A volcano is an opening in the surface of a planet through which molten rock, gas and fragments of solid rock come out. A mountain is often formed around such openings with this material. The times when material is ejected from a volcano are called eruptions.
Volcanoes are mainly found in areas where tectonic plates adjoin each other, which are the places where earthquakes also occur. An exception to this are hotspots. These are areas where hot material from the mantle emerges through the Earth's crust. It is estimated that worldwide there are about 1500 active volcanoes on land, of which about 55 erupt annually.
