Structure of the Earth
In 1970, Soviet scientists drilled the Kola Superdeep Borehole in Russia, reaching 12.3 km down, yet that's less than 0.2% of Earth's 6,371 km radius. In this lesson you'll learn how seismic waves from earthquakes have given us a complete X-ray of our planet's four layers without drilling a single metre further.
Printable Worksheets
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Q1 · If you drilled straight down through Earth, what layers would you pass through? How hot do you think it gets?
Q2 · We've never physically reached Earth's core, how do scientists know what's inside? What evidence could tell them?
● Know
- The four layers of Earth: crust, mantle, outer core, inner core
- The state of matter and temperature of each layer
- That seismic waves reveal Earth's interior structure
● Understand
- Why density increases from crust to core
- How P-waves and S-waves behave differently in solids vs liquids
- Why the inner core is solid despite being the hottest layer
● Can do
- Label Earth's layers on a cross-section diagram
- Explain the seismic wave evidence for a liquid outer core
- Identify Australian geological context (Jack Hills, Indo-Australian Plate)
- Crust
- Mantle
- Outer core
- Inner core
- Seismic wave
- Liquid iron and nickel that generates Earth's magnetic field
- Thin, solid, outermost rocky layer
- Energy wave from an earthquake that reveals interior structure
- Solid iron-nickel at Earth's centre despite extreme heat
- Thick semi-solid rock layer that flows slowly over millions of years
The deepest humans have ever drilled is 12.3 km, the Kola Superdeep Borehole in Russia. Earth's radius is 6,371 km. We've scratched less than 0.2% of the way to the centre. Yet we know what's in there. How? Earthquakes.
Working from the outside in, Earth has four layers:
| Layer | Depth | State | Temperature | Composition |
|---|---|---|---|---|
| Crust | 0–70 km | Solid | Cool → ~900°C | Rocky silicates; oceanic crust thinner (~7 km), continental thicker (~30–70 km) |
| Mantle | 70–2900 km | Semi-solid (flows slowly) | 1000–3700°C | Silicate rock (mostly olivine) |
| Outer core | 2900–5150 km | Liquid | ~4000–5000°C | Liquid iron and nickel |
| Inner core | 5150–6371 km | Solid | ~5000°C | Solid iron-nickel |
Note: the lithosphere = crust + uppermost solid mantle. Tectonic plates are lithospheric slabs. The mantle below (asthenosphere) flows slowly, driving plate movement.
After an earthquake, energy travels outward as seismic waves. Two main types reveal Earth's structure:
- P-waves (Primary / compressional): push-pull motion. Travel through solids AND liquids. Arrive first at a seismograph station.
- S-waves (Secondary / shear): side-to-side motion. Travel through solids ONLY. Arrive second.
The key discovery: after a large earthquake, P-waves reach the other side of Earth, but S-waves do not. There is an S-wave shadow zone. This means S-waves encountered a liquid layer that blocked them, the outer core must be liquid. S-waves cannot travel through liquid because liquids don't transmit shear stress.
Using data from many seismograph stations worldwide, scientists can map exactly where wave speeds change, revealing the boundaries between layers. This "X-ray" of Earth has been refined for over a century.
Density increases from the crust to the core:
| Layer | Density (g/cm³) |
|---|---|
| Crust (granite) | ~2.7 |
| Mantle (silicates) | 3.3–5.5 |
| Outer core (liquid iron) | ~10 |
| Inner core (solid iron) | ~13 |
The inner core is solid because the pressure there reaches 3.6 million atmospheres, this enormous squeezing force overcomes the heat, forcing iron atoms together into a solid even at 5000°C.
Australian connection: Australia sits on the Indo-Australian Plate. Our crust is ancient, the Jack Hills of Western Australia contain zircon crystals that are 4.4 billion years old, the oldest known geological material on Earth. Australian continental crust is some of the most stable on the planet.
S-waves from an earthquake travel through solid rock but stop at Earth's outer core. Predict: what does this tell us about the state of matter of the outer core?
How close was your prediction?
Well done, connecting S-wave behaviour to liquid is the key insight.
Remember: S-waves only pass through solids. If they stop, there must be a liquid layer in the way.
In your workbook, draw a table with columns: Layer | State of matter | Temperature range | Composition | One key fact. Fill in all four layers (crust, mantle, outer core, inner core) using information from this lesson.
In your workbook, draw a circle representing Earth. Divide it into four layers (like an onion). Label each layer with its name, approximate depth range, and state of matter. Use a ruler and draw neatly, include an arrow showing direction from surface to centre.
At the start of the lesson, you read that Earth's inner core is hotter than the surface of the Sun, yet it's solid. That probably seemed impossible!
Now that you've worked through the lesson, explain exactly why that is. Your answer should use the word pressure and include a number from the lesson that makes it click.
Q1. List Earth's four layers in order from the surface to the centre. For each, state its approximate state (solid, liquid, semi-solid) and one other property. (4 marks)
Q2. Explain how scientists use seismic waves to determine that Earth has a liquid outer core. (3 marks)
Q3. Why is the inner core solid despite being at approximately 5000°C? (2 marks)
Answers
▾MCQ 1
B Crust, Mantle, Outer Core, Inner Core is the correct order from Earth's surface to its centre.
MCQ 2
B The outer core is liquid iron and nickel. This is known because S-waves cannot pass through it.
MCQ 3
B S-waves travel through solids only. They cannot pass through the liquid outer core, creating a shadow zone on the far side of Earth from an earthquake.
MCQ 4
C The outer core. Movement of liquid iron and nickel in the outer core generates electric currents, which produce Earth's magnetic field.
MCQ 5
C Pressure at Earth's centre reaches 3.6 million atmospheres. This enormous squeezing force compels iron atoms to remain in a solid arrangement even at 5000°C.
Short Answer 1
Model answer: (1) Crust, solid, rocky silicates; thinnest layer, 0–70 km. (2) Mantle, semi-solid (flows slowly), mostly silicate rock; temperatures 1000–3700°C. (3) Outer core, liquid, iron and nickel; generates Earth's magnetic field. (4) Inner core, solid, iron and nickel; most dense layer at ~13 g/cm³; ~5000°C.
Short Answer 2
Model answer: P-waves can travel through both solids and liquids, while S-waves can only travel through solids. After a major earthquake, P-waves are detected on the opposite side of Earth but S-waves are not, they are blocked by a liquid layer. This missing S-wave zone (shadow zone) tells scientists that the outer core must be liquid, stopping S-waves from passing through.
Short Answer 3
Model answer: The inner core is solid because the pressure there is approximately 3.6 million atmospheres. This extreme pressure forces iron atoms so tightly together that they cannot flow as a liquid, even though the temperature (~5000°C) would normally melt iron. Pressure "wins" over temperature at Earth's centre.