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Evolution requires millions of years to work. Creationist believe the earth is less than 10,000 years old. The earth's magnetic field is decaying rapidly, at a constant (if not decreasing) rate. At this rate, 8000 years ago the earth's magnetism would have equaled that of a magnetic star, a highly unlikely occurrence. Also, if electric currents in the earth's core are responsible for the earth's magnetism, the heat generated by these currents 20,000 years ago would have dissolved the earth.
Clearly the magnetism of the earth is the result of a dynamic rather than a passive condition, which would be the case if the iron core of the earth were solid and passively magnetized. Iron does not retain a permanent magnetism at temperatures above 540° C (1000° F), however, and the temperature at the center of the earth may be as high as 6650° C (12,000° F). The dynamo theory suggests that the iron core is liquid (except at the very center of the earth where the pressure solidifies the core), and that convection currents within the liquid core behave like the individual wires in a dynamo, thus setting up a gigantic magnetic field. The solid inner core rotates more slowly than the outer core, thus accounting for the secular westward drift. The irregular surface of the outer core may help to account for some of the more irregular changes in the field. This dynamic system is the reason the magnetic poles change location with time, a phenomenon known as polar wandering, and the direction of wandering has been observed to reverse. During geologic history the polarity of the earth's magnetic field has periodically reversed. They know this by studying ancient volcanic rocks. As they cooled, they "froze" with their minerals oriented in the magnetic field existing at that time.