Tectonic Hazards (AQA GCSE Geography)

The structure of the Earth

• The Earth is composed of four main layers:
  • Inner core: About 1400km in diameter, a solid and dense layer composed of iron and nickel with temperatures of about 5500°C
  • Outer core: 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: The thickness varies, and is made up of two types of crust 

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, where it is denser and so subducts under the continental crust
• This is why continental crust is much older than oceanic crust, as it isn't destroyed

Plate tectonics

• The crust is broken into a number of tectonic plates

Distribution of the major tectonic plates

• These 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 slab pull theory which suggests that the movement is the result of the weight of the denser oceanic plates subducting and dragging the rest of the plate along
• 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%)

Global earthquake distribution

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

Active volcano distribution

Types of plate boundary

• Volcanic eruptions and earthquakes most commonly occur at or near plate boundaries
• There are three main types of plate boundaries:
  • Constructive
  • Destructive
  • Conservative (transform)

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

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

Conservative (transform) boundary

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

Conservative (transform) boundary

Primary and secondary hazards

• When earthquakes and volcanic eruptions occur, there are primary and secondary hazards

The Primary and Secondary Hazards of Tectonic Activity

	Primary Hazards	Secondary Hazards
Volcanic Eruption	Ash Pyroclastic flow Lava flow Gas emissions Volcanic bombs	Lahars Acidification Landslides Climate change Fires Floods
Earthquake	Ground shaking Gas emissions	Building collapse Landslides Gas leaks Fires  Soil liquefaction Subsidence Mudflows Tsunami