Minerals and rocks
The solid part of the Earth is made of rocks. Every rock is made of one or more minerals. A mineral is a natural solid with a fixed chemical make-up, such as quartz or feldspar. Rocks are grouped into three types by how they form. Igneous rock forms when hot melted rock (magma) cools and hardens, like granite. Sedimentary rock forms when small pieces of rock and shell are pressed together over a long time, like sandstone. Metamorphic rock forms when existing rock is changed by heat and pressure, like marble. Each type tells a story about where and how it was made, which is why geologists study rocks so closely.
Example
Limestone is a sedimentary rock. When it is heated and squeezed deep underground, it changes into marble, a metamorphic rock.
Layers and their densities
The materials of the Earth are not spread evenly. Denser materials sank to the centre long ago, while lighter materials stayed near the surface. This is why the crust is made of lighter rock, while the core is made of heavy iron and nickel. Density tells us how much mass is packed into a certain volume, using the idea density = mass ÷ volume, measured in g cm⁻³.
- Crust: light rocks such as granite and basalt.
- Mantle: rock rich in silicon, oxygen and magnesium.
- Core: mostly iron and nickel, the densest part.
Remember
- Rocks are made of minerals.
- Three rock types: igneous, sedimentary, metamorphic.
- Density increases from the crust to the core.