Deep within the Earth it is so hot that some rocks slowly melt and become a thick flowing substance called magma. Because it is lighter than the solid rock around it, magma rises and collects in magma chambers. Pressure builds up until the gases escape violently and explode. Eventually some of the magma pushes through vents and cracks in the Earth's surface causing a volcanic eruption to occur!
Volcano Eruption National Geographic
How Volcanoes Erupt Ignite Learning