What this lesson teaches
I can explain why disaster risk from earthquakes and eruptions differs between places.
Syllabus K329, Topic 4.3, Key Question 2. Why do disaster risks related to earthquakes and volcanic eruptions vary across places?
Make a guess
Two towns are the same distance from an erupting volcano. Do they face the same ash risk?
- Yes, because distance decides the risk.
- Yes, because ash spreads evenly in a circle.
- No, not necessarily. Ash falls where the wind takes it.
- No, the town nearer the sea is safer from ash.
Show the answer
No, not necessarily. Ash falls where the wind takes it.
Wind direction, eruption height and particle size decide where tephra lands. A town downwind can be buried while one upwind gets little.
Volcanic risk follows each hazard's route: ash goes where the wind blows, and lahars follow valleys.
Magma chemistry shapes the eruption. Magma composition affects its viscosity and how easily gas escapes. Silica-rich, viscous magma often erupts explosively. Runnier magma can send lava flows a long way. But losses still depend on the people and property in each hazard's path.
Water adds more hazards. Rain, rivers and melting snow or ice can mix with loose volcanic material to form lahars. Groundwater and waterlogged, weakened rock can make slopes collapse into wet debris flows. So check drainage routes, loose material and water sources, not just the eruption.
Wind decides where tephra falls. Particle size, eruption height and changing weather affect how far ash travels and how thick it lands. Two towns the same distance from a volcano can get very different amounts of ash. Changing winds mean warnings must be updated.
Where people live matters. Towns in valleys, on slopes or downwind may expose many people and services. Farmers depend on crops that ash can destroy. Weak roofs can collapse under wet ash, and poor roads slow evacuation. Risk maps must combine the hazards with people, infrastructure and the ability to respond.
Past deposits, monitoring and models help identify likely danger zones, but future eruptions can differ. When comparing risks, explain the evidence and the process it points to. Say what else you would need to know.
- Magma chemistry
- Viscous, silica-rich magma tends to explode.
- Water
- Turns loose debris into lahars.
- Wind
- Decides where ash falls.
Step by step
The eruption
Magma type and gas shape the hazards.
Routes
Wind carries ash; valleys channel lahars.
People and buildings
Location, roofs and roads affect vulnerability.
Real examples to use
Mount Pinatubo, 1991
Scientists spotted warning signs, and tens of thousands of people were evacuated before the main eruption. This saved thousands of lives, though about 850 people died, mostly when wet ash from a passing typhoon collapsed roofs.
Nevado del Ruiz, 1985
Scientists warned of lahars, but the warnings were not acted on in time, and about 23,000 people died in Armero.
Worked example: Dry today, dangerous later
A valley may be dry when a hazard map is made. But loose volcanic material lies upstream. Heavy rain later can turn it into a lahar. Plans must consider future conditions.
Watch out for this
Two towns the same distance from a volcano have equal risk.
Wind, valleys, buildings and preparation can make their risks very different.
Check your understanding
Which information matters most for a downstream town's lahar risk?
- Water sources, loose volcanic deposits and valley routes.
- The town's height above sea level and its population.
- The town's distance from the coast and the nearest port.
Show the answer
Water sources, loose volcanic deposits and valley routes.
Yes. These decide whether and where a lahar can flow.