Temperature in the Earth's interior at different times in its history compared with the melting point of iron. The molten iron collected and began to sink under its own great weight. About one third of the primitive planet's material sank to the center, and in the upheaval, heating rates increased and most of the planet was liquified. Figure 2 provides the results of a calculation comparison of predicted Earth temperature profiles and the melting point curve for iron under the corresponding conditions of pressure. The melting of iron leads to the formation of a heavy liquid layer.
Drops begin to develop in later stages and sink toward the center.
The formation of a molten iron core was the first stage of the differentiation of the Earth, in which it was converted from a homogenous body, with roughly the same kind of material at all depths, to a layered body, with a dense iron core, a crust composed of lighter materials with relatively lower melting points, and between them the mantle. The first stage of differentiation, iron melting led to the onset of a mechanism that speeded up the differentiation process. The hotter material expands, becomes lighter than the material above it, and floats upward, carrying its heat to the surface, where the hot matter cools and sinks again. Convective overturn probably was the mechanism that produced the thin crust, and ultimately determined the different chemical composition of the crust. There is evidence that the hot outgassing that occurred during the first billion years led to the first atmosphere of the Earth.