Different rocks are formed in different ways:
- Igneous rocks are formed from cooled molten lava. They are made from crystals that have grown into each so are hard. ('Ignis' is the Latin word for fire.)
- Sedimentary rocks are formed when other rocks are worn away and their remains are deposited as sediments. They often have clear layers which build up over time and are soft because their grains can be rubbed off easily. They often contain fossils. ('Sedo' is the Latin word for settle down.)
Examples: sandstone, chalk and limestone
- Metamorphic rocks are rocks which have been changed by extreme heat or pressure over long periods of time. They are often streaked and are usually hard. ('Meta' is the Greek for change.)
Examples: marble and slate
Some rocks, such as sandstone or chalk, let water soak through them. They are called permeable rocks.
Other rocks, such as marble and slate, do not let water soak through them. They are called impermeable rocks.