Earth’s inner core is a solid ball of iron alloyed with roughly 5% nickel and a small fraction of lighter elements like silicon, sulfur, oxygen, and carbon. It sits at the very center of the planet, about 5,150 kilometers beneath your feet, under pressures so extreme that metal stays solid even at temperatures rivaling the surface of the sun.
Iron, Nickel, and a Few Mystery Ingredients
Iron dominates the inner core’s composition. The ratio of iron to nickel is about 16 to 1, a proportion scientists have worked out by studying iron meteorites, which are remnants of planetary cores that broke apart early in the solar system’s history. Those meteorites serve as natural samples of the kind of metal alloy that sinks to a planet’s center during formation.
Pure iron and nickel alone don’t fully explain what seismic waves reveal about the inner core’s density and behavior. The inner core is slightly less dense than a pure iron-nickel ball would be at the same pressure, which means lighter elements are mixed in, probably making up about 2 to 3% of the total. Silicon is the leading candidate: recent modeling suggests the inner core contains between 1 and 2% silicon. Carbon, oxygen, and sulfur are also possibilities, but pinning down exact amounts is difficult when you can’t collect a direct sample from 5,000 kilometers down.
For comparison, the liquid outer core surrounding it contains a much higher proportion of light elements, roughly 5 to 10%. That difference matters. When molten iron crystallizes onto the inner core’s surface, it sheds lighter elements into the surrounding liquid, and that process helps drive the churning currents in the outer core that generate Earth’s magnetic field.
Solid Metal at 6,000°C
The inner core’s temperature is nearly uniform throughout, sitting at approximately 6,150 kelvin (about 5,880°C or 10,600°F) at the center and dropping only slightly to around 6,130 kelvin at its outer boundary. That’s hotter than the surface of the sun. Yet the inner core remains solid. The reason comes down to pressure rather than temperature.
At the center of the Earth, pressure reaches about 364 gigapascals, or roughly 3.6 million times the atmospheric pressure at sea level. Under that kind of squeeze, iron atoms are packed so tightly that they lock into a solid crystal structure even at extreme heat. The inner core didn’t freeze because the planet cooled down. It froze because pressure at the center became high enough to force molten iron into a solid state. The density of this compressed metal ranges from about 9.9 to 12.2 grams per cubic centimeter, making it significantly denser than iron at the surface (which runs about 7.9 g/cm³).
How Iron Atoms Arrange Themselves
At everyday pressures, iron atoms arrange in a cubic pattern. Under the extreme conditions of the inner core, they shift into a hexagonal close-packed structure, where atoms stack in tightly layered sheets. This arrangement has been confirmed through seismic evidence: the inner core transmits vibrations slightly faster in certain directions than others, a property called anisotropy. That directional behavior is consistent with hexagonal crystals aligned roughly along Earth’s rotational axis, and it rules out the cubic structures iron normally forms at lower pressures.
A Surprisingly Young Part of the Planet
Earth itself is about 4.5 billion years old, but the inner core is much younger. Laboratory experiments recreating core-like pressures and temperatures place its age at roughly 1 billion to 1.3 billion years. Before that, the entire core was liquid. As the planet slowly lost heat, conditions at the center eventually crossed the threshold where iron began crystallizing out of the molten outer core, and the inner core has been growing ever since, adding a few millimeters to its radius each year.
That birth was a pivotal moment for life on Earth. Before the inner core formed, the magnetic field was generated solely by convection in the liquid core, and evidence suggests it was weakening. Once solid iron began crystallizing at the center, the process released heat and expelled lighter elements upward into the outer core, turbocharged the convection currents, and revived the magnetic dynamo. The strengthened magnetic field restored Earth’s protective shield against solar radiation at what geophysicist John Tarduno of the University of Rochester has called “a really interesting time in evolution.”
How Scientists Study Something They Can’t Reach
No drill has ever come close to the inner core. Everything known about it comes from indirect evidence, primarily seismic waves generated by earthquakes. When these waves travel through the planet, they bend, speed up, slow down, or reflect depending on the material they pass through. The inner core was discovered in 1936 by Danish seismologist Inge Lehmann, who noticed that certain pressure waves appeared at seismic stations where they shouldn’t have been. If the entire core were liquid, those waves would have been deflected into a “shadow zone” with no signal. Lehmann proposed that a solid inner boundary was reflecting some waves back, and her model has held up for nearly 90 years.
Modern seismology has gotten far more precise. Scientists now track waves that pass directly through the inner core (called PKIKP waves) and compare how they change over time. By studying pairs of nearly identical earthquakes recorded years apart, researchers can detect tiny shifts in how the inner core is oriented relative to the surface. A 2024 study published in Nature used this technique to show that the inner core gradually rotated slightly faster than the mantle from 2003 to 2008, then reversed direction and crept backward through the same path at a slower rate from 2008 to 2023. The forward rotation was roughly 0.05 to 0.15 degrees per year, and the backward motion was two to three times slower. This back-and-forth hints at a complex push and pull between the solid inner core, the liquid outer core, and the rocky mantle above, though scientists are still working out the mechanics.
Why the Inner Core’s Composition Matters
Knowing what the inner core is made of isn’t just geological trivia. Its composition controls how fast it grows, how much heat it releases, and how effectively it powers the magnetic field that shields the planet from charged particles streaming off the sun. The exact mix of light elements in the inner core also helps scientists understand how elements were distributed when Earth first formed and how they’ve been redistributed over billions of years of cooling.
The proportion of silicon versus sulfur versus oxygen in the core, for instance, carries clues about how much oxygen was available deep inside the early Earth, which in turn constrains models of how the planet accreted from the disk of dust and gas around the young sun. Getting the recipe right for the inner core helps scientists get the recipe right for the whole planet.