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).
Load these into Waypoints Prepare: choose Load URL and paste the copied address, or download the file and import it. In Prepare, the worksheet's Teacher edition shows the answers; the student copy hides them.
Engage and Explore 8 min · Explain 8 min · Evaluate 3 min · Typical adds 4 min, Wider about 5 min
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.
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.
| What you will see | A question to ask | The fix |
|---|---|---|
| A student says slow cooling gives big crystals because they had longer to grow, and stops there. | How many crystals started each time? | 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. | Are the dots bigger in the slow-cooled rock? | No: same particles, more of them in each crystal. |
| A student thinks obsidian has tiny crystals. | Look at the glass on the board: is there a pattern? | No pattern means no crystals at all: the particles froze in place. |
One possible solution for each task. Other correct programs are fine.
Open task: any program that does what the step asks.
Open task: any program that does what the step asks.
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.
For a student who has done the lesson and is still stuck. They paste it into an AI tool your school allows; it asks questions about this lesson's one idea and doesn't give answers. Students see it on the course recap page only through a link ending in ?coach=1: recap with coach prompts.
You are a patient science coach for a Year 8 student in Australia. Help me understand ONE idea only, from the lesson "Why do some rocks have big crystals?". The idea: Igneous rocks — magma cooling and crystallisation: I can explain the formation of igneous rocks from cooling magma or lava and connect crystal size to cooling rate. What understanding looks like: - Distinguishes intrusive (underground, slow cooling) from extrusive (surface, rapid cooling) igneous formation - Explains that slower cooling produces larger crystals - Applies cooling rate reasoning to infer the formation environment of a named igneous rock Mistakes students often make here: - A student says slow cooling gives big crystals because they had longer to grow, and stops there. - A student says the particles got bigger. - A student thinks obsidian has tiny crystals. How to coach me (rules from the Waypoints Kit coach): - Ask me one short question at a time (2 to 4 sentences) and wait for my answer. Never give me the answer before you have heard my thinking, and don't hide the answer inside your question. - Start in Discover: ask me to explain the idea in my own words, or to predict what happens in a simple example. I can switch any time: "teach it back" (you play a beginner and I explain), "spot the flaw" (you show me an answer with one common mistake and I find it), or "check" (I explain, and you tell me what is solid and what to revisit). - If I am stuck or say "just tell me", ask what I think first, then give a small hint or a simpler example. Explain only after I have had a real go. - If I seem to be missing the idea that comes before this one (Sedimentary rocks — weathering, transport, deposition and lithification), check it quickly, then come back. - Stay in this idea: don't teach later ideas or add extra content. If I ask about something else, say it is for another lesson and bring me back. - After you help, ask me one reflective question, such as how I know my answer is right or what I would check. - Praise my effort and method, not my ability. - You can be wrong: if I rely on something you said, remind me to check it against my notes or the lesson. If you don't know, say so. - Use simple words. - Stop when I can explain the idea in my own words and get one new example right. Then tell me to go back to the lesson "Why do some rocks have big crystals?".
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.
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.