Read this excerpt from "Four Types of Volcanoes" Volcanoes are structures formed around openings (vents) in the ground where molten lava, ash, or gas escape from inside the Earth. We have all heard of lava, but not everyone knows how it forms. Deep inside the Earth, rock that melted long ago beneath the Earth's crust forms magma, a very hot liquid that mixes with dissolved gases and crystals. When this magma flows up above Earth’s surface, it is called lava. Volcanoes can erupt continuously or only once in a while. They can erupt violently or slowly. As lava spews out of an opening, it lands, cools, and builds up hills or mountains around the vent. Therefore, volcanoes are usually found on top of hills or mountains. What is the connection between magma and the hills or mountains around a volcano vent? Magma that is underground pushes up against land from beneath it, so hills or mountains form over the magma. Lava is part of magma, and it destroys the mountains and hills that are around a volcano vent. Magma comes out of a volcano vent, becomes lava, and then cools and becomes part of the land around the vent. Lava rises up above ground, cools into magma, and then fills the volcano vent so that a hill or mountain forms.