What is a volcano?

What is a volcano? A volcano is an opening in the Earth's surface through which molten rock, ash, and gas can escape. Volcanoes form when magma, which is molten rock beneath the Earth's surface, rises to the surface and erupts. Volcanoes can take on many different forms, ranging from small cones to massive mountains. The shape and size of a volcano depend on the type of magma, the frequency of eruptions, and the type of eruption.