I have always been told that the geomagnetic field acts "as if" there were a bar magnet inside the Earth. I now this is not true, but I feel that knowing what is actually going on would help me understand the answer to the question that I'm about to ask:
Why does geomagnetic reversal happen? Is it something gradual or relatively fast?
(What is the origin of the geomagnetic field? Can we answer my questions starting from this mechanism?