24.2. Earth as a Magnet http://www.ck12.org
Magnetic Field Reversals
Do you like to read science fiction? Science fiction writers are really creative. For example, an author might write
about a time in the distant past when compasses pointed south instead of north. Actually, this idea isn’t fiction—it’s
a fact! Earth’s magnetic poles have switched places repeatedly over the past hundreds of millions of years, each time
reversing Earth’s magnetic field. This is illustrated inFigure24.13.
FIGURE 24.13
We think of today’s magnetic pole orientation as "normal" only because that’s what we are used to.Scientists don’t know for certain why magnetic reversals occur, but there is hard evidence showing that they have
occurred. The evidence comes from rocks on the ocean floor. Look atFigure24.14, which shows a ridge on the
ocean floor. 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. The newly hardened rock
is then gradually pushed away from the ridge in both directions as more magma erupts and newer rock forms. Rock
samples from many places on the ocean floor reveal that magnetic domains of rocks from different time periods are
aligned in opposite directions. The evidence shows that Earth’s magnetic field reversed hundreds of times over the
last 330 million years. The last reversal was less than a million years ago. What might happen if a magnetic reversal
occurred in your lifetime? How might it affect you? You can learn more about Earth’s magnetic reversals at this
URL: http://www.pbs.org/wgbh/nova/earth/when-our-magnetic-field-flips.html.
Why Is Earth a Magnet?
The idea that Earth is a magnet is far from new. It was first proposed in 1600 by a British physician named William
Gilbert. However, explaining why Earth acts like a magnet is a relatively recent discovery. It had to wait until the
development of technologies such as seismographs, which detect and measure earthquake waves. Then scientists
could learn about Earth’s inner structure (seeFigure24.15). They discovered that Earth has an inner and outer core
and that the outer core consists of liquid metals, mainly iron and nickel. Scientists think that Earth’s magnetic field
is generated by the movement of charged particles through the molten metals in the outer core. The particles move