5.81. Magnetic Field Reversal http://www.ck12.org
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 atFigure5.178. They show the same
ridge on the ocean floor during different periods of time.
FIGURE 5.178
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.