Inside a planet: cores, mantles and crusts
Nobody can drill to Earth's core, but meteorites let us hold pieces of the cores, mantles and crusts of other worlds. This lesson explains how layered worlds form.
- Describe the layered structure of the Earth.
- Explain how differentiation separates a planet into core, mantle and crust.
- Use Earth's average density as evidence for a dense core.
Links to: KS3 chemistry, Earth and atmosphere: the composition and structure of the Earth; physics: density; maths: standard form.
Earth's layers
Earth has a thin rocky crust, a thick rocky mantle and a core made mostly of iron and nickel. The core is about 3,500 km in radius, more than half of Earth's radius, but deep drilling has only ever reached about 12 km down. Our knowledge of the deep Earth comes from earthquake waves, from measurements of Earth's mass and gravity, and from meteorites.
How worlds become layered
When a young asteroid or planet heats up enough to melt, its materials separate by density. Dense molten metal sinks to the centre to form a core. Less dense rock floats above it as a mantle, and the lightest material rises to form a crust. This separation is called differentiation.
Early in the Solar System's history, many asteroids differentiated. The heat came mainly from a short-lived radioactive form of aluminium, aluminium-26, which was plentiful at the time and released energy as it decayed.
Meteorites from every layer
Later collisions smashed some differentiated asteroids apart, scattering pieces from every layer. Iron meteorites are pieces of metal cores. Pallasites, olivine crystals in metal, probably come from where metal met rock. Eucrites and diogenites are crust and deeper rock from the asteroid Vesta, which is still in one piece and was visited by the Dawn spacecraft from 2011 to 2012.
Iron meteorites give us a direct look at the sort of material Earth's core is made of.
Evidence for a dense core
Earth's mass is 6.0 × 10²⁴ kg and its volume is 1.1 × 10²¹ m³. Calculate its average density, and compare it with crustal rock, about 2,700 kg/m³.
- density = mass ÷ volume = (6.0 × 10²⁴) ÷ (1.1 × 10²¹).
- Divide the numbers: 6.0 ÷ 1.1 ≈ 5.5. Subtract the powers: 10²⁴ ÷ 10²¹ = 10³.
- Average density ≈ 5.5 × 10³ kg/m³ = 5,500 kg/m³.
- This is about twice the density of crustal rock, so the inside must contain something much denser.
Answer: About 5,500 kg/m³. Rock alone cannot explain this, which points to a dense iron-nickel core, the same material as iron meteorites (about 7,800 kg/m³ at the surface, and denser still when squeezed in a core).
Model differentiation safely: in a clear jar, mix some sand, a few metal ball bearings or washers, and some small pieces of cork or polystyrene with water. Shake, then let it settle. Draw and label what you see, and match each layer to a part of a differentiated asteroid and the type of meteorite it would produce.
Quiz
5 questions. Choose an answer to see whether you are right and why. Finish the quiz to complete the lesson.
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Question 1What is Earth's core made of?
Earth's core is mostly iron with some nickel, like iron meteorites.
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Question 2What is differentiation?
When a body melts, dense metal sinks to make a core and lighter rock rises to form a mantle and crust.
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Question 3What heated the early asteroids enough to melt some of them?
Aluminium-26 was abundant in the first few million years and released heat as it decayed.
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Question 4Which meteorites are pieces of the crust of the asteroid Vesta?
Eucrites are basalts from Vesta's crust; diogenites come from deeper down.
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Question 5Why does Earth's average density of about 5,500 kg/m³ suggest a dense core?
The whole Earth is about twice as dense as its surface rocks, so its interior must contain denser material such as iron.