Structure of the Earth (AQA A Level Geography): Revision Note

Earth's Structure

• The Earth has four main layers:

  • The crust 

  • The mantle 

  • The outer core

  • The inner core

Crust

• There are two types of crust:

  • Continental - a thicker (up to 70km), less dense layer mainly composed of granite

  • Oceanic - a thinner (5-10km) , denser layer mainly composed of basalt 

Mantle

• The mantle is between the crust and core and is the widest layer (2900km)

  • The upper mantle has two layers

    • The rigid layer above the asthenosphere which together with the crust makes up the lithosphere

    • The asthenosphere is a plastic-type layer which moves very slowly under high pressure and carries the lithosphere on top

  • The lower mantle is hotter and denser than the upper mantle 

  • The intense pressure, at depth, keeps the lower mantle solid

Core

• The core is made up of two parts:

  • Inner Core - solid centre, mostly composed of iron 

  • Outer core - semi-molten, mostly liquid iron and nickel

• The lithosphere consists of seven major and several minor tectonic plates which float on top of the asthenosphere  

• The core is the hottest part of the Earth, and this heat is the main driver of tectonic activity

• There are two key causes of this extreme heat:

  • Radioactive decay of elements such as uranium and potassium inside the earth’s core

  • Heat left over from the collisions of asteroids and other small bodies which resulted in the formation of Earth