Explain decompression melting at a spreading ridge.

As the plates part, hot mantle rock rises and the pressure on it falls, which lowers its melting point enough for it to melt without any additional heat.

Why are eruptions at a divergent margin usually effusive rather than explosive?

The magma is basaltic, low in silica and poor in gas, so it is runny and the lava escapes steadily instead of building up pressure.

Why are earthquakes at a mid-ocean ridge shallow and generally small?

The lithosphere there is thin, hot and weak, so it cannot store much strain before it fractures.

What is a transform fault at a mid-ocean ridge?

It is a fracture that offsets the ridge into separate segments, along which the two sides slide past one another as spreading continues.

WJEC A-Level Geography 601-8455-3

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Tectonic Hazards

Divergent and conservative plate margins

3 things to know
AS Unit 1 and A2 Unit 4

Overview

Decompression melting at a spreading ridge, transform fault at a mid-ocean ridge and continental example of a divergent margin.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Explain elastic rebound theory.

Key term

What happens at a divergent plate margin?

Key term

What landform is produced where a divergent margin runs through an ocean basin?

Questions from this subtopic

Each one with the model answer underneath.

1
What happens at a divergent plate margin?
Model answer
Two plates move apart and magma rises to fill the gap and solidify, so new crust is created, which is why it is also called a constructive margin.
2
What landform is produced where a divergent margin runs through an ocean basin?
Model answer
A mid-ocean ridge, such as the Mid-Atlantic Ridge.
3
Explain decompression melting at a spreading ridge.
Model answer
As the plates part, hot mantle rock rises and the pressure on it falls, which lowers its melting point enough for it to melt without any additional heat.
4
Why are eruptions at a divergent margin usually effusive rather than explosive?
Model answer
The magma is basaltic, low in silica and poor in gas, so it is runny and the lava escapes steadily instead of building up pressure.
5
Why are earthquakes at a mid-ocean ridge shallow and generally small?
Model answer
The lithosphere there is thin, hot and weak, so it cannot store much strain before it fractures.
6
What is a transform fault at a mid-ocean ridge?
Model answer
It is a fracture that offsets the ridge into separate segments, along which the two sides slide past one another as spreading continues.

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