Can anybody explain pole shift?

Question

is it the earths crust moving or the magnetic fields changing? like north becomes south and south becomes north?

Answer 1

A planet's axis can shift if, for example, a mountain is thrust up in high lattitudes.  The mountain's mass is an imbalance, and tends to shift the pole so that the mountain is on the equator.

Answer 2

If you're talking about the magnetic poles, they aren't fixed, they slowly move around and change in intensity and this is believed to be due to changes in the eddy currents within the Earth's iron core.  They are independent of the geographic poles and often reverse every 10,000 years or so.

Answer 3

Popular opinion is that it's caused by the molten core of the earth but I'm sure there is more to it than that.

Answer 4

I don't believe that anyone can accurately explain pole shift since the mechanism of earth's magnetic field isn't even fully understood.  There are some great theories out there that have yet to be substantiated.  

Wikipedia's page on polar reversal does touch on pole shift (see link below)