1Igneous rock
Igneous rock forms when molten rock cools and crystals grow. Magma cools slowly underground; lava cools quickly at the surface.
igneousmagmalava
Lesson 26.2 notes
The rock cycle
Igneous, sedimentary and metamorphic rocks, and how each turns into the others.
2Crystal size
Slow cooling gives crystals time to grow large. Melted salol on a warm slide makes big crystals; on a cold slide, lots of small ones.
crystal
3Weathering
Weathering breaks rock down. Water in a crack freezes and expands, forcing the crack wider until pieces break off.
weathering
4Erosion and deposition
Rivers carry broken rock away: erosion. The pieces are worn smooth, then dropped in layers on the sea bed: deposition.
erosiondeposition
5Sedimentary rock
Layers of sediment are squashed by the layers above and cemented together, making sedimentary rock such as sandstone and limestone.
sedimentary
6Metamorphic rock
Deep underground, heat and pressure change rocks without melting them, often lining up their crystals in bands. Limestone becomes marble and mudstone becomes slate: metamorphic rocks.
metamorphic
7The rock cycle
Over millions of years, rocks are weathered into sediment, buried, changed, melted and cooled again. This is the rock cycle.
rock cycle
Explore: Rock cycle journey
Start with a grain of rock and choose what happens next: weathering, burial, heat and pressure, melting or cooling. See how long each step takes.
Common mistake: Metamorphic rock forms without melting
Metamorphic rocks are changed by heat and pressure while they stay solid. If a rock melts, it becomes magma, and when that cools it makes igneous rock.
Remember
- Igneous rock forms when magma or lava cools. Slow cooling underground gives large crystals; fast cooling gives small ones.
- Sedimentary rock forms from layers of sediment that are squashed and cemented together. It can contain fossils.
- Metamorphic rock forms when rock is changed by heat and pressure without melting. The rock cycle links all three.
Going further at GCSE