The Earth is structured like an onion, composed of distinct, concentric layers that vary dramatically in density and chemical makeup. This organization, driven by gravity sorting materials during the planet’s formation, results in three primary compositional shells: the Crust, the Mantle, and the Core. These layers interact constantly, with the deepest structures influencing the surface. Understanding these divisions reveals the geological processes that shape our world.
The Earth’s Outermost Shell
The Crust is the outermost and thinnest layer, representing less than one percent of the Earth’s total volume, yet it is the surface upon which all life exists. This shell is broadly divided into two major types. Continental crust is relatively thick, ranging from 25 to 70 kilometers, and is composed mainly of less dense, silica-rich (felsic) rocks like granite.
Oceanic crust, by contrast, is much thinner, typically only 5 to 10 kilometers thick, and is made of denser, iron- and magnesium-rich (mafic) rocks, primarily basalt. The density difference means that continental crust essentially “floats” higher, forming the continents, while the denser oceanic crust sits lower, creating the ocean basins. This fragile outer layer is broken into tectonic plates, which are moved by forces from below.
The Vast Middle Layer
Beneath the crust lies the Mantle, a thick shell extending to nearly 2,900 kilometers deep and constituting about 82% of the Earth’s total volume. Its composition is dominated by silicate rocks rich in iron and magnesium, making it denser than the crust. Although the mantle is mostly solid, the intense heat and pressure cause the rock to behave plastically over geological timescales.
This unique physical state allows the material to undergo slow-moving convection currents, where hotter, less dense material rises while cooler, denser material sinks. This circulation transfers heat from the planet’s interior toward the surface. The movement of this material in the upper mantle is the fundamental driver of plate tectonics, influencing surface features like earthquakes and volcanism.
The Deepest Internal Structure
The Core is the Earth’s innermost structure, composed primarily of a nickel-iron alloy, which sank to the center due to its high density early in the planet’s history. This region is subject to extreme temperatures and immense pressure. The core is distinctly separated into two parts: the liquid Outer Core and the solid Inner Core.
The Outer Core is a layer of molten iron and nickel, and its turbulent, convective motion generates electric currents. This flow of electrically conducting fluid creates the geodynamo, which generates the Earth’s protective magnetic field. Below this liquid shell, the Inner Core remains solid despite the extreme heat. The crushing pressure forces the iron atoms into a rigid, crystalline structure, preventing them from melting. The heat driving these core dynamics is supplied by residual heat from the planet’s formation and the decay of radioactive elements within the interior.
Inferring Earth’s Interior Structure
Since direct sampling of the mantle and core is impossible, scientists rely on the study of seismic waves generated by earthquakes to map the planet’s interior. These waves, specifically Primary (P-waves) and Secondary (S-waves), travel through Earth and are recorded by seismographs across the globe. The speed and path of these waves change depending on the density, composition, and physical state (solid or liquid) of the material they pass through.
Researchers track how P-waves and S-waves are refracted, reflected, or slowed down at boundaries between layers. For instance, S-waves cannot travel through liquid, so their complete absence in certain regions confirmed that the Outer Core is molten. Sudden shifts in wave speed, known as discontinuities, mark the precise boundaries between the crust, mantle, and core, allowing scientists to construct a detailed model of the Earth’s internal architecture.