Planet Earth

In Grade 6 you learned the Earth has four layers, with a thin solid crust on the outside. This year comes the surprising part: that crust is not one piece. It is cracked into about a dozen huge slabs called tectonic plates.

The plates sit on top of the mantle. The mantle is solid rock, but it is so hot that it can flow very slowly, like extremely thick treacle. Slow movement inside the mantle drags the plates along with it.

How fast do plates move? Only a few centimetres each year — about the speed your fingernails grow. It feels like nothing, but over millions of years it adds up to thousands of kilometres. Continents that are now far apart were once joined together.

• Tectonic plates are huge slabs that make up the Earth's crust.
• They float on the slowly flowing mantle beneath.
• They move only a few centimetres per year.
• Over millions of years, this slow drift moves whole continents.

The places where two plates meet are called plate boundaries. As you will see, almost all of the Earth's most dramatic events happen there.

Plate boundaries are where the action happens. At a boundary, two plates can do one of three things.

• Pulling apart — the plates move away from each other and magma rises into the gap.
• Pushing together — the plates collide; one may be forced down, crumpling the crust into mountains.
• Sliding past — the plates grind sideways against each other.

Earthquakes happen because plates do not slide smoothly. They lock together, pressure builds up, then they suddenly jerk free. That sudden movement shakes the ground.

Volcanoes form where magma from the mantle can reach the surface — most often where plates pull apart or one plate is pushed under another, melting rock as it goes.

In Grade 6 you used the word 'weathering' to start the rock cycle. Now let's look at it properly. Weathering is the breaking down of rock where it sits — nothing is moved yet.

There are two main kinds.

Physical weathering breaks rock without changing what it is made of:

• Water seeps into a crack, freezes, expands, and forces the crack wider. Repeated over and over, this shatters the rock.
• Rock heats up by day and cools at night. The surface expands and contracts until pieces flake off.

Chemical weathering changes the rock chemically:

• Rainwater is slightly acidic. It slowly dissolves certain rocks, such as limestone.
• This is why old stone statues lose their sharp detail and caves form underground.

Living things help too — plant roots prise cracks apart, a process sometimes called biological weathering.

The key idea: weathering happens in place. The rock is broken, but the pieces have not gone anywhere yet. Moving them is the job of erosion and transport.

Once rock is broken into pieces, two more processes take over.

Erosion is the wearing away of rock and the removal of the loosened pieces. Transport is the carrying of those pieces from one place to another. The same agents do both.

The main agents are:

• Moving water — rivers and waves loosen, lift and carry rock fragments.
• Wind — picks up sand and grit, especially in deserts.
• Ice — glaciers grind rock and drag it downhill.
• Gravity — pulls loose material down slopes.

As pieces are transported they get smaller and more rounded, knocked smooth as they tumble. When the moving water or wind slows down, the pieces are dropped — this dropping is called deposition.

In Grade 6 the rock cycle could feel like a diagram that 'just happens'. Now you can name the forces that drive it.

• Weathering breaks exposed rock into fragments.
• Erosion and transport carry those fragments away — usually downhill, toward seas and lakes.
• The fragments settle in layers (deposition) and are slowly squashed and cemented into sedimentary rock.
• Plate movement then takes over: collisions bury rock deep, where heat and pressure make metamorphic rock, or where rock melts and later cools into igneous rock.
• Plate movement also pushes rock back up to the surface, exposing it to weathering again — and the cycle repeats.

So the rock cycle is powered by two engines working together:

• Energy from the Sun drives the weather — rain, frost and wind — that causes weathering, erosion and transport at the surface.
• Heat from inside the Earth drives the slow movement of plates that buries, melts and lifts rock.

That is the big Stage 8 idea: surface processes and deep-Earth processes are two halves of one never-ending cycle.

Understanding the moving Earth opens up a lot of later science:

• Cycles on Earth — the rock cycle is one of several Earth cycles; the carbon and water cycles work the same loop-of-processes way.
• Forces and energy — plate movement, earthquakes and gravity-driven erosion are all forces in action.
• Materials and their structure — how a rock formed decides its properties as a building material.
• Earth in space — the same gravity that pulls rock down slopes holds the Earth itself in orbit.

Whenever a later topic mentions mountains, oceans, fossils or natural disasters, the moving plates from this guide are usually behind it. Take your time here — it explains the shape of the whole planet.