A volcano is an opening in the surface of a planet through which molten rock (magma) such as lava, gas and fragments of solid rock (tephra) emerge. A mountain is often formed around such openings with this material. That is why people used to speak of a fire-breathing mountain. The times when material is ejected from a volcano are called eruptions or eruptions.
On Earth, volcanoes are mainly found in areas where tectonic plates adjoin each other, so 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 there are about 1500 active volcanoes on land worldwide, of which about 55 erupt annually. Volcanoes also occur on other stony planets and moons within the solar system. For example, volcanoes have been observed on the planets Venus and Mars and the moons Io, near Jupiter, and Triton, near Neptune. The latter two are suspected of emitting liquid nitrogen or methane, the volcanism where this occurs is called cryovolcanism.
Volcanic plains, geysers, hot springs and boiling mud springs are phenomena that can be found near volcanoes, but because they have no pronounced relief, they are not volcanoes. They arise because the crust near volcanoes is thinner than in other places.
The science of studying volcanoes and phenomena related to volcanism is called volcanology.
A common classification for volcanoes is based on the composition of the rock that forms.
Acidic volcanoes - if the magma is felsic (or acidic) in composition, with a concentration of silica above 65%, it is viscous. This makes it difficult for this mass to leave the crater mouth or it can even clog the opening due to plug formation. The latter is the reason that the eruptions at such volcanoes are often very explosive and accompanied by glow clouds. Once on the surface, the magma, which thus becomes lava, solidifies relatively quickly. Lassen Peak in California's Lassen Volcanic National Park is a prime example of an acidic volcano. The igneous rock formed from Mount Saint Helens in Washington State also has an acidic composition.
Basic volcanoes - if the magma is mafic (or basic) in composition (at low silica concentrations), the magma is less viscous and the eruption is less explosive because the magma can leave the crater opening more easily. The low viscosity allows the lava to travel longer distances and flows much faster than acidic lava. Volcanoes of this type are found in Iceland and Hawaii, among others.
Another classification method looks at the shape of the volcano.
Shield volcanoes are formed by low-viscosity (i.e. mafic) lava that can flow far out, so that the volcanoes are characterized by a broad base and slowly ascending, gentle slopes. The largest volcanoes on Earth are of this type. The Mauna Loa in Hawaii with a diameter of 120 km is a clear example of this.
Cinder cones, or cinder cones, are formed as grit, debris and mostly small boulders that are ejected through the volcanic vent and accumulate around it. This creates a cone with a crater in the middle. An example is the Hverfell near Mývatn in Iceland.
Stratovolcanoes, or dome volcanoes, are the opposite of shield volcanoes. The lava that comes out of this type of volcano is much more viscous, so more acidic in composition, and tougher than the lava produced by shield volcanoes. This prevents the lava from flowing out very far. The lava accumulates so that the volcano forms very steep walls. Fuji in Japan is a well-known stratovolcano.
Caldera volcanoes are volcanoes that form in the filled cone of