The Rock Cycle
- Earth & Space
- Ages 11–14 · Grades 6–8
What you’ll find out
- Rocks are igneous, sedimentary or metamorphic according to how they formed.
- Crystal size, layers and fossils reveal the conditions a rock formed in.
- In the rock cycle, any rock type can become any other, driven by the Sun and Earth’s interior.
No sign-up and no real names. In the lesson you check your understanding as you read, and practise until it sticks.
Three families of rock
Geologists sort rocks into three families based on how they formed. Igneous rocks form when molten rock, called magma underground and lava at the surface, cools and hardens; granite and basalt are examples. Sedimentary rocks form when layers of sediment, such as sand, mud and shells, are buried, squeezed and cemented together; sandstone, limestone and chalk are sedimentary. Metamorphic rocks form when heat and pressure deep underground transform existing rocks without melting them; marble and slate are examples.
Rocks that change
Rocks are not permanent; over millions of years they change from one type into another in a never-ending rock cycle. Weathering and erosion break down rocks at the surface, and rivers carry the fragments away and deposit them as layers of sediment, which harden into sedimentary rock. Buried deeply, rock is squeezed and heated into metamorphic rock. If it melts completely, the magma may later cool into igneous rock, which can be pushed back to the surface and weathered again.
Curriculum links: NGSS MS-ESS2-1 · England KS3: Earth and atmosphere · Cambridge Lower Secondary Stages 7–9: Earth and space · NCERT Social Science, Classes 6–8