The Earth is layered

Cut the Earth open and you find four layers, plus the thin skin of gas around it — the atmosphere:

Key idea: the crust is thin compared with the layers below — like the skin on an apple. The mantle is the thickest layer. The core is mostly iron and nickel: the outer core is liquid, the inner core is solid and is the hottest part.

Name the layer

What is the air made of?

The atmosphere is the layer of gases held around the Earth by gravity. Most of the air is nitrogen, not oxygen:

Common mistake: many people think the air is mostly oxygen because that's the gas we breathe in. In fact nitrogen makes up nearly four-fifths of the air — oxygen is only about a fifth.

Mostly which gas?

How rocks are made

The crust is made of rock, and all rocks fall into three families depending on how they formed:

• Igneous — forms when molten rock (magma or lava) cools and hardens. Often has interlocking crystals (e.g. granite, basalt).
• Sedimentary — forms when tiny bits of rock, shells or sand are squashed and cemented in layers over a long time (e.g. limestone, sandstone). Can contain fossils.
• Metamorphic — forms when an existing rock is changed by heat and pressure without melting (e.g. marble, slate). Marble, the stone of many statues, forms from limestone in this way.

Memory hook: Igneous = ignis, "fire" (molten rock that cooled); Sedimentary = settled layers; Metamorphic = "changed form": rock changed by heat and pressure, without melting.

Rocks change over time

Rocks are not permanent — over millions of years they are turned from one type into another. This is the rock cycle:

Common mistake: a rock's type is not fixed forever. Given enough time and the right conditions, any rock can be turned into a different type — that's the whole point of the rock cycle.

Rock ↔ how it forms

Tap the way each rock type forms.

Which rock type?

How did limestone become marble?

How can a tiny bit of CO₂ change the climate?

The Earth's supplies are finite

The Earth gives us useful raw materials — metals from ores, fossil fuels (coal, oil, gas), and materials for glass, plastics and building. But most of these are limited: they took millions of years to form, so they are not replaced as fast as we use them.

Key idea: because these resources are non-renewable, we need to use them carefully so future generations still have some.

Why recycling helps

Recycling means collecting used materials and reprocessing them (for example, melting scrap metal) to make new products, instead of digging up new raw material. It is often efficient — it saves resources and usually uses less energy.

• Recycling a metal like aluminium uses far less energy than extracting it from its ore.
• It cuts down the amount of ore we need to mine, making our limited supplies last longer.
• It reduces waste going to landfill.

But it's not free: recycling still needs energy to collect, clean, sort and melt materials — and some things (like certain plastics) are hard to recycle well. That's why "reduce" and "reuse" matter too.

The point of recycling

Carbon goes round and round

Carbon is constantly moved between the air, living things and the ground. This is the carbon cycle:

Key idea: naturally the carbon cycle stays roughly balanced — the CO₂ taken out roughly equals the CO₂ put back.

Which process removes CO₂?

Humans are adding extra CO₂

People add extra carbon dioxide to the air, mainly by burning fossil fuels (coal, oil and gas) in power stations, cars and factories, and by cutting down forests (the trees are burned or rot, releasing CO₂, and fewer trees are left to take CO₂ in).

Why a tiny amount matters: nitrogen and oxygen make up almost all of the air, but they absorb almost none of this heat. CO₂ does absorb some of it, so even a small amount of CO₂ makes a difference. Gases that absorb this heat, such as CO₂, methane, water vapour and nitrous oxide, are called greenhouse gases.

Common mistake: CO₂ does not "burn" the Earth or make a hole in anything — it absorbs heat given off by the Earth. More CO₂ means more heat absorbed and sent back, which raises the average temperature (global warming) and changes the climate.

Greenhouse gas or raw resource?

Tap a card, then tap the box it belongs in.

Cause of the extra CO₂

How can a tiny bit of CO₂ change the climate?

The big ideas to know

You've covered the key ideas in the KS3 "Earth and atmosphere" topic. Press Finish to see your score.