--- title: "Why do some rocks have big crystals?" subtitle: "Year 8 Earth and space · The rock cycle · Lesson 1" --- # Why do some rocks have big crystals? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** 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? :::teacher-note **In one sentence:** 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). **Students should be able to say:** 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. **Width:** the unmarked parts are the floor every student does (Engage, Explore, Explain, Evaluate). Typical and Wider sections add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Engage :::keep-together **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 My prediction, and why: {{lines-3}} ::: :::teacher-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. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: crystal size records cooling rate :::keep-together ### 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 ::: :::teacher-note **Answer:** Only a few crystals started, and the particles had time to join them With little time below the melting point, few crystals start. Each one has time to collect particles from the melt and grow large. ::: :::keep-together ### 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 ::: :::teacher-note **Answer:** Many crystals started at once, and the melt set before any could grow large 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. ::: :::keep-together ### 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: {{field-15}} - Basalt: crystals too small to see: {{field-15}} - Obsidian: glass, no crystals: {{field-15}} - Gabbro: large crystals, dark: {{field-15}} ::: :::teacher-note **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) ::: :::typical ## Elaborate: the extremes :::keep-together ### 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 ::: :::teacher-note **Answer:** The particles froze where they were, with no pattern Obsidian forms when lava cools so fast that particles can't move into a pattern at all. ::: ::: :::wider ## Elaborate: read a rock :::keep-together ### 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 ::: :::teacher-note **Answer:** It cooled slowly underground for a while, then erupted and cooled quickly Two stages: slow cooling grew the big crystals, then fast cooling set the rest as tiny crystals around them. ::: :::keep-together ### 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 ::: :::teacher-note **Answer:** It cooled very fast, while gas bubbles were still escaping Frothy lava blasted out of a volcano: cooled almost instantly, trapping gas bubbles in glass. ::: ::: ## 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.** ## Exit ticket :::keep-together **A rock has crystals too small to see. Did it form underground or at the surface? Why?** {{lines-3}} ::: :::teacher-note **Answer:** At the surface: it cooled quickly, so many crystals started and none had time to grow large. :::