What this lesson teaches
I can link where earthquakes and volcanoes occur to plate boundaries and hotspots.
Syllabus K229, Topic 3.2, Key Question 3. How might distribution of earthquakes and volcanoes influence the location of tectonic hazards?
I can explain the hazards, impacts and benefits of earthquakes and volcanoes for nature and people.
Syllabus K229, Topic 3.2, Key Question 4. How might tectonic hazards affect the natural and human systems?
Make a guess
A volcano stopped erupting a year ago. Then heavy rain falls on its slopes. Can the volcano still be dangerous?
- Yes. Rain can turn loose debris into a lahar.
- Not unless the rain makes it erupt again.
- No, because rain cools the volcano safely.
- No. The danger ends once the eruption has ended.
Show the answer
Yes. Rain can turn loose debris into a lahar.
Water mixed with loose volcanic deposits forms a fast mudflow that rushes down valleys. Lahars can start months or years after an eruption.
Wind spreads volcanic ash, and water can turn loose volcanic debris into lahars, even long after an eruption ends.
Tephra is any material thrown out by a volcano, from big blocks to fine ash. Ash is made of tiny bits of rock and glass. Wind carries it far. It cuts visibility, spoils water, damages engines and grounds flights. Thick ash kills crops. Wet ash is heavy and can collapse roofs.
Volcanoes release water vapour, carbon dioxide and sulfur dioxide. How dangerous they are depends on how concentrated they are and where they collect. Carbon dioxide can gather in low places and push out breathable air. Sulfur dioxide irritates lungs and causes acid pollution.
A lahar is a fast mudflow of water mixed with volcanic debris. The water may come from heavy rain, melting snow and ice, or a crater lake. Lahars rush down valleys, bury farms and towns and destroy bridges. A lahar is wet and water-rich, unlike a hot pyroclastic flow.
Rain can start lahars months or years after an eruption. At Mount Rainier in the USA, snow, ice and weak rock make lahars a key danger to towns downstream.
- Tephra
- Any material a volcano throws out, including ash.
- Lahar
- Mudflow of water and volcanic debris.
- After the eruption
- Rain can trigger lahars much later.
Step by step
Material is released
Eruptions spread tephra and gases.
Wind spreads ash
Places and flights downwind are affected.
Water moves debris
Rain or meltwater turns loose deposits into lahars.
Real examples to use
Nevado del Ruiz, Colombia, 1985
A small eruption melted snow and ice. The resulting lahar buried the town of Armero, killing about 23,000 people.
Eyjafjallajokull, Iceland, 2010
Ash from this eruption closed much of Europe's airspace for days, and about 100,000 flights were cancelled.
Lake Nyos, Cameroon, 1986
A sudden release of carbon dioxide from a volcanic lake suffocated about 1,700 people.
Worked example: Wind decides where ash falls
Two towns are the same distance from a volcano. If the wind blows towards one, it gets much heavier ash. Distance matters, but wind direction explains the difference.
Watch out for this
Lahars can only happen during an eruption.
Rain or meltwater can turn old volcanic deposits into lahars long after the eruption.
Check your understanding
Heavy rain falls on loose volcanic deposits months after an eruption. What hazard can form?
- A lahar flowing down the river valleys.
- A new pyroclastic flow from the crater.
- A new earthquake under the volcano.
Show the answer
A lahar flowing down the river valleys.
Yes. Water turns loose debris into a mudflow.
Check your understanding
How did the 1985 eruption of Nevado del Ruiz kill about 23,000 people in Armero?
- A lava flow ran down the valley and reached the town.
- It melted snow and ice into a lahar that buried the town.
- Poisonous gas from the crater collected in the valley around the town.
Show the answer
It melted snow and ice into a lahar that buried the town.
A small eruption triggered a deadly mudflow.