Geophysical hazards: earthquakes, volcanoes, tsunami and landslides

Plate boundaries and hazards

The lithosphere is broken into tectonic plates that move slowly relative to each other. Most earthquakes and volcanoes occur at plate boundaries.

• Divergent (constructive): plates move apart; magma rises, giving frequent, mostly gentle basaltic eruptions and shallow earthquakes (for example Iceland on the Mid-Atlantic Ridge).
• Convergent (destructive): an oceanic plate subducts beneath another plate. This gives the largest earthquakes (megathrust earthquakes on the subduction interface), explosive volcanoes and most large tsunami.
• Collision: two continental plates meet; powerful shallow earthquakes but little volcanic activity (for example the Himalaya).
• Conservative (transform): plates slide past each other; earthquakes but no volcanoes (for example the San Andreas Fault in California).
• Hotspots: plumes of hot mantle away from boundaries (for example Hawaii) give effusive volcanoes.

Earthquakes and tsunami

Stress builds up where plates lock together. When rocks suddenly slip, energy is released as seismic waves, which cause ground shaking.

Secondary hazards include liquefaction (saturated sediments lose strength and behave like a liquid), landslides and fires.

A tsunami forms when the sea bed is suddenly displaced vertically, lifting a huge column of water. The 2004 Indian Ocean tsunami came from a magnitude 9.1 earthquake on the interface between the India and Burma plates; sea-floor uplift displaced the water above it (USGS).

Volcanic hazards

• Lava flows: usually slow, destroy property more than lives.
• Pyroclastic flows: fast, very hot avalanches of gas, ash and rock; the deadliest primary hazard.
• Tephra and ash fall: collapse roofs, ground aircraft, damage crops.
• Lahars: mudflows of ash and water (from rain or melting snow) that rush down river valleys.
• Volcanic gases: such as carbon dioxide and sulphur dioxide.

Explosivity depends on magma: viscous, silica-rich, gas-rich magma at subduction zones erupts explosively; runny basaltic magma at divergent boundaries and hotspots is usually effusive.

Landslides and avalanches: complex hazards

A slope fails when the forces pulling material downslope (shear stress) exceed the strength holding it in place (shear strength).

They are complex hazards because the trigger is often another hazard:

• heavy or prolonged rain saturates the slope, adding weight and reducing friction
• heavy snowfall or rapid warming loads or weakens a snowpack (avalanches)
• earthquake shaking or volcanic activity shakes material loose.

Human actions such as deforestation, cutting into slopes for roads and building on steep land raise the risk.

Worked example

Explain why the 2004 Sumatra-Andaman earthquake produced a tsunami.

1. The earthquake was on a subduction (convergent) boundary between the India and Burma plates.
2. Rupture of the plate interface uplifted the sea floor suddenly.
3. This vertical movement displaced the whole column of water above it, generating waves that spread across the Indian Ocean.

Answer: Sudden vertical displacement of the sea bed at a subduction zone displaced the water above it, creating a tsunami.

Check yourself

1. At which type of plate boundary are earthquakes common but volcanoes absent?
   • Divergent
   • Conservative (transform)
   • Oceanic-continental convergent
   • Oceanic-oceanic convergent
   Show answer

   Conservative (transform). Plates slide past each other, so no magma is generated.

2. What is the essential cause of an earthquake-generated tsunami?
   • Strong winds blowing over the ocean
   • Horizontal sliding of plates along a transform fault
   • Very high tides at full moon
   • Sudden vertical displacement of the sea bed
   Show answer

   Sudden vertical displacement of the sea bed. The water column is lifted or dropped; horizontal slip moves little water.

3. A mudflow of volcanic ash and water rushing down a river valley is called
   • a pyroclastic flow
   • a lahar
   • a lava flow
   • tephra
   Show answer

   a lahar. Lahars can travel tens of kilometres down valleys.

4. Which volcanic hazard is usually the deadliest because of its speed and temperature?
   • Lava flow
   • Gas emissions
   • Pyroclastic flow
   • Ash fall
   Show answer

   Pyroclastic flow. Pyroclastic flows are fast, hot and leave no time to escape.

5. Liquefaction during an earthquake happens when
   • saturated loose sediments lose strength and behave like a liquid
   • lava melts surface rocks
   • a tsunami floods coastal land
   • groundwater boils from friction
   Show answer

   saturated loose sediments lose strength and behave like a liquid. Shaking raises water pressure between grains, so buildings sink or tilt.

6. Why are landslides described as complex hazards?
   • They only happen in developing countries
   • They are measured on several scales at once
   • They are often triggered by another hazard, such as heavy rain or an earthquake
   • They always occur at plate boundaries
   Show answer

   They are often triggered by another hazard, such as heavy rain or an earthquake. Slope processes combine with weather or tectonic triggers.

7. Explain why volcanoes at subduction zones tend to erupt explosively.
   Show answer

   Subduction produces viscous, silica-rich magma that traps gas, so pressure builds until it is released in a violent explosion.. Viscous, silica-rich magma traps gas until pressure is released explosively.

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Sources

• Pearson Edexcel International Advanced Subsidiary/Advanced Level in Geography specification (Issue 3, September 2025)
• USGS: Indian Ocean tsunami remembered (23 December 2014)

Last updated 8 November 2026. Spotted a mistake? Tell us.