1. Why Earth Has Layers: Density & Differentiation

Earth is not a uniform ball of rock. It is sorted into layers — dense metal in the center, lighter rock on the outside. But why? The answer goes back to how Earth formed about 4.6 billion years ago.

Early Earth was extremely hot and partly molten, heated by the energy of countless collisions as it grew (accretion) and by the decay of radioactive elements inside it. When much of the planet is molten, gravity does what it does to oil and water: it sorts material by density.

• Dense materials — mostly iron and nickel — sank to the center, forming the core.
• Lighter silicate rock floated upward, forming the mantle and, at the very top, the thin crust.

This sorting of a planet's material by density is called differentiation. It is the reason Earth ended up with a metal core wrapped in rocky layers, and the reason it still gets hotter as you go down.

⚠️ Common misconception: the layers did not form by being stacked up from the outside, like sediment settling. They separated from the inside out by density while Earth was molten — heavy stuff sank, light stuff rose.

💡 Tip: think 'heavy sinks, light floats.' Iron is heavy → core; rock is lighter → mantle and crust.