The structure of our planet, characterized by distinct layers, is a direct result of planetary differentiation. This process describes how a celestial body’s materials separate based on density and chemical properties. Earth’s internal structure is arranged into four primary compositional layers: the thin Crust, the thick Mantle, the liquid Outer Core, and the solid Inner Core. This density-driven sorting, where the heaviest elements sank to the center, occurred very early in Earth’s history, powered by immense internal heat and gravity.
Earth’s Molten Beginning: Accretion and Initial Heat
The proto-Earth began as a collection of dust and rock fragments, called planetesimals, which gradually clumped together through accretion about 4.54 billion years ago. As the mass of the growing planet increased, it was subjected to three intense sources of internal heating that led to a fully molten state, often described as a global Magma Ocean.
The most immediate source of heat was the kinetic energy released from the constant, high-velocity impacts of accreting planetesimals. Each collision transformed mechanical energy into thermal energy, rapidly heating the planet’s interior. As the planet grew, gravitational compression caused by its own mass generated additional heat by squeezing the interior materials.
A third major heat source was the radioactive decay of short-lived, unstable isotopes, such as Aluminum-26. This decay released thermal energy throughout the planet’s interior. These combined heat sources raised the internal temperature above the melting points of rock and metal, allowing materials to flow and separate according to density.
The Great Density Separation: Forming the Core
Once the planet was largely molten, the “Iron Catastrophe” began the separation of layers. The densest elements, primarily iron and nickel, were no longer bound within solid rock and began to sink rapidly toward the planet’s center, driven by gravity. This occurred within the first tens of millions of years of Earth’s formation.
The sinking of this heavy metallic material released enormous amounts of gravitational potential energy, which converted into heat, further raising the planet’s temperature by an estimated 2,000 degrees Celsius. This heating ensured that the heavy metals quickly coalesced into the planetary core, which is composed of a liquid Outer Core and a solid Inner Core.
The Outer Core is a shell of molten iron and nickel. Below it, the Inner Core, also made of iron and nickel, remains solid despite being hotter (up to 5,400 degrees Celsius). This paradoxical solidity is due to the extreme pressure at the planet’s center, which compresses the metal atoms into a crystalline structure.
Cooling and Layering: Defining the Mantle and Crust
Following the formation of the dense metallic core, the less dense silicate materials rich in oxygen, silicon, and magnesium were left to surround it. This vast volume of molten rock began to cool and solidify, forming the Mantle, which constitutes the thickest layer of the planet.
The final layer to form was the thin, outermost Crust, which separated from the mantle through subsequent, slower processes. As the mantle cooled, lighter silicate compounds, particularly those rich in aluminum and potassium, were chemically separated from the main body of rock. These less dense materials rose to the surface through partial melting and volcanic activity.
The cooling and solidification of these lightest silicate melts created the Earth’s rigid, outermost shell. This differentiation resulted in a final structure where the least dense materials form the crust, the moderately dense silicates make up the mantle, and the densest iron-nickel alloy forms the core.