Volcanoes exist all over the world. They have existed for many thousands of years and 660 of them are still active: they can be triggered by an eruption. The ground we live on consists of a layer of rocks called the lithosphere. This layer is made up of many large plates. Beneath these plates are molten materials called magma. Sometimes the molten materials reach the surface, causing volcanic eruptions. A volcanologist is the scientist who studies volcanoes. They can predict a volcanic eruption and warn people living nearby in time.