A volcano is a crack in the crust of a planet like the earth. Magma can rise through the crack and erupt as lava on the surface of the planet. The ejected lava accumulates around the crack and a volcanic edifice grows. The shape of the volcano edifice depends on the amount of lava erupted and the lava type produced. Sometimes only a small cinder cone is created, or a hornito, another time a big mountain. When the lava is low viscous, it creates a shield volcano. When highly viscous lava eruptes, a steep stratovolcano forms.
Apart from lava, gases and volcanic ash can also be erupted. Before the volcano erupts, the magma accumulates in a magma chamber that lies beneath the volcano.
The magma is generated in the Earth’s mantle, by partial melting of mantle rocks, or subducted earth crust.
Where the most volcanoes are
There are about 1900 volcanoes on earth that are classified as potentially active. Most volcanoes are grown along the tectonic plate boundaries. 70% of the volcanoes are located in the Pacific Ring of Fire. This is marked by the plate boundaries to the Pacific plate. Most volcanoes along the Pacific Coast are dangerous subduction volcanoes. These volcanoes usually erupt explosively or produce lava domes. The number of underwater volcanoes is unknown. Most likely, their number is significantly higher than the known volcanoes on land. Sometimes it happens that an underwater volcano appears and a new volcanic island is formed. The birth of such an island is a tremendous natural phenomenon.
The simplest volcanic form is the fissure volcano. It consists only of fissures in the ground. These fissures can be several kilometres long. Low-viscosity lava erupts from them. Often, lava fountains are produced, which then feed lava flows.
Some of the most spectacular volcanoes were created by mantle-plumes. These volcanoes are called hot spot volcanoes. One of the most famous hot spot volcanoes is the Kilauea in Hawaii. Its last big eruption took place in 2018. Most hot spot volcanoes erupt effusively. That is, they produce low-viscosity lava. This can form lava lakes, or lava fountains and lava flows. In these eruptions rarely people die, because you can escape from the lava flows. They usually do not move very fast. But there are also hot spot volcanoes like the Yellowstone caldera that produce a highly viscous lava. These volcanoes can cause gigantic explosive eruptions called super volcano eruptions. These have a VEI 8 and could trigger a global winter and thus the downfall of human civilization. While the less dangerous hotspot volcanoes form in the oceanic crust, the highly explosive erupting hot spot volcanoes are found in the continents.
Most of these supervolcanoes are caldera volcanoes. Even less dangerous volcanoes can form calderas on their summits. Typical examples are the shield volcanoes in Hawaii.
Another type of volcano is the stratovolcano. They are mainly formed at subduction zones. After particularly strong explosive eruptions, summit calderas can form, as happened on Mount Tambora in 1815.
Dome volcanoes are some of the most dangerous volcanoes. They build up lava domes, which consist of highly viscous lava. If these domes collapse, pyroclastic flows are created. These hot avalanches of gas and tephra flow down the volcanic slope and destroy everything in their path.
Complex volcanoes often represent a mixture of different volcanic types and have several eruption centres. So some lava domes can exist next to cinder cones. Cinder cones are a relatively small volcanic form, often only active once. Cinder cones are often found as parasitic cones on the flanks of larger volcanic structures and are monogenetic. They rarely occur solitary, but form entire fields.
Up to 50 volcanoes erupt worldwide every year. Some of them are permanently active. These volcanoes include Stromboli in Italy and Dukono in Indonesia. For a list with erupting volcanoes follow the link to the volcano news page, or jump to the daily updates.