- Magma is molten igneous rock.
- 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.