The Rock Cycle

Rocks are not as solid as they seem

A rock looks like it could last forever. But over millions of years, even the hardest mountain is slowly worn down, and brand-new rock is created. This never-ending journey is called the rock cycle.

There are three main families of rock — igneous, sedimentary and metamorphic — and the rock cycle explains how each one forms and how they change into one another.

Igneous rock: born from fire

Deep inside the Earth it is so hot that rock melts into a thick, glowing liquid called magma. When magma cools and hardens, it forms igneous rock.

• If magma cools slowly deep underground, large crystals grow. Granite is an example.
• If magma erupts from a volcano as lava and cools quickly in the air, the crystals are tiny. Basalt is an example.

The word "igneous" comes from the Latin word for fire — a good reminder of how this rock is born.

Sedimentary rock: built in layers

Rocks at the surface get broken into small pieces by weathering — the action of wind, rain, ice and changing temperatures. Rivers and wind then carry these pieces away; this movement is called erosion.

The small pieces, called sediment, settle in layers at the bottom of lakes, rivers and seas. Over thousands of years, more layers pile on top, squashing the lower layers. Minerals act like glue, cementing the particles together. The result is sedimentary rock, such as sandstone, limestone and shale.

Sedimentary rocks often have visible layers, and they are the rocks most likely to contain fossils, because dead plants and animals can be buried in the sediment.

Metamorphic rock: changed by heat and pressure

Sometimes rock gets buried very deep, or pushed close to hot magma. The intense heat and pressure change the rock without fully melting it. This transformed rock is called metamorphic rock ("metamorphic" means "changed form").

For example:

• Limestone (sedimentary) becomes marble.
• Shale (sedimentary) becomes slate.
• Granite (igneous) can become gneiss.

The minerals rearrange and the rock often becomes harder and more crystalline.

The cycle that never ends

Here is the clever part: any rock can become any other type. A mountain of igneous granite can be weathered into sand, which becomes sedimentary sandstone, which can be buried and heated into metamorphic gneiss, which can melt back into magma and start again.

Igneous → weathered into sediment → Sedimentary → heated and pressed → Metamorphic → melted → back to magma → Igneous…

There is no beginning and no end. That is why we call it a cycle, just like the way water moves around our planet in the water cycle. These slow Earth processes also help shape the landscapes and habitats that living things depend on — see food chains and ecosystems.

Try it yourself: become a rock detective

You can practise spotting the three rock types with a simple collection.

1. Collect 5–10 rocks or pebbles from a garden, park or beach (ask permission first).
2. Wash them and look closely, using a magnifying glass if you have one.
3. For each rock, ask:
4. Can I see layers? It may be sedimentary.
5. Can I see shiny crystals or speckles? It may be igneous.
6. Does it look shiny, banded or very hard, like slate or marble? It may be metamorphic.
7. Sort your rocks into three groups and write down your reasons.

Real geologists use the same clues — texture, layers and crystals — to read the long history hidden inside every rock.