Rocks
There are three main types of rock: igneous, sedimentary and metamorphic. They are formed in different ways and have different properties.
Igneous rocks
Igneous rocks are formed from molten rock that has cooled and solidified.
The inside of the Earth is very hot - hot enough to melt rocks. Molten (liquid) rock is called magma. When the magma cools enough, it solidifies and igneous rock forms.
What are igneous rocks like?
Igneous rocks contain randomly arranged interlocking crystals. The size of the crystals depends on how quickly the molten magma solidified:
- magma that cools slowly will form an igneous rock with large crystals
- lava that cools quickly will form an igneous rock with small crystals
This means that we get two main types of igneous rock, extrusive and intrusive, as shown in the table:
| Extrusive | Intrusive | |
|---|---|---|
| Where the magma cooled | On the surface | Underground |
| How fast the magma cooled | Quickly | Slowly |
| Size of crystals | Small | Large |
| Examples | Obsidian and basalt | Granite and gabbro |
Extrusive igneous rocks form from magma that erupted onto the surface as lava, where it cooled quickly. On the other hand, intrusive igneous rocks form from magma that cooled slowly, deep underground.
Igneous rocks do not contain any fossils. This is because any fossils in the original rock will have melted when the rock melted to form magma.
Modelling the cooling of magma
You may have done an experiment at school with a substance called salol. If molten salol cools slowly, you get big crystals. If it cools quickly, you get small crystals.