The result of this was that large portions of Earth's mass melted. When that happened, material could move around inside the Earth, driven by differences in density. Dense material sank toward Earth's center, while lighter material floated toward the surface. Since iron is much denser than regular rocks, it preferentially sank toward the center. This process is called differentiation , and the final result of differentiation is that Earth's internal structure is not uniform, but layered.
Lighter material is near the surface, and denser material is found near the center.
Incidentally, in lecture I used the analogy of freezing golf balls and tennis balls in ice, and then melting the ice. Earth Structure Earth's internal structure is layered. The figure above shows a generalized model of the structure, with the parts of Earth's interior shown in various colors. The four major parts of Earth's interior are, from outermost to innermost, the crust , mantle , outer core , and inner core . It is made of cold, brittle, and relatively light material.