Key Stage 3 science · Chemistry – Earth and atmosphere

KS3 rock cycle and structure of the Earth: a guide for parents

The Earth is made of layers, and its rocks are constantly being formed, broken down and changed over millions of years. In Years 7 to 9 children learn what the Earth is made of, how its structure is known, how the three types of rock form in the rock cycle, and why the planet’s resources are limited.

The Earth, the rock cycle and resources, explained

Pupils learn that the Earth has four main layers. The crust is the thin, solid outer layer we live on, only about 5 to 70 kilometres thick, compared with around 6,400 kilometres to the centre of the planet. Beneath it is the mantle, a thick layer of hot rock that is solid but can flow very slowly over millions of years. At the centre is the core: a liquid outer core and a solid inner core, both made mostly of iron with some nickel. The crust is made mostly of compounds of oxygen and silicon, with metals such as aluminium, iron and calcium, while the core is mainly metal. Nobody has drilled further than a tiny part of the crust, so scientists learn about the deep Earth mainly from how earthquake waves travel through it.

The rock cycle describes how rocks are changed from one type to another. Igneous rocks form when molten rock cools and solidifies. If it cools slowly underground, as granite does, large crystals have time to grow; if it cools quickly at the surface, as basalt does, the crystals are small. Sedimentary rocks form when rock is broken down by weathering, carried away by erosion, deposited in layers and then squashed and cemented together over time. Sandstone, mudstone and limestone are examples, and they are the rocks in which fossils are usually found. Metamorphic rocks form when existing rock is changed by heat and pressure deep underground without melting, as limestone becomes marble and mudstone becomes slate.

The word “cycle” is important. Any rock can become another type, and the journey takes millions of years. Igneous rock can be weathered into sediment, sediment can be buried and turned into metamorphic rock, and metamorphic rock can melt and cool into igneous rock. A sedimentary rock can also be heated and become metamorphic, and any rock can be uplifted to the surface again by movements of the crust. Children draw it as a loop with many arrows rather than a single chain.

The Earth supplies the materials we use, such as metal ores, fossil fuels, clay, sand and limestone, and they are finite, because they take millions of years to form. Recycling reduces the need to extract new material, and cuts the energy needed: making aluminium from recycled cans takes a small fraction of the energy needed to extract it from ore. Pupils are also asked to evaluate how well recycling works. Collecting and sorting takes energy, some materials cannot be reprocessed many times, and recycling reduces rather than removes the need for new resources.

Words your child will hear

Crust: The thin, solid outer layer of the Earth.
Mantle: The thick layer of hot rock beneath the crust that flows very slowly.
Igneous rock: Rock formed when molten rock (magma or lava) cools and becomes solid.
Sedimentary rock: Rock formed from layers of sediment that are pressed and cemented together.
Metamorphic rock: Rock changed by heat and pressure without melting.
Weathering and erosion: Weathering breaks rock into pieces; erosion carries the pieces away.
Finite resource: A resource that cannot be replaced within a human lifetime, so it can run out.

What the National Curriculum says

The programme of study for Key Stage 3 says:

Pupils should be taught about the composition of the Earth
Pupils should be taught about the structure of the Earth
Pupils should be taught about the rock cycle and the formation of igneous, sedimentary and metamorphic rocks
Pupils should be taught about Earth as a source of limited resources and the efficacy of recycling

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

A model answer: how limestone can become marble

Explain how limestone forms and how it could become marble.

Start with the sediment. Over millions of years the shells and skeletons of tiny sea creatures sink to the sea floor and build up in layers.
Explain how it becomes rock. The weight of the layers above squashes the sediment together (compaction), and minerals dissolved in the water bind the pieces together (cementation). The result is limestone, a sedimentary rock.
Move on to the change. If limestone is buried deep underground, or comes into contact with hot magma, it is exposed to high temperature and pressure.
Say what happens to the rock. The heat and pressure change the rock’s structure and make the small grains grow into a network of interlocking crystals. The rock does not melt.
Name the new rock. The new rock is marble, a metamorphic rock. Mention that if it melted it would become igneous rock when it cooled.

Limestone is a sedimentary rock formed from compacted and cemented sea-creature remains. Heat and pressure change it, without melting, into marble, a metamorphic rock.

Common mistakes and how to help

Thinking the mantle is liquid and that we live on a floating crust of molten rock

Volcanoes and diagrams of glowing lava give the impression that the inside of the Earth is a sea of liquid rock.

What helps: Explain that the mantle is hot but under such great pressure that it behaves as a solid, and only flows over very long periods, like very thick putty. Molten rock forms only in particular places, such as beneath some volcanoes.

Believing the rock cycle is a fixed one-way sequence

Diagrams are often drawn as a circle, so igneous, sedimentary and metamorphic appear to be steps in order.

What helps: Ask your child to find as many different routes between the three types as they can. Any rock can become any other, or change into the same type again.

Saying metamorphic rocks form when rock melts

Heat is part of the process, and children link heat with melting.

What helps: Compare clay being fired in a kiln: it becomes hard pottery without melting. If rock melts, it becomes magma, which then forms igneous rock when it cools.

Assuming recycling means resources will never run out

Recycling is often presented as the solution, so the loop looks endless.

What helps: Talk about losses at each stage. Some items are never collected, some materials weaken each time they are reprocessed, and demand is growing. Recycling slows the use of resources, but it does not remove it.

How to help at home

Make sediment layers. Shake a jar of water containing soil, sand and small stones, and then leave it to settle for a day. Your child can describe which material sinks first and why, and how layers like these could become sedimentary rock.
Rock walk. On a walk with an adult, look at stone walls, buildings, gravestones and pebbles. Ask your child to notice crystals, layers or fossils and decide whether each stone looks igneous, sedimentary or metamorphic. Rocks should be looked at, not hammered.
Model the Earth’s layers. Draw a line 64 centimetres long from the surface to the centre, so that 1 centimetre stands for 100 kilometres. The mantle takes up about 29 centimetres, the outer core about 22 and the inner core about 12, while the crust is less than a centimetre, under a millimetre beneath the oceans. Label each layer and talk about how thin the part we live on really is.
A crayon rock cycle. Use wax crayons as model rock. An adult grates shavings of a few colours into layers to stand for sediment. Wrap them in foil and press hard to make “sedimentary rock”. Warm the parcel in your hands, press again and see the layers squash and bend, a little like metamorphism. Melting the wax, to model igneous rock, is a job for an adult only.
Recycling audit. Look through the recycling bin and ask what each item is made of, where the material originally came from and whether it can be recycled many times. Ask your child to argue for and against recycling, using evidence.

Leads on to

Key Stage 4: The atmosphere and climate change
Key Stage 4: Air pollutants and potable water