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Magnetic Field Reversal

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Magnetic Field Reversal

Do you like science fiction? This is a fictional scene that is set in the future. Science fiction artists are so creative! They might even describe a time when compasses point south instead of north. Actually, this idea isn’t fiction—it’s a fact.

Changing Places

Earth’s magnetic poles have switched places repeatedly in the past. As you can see in the Figure below , each time the switch occurred, Earth’s magnetic field was reversed. The magnetic field is the region around a magnet over which it exerts magnetic force. We think of today’s magnetic field direction as “normal,” but that’s only because it’s what we’re used to. You can watch a simulation of a magnetic field reversal at this URL: http://www.pbs.org/wgbh/nova/earth/when-our-magnetic-field-flips.html

Fact, not Fiction

Scientists don’t know for certain why magnetic reversals occur, but there is hard evidence that they have for hundreds of millions of years. The evidence comes from rocks on the ocean floor. Look at Figure below . They show the same ridge on the ocean floor during different periods of time.

A. At the center of the ridge, hot magma pushes up through the crust and hardens into rock. Once the magma hardens, the alignment of magnetic domains in the rock is frozen in place forever. Magnetic domains are regions in the rocks where all the atoms are lined up and pointing toward Earth’s north magnetic pole.

B. The newly hardened rock is gradually pushed away from the ridge in both directions as more magma erupts and newer rock forms. The alignment of magnetic domains in this new rock is in the opposite direction, showing that a magnetic reversal has occurred.

C. A magnetic reversal occurs again. It is frozen in rock to document the change.

Rock samples from many places on the ocean floor show that the north and south magnetic poles reversed hundreds of times over the last 330 million years. The last reversal was less than a million years ago.

Q: What might happen if a magnetic reversal occurred in your lifetime? How might it affect you?

A: For ideas, learn more about Earth’s magnetic reversals at this URL: http://www.pbs.org/wgbh/nova/earth/when-our-magnetic-field-flips.html

Summary

  • Earth’s magnetic poles have switched places repeatedly over hundreds of millions of years, each time reversing Earth’s magnetic field.
  • Magnetic domains in rocks on the ocean floor provide evidence for magnetic field reversals in the past.

Vocabulary

  • magnetic field : Area around a magnet where it exerts magnetic force.

Practice

At the following URL, listen to the streaming audio (or read the article) about magnetic field reversals. Then answer the questions below. http://science.nasa.gov/science-news/science-at-nasa/2003/29dec_magneticfield/

  1. How has Earth’s magnetic field changed during the past 200 years?
  2. How often do magnetic field reversals occur? When did the last one take place?
  3. How long does it take for a magnetic field reversal to occur? Describe what happens to Earth’s magnetic field during that time.

Review

  1. What is a magnetic field reversal?
  2. Explain how rocks on the ocean floor provide hard evidence that magnetic reversals occurred in the past.

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