The core has a radius of 3480 km, which is a little more than half of the radius of the Earth. It consists of a solid inner core (radius of 1215 km) and a liquid outer core. The core is composed of 90% iron. It is a fairly good electrical conductor, with a conductivity estimated at 300,000 Siemens/m. To a good approximation (details), the flow of the core fluid carries along the magnetic field, as if it were frozen in. This leads to the gradual changes observed in the geomagnetic field at the Earth's surface.

The Main Magnetic Field originates from a dynamo process in the fluid outer core of the Earth. It strongly dominates over the various other contributions to the geomagnetic field, accounting for over 95% of the field strength observed at the Earth's surface.