectonic boundary we find along the plate margin where
California lies today.
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2.
During your introduction to geologic maps (the in-class exercise, “Questions
about Geologic Maps”); the field trip to Sutro Heights and Reading Assignment #2 from
Konigsmark’s “Geologic Trips”; and your investigation of Stage 1 of Problem #1,
“Living on the Edge”, you encountered geologic maps showing areas of bedrock in S.F.
Typical bedrock in S.F. is mapped as the Franciscan Complex, which includes a diverse
and complex suite of igneous, sedimentary, and metamorphic rocks. The link below, to
the class handout “Geologic Maps of San Francisco”, includes a list of rock types in the
Franciscan complex:
http://funnel.sfsu.edu/courses/gm309/F05/problems/LivingOnTheEdge/SFGeo
Maps.html
In particular, see the lowermost of the three snippets that constitute Legend #1.
Questions:
Answer the questions posed below. You should work largely independently. Turn in your
work at the end of class.
Examine the accompanying color diagram, "How it looked in the Mesozoic Era", and
notice the four main rocks forming the Franciscan Complex and where/how they
originated.
1.
Examine the samples of Franciscan Complex rocks in the classroom. Briefly
describe each distinct type of rock type, and classify each type as igneous, clastic
sedimentary, chemical sedimentary, or metamorphic:
2.
Refer to your paper map of the Pacific Ocean about 100 million years ago. Read
the legend and note the different features on the seafloor. Based on this map, pose a
hypothesis to explain where on the map the rocks of the Franciscan Complex likely first
formed and how they got where we find them today. (You will test your hypothesis later
using plate tectonic animations.)
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3.
The three Quicktime movies that should reside on the desktop of computers in
TH 604 and 607 show animations of the evolution of parts of the Pacific Ocean. Look
for examples of divergent, convergent, and transform plate boundaries as you watch the
animations.
The file “GlobPacNoAm.mov” contains an animation of the evolution of the
Pacific Ocean over the last 80 million years. Double-click on this file and start the
animation. Note the following about the animation:
a. the time (in millions of years ago, or “Ma”) is shown at the upper right;
b. plate boundaries are shown in red; and
c. shades of blue are used to represent the age of the sea floor (lighter shade
means younger rocks)
Based on this movie, answer the following questions:
(A) At the start of the movie, the plate beside what is today California is the
Farallon plate; the plate at the lower left is the Pacific plate; and the northernmost plate is
the Kula Plate. Based on what you see happening in the movie and your understanding of
convergent plate boundaries, what has happened to the Kula plate?
(B) When does the Pacific plate first "touch" the North American plate?
4.
The file “EPacNoAm.mov” contains an animation of the evolution of the eastern
Pacific Ocean over the last 40 million years. Double-click on this file and start the
animation. Note the following about the animation:
a. the time (in millions of years ago, or “Ma”) is shown at right;
b. plate boundaries are shown in red, and different symbols are used for
convergent boundaries on one hand and divergent and transform fault boundaries on the
other hand; and
c. shades of blue are used to represent the age of the sea floor (lighter shade
means younger rocks)
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Based on this movie, answer the following questions:
(A) What happens to the plate margin in California starting about 25 million years
ago? How does this change in plate margin in California shed light on the existence of the
San Andreas fault today?
(B) When does the Gulf of California, which separates Baja Mexico from
mainland Mexico, begin to form?
5.
The file “MesozoicSubduction.mov” contains an animation of a typical oceaniccontinental convergent plate boundary of the sort found along California until 20-25
million years ago. Based on this animation, answer the questions below.
(A) How does the process of subduction shown in the animation help explain the
origin of the various rocks of the Franciscan Complex?
(B) How has your original hypothesis developed at the beginning of class been
supported or contradicted by what you have viewed in the animations? Are there any
modifications to your original hypothesis that you would make in light of what you have
learned?
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