Lesson 5.5
Magnetic Evidence for Seafloor Spreading
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Use the resource below to answer the questions that follow.
 Plate tectonics: Evidence of plate movement at
1. What is the most obvious piece of evidence that the plates were once joined
together? Does this mean without doubt that they were once joined together? Why or
why not?
2. What is in the middle of the Atlantic Ocean?
3. What does a cooling basalt do with respect to the magnetic field?
4. What pattern of magnetic minerals would you expect in basalt rock if the magnetic
field never changed?
5. How does the magnetic orientation of minerals in basalts older than around 780,000
years differ from those that are younger than that age?
6. What happens to magnetic orientation in rocks in a sequence?
7. How are rocks right at the Mid-Atlantic Ridge oriented magnetically? How about
rocks just out from those?
8. What pattern in magnetic orientation is seen across the seafloor?
9. What do scientists say about rocks that point in the opposite direction to the current
magnetic field and are the same distance from the ridge?
Review
1. Describe the pattern the magnetic stripes make in the ocean floor.
2. How does magnetic polarity reveal the age of a piece of seafloor?
3. What other indications do scientists have regarding the age of the seafloor in various
locations?