Earth: Magnetism

It is not known what causes the magnetic field of the Earth. Ancient navigators knew of the wonders of the compass but not the science behind it, so they told of a large magnetic rock located in the north. In 1600, William Gilbert first proposed the theory that the Earth itself had a magnetic field. Today, scientists believe that the motions of the ions within the planet's liquid core causes the phenomenon.
The Earth has a strong enough magnetic field to be detected by a wide range of instruments. One of the most common tools utilized in navigation is the compass. The needle of a compass points toward the magnetic north pole.

The magnetic north pole, however, should not be misinterpreted as true north, where the North Pole is. The North and South Poles are defined to be the northern and southern ends of the Earth's axis about which it rotates constantly. The north which a compass needle points to is actually 1,300 km from true North. The angle difference between these two norths is termed the declination angle. Similar to its counterpart, magnetic south is approximately 2,550 km from true south.
Of course, if this isn't enough, there is a phenomenon known as magnetic pole wandering. Paul Serson and Jack Clark, Canadians in origin, first discovered this on accident. Solar winds cause daily shifts of up to 80 km. Irregularities in the particle motions in the liquid core also play an important role. On top of the magnetic pole movement over time, the Earth's magnetic field also changes direction every half a million years. It is not an overnight change, but a gradual one that may take a few thousands years to complete. There have been at least 171 north-south pole reversals in the past 75 million years.
Paleomagnetism is the study of the Earth's magnetic field in the past. Like geologists, paleomagnetists study rocks. When molten rock crystallizes, its crystals align with the Earth's magnetic field. By knowing this orientation, the place the rock was found, and its age (from radioactive dating or some other method), researchers can find out intriguing information from them.

Copyright © 2000 by Gary Chan and Matthew McDermott. All rights reserved.