What controls the viscosity of magma?

Its silica content and its temperature, so that silica-rich magma at lower temperatures is stiff and viscous while hot, silica-poor magma is runny.

Why is rhyolitic magma so dangerous?

It contains over about 69% silica, so it is extremely viscous, holds its dissolved gas until the pressure becomes enormous, and then erupts violently.

Describe the shape of a shield volcano and explain why it takes that form.

It has a very broad base and gentle slopes of under about 10°, because the runny basaltic lava travels a long way from the vent before it solidifies.

Name a shield volcano.

Mauna Loa in Hawaii, which is the largest active volcano on Earth.

How is a composite volcano built up?

Successive eruptions lay down alternating layers of lava and pyroclastic material, and these build the classic steep-sided cone.

WJEC A-Level Geography 601-8455-3

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Tectonic Hazards

Types of volcano and styles of eruption

3 things to know
AS Unit 1 and A2 Unit 4

Overview

Approximate silica content of basaltic magma, shield volcano and two composite volcanoes.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

What is a cinder cone?

Key term

Explain how the same volcano can erupt effusively at one time and explosively at another.

Key term

State the approximate silica content of basaltic magma.

Questions from this subtopic

Each one with the model answer underneath.

1
What controls the viscosity of magma?
Model answer
Its silica content and its temperature, so that silica-rich magma at lower temperatures is stiff and viscous while hot, silica-poor magma is runny.
2
State the approximate silica content of basaltic magma.
Model answer
Around 45 to 52%, which is why it flows so readily.
3
Why is rhyolitic magma so dangerous?
Model answer
It contains over about 69% silica, so it is extremely viscous, holds its dissolved gas until the pressure becomes enormous, and then erupts violently.
4
Describe the shape of a shield volcano and explain why it takes that form.
Model answer
It has a very broad base and gentle slopes of under about 10°, because the runny basaltic lava travels a long way from the vent before it solidifies.
5
Name a shield volcano.
Model answer
Mauna Loa in Hawaii, which is the largest active volcano on Earth.
6
How is a composite volcano built up?
Model answer
Successive eruptions lay down alternating layers of lava and pyroclastic material, and these build the classic steep-sided cone.

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