Earth's inner core and iron crystallization (IMAGE)
Caption
A cut-away of Earth's interior shows the solid iron inner core (red) slowly growing by freezing of the liquid iron outer core (orange). Seismic waves travel through the Earth's inner core faster between the north and south poles (blue arrows) than across the equator (green arrow). The researchers concluded that this difference in seismic wave speed with direction (anisotropy) results from a preferred alignment of the growing crystals -- hexagonally close packed iron-nickel alloys, which are themselves anisotropic -- parallel with Earth's rotation axis.
Credit
Graphic by Daniel Frost
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