Think about the ground beneath your feet. It feels solid, permanent, and honestly, pretty dull. But if you could tunnel down past the dirt, the bedrock, and the shifting tectonic plates, you'd eventually hit a place that defies common sense. It’s a ball of fire and metal. Specifically, what is the inner core of the earth made of is a question that involves pressures so intense they turn white-hot metal into a solid.
It’s weird.
We live on the crust, a thin "eggshell" of rock. Beneath that is the mantle, which is basically a slow-moving, hot plastic. Then you hit the outer core—a literal ocean of liquid iron and nickel. But at the very center, 3,960 miles down, sits the inner core. It's about the size of the Moon.
The Recipe for a Planet’s Heart
Mostly, it's iron.
When the Earth was just a messy, molten blob 4.5 billion years ago, the heavy stuff sank. Gravity pulled the dense metals toward the center while the lighter rocks floated to the top. This process, called planetary differentiation, is why we have a core at all. According to seismic data analyzed by researchers like Dr. Hrvoje Tkalčić at the Australian National University, the inner core is roughly 80% to 85% iron. The rest is mostly nickel.
But there’s a catch.
Pure iron-nickel alloys are actually denser than the actual core. This tells geophysicists that there must be "light elements" mixed in there. We’re talking about sulfur, oxygen, silicon, and maybe a dash of hydrogen. These elements act like a seasoning that thins out the metal just a bit. Without them, the math doesn't work.
The temperature is staggering. We’re talking $5,400^\circ\text{C}$ to $6,000^\circ\text{C}$ ($9,800^\circ\text{F}$ to $10,800^\circ\text{F}$). That is roughly the temperature of the surface of the Sun. You’d think it would be a gas or a liquid, right? Wrong.
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Pressure wins.
The weight of the entire planet is pressing down on that center point. It’s over 3 million times the atmospheric pressure we feel at sea level. This crushing force squeezes the atoms so tightly together that they cannot move into a liquid state. They stay locked in a crystalline lattice.
Wait, the Core is a Giant Crystal?
Sorta.
It’s not a diamond, but it is crystalline. Scientists believe the iron atoms in the inner core are arranged in a hexagonal close-packed (hcp) structure. Imagine a bunch of oranges stacked in a crate in a very specific, tight pattern. That’s the inner core.
What’s even crazier is that the inner core isn't just one big, uniform lump. In 2023, researchers including Thanh-Son Phạm published a study in Nature Communications suggesting there might be an "innermost inner core." This is a distinct central zone about 400 miles wide where the iron crystals are oriented differently than the rest of the inner core. It’s like a Russian nesting doll of geology.
Why we can't just drill down and check
We’ve never been there. The deepest hole ever dug, the Kola Superdeep Borehole in Russia, only went down about 7.6 miles. That’s barely a scratch.
So, how do we know what is the inner core of the earth made of?
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Earthquakes.
When a big quake hits, it sends shockwaves (seismic waves) through the planet. P-waves (primary waves) can travel through the core, but they change speed and direction depending on what they hit. By measuring these waves at stations all over the globe, scientists can "see" the interior of the Earth like an ultrasound of a baby.
- If the wave speeds up, the material is denser.
- If it slows down, it might be hotter or slushier.
- If it disappears entirely (like S-waves in the liquid outer core), we know it’s hitting a liquid.
The Dynamo and the Magnetic Shield
The fact that the inner core is solid and the outer core is liquid is the only reason you’re alive to read this.
The liquid outer core is constantly swirling. Because it’s made of conductive iron, this movement creates electric currents. This is the Geodynamo. The solid inner core plays a huge role here by acting as an anchor. As the Earth slowly cools, the inner core actually grows by about a millimeter every year as the liquid outer core freezes onto it.
This freezing process releases "latent heat." This heat fuels the convection in the outer core, keeping the magnetic field strong. Without that magnetic field, the solar wind would strip away our atmosphere. We’d be a dead rock like Mars.
Common Myths About the Center of the Earth
People love a good story, but Jules Verne got a lot wrong.
First, it’s not hollow. Gravity wouldn't allow that. Second, it’s not "burning" like a fire. Fire requires oxygen and a chemical reaction. The core is hot because of leftover heat from the Earth's formation and the decay of radioactive elements like Uranium-238 and Thorium-232.
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Third, the inner core isn't stationary. It actually rotates.
For a long time, we thought it rotated faster than the rest of the planet (super-rotation). However, recent studies using data from old nuclear test sites suggest it might speed up and slow down in cycles. Some researchers even think it might have paused its rotation relative to the surface recently. It’s not "stopping" in space; it’s just moving at the same speed as the crust for a bit.
The Mystery of the Light Elements
The biggest debate in geology right now isn't if there is iron, but what else is down there.
- Silicon: A top candidate because it dissolves easily in iron at high pressures.
- Sulfur: We see a lot of it in meteorites (which are basically leftovers from planet building), but there seems to be less of it in the core than expected.
- Oxygen: This is the wildcard. Some models say oxygen can't exist in the core; others say it's the primary light element.
Determining the exact ratio is vital. If we know the chemistry of the core, we know exactly how the Earth was "baked" billions of years ago. It’s a forensic investigation of our origin.
Practical Insights: Why This Matters to You
You probably won't ever see a piece of the inner core. Even the lava from volcanoes comes from the upper mantle, not the core. But understanding what is the inner core of the earth made of has real-world applications.
- Satellite Protection: Our magnetic field protects the satellites that run your GPS and cell service. Understanding the core helps us predict "magnetic flips" or weak spots like the South Atlantic Anomaly.
- Space Exploration: When we look at exoplanets, we use Earth as the template. If a planet doesn't have a solid-liquid core setup, it likely doesn't have a magnetosphere, meaning it’s probably not habitable.
- Predicting the Future: Eventually, the Earth will cool down completely. The outer core will freeze solid. When that happens, the magnetic field dies. Don't panic—that’s billions of years away.
If you want to stay updated on this, keep an eye on "seismic tomography" reports. New AI models are currently re-analyzing decades of earthquake data to map the inner core in 3D with more detail than we ever thought possible. We are effectively building a high-definition map of a place we can never visit.
To deepen your understanding of the Earth's interior, you should look into the "PREM" model (Preliminary Reference Earth Model). It's the gold standard for how scientists divide the Earth's layers by density. You can also track real-time seismic activity through the USGS website to see the "probes" scientists use to study the core in action.