Tectonic Hazards (Edexcel IGCSE Geography)

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 (divergent)
  • Destructive (convergent)
  • Collision
  • Conservative (transform)

Constructive (Divergent) Plate Boundary

• At the 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 (Divergent) plate boundary

Destructive (Convergent) Plate Boundary

• At a destructive (convergent) 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 an example
• Both volcanic eruptions and earthquakes occur at this type of plate boundary

Destructive (Convergent) Plate Boundary

Collision Boundary

• At a collision boundary, two plates of similar density move towards each other
• Neither is dense enough to subduct so the land is pushed upwards
• This process forms fold mountains such as the Himalayas
• Earthquakes are the main hazard at this type of plate boundary

Collision Boundary

Conservative (Transform) Boundary

• At a conservative (transform) 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 (Transform) Boundary

• Volcanoes occur at constructive (divergent), destructive (convergent) plate boundaries and hot spots
• Volcanoes do not occur at collision boundaries or conservative (transform) boundaries

Volcanoes at Constructive Boundaries

• At a constructive (divergent) boundary, the tectonic plates are moving away from each other:
  
  • Constructive plate boundaries often occur under the sea/ocean
  • The lava escapes through the gap left as the plates move apart
  • The lava cools and hardens, forming a new crust
• At constructive plate boundaries, the lava tends to be runny and eruptions are less explosive
• These types of eruption form shield volcanoes, which have gently sloping sides

Volcanoes at Destructive Boundaries

• At a destructive (convergent) boundary, the tectonic plates are moving towards each other:
  • The heavier, denser oceanic plate subducts under the lighter continental plate
  • In the subduction zone, the two plates come together, causing friction
  • Friction causes heat and the plate material melts, forming magma
  • The magma rises to the surface through cracks in the crust
  • The cooling lava and ash build up, forming a volcano
• At destructive plate boundaries, the lava tends to be sticky and produces explosive eruptions
• These eruptions tend to form composite or stratovolcanoes 

Volcanoes at Hot Spots 

• At a hot spot, the tectonic plate passes over a plume of magma:
  • The magma rises to the surface through cracks in the crust
  • As the tectonic plate moves slowly over the magma plume, a line of islands may form, e.g. Hawaii

Hot spot and island formation

Volcano Primary and Secondary Hazards

• Volcanic eruptions only become hazards when they affect people 
• The hazards from the volcanic eruption itself are primary hazards:
  • Ash
  • Pyroclastic flow
  • Lava flow
  • Gas emissions
  • Volcanic bombs
• The hazards created that happen as a result of the primary hazards are secondary hazards:
  • Lahars 
  • Acidification
  • Landslides
  • Climate change
  • Fires
  • Floods

Earthquakes and Plate Boundaries

• Earthquakes can occur anywhere but mostly occur at or near plate boundaries
• Earthquakes happen at all plate boundaries - constructive (divergent), destructive (convergent), collision and conservative (transform)

• At a constructive (divergent) plate boundary, earthquakes tend to be weaker as the plates are moving apart
• At destructive (convergent), collision and conservative (transform) plate boundaries earthquakes tend to be stronger

Earthquakes Primary and Secondary Hazards

• The primary hazard of an earthquake is the ground shaking all other hazards then follow on from this as secondary hazards
• Secondary hazards can include:
  • Collapse of buildings and other structures
  • Landslides
  • Gas leaks 
  • Fires
  • Soil liquifaction
  • Subsidence
  • Mudflows
  • Tsunami

Earthquake Sequence

• The sequence of an earthquake is the same regardless of the boundary at which it happens:
  • As the tectonic plates move, they can get stuck
  • Pressure builds as the plates continue to try to move
  • Eventually, the plates jolt free and the pressure is released as energy
  • The point at which the earthquake starts is the focus
  • The epicentre is the point directly above the focus on the earth's surface
  • The energy passes through the Earth's crust as waves which is the earthquake
• Earthquakes can happen as a result of human activity such as drilling into the crust or mining