GCSE · Geography · AQA · Spec 8035

Freeze-thaw weathering

Rock looks unbreakable. So how does plain water, soft enough to pour through your fingers, end up splitting it into pieces, with no hammer and no chemicals?

Rebuild the freeze-thaw loop

2 stages are missing. Use the rest of the loop to work out which goes where.

Stage 1: Water gets into a crack
The starting point. On every trip round the loop, the water reaches a little further in.
Stage 3: The ice widens the crack
Jammed inside the crack, the growing ice pushes outwards on the rock and forces it open a little more.

…and stage 4 leads back to stage 1.

What really breaks the rock?

Which idea is closest to yours?

A crack in a steep rock face has been through many rounds of freezing and thawing. Now a piece of the rock has broken off.

What do you think actually broke the rock?

The ice reacted with the rock and turned it into something weaker.One really hard frost froze the water and split the rock in one go.The freezing did all the work. When the ice melted, nothing much happened until the next frost.The water kept freezing and thawing. Each time, the crack got a bit wider, until the rock broke apart.

How sure are you?

I'm sureI think soI'm guessing

The ice reacted with the rock and turned it into something weaker.

This is a common misconception

Nothing about what the rock is made of has changed. The ice doesn't react with the rock. It pushes on it. Breaking rock into smaller pieces without any chemical change is called mechanical weathering, and freeze-thaw is one type of it.

One really hard frost froze the water and split the rock in one go.

This is a common misconception

A freeze widens the crack; that's its job in the loop. What breaks the rock is repeated cycles of freezing and thawing, each one widening the crack a little more, until the rock eventually breaks into smaller pieces.

The freezing did all the work. When the ice melted, nothing much happened until the next frost.

This is a common misconception

The thaw has a job too. When the ice melts, the water seeps deeper into the crack, so it's further in when the next freeze comes. Freeze and thaw work as a pair.

The water kept freezing and thawing. Each time, the crack got a bit wider, until the rock broke apart.

This matches the evidence

Exactly. Nothing chemical happened: the rock was physically broken into smaller pieces, which makes freeze-thaw mechanical weathering. And it was the repetition that did it: freeze, thaw, freeze again, until the rock gave way.

Weathering is the breakdown or disintegration of rocks at or close to the Earth's surface. Freeze-thaw is the mechanical kind: rock physically broken into smaller pieces, with no chemical change, over many cycles.

Predict, then check

Think about the loop you just rebuilt. What does it need to keep going round?

Freeze-thaw weathering is very common on coasts with cold climates. Why?

Where the broken rock goes

From cracked rock face to scree

View: 1. Weakened. Showing 2 layers: Rock face, Weakened rock

View

Explore

step through the views →

Repeated freezing and thawing have widened a crack near the top of this steep rock face.

Step through the three views, then open each part to read what it is.

Your turn

Explain it in your own words

Explain how freeze-thaw weathering breaks rock into smaller pieces. [4 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

Water enters cracks in the rock. (1 mark)
Below freezing, the water turns to ice and expands. (1 mark)
This puts pressure on the surrounding rock, widening the crack. (1 mark)
On thawing, the water seeps deeper into the crack. (1 mark)
Repeated cycles widen the crack until the rock breaks into smaller pieces. (1 mark)

Model answer

Water enters a crack in the rock. When the temperature falls below freezing, it turns to ice and expands, which puts pressure on the surrounding rock and widens the crack. When the ice thaws, the water seeps deeper. Repeated cycles of freezing and thawing widen the crack until the rock breaks into smaller pieces.

A strong answer is a chain: each step causes the next, and then the chain goes round again. If your answer stops at the first freeze, it's missing the loop.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

How water, freezing and thawing over and over, breaks solid rock apart.

What you need to know

Weathering is the breakdown or disintegration of rocks at or close to the Earth's surface.
Freeze-thaw is a type of mechanical weathering: rock is physically broken into smaller pieces, with no chemical change to the rock.
The loop: water enters a crack → the temperature drops below freezing, so the water turns to ice and expands → the ice puts pressure on the rock and widens the crack → the ice thaws and the water seeps deeper → it freezes again.
Repeated cycles of freezing and thawing widen the cracks until the rock eventually breaks into smaller pieces.

The big picture

Weathering is the breakdown or disintegration of rocks at or close to the Earth's surface. Freeze-thaw is a type of mechanical weathering: it breaks rock physically into smaller pieces, with no chemical change. Water gets into a crack, freezes and expands, and the pressure widens the crack. When the ice thaws, the water seeps deeper. After many cycles the rock breaks apart. It is very common on coasts with cold climates, where temperatures fluctuate and waves and rain supply water. On a steep rock face, the weakened rock can break away and fall (a rock fall), and the fragments pile up at the base as scree.

Key points

1The thaw matters as much as the freeze: it lets the water seep deeper, so the next freeze widens the crack further.

2Freeze-thaw is very common on coasts with cold climates, because temperatures there fluctuate and waves and rain supply water.

3Rock weakened by freeze-thaw can break away from a steep rock face and fall to its base (a rock fall). The fragments that pile up at the base are called scree.

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Worked example

Problem

Coast A has a cold climate, and its temperature often rises above and falls below freezing. Coast B's temperature stays below freezing all winter. Explain why freeze-thaw weathering is likely to be more active at Coast A. (3 marks)

⚠ Watch out

Calling freeze-thaw chemical weathering. The ice doesn't react with the rock or change what it's made of. It physically pushes the crack wider, so freeze-thaw is mechanical weathering.

🧠

Memory hook

Freeze pushes, thaw sneaks, and round it goes. Each freeze pushes the crack wider; each thaw sneaks the water in deeper. Repeat until the rock gives way.

✓

Check yourself

Cover the page. Start from 'water gets into a crack' and talk yourself round the loop twice. On the second time round, say what's different about the crack, and where the water is.

Flashcards

(11)

What is weathering?

The breakdown or disintegration of rocks at or close to the Earth's surface.

Which type of weathering is freeze-thaw?

Mechanical weathering.

What makes weathering 'mechanical'?

The rock is physically broken into smaller pieces, without any chemical change to the rock.

In freeze-thaw weathering, where does the water collect?

In cracks in the rock.

What happens to water in a crack when the temperature drops below freezing?

It turns to ice and expands.

How does freezing make a crack wider?

The expanding ice puts pressure on the surrounding rock, which widens the crack.

What happens to the water when the ice in a crack thaws?

It seeps deeper into the crack, ready for the next freeze.

When does the rock finally break into smaller pieces?

After repeated cycles of freezing and thawing have widened the cracks.

Why is freeze-thaw very common on coasts with cold climates?

Temperatures fluctuate, and waves and rain supply water.

What is a rock fall on a steep rock face?

Rock breaking away from the face and falling to its base, for example rock weakened by freeze-thaw.

What is scree?

The fallen rock fragments that pile up at the base of a steep rock face.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Rebuild the freeze-thaw loop

Which idea is closest to yours?

Explain it in your own words

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