Igneous rocks 

The word igneous comes from the Latin word 'ignis' which means 'of fire'. Igneous rocks can be formed in many parts of the world but they are mostly found near volcanoes.

When hot lava or magma cools down it becomes an igneous rock. If the magma cools down and solidifies inside the Earth it is called an intrusive igneous rock; this happens slowly and forms large crystals. An example is granite, you will often see granite statues at museums.

When lava erupts from a volcano and solidifies on the surface of the Earth, it is called an extrusive igneous rock. This happens quickly and forms tiny crystals. Basalt is an extrusive igneous rock that is used to make roads and pavements.

Sedimentary rocks 

Sedimentary rocks are only formed on the crust of the Earth, in water, or on land. They are formed by years and years of sediment compacting together, layer after layer until a solid rock is formed. These layers are called strata. Sedimentary rocks can take millions of years to form.

Examples of sediments are sand, minerals, and animal or plant material. Rivers often carry sediments with them which settle at the bottom of the water and become hard.

Some sedimentary rocks are compacted loosely and others are cemented like glue by minerals and chemicals. Unlike igneous rocks which have crystals, sedimentary rocks are grainy. Sandstone is a common sedimentary rock, it is made when sand and water get pressed together.

Metamorphic rocks 

Metamorphic rocks are igneous or sedimentary rocks that have been changed by heat or pressure. When they change or metamorphose they are transformed into a harder, more compact rock. Metamorphic rocks are normally found inside the Earth’s crust where there is a lot of heat and pressure. Metamorphic rocks can be foliated (stripey) or non-foliated (non stripey). Marble is an example of a metamorphic rock. 

Rock Cycle

Rocks are constantly changing from one type of rock into another; they do this in cycles over millions of years. Now that we know the three types of rocks, let’s have a look at the rock cycle.

A volcano erupts and carries magma to the Earth’s surface. When it cools down it forms an igneous rock.

The rocks on the surface of the Earth are broken down into small pieces of sediment by weathering (wind, rain, snow) or by a river. These sediments build up layer after layer over many years and become hard. The result is a sedimentary rock. The sedimentary rocks will eventually get covered over by other rocks and end up in the Earth’s crust. When they face high temperatures and pressure they will metamorphose into a metamorphic rock. If hot magma touches them they will melt and when they cool down they will become an igneous rock. The cycle will keep repeating but it can follow any order. 

Remember an igneous rock can also transform into a metamorphic rock.