Earth’s solid inner core rotates slightly faster than the rest of the Earth
The solid iron inner core is about the size of the Moon. The inner core rotates relative to the mantle in the eastward direction at an angular velocity of 0.3° to 0.5° per year, or about one extra revolution each 900 years (Zhang et al., 2005) 
The super-rotation was predicted by models that explain Earth’s inherent magnetism (Glatzmaier and Roberts, 1996) 
The evidence for super-rotation is in the recording of seismic waves from earthquakes that are more or less on the opposite side of Earth. Refraction of the waves at interfaces between mantle and core, and bending of the waves through material of varying density, lead to multiple paths from source to receiver. Some of the rays go through the inner core, some go around it, and these will have different arrival times. This allows geophysicists to measure the relative speed of the super-rotation. 
The super-rotation of the inner core is the main driver of the geodynamo which creates Earth’s magnetic field.
- hot bouyant liquid iron in the lower part of the outer core rises upward
- liquid iron loses some of its heat at the mantle boundary and cools and sinks.
- this process (thermal convection) transfers energy from bottom to top.
- heat escaping from the upper core causes the solid inner core to grow larger, the chemical process involving iron crystals and the growing inner core enhances the convection mechanism.
- Earth’s rotation through the Coriolis effect deflects the rising iron fluid into spiral or helical paths.
- Three-dimensional numerical simulations of the geodynamo suggest that a super-rotation of Earth’s solid inner core relative to the mantle is maintained by magnetic coupling between the inner core and an eastward thermal wind in the fluid outer core. This mechanism, which is analogous to a synchronous motor, also plays a fundamental role in the generation of Earth’s magnetic field.
- The liquid iron carries electric charge and when electric charges move in curved paths they produce magnetic field. This is how the Earth’s magnetic field is created.
- The convection mechanism and the super-rotation of the inner core keeps the geodynamo going.
The thickness of Earth’s crust (litosphere) is between 5 to 30 km. The mantle is 2900 km thick. The fluid outer core is about 2200 km thick. There are turbulent convection currents inside the fluid outer core. The temperature in the core is about 5000° Celcius.
Formation of the Inner Core
During the formation of Earth the molten metallic iron separated from the surrounding mixture of silicates and oxides and sank to the center. This is how the liquid iron core was formed.
How do we know that the core is liquid and the core contains iron? The evidence comes from Seismological measurements which indicate that the density of the core is just 10% less than the density of pure iron. This means the core is mostly iron. Light elements rise into the fluid outer core, while denser elements form the solid inner core.
 Zhang, J., Song, X., Li, Y., Richards, P., Sun, X., and Waldhauser, F., 2005, Inner core differential motion confirmed by earthquake waveform doublets: Science, v. 309, p. 1357–1359.
 Glatzmaier, G., and Roberts, P., 1996, Rotation and magnetism of Earth’s inner core: Science, v. 274, p. 1887–1891.