How did heavy and light elements separate to form Earth’s core and crust?

The separation of Earth’s internal layers, known as planetary differentiation, occurred as the early planet heated up due to radioactive decay, gravitational energy, and frequent impacts. This heat caused the interior to partially melt, facilitating a massive segregation of elements based on their chemical affinity and density.12

Metallic elements, primarily iron and nickel, were reduced to a liquid state and sank toward the center to form the core, carrying with them siderophile (iron-loving) elements. In contrast, lithophile elements—those with a greater affinity for oxygen—combined into silicate compounds that remained in the mantle and eventually formed the crust.12

The formation of the crust involved further differentiation through partial melting, where less-dense liquids rose toward the surface and crystallized. This process is distinct from core formation, as it relies on subtle geochemical sorting mechanisms rather than the simple physical separation of immiscible metallic liquids. Earth continues to differentiate today through plate tectonics, which cycles material between the surface and the interior.32