A lahar is a fast-moving flow of volcanic ash and debris mixed with water, with roughly the consistency of wet concrete.

Why was the Nevado del Ruiz disaster considered particularly avoidable?

The eruption itself was small, hazard maps had already shown Armero to lie in the path of lahars, and the warnings that were issued were not acted upon.

Why do lahars follow river valleys?

They are dense fluid flows, so gravity channels them along the existing drainage lines, which are also where roads and settlements tend to be built.

What is a jökulhlaup?

A jökulhlaup is a sudden glacial outburst flood caused when an eruption beneath an ice cap melts a large volume of ice.

WJEC A-Level Geography 601-8455-3

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Tectonic Hazards

Secondary volcanic hazards

3 things to know
AS Unit 1 and A2 Unit 4

Overview

Three ways a lahar can be triggered, volcanic landslide or sector collapse and eruption that began with the largest landslide ever recorded.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

What is a lahar?

Key term

What are three ways a lahar can be triggered?

Numbers

Name the eruption whose lahars caused one of the worst volcanic disasters of the twentieth century.

Questions from this subtopic

Each one with the model answer underneath.

1
What is a lahar?
Model answer
A lahar is a fast-moving flow of volcanic ash and debris mixed with water, with roughly the consistency of wet concrete.
2
What are three ways a lahar can be triggered?
Model answer
Heavy rain falling on fresh ash deposits, the rapid melting of snow and ice by an eruption, and the sudden failure of a crater lake.
3
Name the eruption whose lahars caused one of the worst volcanic disasters of the twentieth century.
Model answer
The 1985 eruption of Nevado del Ruiz in Colombia, whose lahars buried the town of Armero and killed about 23,000 people.
4
Why was the Nevado del Ruiz disaster considered particularly avoidable?
Model answer
The eruption itself was small, hazard maps had already shown Armero to lie in the path of lahars, and the warnings that were issued were not acted upon.
5
Why do lahars follow river valleys?
Model answer
They are dense fluid flows, so gravity channels them along the existing drainage lines, which are also where roads and settlements tend to be built.
6
What is a jökulhlaup?
Model answer
A jökulhlaup is a sudden glacial outburst flood caused when an eruption beneath an ice cap melts a large volume of ice.

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