Magnetism Measurements Reveal the Earth's Metallic Core
Earth's magnetic field is believed to be formed by the convection of hot, molten iron in the outer core. There must be significant amounts of iron for Earth to have such a strong magnetic field.
The Geographic North Pole is the axis of Earth's rotation. It is is currently offset from the Magnetic North Pole by about 11.5 degrees (same with the South Poles). The magnetic poles are very slowly wandering relative to the geographic poles. This wondering of the magnetic poles is caused by gradual changes of Earth's magnetic field. Current thought is that shifts in Earth's magnetic field are probably caused by changes in gravity-driven flowing currents in the planet's liquid-metallic outer core.
Magnetic Reversals: The Earth’s magnetic field reverses, causing the locations of the north and south magnetic poles to switch. If a magnetic reversal were to occur today, then a magnetic compass would point to the South Magnetic Pole instead of the North Magnetic Pole. Geologists and geophysicists have determined that magnetic reversals have happened many times through geologic time. Magnetic reversals are preserved in the “paleomagnetic record” - preserved as weak magnetic fields locked into rocks bearing magnetic minerals at the time they form (see the Paleomagnetism discussion below). We think that the magnetic reversals are probably caused by shifting currents in Earth's liquid metallic outer core. When a magnetic reversal occurs, basically what happens is the north magnetic pole becomes the south magnetic pole, and vice-versa. This switching of magnetic poles can last for periods ranging from thousands millions of years. Hundreds of magnetic reversals are recorded in the geologic records, observed in rocks on continents and the seafloor in many regions around the globe where rocks of all ages are preserved.