Geophysicists realized decades ago that a solid inner core exists, but they knew precious little else about it. They believed the inner core and the liquid shell surrounding it were made largely of iron, yet other features of the heart of the planet remained enigmatic. But during the 1980s, seismologists examining earthquake waves that pierce the inner core made a startling find. Waves traveling along the planet's north-south axis go 3 to 4 percent faster through the inner core than those that follow paths close to the equatorial plane. The leading theory is that at the immense pressures of the inner core, iron takes on a hexagonal crystal form that has inherently directional physical properties.
Some force apparently keeps the hexagonal iron crystals all in close alignment.
Otherwise the seismic anisotropy would not be as large as measured. The central core may have grown gradually to its present size as liquid iron at the bottom of the outer core solidified and attached itself to the inner core. Slow solidification of iron might have allowed the inner core to grow quietly for billions of years, becoming in the end a gargantuan single crystal, more than 2,400 kilometers across. But slow crystal growth does not explain the alignment of the inner core's axis of anisotropy with the earth's rotation axis. The process also fails to account for the seismological evidence that the anisotropic grain is not uniform.