Structure of the Earth (Edexcel GCSE Geography: B (1GB0))

The structure of the Earth

• The Earth is composed of a number of layers
  • Inner core: About 1400km in diameter, a solid and dense layer composed of iron and nickel with temperatures of about 5000 - 6000°C
  • Outer core: About 2100km thick, a semi-molten metal layer with temperatures between about 4000-6000°C
  • Mantle: About 2900km thick, a semi-molten layer which is less dense than the outer core this is split into two parts 
    • Asthenosphere
    • Lower mantle
• Lithosphere: The thickness varies, and is made up of two types of crust together with the rigid upper part of the mantle.
• The two types of crust are:
• • • Oceanic
    • Continental

Structure of the Earth

Crust 

• There are two types of crust and they are part of the lithosphere:
  • The oceanic crust is thinner (5-10km) but heavier, denser and is composed mainly of basalt 
  • The Continental crust is thicker (25-100km) but is older, denser and is composed mainly of granite
• Oceanic crust is continually being created and destroyed as a result of plate movement where it is denser and so subducts under the continental crust

Mantle

• The mantle is composed of the:
• Upper mantle which has two layers
  • The rigid upper layer which is part of the lithosphere 
  • The Asthenosphere which is partially molten plastic type layer which moves under high pressure and is between 35-250km
• Lower mantle which is solid and the largest of the Earth's layers (2900km) 

Convection currents

• The heat within the Earth (geothermal) which is estimated to reach up to between 5-6000^oC is generated by radioactive decay
• Heat rises from the Earth's core and creates convection currents in the mantle - the tectonic plates which make up the crust (lithosphere) move on top of the semi-molten mantle below
• Current theory is called slab pull theory 
  • This suggests that in addition to the convection currents, plate movement is a result of the weight of the denser oceanic plates subducting and dragging the rest of the plate along

Convection currents and slab pull

Mantle plumes

• Within the mantle there are also mantle plumes which are upwellings of superheated rock
• These can lead to either:
  • Divergent plate boundaries
  • Hot spots located at random points on a tectonic plate