Rocks, soil and fossils. Attnom Academy

S3.11.1 Chapter 1

Three kinds of rock

Why do some rocks sparkle and others crumble?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A All rocks are made the same way. They only differ in colour.
You picked A. Read on. At the end, the chapter comes back to your guess.
B Rocks form in different ways, and that changes their grains and how hard they are.
You picked B. Read on. At the end, the chapter comes back to your guess.
C Soft rocks are young rocks that have not finished growing.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Describe a rock by its grains, colour and hardness
Test the hardness of a rock by scratching it with a nail and a coin
Sort rocks into igneous, sedimentary and metamorphic by how they formed

Pick up a stone and look at it closely. Does it sparkle? Is it rough or smooth? Can you scratch it? Rocks are not all the same, and the differences tell us how each one was made.

A rock is made of small pieces. Each piece is a grain. A grain may be a crystal. A crystal is a small solid with flat sides that grew in one place. Or a grain may be a bit of older rock. You can describe any rock by three things: its grains, its colour and how hard it is.

Draw it first

Draw four rocks, close up

1 Draw a square. Fill it with a few large shapes that touch. These are the big crystals of granite.
2 Draw another square with many small shapes that touch. These are the tiny crystals of basalt.
3 Draw a third square with round grains. Leave a little space between them. This is sandstone, and the space holds the cement.
4 Draw a fourth square with wavy bands that run the same way. This is gneiss.

Granite has big crystals that lock together. Basalt has crystals so small that you need a lens to see them. Sandstone is round grains stuck together. Gneiss shows wavy bands.

Colour helps, but it can fool you. Granite can be pink, grey or white. Two rocks that look the same colour can be different kinds.

The hardness of a rock tells how easily it is scratched. A harder thing scratches a softer thing, and the softer thing cannot scratch it back. A soft rock crumbles. A hard rock does not.

A fingernail is about 2.5 on the scale. A copper coin is about 3.5. A steel nail is about 5.5.

Use the three things in order. Try the fingernail first. If it scratches the rock, the rock is softer than 2.5. If not, try the coin, then the nail. Some coins are steel with a thin coat of copper, and they scratch harder rock. Use a coin of real copper, or copper wire.

The coin scratches this rock and the fingernail does not. So its hardness lies between 2.5 and 3.5.

Try it

Scratch a rock

You need: a rock with a flat side, such as sandstone or chalk, your fingernail, a coin of real copper, or a piece of copper wire, a steel nail, a cloth.

Be safe: Never hit a rock. Chips can fly into your eyes. Push the nail along the rock with its point away from you. Wash your hands afterwards. Never go to cliffs, quarries or riverbanks alone to collect rocks.

Hold the rock firmly on a table. Rub your fingernail along the flat side with a firm push. Wipe the spot with the cloth and look.
Do the same with the copper coin. Always push the tool away from your body.
Do the same with the nail. Hold the nail so that its point faces away from you.
Write down which tool made a scratch and which did not.

What you will see: A soft rock is scratched by the fingernail. A harder one is not scratched by the fingernail, but the coin or the nail scratches it. The first tool that makes a scratch tells you the rock is softer than that tool.

If you cannot try it, this is what happens.

Now we can sort rocks. Scientists sort them by how they formed. Igneous rock formed when melted rock cooled and turned hard. Cooling slowly gives big crystals, like granite. Cooling fast gives small crystals, like basalt.

Sedimentary rock formed from layers of small bits, such as sand and mud. Over a very long time the layers were pressed and stuck together. Sandstone is one. Shale is made from pressed mud, and limestone mostly from the shells of small sea animals.

Metamorphic rock is rock that heat and pressure changed. It did not melt. Limestone becomes marble this way, and shale becomes slate. Gneiss is metamorphic too. Heat and pressure lined up its grains in bands.

Heat and pressure change limestone into marble. The rock never melts.

See it three ways

As a picture

In words

Igneous rock cooled from melted rock, such as granite. Sedimentary rock was pressed from layers of bits, such as sandstone. Metamorphic rock was changed by heat and pressure, such as marble.

A wrong idea many people have

You can tell the kind of rock by its colour.

Why it feels right: Many rocks you see first, such as black basalt or white marble, seem to keep one colour.

The test that breaks it: Look at granite in different places. It can be pink, grey or white. Marble can be white, pink or green. Rocks of one kind come in many colours.

What is true: Grains tell you much more than colour. Look for locked crystals, round grains or bands, and test the hardness. Marble and basalt both have locked crystals. But a nail scratches marble, and it does not scratch basalt.

Key words

grain: one small piece that a rock is made of.
crystal: a small solid with flat sides that grew in one place.
hardness: how hard it is to scratch a rock.
igneous: rock that formed when melted rock cooled.
sedimentary: rock made of small bits pressed and stuck together.
metamorphic: rock changed by heat and pressure, without melting.

Back to your guess

The answer is B: Rocks form in different ways, and that changes their grains and how hard they are.

Rocks are made in three main ways. How a rock formed decides its grains and its hardness.

Keep it

Copy this into your exercise book.

Igneous rock cooled from melted rock. Sedimentary rock is bits pressed together. Metamorphic rock was changed by heat and pressure. A harder thing scratches a softer one.

Draw: Draw three squares: locked crystals, round grains with cement, and wavy bands. Name the kind of rock under each.

S3.11.2 Chapter 2

Inside the Earth

What would you meet if you could dig down and down?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A Solid rock all the way down to the centre.
You picked A. Read on. At the end, the chapter comes back to your guess.
B A sea of liquid fire just under the ground.
You picked B. Read on. At the end, the chapter comes back to your guess.
C Layers: a thin crust, a thick mantle, a liquid outer core and a solid inner core.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Label the crust, mantle, outer core and inner core
Explain how volcanoes and earthquakes happen where plates meet
Describe how to stay safe in an earthquake

Imagine a hole that goes down and down. What would you meet? The Earth has layers, a little like an onion cut in half.

The top layer is the crust. It is a thin skin of rock, and we live on it. Under the crust is the mantle, a very thick layer of hot rock.

Draw it first

Draw a slice through the Earth

1 Draw a big circle for the Earth. Draw a thin ring inside its edge. The ring is the crust. It is drawn much too thick, so that you can see it.
2 Draw a smaller circle inside. The thick layer between the two circles is the mantle.
3 Draw a circle for the outer core. It is hot liquid metal.
4 Draw a small black circle in the middle. This is the inner core. It is solid metal.

The mantle is mostly solid rock. Very slowly, over a long time, it flows a few centimetres each year, like thick dough. It is not a sea of liquid.

In the middle is the core. It is mostly iron. The outer core is liquid. The inner core is solid, because the weight above squeezes it so hard.

See it three ways

As a picture

In words

Only the outer core is liquid. The crust and the inner core are solid. The mantle is mostly solid and flows very slowly.

In symbols

Distance from the centre: inner core 1220 km, outer core 3480 km, whole Earth 6371 km.

A wrong idea many people have

Under the ground there is a sea of liquid fire.

Why it feels right: A volcano pours out melted rock, so the rock under it must be liquid.

The test that breaks it: Scientists study the waves that an earthquake sends through the Earth. The waves pass through the mantle the way they pass through solid rock. The liquid outer core stops one kind of wave.

What is true: The mantle is mostly solid. Small pockets of it melt. That melted rock is what a volcano pours out.

The crust and the top of the mantle are cracked into big pieces. Each piece is a plate. Plates move very slowly, a few centimetres a year. That is about as fast as your fingernails grow.

A fingernail grows about 4 cm in a year. A plate moves about 5 cm.

Where two plates push together, one can slide under the other. The sinking plate carries water down with it. The water helps the hot rock above it to melt. The melted rock rises and a volcano can form. A volcano is a place where melted rock, gas and ash come out of the Earth.

One plate sinks under another. Melted rock rises to a volcano, and the grinding plates cause earthquakes.

Plates also grind against each other. They stick, and then they suddenly slip. The ground shakes. This shaking is an earthquake.

Try it

Make a model of sliding plates

You need: your two hands.

Be safe: Push gently so that you do not hurt your hands.

Put your hands flat together, with the fingers pointing forward.
Push one hand forward and the other hand back, gently, but with the palms pressed together.
Keep pushing until the hands suddenly slip.

What you will see: Your hands stick and then jerk past each other. Plates do the same. The sudden jerk is an earthquake.

If you cannot try it, this is what happens.

Most volcanoes and earthquakes happen where plates meet. The middle of a plate is usually quiet. Where plates pull apart, melted rock rises into the gap, mostly under the sea.

If an earthquake starts while you are indoors, drop to the floor. Cover your head and neck under a strong table, and hold on. Stay away from windows. Stay there until the shaking stops.

Drop, cover and hold on. Keep away from windows.

If there is no strong table, crouch next to an inside wall, away from windows. Cover your head and neck with your arms.

If you are outside, move to an open place away from buildings, trees and wires. After the shaking, watch for things that may fall. Stay away from damaged buildings.

Key words

crust: the thin top layer of the Earth, made of rock.
mantle: the thick layer of hot, mostly solid rock under the crust.
core: the middle of the Earth, made mostly of iron.
plate: one big piece of the crust and top of the mantle.
volcano: a place where melted rock, gas and ash come out of the Earth.
earthquake: a shaking of the ground when rock suddenly slips.

Back to your guess

The answer is C: Layers: a thin crust, a thick mantle, a liquid outer core and a solid inner core.

The Earth has four layers. Only the outer core is liquid. The mantle is mostly solid rock that flows very slowly.

Keep it

Copy this into your exercise book.

The Earth has a crust, a mantle, a liquid outer core and a solid inner core. Plates move a few centimetres a year. Volcanoes and earthquakes happen where plates meet.

Draw: Draw a circle with four rings: crust, mantle, outer core and inner core. Draw two arrows where two plates push together.

S3.11.3 Chapter 3

Weathering and erosion

How does a mountain turn into sand?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A One big flood washes the whole mountain away.
You picked A. Read on. At the end, the chapter comes back to your guess.
B Rocks shrink by themselves as they get old.
You picked B. Read on. At the end, the chapter comes back to your guess.
C Heat, ice, water and roots break rock into small pieces, and then the pieces are carried away.
You picked C. Read on. At the end, the chapter comes back to your guess.

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.

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.

S3.11.4 Chapter 4

The rock cycle

Can one kind of rock turn into another kind?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A No. A rock stays the same kind for ever.
You picked A. Read on. At the end, the chapter comes back to your guess.
B Yes, but only in a few weeks.
You picked B. Read on. At the end, the chapter comes back to your guess.
C Yes. Over millions of years, heat, pressure, weathering and melting can turn it into another kind.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Draw the rock cycle with melting, cooling, burying and lifting
Explain how one rock can become another over millions of years

Does a rock stay the same for ever? It seems so, because we cannot see it change. But over millions of years, rocks change from one kind into another. The path they follow is the rock cycle.

Start deep underground. There, rock can get so hot that it melts. Melted rock under the ground is magma. Magma that reaches the surface is called lava.

Draw it first

Draw the rock cycle

1 Draw a box for magma and a box for igneous rock. Draw an arrow from magma to igneous rock and write cools on it.
2 Add a box for sediment. The arrow into it says lifted and worn away.
3 Add a box for sedimentary rock. The arrow into it says buried and pressed.
4 Add a box for metamorphic rock. Heat and pressure make it. Draw an arrow from it back to magma and write melts.

When magma cools, it hardens into igneous rock. Magma that cools slowly under the ground makes big crystals, like granite. Lava that cools fast on the surface makes small crystals, like basalt.

Slow cooling gives big crystals. Fast cooling gives small ones.

Rock deep down can be lifted up when plates push together. At the surface, weathering and erosion wear it into sediment.

Plates that push together can lift rock up. Then weathering and erosion wear the top away.

Rivers drop sediment in layers on a lake floor or the sea floor. New layers bury the old ones. The weight presses the lower layers, and minerals in the water glue the bits together. The layers become sedimentary rock.

The lower layers are buried and pressed. They have become rock.

Rock buried deeper still gets hot and is squeezed hard. Heat and pressure can change it without melting it. Shale becomes slate. Limestone becomes marble. These are metamorphic rocks.

If the rock gets hotter, it melts into magma. Then the cycle can start again. One turn of the cycle takes millions of years.

The ring is not the only way round. A rock can take a shortcut. Igneous rock can be buried and heated into metamorphic rock, without becoming sediment first. Metamorphic rock can be lifted and worn away to sediment.

Solid arrows are the ring. Dashed arrows are shortcuts.

See it three ways

As a picture

In words

Each row changes one rock into the next, and each row ends where the next begins. Lifting brings deep rock up, so that it can be worn away. Cooling, wearing, pressing, heating and melting are the changes.

Try it

Make a block of sedimentary rock

You need: sand, a spoon of mud or clay soil, water, a small cup, a sunny place.

Be safe: Wash your hands after you touch mud. Never look at the Sun.

Mix a cup of sand with a spoon of mud. Add a little water and stir to a thick paste.
Press the paste firmly into the cup. Leave it in the sun for two days.
Tip the block out. Press on it gently with your thumb.

What you will see: The mud has glued the sand into a block, as minerals glue sand into sandstone. Your block crumbles more easily than real rock, because real rock was pressed for millions of years.

If you cannot try it, this is what happens.

A wrong idea many people have

Rocks must follow the cycle in one fixed order.

Why it feels right: A picture of a ring with arrows seems to show only one path.

The test that breaks it: A sedimentary rock can be lifted and worn away again without being heated. An igneous rock can be buried and changed into a metamorphic rock. Any rock can be melted.

What is true: There are many shortcuts. Any kind of rock can become any other kind, given enough time.

Key words

rock cycle: the slow change of rock from one kind to another and back.
magma: melted rock under the ground.
lava: melted rock that has reached the surface.

Back to your guess

The answer is C: Yes. Over millions of years, heat, pressure, weathering and melting can turn it into another kind.

Rocks do change kind, but very slowly. One full turn of the rock cycle takes millions of years.

Keep it

Copy this into your exercise book.

Magma cools to igneous rock. Rock wears into sediment. Sediment is buried and pressed into sedimentary rock. Heat and pressure make metamorphic rock. More heat melts it.

Draw: Draw five boxes in a ring: magma, igneous, sediment, sedimentary, metamorphic. Add an arrow with a word between each pair. Add one dashed shortcut.

S3.11.5 Chapter 5

Soil

What is soil made of?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A Soil is bits of rock mixed with humus, water and air.
You picked A. Read on. At the end, the chapter comes back to your guess.
B Soil is just crushed rock.
You picked B. Read on. At the end, the chapter comes back to your guess.
C Soil is the same everywhere in the world.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Separate soil into layers by shaking it in a jar of water
Compare how fast water drains through sandy soil and clay soil

Take a handful of soil and look at it closely. What is in it? Soil is the thin top layer of the land. It is not just crushed rock. It is a mix of four things.

Soil has tiny grains of rock. It has humus, which is the rotted bits of dead plants and animals. It has water. And it has air in the gaps between the grains.

Dig down and you can see that soil has layers. The top layer is dark, with the most humus. Under it the soil has less humus and more rock. Far down there is usually solid rock.

Soil comes from rock that weathering broke up, and from humus.

The rock grains in soil come in different sizes. Sand grains can be seen, and they feel gritty. Silt grains are smaller and feel like flour. Clay grains are the smallest, too small to see one by one. Clay feels sticky when it is wet.

Draw it first

Draw a jar of soil, shaken and settled

1 Draw a jar with soil and water in it. Just after shaking, the grains and water are mixed through.
2 Draw it again after it has stood still. The biggest grains have sunk to the bottom, and the finest are at the top. Draw bits of dead plant floating in the water.

Try it

Shake soil in a jar

You need: a clear plastic jar with a lid, a few spoons of soil, water, a ruler.

Be safe: Use a plastic jar, never glass. Close the lid tight. Wash your hands after you touch soil.

Fill the jar about one third with soil. Pour in water until the jar is nearly full. Close the lid tight.
Shake the jar hard for one minute. Stand it somewhere that nobody will knock it.
Look at the jar after one minute, after one hour and after one day.
Measure each layer with the ruler and write down how many cm thick it is.

What you will see: The biggest grains sink first and the finest settle last. So the layers go small stones, sand, silt and clay from the bottom. Bits of dead plant float. The water may stay cloudy for a day, because clay is so fine.

If you cannot try it, this is what happens.

Why do the layers go in that order? A big grain is heavier than a small one. It sinks fast through the water. A tiny clay grain sinks very slowly. The humus is light, so it floats.

Grain sizes get smaller from stones to clay.

The grains also decide how well water can drain, which means run down through the soil. Sandy soil has big grains with big gaps between them. Water drains through it fast.

Clay soil has tiny grains packed tight, with tiny gaps. Water drains through it slowly, so clay soil stays wet and sticky.

See it three ways

As a picture

In words

Water runs through sandy soil fast and through clay soil slowly. The soil with the bigger gaps drains faster.

In symbols

One test, for example: 100 mL poured on each soil. Sandy soil let 70 mL through in a minute. Clay soil let 20 mL through. Your numbers will differ.

Try it

Compare how soils drain

You need: two plastic cups, two jars, sandy soil and clay soil, water, a measuring jug, a clock.

Be safe: A grown up must make the holes and put the nail away. Wash your hands after you touch soil.

Ask a grown up to poke three small holes in the bottom of each cup with a nail.
Fill one cup half full of sandy soil and the other half full of clay soil. Stand each cup on top of a jar.
Pour 100 mL of water into each cup at the same time. Wait one minute.
Read how much water has dripped into each jar.

What you will see: More water comes through the sandy soil than through the clay soil. The clay holds the water back.

If you cannot try it, this is what happens.

A wrong idea many people have

Soil is just crushed rock.

Why it feels right: Soil is brown and gritty, like ground-up stone.

The test that breaks it: Shake soil in a jar of water. Bits of dead plant float on top. Crushed rock alone would all sink. Plants grow poorly in pure sand.

What is true: Soil is rock grains mixed with humus, water and air. The humus holds water, and as it rots it gives plants the minerals they need.

Key words

soil: the thin top layer of land, made of rock grains, humus, water and air.
humus: the rotted bits of dead plants and animals in soil.
sand: rock grains you can see, between 0.06 and 2 mm.
silt: grains smaller than sand that feel like flour.
clay: the smallest grains, which feel sticky when wet.
drain: to run down through soil.

Back to your guess

The answer is A: Soil is bits of rock mixed with humus, water and air.

Soil is more than crushed rock. It holds humus from dead plants and animals, and it holds water and air. Its grains can be big or tiny.

Keep it

Copy this into your exercise book.

Soil is rock grains, humus, water and air. Shaken in water, big grains sink first: stones, sand, silt, clay. Sandy soil drains fast. Clay soil drains slowly.

Draw: Draw a jar with four layers from the bottom: stones, sand, silt, clay. Add water on top with humus floating.

S3.11.6 Chapter 6

Caring for soil

Why do farmers worry when soil washes away?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A Soil grows back in a few days, so it does not matter.
You picked A. Read on. At the end, the chapter comes back to your guess.
B Soil takes hundreds of years to form, so lost soil cannot be replaced soon.
You picked B. Read on. At the end, the chapter comes back to your guess.
C Soil that washes away always lands somewhere better for plants.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Explain how trees and grass stop soil washing away
Describe how compost and changing crops keep soil healthy

Soil is the thin skin of land where plants grow. A thin layer of it can take hundreds of years to form. But one storm can wash it away. So soil needs care.

Draw it first

Draw rain on a bare slope, then on grass

1 Draw a slope with no plants. Draw rain falling and soil grains running down the slope with the water.
2 Draw the slope again with grass. Draw the roots reaching down into the soil. The grains stay where they are.

On bare soil, rain runs downhill and carries soil with it. This is erosion. Wind can carry dry, bare soil too.

Grass and trees protect soil. Their roots hold the soil together like a net. Their leaves slow the rain, so the drops do not hit the soil so hard.

See it three ways

As a picture

In words

A hill with grass loses far less soil than a bare hill in the same storm. These numbers are an example.

In symbols

Bare strip: 40 kg lost. Grass strip: 4 kg lost. The bare strip lost 10 times as much.

Try it

Wash bare soil and grass

You need: two trays, bare soil, a square of grass with its roots, cut by a grown up with a spade, two clear jars, a jug of water.

Be safe: Do this outside or over a sink. Wash your hands after you touch soil.

Put bare soil in one tray. Put the square of grass in the other tray.
Tilt both trays the same way. Stand a jar at the low end of each.
Pour the same amount of water slowly over each tray, like rain.
Look at the water that runs into each jar.

What you will see: The water from the bare soil is brown and muddy, because it carries soil. The water from the grass is nearly clear. The roots hold the soil.

If you cannot try it, this is what happens.

Plants make their own food in their leaves, using light. But they take water and minerals from the soil, so soil can wear out. We can give some back. Compost is made from old plant scraps, such as leaves and vegetable peels, left to rot. Worms and tiny living things turn them into dark, crumbly humus.

The oldest scraps at the bottom have turned to compost.

Humus holds water like a sponge. As it rots, it gives plants minerals. Adding compost makes soil richer. Soil that has what plants need is fertile.

Suppose a farmer grows the same crop in a field every year. That crop uses up the same minerals each time. So farmers change the crop from year to year. This is crop rotation.

Each field grows a different crop every year.

Beans, peas and lentils have small lumps on their roots. Tiny living things in the lumps take nitrogen from the air. Plants use some of it. When the roots and leaves rot, the rest stays in the soil. Nitrogen is a mineral that plants need to grow. So a field of beans adds nitrogen to the soil.

Roots of beans and peas carry small lumps. Tiny living things in the lumps take nitrogen from the air.

A wrong idea many people have

Soil lasts for ever, so we can use it any way we like.

Why it feels right: There seems to be a lot of it, and it looks the same from year to year.

The test that breaks it: In an example storm, a bare strip of hill lost 40 kg of soil. A grass strip lost 4 kg. A thin layer of soil can take hundreds of years to form.

What is true: Soil forms very slowly and can be lost quickly. Plants, compost and changing crops help us keep it.

Key words

compost: old plant scraps rotted into dark, crumbly humus.
fertile: having what plants need to grow well.
crop rotation: growing a different crop in a field each year.
nitrogen: a mineral that plants need to grow, which beans add to the soil.

Back to your guess

The answer is B: Soil takes hundreds of years to form, so lost soil cannot be replaced soon.

A thin layer of soil can take hundreds of years to form. Plants, compost and changing crops help to keep it.

Keep it

Copy this into your exercise book.

Roots hold soil and leaves slow rain. Compost adds humus. Changing crops stops soil wearing out. Beans add nitrogen.

Draw: Draw a slope with grass and roots. Beside it draw three fields with a different crop in each, and mark the beans.

S3.11.7 Chapter 7

Fossils

How did a shell get inside a rock on a mountain?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A Someone put it there.
You picked A. Read on. At the end, the chapter comes back to your guess.
B The shell grew inside the rock.
You picked B. Read on. At the end, the chapter comes back to your guess.
C It was buried in mud long ago. The mud became rock, and the rock was lifted up.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Explain how a fossil forms in sedimentary rock
Make a model fossil by pressing a shell or leaf into clay

A fossil is the remains or the print of a living thing from long ago, kept in rock. It can be a shell, a bone, a leaf or even a footprint. Fossils show us what lived on Earth before people.

Draw it first

Draw how a fossil forms

1 A sea animal dies. Its shell falls to the sea floor. Draw the shell on the ground under the water.
2 Mud and sand cover the shell, one layer after another. Draw the shell with ground on top of it.
3 Over a very long time the layers become rock. Bit by bit, a mineral takes the place of the shell and turns it to stone. Draw the shell with crossed lines.
4 Later the rock is lifted up. Draw an arrow that points up.
5 Wind and water wear the rock above away. The fossil shows at the surface, and someone finds it.

For this to happen, the animal must be buried quickly. If it lies on the surface, other animals eat it or it rots away. Hard parts, such as shells, bones and teeth, become fossils most often. Soft parts rot away.

Shells, bones and leaves can all become fossils.

Most fossils are found in sedimentary rock. Sediment settles in gentle layers and buries animals without crushing them.

Fossils sit inside the layers of sedimentary rock.

See it three ways

As a picture

In words

Igneous rock forms from melted rock, which is far too hot for a living thing to survive. Metamorphic rock is heated and squeezed, which usually destroys fossils. Sedimentary rock keeps them.

Some fossils are not the shell itself. A shell pressed into soft mud leaves a hollow print. The print is a mould. If the hollow fills up with mud or minerals, the filling is a cast. A cast has the shape of the shell.

A mould is a hollow print. A cast is the shape that fills it.

Try it

Make a model fossil

You need: modelling clay or soft clay from the ground, a small shell or a leaf, a flat board.

Be safe: Wash your hands after you handle clay or shells. Never go to cliffs, quarries or riverbanks alone to look for fossils.

Roll the clay into a flat slab on the board.
Press the shell firmly into the clay. Press the whole shell, but do not push it right through.
Lift the shell out slowly.
Look at the print in the clay. You have made a mould.

What you will see: The print has the shape of the shell, with its ridges. A real mould forms in mud in the same way. But real mud needs millions of years to turn into rock.

If you cannot try it, this is what happens.

A wrong idea many people have

A fossil is the real shell or bone, just very old.

Why it feels right: It looks like the shell and it has the same shape.

The test that breaks it: Pick up a fossil shell. It is heavy and hard, like a stone. Minerals took the place of the shell bit by bit.

What is true: In many fossils the original material has been replaced by minerals. The shape stays, and the fossil is stone.

Key words

fossil: the remains or print of a living thing from long ago, kept in rock.
mineral: a solid that forms in nature. Rocks are made of minerals.
mould: a hollow print left in mud or clay by a shell or leaf.
cast: the shape that fills a mould and keeps its form.

Back to your guess

The answer is C: It was buried in mud long ago. The mud became rock, and the rock was lifted up.

A shell can be buried in mud on a sea floor. Over a very long time the mud becomes rock. Later the rock can be lifted high, even into mountains.

Keep it

Copy this into your exercise book.

A fossil is a living thing from long ago, kept in rock. It is buried quickly, the layers become rock, and minerals turn it to stone. Most fossils are in sedimentary rock.

Draw: Draw five small boxes in order. The shell lies on the ground. It is buried. It turns to stone. The rock is lifted. It is found.

S3.11.8 Chapter 8

Reading Earth's history

How can a rock tell us about a time long before people?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A Layers show which rock is older, and fossils show what lived there and what the place was like.
You picked A. Read on. At the end, the chapter comes back to your guess.
B It cannot. Nobody saw the past, so rocks tell us nothing.
You picked B. Read on. At the end, the chapter comes back to your guess.
C The oldest rocks are always the ones on top.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Explain why lower rock layers are usually older
Explain what fossils tell us about life long ago

A cliff by the sea often shows stripes of rock. So does the side of a road cut through a hill. Each stripe is a layer. The layers are like the pages of a book about Earth. A geologist is a scientist who reads that book.

Draw it first

Draw the layers as they formed

1 Draw the first layer, at the bottom. It formed first, so it is the oldest.
2 Draw the second layer on top of it. It formed later.
3 Draw the third layer on top. It formed last, so it is the newest.

Layers form one on top of another. A new layer cannot settle under one that is not there yet. So the lowest layer is usually the oldest, and the top layer is usually the newest.

Sometimes mountains build and layers are tipped or even turned over. Then the order may be wrong. Fossils help scientists check.

The layers also give us time. Here is an example. In one quiet lake, mud settles about 2 mm each year. A layer of mud 30 cm thick holds 300 mm of mud. It took about 150 years to build.

A thicker layer took more years to build. Other places build up faster or more slowly.

Fossils are evidence of the past. Evidence is a fact that helps us find out what is true. A fossil shows that a living thing was once in that place.

See it three ways

As a picture

In words

Read the cliff from the bottom up. First there was water, with fish. Then shells settled in the sea. At the top the layers hold a fern, a land plant. The place changed from water to land.

A fossil tells us what the place was like when the animal or plant was alive.

Shell fossils are found on some high mountains. This tells us that the rock was once sea floor. Plates pushed it up over millions of years.

Some fossils are of animals that no longer live anywhere. We say they are extinct. The dinosaurs are one example.

The Earth is about 4.6 billion years old. People have lived on it for only a tiny part of that time.

The dates are rounded. Scientists find them from rocks and fossils. Life may be older than its oldest fossils.

The whole bar is 4600 million years. People fit in a line thinner than a pencil mark.

Try it

Build layers in a jar

You need: a clear plastic jar, dry sand, dry soil, dry rice, a spoon, a ruler.

Be safe: Wash your hands after you touch soil. Do not taste anything in the jar.

Pour a layer of sand about 2 cm thick into the jar. Level it with the spoon.
Pour a layer of soil about 2 cm thick on top of the sand.
Pour a layer of rice about 2 cm thick on top of the soil.
Look at the jar from the side. Which layer did you pour first? Which did you pour last?

What you will see: The sand, poured first, is at the bottom. The rice, poured last, is on top. The lowest layer is the oldest, just as in a cliff.

If you cannot try it, this is what happens.

A wrong idea many people have

The top layer of rock is always the oldest.

Why it feels right: The top is what we see first, and old things seem to be on the surface.

The test that breaks it: Pour sand, soil and rice into a jar in that order. The sand was poured first, and it is at the bottom.

What is true: Layers settle on top of each other, so the lowest is usually the oldest. Only if layers are turned over is the order reversed.

Key words

geologist: a scientist who studies rocks and the history of the Earth.
evidence: a fact that helps us find out what is true.
extinct: no longer living anywhere on Earth.

Back to your guess

The answer is A: Layers show which rock is older, and fossils show what lived there and what the place was like.

Layers of rock are laid down in order, so lower layers are usually older. Fossils in the layers show what lived there long ago.

Keep it

Copy this into your exercise book.

Layers settle one on another, so the lowest is usually the oldest. Fossils are evidence of what lived there. The Earth is about 4.6 billion years old.

Draw: Draw a cliff of three layers. Mark the oldest at the bottom and the newest at the top. Put a fossil in one layer.

Draw four rocks, close up

Scratch a rock

Draw a slice through the Earth

Make a model of sliding plates

Draw ice breaking a rock

Shake stones in a jar

Draw the rock cycle

Make a block of sedimentary rock

Draw a jar of soil, shaken and settled

Shake soil in a jar

Compare how soils drain

Draw rain on a bare slope, then on grass

Wash bare soil and grass

Draw how a fossil forms

Make a model fossil

Draw the layers as they formed

Build layers in a jar

Key words, A to Z