Planet Earth

If you could slice the Earth in half like an orange, you would see it is not the same all the way through. It is made of four layers.

• The crust is the thin, solid, rocky layer on the outside — the part we live on.
• The mantle is a thick layer of very hot rock. Some of it can flow slowly, like thick treacle.
• The outer core is a layer of liquid metal — mostly iron and nickel.
• The inner core is a ball of solid metal at the centre, even though it is the hottest part. The huge pressure squashing it keeps it solid.

It gets hotter the deeper you go. The crust feels solid and ordinary, but only a few kilometres below your feet things start to get extremely hot.

Pick up any rock — a pebble on a path, a stone wall, a cliff — and it will belong to one of three families.

• Igneous rocks are made of interlocking crystals. Granite and basalt are examples.
• Sedimentary rocks are made of flat layers, often with tiny grains you can rub off. Sandstone and limestone are examples.
• Metamorphic rocks often have wavy bands or a hard, shiny look. Marble and slate are examples.

You do not need to memorise every rock name. What matters is learning to look at a rock and ask: crystals, layers, or bands?

The three rock families look different because they form in completely different ways.

Igneous rock forms when molten rock cools and hardens. Deep underground, rock can be hot enough to melt — this is called magma. When magma cools, it turns solid. As it cools, crystals grow and lock together. Magma that reaches the surface, for example from a volcano, is called lava.

Sedimentary rock forms from tiny pieces called sediment — bits of broken rock, sand, mud and shells. These pieces are carried by rivers and settle in layers at the bottom of seas and lakes. Over millions of years the layers are pressed down and stuck together into solid rock.

Metamorphic rock forms when an existing rock is changed by heat and pressure — but without melting. The heat and squeezing change the rock's appearance and make it harder. For example, soft limestone can be changed into hard marble.

Notice the pattern: igneous needs cooling, sedimentary needs layers being squashed, and metamorphic needs heat and pressure changing a rock.

Rocks may look like they last forever, but they slowly change. The rock cycle is the never-ending journey a rock takes between the three types.

Here is one trip around the cycle:

• An igneous rock at the surface is slowly broken into small pieces by wind, rain and ice. This is called weathering.
• The pieces are washed away, settle in layers and are squashed into sedimentary rock.
• If that sedimentary rock is buried and squeezed by heat and pressure, it becomes metamorphic rock.
• If a rock is buried deep enough it can melt into magma — and when the magma cools it forms igneous rock again.

There is no real start or end. The cycle has been turning for billions of years.

Soil is not just 'dirt'. It is a living, important material that takes a very long time to form.

Soil forms in two steps:

• Weathering slowly breaks solid rock into tiny pieces of grit and sand. Rain, frost and plant roots all help.
• These rock pieces mix with humus — the dark, rotted remains of dead plants and animals. Small creatures like worms help mix it all together.

Good soil also holds air and water in the gaps between its pieces. So a handful of soil contains broken rock, humus, water, air and living things, all mixed together.

Soil matters more than most people realise:

• It gives plants a place to anchor their roots so they do not fall over.
• It supplies plants with water and the minerals they need to grow.
• It is home to countless living things, from worms to tiny microbes.
• Almost all the food you eat depends on soil — directly or indirectly.

Because soil takes hundreds of years to form, looking after it is really important.

Knowing how the Earth and its rocks work opens the door to lots of other science:

• Cycles on Earth — the rock cycle is one of several natural cycles you will study, alongside the water cycle.
• Earthquakes and volcanoes — these happen because of movement in the Earth's crust and mantle.
• Materials and their properties — rocks are used as building materials, and their properties depend on how they formed.
• Living things and habitats — soil is the base of food chains on land, so it links biology and Earth science together.

Whenever a later topic mentions rocks, soil or the inside of the Earth, come back to this guide. Taking the time to really understand it now will make those topics far easier.