The center of the Earth is a ball of iron and nickel, roughly 5% nickel by composition, with small amounts of lighter elements mixed in. This inner core sits at the very center of the planet with a radius of about 1,216 kilometers (roughly 760 miles), making it slightly smaller than the Moon. It is surrounded by a liquid outer core of similar composition that extends another 2,200 kilometers outward. Together, these two layers tell a story about how our planet formed, why it has a magnetic field, and how scientists figured all of this out without ever drilling deeper than about 12 kilometers.
Iron, Nickel, and a Few Mystery Ingredients
The core is overwhelmingly iron. Nickel makes up about 5% of the mix, and the remaining fraction consists of lighter elements. In the inner core, these lighter elements account for only 2 to 3% of the total composition. The outer core contains more of them, roughly 5 to 10%, which partly explains why it’s less dense than the inner core.
Exactly which light elements are present is one of the bigger open questions in geoscience. The leading candidates are silicon, oxygen, sulfur, carbon, and hydrogen. Recent studies have proposed that the inner core is specifically an iron-nickel-silicon alloy containing 1 to 2% silicon. There’s no reason to think only one of these elements is present; the outer core likely contains a cocktail of several. Pinning down the exact recipe matters because it affects everything from how heat flows through the planet to how the magnetic field behaves.
Why the Inner Core Is Solid and the Outer Core Is Liquid
Both layers are extraordinarily hot, reaching temperatures between 7,200 and 9,000°F (4,000 to 5,000°C). At the surface, iron melts at about 2,800°F, so you’d expect the entire core to be liquid. The inner core stays solid because of pressure. At a depth of over 5,000 kilometers, the weight of everything above squeezes atoms so tightly together that they lock into a solid crystalline structure despite the extreme heat.
The outer core doesn’t experience quite as much pressure, so it remains a churning ocean of liquid metal. The boundary between the two sits at a depth of about 5,155 kilometers. At that transition, there’s a density jump of roughly 4.5%, too large to be explained by the simple difference between solid and liquid iron. The mismatch confirms that the outer core holds a higher concentration of lighter elements than the inner core does.
How Scientists Know What’s Down There
No one has ever sampled the core directly. Everything we know comes from seismic waves, the vibrations that radiate outward from earthquakes. These waves travel faster through denser material and slow down in hotter or molten regions. Two types matter most: P-waves (pressure waves), which can travel through both solids and liquids, and S-waves (shear waves), which can only move through solids.
When an earthquake occurs, seismometers around the world record which waves arrive and which don’t. At distances greater than about 103 degrees of arc from the earthquake’s origin, S-waves disappear entirely. P-waves also vanish in a band between roughly 103 and 143 degrees. The German seismologist Beno Gutenberg explained this pattern back in 1914: a molten layer beginning at about 2,900 kilometers deep blocks S-waves and bends P-waves, creating a “shadow zone” on the far side of the planet. That molten layer is the outer core.
Deeper still, P-waves that do penetrate the core suddenly speed up again at about 5,155 kilometers, revealing the solid inner core within. By measuring how fast waves travel at different depths and in different directions, scientists can calculate the density and stiffness of each layer, then work backward to figure out what materials match those properties. Iron-nickel alloys fit the data better than anything else.
A Core Within the Core
In 2002, researchers analyzing seismic waves that pass through the deepest part of the inner core found evidence of a distinct region at the very center, sometimes called the “innermost inner core.” This zone constitutes only about 0.01% of Earth’s total volume, but the seismic waves passing through it behave differently. Specifically, the pattern of anisotropy changes: waves traveling in certain directions speed up or slow down in ways that don’t match the rest of the inner core.
This suggests the iron crystals at the very center are aligned differently than those in the surrounding inner core. One possibility is that the innermost inner core formed under different conditions early in Earth’s history, preserving a distinct crystalline texture as a kind of geological fossil. The evidence is still limited because seismic waves reaching this depth travel nearly radially, making it hard to gather detailed directional information.
How the Core Formed
Earth’s core is the product of the most significant event in the planet’s history: differentiation. During Earth’s formation roughly 4.5 billion years ago, the planet was largely molten. Heavy elements like iron sank toward the center under gravity, while lighter silicate minerals floated upward to form the mantle and crust. This separation established the basic layered structure we see today.
The conditions during this process have been constrained through lab experiments and simulations: pressures of 40 to 60 billion pascals (gigapascals) and temperatures of 3,000 to 4,000 kelvin. Water played a surprisingly important role. Recent planet formation models suggest that 10 to 100 times the volume of today’s oceans may have been delivered to Earth during its assembly. Research published in Science Advances using machine-learning molecular dynamics simulations found that water’s presence during core formation changed which elements ended up where, promoting magnesium partitioning into the metallic core while pushing silicon and hydrogen into the silicate mantle.
The Core Powers Earth’s Magnetic Field
The liquid outer core is responsible for the magnetic field that shields the planet from solar radiation. The process works like a self-sustaining dynamo. As the outer core loses heat to the mantle above, convection currents develop in the liquid iron, much like water circulating in a boiling pot. Because the iron is electrically conductive and Earth is rotating, these flowing currents generate electrical currents, which in turn produce magnetic fields, which further influence the flow of liquid metal. This feedback loop, called the geodynamo, has kept Earth’s magnetic field running for billions of years.
The inner core contributes to this process as well. As it slowly solidifies and grows, it releases heat and expels lighter elements into the outer core. Both of these effects drive additional convection, helping to sustain the churning motions that power the dynamo. Without a liquid, electrically conductive outer core, Earth would have no global magnetic field, and the planet’s surface would be far more exposed to charged particles from the Sun.