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# Why do some rocks have big crystals?

Year 8 Earth and space · The rock cycle

<!-- Speaker note: 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 (Engage and Explore 8 min · Explain 8 min · Evaluate 3 min · Typical adds 4 min, Wider about 5 min). -->

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## Today

**We are learning to** 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.

- 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.

<!-- Speaker note: 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. Teacher: In your own words: the granite and basalt paragraph is the evidence for the node. -->

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## Why it matters

> 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?

<!-- Speaker note: Rocks keep a record of how they formed. This lesson teaches you to read one part of it. -->

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<span style="background:#dbeafe; color:#1a1a1a; border:1.5px solid #3b82f6; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">💬 Discussion</span>

## Engage: predict

**Molten rock cools and sets into solid rock full of crystals. Which do you predict gives bigger crystals?**

- Cooling slowly, deep underground
- It makes no difference
- Cooling quickly, at the surface
- It depends only on how much molten rock there is

<!-- Speaker note: What happens: Cooling slowly. Few crystals start, and they have time to grow large. Fast cooling starts many crystals at once and none can grow. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: if you have them, pass round granite and basalt samples before predicting. -->

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## Explore it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-science/index.html?id=deck%2Fscience%2Fearth-8%2Figneous%2Fstep-2&code=widget%3A+cooling%0A}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/earth-8/igneous/?full=step-2#step-2)

<!-- Speaker note: Choose a cooling speed, then press **Cool the magma**. Each colour is one crystal. Do it again at a very different speed. -->

<!-- Speaker note: Teacher: insist on two very different speeds. Ask: did slow cooling take longer to start crystals, or start fewer? -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## What happened?

* Cooling slowly. Few crystals start, and they have time to grow large. Fast cooling starts many crystals at once and none can grow.

<!-- Speaker note: Reveal after running. Ask who predicted it, and what made them think so. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Slow cooling

**Why did slow cooling give bigger crystals?**

- Slow cooling melts the small crystals
- There was more magma
- Only a few crystals started, and the particles had time to join them
- Slow cooling makes each particle bigger

* Answer: Only a few crystals started, and the particles had time to join them

<!-- Speaker note: With little time below the melting point, few crystals start. Each one has time to collect particles from the melt and grow large. -->

<!-- Speaker note: Teacher: the two mechanism questions carry the checkpoint; the sort is the application. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Fast cooling

**Why did fast cooling give small crystals?**

- The particles shrank as they cooled
- Many crystals started at once, and the melt set before any could grow large
- Fast cooling breaks crystals apart

* Answer: Many crystals started at once, and the melt set before any could grow large

<!-- Speaker note: Cooling fast, the melt is soon far below its melting point and crystals start everywhere. They share out the particles and the rock sets before any gets big. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Intrusive or extrusive

**Where did each rock form?**

Groups: Intrusive (underground, slow cooling) · Extrusive (at the surface, fast cooling)

Granite: large crystals you can see · Basalt: crystals too small to see · Obsidian: glass, no crystals · Gabbro: large crystals, dark

* Answer: Granite: large crystals you can see → Intrusive (underground, slow cooling); Basalt: crystals too small to see → Extrusive (at the surface, fast cooling); Obsidian: glass, no crystals → Extrusive (at the surface, fast cooling); Gabbro: large crystals, dark → Intrusive (underground, slow cooling)

<!-- Speaker note:  -->

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<span style="background:#fef3c7; color:#1a1a1a; border:1.5px solid #f59e0b; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✍️ Activity</span>

## Elaborate: the extremes

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-science/index.html?id=deck%2Fscience%2Fearth-8%2Figneous%2Fstep-4&code=widget%3A+cooling%0Arates%3A+1%2C+5%0Acaption%3A+%22Very+slow%2C+or+instant%3A+compare+the+two+extremes.%22%0A}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/earth-8/igneous/?full=step-4#step-4)

<!-- Speaker note: What this step is for: Pushing to instant cooling produces glass, which shows the limit of the mechanism: no time for any pattern. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Glass

**With instant cooling, most of the rock was glass. What does glass mean at the particle level?**

- The particles evaporated
- The particles froze where they were, with no pattern
- The particles made one giant crystal

* Answer: The particles froze where they were, with no pattern

<!-- Speaker note: Obsidian forms when lava cools so fast that particles can't move into a pattern at all. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Big and small together

**A rock has large crystals surrounded by tiny ones (geologists call it porphyry). What is the most likely history?**

- It cooled slowly underground for a while, then erupted and cooled quickly
- It cooled quickly and then slowly
- Two different rocks were glued together

* Answer: It cooled slowly underground for a while, then erupted and cooled quickly

<!-- Speaker note: Two stages: slow cooling grew the big crystals, then fast cooling set the rest as tiny crystals around them. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Pumice

**Pumice is a glassy rock full of holes, so light it can float. What can you infer about how it formed?**

- It cooled very fast, while gas bubbles were still escaping
- It was worn into shape by water
- It cooled slowly deep underground

* Answer: It cooled very fast, while gas bubbles were still escaping

<!-- Speaker note: Frothy lava blasted out of a volcano: cooled almost instantly, trapping gas bubbles in glass. -->

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## 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.**

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Exit ticket

**A rock has crystals too small to see. Did it form underground or at the surface? Why?**

* At the surface: it cooled quickly, so many crystals started and none had time to grow large.

<!-- Speaker note: Students answer on paper or a mini whiteboard before the reveal. Anyone who cannot answer needs the main idea again next lesson. Where this 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. -->