# magnetic field reversal

## fact not fiction

Scientists dont 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 1.2. 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 Earths 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.

## changing places

Earths magnetic poles have switched places repeatedly in the past. As you can see in the Figure 1.1, each time the switch occurred, Earths magnetic field was reversed. The magnetic field is the region around a magnet over which it exerts magnetic force. We think of todays magnetic field direction as normal, but thats only because its what were used to.

## instructional diagrams

No diagram descriptions associated with this lesson

## questions

earths magnetic poles have switched places repeatedly in the past.

``````-->  a. true

b. false
``````

scientists know for certain why magnetic field reversals occur.

``````a. true

-->  b. false
``````

evidence for magnetic field reversals

``````a) covers 100s of millions of years.

b) was discovered on the ocean floor.

c) comes from the magnetic domains of rocks.

-->  d) all of the above
``````

when did the last magnetic reversal occur?

``````a) more than 330 million years ago

b) about 300 million years ago

c) about 100 million years ago

-->  d) less than 1 million years ago
``````

rocks on the ocean floor far from a ridge

``````-->  a) are older than rocks close to the ridge.

b) always have reversed polarity.

c) keep reversing the polarity of their magnetic domains.

d) two of the above
``````

after magma hardens, the alignment of magnetic domains reverses with each magnetic field reversal.

``````a. true

-->  b. false
``````

## diagram questions

No diagram questions associated with this lesson