If we could travel to the center of Earth, we would leave behind the rocky outer shell and enter the liquid iron core about halfway down. After we traversed 80% of the distance to the center, the liquid iron core would transform into a solid due to the crushing influence of pressure, even though the temperature is comparable to the surface temperature of the Sun. Crossing into the solid region brings us to Earth’s inner core. Figure 1 illustrates the principal subshells that make up the planet.

Despite its small size–less than 1% of Earth’s volume, with a radius the width of Texas–the inner core has a surprisingly large role in the dynamics of our planet’s interior. Earth’s gradual cooling causes the inner core to grow at roughly 1 mm/yr by solidification of the liquid surrounding it. The release of latent heat and the exclusion of impurities in the liquid...

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