The Earth’s interior is organized into distinct layers, including the crust, mantle, and core. This internal architecture governs almost all geological phenomena on our planet, from the magnetic field to plate tectonics. The layering is the result of a fundamental scientific principle that provides insight into the history and ongoing dynamics of our world.
Planetary Differentiation
The process responsible for the Earth’s layered structure is known as planetary differentiation: the separation of a planetary body’s internal materials based primarily on their density. This process is driven by the planet’s gravitational force acting on mobile or fluid materials. Denser materials are pulled toward the center, while less dense materials rise toward the surface.
This sorting causes the planet to evolve from a homogeneous mixture to a series of concentric shells. Denser elements, such as iron and nickel, sink to form the core, while lighter silicate compounds create the outer layers. Differentiation requires the materials to be in a partially or wholly molten state for this movement and sorting to occur.
The Early Molten Earth
Differentiation was possible because the early Earth was largely molten, allowing materials to move freely. This heat originated from multiple sources shortly after the planet’s formation approximately 4.6 billion years ago. One source was the kinetic energy from accretion, as planetesimals and debris collided violently, converting their energy of motion into heat.
Gravitational compression, caused by the planet’s growing mass, also heated the interior. The most significant and continuous internal heat source came from the decay of radioactive isotopes, such as potassium-40, uranium, and thorium. This radioactive decay released thermal energy, which was trapped within the interior. This combination of impact heating, compression, and radiogenic heat ensured that the early Earth existed as a global magma ocean, facilitating the movement required for differentiation.
Composition and Density of the Major Layers
Planetary differentiation resulted in the three major compositional layers we observe today: the core, the mantle, and the crust. The core accounts for about 31% of the Earth’s mass and is the most dense layer, primarily composed of iron and nickel. This high-density metallic material sank to the center compared to the surrounding silicates.
The core is divided into a liquid outer core and a solid inner core. The outer core has a density range of about 9.9 to 12.2 grams per cubic centimeter, while the solid inner core is denser still, estimated between 12.6 and 13.0 grams per cubic centimeter. The mantle surrounds the core, constituting about 68% of the Earth’s mass. This layer is composed of silicate rocks rich in iron and magnesium, and its density ranges from approximately 2.9 grams per cubic centimeter in the upper region to 5.7 grams per cubic centimeter deeper down.
The crust is the outermost and least dense layer, making up less than 1% of the planet’s mass. It is composed of silicate rocks rich in elements like silicon, oxygen, and aluminum. Continental crust density is low (around 2.6 to 2.8 grams per cubic centimeter), while oceanic crust is slightly denser (about 3.0 grams per cubic centimeter). The distinct chemical compositions and densities of these three major layers serve as direct evidence of the sorting mechanism of planetary differentiation.