Event: Formation of Earth’s Core and Internal Layers
Date: ~4.5 to 3.5 billion years ago

“The crust of the Earth must be a shell floating on a fluid interior…. Thus the surface of the globe would be capable of being broken and distorted by the violent movements of the fluids on which it rested.”
— Benjamin Franklin

Dear Human,

Beneath your feet, Earth is not still. It never was.

In its earliest days, the young Earth was a sphere of searing energy—formed from collisions, accretion, and relentless impacts with planetesimals and protoplanets. Each impact delivered kinetic energy that transformed into heat, while gravitational compression added more. The result was a world incandescent from within—a global magma ocean suspended in space.

As this molten sphere churned, a quiet revolution began. Planetary differentiation—the great sorting of matter—took hold. The densest elements, especially iron and nickel, began to migrate inward, falling through the molten layers toward the core. With them came rare and heavy elements like gold, platinum, and iridium, now locked deep within Earth’s inaccessible center. Lighter materials—oxygen, silicon, aluminum, magnesium—floated upward to form the mantle and crust.

Yet the Earth’s heart did not simply cool and die once its birth heat began to fade. Trapped deep within its iron belly and silicate bones were unstable isotopes forged in ancient supernovae—uranium-238, uranium-235, thorium-232, and potassium-40. These atoms began to split apart, releasing a constant trickle of heat. Over billions of years, this radiogenic warmth has kept the mantle restless, the outer core molten, and the geodynamo alive. Without it, the mantle would have stiffened, the magnetic field collapsed, and the atmosphere and oceans been stripped away, leaving Earth as silent and frozen as Mars.

What had once been a chaotic mixture of elements slowly organized into a layered body: dense metals at the center, silicate rock above, and gases eventually escaping to form the atmosphere. Gravity, guided by heat, had sculpted the first interior architecture of Earth.

At the heart of it all, the inner core much later began to crystallize—about 1,220 kilometers in radius. Composed primarily of solid iron and nickel alloy, it formed under pressures exceeding 3.5 million atmospheres. Temperatures here exceed 5,000°C, yet the core remains solid—not because it is cool, but because pressure forbids it to melt. It is a buried star of metal, locked in stillness by its own weight.

Surrounding it is the outer core—a roiling ocean of molten iron and nickel roughly 2,200 kilometers thick. Here, movement is everything. The convection of liquid metal, stirred by heat escaping from the inner core, creates swirling currents. As these currents twist through Earth’s rotation, they generate the geodynamo—the source of Earth’s magnetic field. This invisible shield deflects charged solar particles and preserves our atmosphere. Without it, the surface would be battered by solar radiation and winds, and our atmosphere would be far more vulnerable to escape.

Above the core lies the mantle, a vast layer of solid yet plastic rock extending up to the base of the crust. It is not liquid, but over long timescales, it moves. Mantle convection—driven by deep heat and aided by density variations—pushes tectonic plates around the globe. The continents drift. Oceans open and close. Mountains rise and erode. All of it, powered from below.

Each layer—core, mantle, crust—emerged from the same planetary fire, shaped not only by composition but by motion. The Earth became a self-organizing machine, converting internal chaos into geologic order.

You live atop a crust only a few to a few dozen kilometers thick, thinner, in scale, than the skin on an apple. Beneath it lies a planetary engine of stone and metal, spinning and shifting in slow, eternal breath. Every mountain you see, every earthquake you feel, every continent that drifts across the sea owes its movement to the molten heart beneath your feet.

It is hidden, but it is not silent.
It is ancient, but it is not still.
It is buried, but it breathes heat and motion into all the world above.
And though you may never see it, you would not be here without it.

Pathfinder

Internal structure of Earth