The Earth has a core composed of two parts: the inner and the outer portion. The outer core is primarily liquid iron, and some nickel, while the inner is primarily solid iron. For a while, it was thought that the inner portion was a single iron crystal, though more recent theories predict it is more likely made of dissimilar parts, with irregular features. What scientists 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 1,790 to 3,160 miles (2,890 to 5,150 kilometers), and there is a degree of uncertainty about the precise depth. The inner section begins at a depth of 3,160 to 3,954 miles (5,150 to 6,360 kilometers). Above the outer section is the mantle, the largest portion of the Earth’s subterranean regions. In comparison to the deeper layers, the mantle is highly viscous and continuously circulates.
It is the outer section that is responsible for the Earth’s magnetic field. It freely circulates due to stirring caused by the Earth’s rotation, with its dynamics 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 referred to as 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 portion.
The Earth has not always had a solid inner core. Although the pressure there is extremely high, 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 section 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 it rotates about a third of an additional degree over the surface every year. So for every 1,000 or so rotations by the Earth’s surface, the inner portion rotates 1,001 times.