A MAGNET INSIDE THE EARTH?
When it became known that the interior of the earth is hot, it was obvious that the earth's magnetic field could not be a permanent magnet. This is because heating disorients various parts of a magnet destroying the ability to produce a coherent magnetic field. Thus arose the puzzle of where the earth's magnetic field comes from. Scientists now believe it to be caused by circulation of liquid metal in the outer core:
If friction can ionize the metal atoms, then the flow of ionized material becomes an electric current, which produces the magnetic field. Such a mechanism is known as a dynamo, a device that converts mechanical energy of motion into electrical energy. Thus the earth is more of an electromagnet than a permanent magnet.
I n appearance the magnetic field of the earth resembles that of a bar magnet inclined slightly to the earth's axis of rotation. The magnetic lines of force run between the north and south polar regions of the earth, much as the pattern formed by iron filings sprinkled around a bar magnet does. The intensity of the magnetic field decreases away from the earth's surface, but the magnetic field can still be measured many tens of thousands of kilometers out in space.
However it began, the earth's magnetic field has changed polarity (the north magnetic pole becomes the south magnetic pole and vice versa) many times over geologic time. Scientists trace the history of these changes by studying the magnetism frozen into rocks of different ages: Iron particles in molten lava beds align themselves along the lines of the existing magnetic force, and after the rocks solidify, they retain the orientation of the magnetic field indefinitely. Such rocks show that magnetic reversals have come at intervals as short as 35,000 years. Why the reversals? We do not know. One suggestion is that they may be related in some way to changes in the earth's rotation or in the fluid state of its outer core.
