Rocks, soils and fossils · Year 9
Rocks are made of minerals. They form in three ways, so there are three types: igneous, sedimentary and metamorphic. Over millions of years rocks are broken down, moved and made into new rocks: the rock cycle. Soil is made from tiny pieces of rock mixed with rotted living things, and fossils are the remains of living things kept in rock.
Key facts
- Igneous rocks (granite, basalt) form when molten rock cools. They are made of interlocking crystals: slow cooling underground gives large crystals, fast cooling at the surface gives small ones.
- Sedimentary rocks (sandstone, limestone, chalk) form from layers of sediment that are squashed and cemented together. They are often porous and may contain fossils.
- Metamorphic rocks (marble, slate) form when rocks are changed by heat and pressure without melting.
- Weathering breaks rock where it is (freeze–thaw, acid rain, roots); erosion carries the pieces away; deposition drops them as layers of sediment.
- Soil contains small pieces of rock, humus (rotted plant and animal material), air, water and living things. Sandy soil drains quickly; clay soil holds water; loam is a good mixture.
- A fossil forms when a living thing is buried quickly in sediment and its hard parts are slowly replaced by minerals. In layers that have not been disturbed, the lowest layers are the oldest.