Earth’s core is a superheated ball of metal roughly the size of Mars, sitting about 2,900 kilometers beneath your feet. You can’t photograph it or send a probe there, but decades of seismic data, lab experiments, and computer simulations give scientists a detailed picture of what’s down there: two distinct layers of iron-nickel alloy, one liquid and one solid, reaching temperatures comparable to the surface of the Sun.
Two Layers, Two States of Matter
The core has two parts. The outer core is a 2,200-kilometer-thick shell of liquid metal. Beneath it, the inner core is a solid sphere about 1,250 kilometers thick, roughly 70% the size of the Moon. Both layers are made primarily of iron alloyed with about 5% nickel, plus smaller amounts of lighter elements like oxygen, silicon, and sulfur.
The reason one layer is liquid and the other solid comes down to pressure. Both layers are extraordinarily hot, but the inner core experiences so much pressure from the weight of everything above it that the iron is forced into a solid state. The outer core, while still under immense pressure, doesn’t cross that threshold, so the metal stays molten. Think of it as a tug-of-war between heat (which wants to melt things) and pressure (which wants to compress them into a solid). At the center of the planet, pressure wins.
How Hot and How Dense
The inner core is nearly the same temperature throughout: approximately 6,150 kelvin (about 5,880°C or 10,600°F) at the center, dropping only slightly to around 6,130 kelvin at the boundary with the outer core. That’s hotter than the surface of the Sun, which sits around 5,500°C. The pressure at the center reaches roughly 364 gigapascals, which is about 3.6 million times atmospheric pressure at sea level.
If you could somehow see the inner core, it wouldn’t look like a lump of steel. Under those extreme conditions, iron takes on a crystalline structure. Seismic waves travel through it faster in the north-south direction than east-west, suggesting the iron crystals are aligned along Earth’s rotation axis. Scientists have also detected a possible “innermost inner core” with a slightly different crystal alignment, hinting that the core may have grown in at least two distinct phases over geologic time.
A Churning Ocean of Liquid Metal
The outer core is where things get dynamic. Picture a turbulent ocean of molten iron, constantly convecting as heat escapes outward. This churning is driven by radioactive decay and by chemical differentiation, a process where lighter elements separate from heavier ones as the inner core slowly crystallizes and grows.
That convection is what keeps Earth habitable. The motion of electrically conducting liquid iron generates electric currents, and those currents produce Earth’s magnetic field through a self-sustaining feedback loop. As long as the outer core keeps convecting, the magnetic field keeps regenerating. Without it, the solar wind would strip away the atmosphere over time, much as it did on Mars after that planet’s core cooled and its magnetic field died.
The Boundary Between Core and Mantle
The transition from the rocky mantle to the liquid metal core isn’t a clean line. Just above the outer core sits a roughly 250-kilometer-thick zone called the D” layer (pronounced “D double prime”). This is technically the bottommost slice of the mantle, but it behaves unlike anything above it. Rock here transforms into an unusual mineral phase called post-perovskite, which can absorb large amounts of iron from the core below.
Scattered across this boundary are thin patches, only 5 to 40 kilometers thick, where seismic waves slow down dramatically. These ultralow-velocity zones may represent partially melted rock, regions where core material has chemically infiltrated the mantle, or some combination of both. They’re among the most puzzling features inside the planet and likely play a role in how heat transfers from the core into the mantle, ultimately driving plate tectonics at the surface.
What It’s Made Of
Iron dominates, making up roughly 85% of the core by weight. Nickel accounts for about 5%. The remaining fraction is lighter elements, and pinning down exactly which ones has been a long-standing puzzle. Modeling published in the Proceedings of the National Academy of Sciences found that the best fit to seismic data is a core containing about 3.7% oxygen and 1.9% silicon. Sulfur and carbon may be present in smaller amounts, though some models find little need for them. Critically, no oxygen-free composition matches the seismic observations, making oxygen a near-certain ingredient in the outer core.
The inner core is slightly different in composition. As it crystallizes from the liquid outer core, lighter elements get expelled into the surrounding melt. This means the inner core is purer iron-nickel, while the outer core is enriched in those lighter elements. That chemical separation releases energy, helping to fuel the convection that powers the magnetic field.
The Inner Core Is Slowing Down
For decades, the inner core appeared to rotate slightly faster than the rest of the planet. But a 2024 study from the University of Southern California, analyzing seismic data from 121 repeating earthquakes recorded between 1991 and 2023, confirmed that the inner core began slowing around 2010. For the first time in approximately 40 years, it’s now rotating slightly slower than Earth’s surface.
The cause appears to be a combination of forces: the churning liquid outer core exerts drag on the solid inner core, while gravitational pulls from dense regions in the rocky mantle above also tug on it. The practical effects at the surface are subtle. Changes in core rotation could slightly alter the length of a day by fractions of a millisecond and may influence long-term patterns in Earth’s magnetic field, though scientists are still working out the details.
How Scientists “See” the Core
No drill has come close to the core. The deepest borehole ever made, Russia’s Kola Superdeep Borehole, reached only about 12 kilometers, barely scratching the crust. Instead, scientists rely on seismic waves generated by earthquakes. Two types matter most: compression waves (P-waves) and shear waves (S-waves). P-waves can travel through both solids and liquids, but S-waves only move through solids. When S-waves hit the outer core, they stop. That shadow zone is how scientists first confirmed the outer core is liquid, back in the early 20th century.
Modern techniques go well beyond simple wave arrival times. Researchers now analyze the way core-sensitive seismic signals split, reflect, and scatter, extracting information about crystal alignment, density variations, and even layering within the inner core itself. Combine that with lab experiments that crush iron samples to core-like pressures using diamond anvils and lasers, plus computational simulations of how metals behave under extreme conditions, and the picture becomes remarkably detailed for a place no human will ever visit.