The Earth and its atmosphere · Classify
Three ways to make a rock
Every rock on Earth was made in one of three ways, and the rock still carries the evidence of which one — if you know what to look at.
Start here
A kitchen worktop of polished granite, and a statue of polished marble. Both hard, both shiny, both full of crystals.
Look closely and the granite has separate specks of pink, white and black locked together like a jigsaw. The marble is one colour running in soft swirls. Drip acid on each: the granite ignores it and the marble fizzes.
What does that fizzing tell you about the marble?
That it is a carbonate — and that is a clue to its history. Marble started life as limestone, made from crushed shells on a sea floor, and was then cooked and squeezed deep in the crust until its crystals grew and rearranged. The granite crystallised from molten rock and was never anything else. Same look, entirely different life story, and the acid found the difference in two seconds.
Rocks are grouped by how they formed, not by how they look. There are three routes.
Igneous rocks cooled from molten rock. Sedimentary rocks were built up from fragments, shells or crystals left behind by water. Metamorphic rocks were something else first, and were changed by heat and pressure without ever melting.
Reference · keep this one open
What settles it is how the rock formed, and the rock still shows it
Igneous
- Cooled from molten rock
Interlocking crystals like a jigsaw, and no layers anywhere. Large crystals if it cooled slowly deep underground; crystals too small to see if it cooled fast at the surface.
Fossils: never. Nothing survives the melt.
Granite, basalt
Sedimentary
- Settled in layers and cemented
Visible layers, and separate rounded grains rather than an interlocking mesh. Often crumbly, and full of tiny spaces that soak up water.
Fossils: yes, and this is the only group in which they survive.
Sandstone, limestone, mudstone
Metamorphic
- Changed by heat and pressure, without melting
Interlocking crystals with bands or swirls running through them, or a rock that splits cleanly into flat sheets.
Fossils: rarely, and usually distorted beyond recognition.
Marble, slate
Crystal size in an igneous rock is a clock. Big crystals mean the melt cooled slowly, deep underground, with time for them to grow. Crystals too small to see mean it cooled in days at the surface. Size is set by cooling rate, not by what the rock is made of.
Your turn · six samples
Read the evidence. Igneous, sedimentary or metamorphic?
0 of 6 decided
Two of these six are made of the same compound as each other, and they are not in the same group.
Sample 1
quarry in Cornwall
- Interlocking crystals of pink, white and black, a few millimetres across
- No layers of any kind
- Very hard; does not fizz with acid
You said igneous — correct.
Granite. Large interlocking crystals with no layering means it crystallised from molten rock, and the size of the crystals means it cooled slowly, deep underground.
You said sedimentary. It is igneous.
Granite. Large interlocking crystals with no layering means it crystallised from molten rock, and the size of the crystals means it cooled slowly, deep underground.
You said metamorphic. It is igneous.
Granite. Large interlocking crystals with no layering means it crystallised from molten rock, and the size of the crystals means it cooled slowly, deep underground.
Sample 2
cliff face, Yorkshire
- Clear horizontal layers
- Made of rounded grains that rub off on your fingers
- Soaks up water; a drop disappears into it
You said igneous. It is sedimentary.
Sandstone. Rounded separate grains and visible layers mean this was sand carried by water or wind, dropped, buried and cemented. The gaps between the grains that the cement never filled are why it soaks up water.
You said sedimentary — correct.
Sandstone. Rounded separate grains and visible layers mean this was sand carried by water or wind, dropped, buried and cemented. The gaps between the grains that the cement never filled are why it soaks up water.
You said metamorphic. It is sedimentary.
Sandstone. Rounded separate grains and visible layers mean this was sand carried by water or wind, dropped, buried and cemented. The gaps between the grains that the cement never filled are why it soaks up water.
Sample 3
near an old volcano
- Dark grey, almost black
- Crystals too small to see even with a hand lens
- Solid all the way through, no layers
You said igneous — correct.
Basalt. Crystals that small mean the melt cooled in days rather than millennia — this is lava that reached the surface. Cooled from a melt, exactly like the granite, and at the opposite speed.
You said sedimentary. It is igneous.
Basalt. Crystals that small mean the melt cooled in days rather than millennia — this is lava that reached the surface. Cooled from a melt, exactly like the granite, and at the opposite speed.
You said metamorphic. It is igneous.
Basalt. Crystals that small mean the melt cooled in days rather than millennia — this is lava that reached the surface. Cooled from a melt, exactly like the granite, and at the opposite speed.
Sample 4
building stone, quarried
- Interlocking crystals with pale swirls running through it
- No fossils, though the surrounding rock is full of them
- Fizzes with dilute acid
You said igneous. It is metamorphic.
Marble. It fizzes because it is calcium carbonate, and it sits among fossil-bearing limestone — so it was limestone, cooked and squeezed until the crystals grew and the fossils were destroyed. It never melted.
You said sedimentary. It is metamorphic.
Marble. It fizzes because it is calcium carbonate, and it sits among fossil-bearing limestone — so it was limestone, cooked and squeezed until the crystals grew and the fossils were destroyed. It never melted.
You said metamorphic — correct.
Marble. It fizzes because it is calcium carbonate, and it sits among fossil-bearing limestone — so it was limestone, cooked and squeezed until the crystals grew and the fossils were destroyed. It never melted.
Sample 5
coastal cliff
- Pale grey, and shell shapes are visible on the broken surface
- Fizzes vigorously with dilute acid
- Soft enough to scratch with a coin
You said igneous. It is sedimentary.
Limestone. Fossils are the giveaway — no fossil survives melting or heavy metamorphism, so a rock full of shells was built up gently from the remains of sea creatures. Same compound as sample 4, different group.
You said sedimentary — correct.
Limestone. Fossils are the giveaway — no fossil survives melting or heavy metamorphism, so a rock full of shells was built up gently from the remains of sea creatures. Same compound as sample 4, different group.
You said metamorphic. It is sedimentary.
Limestone. Fossils are the giveaway — no fossil survives melting or heavy metamorphism, so a rock full of shells was built up gently from the remains of sea creatures. Same compound as sample 4, different group.
Sample 6
roof of an old house
- Dark grey and splits cleanly into thin flat sheets
- Grains far too small to see
- The sheets it splits into cut across the faint traces of the original layers
You said igneous. It is metamorphic.
Slate. Mudstone put under pressure has its flat grains rotated until they all point the same way, and the rock then splits along that direction. It never melted — which is exactly what metamorphic means.
You said sedimentary. It is metamorphic.
Slate. Mudstone put under pressure has its flat grains rotated until they all point the same way, and the rock then splits along that direction. It never melted — which is exactly what metamorphic means.
You said metamorphic — correct.
Slate. Mudstone put under pressure has its flat grains rotated until they all point the same way, and the rock then splits along that direction. It never melted — which is exactly what metamorphic means.
Notice which clues actually decided it.
Not colour, not hardness, not weight. What settled every one of those six was texture — whether the pieces interlock like a jigsaw or sit as separate grains — plus layers, fossils and how the rock breaks.
Two of the samples were the same chemical compound as each other and ended up in different groups. A rock is not what it is made of. A rock is what happened to it.
Key fact
Igneous rocks cooled from molten rock and have interlocking crystals. Sedimentary rocks are layers of grains and can hold fossils. Metamorphic rocks were changed by heat and pressure without melting.
Five words
Say your answer out loud before you turn each card over. If you cannot say it, you do not know it yet.
Think again
“If a rock has crystals in it, it must be igneous.”
Crystals do form when molten rock cools. Commit before you read on.
Marble is packed with interlocking crystals and never melted. Heat and pressure let the crystals in the original limestone grow and re-form while the rock stayed solid — the same thing that happens to slate, and to the bands in a piece of gneiss.
So crystals narrow the answer to igneous or metamorphic and no further. What separates the two is the rest of the evidence: banding, swirls or a rock that splits into sheets says metamorphic; a random jigsaw of different minerals with no layering says igneous. One clue rarely closes a classification. It just removes an option.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
Which type of rock can contain fossils, and why only that one?
Rung 2 · The one that catches people
Two igneous rocks have the same composition, but one has crystals 5 mm across and the other has crystals too small to see. What is the difference between them?
Rung 3 · Explain
You are handed an unknown rock. Describe the observations you would make and explain how each one narrows down which of the three types it is.
Rung 4 · Take it somewhere new
A geologist finds a band of marble running through a region of limestone, and an old igneous intrusion nearby. Explain how the marble got there, and what the arrangement tells you about the order of events.
Key note
Igneous rocks form when molten rock cools and crystallises; slow cooling underground gives large crystals and fast cooling at the surface gives small ones. Sedimentary rocks form when fragments settle in layers and are cemented together, and fossils are preserved in them and almost nowhere else. Metamorphic rocks form when existing rock is changed by heat and pressure without melting.
Going further
Sedimentary rock is history written in order. The layers were laid down oldest at the bottom, and each one records the conditions of its own moment — a desert leaves rounded wind-blown sand, a warm shallow sea leaves shell limestone, a swamp leaves coal. Read a cliff face from bottom to top and you are reading millions of years in sequence, with the fossils in each layer telling you what was alive when it formed.
Metamorphic rock is why slate roofs exist. Mudstone squeezed under a mountain range has its flat mineral grains rotated until they all line up in the same direction, and a rock whose grains are all aligned splits cleanly along that direction into sheets. Those sheets are not the mud's original layers — the split follows the pressure, and it can cut clean across the bedding. Nobody designed that property; it is a record of the direction the squeeze came from, and Welsh slate roofed half of Victorian Britain because of it.
Before this lesson
At GCSE this becomes
- Mineral identification, grain size and cooling rate, and using rock sequences to date events in the Earth's history.
Where to next
Ask Mr Badmus AI
Still unsure how marble and limestone are related?
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