Planet Earth

In earlier years you learned the Earth has four layers and that its crust is cracked into tectonic plates that drift a few centimetres a year. This year comes the deeper question: what actually moves them?

The answer is heat. The Earth's core is extremely hot, partly left over from when the planet formed and partly released by radioactive decay. That heat slowly escapes outwards through the mantle.

The mantle is solid rock, but it is hot enough to flow very slowly. Hot mantle rock near the core is less dense, so it rises. As it nears the cool crust it loses heat, becomes denser, and sinks again. This continuous loop is a convection current — the same idea as warm air rising above a heater.

These slow-moving currents act like a giant conveyor belt. The plates sit on top and are dragged along by the flowing rock beneath. So plate movement is not random — it is the visible result of heat escaping from deep inside the Earth.

You met the rock cycle before as a diagram of three rock types with arrows between them. Now you can see it as a true system, with named processes and energy sources.

The deeper idea is that two energy sources keep the whole cycle turning:

• Energy from the Sun powers the weather. Rain, frost and wind cause the weathering and erosion that break rock into sediment at the surface.
• Heat from inside the Earth powers plate movement. This buries rock deep (making metamorphic rock), melts it into magma, and pushes rock back up to be weathered again.

Notice there is no single "first" rock. A metamorphic rock can melt and become igneous; an igneous rock can be weathered into sedimentary rock; sedimentary rock can be squeezed into metamorphic rock. Every path is two-way over a long enough time.

A natural resource is any useful material we take from the Earth. Once you start looking, they are everywhere.

• Rocks and minerals — limestone for cement, sand and gravel for concrete, clay for bricks.
• Metal ores — rocks rich enough in metal to be worth extracting, such as iron ore and aluminium ore.
• Fossil fuels — coal, oil and natural gas, formed from the buried remains of living things over millions of years.
• Water — for drinking, farming and industry.
• Soil — the fertile layer that almost all our food depends on.

The crucial new idea at Stage 9 is that resources differ in how quickly they are replaced. Some are renewed in days; others took millions of years to form and, once used, are effectively gone.

This matters because the human population keeps growing and each person uses more resources than ever before. The Earth is not getting any bigger — so understanding which resources can last, and which cannot, is one of the most important ideas in science today.

Resources fall into two groups, and the dividing line is time.

A renewable resource is replaced by natural processes about as fast as we use it — for example sunlight, wind, flowing water and timber from replanted forests. Use them today and they will still be there tomorrow.

A non-renewable resource forms so slowly that, on a human timescale, it cannot be replaced. Once used, it is gone for thousands or millions of years.

Fossil fuels are the classic non-renewable example. Coal, oil and gas took millions of years to form from buried living things — yet we are burning them within just a few centuries. Metal ores are non-renewable too: there is only so much rich ore in the crust.

A tricky case is soil. A renewable resource on paper, soil takes hundreds of years to build up — so if it is eroded faster than that, it behaves as if it were non-renewable.

If non-renewable resources keep running down, what can we do? The goal is sustainability: using resources in a way that can carry on into the future.

A simple test: would there still be enough for our children and grandchildren? If yes, the use is sustainable. If we are using something far faster than it is replaced, it is not.

There are several practical strategies:

• Reduce — use less in the first place: insulate buildings, switch off devices, design products that last longer.
• Reuse — use an item again instead of throwing it away.
• Recycle — turn used materials back into useful ones. Recycling aluminium uses only a small fraction of the energy of extracting it from ore.
• Switch to renewables — generate electricity from sunlight, wind and flowing water instead of burning fossil fuels.
• Protect soil — plant trees and crops that hold soil in place, so it is not eroded faster than it forms.

No single action solves everything. Sustainability comes from many choices, made by individuals, communities and whole countries. The science you are learning now is exactly what those decisions should be based on.

Seeing the Earth as a system of resources prepares you for a lot of later science:

• Cycles on Earth — the carbon cycle and climate change are deeply tied to how we use fossil fuels.
• Energy resources — the choice between renewable and non-renewable energy is a major theme in physics.
• Chemistry of materials — extracting metals from ores, and the cost of doing so, builds on resource ideas.
• Living things and ecosystems — protecting soil, water and habitats links Earth science to biology.

Whenever a later topic mentions fuels, energy choices, recycling or the environment, the ideas of renewable, non-renewable and sustainable use will be behind it. Take your time here — these are some of the most important ideas you will ever study.