Pick one before you read on. It is fine to be wrong.
By the end you can
Explain how heat, water, ice and roots break up rocks
Explain how rivers, wind and waves carry rock away
Look at an old stone wall or a big rock. Have you seen small cracks in it? Rock is hard, but over a very long time it breaks into smaller pieces. That is how a mountain can turn into sand.
Weathering is the breaking of rock into smaller pieces, in the place where it lies. The pieces do not go anywhere yet. Weathering has several causes.
Draw it first
Draw ice breaking a rock
1 Draw a rock with a narrow crack. Draw rain water in the crack. The water takes up a small space.
2 Draw the crack again on a freezing night. The water has frozen into ice. The ice is bigger, so the crack is wider, and it pushes the sides apart.
Rain water seeps into cracks. In cold places it freezes at night. Water gets bigger when it freezes, so the ice pushes on the sides of the crack. After many freezing nights the crack grows wider, and the rock breaks.
See it three ways
As a picture
In words
Water gets about 9 percent bigger when it freezes. The ice needs more room than the water did, so it pushes on the crack. A millilitre (mL) holds the same space as a cubic centimetre (cm³).
In symbols
100 cm³ of water makes about 109 cm³ of ice.
Rain water is a little acid. Over a very long time it can slowly dissolve some rock, such as limestone.
Heat breaks rock too. Some places have hot days and cold nights. There a rock gets a little bigger by day and a little smaller by night. Over many years the rock cracks and flakes.
The change in size is small, and it is drawn bigger here so that you can see it.After many days of heating and cooling the rock cracks.
Living things break rock as well. A seed can land in a crack. When it grows, its roots push into the crack. As the roots get thicker, they push the sides apart.
Roots grow thicker and push the sides of the crack apart.
Erosion is the carrying away of rock pieces. Rivers, wind and waves do this work. The pieces they carry are called sediment.
A river carries sand and stones downhill. The pieces knock against each other and against the river bed. They break into smaller bits, and their sharp corners wear off.
Far down the river, the pieces are smaller and rounder.
Wind lifts dry sand and dust and drops it in a new place. Waves hit the foot of a cliff and drag loose pieces back out to sea.
Rivers, wind and waves all carry rock pieces away.
Try it
Shake stones in a jar
You need: a plastic jar with a lid, three soft stones, such as sandstone or chalk, water, a lens, if you have one.
Be safe: Use a plastic jar, never glass. Close the lid tight and shake away from other people. Wash your hands afterwards.
Put the stones in the jar. Pour in water until the jar is half full. Close the lid tight.
Shake the jar hard for five minutes. A grown up can count the minutes.
Stand the jar still for an hour. Look at the bottom and at the water.
Look at the grit at the bottom, with the lens if you have one. Look at the stones. Are their corners the same as before?
What you will see: A thin layer of grit settles at the bottom, perhaps a millimetre or two thick. The picture draws it much thicker so that you can see it. The stones have rounder corners, and the water may stay cloudy for a while. The stones knocked bits off each other, as stones do in a river.
If you cannot try it, this is what happens.
A wrong idea many people have
Weathering and erosion are the same thing.
Why it feels right: Both wear rock down, and both happen at the same time.
The test that breaks it: A boulder on a hill can be full of cracks. It is weathered, but it has not moved. Only when a river or the wind carries a piece away is that erosion.
What is true: Weathering breaks rock where it lies. Erosion carries the pieces away.
Key words
weathering
the breaking of rock into smaller pieces, where it lies.
erosion
the carrying away of rock pieces by a river, wind or waves.
sediment
the small pieces of rock that are carried and dropped.
Back to your guess
The answer is C: Heat, ice, water and roots break rock into small pieces, and then the pieces are carried away.
Rock is broken into pieces where it lies. That is weathering. Rivers, wind and waves then carry the pieces away. That is erosion.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
What is weathering?
See the answer
Rock breaking into pieces where it lies
Weathering breaks rock in the place where it lies. Erosion carries the pieces away.
If you chose A: That is erosion. Weathering is the breaking that comes first, in the same place. Go to S3.11.3.
If you chose B: Melting is something else. Weathering breaks rock without melting it. Go to S3.11.3.
Why can freezing water break a rock?
See the answer
Water gets bigger when it freezes, and it pushes the crack wider.
About 100 cm³ of water makes about 109 cm³ of ice. The extra room pushes the crack wider.
If you chose B: It is not the weight. Ice needs more room than the water did, so it pushes the sides of the crack apart. Go to S3.11.3.
If you chose C: Water gets bigger when it freezes, not smaller. That is why it pushes. Go to S3.11.3.
How can a tree help to break a rock?
See the answer
Its roots grow into a crack and push the sides apart.
As roots grow thicker, they push the sides of a crack apart.
If you chose A: Leaves give shade. It is the roots, growing into cracks, that break rock. Go to S3.11.3.
If you chose C: A falling fruit is far too light. The roots do the work, slowly. Go to S3.11.3.
Some places have hot days and cold nights. Why can a rock crack there?
See the answer
It gets a little bigger by day and a little smaller by night, again and again.
Heat makes the rock expand and cold makes it contract. Many years of this makes cracks and flakes.
If you chose A: The Sun does not melt rock. The rock only expands when it warms and contracts when it cools. Go to S3.11.3.
If you chose B: Rock does not turn to ice. It is the repeated change of size, bigger and smaller, that cracks it. Go to S3.11.3.
A river carries sand and stones from the hills to the sea. What is this called?
See the answer
Erosion
Erosion is the carrying away of rock pieces by a river, wind or waves.
If you chose B: Weathering breaks the rock where it lies. Carrying it away is erosion. Go to S3.11.3.
If you chose C: Sediment is the name for the pieces that are carried. The carrying itself is erosion. Go to S3.11.3.
Why are pebbles far down a river smaller and rounder than boulders near the top?
See the answer
They knock against each other and the river bed as they are carried.
The long journey breaks pieces into smaller bits and wears off their corners.
If you chose A: Rock does not form in small round pebbles. Pieces are worn into that shape on the way. Go to S3.11.3.
If you chose C: Warm water does not shrink rock. Knocking and rubbing wear the pieces down. Go to S3.11.3.
From an earlier chapter
Which layer of the Earth is mostly solid rock that flows very slowly?
See the answer
The mantle
The mantle is hot rock that is mostly solid. It flows a few centimetres each year.
If you chose B: The outer core is liquid metal. The mantle is the thick layer of mostly solid rock. Go to S3.11.2.
If you chose C: The crust is the thin top layer. It does not flow. Go to S3.11.2.
A crack holds 200 cm³ of water. Water makes 109 cm³ of ice from every 100 cm³. How much room does the ice need?
See the answer
218 cm³
200 cm³ is 2 lots of 100 cm³. Each lot makes 109 cm³ of ice, and 2 × 109 = 218.
Keep it
Copy this into your exercise book.
Weathering breaks rock where it lies. Ice, heat and roots all do it. Erosion carries the pieces away. Rivers, wind and waves do it. The pieces get smaller and rounder.
Draw: Draw a rock with a crack and ice in it. Draw arrows that push the sides apart. Beside it draw a river with big pieces at the top and small round ones below.
