Join our rock school!
Discover the world beneath your feet to become a geologist
In this Nature Club session for primary school age children we find out all about the three types of rock: sedimentary, metamorphic and igneous. We also try some experiments to take a closer look at rocks, find out how they were made and test their key properties.
Video: Join our education manager Vicky as she talks about the different kinds of rock and takes you through some simple experiments. Download our rock experiments worksheet below to carry out the same tests on the rocks you find in your garden or school grounds or when out exploring. You could even play a game of 'What Rock's in My Sock?!' We also have a 'rock names' activity sheet.
What are rocks?
Rocks are made of grains that fit together. Each grain in the rock is made from a mineral, which is a chemical compound. The grains in a rock can be different colours, shapes and sizes.
Rocks with rounded grains are more likely to absorb water than rocks with interlocking grains. This is because the water can get into the gaps between the grains. Imagine you have a rock made up grains shaped like little round marbles, and another made up grains fitted together like pieces of Lego. Which one would let in water? Rocks that absorb water are called porous.
Formation of sedimentary rocks
A river carries, or transports, pieces of broken rock as it flows along. When the river reaches a lake or the sea, its load of transported rocks settles to the bottom. We say that the rocks are deposited. The deposited rocks build up in layers, called sediments. This process is called sedimentation.
The weight of the sediments on top squashes the sediments at the bottom. This is called compaction. The water is squeezed out from between the pieces of rock and crystals of different salts form. The crystals form a sort of glue that sticks or cements the pieces of rock together. This process is called cementation.
These processes eventually make a type of rock called sedimentary rock. It may take millions of years for sedimentary rocks to form.
Formation of igneous rocks
The inside of the Earth is very hot - hot enough to melt rocks. Molten (liquid) rock forms when rocks melt. The molten rock is called magma. When the magma cools and solidifies, a type of rock called igneous rock forms.
Igneous rocks contain randomly arranged interlocking crystals. The size of the crystals depends on how quickly the molten magma solidified. The more slowly the magma cools, the bigger the crystals.
Formation of metamorphic rocks
These are formed from other rocks that are changed because of heat or pressure. Earth movements can cause rocks to be deeply buried or squeezed. As a result, the rocks are heated and put under great pressure. They do not melt, but the minerals they contain are changed chemically, forming metamorphic rocks. Sometimes, metamorphic rocks are formed when rocks are close to some molten magma, and so get heated up.