Earth's structure, volcanoes and earthquakes

To explain why volcanoes and earthquakes happen where they do, you need to know what lies beneath the crust and how plates move.

Practise this lesson free (7 questions)

Inside the Earth

Inner core: solid iron and nickel, extremely hot.
Outer core: liquid iron and nickel.
Mantle: the thickest layer, hot rock that flows very slowly.
Crust: the thin outer layer. Oceanic crust is thinner and denser (mainly basalt). Continental crust is thicker and less dense (mainly granite).
The crust is broken into tectonic plates. Heat from the core drives convection currents in the mantle, and these, together with the pull of sinking plates, move the plates a few centimetres a year.

Two types of volcano

Shield volcanoes: runny (low-viscosity) basalt lava flows a long way, building wide, gently sloping cones. Eruptions are frequent but gentle. Found at constructive margins and hotspots, for example Mauna Loa in Hawaii.
Composite volcanoes (stratovolcanoes): thick, sticky (viscous) lava traps gas, so eruptions are explosive. Layers of ash and lava build steep cones. Found at destructive margins, for example Mount St Helens, USA, which erupted violently in 1980.
Hazards: lava flows, pyroclastic flows (fast, hot clouds of gas and ash), ash falls and gases; secondary hazards include lahars (mudflows).

How earthquakes happen

Pressure builds as plates lock together, then is released suddenly as the rock breaks or slips.
The point underground where this happens is the focus. The point on the surface directly above it is the epicentre.
Energy travels as seismic waves; shaking is usually strongest near the epicentre.
Shallow-focus earthquakes usually cause more damage at the surface than deep ones.
Secondary hazards include landslides, fires, liquefaction of wet ground and tsunamis.
Worked example

Explain why composite volcanoes erupt more explosively than shield volcanoes.

  1. Composite volcanoes have thick, viscous lava.
  2. Viscous lava traps gas, so pressure builds up inside the volcano.
  3. When it is released the eruption is explosive; shield volcanoes have runny lava, so gas escapes easily and eruptions are gentle.

Answer: Their viscous lava traps gas until pressure builds and is released explosively, whereas runny shield lava lets gas escape.

Key idea

Layers: inner core (solid), outer core (liquid), mantle, crust (oceanic: thin, dense; continental: thick, less dense). Convection currents move plates. Shield volcanoes: runny lava, gentle slopes and eruptions, constructive margins and hotspots (Mauna Loa). Composite: viscous lava, steep, explosive, destructive margins (Mount St Helens, 1980). Earthquakes: focus underground, epicentre above; shallow focus is more damaging.

Quick check

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Extra practice

2 more questions

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  1. Why is the inner core solid even though it is very hot?

    1. Immense pressure keeps it solid
    2. It is cold
    3. It is made of ice
    4. It is made of granite
    Show the answer

    Answer: Immense pressure keeps it solid

    Pressure prevents melting.

  2. Which crust is made mainly of granite?

    1. Continental crust
    2. Oceanic crust
    3. The mantle
    4. The inner core
    Show the answer

    Answer: Continental crust

    Continental crust is lighter and thicker.

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Sources and references

Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.

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