WHAT THE GEOLOGISTS SAY

Every rock
has a past.

Rocks are sorted by how they formed.

Hexagonal dark basalt columns stepping down to the sea on a rocky coastline under a soft grey sky — AI-generated image, not a photograph of a real person or place.

Cooled from melt
Laid down in layers
Changed by heat
Three ways to make a rock

Some cooled from molten rock, some built up from sand and mud, and some were changed by heat and pressure. The British Geological Survey describes how rocks are slowly recycled from one type to another over millions of years.

Updated 14 October 2026

THE SHORT ANSWER

Three rock types,
one slow cycle.

Geologists group rocks by how they formed.
Each type can, given time, become another.

IGNEOUS

Cooled from molten rock

Magma that cools underground slowly grows large crystals, like granite. Lava that cools at the surface sets fast and fine-grained, like the basalt of the Giant's Causeway.

Read a rock

SEDIMENTARY

Built up in layers

Sand, pebbles and mud worn from older rock are carried away and laid down, usually in seas and lakes. The layers, called strata, are compacted and cemented over time.

Spot one

METAMORPHIC

Changed, not melted

Deeply buried rock under intense heat and pressure changes without melting. Limestone becomes marble and shale may become slate.

See the numbers

READ A ROCK

What can you see
in the stone?

Pick what you notice in a rock to see which type it probably is. The bar shows how much heat the rock has been through: melted, heated without melting, or not heated at all.

Based on British Geological Survey and Sedgwick Museum guidance. A rough field guide, not a lab identification.

Rock reader

Choose what you seeLarge crystals you can seeDull, grey and fine-grainedLayers, grains or fossilsStripes or folds in crystals

Large crystals you can seeIgneous, cooled slowly

Large, interlocking crystals suggest magma that cooled slowly underground, like granite. Peterhead granite shows pink feldspar, grey quartz and black mica.

Dull, grey and fine-grainedIgneous, cooled fast

Crystals too small to see suggest lava that cooled quickly at the surface. Basalt is the classic example, and often looks dull and grey.

Layers, grains or fossilsSedimentary

Rock laid down in layers (strata), sometimes with fossils, is sedimentary. It can be soft and crumbly or hard, like sandstone, limestone or conglomerate.

Stripes or folds in crystalsMetamorphic

Interlocking crystals that look stripy, sometimes folded, point to rock changed by heat and pressure without melting, such as schist or gneiss.

Sources: British Geological Survey, Rocks and minerals; Sedgwick Museum of Earth Sciences, University of Cambridge, Investigating rocks.

TWO NUMBERS WORTH KNOWING

Rock keeps time
on a grand scale.

One UK landmark, two numbers:
how big it is and how old it is.

≈ 40,000 columns

Basalt columns standing out from the sea

UNESCO describes some 40,000 massive black basalt columns at the Giant's Causeway in County Antrim, a World Heritage Site since 1986. The BGS says the columns formed from cracks as the lava cooled.

GIANT'S CAUSEWAY · UNESCO

50–60 million years ago

When the volcanic activity happened

UNESCO dates the Causeway's volcanic landscape to the Tertiary period, around 50 to 60 million years ago. The BGS says the rock cycle itself plays out over millions of years.

AGE · UNESCO

The Geological Society says Britain has a larger variety of rocks than any other similar-sized area of the world, formed over many millions of years as the land moved slowly around the globe.

A stack of grey-blue slate pieces beside a hillside quarry, with thin flat sheets catching the light — AI-generated image, not a photograph of a real person or place.

FOUR WAYS TO LOOK

Read the rocks
on your next walk.

You do not need a hammer or a lab. Walls, cliffs, beaches and old buildings are full of rock, and a careful look tells you a lot.

Look for crystals
Interlocking crystals mean igneous or metamorphic rock. Large crystals in an igneous rock mean slow cooling underground.
Look for layers
Bands of sand, pebbles or mud stacked on top of each other are strata, the signature of sedimentary rock.
Look for fossils
Shells and plant remains turn up in sedimentary rock. Limestone forms from the shell fragments of sea creatures.
Look for stripes and folds
Stripy, folded crystals suggest metamorphic rock, squeezed and heated deep underground.

THE ROCK CYCLE

Any rock can
become another.

Weathering breaks rock down where it stands, and erosion moves the pieces by gravity, wind, rain, rivers, the sea and glaciers. The pieces are deposited, buried and hardened into sedimentary rock.

Burial brings heat and pressure that make metamorphic rock, and melting makes magma that cools into new igneous rock. Uplift brings it all back to the surface, and the cycle starts again.

A close-up of a cliff face showing horizontal layers of sandstone and pebbly rock in bands of cream, orange and brown — AI-generated image, not a photograph of a real person or place.

Questions

Questions people ask

What are the three types of rock?

Igneous, sedimentary and metamorphic. The British Geological Survey classifies rocks by how they formed: from cooled molten rock, from layers of sediment, or by heat and pressure changing an existing rock.

What is the rock cycle?

It describes how rocks are slowly recycled from one type to another over millions of years, through weathering, erosion, deposition, burial, metamorphism, melting and uplift.

Why does granite have big crystals and basalt tiny ones?

Cooling speed. Magma that cools very slowly underground, possibly over thousands or millions of years, grows large crystals. Lava that cools quickly at the surface forms small, fine-grained crystals.

Where can I see each rock type in the UK?

Basalt at the Giant's Causeway in County Antrim, granite at Peterhead in Scotland, slate in North Wales and Cornwall, and schist and gneiss in Scotland, according to the BGS and the Geological Society.

Does metamorphic rock melt?

No. Metamorphic rock changes under heat and pressure without melting. If rock does melt, the magma cools into igneous rock instead.

Every rock
has a past.