In the geologic time scale you met deep time: Earth is about 4.54 billion years old. This lesson asks two questions. How did a planet with layers, a metal core, a rocky mantle and a thin crust come to be? And how can we know its age so precisely when the planet keeps destroying its own oldest rocks?
From dust to layers
About 4.6 billion years ago, a disk of gas and dust circled the young Sun. Dust grains stuck together into pebbles, pebbles into kilometer-sized planetesimals, and those crashed together into planets. Every collision turned motion into heat. Add the heat of short-lived radioactive elements, and the young Earth grew hot enough to melt in large part.
💡 Why it works
Melted rock and metal separate the way oil and water do: the denser liquid sinks. Iron and nickel (about 7 to 8 g/cm³ at the surface, more under pressure) drained toward the center, while lighter silicate rock (about 3 g/cm³) floated up. Geologists call this sorting by density differentiation. It left Earth layered: a metal core, a rocky mantle, and the lightest rock as a thin crust. A planet that never melted would be an even mix all the way through.
🔑 Key idea
Earth formed about 4.54 billion years ago from colliding rock and metal. While it was hot, dense iron sank to make the core and lighter rock rose to make the mantle and crust. Its age comes from meteorites and Moon rocks, because Earth's own first crust has been recycled.
Earth's layers by composition. Drag the probe down to see how density, pressure and temperature rise toward the center.
Worked examples
✏️ Worked example: Why the inside must be dense
Earth's mass is $5.97 \times 10^{24}\,\text{kg}$ and its volume is $1.08 \times 10^{21}\,\text{m}^3$. Surface rocks average about $2.7$ to $3.0\,\text{g/cm}^3$. What does Earth's mean density tell us?
Divide by 1,000 to get g/cm³: about $5.5\,\text{g/cm}^3$.
That is about twice the density of surface rock. Rock squeezed by pressure gets a little denser, but not nearly enough: there must be something much denser deep inside, an iron core.
Answer: About $5.5\,\text{g/cm}^3$: far denser than surface rock, so Earth has a heavy metal core.
✏️ Worked example: How long before the oldest rocks?
Earth formed about 4,540 million years ago. Its oldest known rocks, in northern Canada, are about 4,000 million years old, and its oldest known mineral grains (zircons from Australia) about 4,400 million years old. How much time passed before each?
Before the oldest grains: $4,540 - 4,400 = 140$ million years.
Before the oldest rocks: $4,540 - 4,000 = 540$ million years.
For more than half a billion years, no rock survived intact. The early crust was melted by impacts, eroded and pulled back into the mantle.
Answer: About 140 million years before the oldest grains and 540 million years before the oldest rocks.
⚠️ Common mistake
It is natural to think Earth's age must come from its oldest rocks. But those are only about 4.0 billion years old, because plate tectonics and erosion keep recycling crust; almost no ocean floor is older than about 200 million years. The 4.54-billion-year age comes from radiometric dates on meteorites, leftover pieces from when the planets formed, which agree with the oldest Moon rocks and the oldest Earth minerals. Check: if the oldest rocks gave Earth's age, no mineral grain could be older than them, yet zircons of 4.4 billion years exist.
All of Earth's history squeezed into one year. Earth forms at the first moment of January 1; the oldest rocks appear in mid-February.
Practice
Pass with 3 right answers, the last 2 in a row. Each right answer earns 2 XP.