Lesson Title: Magnetic Polarity and Sea Floor Spreading
Motivating Questions:
How is our understanding of plate tectonics applicable to the study of geology,
environmental issues, societal problems or even the exploration for oil and commercially
important minerals?
Key Vocabulary:
Continental Drift Theory: A theory that originally proposed that the continents are rafted
about. It has essentially been replaced by the plate tectonics theory.
Convergent boundary: A boundary in which two plates move together, causing one of
the slabs of lithosphere to be consumed into the mantle
as it descends beneath an overriding plate.
http://www.cotf.edu/ete/modules/msese/earthsysflr/plates2.html
Divergent boundary: A region where the rigid plats are moving apart, typically by the
mid-oceanic ridges. http://www.cotf.edu/ete/modules/msese/earthsysflr/plates2.html
Hot spots: A concentration of heat in the mantle capable of producing magma which, in
turn, extrudes onto the Earth’s surface. A good example is the Hawaiian Islands.
Normal polarity: A magnetic field that is the same as that which exists today.
Paleomagnetism: The natural remnant magnetism in rock bodies. The permanent
magnetization acquired by rock which can be used to determine the location of the
magnetic poles and the latitude of the rock at the time it became magnetized.
Image courtesy of US Geological Survey.
Pangaea: The proposed super-continent which 200 million years ago began to break
apart and form the present land masses. http://geology.com/pangea.htm
Plate: One of numerous rigid sections of the lithosphere that moves as a unit over the
material of the asthenosphere.
Plate tectonics: The theory which proposes that Earth’s outer shell consists of individual
plates which interact in various ways, thereby creating earthquakes, volcanoes, mountains
and the crust itself. http://geology.er.usgs.gov/eastern/plates.html
Polar wandering: As the result of paleomagnetic studies in the 1950’s, researchers
proposed that either the magnetic poles migrated greatly through time or the continents
had gradually shifted their positions.
Reverse polarity: A magnetic field opposite to that which exists at present.
Seafloor spreading: The process of producing new sea floor between two diverging
plates. http://www.pmel.noaa.gov/vents/nemo/explorer/concepts/spreading.html
Instructor Notes:
You will need to have compasses available for students. Make sure they understand why
the needle points to the north and how the compass is used for navigation today. Use
peer teaching to help those students who are not familiar with working with a compass.
Procedure:
ENGAGE
Show the following photos and have students identify and discuss what is happening
between South America and Africa.
In 1858, geographer Antonio Snider-Pellegrini made these two maps showing his version
of how the American and African continents may once have fit together, then later
separated. Left: The formerly joined continents before their separation. Right: The
continents after the separation. (Reproductions of the original maps courtesy of
University of California, Berkeley.)
EXPLORE
Students will work in teams of two using the Seafloor Spreading model made for this
activity to study the floor of the Atlantic Ocean. Their study will provide unusual
information about the Earth’s crust in this area. Students will have the opportunity to
make decisions about where to place the compass to conduct their study. As the students
use the compass and pass it over the magnets they will observe that at times the compass
needle points to the north and at other times it points to the south.
EXPLAIN
Explain to the students that they need to stop and mark any spots along the dashed line
where the compass needle is perfectly aligned in the north-south direction. They will
need to make a mark with a pencil to illustrate the spot where they stopped. Make sure
they do not lift their compasses. As they were moving the compass they were modeling
traveling over an area that contains rock with normal polarity and that the north-south
alignment of the compass needle is a clue that they are about to move over an area that
has reverse polarity. On the west side of the mark, write “N” for normal and on the east
side write “R” for reverse polarity. Let them know that on the other side of the mark the
polarity has switched again, back to normal. They will write “N” on the east side of the
mark. See illustration below.
Side A West
N
-----------------R R
N
------------
Students will now continue with the same procedure along the entire top and bottom edge
of the cardboard. They will need to stop and mark each point where the needle is aligned
in a north-south direction. If they have time, let them double check each others work
within their teams.
Students should be able to identify and discuss the fact that they discovered similar
patterns on the top and bottom of the cardboard. Have them draw vertical lines through
the marks on the top and bottom of the cardboard. Students will hypothesize what these
lines represent.
Side A West
N
A
R
R
B
Side B East
N
N
C
D
R
R
E
N
F
Inform the students that volcanic vents are located along the middle of the Atlantic
Ocean. This model tries to illustrate the divergent boundaries where the magma rises to
the surface. By studying the patterns of polarities of rock that makes up the sea floor, they
can estimate how the sea floor has moved over time. Students will be able to discuss the
polarity of the rock in sections C and D (nearest the ridge). They should also be able to
discuss the type of polarity found in sections B and E and what it means.
Let students identify the oldest and youngest rocks in this model. Discuss how this
model represents the Atlantic Ocean. If continental drift and seafloor spreading is
correct, then what is the rate at which South America and Africa are drifting apart.
Students will measure the distance of each mark form the ridge crest. One cm is
equivalent to 10 km. Every 80 km equals 4 million years. Students can complete the
math and record their results in a table of their creation.
Here are the results the students could obtain. These distances are approximate; theirs
may differ depending on where you located the magnets.
Section Distance from center Distance from Ridge Age
Of model (cm)
Crest in Km
(millions of years)
A
20
200
10
B
8
80
4
C
0
0
0 (current)
D
0
0
0 (current)
E
8
80
4
F
20
200
10
ELABORATE
Students will now decide a research topic and describe the historical development of
the theories of plate tectonics, including continental drift and sea floor spreading. A
written report or oral presentation will be conducted. Their research will:
 focus on Wegner and his supporters that gave rise to the theory of continental
drift, or
 identify the first evidence that led some to suspect the continents were once
connected, or
 identify the super-continent Pangaea and how and when it split apart, or
 explain why the discovery of the fossil remains of Mesosaurus in both South
America and Africa, but nowhere else, supports the continental drift hypothesis.
Students will share their information to the class.
EXTENSIONS/ADAPTATIONS
Students are encouraged to use the internet and make a power point presentation to
accompany their report.
EVALUATE
Students will be evaluated on their research project/presentation. A basic rubric for
presentations will be used.
Students will also respond to the following questions.
1. What is seafloor spreading? Where is active seafloor spreading occurring today?
2. What did you discover about the polarity of rocks on the ocean floor in the
Atlantic Ocean?
3. Models are a good thing to use to illustrate concepts that are too small, too large
or impossible to visit. What was good about this Sea-floor spreading model and
what were its limitations?
4. What is one new thing that you learned from completing this lesson? Why is it
important
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