Earth's inner core grows by approximately 1 millimeter every year as the planet cools.

The Earth's inner core is a solid ball of iron and nickel that sits approximately 5,150 kilometers below the surface. Research published in journals like Nature Geoscience suggests the inner core only began to solidify between 500 million and 1.5 billion years ago. Before this period, Earth's entire core was likely a molten liquid. The growth occurs through a process called latent heat release. As the planet's internal temperature drops, the liquid iron at the boundary of the outer core reaches its freezing point and crystallizes. This crystallization releases heat, which helps drive the convection currents in the liquid outer core. These currents are essential because they generate Earth's protective magnetic field through the geodynamo process. Seismologists use data from earthquake waves to measure the inner core's size, which is currently about 1,220 kilometers in radius. While the 1 millimeter annual growth rate seems tiny, it represents an enormous volume of metal when calculated across the entire surface of the core. Scientists estimate that it will take another 4.5 billion years for the core to cool completely, roughly the same amount of time the Earth has already existed.