Earth became itself by coming apart. As material orbiting the young Sun gathered into a growing planet, collisions and radioactive decay supplied heat; some rock and metal melted, allowing gravity to pull dense iron downward while lighter silicate material remained above. This was planetary differentiation, a separation whose consequences have never stayed buried.
The process was messier than a neat settling of layers. Iron likely moved through molten rock in droplets and larger pools, carrying elements such as nickel with it, while repeated impacts stirred and reheated the growing planet. The details remain disputed. Evidence comes partly from meteorites, which preserve clues to the ingredients available during planetary formation, and partly from experiments that test how elements behave under extreme pressure and temperature. The result is an iron-rich core beneath a rocky mantle, not two perfectly pure substances.
That ancient sorting still governs the planet's restless interior. Heat escaping from the core helps drive convection in its liquid outer layer; the motion of electrically conducting metal generates much of Earth's magnetic field through a process called the geodynamo. The field deflects many charged particles from the solar wind. Meanwhile, heat moving through the mantle contributes to convection and plate tectonics, which reshape the surface over immense spans of time. The division is not destiny. Cooling, chemistry and the slow movement of rock all complicate the story, yet the first descent of metal left Earth with an interior capable of motion. Far below the crust, that first separation still casts its long shadow.
