Magma forms in the hot, soft layer of rock below the lithosphere.

First a little extra heat is added (could be from radioactivity or decrease in pressure).

The magma then pushes its way up and it cools as it travels.

If it hardens before it gets to the surface, it forms intrusive igneous rock.

An example of intrusive igneous rock is Granite (large crystals).

But other magma which is more runny, makes its way above the surface and spews out of a volcano as lava. It is often mixed with gases and rock.

After this magma has penetrated the surface, it is cooled and hardens rapidly.

This forms extrusive igneous rock, an example of this is Basalt (small crystals).

If a sample of igneous rock has large crystals it means it has cooled slowly. Small crystals form as a result of rapid cooling.

Shapes of intrusive igneous rock - name depends on shape and direction.

e.g. a dike intrudes across rock layers.
a sill intrudes between rock layers.