Plate Tectonics (AQA A Level Geography)

• The tectonic theory of crustal evolution is a scientific theory that revolutionised people’s understanding and study of geological processes and the Earth
• It helps to explain geological phenomena such as:
• the occurrence of hazards such as earthquakes and volcanic eruptions
• the formation of mountain ranges
• the movement of continents
• the distribution of some mineral resources such as diamonds

Continental Drift Theory

Overview of Continental Drift Theory

• The tectonic plates move slowly over the asthenosphere
• Scientists agree that the plates move but there is still debate over the mechanisms that cause the movement

Convection Currents

• In the past the theory of convection currents was used on its own to explain tectonic plate movement
• Heat from radioactive decay in the core moves upwards in the mantle
• It creates convection currents which rise towards the surface before spreading in the asthenosphere, cooling and sinking
• As they reach the asthenosphere they carry the lithospheric plates above with them

Convection currents in the mantle

• Other processes are now recognised as being important in plate movement

Slab pull and gravitational sliding/ridge push

• These processes are now thought to be very significant in driving plate movement
• A subduction zone is formed when two plates move towards each other 
• The heavier, denser oceanic plate subducts under the lighter, less dense plate
• As the plate sinks, gravity pulls the plate down into the mantle
• This is known as slab pull
• New oceanic crust is formed at mid-ocean ridges and stands 2-3km above the ocean floor
• As the crust moves away from a mid-ocean ridge, it cools and becomes denser and thicker
• This causes the lithosphere to slope away from the ridge and gravity pulls the lithosphere down this slope, pushing it forwards
• This is known as gravitational sliding (or ridge push)
• Ridge push is much less important than slab pull in causing the movement of tectonic plates

Processes of slab pull and ridge push (gravitational sliding)

Sea floor spreading

• Palaeomagnetism provides evidence that the sea floor has gradually moved apart at a mid-ocean ridge
• Lava cools and solidifies with the minerals lining up with the magnetic field
• The direction of the minerals on either side is a mirror image

Sea floor spreading and palaeomagnetism