Igneous rock forms when magma or lava cools. Slow cooling underground gives large crystals (granite); fast cooling at the surface gives small crystals (basalt) or glass (obsidian). Crystal size is a record of cooling rate.
Granite kitchen benches have crystals you can see from across the room. Basalt has crystals too small to see. Both crystallised from molten rock: what was different?
Rocks keep a record of how they formed. This lesson teaches you to read one part of it.
Engage: Commit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Explore: Try it on the board and compare what happens with your prediction.
Explain: Use the particle model to explain what you saw.
Elaborate: Use the same idea in a new context.
Evaluate: Look back at your first ideas and what changed.
WidthMore contexts, same idea.
Make your prediction first. Help opens once you've locked it in.
EngageCommit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Engage: big crystals or small?
ExploreTry it on the board and compare what happens with your prediction.
Explore: cool the magma
Choose a cooling speed, then press Cool the magma. Each colour is one crystal. Do it again at a very different speed.
Try it yourselfNothing is saved: start again as often as you like.
Cool the magma at two different speeds and compare the crystals.
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: crystal size records cooling rate
Intrusive rocks cool underground, inside other rock. Extrusive rocks cool after lava erupts onto the surface.
In your book: explain why granite has large crystals and basalt small ones. Use "cooling", "crystals start" and "time to grow".
Where did each rock form?
Granite: large crystals you can see
Basalt: crystals too small to see
Obsidian: glass, no crystalsCooled almost instantly: lava into water or air.
Gabbro: large crystals, darkSame make-up as basalt, but it cooled slowly underground.
Elaborate TypicalUse the same idea in a new context.
Elaborate: the extremes
Try it yourselfNothing is saved: start again as often as you like.
Cool the magma very slowly, then instantly.
Use the board. This ticks when you get there.
Elaborate WiderUse the same idea in a new context.
Elaborate: read a rock
EvaluateLook back at your first ideas and what changed.
Evaluate: look back
Write one sentence: "At first I thought… Now I think… because…"
Revision summary
Igneous rock forms when magma (underground) or lava (at the surface) cools and sets.
Slow cooling (intrusive, underground): few crystals start and grow large. Granite, gabbro.
Fast cooling (extrusive, at the surface): many crystals start, all stay small. Basalt.
Instant cooling: no time for a pattern, so glass. Obsidian, pumice.
Crystal size is a record of cooling rate.
For teachers
Students explain igneous rock formation from magma or lava and connect crystal size to cooling rate, then use crystal size to infer where a rock formed (AC9S8U04, node 2, a hinge node). About 25 minutes.
I can tell intrusive (slow cooling underground) from extrusive (fast cooling at the surface) igneous rock.
I can explain why slower cooling gives larger crystals.
I can infer where an igneous rock formed from its crystal size.
Before class
A concept tutorial for introducing or revising the idea; it does not replace handling rock samples or a crystallisation practical. Set the width with ?width=minimum, typical (default) or wider.
The hinge is the mechanism, not the rule: slow cooling means few crystals start and they grow. Students who only memorise "granite big, basalt small" can't read an unfamiliar rock.
Model notes: each colour is one crystal. The model shows only cooling speed; granite and basalt also differ in composition (felsic and mafic), which is why gabbro (slow-cooled basalt-type magma) is in the sort. Typical results: very slow, about 5 crystals of 60 particles; fast, about 23 of 14; instant, mostly glass.
Running the lesson
Engage: if you have them, pass round granite and basalt samples before predicting.
Explore: insist on two very different speeds. Ask: did slow cooling take longer to start crystals, or start fewer?
Explain: the two mechanism questions carry the checkpoint; the sort is the application.
In your own words: the granite and basalt paragraph is the evidence for the node.
The hard part (on purpose)
Explaining crystal size by how many crystals start and how long they grow. When a student stops at 'longer to grow', ask how many crystals started each time.
When students get stuck
A student says slow cooling gives big crystals because they had longer to grow, and stops there. Ask: How many crystals started each time? Fix: Few start in slow cooling, so the particles are shared between few crystals. That is why each one is large.
A student says the particles got bigger. Ask: Are the dots bigger in the slow-cooled rock? Fix: No: same particles, more of them in each crystal.
A student thinks obsidian has tiny crystals. Ask: Look at the glass on the board: is there a pattern? Fix: No pattern means no crystals at all: the particles froze in place.
Stretch and support
Stretch: Columnar basalt (like the Organ Pipes in Victoria) has small crystals but forms giant columns. Is crystal size the same thing as the size of the rock's shapes?
Support: Give a two-row table to fill while running the board: cooling speed, number of crystals, average size, what rock it is like.
Where it leads
Next: metamorphic rocks (node 3), changed by heat and pressure without melting; then the full rock cycle.
In class: salol (phenyl salicylate) or a saturated alum solution cooled on a warm and a cold slide under a hand lens; or salt solution evaporated fast and slow.
What each step is for
Engage: big crystals or small?: Students often link crystal size to the amount of magma or to time 'growing' without a mechanism; the board shows the competition between starting and growing crystals.
Explore: cool the magma: Running the same melt at two cooling speeds and reading the results table makes the cause visible: number of crystals started, not time alone.
Explain: crystal size records cooling rate: Explaining the mechanism (few crystals start, particles join them) lets students infer formation from crystal size instead of memorising rock names.
Elaborate: the extremes: Pushing to instant cooling produces glass, which shows the limit of the mechanism: no time for any pattern.
Elaborate: read a rock: Applying the crystal size rule to unfamiliar samples is the transfer the node asks for.
Now you can
Tell intrusive (slow cooling underground) from extrusive (fast cooling at the surface) igneous rock.
Explain why slower cooling gives larger crystals.
Infer where an igneous rock formed from its crystal size.