Final answer:
The process whereby planets separate into core, mantle, and crust layers due to the melting and differentiation of their materials by density is known as differentiation.
Step-by-step explanation:
The process by which planets (including Earth) separate into core-mantle-crust layers is called differentiation. Differentiation occurs when a planet's interior is hot enough to melt its components, which allows materials to separate based on their density. The heavy metals sink towards the planet's center to form the dense core, while the lighter materials rise to form the mantle and the crust. This process is confirmed by evidence that Earth's bulk density (5.5 g/cm³) is greater than the density of its surface materials (3 g/cm³), suggesting denser materials lie below the surface in the core.