Central volcanoes are the simplest kind of volcano. A long pipe reaches up from the magma chamber deep below the surface. Here the rock is molten and under great pressure. From time to time the pressure builds up so much that the molten magma bursts up to the surface through its pipe (also called a vent). The magma is liquid and full of gas. When it reaches the air, the gas expands, blowing the liquid apart. The tiny bits of liquid cool and form ash. This is what falls from a volcano during the first part of the eruption.

Later, as the pressure in the magma chamber gets less, the magma rises to the surface under less pressure and the liquid does not get blown apart. Instead it flows from the volcano vent and is called lava. Together, ash and lava make the volcano cone higher.