KS3 · Chemistry

Types of rocks (sedimentary, igneous, metamorphic)

Some rocks were once melted, some were squeezed for ages without melting, and some are grains glued together. Every rock leaves clues. Let's learn to read them.

Be a rock detective

Which clue points to which rock type?

Rocks are mixtures of minerals, and there are three main types. Each type is made in a different way, and that leaves clues you can spot with a hand lens. For each clue below, tick EVERY rock type it can point to. Careful: some clues belong to more than one type.

  • A Sedimentary
  • B Igneous
  • C Metamorphic
  1. Separate grains stuck together
  2. Fossils or bits of shell
  3. Chunks of grit or pebbles among the grains
  4. Crystals pointing in every direction
  5. Crystals lined up in one direction
  6. Layers
  7. Made of crystals, not grains

Igneous rock · Where it starts

Is there a sea of melted rock under our feet?

Igneous rock forms when melted rock cools. So first: where does melted rock come from?

Which is closest to what you think right now?
How sure are you?

Predict, then check

You can model cooling rock in the lab with salol, a substance that forms crystals as it cools between two microscope slides.

Melted salol is put between two warm microscope slides, and some more between two cool slides. Which will grow the larger crystals?

Igneous rock · Inside or outside?

Where melted rock cools decides its crystal size

Step down from the surface. Choose a zone to see how the melted rock behaves there, and what kind of igneous rock it leaves behind.

↑ Earth's surface

↓ Deep below the surface

Zone 1 of 3

At the surface

Lava crystallises outside Earth's crust. It cools quickly, so its crystals stay small. Rock made this way is called extrusive. Think 'exit': it formed after the molten rock came out.

  • Small crystals
  • Extrusive igneous rock
  • e.g. basalt, pumice, rhyolite

Outside the crust: quick cooling, small crystals (extrusive). Inside the crust: slow cooling, large crystals (intrusive). And what the molten rock is made of helps decide whether the rock ends up pale or dark.

Metamorphic rock

?

Reason it through

How can one rock turn into a completely different rock without ever melting?

Link 1 of 4

First link · your turn

Start with an existing rock: sedimentary, igneous or even metamorphic. It ends up near a tectonic plate boundary. What does it face there?

2
Locked — reveal the link above first
3
Locked — reveal the link above first
4
Locked — reveal the link above first

Properties and uses

Why is a worktop made of granite, not sandstone?

A porous rock has tiny gaps (pores) between its grains that can soak up air, water and other liquids. Drop one in water and you'll see air bubble out. Tick each property you think each rock has, then check, and read down each column for the pattern.

Granite
Sandstone
Slate
Chalk

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Every rock carries clues to how it was made. Learn to read them and you can tell sedimentary, igneous and metamorphic rock apart.

What you need to know

  • Rocks are mixtures of minerals. The three main types are sedimentary, igneous and metamorphic.
  • Clues: grains glued together (and fossils) point to sedimentary; jumbled crystals point to igneous; lined-up crystals point to metamorphic.
  • Layers can mean sedimentary or metamorphic, so always use several clues together.
  • Igneous rock forms when magma or lava cools. Slow cooling inside the crust gives large crystals (intrusive); quick cooling outside gives small crystals (extrusive).
  • Metamorphic rock forms when heat and pressure change an existing rock without melting it.

The big picture

Rocks are mixtures of minerals, and the three main types each leave clues to how they formed. Sedimentary rocks have grains glued together, often in layers and sometimes with fossils. Igneous rocks have jumbled crystals and form when magma or lava cools: slowly inside the crust gives large crystals, quickly outside gives small ones. Metamorphic rocks have lined-up crystals, made when heat and pressure change a rock without melting it. Layers fit two types, so use clues together.

Key points

1Rock melts only where temperature and pressure are right, near plate boundaries and at hot spots; the mantle's rock stays solid.
2Magma is molten rock below the surface; lava is molten rock on the surface.
3Examples: intrusive igneous — granite, gabbro, diorite; extrusive igneous — basalt, pumice, rhyolite.
4Molten rock rich in silicon tends to form pale rock; rich in iron and magnesium, darker rock such as gabbro.
5With more heat, pressure and time, shale becomes slate, then schist, then gneiss. Limestone can become marble.
6Porous rocks (usually sedimentary) soak up liquids and crumble more easily; rocks with interlocking crystals are non-porous.

Worked example

Problem

You're handed two mystery rocks. Rock A, through a hand lens: it shows layers, it's made of crystals rather than grains, the crystals are lined up, and there are no fossils. Rock B: large crystals pointing in every direction. Classify each rock, and say whether Rock B cooled inside or outside the crust.

⚠ Watch out

Naming a rock type from one clue, usually layers. Layers can mean sedimentary or metamorphic, so check grains versus crystals, and how the crystals sit. And the mantle isn't liquid: rock melts only where temperature and pressure are right.

🧠

Memory hook

Glued, Jumbled, Lined up: Sedimentary, Igneous, Metamorphic. And for igneous crystals: slow grows big.

✓

Check yourself

Close the lesson and, from memory, explain in three sentences how limestone could become marble, and why it never melts on the way.

Flashcards

(15)
What are rocks made of?
A mixture of minerals.
Name the three main types of rock.
Sedimentary, igneous and metamorphic.
What does a sedimentary rock look like close up?
Separate grains stuck together by other minerals. Most show layers, and some contain fossils, bits of shell, grit or pebbles.
Which crystal clue points to igneous rock?
Crystals pointing in many directions (jumbled), not grains.
Which crystal clue points to metamorphic rock?
Crystals that are mostly lined up, so the rock tends to show layers.
Why can't layers alone tell you the rock type?
Layers could mean sedimentary or metamorphic. You need another clue, such as grains versus crystals.
Magma or lava: what's the difference?
They're the same molten rock. Magma is below Earth's surface; lava is on the surface.
Where does rock melt?
Only where temperature and pressure are right: near tectonic plate boundaries and at hot spots. Most minerals in rock melt at about 600 to 1,300 °C.
How does cooling rate affect crystal size in igneous rock?
Quick cooling gives small crystals; slow cooling gives large crystals.
Extrusive versus intrusive igneous rock?
Extrusive: lava crystallises outside the crust, quickly, giving small crystals (basalt, pumice, rhyolite). Intrusive: magma crystallises inside the crust, slowly, giving large crystals (granite, gabbro, diorite).
What makes an igneous rock pale or dark?
Its make-up: molten rock rich in silicon tends to form pale rock; rich in iron and magnesium, darker rock such as gabbro.
How does a metamorphic rock form?
Sustained extreme pressure and/or temperature change an existing rock without melting it. Its minerals recrystallise and line up in layers.
What does shale turn into as heat, pressure and time increase?
Slate, then schist, then gneiss. Beyond that, it would melt to magma.
What does porous mean?
A porous rock has small gaps between its grains that can absorb and hold air, water and other liquids.
Why is granite good for a kitchen worktop?
It is hard and non-porous, so it doesn't soak up spills or crumble.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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