The rock cycle is the slow, continuous process by which rocks form, break down, and reform over millions of years. All rocks on Earth are made of minerals and belong to one of three types — igneous, sedimentary, or metamorphic — depending on how they formed. Understanding the cycle shows how materials are recycled in the Earth's crust and mantle.
The three types of rock
1. Igneous rocks
Igneous rocks form when magma (molten rock underground) or lava (molten rock at the surface) cools and solidifies.
The key factor determining the texture of an igneous rock is the rate of cooling:
| Cooling rate | Crystal size | Where it solidifies | Example |
|---|---|---|---|
| Slow (underground) | Large crystals | In the crust (intrusive) | Granite |
| Fast (at the surface) | Small crystals | Lava flows, volcanic eruptions (extrusive) | Basalt |
| Very fast (in air/water) | Glassy, no crystals | Volcanic eruptions | Obsidian |
Granite is a pale, coarse-grained intrusive igneous rock. You can see individual crystals of quartz, feldspar, and mica with the naked eye because the magma cooled slowly deep underground over thousands to millions of years.
Basalt is dark, fine-grained extrusive igneous rock. The crystals are too small to see without a microscope because the lava cooled rapidly on the Earth's surface (minutes to years).
The rule: the slower the cooling, the larger the crystals.
2. Sedimentary rocks
Sedimentary rocks form at or near the Earth's surface from layers of sediment (particles of rock, mineral, shell, or organic matter) that build up over time.
The process of sedimentary rock formation:
- Weathering: Existing rocks are broken down into fragments (sediment) by physical processes (freeze-thaw, wind, waves) or chemical processes (acid rain dissolving limestone).
- Erosion and transport: Water, wind, and ice carry sediment from its source towards seas, lakes, or rivers.
- Deposition: Sediment settles in layers at the bottom of a body of water. Heavier particles settle first; lighter ones settle on top.
- Compaction and cementation: Over millions of years, the weight of overlying layers compresses the sediment. Minerals precipitate from water in the pore spaces, gluing the particles together — a process called lithification.
Properties of sedimentary rocks:
- Often contain fossils (remains of organisms preserved in the sediment)
- Formed in layers (strata) — visible in cliffs and road cuttings
- Usually porous (contain pore spaces that can hold water or oil)
- Generally softer than igneous or metamorphic rocks
Examples: Limestone (formed from shells and coral), sandstone (formed from sand grains), mudstone/shale (formed from fine clay particles), chalk (formed from microscopic shells of marine plankton).
3. Metamorphic rocks
Metamorphic rocks form when existing rocks (igneous, sedimentary, or even other metamorphic rocks) are subjected to intense heat and/or pressure inside the Earth's crust. The original minerals recrystallise into new minerals without the rock melting (if it melted it would become magma and eventually an igneous rock).
Examples:
| Original rock | + Heat and pressure → | Metamorphic rock |
|---|---|---|
| Limestone | → | Marble |
| Shale/mudstone | → | Slate |
| Granite | → | Gneiss |
| Sandstone | → | Quartzite |
Properties of metamorphic rocks:
- Very hard and dense because they have been recrystallised under pressure
- May show foliation (aligned layers of minerals) in rocks such as slate
- Do not usually contain fossils (the original fossils are destroyed by the heat and pressure)
Slate splits into flat sheets because the minerals realign perpendicular to the direction of pressure, giving it a clear cleavage. This makes it useful as roofing material and for snooker tables.
The rock cycle diagram: how rocks change into each other
The rock cycle can be summarised as a set of transformations:
| Starting rock | Process | Resulting rock |
|---|---|---|
| Any rock | Melting (extreme heat in mantle/crust) | Magma |
| Magma | Cooling and solidifying | Igneous rock |
| Any rock | Weathering, erosion, deposition, compaction, cementation | Sedimentary rock |
| Any rock | Heat and pressure (without melting) | Metamorphic rock |
Note: The cycle can be entered at any point and rocks can bypass some stages. For example, an igneous rock can be metamorphosed directly without becoming sedimentary first; a metamorphic rock can be eroded and become sedimentary.
How the rock cycle fits the KS3 national curriculum
The Department for Education's KS3 science programme of study requires pupils to describe "the rock cycle and the formation of igneous, sedimentary, and metamorphic rocks." BBC Bitesize KS3 Chemistry covers the rock cycle with diagrams, video clips, and questions on rock formation for Year 8 pupils. Rocks and the rock cycle also connect to geography topics such as plate tectonics and landscape formation.
Comparing the three rock types: a summary table
| Property | Igneous | Sedimentary | Metamorphic |
|---|---|---|---|
| How formed | Cooling magma/lava | Compaction of sediment | Heat and/or pressure |
| Crystal size | Large (slow cool) or small (fast cool) | No crystals — grains | Recrystallised, often fine |
| Contains fossils? | No | Often yes | Rarely (destroyed) |
| Layers visible? | No | Yes (strata) | Sometimes (foliation) |
| Examples | Granite, basalt | Limestone, sandstone | Marble, slate |
| Hardness | Hard | Generally softer | Very hard |
Common mistakes
Mistake 1 — Saying granite has large crystals because it erupted from a volcano. Granite never reaches the surface as lava. It cooled slowly deep underground (intrusive). Extrusive (volcanic) igneous rocks like basalt have small crystals.
Mistake 2 — Saying metamorphic rocks have melted. Metamorphic rocks form below melting point. If they melted, they would become magma and eventually igneous rock. The distinction is important in exam questions.
Mistake 3 — Forgetting that any rock type can become any other type. A sedimentary rock can be subducted, melted, and become igneous. An igneous rock can be weathered and become sedimentary. Metamorphic rock can be melted and become igneous. The cycle is flexible, not a one-way sequence.
Mistake 4 — Confusing weathering with erosion. Weathering is the breakdown of rock in place (no movement). Erosion is the removal and transport of that material away from the site. Both are needed to produce and move sediment.
Frequently asked questions
What is the difference between magma and lava?
Magma is molten rock that remains underground — inside the mantle or within magma chambers in the crust. Lava is magma that has erupted onto the Earth's surface through a volcano or a fissure. The chemical composition may be identical, but the term changes depending on location. When magma cools underground it forms intrusive (plutonic) igneous rocks; when lava cools on the surface it forms extrusive (volcanic) igneous rocks.
Why do sedimentary rocks often contain fossils but igneous and metamorphic rocks do not?
Fossils form when an organism is buried in sediment quickly after death, preventing decay. As the sediment lithifies into rock over millions of years, the remains (or their mineral replacement) are preserved. Igneous rocks form from molten magma at temperatures that would destroy any organic material. Metamorphic rocks are subjected to intense heat and pressure that crush and destroy any fossil structures. So fossils are almost exclusively a feature of sedimentary rocks and are rarely (if ever) found in the other types.
How long does the rock cycle take?
The rock cycle operates on geological timescales — typically millions to hundreds of millions of years. For example, sediment deposited on the ocean floor may take tens of millions of years to be compressed into rock, subducted into the mantle, and eventually returned to the surface as igneous or metamorphic rock. Some rock cycles are faster: volcanic rocks at mid-ocean ridges can form and be recycled within tens of millions of years, while continental granite can persist for billions of years. By comparison, a human lifetime is a tiny fraction of even the fastest rock cycle processes.
What is the difference between weathering and erosion?
Weathering is the in-place breakdown of rock at or near the Earth's surface. It can be physical (freeze-thaw action, where water expands as it freezes and fractures the rock), chemical (acid rain dissolving calcium carbonate in limestone), or biological (plant roots splitting rocks, lichens releasing acids). Erosion is the removal of the weathered material from its original location by agents such as running water, wind, waves, and glaciers, and its transport elsewhere. Weathering loosens the material; erosion moves it.
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