The oldest known rocks on Earth are 3900 million years old – they are the Acasta gneiss rocks from Canada.
There are three main kinds of rock: igneous rock, sedimentary rock and metamorphic rock.
Igneous rocks (igneous means ‘fiery’) are made when hot molten magma or lava cools and solidifies.
Volcanic rocks, such as basalt, are igneous rocks that form from lava that has erupted from volcanoes. Metamorphic rocks are rocks that have changed over time, such as limestone which is made into marble because of the heat generated by magma.
Sedimentary rocks are rocks that are made from the slow hardening of sediments into layers, or strata.
Some sedimentary rocks, such as sandstone, are made from sand and silt. Other rocks are broken down into these materials by weathering and erosion.
Most sediments form on the sea-bed. Sand is washed down onto the sea-bed by rivers.
Limestone and chalk are sedimentary rocks made mainly from the remains of sea creatures.