Lesson focus: Why do some rocks contain large crystalA solid where the particles are arranged in a neat, repeating pattern, giving it flat faces.?
Compare igneous and metamorphic rock formation.
Build on earlier learning
Melting and freezingConnect cooling and solidification with particle arrangement.
Practical
Crystal size and cooling rate
- Melt salol gently in a water bath.
- Drop some onto a warm slide and some onto a cold slide.
- Observe the crystals with a hand lens.
Record: Crystal size for each slide and the link to how fast magma cools.
Safety: Use a water bath, not a direct flame, to melt salol.
You will be successful when you can…
- Explain igneous solidification.
- Relate cooling rate to crystal size.
- Explain metamorphism without melting.
- Use more than one feature to classify.
Words that carry the science
- igneous rock
- Rock formed when molten rock cools and solidifies.
- magma
- Molten rock below Earth’s surface.
- crystal
- Solid with particles arranged in a regular structure.
- metamorphism
- Change to existing solid rock through heat, pressure or reactive fluids.
- texture
- Size, shape and arrangement of grains or crystals.
Use process evidence to separate igneous formation from metamorphism
01Cooling produces igneous rock
Slower cooling usually allows larger crystalA solid where the particles are arranged in a neat, repeating pattern, giving it flat faces. to grow; rapid cooling leaves less time and can produce small crystals or glass. Why? In melted rock, the particles can move about. As the rock cools, they join onto crystals. If it cools slowly, the particles have more time to join on, so the crystals grow bigger. If it cools quickly, there is less time, so the crystals stay small.
02Metamorphism stays solid
Heat, pressure and chemically active fluids can recrystallise minerals and change textureThe size, shape and arrangement of the grains or crystals in a rock. while the rock remains solid. If it melts, later cooling makes igneous rockRock formed when hot liquid rock (magma or lava) cools and becomes solid..
03Features need context
Large interlocking crystals support slow crystallisation but can also occur in metamorphic rocks. Mineral composition, field relationships and texture strengthen identification.
Crystal size and cooling rate
As magma cools, crystals start to grow and keep growing until they meet their neighbours. Slow cooling (deep underground) gives few seeds and lots of time: big crystals. Fast cooling gives many tiny crystals, or glass.
Orange: molten rock. Grey, pink and black: crystals of different minerals.
Controls
Crystal counts are illustrative. Metamorphic rocks form differently: heat and pressure change existing rock without melting it.
Compare two igneous textures
Samples have the same mineral composition. A has 5 mm crystals; B is glassy.
- Hold composition constant as stated.
- Larger crystals in A indicate more growth time.
- A therefore cooled more slowly than B.
- B’s glassy texture supports very rapid cooling.
How to make the reasoning visible: The cooling inference is strongest because composition is controlled; crystal size alone is not a universal identifier.
Formation evidence table
Choose a process and identify what extra evidence is needed.
| Observation | Possible process | Extra evidence | Unsafe conclusion |
|---|---|---|---|
| Large interlocking crystals | Slow cooling or recrystallisation | Composition/context | Definitely igneous |
| Tiny crystals | Rapid cooling | Same composition/location | Always volcanic |
| Aligned minerals | Directed pressure | Mineral bands/context | Rock melted |
| Glassy texture | Very rapid cooling | Chemistry/flow context | No crystals anywhere |
Why does melting rule out metamorphism for that stage?
Why compare samples of similar composition?
Use the table. Why do tiny crystals suggest rapid cooling?
Check your understanding
Answer from memory, then check against the suggested answer.
What is igneous rock?
What is magma?
What is crystal?
What is metamorphic rock?
What is lava?
Explain why granite commonly has larger crystals than basalt.
Why can rock shape alone not identify metamorphism?
Granite has large crystals and basalt has small crystals. Which one cooled underground? Explain.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
When melted rock cools slowly, it makes bigger crystals than when it cools quickly. Answer the parts to explain why.
In melted rock, what can the particles do?
(1)What does slow cooling give the particles before the rock goes solid?
(1)What do the particles do in that time?
(1)What happens when melted rock cools quickly?
(1)Total for this question: 4 marks
Command word: Link the cooling time to the size of the crystals.