What do you think when I say magnet?
Maybe something like this:
So what do you think If I tell you that this is a magnet?
Our Earth behaves as a big magnet. It is like there is a magnetic bar inside, with two poles.
The hypothesis that there is a magnetic bar inside the Earth is inconceivable because at the Curie temperature (768 °C for the iron) the magnetic field destroys, at the centre of the Earth the temperature is much bigger! (around 5000 °C). So the magnetic field could not conserve.It was discovered that the magnetic field is generated by the convection of the outer nucleus. So why the convection of the mantle doesn’t generate the magnetic field? First of all because the material in the mantle is viscous while in the nucleus it is very fluid, secondly because in the nucleus there is the presence of iron that is a good electrical conductor. Rapid movements in the outer nucleus can produce electrical currents in the iron, generating a geodynamo with a big magnetic field.
The rocks record the field magnetic orientation, in fact studying the ocean floor it was discovered that the magnetic field was reversed many times! When the lava comes out from the oceanic ridges some of her crystals orient according to the current magnetic field and when the lava cools and forms rocks they maintain the magnetic field orientation.
The chrons (normal of inverse epochs) last about a half million years. The chrons are interrupted by some subchrons that are short.