Download G2S15Lesson8 Tectoni..

Geology 2, Historical Geology
Lesson 8 - Plate Tectonics and Paleogeography
Your Name___________________
Readings:
Earth Through Time:
p. 170-211 (Chap 7) – Plate Tectonics. Also the following web page provides information:
http://tinyurl.com/g1tectonics
PART I - Questions
1. Which of the following are parts of a lithospheric plate (circle the ones included in the
lithosphere):
Continental Crust, Inner Core, Lower Mantle, Oceanic Crust, Outer Core, Upper Rigid
Mantle.
2. List one of Earth’s major plates that includes mostly oceanic crust:
3. List one of Earths major plates that includes both oceanic and continental crust:
4. Read about Ophiolites (p.192-194). The following web page also has information:
http://tinyurl.com/ophiolite
Draw a diagram of an Ophiolite and label the different levels:
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5. Complete the following table to list the characteristics of each type of plate boundary (Divergent, Convergent, Transform).
Plate Boundary Information
Boundary Type
Sense of relative
motion
Map Symbol
Topographic Features
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Geologic Events
Modern Example
6. Plate boundaries and continental boundaries
a. Analyze the diagram below. Circle the portion that represents a plate boundary and
state the type of plate boundary.
b. Locate the two regions shown by the letter A in the figure above.
Which of the following best describes this area: Plate boundary, Convergent plate
boundary, active tectonic boundary, passive continental margin.
c. Describe the different characteristics of the following and provide a modern day
example for each type of margin.
A Passive Continental Margin
An Active Continental Margin.
d. For each of the following descriptions, indicate if it is an active or passive boundary. If
it is active, indicate the type of plate boundary.

Mountain building, volcanism, oceanic trench

Rift valleys, basaltic volcanism

Well sorted sediment that covers the boundary between continental and oceanic
crust. The sediment changes from quartz sandstone, to shale, to limestone. Few
earthquakes and no volcanism.

Distinct horizontal fault at the boundary. Many linear valleys and crustal
deformation along the fault. No volcanism.
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7. Examine the maps showing the continental configurations during the breakup of Rodinia
(Pannotia is a portion of Rodinia) and the formation of Pangea (600 million years ago to 302
million years ago).
Explain how the sequence shown is evidence of a Wilson Cycle
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Part II - Background information for using rock magnetism to determine
paleogeography and the history of plate motion
When rocks form they can acquire and record the current state of the Earth’s magnetic field at
the location of their formation. Minerals of igneous rocks align with the prevailing magnetic
field to produce a record of the magnetic field orientation at the time of crystallization. We think
of the current magnetic field orientation as Normal. This means there is a magnetic attraction
toward the north magnetic pole (near the north geographic pole) and a magnetic repulsion away
from the south magnetic pole. An iron-rich igneous rock forming today would record a normal
orientation.
(USGS Public Domain)
At various times in the past the orientation has been reversed. A rock crystallizing at this time
would have a magnetism pointed toward the south magnetic pole. The magnetic time scale
below provides a record of when the Earth’s magnetic field was either normal (black bars) or
reversed (white bars). Each time period is given an anomaly number. The age is shown in
millions of years (Ma).
(USGS, Public Domain)
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The magnetic time scale described on the previous page can be used to date rocks. The map below
shows the patterns as observed on the floor of the southern Atlantic Ocean, between South America
(SA) and Africa (AF). The darker double line in the center represents the current position of the midAtlantic ridge (divergent plate boundary).
1000
500
Scale
00
1000
2000
3000
Continen
t AF
Continent
SA
Ocean
Floor
Fracture Zone
8
21
Fracture Zone
21
21
8
8
5
5
13
5
5
5
8 13
5
8
21
21
21
1. Use the magnetic anomaly time scale to determine the age (in millions of years) of
Anomalies (sometimes called Chrons) 21, 8, and 5. List those ages:
2. Use the following steps to reconstruct the position of Africa (AF) relative to South America (SA) at
the time of Anomaly 21. Tracing paper will be provided.

Draw a colored line over each of the magnetic lineations of anomaly number 21 on the South
American side of the Mid-Atlantic Ridge (do not color the African side).

Place a piece of tracing paper over the figure and hold it in place with tape or paper clips.
Repeat the process described above for anomaly 21 on the African side of the Mid-Atlantic
Ridge. Note that anomaly 21 should be colored on the figure only for the South American side
and it should be colored on the tracing paper only for the African side.

With the tracing paper in place, trace the coastlines of Africa and South America on the tracing
paper with black pencil.

Detach the tracing paper and slide it toward the South American side until the colored line on
the tracing paper matches the colored line on the figure. When the two lines are matched as
closely as possible, hold the tracing paper in place and trace the coastline of South America in
colored pencil on the tracing paper.
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3. Use the tectonic map provided in the classroom to reconstruct the oceanic plate configuration
west of North America. Perform the following activities and answer the questions.
a. What two locations on the map have active Divergent Boundaries producing new oceanic plate
on both the east and west side of the boundary?
b. Look very closely at the symbols for subduction zone (open triangles vs filled triangles).
Which portion of western North America is shown as having an “active” subduction zone?
Which portion is “inactive”?
c. Where are the oceanic “magnetic anomalies” (chrons) the most offset? Along which Fracture
Zone?
d. Find Chrons (magnetic anomalies) 15, 10, 5A, and 0 on the top portion of the map. Each of
these chrons represents the divergent boundary at a specific time. Use the map magnetic time
scale to determine when each of these chrons was an active divergent boundary:
Chron 15:
Chron 10:
Chron 5A:
Chron 0:
Use this information about the Chrons to answer the following:
Which way is the Divergent Boundary moving over time (east or west)?
Describe what has happened to the oceanic plate that lies east of the divergent boundary
(toward the North American side).
e. Look at the Chrons below the lower most portion of the modern Gorda Ridge. Note the
positions of Chron 15, Chron 10, Chron 6C, and the San Andreas Fault.
Why are there no Chrons shown that are younger than 5C?
f. What geologic features in California are evidence of that the west coast of California was once
an active convergent plate boundary (consult the CA geologic maps in the classroom).
The animations of Tanya Atwater may help with understanding these questions. As a graduate
student, Tanya used the same magnetic anomaly information to “reconstruct” the tectonic
history of western North America.
http://tinyurl.com/g2atwater
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