c. 4.54 Billion BCE

Earth’s Core Forms

Frequent collisions in the early history of the solar system no doubt resulted in the catastrophic disruption and even vaporization of many asteroids and planetesimals (small protoplanetary objects that could grow into full-size planets). Sometimes these collisions must also have led to the accretion (growth) of some of these bodies. And as some of these lucky survivors grew, their gravity became stronger, helping them to attract even more incoming materials to help them grow even larger. Above a certain size (usually thought to be around 250–375 miles or 400–600 km across), a planetesimal’s self-gravity pulls it into a relatively spherical shape. The resulting overlying pressures cause the temperatures inside those bodies to start to increase with depth, while at the same time considerable energy is continuing to be added from additional impact events at the surface.

The result of this accretion and internal heating is that growing bodies can differentiate, or segregate, with denser elements and minerals (like those dominated by iron) sinking to the interior and less-dense elements and minerals (like those dominated by silicon) floating to the top. Geophysical models show that this process—which ultimately led to the formation of the typical core/mantle/crust structure that we see in rocky and icy planetary bodies today—happened relatively quickly to many growing planets early in the history of the solar system.

In growing rocky planets like our own, additional internal heat was provided by the radioactive decay of certain elements (such as certain isotopes of aluminum or uranium, for example) that release heat as they decay. The resulting internal buildup of heat eventually entirely melted the iron-rich cores of some of these planets. Spinning, molten, electrically conducting cores led to the formation of strong magnetic fields on some of these worlds; on Earth, those fields would ultimately help to make the surface habitable by shielding the surface from much of the most harmful radiation from the Sun.

The Earth’s core has continued to evolve with time. While the outer core is still molten, the inner region of the core is thought to have cooled and solidified about 1 to 1.5 billion years ago.

SEE ALSO Earth’s Mantle and Magma Ocean (c. 4.5 Billion BCE), Continental Crust (c. 4 Billion BCE), Magnetite (c. 2000 BCE), Solar Flares and Space Weather (1859), Earth’s Core Solidifies (~2–3 Billion)

Artist’s concept cutaway view of the very young Earth early in the history of the solar system, continually bombarded by impacts but still segregating into a core, mantle, and crust.