What is the epicentre of an earthquake?

the point on the surface directly above the focus. The focus is the point underground.

Which scale measures the magnitude of an earthquake?

the Moment Magnitude Scale. It measures the energy released.

What is a pyroclastic flow?

a fast cloud of very hot gas, ash and rock. It moves too fast to outrun.

Which gas from volcanoes causes acid rain?

sulfur dioxide. It reacts with oxygen and water vapour in the air.

Why are soils near volcanoes fertile?

weathered lava and ash are rich in minerals. The minerals are nutrients for crops.

Earthquakes and volcanic eruptions · Cambridge IGCSE Environmental Management

In plain words

Plates do not slide smoothly. They stick, the strain builds for years, and then they jerk. That is an earthquake. And where molten rock finds a way up through the crust, it builds a volcano, which is both a danger and, for millions of farmers, a gift.

9 things to know

An earthquake is a sudden release of energy, when rocks under strain slip. The energy spreads out as waves that shake the ground.
The focus is the point inside the Earth where the earthquake starts. The epicentre is the point on the surface directly above the focus. Shaking is usually strongest there.
Magnitude is the size of an earthquake, a measure of the energy released. It is measured on the Moment Magnitude Scale. A higher number means a much larger earthquake.
In a volcanic eruption, magma rises to the surface. Once it is out, it is called lava.
Eruptions throw out ash, which is fine particles of rock, and volcanic bombs, which are lumps of molten rock that harden as they fly.
Gases are released: water vapour, carbon dioxide and sulfur dioxide. The sulfur dioxide causes acid rain.
A pyroclastic flow is a cloud of very hot gas, ash and rock that rushes down the side of a volcano at great speed. It is the deadliest part of an eruption.
Volcanoes also give opportunities. The soils are fertile, because weathered lava and ash are rich in minerals. Minerals can be extracted, and the rock is used as building material.
Hot rocks near the surface supply geothermal power, and volcanoes attract tourism.

Worked example

Two earthquakes have the same magnitude. One causes far more damage in a nearby city. Suggest why, using the terms focus and epicentre.

Magnitude measures the energy released, so that is the same for both.
If the focus is shallow, the waves have little rock to pass through before they reach the surface, so the shaking is stronger.
If the epicentre is close to the city, the strongest shaking is where the people and buildings are.

Tips and tricks

Focus: underground, where it starts. Epicentre: on the surface, directly above. Do not swap them.
Magma is underground. Lava is on the surface. It is the same material with two names.

6 questions, about 2 minutes.

It lands in your notebook with its questions as flashcards.

Earthquakes and volcanic eruptions: 6 questions and answers

These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.

What is the epicentre of an earthquake?
- the point underground where it starts
- the point on the surface directly above the focus (the answer)
- the edge of a tectonic plate
- the centre of the Earth
The focus is the point underground.
Which scale measures the magnitude of an earthquake?
- the pH scale
- the Celsius scale
- the Moment Magnitude Scale (the answer)
- the Beaufort scale
It measures the energy released.
What is magma called once it reaches the surface?
- ash
- basalt
- a volcanic bomb
- lava (the answer)
Same material, new name.
What is a pyroclastic flow?
- a fast cloud of very hot gas, ash and rock (the answer)
- a flood of melted snow
- a slow river of lava
- a crack in the ground
It moves too fast to outrun.
Which gas from volcanoes causes acid rain?
- sulfur dioxide (the answer)
- water vapour
- nitrogen
- oxygen
It reacts with oxygen and water vapour in the air.
Why are soils near volcanoes fertile?Stretch
- they are always wet
- they contain no rock
- weathered lava and ash are rich in minerals (the answer)
- eruptions add organic matter
The minerals are nutrients for crops.

6 questions

Tap an answer and you’ll see straight away whether it’s right, and why.

Worksheet

3 questions, 8 marks. Write your answers on paper, then check them.

Earthquakes and volcanic eruptions

Cambridge IGCSE Environmental Management 0680 · 8 marks · papermunch.org

Name ______________________________ Date ______________

Define the terms focus and epicentre.[2]
Show answerHide answer
The focus is the point inside the Earth where an earthquake starts. The epicentre is the point on the Earth's surface directly above the focus.
I got this one
Name three gases released in a volcanic eruption.[3]
Show answerHide answer
Water vapour, carbon dioxide and sulfur dioxide.
I got this one
Explain why people farm on the slopes of volcanoes despite the danger.[3]
Show answerHide answer
Lava and ash weather into fertile soil that is rich in minerals. Crop yields are high. Eruptions are rare, so people judge the benefit to be greater than the risk.
I got this one

Answers: Earthquakes and volcanic eruptions

1. The focus is the point inside the Earth where an earthquake starts. The epicentre is the point on the Earth's surface directly above the focus.
2. Water vapour, carbon dioxide and sulfur dioxide.
3. Lava and ash weather into fertile soil that is rich in minerals. Crop yields are high. Eruptions are rare, so people judge the benefit to be greater than the risk.