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?