What you will learn
- the three rock types — igneous, sedimentary, metamorphic — and how each forms,
- how to identify a rock from its features (crystals, layers, fossils, grain size),
- the rock cycle as a loop driven by heat, pressure, weathering and deposition,
- why the timescale of the rock cycle is millions of years,
- how rock properties decide whether a rock is useful (building stone, ore, fuel).
Worked example 0 Real-world example: identifying a mystery rock
A student finds a rock that is grey, has small flat layers, splits cleanly into sheets, and contains no visible crystals.
- Layers that split cleanly suggest a metamorphic origin, not igneous.
- No visible crystals → fine-grained. Matches slate.
- Slate forms when shale (a sedimentary rock) is squeezed and heated underground.
The rock is slate. Its ability to split in sheets is why it is used for roofing.
Key idea: classification uses observable features (crystals, layers, fossils) as a fingerprint for how the rock formed.
1. Three rock types
- Igneous — solidified from molten rock (magma or lava). Examples: granite, basalt, pumice, obsidian.
- Sedimentary — formed from compressed layers of sediment (often in water). Examples: sandstone, limestone, mudstone, coal.
- Metamorphic — any rock changed by heat and/or pressure without melting. Examples: marble (from limestone), slate (from shale), gneiss, schist.
2. Identifying rocks
| Feature | Likely rock type |
|---|---|
| Visible crystals, interlocking | Igneous (e.g. granite) |
| Glassy, no crystals | Igneous (e.g. obsidian) |
| Small holes (vesicles), very light | Igneous (e.g. pumice) |
| Clearly layered, gritty | Sedimentary (sandstone) |
| Contains fossils | Sedimentary |
| Fizzes with acid (contains calcium carbonate) | Sedimentary (limestone) or metamorphic (marble) |
| Layered and splits cleanly along layers | Metamorphic (slate, schist) |
| Banded with dark and light stripes | Metamorphic (gneiss) |
Worked example 1 Crystals big or small?
Two igneous rocks: one with large visible crystals (granite) and one with crystals too small to see (basalt).
- Granite formed deep underground where magma cooled slowly over thousands to millions of years. Slow cooling → large crystals.
- Basalt formed from lava on the surface, cooling in days to weeks. Fast cooling → small crystals.
Key idea: the cooling rate of molten rock is recorded in its crystal size. Slow = big; fast = small; instant (obsidian) = none.
3. How each type forms
Igneous: magma from inside Earth pushes upward. If it cools underground → intrusive (granite). If it erupts as lava and cools on the surface → extrusive (basalt).
Sedimentary formation has three stages:
- Weathering and erosion — wind, water and ice break up rock into fragments.
- Transport and deposition — fragments settle in layers (often in lakes, rivers, or oceans).
- Compaction and cementation — weight of overlying layers squeezes the sediment; minerals in water glue the grains together.
Metamorphic: existing rock is buried, heated and pressed (often near plate boundaries) until its minerals reorganise without fully melting. Very high heat would melt it, producing igneous rock instead.
Worked example 2 Why fossils are almost always in sedimentary rock
A fossil forms when plant or animal remains are buried in sediment and slowly replaced by minerals.
- Igneous rock formed from molten material — any remains would burn up at over °C.
- Metamorphic rock was heated and pressed strongly — fossils are destroyed.
- Sedimentary rock forms at low temperatures — remains can survive and mineralise.
So to find dinosaur bones, geologists look at sedimentary rock layers of the right age.
4. The rock cycle
The rock cycle is a loop: any rock type can become any other type given enough time.
Worked example 3 Tracing a rock cycle journey
A grain of quartz in a granite outcrop is exposed by erosion.
- Wind and rain weather the granite; the quartz grain washes into a river.
- The river carries it to the sea, where it settles on the seabed.
- Over millions of years, layers bury it; pressure cements it into sandstone.
- Tectonic forces push the sandstone deep underground; it is heated and compressed into quartzite (a metamorphic rock).
- Deeper still, it may melt back to magma, which cools as new igneous rock.
Key idea: the cycle has no beginning or end. Any one grain has been through many rounds.
5. Timescales and usefulness
- Sedimentary layers accumulate at roughly mm per year in ocean sediments — a -metre cliff of sandstone represents about years.
- Metamorphic transformations can take millions of years.
- Uplift of a mountain range from sea level to m can take - million years.
Rock properties decide usefulness:
- Granite is hard and weather-resistant → benchtops, kerbstones.
- Limestone is soft and fizzy in acid → cement and lime mortar.
- Coal is an energy-rich sedimentary rock formed from ancient plants.
- Slate splits cleanly → roofing tiles.
- Marble is soft enough to carve but dense enough to polish → sculpture and tiles.
Practice: Year 7
Tier 1: recall and identify
- Name the three rock types.
- How does an igneous rock form?
- How does a sedimentary rock form?
- How does a metamorphic rock form?
- Give one example of each rock type.
- What does crystal size in an igneous rock tell you about cooling?
- In which rock type would you most likely find fossils? Why?
- Which rock type tends to be layered from settling, and which is layered from pressure?
- Name a rock made from compressed plant material.
- State one reason granite is used as a kitchen benchtop.
Tier 2: explain and reason
- Explain why basalt has smaller crystals than granite.
- Why is coal a sedimentary rock and not an igneous rock?
- A rock fizzes when a drop of vinegar is added. What chemical is it likely to contain, and which two rock types are possible?
- A student claims “the rock cycle is a one-way process from igneous to sedimentary to metamorphic.” Correct the claim.
- Marine fossils are found in rock at the top of the Himalayas. Explain using the rock cycle.
- Why are metamorphic rocks almost never formed on Earth’s surface?
Tier 3: apply to a novel context
- A builder needs a stone that splits into thin flat slabs for roofing. Which rock is most suitable and why?
- You find a grey rock with embedded visible crystals and no layering. Propose a classification and a likely formation story.
- Describe the full journey a grain of sand could take through the rock cycle, listing every rock type it becomes.
- A m thick sedimentary layer deposited at mm per year represents how many years? What might have caused that layer to form?
Challenge
Harder reasoning
- Zircon crystals from Western Australia are over billion years old — older than any known rock. Explain how a crystal can be older than the rock it is found in, using rock cycle reasoning.
- If Earth had no plate tectonics, which part of the rock cycle would still operate and which would stop? Justify.
- A diamond is formed deep in Earth’s mantle under extreme pressure. A piece of charcoal (carbon) is chemically the same element. Explain using the rock cycle concepts why their form and value differ so greatly.
- A sedimentary rock contains flat fish fossils in its lower layer and desert dune patterns in its upper layer, with no mixing between. What does this tell you about the environment at this site, and in what order did events occur?
Answers
Answer key
Attempt the practice first. When you're ready to check, expand the answers below.
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Year 7 answers
Tier 1: recall and identify
- Igneous, sedimentary, metamorphic.
- From the cooling and solidification of molten rock (magma or lava).
- Sediment is deposited in layers, compacted and cemented together into rock.
- An existing rock is heated and/or pressed without melting, changing its minerals and texture.
- Igneous: granite or basalt. Sedimentary: sandstone or limestone. Metamorphic: slate or marble.
- Large crystals mean slow cooling (underground); small crystals mean rapid cooling (on the surface).
- Sedimentary rock. It forms at low temperatures that do not destroy plant or animal remains, and sediment often buries them before decay is complete.
- Sedimentary layers form from sediment settling (bedding). Metamorphic layers form from pressure alignment (foliation).
- Coal.
- Granite is hard, weather-resistant and takes a polish — durable and attractive for benchtops.
Tier 2: explain and reason
- Basalt forms from lava on the surface that cools in days or weeks — crystals have no time to grow large. Granite forms underground where magma cools over thousands of years, allowing large interlocking crystals.
- Coal forms from plant material compressed in layers over millions of years — a sedimentary process. It is not formed from molten rock.
- Likely contains calcium carbonate. Could be limestone (sedimentary) or marble (metamorphic version of limestone).
- The rock cycle is a loop, not a one-way path. Any rock type can transform into any other given the right conditions — including melting sedimentary back to igneous, or weathering igneous to sediment.
- The rock was once a marine sedimentary layer. Tectonic forces pushed the layer upwards as plates collided, eventually raising it to mountain heights. The fossil is evidence that the rock formed underwater.
- Metamorphism requires high temperature and/or pressure. These conditions exist at depth (buried by kilometres of rock) or near plate boundaries — not at the surface.
Tier 3: apply to a novel context
- Slate. It is a metamorphic rock that splits cleanly along flat planes (foliation), making it ideal for waterproof roofing tiles.
- Likely igneous (granite or similar) — visible interlocking crystals with no layering suggest slow cooling underground.
- Example: granite weathers → quartz grains → deposited in river → compacted as sandstone (sedimentary) → buried and metamorphosed to quartzite → deeper still, melted to magma → cools as granite again.
- m = mm. At mm/year: years. The sediment may have been deposited in a stable lake or ocean environment, possibly during a long period of steady erosion upstream.
Challenge
- Zircon is a very hard mineral that can survive multiple trips through the rock cycle. A crystal can be released when its parent rock weathers, then redeposited in a new rock. The crystal’s age is measured by radioactive decay and reflects when it first crystallised, not when its current rock formed.
- Without plate tectonics: weathering, erosion, deposition and compaction (sedimentary formation) still work because they depend on water, wind and gravity. Metamorphism largely stops — there would be no mountain-building or subduction, so pressure-heating of deep rock is minimal. Igneous activity would also be reduced; most volcanism is plate-boundary related.
- Both are carbon. Charcoal is light, crumbly carbon formed at the surface from burnt wood. Diamond forms km deep under immense pressure and heat, arranging carbon atoms into a hard, transparent crystal. The rock-cycle context — the depth, pressure and slow growth — turns the same element into vastly different materials.
- Fish fossils in the lower layer indicate an aquatic environment (lake or sea) at that time. Desert dune patterns above indicate later dry, windy conditions. The absence of mixing suggests a rapid environmental change. Order: water environment first, then drying, then desert conditions, all preserved in the sequence of layers.
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