<P> Growth of the inner core is thought to play an important role in the generation of Earth's magnetic field by dynamo action in the liquid outer core . This occurs mostly because it cannot dissolve the same amount of light elements as the outer core and therefore freezing at the inner core boundary produces a residual liquid that contains more light elements than the overlying liquid . This causes it to become buoyant and helps drive convection of the outer core . The existence of the inner core also changes the dynamic motions of liquid in the outer core as it grows and may help fix the magnetic field since it is expected to be a great deal more resistant to flow than the outer core liquid (which is expected to be turbulent). </P> <P> Speculation also continues that the inner core might have exhibited a variety of internal deformation patterns . This may be necessary to explain why seismic waves pass more rapidly in some directions than in others . Because thermal convection alone appears to be improbable, any buoyant convection motions will have to be driven by variations in composition or abundance of liquid in its interior . S. Yoshida and colleagues proposed a novel mechanism whereby deformation of the inner core can be caused by a higher rate of freezing at the equator than at polar latitudes, and S. Karato proposed that changes in the magnetic field might also deform the inner core slowly over time . </P> <P> There is an East--West asymmetry in the inner core seismological data . There is a model which explains this by differences at the surface of the inner core--melting in one hemisphere and crystallization in the other . The western hemisphere of the inner core may be crystallizing, whereas the eastern hemisphere may be melting . This may lead to enhanced magnetic field generation in the crystallizing hemisphere, creating the asymmetry in the Earth's magnetic field . </P> <P> Based on rates of cooling of the core, it is estimated that the current solid inner core started solidifying approximately 0.5 to 2 billion years ago out of a fully molten core (which formed just after planetary formation). If true, this would mean that the Earth's solid inner core is not a primordial feature that was present during the planet's formation, but a feature younger than the Earth (the Earth is about 4.5 billion years old). </P>

What 2 elements make up the inner core