Our earth is hot inside. Very hot rock, called "magma," is deep underground. Magma is a runny liquid, kind of like melted ice cream, but it is so hot that it smokes and steams. It is hot enough to melt steel. Now, that's hot!

Sometimes, the magma in the earth builds up great pressure, and starts pushing upward. The pressure pushes the magma through a weak spot in the ground, and sometimes it rises up through "pipes," which are like tunnels. It can come from deep inside the earth all the way to the surface of the ground. It may push up the ground, forming a mountain. If it breaks through the top of the ground or the mountain it has created, a volcano is formed.

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