Volcanoes are formed when a vent in the earth reaches all the way down into pockets of molten rock called magma. This magma escapes from the vent and builds up a mound around the vent when it cools off and hardens. This mound might be the size of a small hill, or it could grow to be a giant mountain. There are different types of volcanoes, each formed by different types of eruptions. 

Volcanoes