BOX 3.2 Rock magnetism

There are two different kinds of magnetization found in rocks: induced and remanent. A rock has induced magnetization if the rock ceases to be magnetic when you place the rock in an area that has no magnetic field. On the other hand, remanent magnetization is frozen in the rock when the rock solidifies from molten materials (or more rarely, when magnetite grains are cemented in place in a sedimentary rock); in a field-free area, it will still be magnetic and the direction of the magnetization will be that of the Earth's magnetic field at the time the magnetization was acquired. Studies of remanently magnetized rocks show that the magnetic north and south poles have flipped back and forth through geologic time. About 730,000 years ago, our ancestors' compass needle would have pointed south instead of north. On map GP-1007, almost all of the magnetic anomalies are caused by rocks with induced magnetization.


You can take a paper clip or steel nail and cause it to be magnetic for a short time. Take two paper clips. Do they attract each other? Now take a bar magnet. Hold the magnet in one hand and carefully put a paper clip next to the magnet's end so that the magnet's attraction holds one end of the paper clip. Now carefully place a second paper clip at the end of the first paper clip so that the attraction of the magnet can also hold it. Attach a third paper clip to the end of the second paper clip. How many paper clips can you connect in this magnetic chain? What determines how many paper clips you can connect in this fashion? Now carefully separate the first paper clip from the magnet. What happens? Even without the magnet, the three paper clips will remain attached to each other for a short period of time. How long did they stay attached?

This exercise helps illustrate how something that is not magnetic can become magnetic when placed in a strong magnetic field (the magnet's), but, once removed, loses its magnetization.

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