Rock and soil
When you run about in a park or on a playing field, the ground may seem soft and springy. It is covered in grass, which makes a thick mat of roots and stems. The grass grows in the soil. This is a powder made from particles of rock and spongy material. The spongy material is made from tiny pieces of dead plants that are rotting away in the ground.

In wet weather the soil may swell with water and as you run across the ground you may slide and slip. In dry weather the soil may hold little water and feel hard if you fall on it. If you were to dig down into the soil at any time, you would not go very deep before you came across rock. This is a solid substance, which is much harder than soil. It is also a much thicker layer in the ground than soil. A layer of soil may be a metre thick but the rock beneath it is many kilometres thick. If you could travel down through it in a fast lift it would take you over half an hour to reach the bottom.

Is the Earth just one big lump of rock?
No. It is divided into three parts. At the centre is the core. This is made from a huge ball of iron and nickel. This is about 6,000 kilometres across. Around the core is a mixture of many rocky substances. It forms a layer about 3,300 kilometres thick. This layer is called the mantle. Above the mantle is the rocky part of the Earth called the crust. This is the part that is just underneath the soil. The crust covers the mantle like a shell covers the inside of an egg. The crust is broken into huge slabs of rock just like you can break the shell of a boiled egg into large pieces. These huge pieces of rock are called plates and are slowly moving. Some move as fast as your fingernails grow.

What makes the plates of rock move?
The rocks in the mantle are very hot. They churn about beneath the crust and as they move they push the plates around.

Can people go to the centre of the Earth?
No. People can only go down into the caves in the crust. The deepest cave that has been found so far is just over 1,300 metres below the surface of the ground. The deepest hole that has been drilled in the ground only reached 15 kilometres down into the crust. It did not reach the mantle.

Could people go to the centre of the Earth one day?
It would be very unlikely. They would have to go in some sort of submarine that could move through molten rock. If the heat did not make it too hot to survive in the submarine the rocks would probably crush the submarine flat in seconds.

How do we know what it is like in the Earth, if we can't go there?
As the plates of rock in the crust move, they rub against each other. This rubbing causes earthquakes. When an earthquake occurs it sends vibrations called shockwaves through the Earth to other places on its surface. The ways the shockwaves travel through the Earth tell us about the arrangement of substances in it. It may seem odd to be able to tell about something without seeing it but you may have tried this activity with a tin. You can tell something about the contents of a tin if you shake it. If you hear long, loud sounds they tell you that the tin may contain something hard. If you hear a single very quiet sound, it tells you that the tin may contain something soft.

Did people in the Stone Age make quarries?
They did not dig huge holes in the ground as we do today. At the foot of cliffs there are huge piles of stones called scree. They form when rocks on the cliff face break up owing to the forces of the wind, rain and ice. Some people searched the scree to find stones for axes. When they found a suitable stone they chipped it into a rough axe shape. They made large numbers of rough axes, which were then carried to other places to be given a sharp cutting edge. The places on the scree where axes were collected were called axe factories. One kind of stone that was used for making arrowheads as well as axes was a stone called flint. Mines were made to dig out the flint. Some mines were simply large holes while others were tunnels a few metres below the surface of the ground.

What are the tiny pieces that make up the soil?
There are three sizes of rocky pieces in the soil. The largest ones are sand grains. Smaller sized particles are called silt and the smallest particles are called clay. There is also a substance in soil that does not come from rock. It is called humus and is made from the remains of living things - particularly plants. This substance binds the rocky particles together to make larger lumps called soil crumbs. The humus makes the soil crumbs dark in colour.