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4.3: Magnetic Polarity Evidence for Continental Drift

Difficulty Level: Basic Created by: CK-12
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"How did technology play a role in developing Wegener's idea?"

After Wegener's death, the continental drift idea was pretty much dead. It would have remained that way except for the development of technology. Using technology, scientists would find more evidence that continents had drifted. They would also be able to find the mechanism. This type of magnetometer was one of their important tools.

Magnetic Polarity

Some important evidence for continental drift came after Wegener's death. This is the magnetic evidence. Earth's magnetic field surrounds the planet from pole to pole. If you have ever been hiking or camping, you may have used a compass to help you find your way. A compass points to the magnetic North Pole. The compass needle aligns with Earth’s magnetic field.

Some rocks contain little compasses too! As lava cools, tiny iron-rich crystals line up with Earth’s magnetic field. These crystals are magnetite crystals. Anywhere lavas have cooled, these magnetite crystals point to the magnetic poles. The little magnets point to where the north pole was when the lava cooled. A magnetometer is a device capable of measuring the magnetic field. A magnetometer can be used on land. A magnetometer also can be dragged behind a ship.

Evidence for Continental Drift

Geologists used magnetometers to look at rocks on land. They wanted to know which direction magnetite crystals pointed. This would tell them where magnetic north was at the time the rocks cooled. This is what they learned:

  • Magnetite crystals in fresh volcanic rocks point to the current magnetic north pole ( Figure below ).

Earth’s current north magnetic pole is in northern Canada.

  • Older rocks that are the same age and are on the same continent point to the same location. However, the location they point to is not the current north magnetic pole.
  • Older rocks that are different ages and are on the same continent do not point to the same location. None of them point to the current magnetic north pole.
  • Rocks on different continents that are the same age point to different locations. Only recent rocks point to the current north magnetic pole.

How did the geologists explain this? There is only one logical explanation. There was almost certainly only one north magnetic pole through Earth history. The north magnetic pole is very likely in the same spot it has always been. If these two things are true, then the continents have moved.

Support for Wegener's Idea

Geologist tested the idea that the pole remained fixed but the continents moved. They fitted the continents together as Wegener had done. They moved them into the positions where they had been at the time the magnetite crystals cooled. The magnetite crystals pointed to the current north magnetic pole ( Figure below ). The magnetic pole seemed to have moved, but had not. They named the phenomenon apparent polar wander .

On the left: The apparent north pole for Europe and North America if the continents were always in their current locations. The two paths merge into one if the continents are allowed to drift.

Geologists were now more interested in continental drift. More than ever, they needed a mechanism.

Vocabulary

  • apparent polar wander : The path on the globe showing where the magnetic pole appeared to move over time.
  • magnetite : A magnetic mineral that takes on Earth's magnetic polarity as it crystallizes.
  • magnetometer : An instrument that measures the magnetic field intensity.

Summary

  • Magnetite is a magnetic mineral found in lava. The magnetite points to the magnetic north pole when it cools.
  • Scientists used magnetometers to show where the north magnetic pole had been when magnetite crystals cooled.
  • Magnetite crystals of different ages and on different continents pointed to different spots. The simplest explanation is that the continents have moved.
  • Apparent polar wander is another line of evidence for drifting continents.

Practice

Use this resource to answer the questions that follow.

http://www.grossmont.edu/garyjacobson/animations.htm :

  1. Explain what appears to be occurring at the north pole.
  2. What direction is the pole moving?
  3. Describe the movement of the south pole.
  4. What direction is the south pole moving?

Review

  1. What is apparent polar wander?
  2. Describe how magnetite indicates magnetic pole. Why does it sometimes point to a spot that is not where the pole is located?
  3. How did scientists use magnetic evidence to conclude that the continents moved?

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Difficulty Level:

Basic

Grades:

6 , 7

Date Created:

Jan 04, 2013

Last Modified:

Aug 24, 2014
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