Igneous and Metamorphic Rocks

The three main types of rock are: sedimentary, igneous and metamorphic.

We will cover igneous and metamorphic rocks

Igneous rocks

Igneous rock is formed from magma or lava that has been cooled down and solidified.

  • Magma is the name for molten rock, which is a liquid.

Igneous rocks are made up of interlocking crystals that are randomly arranged. They are called non porous because water can’t get into the rock.

Some examples of igneous rocks are granite, basalt and obsidian.

Forming Igneous Rock

Igneous rocks are formed from volcanic activity Molten rock can come in two forms:

  • Magma is molten rock that is underground
  • Lava is molten rock that has broken through the Earth’s surface

Igneous rocks can be formed from either magma or lava and can form either underground or over ground.

  • Lava – Igneous rock can form outside the volcano when lava cools. These are called extrusive igneous rocks. Lava that falls in water or on ground outside the volcano cools quickly to form igneous rock. As the lava cools quickly, there is not much time for crystals to form, which is why the crystals are quite small.

Basalt is an example of of an igneous rock, which forms from lava that cools quickly. This means it has very small crystals. It is strong a material used in construction (e.g. building blocks or groundwork).

  • Magma – Igneous rock can also form underground when magma cools – These are called intrusive igneous rocks. As magma cools down slowly, it takes longer for crystals to form, which results in larger crystals forming

Magma is molten rock beneath the surface of the earth. When magma cools and solidifies at or near the surface, it creates igneous rock. Magma cools underground slowly, which means it takes a long time for crystals to form, this means larger crystals form.


Many factors cause the igneous rocks to form in different ways. We can categorise igneous rocks into extrusive and intrusive.

Intrusive Extrusive
How fast the magma cools down slow Fast
Size of crystals formed big small

Granite is an example of an igneous rock that contain large crystals. It is used for construction and also for a decorative stone, as its crystals give it a sparkling effect.

Metamorphic rocks

Metamorphic rocks are formed from other rocks that change due to an increase in heat/pressure.

Some examples of metamorphic rocks are gneiss, schist and slate.

When sedimentary or igneous rocks are subjected to extreme heat and pressure, their mineral structures transform, resulting in metamorphic rock.

An example of this is when heat and pressure is applied to limestone (the sedimentary rock), marble is made.