Any natural opening in the Earth's crust, called a fissure, where melted rock, ash, gases, and steam come out is called a volcano.Volcanoes look like mountains, but they are really flat land that is pushed upward because of the pressure below the earth. The pressure comes from hot magma and gases that build up below the earth's surface and that work up to the surface through cracks in the earth's plates. As it is forced up to the surface, the gases and magma push the earth up with it forming the mountain shape. When the magma gets near the surface of the earth it sometimes erupts violently, but can also erupt without violence.

Volcanoes are not always erupting. Sometimes they lie quietly for thousands of years in between eruptions. Some volcanoes have areas around them that experience earthquakes and release gases, but do not erupt with magma.