The molten rock that supplies a volcano is called magma. The place where the magma is stored is called a magma chamber.

The word 'magma' comes from the Greek word for 'dough', which is another slightly sticky mixture. Magma is full of trapped gas bubbles that expand as the magma rises and becomes under less pressure.

A volcano stores its magma in vast, underground magma chambers. Each chamber may be tens of kilometres across, and more than a kilometre deep.

The magma has to find a way to the surface before it can erupt and turn into lava, ash and gas. If the surface rocks are thick and tough, the molten rock never reaches the surface. But where the rocks are weak and cracked, perhaps due to earthquakes, then the magma will push to the surface, causing a volcanic eruption.