The Earth is structured like an onion, composed of several distinct layers that transition from the surface to the center. Each subterranean layer possesses unique physical properties, characterized by a progressive increase in density with depth. This layered structure results from planetary differentiation, where heavier materials sank toward the center early in its formation, while lighter materials rose toward the surface. The density of any layer is determined by its chemical makeup and the intense physical compression from the weight of the layers above.
The Earth’s Densest Region
The densest layer of the Earth is the Inner Core, a superheated, solid sphere found at the planet’s center. This innermost region exhibits the highest density of any natural material within the Earth’s structure. Based on seismic wave measurements, the density of the inner core ranges from approximately 12.8 grams per cubic centimeter (g/cm³) at its boundary to about 13.1 g/cm³ at the center. By comparison, surface materials like basalts barely reach 3.0 g/cm³. This tremendous density is achieved beneath the molten Outer Core, which is already significantly denser than the rocky layers above it.
The Role of Composition: Heavy Elements
The core’s extreme density is primarily due to its elemental composition, dominated by heavy metals. Scientific models suggest the core is predominantly an alloy of Iron (Fe) and Nickel (Ni), elements much heavier than the silicon, oxygen, and magnesium compounds of the outer layers. These metallic elements sank to the center during the planet’s early, molten stage, segregating them from lighter silicates. The solid inner core, however, is not made of pure iron and nickel; its observed density is slightly lower than expected, indicating the presence of a small percentage of lighter elements like sulfur, silicon, or oxygen. This concentration of metals means the core, despite being less than 20% of the Earth’s radius, accounts for a substantial fraction of the planet’s total mass.
How Extreme Pressure Increases Density
The distinction between the solid Inner Core and the liquid Outer Core, which share a similar composition, is explained by the immense pressure at the center of the Earth. The pressure at the boundary of the inner core is estimated to be around 330 to 360 Gigapascals (GPa), equivalent to 3.3 to 3.6 million times the atmospheric pressure at sea level. This colossal force is the weight of all the overlying crust, mantle, and outer core pressing down on the innermost layer. Although the inner core temperature is extremely high—up to 5,400 °C—the intense pressure prevents the iron-nickel alloy from melting. The pressure compresses the atoms into a tightly packed, solid crystalline structure, making it measurably denser than the molten outer core.
Density Comparison: Crust and Mantle
The Inner Core’s density provides a stark contrast to the Earth’s outer layers. The outermost layer, the Crust, is the least dense, with an average density ranging from 2.2 to 2.9 g/cm³, composed primarily of light silicate-rich rocks. The Mantle, which sits beneath the crust, has an intermediate density that increases with depth, starting at about 3.4 g/cm³ and increasing to around 5.6 g/cm³ just above the core. This layer consists of denser iron and magnesium silicate minerals. A sharp increase in density occurs at the Core-Mantle Boundary (CMB), where the rocky mantle meets the metallic outer core, which starts at approximately 9.9 g/cm³.