The core is Earth’s innermost and densest layer, a region of intense heat and pressure located over 2,890 kilometers beneath the surface. It extends to the planet’s center, approximately 6,371 kilometers deep. Directly accessing this region is impossible with current technology, as the deepest human-made boreholes penetrate only a fraction of the distance into the crust. Scientists must therefore rely on indirect methods, primarily the study of seismic waves generated by earthquakes, to deduce the physical properties and chemical composition of the core. These waves act like a planetary ultrasound, revealing how materials behave under conditions that cannot be replicated on the surface.
The Core’s Two Distinct Layers
The core is divided into two distinct regions: a liquid Outer Core surrounding a solid Inner Core. This division results from the competing effects of temperature and pressure acting on the material. The Outer Core begins at the boundary with the mantle and is a molten metal layer estimated to be about 2,200 kilometers thick.
The temperature in this region is high enough to keep the iron-nickel alloy in a liquid state, allowing it to flow and convect. The Inner Core, lying at the center, is a sphere with a radius of approximately 1,221 kilometers. Here, the pressure is so overwhelming that it compresses the atoms into a rigid, solid structure despite the even higher temperatures. The increasing weight of the overlying layers causes this phase change from liquid to solid.
Primary Chemical Ingredients
The core’s composition is a metallic alloy dominated by Iron (Fe) and a substantial presence of Nickel (Ni). This dense mixture formed early in Earth’s history through planetary differentiation, where the heaviest elements sank toward the center while the planet was molten. Although a pure iron-nickel alloy accounts for the core’s overall density, seismic data reveals a slight “density deficit” in the Outer Core, suggesting the presence of lighter elements alongside the metals.
Scientists hypothesize that up to 10% of the core’s mass consists of these lower-atomic-number elements, incorporated during formation. Candidates for these light elements include Silicon (Si), Oxygen (O), Sulfur (S), Carbon (C), and Hydrogen (H). Their inclusion is necessary to explain the transmission speeds of seismic waves observed deep within the planet.
Extreme Conditions Heat and Pressure
The physical environment within the core involves extreme heat and pressure. Temperatures are comparable to those found at the surface of the Sun, with estimates for the Inner Core reaching around 5,200 degrees Celsius. This thermal energy is a remnant of the planet’s formation and is supplemented by the decay of radioactive isotopes and the slow solidification of the Inner Core.
The pressure is equally extreme, starting at approximately 135 Gigapascals (GPa) at the core-mantle boundary and rising to about 360 GPa at the center. This immense force dictates the physical state of the core material. It is the pressure that compresses the iron atoms into a crystalline solid in the Inner Core, overriding the melting effects of the high temperature.
The Geodynamo How the Core Protects Earth
The core’s liquid nature is responsible for Earth’s magnetic field, generated through a self-sustaining process known as the geodynamo, which operates within the Outer Core. As heat radiates outward from the Inner Core, it creates thermal and compositional buoyancy in the molten iron-nickel alloy. This energy drives large-scale convective currents, causing hot, less-dense fluid to rise and cooler, denser fluid to sink.
Because the molten metal is electrically conductive, Earth’s rotation acts on these currents, twisting them into spiraling columns. This organized movement generates powerful electric currents, which induce the planet’s dipole magnetic field. This field extends thousands of kilometers into space, creating the magnetosphere. The magnetosphere acts as a shield, deflecting harmful charged particles, such as solar wind and cosmic radiation, away from the atmosphere.