The magnetic axis of celestial objects flips periodically, including a complete reversal (by 180 degrees). For example, the sun's magnetic axis reverses direction every 11 years. However, the Earth's geomagnetic axis flips much less often. The last time it reversed completely was 780,000 years ago, a phenomenon called the Matuyama-Bunhes reversal after the two scientists who described it. Since then, the Earth's magnetic field has tried to flip on 10 occasions, but on each occasion it has reverted back to its current axis.
It's fair to say therefore that such reversals happen very infrequently for the Earth. When it does happen though, it affects the polarity of magnetic material in lava flows, sea beds etc which can be detected in rocks and fossils.
The Curie temperature is one above which magnetic substances (which includes all minerals containing iron, nickel and cobalt) lose their magnetic properties. This varies between 580 and 680 degrees Celsius for the oxides of iron -Fe2O3 and Fe3O4. Conversely, when cooled below this temperature, such objects regain their magnetism. Thus, igneous rocks have inherent magnetism dating back to the time when they were formed from cooling lava flows millions of years ago.
The reversal of Earth's magnetic axis is not instantaneous- it occurs slowly- over thousands of years. During this period, magnetic material in cooling magma will take up the polarity of the reversed magnetic polarity of Earth, and the rocks formed therefrom would reflect this reversed polarity for ever. Thus, geologists can analyse such rocks, or fossils which have enclosed magnetic material in sea beds and make a fair guess as to their age. This is one way of fossil or rock dating.
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