Earth’s structure and plate tectonics

The Earth is made of layers: a thin rocky crust, a thick mantle of hot rock, a liquid outer core and a solid inner core. The crust and the top of the mantle are cracked into tectonic plates that move a few centimetres a year, causing earthquakes, volcanoes and mountains where they meet.

Key facts

  • Crust: thin, solid rock (about 5 to 70 km thick). Mantle: hot rock that can flow very slowly (about 2900 km thick). Outer core: liquid iron and nickel. Inner core: solid iron and nickel, hotter than 5000°C.
  • Tectonic plates move a few centimetres a year, carried by convection currents in the mantle.
  • Plates moving apart: magma rises and makes new crust (the Mid-Atlantic Ridge). An ocean plate sinking under a continental plate: volcanoes and earthquakes (the Andes). Two continental plates colliding: fold mountains (the Himalayas). Plates sliding past each other: earthquakes (the San Andreas Fault).
  • Alfred Wegener (1912) suggested continental drift: coastlines that fit together, matching fossils and rocks on different continents, and coal in Antarctica.
  • Earthquakes are recorded with seismometers. In a volcano, magma rises from the magma chamber up the vent and comes out of the crater as lava and ash.
  • Earthquake waves showed us the layers: some waves cannot pass through the liquid outer core.

Worked examples

Example 1

Name the layers of the Earth labelled A to D.

  1. A is the thin outer skin of solid rock: the crust.
  2. B is the thick layer of hot rock that flows very slowly: the mantle.
  3. C is liquid iron and nickel: the outer core.
  4. D, at the centre, is solid iron and nickel: the inner core.

Answer: A crust, B mantle, C outer core, D inner core

Example 2

A tectonic plate moves 5 cm a year. How far does it move in a million years? Give your answer in kilometres.

  1. Distance in centimetres: 5 × 1000000 = 5000000 cm
  2. Change to metres (÷ 100): 5000000 ÷ 100 = 50000 m
  3. Change to kilometres (÷ 1000): 50000 ÷ 1000 = 50 km
  4. Slow movements add up to huge distances over millions of years.

Answer: 50 km

Example 3

Label the magma chamber, the vent, the crater and the lava on this volcano.

  1. The magma chamber is the store of molten rock deep underground.
  2. The vent is the pipe the magma rises up through.
  3. The crater is the hollow at the top where it comes out.
  4. Lava is molten rock that flows out over the surface.

Answer: magma chamber at the bottom, vent up the middle, crater at the top, lava flowing down the side

Example 4

Give three pieces of evidence that South America and Africa were once joined.

  1. The coastlines of South America and Africa fit together like jigsaw pieces.
  2. Fossils of the same land animals and plants are found on both, though they could not have crossed an ocean.
  3. Rock layers of the same type and age match up across the two continents.

Answer: their coastlines fit together; the same fossils are found on both; matching rock layers are found on both