Plate Tectonics (OCR GCSE Geography)

• Four main layers form the structure of the Earth:
• Inner core: This layer is about 1400km in diameter. It is a solid and dense layer composed of iron and nickel with temperatures of about 5500°C
• Outer core: The outer core is about 2100km thick, a semi-molten metal layer with temperatures between about 5000-5500°C
• Mantle: About 2900km thick, a semi-molten layer which is less dense than the outer core 
• Crust: Made up of two types of crust (continental and oceanic), the thickness varies

Structure of the Earth Diagram

Structure of the Earth

Crust

• There are two types of crust:
  • The oceanic crust is thinner (5-10km) but heavier and denser
  • The continental crust is thicker (25-90km) but is older and lighter
• Oceanic crust is continually being created and destroyed as a result of plate movement
  • This happens because the oceanic crust is denser and so subducts under the continental crust
• As a result continental crust is much older than oceanic crust because it isn't destroyed

Plate tectonics

• The crust is broken into a number of tectonic plates

Distribution of Plate Boundaries Map

Distribution of tectonic plates

• The tectonic plates move on top of the semi-molten mantle below
• Tectonic theory once stated the movement of the plates was the result of convection currents in the mantle
  • Current theory is called ridge push and slab pull theory which suggests that the movement is caused by:
    • Ridge push - the new crust forming at the constructive boundary which then pushes the older crust away
    • Slab pull - the weight of the denser oceanic plates subducting and dragging the rest of the plate along

Diagram of Ridge Push and Slab Pull

Convection currents, ridge push and slab pull

• A plate boundary or margin is where two plates meet

Earthquake distribution

• Earthquakes occur at all types of plate boundaries
  
  • Most occur along the Pacific 'Ring of Fire' (approximately 90%)

Distribution of Earthquakes Map

Distribution of earthquakes

Volcano distribution

• Most volcanoes occur at constructive and destructive plate boundaries
• The majority of active volcanoes (approximately 75%) are located around the rim of the Pacific Ocean called the 'Ring of Fire'
• Hotspots occur away from plate boundaries and are plumes/columns of magma which escape through the Earth's crust

Distribution of Active Volcanoes Map

Distribution of active volcanoes

Types of plate boundary

• Volcanic eruptions and earthquakes most commonly occur at or near plate boundaries
• There are four main types of plate boundaries:
• Constructive
• Destructive
• Conservative
• Collision

Constructive plate boundary

• At a constructive boundary the plates are moving apart
• The Mid-Atlantic Ridge is an example of a constructive plate boundary
• Both volcanic eruptions and earthquakes can occur at this type of plate boundary

Constructive Plate Boundary Map

Constructive plate boundary

Destructive plate boundary

• At a destructive plate boundary the plates are moving together
• The denser, heavier oceanic plate subducts under the lighter, less dense continental plate
• The boundary between the Nazca plate and the South American plate is one example
• Both volcanic eruptions and earthquakes occur at this type of plate boundary

Destructive Plate Boundary Diagram

Destructive plate boundary

Collision boundary

• At a collision boundary two continental plates are moving towards each other
• They are less dense than the mantle below them so they do not subduct
• The crust is forced upwards forming fold mountains such as the Himalayas
• At a collision boundary there are no volcanoes but earthquakes do occur

Collision Plate Boundary Diagram

Collision plate boundary

Conservative boundary

• At a conservative boundary the plates move past each other in opposite directions or in the same direction at different speeds
• Earthquakes are the only hazard at this type of boundary

Conservative Plate Boundary Diagram

Conservative plate boundary