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:

ExtrusiveIntrusive
Where the magma cooledOn the surfaceUnderground
How fast the magma cooledQuicklySlowly
Size of crystalsSmallLarge
ExamplesObsidian and basaltGranite 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.

Obsidian
Obsidian has tiny crystals
Granite
Granite has large crystals

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.