Formed when molten rock (magma) from the mantlecools down and hardens. The rock forms crystals as it cools. Igneous rocks are usually hard, e.g. granite.
Sedimentary rocks
Formed when layers of sediment are compacted together until they become solid rock. There are two main types in the UK: Carboniferous limestone and chalk are formed from tiny shells and skeletons of dead sea creatures. Limestone is quite hard, but chalk is a much softer rock. Clays and shales are made from mud and clay minerals. They are very soft.
Metamorphic rocks
Formed when other rocks (igneous, sedimentary or older metamorphic rocks) are changed by heat and pressure. The new rocks become harder and more compact, e.g. shale becomes slate and, with further pressure and heat,slate becomes schist.
