What is a volcano? For an answer, you need to know something about the earth itself.Below the surface of the earth is a rocky crust. Within the crust are pockets of boiling hot gas and liquid. This churning matter is called magma.Magma has great force and pushes on the rock around it. If there is a weak spot above, the boiling magma the boiling magma blasts through. The magma works its way to the surface and escapes. It bursts into the air. As it falls, it makes a shape like a cone around the opening. A volcano is born!From time to time, a volcano may throw up steam and other gases, fine dust, pieces of rock and lava.Lava can destroy anything in its path. However, there are two kinds of lava. One kind moves faster than a man can run. But the other kind moves slowly, like tar - and people have time to escape from that kind of lava. Sometimes a volcano gets plugged up. This happens when magma hardens and fills in the volcano's openings. Then gases within the crust build and build into a savage push. POW! Again the volcano "pops its cork." It showers its waste like a giant fountain.