The Earth has a core composed of two parts: the inner core and the outer core. The outer core is primarily liquid iron, and some nickel, and the inner core is primarily solid iron. For a while it was thought that the inner core was a single iron crystal, though more recent theories predict it is more heterogeneous, with irregular features on scales as low as one kilometer. What we know about the core has been derived from analysis of seismic waves as well as models based on accepted physics and chemistry.

The outer core begins at a depth of 2890–5150 kilometers (1790 – 3160 miles) – there is a degree of uncertainty about the precise depth. The inner core begins at a depth of 5150-6360 kilometers (3160-3954 miles). Above the outer core is the mantle, the largest portion of the Earth’s subterranean regions. In comparison to the core, the mantle is highly viscous and continuously circulates.

It is the outer core that is responsible for the Earth’s magnetic field. It freely circulates due to stirring caused by the Earth’s rotation, its dynamics being dictated by the Coriolis effect. The effect is similar to the circulation observed in pieces of pasta boiling in a pot. This constant circulation gives rise to the Earth’s magnetic field, in a process described by the dynamo theory. Although the inner core is too hot to maintain a permanent magnetic field, it probably helps stabilize the field generated by the outer core.

The Earth has not always had a solid inner core. Although the pressure there is extremely great, at one point it was so hot that the entire thing was liquid. It has cooled slowly over time, and is thought to be between 2 and 4 billion years old, younger than the Earth itself which is about 4.5 billion years old. The inner core was discovered in 1936 by Inge Lehmann.

Because the inner core is a solid suspended in a liquid, it can rotate independently of the Earth itself. Most geophysicists believe that the core rotates about a third of an additional degree over the surface every year. So for every thousand or so rotations by the Earth’s surface, the core rotates a thousand and one times.