Earth Science - Tuckey
Magnetism Activity
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Name:
Pole:
Patterns of Magnetic Polarity Reversals
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. In fact, oceanic crust has been likened to a tape recorder moving on a giant
seafloor conveyor belt. As magma along seafloor 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 they form. 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.
The figure 13.1 shows the magnetic reversals that have occurred in three different ocean basins over the last 80
million years. In this investigation, you will use patterns of magnetic polarity reversals (north and south switch
places) to compare rates of seafloor spreading.
You will need: at least three different colors of markers/pens/pencils/crayons; a straightedge; some scrap paper.
Procedure
1. Along the time scale at the top of the figure, locate 25, 50, and 75 million years. With a straight edge, draw
a line from those three dates straight down to the patterns for the South Atlantic. Then 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.
2. Using any color, shade in the portion of each pattern between 0 and 25 million years. With a different color,
shade in the portion between 25 and 50 million years on each pattern. Repeat with a different color for the
portion between 50 and 75 million years.
3. 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 below the reversal patterns for the South Atlantic in Figure 13.2. Repeat for
each of the intervals on each of the other patterns.
4. 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 patterns, shade in each column to the height of your line.
Analysis and Conclusion
1. Is Earth's current polarity normal or reversed? What does normal polarity mean?
2. How does your bar graph in Figure 13.2 for the South Atlantic support the idea that the spreading rate has
been relatively uniform there over the last 75 million years?
3. For the North Pacific, in which time interval was the spreading rate
a. fastest?
b. slowest?
4. In Figure 13.1, use the distance scale for each of the three ocean basins (they are all different scales) to
determine to the nearest 100 kilometers, the total distance from the spreading center to the seafloor that is 75
million years old (the oldest dates you have colored) for the
a. South Atlantic.
b. North Pacific.
c. Pacific Antarctic.
5. According to your results in question 5, for which ocean basin has the overall spreading rate been the
slowest? Which basin has had the fastest spreading?
6. Using your values from question 5, calculate the spreading rate, in centimeters per year, for both the slowest
and the fastest spreading centers. Remember: rate is simply distance divided by time; there are 100,000
centimeters in one kilometer.
7. [BONUS] At a location in the Atlantic Ocean where the spreading rate is 1.5 cm/year, the distance across
the ocean is 6,600 kilometers. How many million years has the Atlantic Ocean taken to open to that distance
at that location? Remember: the spreading center is in the center of the ocean, not at the edge; time can be
found by taking distance and dividing by rate.