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Tectonic Hazards

The internal structure of the Earth

  • 3 things to know
  • AS Unit 1 and A2 Unit 4

Overview

Approximate radius of the Earth, mohorovičić discontinuity and how seismic waves show that the outer core is liquid.

What you need to know

Answer each one before you reveal it.

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Key term

What are the four main layers of the Earth, working inwards from the surface?

Key term

How does the thickness of continental crust compare with that of oceanic crust?

Key term

Why is oceanic crust denser than continental crust?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    What are the four main layers of the Earth, working inwards from the surface?

    Model answer

    The crust, the mantle, the outer core and the inner core.

  2. 2

    State the approximate radius of the Earth.

    Model answer

    It is about 6,371 km from the surface to the centre.

  3. 3

    What is the Mohorovičić discontinuity?

    Model answer

    It is the boundary between the base of the crust and the top of the mantle, detected because seismic waves speed up sharply as they cross it.

  4. 4

    How does the thickness of continental crust compare with that of oceanic crust?

    Model answer

    Continental crust is roughly 30 to 70 km thick, while oceanic crust is only about 6 to 10 km thick.

  5. 5

    Why is oceanic crust denser than continental crust?

    Model answer

    Oceanic crust is basaltic and rich in iron and magnesium at about 3.0 g cm⁻³, whereas continental crust is granitic and richer in silica and aluminium at about 2.7 g cm⁻³.

  6. 6

    Why is no oceanic crust older than about 200 million years when continental crust can be billions of years old?

    Model answer

    Oceanic crust is continually destroyed by subduction, while continental crust is too buoyant to be carried down and so survives.

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