All rocks belong to one of three main groups: sedimentary, igneous and metamorphic because of how they are formed. The rocks in each group are very varied but share some common features. Here, you will find the main features of the rocks in the three groups, how they are formed and some examples.
1. Sedimentary Rocks
Rocks are gradually worn down by weathering and the particles that break off are transported by water and end up in lakes, river estuaries and the sea or blown by wind to form sand dunes. Finding rocks made from wind blown particles is a lot less common than finding sedimentary rocks made in water.
Particles of weathered rock that is transported by rivers and streams settle to the bottom of lakes or the sea. Over millions of years, they are buried by more sediments. They are compressed by the weight of the sediments above and become cemented together forming a sedimentary rock. Sometimes, the remains of dead plants and animals are trapped in the sediments and become part of the rock. When you find these, they are called fossils.
The main features of sedimentary rocks:
- They contain layers. Sometimes you have to look very carefully to recognise the layers which can be many tens of metres thick.
- They are the only rock type to contain good fossils.
- When you rub or scratch them over a piece of white card using a metal object, you can usually see lots of the particles fall onto the card. This can happen with the other two rock groups but you only ever get a few pieces.
Examples of sedimentary rocks:
2. Igneous Rocks
Igneous is the Latin word for ‘fire‘ so you can probably guess these rocks are linked with volcanoes. Deep in the Earth it is very hot, hot enough to melt rocks! These molten rocks are squeezed upwards through cracks in the Earth where they cool down and solidify. Some solidify inside the earth’s crust, they are called ‘intrusive igneous rocks‘. Some come out of volcanoes, they are called ‘extrusive igneous rocks‘.
The main features of igneous rocks:
- They consist of a jumble of interlocking crystals that do not usually show any layers. Igneous rocks with big crystals cooled slowly and vice-versa.
- They are very hard. If you rub them with a metal object over a piece of white card, unless the rock is damaged, you will not see any particles fall onto the card.
- They NEVER contain fossils (can you think why not?).
Examples of igneous rocks:
3. Metamorphic Rocks
Any rocks of the Earth’s crust can be subjected to great heat, extremely high pressure or a combination of both. The heat and pressure changes them, these changed rocks are called metamorphic rocks. The most important things to remember are:
- They have not been melted.
- They can be made from sedimentary, igneous or even other metamorphic rocks.
The main features of metamorphic rocks:
Oh dear, this is difficult! Some metamorphic rocks are like very hard sedimentary rocks and others are like igneous rocks, they are the hardest of the rock groups to identify. If you see something with layers of crystals then it is almost certainly metamorphic. Some can even contain fossils, but that is very rare indeed and the fossils are very distorted.
Examples of metamorphic rocks:
- Gneiss (pronounced ‘nice’)