Download Explore 2: Shifting Plates and Wandering Poles

Explore 2: Shifting Plates and Wandering Poles
Target Concept
Certain types of rocks “record” the intensity and direction of Earth’s magnetic field at the time the rocks
form. These slightly-magnetized rocks provide further evidence for plate tectonics.
Targeted Standards
• SA-7
Addressed Standards
• SA-5
Assessment Task
Students will model magnetic reversal patterns and use paleomagnetic data to demonstrate plate motion.
Background
Some rocks are naturally magnetized. How? When igneous rocks cool down from the molten stage, or
when sand grains gently settle down through water, the iron-rich molecules tend to align themselves with
Earth’s magnetic field. It is the alignment of atomic forces within certain rocks that creates a magnetic
“pull.” When a volcano erupts, tiny grains within the cooling rocks “point north,” so to speak. The rocks
record where magnetic north was, relative to them, at the time of formation.
However, magnetic north and south have switched places many times throughout Earth’s history! The
magnetic north pole sometimes becomes the magnetic south pole and vice versa. This, too, is recorded
in the rocks.
In 1963, two English geophysicists, Vine and Matthews, used extremely sensitive magnetometers to
make magnetic measurements of the seafloor across the Mid-Atlantic Ridge. They discovered bands of
rocks, parallel to the Mid-Atlantic Ridge, where the rocks showed “normal” polarity. (That is, the polarity
at the time those rocks formed was the same as it is today: north is north and south is south.). They
also found bands where the rocks showed “reversed” polarity. (I.e. the south magnetic pole was acting
like the north magnetic pole at the time those rocks were formed.) What was interesting to Vine and
Matthews was that (a) the bands formed parallel to the ridge and (b) the pattern on both sides of the ridge
was the same!
Vine and Matthews proposed that lava was oozing out of a crack that ran the length of the ridge, creating
new sea floor (new crust). As the ridge continued to erupt, the slightly older sea floor got pushed aside.
This was a big break-through for the theory of plate tectonics. It gave a mechanism for sea-floor
spreading. It explained the magnetic patterns that Vine and Matthews observed and provided a
mechanism for the continents to have split apart. When Vine and Matthews later measured the ages of
the rocks on either side of the Mid-Atlantic Ridge, what did they find? Rocks right next to the ridge were
very young, and the rocks got progressively older as you moved away from the ridge!
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The following activity is from Seismic Sleuths. It starts by showing how patterns of magnetic polarity can
be recorded in rocks. (Students magnetize strips of cassette tape and form patterns.) Students then
use a paper model of a mid-ocean ridge to “create” new crust and re-create Vine and Matthews’ work.
Finally, students use magnetic data collected from several different continents to establish how those
continents may have moved relative to each other.
Vocabulary
paleomagnetism
Procedure
Follow the instructions in the attached activity, “Shifting Plates and Wandering Poles” which is located in
Appendix K. This activity is used by permission of the Federal Emergency Management Agency.
(FEMA/AGU. “Seismic Sleuths—Earthquakes—A Teacher’s Package on Earthquakes for Grades 7-12,”
American Geophysical Union, Washington, DC, 1994, 367 pp.)
Generalize and Assess
See the lesson.
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Copied fromAGU/FEMA s3eismic Sleuths Earhquakes A Teacher’s Package for Grades 7-12, 1995
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Copied fromAGU/FEMA s3eismic Sleuths Earhquakes A Teacher’s Package for Grades 7-12, 1995
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Copied fromAGU/FEMA s3eismic Sleuths Earhquakes A Teacher’s Package for Grades 7-12, 1995
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Copied fromAGU/FEMA s3eismic Sleuths Earhquakes A Teacher’s Package for Grades 7-12, 1995
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Copied fromAGU/FEMA s3eismic Sleuths Earhquakes A Teacher’s Package for Grades 7-12, 1995
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Copied fromAGU/FEMA s3eismic Sleuths Earhquakes A Teacher’s Package for Grades 7-12, 1995
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Copied fromAGU/FEMA s3eismic Sleuths Earhquakes A Teacher’s Package for Grades 7-12, 1995
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Copied fromAGU/FEMA s3eismic Sleuths Earhquakes A Teacher’s Package for Grades 7-12, 1995
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Copied fromAGU/FEMA s3eismic Sleuths Earhquakes A Teacher’s Package for Grades 7-12, 1995
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Copied fromAGU/FEMA s3eismic Sleuths Earhquakes A Teacher’s Package for Grades 7-12, 1995
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Copied fromAGU/FEMA s3eismic Sleuths Earhquakes A Teacher’s Package for Grades 7-12, 1995
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Copied fromAGU/FEMA s3eismic Sleuths Earhquakes A Teacher’s Package for Grades 7-12, 1995
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Copied fromAGU/FEMA s3eismic Sleuths Earhquakes A Teacher’s Package for Grades 7-12, 1995
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Copied fromAGU/FEMA s3eismic Sleuths Earhquakes A Teacher’s Package for Grades 7-12, 1995
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Copied fromAGU/FEMA s3eismic Sleuths Earhquakes A Teacher’s Package for Grades 7-12, 1995
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Copied fromAGU/FEMA s3eismic Sleuths Earhquakes A Teacher’s Package for Grades 7-12, 1995
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Copied fromAGU/FEMA s3eismic Sleuths Earhquakes A Teacher’s Package for Grades 7-12, 1995
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