Rocks with hotter temperatures sank to the bottom of the Earth. The rocks with the hottest temperature became the core and the rocks with the least amount of heat became the crust.
The stratification of Earth into its geologic layers was brought about by the formation of the Earth's iron core. The iron core was generated by a combination of radioactive decay and gravitation, which raised the temperature enough for molten iron to form. The migration of molten iron to the center of the Earth displaced the less dense materials toward the surface.The early Earth needed lots of energy to trigger the creation of molten iron. Some of this energy came from radioactive decay. Radioactive elements such as uranium and thorium give off heat when they decay. Radioactive elements were present in greater amounts in the early Earth. The radiation emitted by these elements increased the temperature of the Earth by roughly 2,000 degrees Celsius (about 3,600 degrees Fahrenheit).Gravitational forces both helped the iron accumulate in the center of the Earth and helped generate additional temperature. As the early Earth compacted into a planet thanks to gravity, this compaction gave off heat. As a result, gravitational energy helped raise the temperature of the Earth by an additional 1,000 degrees Celsius (about 1,800 degrees Fahrenheit). In turn, this temperature increase helped sustain the presence of molten iron at the Earth's core.
