AQA · GCSE · Physics

    Course version: 8463 · Version from Sept 2016

    S-waves cannot travel through a liquid. P-waves and S-waves provide…

    Full specification wording

    Waves for detection and exploration (physics only) (HT only): S-waves cannot travel through a liquid. P-waves and S-waves provide evidence for the structure and size of the Earth’s core.

    Put the two behaviours together and the inside of the Earth can be mapped without anyone going there.

    Checked against the AQA 8463 specification (opens in a new tab) on .

    • Point 10 of 12
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Put the two behaviours together and the inside of the Earth can be mapped without anyone going there. S-waves are transverse and need a material that resists shearing, so they stop at the boundary of the liquid outer core; that leaves a huge S-wave shadow covering most of the far side of the globe, which is the evidence that the outer core is liquid. P-waves do cross the core but change speed as they enter it, so they refract and leave a narrower P-wave shadow zone. The angular width and position of each shadow is measured from seismometers worldwide, and the only structure that fits is a layered Earth: solid mantle, liquid outer core, and a solid inner core revealed by faint refracted P-wave arrivals inside the shadow.

    What earns the marks

    • Stating that S-waves are not detected on the far side because they cannot travel through the liquid outer core.
    • Saying that P-waves are refracted where the speed changes at the core boundary.
    • Using the size of the shadow zones to work out the size of the core.
    • Concluding that the outer core is liquid and the inner core solid.

    Common mistakes and how to put them right

    • Saying the core absorbs S-waves rather than that S-waves cannot travel through a liquid at all.

    • Giving the conclusion, a liquid core, without the seismic evidence that supports it.

    • Assuming the whole core is liquid and ignoring the solid inner core.

    • Describing a shadow zone as where the waves are weakest rather than where direct waves do not arrive.

    Exam tips

    • Structure the answer as observation then conclusion: what is or is not detected, then what that shows.
    • Use the term shadow zone; it is the wording examiners look for.
    • Mention both wave types, since an answer using only S-waves cannot give the size of the core.

    Try the lesson

    The start of the Waves for detection and exploration (physics only) (HT only): S-waves cannot travel through a liquid. P-waves and S-waves provide evidence for the structure and size of the Earth’s core. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Seismologists record seismic waves from an earthquake using detectors located around the Earth. They observe a large shadow zone on the opposite side of the Earth where no S-waves are detected. What does this shadow zone demonstrate about the Earth's interior?

    Listen

    Put the two behaviours together and the inside of the Earth can be mapped without anyone going there. S-waves are transverse and need a material that resists shearing, so they stop at the boundary of the liquid outer core; that leaves a huge S-wave shadow covering most of the far side of the globe, which is the evidence that the outer core is liquid. P-waves do cross the core but change speed as they enter it, so they refract and leave a narrower P-wave shadow zone. The angular width and position of each shadow is measured from seismometers worldwide, and the only structure that fits is a layered Earth: solid mantle, liquid outer core, and a solid inner core revealed by faint refracted P-wave arrivals inside the shadow. A strong answer earns marks for these points. stating that S-waves are not detected on the far side because they cannot travel through the liquid outer core. saying that P-waves are refracted where the speed changes at the core boundary. using the size of the shadow zones to work out the size of the core. concluding that the outer core is liquid and the inner core solid. Exam tip: Structure the answer as observation then conclusion: what is or is not detected, then what that shows.

    See it

    A top-down flowchart. An earthquake leads to S-waves and P-waves travelling outwards. One branch shows S-waves reaching the liquid outer core, then S-waves cannot travel through a liquid, then an S-wave shadow covers most of the far side. The other branch shows P-waves reaching the core boundary, then P-waves change speed and refract, then a narrower P-wave shadow zone. Both shadow-zone boxes lead to shadow zones measured worldwide, which leads to the conclusion that the outer core is liquid and the inner core solid.
    It shows how S-waves stopping at the liquid outer core and P-waves refracting at the core boundary produce shadow zones that reveal the structure of the core.

    Key words

    Say what each word means before you turn the card.

    S-wave shadow

    Try answering aloud or in your head before turning the card.

    liquid outer core

    Try answering aloud or in your head before turning the card.

    solid inner core

    Try answering aloud or in your head before turning the card.

    refraction at the core boundary

    Try answering aloud or in your head before turning the card.

    See every definition
    S-wave shadow
    A huge shadow covering most of the far side of the globe, left because S-waves stop at the boundary of the liquid outer core.
    liquid outer core
    The layer at which S-waves stop, which is the evidence that this part of the core is liquid.
    solid inner core
    The part of the core revealed by faint refracted P-wave arrivals inside the shadow.
    refraction at the core boundary
    The change of direction of P-waves as they enter the core, where their speed changes.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. An earthquake leads to S-waves and P-waves travelling outwards. One branch shows S-waves reaching the liquid outer core, then S-waves cannot travel through a liquid, then an S-wave shadow covers most of the far side. The other branch shows P-waves reaching the core boundary, then P-waves change speed and refract, then a narrower P-wave shadow zone. Both shadow-zone boxes lead to shadow zones measured worldwide, which leads to the conclusion that the outer core is liquid and the inner core solid.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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