Explain how scientists use P-waves and S-waves produced by earthquakes to determine that the Earth has a liquid outer core.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Seismic waves produced by earthquakes travel through the Earth and are detected at monitoring stations across the world. Scientists have used these waves to build up a picture of the Earth's internal structure.
Model answer (5 marks)
P‑waves are longitudinal waves that can travel through solids and liquids, whereas S‑waves are transverse waves that can only travel through solids.
When an earthquake occurs, the waves are recorded at stations on the far side of the Earth. A shadow zone appears where no S‑waves are detected, showing that S‑waves are blocked by a liquid layer.
P‑waves, however, are still recorded in this zone, indicating that they have passed through the core.
Because the S‑waves are blocked but the P‑waves are not, the layer that blocks S‑waves must be liquid – this is the Earth's outer core.
When an earthquake occurs, the waves are recorded at stations on the far side of the Earth. A shadow zone appears where no S‑waves are detected, showing that S‑waves are blocked by a liquid layer.
P‑waves, however, are still recorded in this zone, indicating that they have passed through the core.
Because the S‑waves are blocked but the P‑waves are not, the layer that blocks S‑waves must be liquid – this is the Earth's outer core.
Examiner tips
- Use the terms ‘longitudinal’ and ‘transverse’ to describe P‑ and S‑waves; this shows understanding of wave types. Explain the existence of the S‑wave shadow zone and the detection of P‑waves in the same region. Link the blocking of S‑waves to a liquid medium. Mention that P‑waves can travel through liquids to justify the conclusion.
Common mistakes
- Confusing the types of waves or using the wrong wave type for the wrong medium. Forgetting to mention the S‑wave shadow zone. Claiming that both waves are blocked or that P‑waves cannot pass through liquids.
Mark scheme (5 marks)
- P-waves are longitudinal seismic waves that can travel through all mediums (solids and fluids)
- S-waves are transverse seismic waves that cannot travel through fluids/liquids
- A shadow zone exists on the opposite side of the Earth where S-waves are not detected
- P-waves are still detected on the opposite side of the Earth, showing they have passed through the core
- Because S-waves are blocked but P-waves pass through, the outer core must be liquid/fluid
Key terms in this question
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