[inquest]

What about the second possibility I raised? It appears that there are at least two main gyres circulating inside the globe in opposite directions from each other. That would explain why at the crust is splitting in two directions at the mid-Atlantic ridge. If they're moving in opposite directions, then the fields they're generating would largely cancel each other out, unless one is a bit stronger than the other. So all it would take for a reversal of polarity would be a change in the relative strength of the two gyres, so that the other one becomes dominant.

[RightWhale]

The problem I have with that is that the material would be molten and electrically conductive, and so would lack a net electrical charge and also lack dipoles set in crystalline structure. Magnets are made of solid-state material or moving charge fields. There would be no mechanism to generate a magfield.