Name: ________________________
Due Date: _____________________
Per: ________
40 minutes
Magnetic Polarity Reversals
Introduction:
While no one is quite sure why reversals in Earth’s magnetic field occur, the
evidence for reversals is permanently recorded in Earth’s crust. As magma along
mid-ocean ridges (spreading centers) cools and solidifies, magnetic minerals
align themselves with Earth’s magnetic field. The magnetic minerals store a
permanent record of the direction of Earth’s magnetic field at the time. These
recordings are not only helpful in understanding a part of Earth’s geologic history,
but they also provide the data necessary to make comparisons of spreading rates
in different ocean basins.
Figure 1 shows the magnetic reversals that have occurred in three different
ocean basins over the last 80 million years. In this investigation, you will use the
patterns of magnetic polarity reversals to compare rates of seafloor spreading.
Materials: Three colored pencils, Scrap paper
Procedures:
1. Along the time scale at the top of Figure 1, locate and mark 25, 50, and 75
million years.
2. Draw a straight line from those three dates down to the pattern for the
South Atlantic Ocean. Using the slanted lines already drawn as guides,
draw straight but similarly slanted lines to match the South Atlantic pattern
to those for the North Pacific and Pacific Antarctic oceans.
3. Using your 3 colored pencils, shade in the portion of each pattern between
0 & 25, 25 & 50, and 50 & 75 million years.
4. Lay the edge of a sheet of scrap paper on the South Atlantic reversal
pattern beside the interval you have just colored for 0 to 25 million years.
Mark off the distance on the edge of the scrap paper. Transfer the marked
distance to the appropriate bar in Figure 2 for the South Atlantic (you may
also use a ruler). Repeat for each of the intervals on each of the other
patterns.
5. Draw a straight line across the top of each bar at the distance you have
marked. Using the same colors you used on the magnetic reversal pattern,
shade in each column to the height of your line.
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Dynamic Crust
Magnetic Polarity Reversals
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Dynamic Crust
Magnetic Polarity Reversals
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Conclusion Questions:
1. Is Earth’s current polarity normal or reversed? _________________________
2. What does polarity mean? _________________________________________
______________________________________________________________
3. How does your bar graph in Figure 2 for the South Atlantic show that the
spreading rate has been relatively constant over the last 75 million years?
______________________________________________________________
4. For the North Pacific, in which time interval was the spreading rate…
Fastest? ________________ my
Slowest? ___________________ my
5. For the Pacific Antarctic, in which time interval was the spreading rate…
Fastest? ________________ my
Slowest? ___________________ my
6. In Figure 1, use the distance scale for each of the three ocean basins to
determine (to the nearest 100 km) the total distance from the spreading
center to the seafloor that is 75 million years old (the oldest dates you have
colored) for the…
South Atlantic. ___________________________________
North Pacific. ____________________________________
Pacific Antarctic. __________________________________
7. According to your results in question 6, for which ocean basin has the overall
spreading rate been slowest? _______________ fastest? _______________
8. Using values from question 6, calculate the spreading rate (in cm/year) for…
Slowest ocean basin
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Dynamic Crust
Fastest ocean basin
Magnetic Polarity Reversals
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9. At a location in the Atlantic Ocean where the spreading rate is 1.5 cm/yr, the
distance across the entire ocean is 6600 km. How many million years has the
Atlantic Ocean taken to open to that distance at that location? (The distance
from the ridge to the coast is half the distance across the ocean.)
Show your work!
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Dynamic Crust
Magnetic Polarity Reversals
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