About Volcanoes Volcanoes are mountains that lava erupts from. When thick magma and large amounts of gas build up under the surface there can be explosions that can be very dangerous. Magma is a hot fluid below or within the earth's crust from which lava and other igneous rocks are formed by the process of cooling. Once magma reaches the the earth's surface it is no longer called magma it is now called lava. Magma is less dense than solid rock, it is under a lot of pressure so it starts t move upward, toward the surface of earth. How do volcanoes form? Volcanoes are formed when rocks become hot enough under pressure that they are able to melt and become magma. For a volcano to form the magma must first reach tot he earth's surface. Volcanoes are not common all around the world They can only occur where there are conditions that release magma from earth's interior.