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.