Tectonic hazards — AQA GCSE Geography revision

Plate tectonics

• The Earth's crust is split into plates that move on the semi-molten asthenosphere, driven by convection and by slab pull at subduction zones (slab pull is now regarded as the dominant force).
• Destructive (convergent) margins: denser oceanic crust subducts beneath continental crust, producing deep earthquakes, ocean trenches, fold mountains and explosive composite volcanoes.
• Constructive (divergent) margins: plates move apart, magma rises to fill the gap, producing shallow earthquakes and gentle shield volcanoes (e.g. mid-ocean ridges). Conservative margins: plates slide past each other, causing earthquakes but no volcanoes.

Effects and responses

• Primary effects happen instantly (buildings collapse, deaths, roads fractured); secondary effects follow (fires from ruptured gas mains, disease from broken water pipes, business closure, homelessness).
• Immediate responses happen in hours/days (search and rescue, emergency aid, temporary shelter); long-term responses take months/years (rebuilding to earthquake-resistant standards, retraining, new hazard plans).
• Impacts are usually more severe in lower-income countries because of weaker building regulations, fewer emergency services, less insurance and slower recovery funding — the hazard is natural, the disaster is largely human.

Key terms

Natural hazard

A natural process that threatens people or property.

Subduction

The sinking of denser oceanic crust beneath another plate at a destructive margin.

Focus

The point underground where an earthquake originates.

Epicentre

The point on the surface directly above the focus.

Primary effect

An impact caused directly and immediately by the hazard event.

Secondary effect

An impact that follows on from the primary effects.

Common exam traps

• Writing that plates move only because of convection currents — slab pull at subduction zones is now seen as the main driver.
• Confusing focus (underground origin) with epicentre (point on the surface above it).
• Calling fires or disease a primary effect — they follow the shaking, so they are secondary.

Practice questions with answers

1. 1. At which plate margin is oceanic crust destroyed?

   • • Constructive
   • • Destructive
   • • Conservative
   • • Collision

   Answer: Destructive

   Denser oceanic crust subducts and melts at a destructive margin.

2. 2. Which landform is typical of a constructive plate margin?

   • • Ocean trench
   • • Mid-ocean ridge
   • • Fold mountains
   • • Rift-free plateau

   Answer: Mid-ocean ridge

   Plates separating allow magma to rise and build a ridge of new crust.

3. 3. A conservative margin produces which combination?

   • • Earthquakes only
   • • Volcanoes only
   • • Both earthquakes and volcanoes
   • • Neither

   Answer: Earthquakes only

   Plates slide past one another; no crust is created or destroyed, so no magma rises.

4. 4. Define the term 'primary effect' of an earthquake and give one example.

   Answer: An immediate, direct impact of the shaking, e.g. buildings collapsing

   Primary effects are caused by the event itself, not by knock-on consequences.

5. 5. Which is a secondary effect of a volcanic eruption?

   • • Lava burning farmland
   • • Ash fall damaging roofs
   • • Long-term loss of tourist income
   • • Pyroclastic flow deaths

   Answer: Long-term loss of tourist income

   Economic decline follows the event rather than being caused directly by it.

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