Why will the Earths magnetic poles aventually reverse?
It all happens in the earth’s molten metal core. The theory of the generation of the net equatorial electric current, which generates the dipole magnetic field, is that it is a residual imbalance from lots of much stronger shorter current loops which interact and feed back into each other in uncomputable ways. It’s like a chaotic system – a tiny internal change can cause a big external one. The historical evidence, as other posters have said, is that the system has enough inertia to keep its net dipole direction stable for about 100,000 years at a time, although the intensity varies over that period. At Newcastle-on-Tyne in the early 1960’s, two experimenters constructed a simple mechanical model with two rotors, whose nett magnetic polarity was seen to oscillate smoothly but unpredictably even though the rotors were going at constant speed.
