Plate Tectonics (OCR GCSE Geography B)

Structure of the Earth

• 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

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

• 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

• A plate boundary or margin is where two plates meet

Plate Boundaries

Earthquake distribution

• Earthquakes occur at all types of plate boundaries

  • Most occur along the Pacific 'Ring of Fire' (approximately 90%)

Distribution of Earthquakes Map

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

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

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

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