The Rock Cycle and the Three Types of Rock, Grade 9
Three questions will identify almost any rock a school hands a Grade 9 class.
Can you see layers? Can you see crystals or gas holes? Is it banded, shiny or unusually hard? That sequence sorts sedimentary from igneous from metamorphic, and it works on a bench with no equipment beyond a hand lens and a drop of vinegar.
This page covers the three rock types, the tests that separate them, the rock cycle that links them, and the layers of the Earth the whole thing happens in. It also fixes a mineral name the textbook gets wrong twice over.
The three questions
| Question | If yes | What you are seeing |
|---|---|---|
| Can you see flat layers or bands of different grain? | Sedimentary | Particles that settled, one layer on top of another |
| Can you see interlocking crystals, or holes where gas escaped? | Igneous | Melt that cooled and set |
| Is it banded, shiny or noticeably hard, and does it split into sheets? | Metamorphic | An older rock squeezed and heated until its minerals rearranged |
Crystals interlock. Layers lie parallel. That difference is the one learners miss, and it is why granite is so often called sedimentary because of its speckles.
Where this fits in the curriculum
| Subject | Natural Sciences |
|---|---|
| Grade | 9 |
| Term | 4 |
| Strand | Planet Earth and beyond |
| Topic | Topic 17, Lithosphere and the rock cycle |
| Status | No formally assessed practical task |
| Marks | None prescribed. Our worksheet is 40 marks |
Four tests you can run on any rock
| Test | How | What it tells you |
|---|---|---|
| Look for layers | Turn the rock so light falls across it at an angle | Flat parallel bands mean sedimentary |
| Look for crystals | A hand lens, or a phone camera at full zoom | Big interlocking crystals mean slow cooling underground. None visible means it cooled fast |
| Scratch it | Drag a steel nail across a fresh surface | A nail is about 5,5 on the hardness scale. It marks shale and limestone. It will not mark granite, slate or quartzite |
| One drop of vinegar | Watch closely for tiny bubbles | Fizzing means a carbonate: limestone or marble. Nothing else in a school set fizzes |
The vinegar test has a trap in it, and it is a good one. Limestone fizzes and it is sedimentary. Marble fizzes and it is metamorphic. They are the same chemistry, and marble is what limestone becomes under heat and pressure. So the fizz proves a carbonate, not a rock type, and the learner has to use a second test.
And one test everybody remembers: pumice floats. It is the only common rock that does, because it is more gas hole than rock.
The rock cycle, as processes rather than a picture
| From | To | Process |
|---|---|---|
| Magma | Igneous rock | Cooling and hardening |
| Any rock at the surface | Loose particles | Weathering. Breaking up where it stands |
| Particles on a slope | Particles in a river or the sea | Erosion. Being carried away |
| Particles in water | Layers of sediment | Deposition |
| Layers of sediment | Sedimentary rock | Compaction under the weight of later layers |
| Any rock | Metamorphic rock | Heat and pressure |
| Any rock pushed deep enough | Magma | Melting |
Weathering and erosion are not the same thing and the marks depend on it. Weathering breaks the rock up where it is. Erosion carries the pieces somewhere else. Frost that splits a boulder is weathering. The river that takes the fragments away is erosion.
The cycle has no starting point
Textbooks draw it as a circle beginning at magma, which makes it look like a queue. It is not. Every step is happening right now, in different places: a mountain is weathering, a delta is compacting, a plate edge is melting. A rock can also skip steps, going straight from igneous to metamorphic without ever being sediment.
Why granite has big crystals and pumice has none
Both are igneous. Both came from melt. The only difference is how fast they cooled.
| Granite | Pumice | |
|---|---|---|
| Where it cooled | Deep in the crust | On the surface |
| How fast | Very slowly, over thousands of years | In minutes |
| Crystals | Large enough to see and name | Too small to see at all |
| The giveaway | Speckles of three different minerals | Holes, and it floats |
Slow cooling gives crystals time to grow. That single sentence answers a four-mark question in every Term 4 paper.
The layers of the Earth
| Layer | Roughly | State |
|---|---|---|
| Crust | About 5 km under the oceans, about 35 km under continents | Solid, and the thinnest layer |
| Mantle | About 2 900 km | Solid rock that flows very slowly. Partly molten near the base |
| Outer core | About 2 300 km | Liquid |
| Inner core | About 1 200 km to the centre | Solid, and about as hot as the surface of the Sun |
Treat those numbers as approximate, whatever your textbook prints. Nobody has been down there. The figures come from the way earthquake waves bend and speed up as they pass through different materials, and they are revised as the models improve.
The lithosphere is the crust plus the rigid top of the mantle. It is the layer the whole rock cycle happens in.
One correction worth knowing
Several Grade 9 textbooks say that gold occurs in the mineral "calverite (AuTe₃)".
Both halves are wrong. The mineral is calaverite, and its formula is AuTe₂, a gold telluride. A learner who searches the printed spelling finds nothing, and concludes the internet is broken.
Two smaller ones from the same page: malachite is Cu₂CO₃(OH)₂ and the brackets matter, and feldspar and mica are compounds of silicon and oxygen. Silica is already silicon dioxide, so "compounds of silica and oxygen" says the oxygen twice.
The bread model, and what it actually shows
Layer slices of brown and white bread with a filling between each, then press the stack down hard with a book.
| In the model | In the ground |
|---|---|
| Each slice | A layer of sediment laid down at a different time |
| Pressing with the book | Compaction under the weight of everything above |
| The bottom slice | The oldest layer |
Ask which slice is oldest before saying anything. That one question is the principle geologists use to read a cliff face, and a class works it out in seconds.
If it does not work
| Problem | Cause | Fix |
|---|---|---|
| All three rocks look the same | The samples are too small or too similar | Fist-sized pieces, and make sure the three are genuinely different types |
| Nothing fizzes | No carbonate in the set | Add limestone, marble or a stick of blackboard chalk |
| Granite called sedimentary because of the speckles | Speckles read as "layers" | Look with the lens: crystals interlock like a jigsaw, layers run parallel |
| Slate and shale confused | They are the same material before and after heat | Slate is harder and splits into clean flat sheets. Shale crumbles between the fingers |
| The bread model is just a sandwich | It was not pressed | Flatten it with a book. The pressure is the lesson |
| The scratch test marks everything | Testing a weathered surface | Break or find a fresh face. Weathered rock is soft rock |
Safety
- Do not hammer rocks in a classroom. Chips fly a long way and nobody wears goggles for a rock lesson
- Rocks get thrown. Say so once, at the start
- The scratch test is a nail on a rock, not a nail on the bench
- Wash hands after handling rocks collected outside
Free worksheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner worksheet, 40 marks. Part A is rock identification and the layers of the Earth, marked on the learner's own observations. Part B is the rock cycle
- Marking memorandum, including how to mark an identification that is reasoned correctly but named wrongly
Related practicals
- Plant and animal cells under a microscope, Grade 9. Term 1, and the same hand lens habit
- The pH scale and indicators, Grade 9. Acids, which is what the vinegar test is
- Solubility and dissolving. Chemical weathering is dissolving, slowly
What you need to run it
Almost nothing, and that is the good news about this practical.
You need three rocks. A stone yard, a quarry, a landscaping supplier or a building site will give you offcuts for nothing. Granite kitchen worktop offcuts are ideal, and a pharmacy sells pumice as a foot stone. Slate comes off any old roof. Sandstone and shale are a roadside cutting.
A hand lens is the one thing worth buying, and we do not currently stock one, which is an obvious gap we are fixing. In the meantime a phone camera at full zoom shows crystals perfectly well, and a student microscope is better still for looking at sand grains.
A dropping pipette at R10 makes the vinegar test tidy. The vinegar comes from the kitchen.
We are putting together a Grade 9 rock identification set: six labelled specimens, granite, pumice, sandstone, shale, slate and marble, in a tray with a blank tray for handing out unknowns. Coming shortly.