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?
Most estimates for the duration of a polarity transition are between 1,000 and 10,000 years.
Source: a not very reliable site (but do check its sources in turn). $\endgroup$ – Renan Jul 25 '14 at 18:41