Rocks and Minerals

Rocks are the solid materials that make up the Earth's crust, the layer we live on. They might all look similar at first, but rocks belong to three main families, each formed in a completely different way: igneous, sedimentary, and metamorphic.

Sedimentary rocks form very slowly over millions of years. Tiny pieces of sand, mud, and the shells of dead sea creatures sink to the bottom of seas and lakes. Layer upon layer builds up and gets squeezed together until it hardens into rock. Sandstone, limestone and chalk are all sedimentary.

Metamorphic rocks start out as igneous or sedimentary rock, but are then changed by enormous heat and pressure deep underground. The word 'metamorphic' means 'changed in form'. Marble was once limestone, and slate was once soft mudstone, before heat and pressure transformed them.

Rocks are made of even smaller building blocks called minerals. A mineral is a natural solid with a particular chemical make-up, like quartz, mica or feldspar. Most rocks are a mixture of several different minerals locked together.

Scientists identify minerals by testing their properties: their colour, how shiny they are (called lustre), how hard they are, and how they break. A common mistake is to judge a mineral only by its colour, but colour alone can be misleading — hardness and lustre are far more reliable clues.

Rocks are never truly destroyed — they are recycled in a never-ending journey called the rock cycle. Over millions of years, rocks are worn away, buried, heated, melted and cooled again, turning from one type into another. The mountain rock of today might become beach sand, then sandstone, far in the future.