The Earth supplies the materials we use and the air we breathe, and both have changed over time, most recently as a result of human activity.
The Earth is made of a mixture of elements and compounds. Oxygen and silicon are the most abundant elements in the crust, mostly combined as silicates in rock, along with aluminium, iron and calcium.
Useful materials are not spread evenly. They are concentrated in particular deposits, and mining takes place where those deposits are found.
The Earth has layers. The thin, solid crust is on the outside. Below it the mantle is mostly solid but can flow very slowly. Inside that is a liquid outer core and a solid inner core, both mainly iron and nickel.
Movement in the liquid outer core is what produces the Earth's magnetic field.
Igneous rock forms when molten rock cools and solidifies; cooling slowly underground gives large crystals, cooling quickly at the surface gives small ones.
Sedimentary rock forms when layers of sediment are compacted and cemented together over long periods, so it often contains fossils and visible layers.
Metamorphic rock forms when existing rock is changed by heat and pressure without melting. Limestone becomes marble, and mudstone becomes slate.
The rock cycle links all three: weathering and erosion break rock into sediment, burial and pressure make sedimentary rock, deeper heat and pressure make metamorphic rock, and melting returns it to magma.
Ores, fossil fuels and many minerals are finite: they took millions of years to form and are being used far faster than they are replaced.
Recycling reduces the demand for new ore, saves the energy that extraction would need, and reduces waste going to landfill. It is not free, since collecting and sorting cost energy too, but for metals such as aluminium the saving is very large.
Today's atmosphere is about 78% nitrogen and 21% oxygen, with the remaining 1% mostly argon, plus small amounts of carbon dioxide and water vapour.
It has not always been this way. The early atmosphere came from volcanic gases and was mostly carbon dioxide with little or no oxygen; oxygen appeared once photosynthesising organisms evolved.
Burning fossil fuels releases carbon dioxide that had been locked away for millions of years, and the amount in the atmosphere has risen sharply since industrialisation.
Carbon dioxide is a greenhouse gas: it absorbs infrared radiated from the Earth’s surface and re-emits it in all directions, including back towards the surface, warming the planet. The consequences include rising sea levels, changes in the frequency and severity of some extreme weather events and changes to where species can live.
Carbon moves continually between the atmosphere, living things, the oceans and the rocks. Photosynthesis removes carbon dioxide from the air; respiration, decay and combustion return it.
Some carbon is locked away for very long periods, in fossil fuels and in carbonate rocks such as limestone, much of which formed from the remains of marine organisms. Burning fossil fuels returns that long-buried carbon to the atmosphere far faster than natural processes remove it.
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