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?
The Earth's magnetic field can't be a permanent bar magnet. Permanent bar magnets aren't that permanent. A bar magnet would only last a few tens of thousands of years before it decayed. That's extremely short in geological terms. The Earth's magnetic field has been around for billions of years. Something has to be constantly regenerating the magnetic field. Per the widely-accepted dynamo theory that "something" is moving conductive liquids in the Earth's liquid outer core.
Recent supercomputer simulations of the Earth's dynamo have finally shown a dynamo that lasts longer than the ohmic decay, and those simulations have also shown field reversals. They appear to be an emergent feature of a chaotic system. There's no cause per se.
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