Volcano
A volcano is commonly defined as a vent or fissure in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.
On Earth, volcanoes are most often found where tectonic plates are diverging or converging, and because most of Earth's plate boundaries are underwater, most volcanoes are found underwater. For example, a mid-ocean ridge, such as the Mid-Atlantic Ridge, has volcanoes caused by divergent tectonic plates whereas the Pacific Ring of Fire has volcanoes caused by convergent tectonic plates. Volcanoes resulting from divergent tectonic activity are usually non-explosive whereas those resulting from convergent tectonic activity cause violent eruptions. Volcanoes can also form where there is stretching and thinning of the crust's plates, such as in the East African Rift, the Wells Gray-Clearwater volcanic field, and the Rio Grande rift in North America. Volcanism away from plate boundaries most likely arises from upwelling diapirs from the core–mantle boundary called mantle plumes, deep within Earth. This results in hotspot volcanism or intraplate volcanism, in which the plume may cause thinning of the crust and result in a volcanic island chain due to the continuous movement of the tectonic plate, of which the Hawaiian hotspot is an example. Volcanoes are usually not created at transform tectonic boundaries where two tectonic plates slide past one another.
Volcanoes, based on their frequency of eruption or volcanism, are referred to as either active, dormant, or extinct. Active volcanoes have a history of volcanism and are likely to erupt again, while extinct ones are not capable of eruption at all as they have no magma source. "Dormant" volcanoes have not erupted in a long time - generally accepted as since the start of the Holocene, about 12,000 years ago - but may erupt again. However, dormant volcanoes are technically considered to be seismically "active". These categories aren't entirely uniform; they may overlap for certain examples.
Large eruptions can affect atmospheric temperature as ash and droplets of sulfuric acid obscure the Sun and cool Earth's troposphere. Historically, large volcanic eruptions have been followed by volcanic winters which have caused catastrophic famines.
Other planets besides Earth have volcanoes. For example, volcanoes are very numerous on Venus. Mars has significant volcanoes. In 2009, a paper was published suggesting a new definition for the word 'volcano' that includes processes such as cryovolcanism. It suggested that a volcano be defined as 'an opening on a planet or moon's surface from which magma, as defined for that body, and/or magmatic gas is erupted.'
This article mainly covers volcanoes on Earth. See and cryovolcano for more information.
Etymology and terminology
The word volcano originates from the early 17th century, derived from the Italian name Vulcano, a volcanic island in the Aeolian Islands of Italy, which in turn comes from the Latin name or, referring to Vulcan, the god of fire in Roman mythology.The set of processes and phenomena involved in volcanic activity is called volcanism . The study of volcanism and volcanoes is called volcanology , sometimes spelled vulcanology.
Plate tectonics
According to the theory of plate tectonics, Earth's lithosphere, its rigid outer shell, is broken into sixteen larger and several smaller plates. These move continuously at a slow pace, due to convection in the underlying ductile mantle, and most volcanic activity on Earth takes place along plate boundaries, where plates are converging or are diverging.During the development of geological theory, certain concepts that allowed the grouping of volcanoes in time, place, structure and composition have developed that ultimately have had to be explained in the theory of plate tectonics. For example, some volcanoes are polygenetic with more than one period of activity during their history; other volcanoes that become extinct after erupting exactly once are monogenetic and such volcanoes are often grouped together in a geographical region.
Divergent plate boundaries
At the mid-ocean ridges, two tectonic plates diverge from one another as hot mantle rock creeps upwards beneath the thinned oceanic crust. The decrease of pressure in the rising mantle rock leads to adiabatic expansion and the partial melting of the rock, causing volcanism and creating new oceanic crust. Most divergent plate boundaries are at the bottom of the oceans, and so most volcanic activity on Earth is submarine, forming new seafloor. Black smokers are evidence of this kind of volcanic activity. Where the mid-oceanic ridge is above sea level, volcanic islands are formed, such as Iceland.Convergent plate boundaries
zones are places where two plates, usually an oceanic plate and a continental plate, collide. The oceanic plate subducts, forming a deep ocean trench just offshore. In a process called flux melting, water released from the subducting plate lowers the melting temperature of the overlying mantle wedge, thus creating magma. This magma tends to be extremely viscous because of its high silica content, so it often does not reach the surface but cools and solidifies at depth. When it does reach the surface, however, a volcano is formed. Thus subduction zones are bordered by chains of volcanoes called volcanic arcs. Typical examples are the volcanoes in the Pacific Ring of Fire, such as the Cascade Volcanoes or the Japanese Archipelago, or the eastern islands of Indonesia.Hotspots
are volcanic areas thought to be formed by mantle plumes, which are hypothesized to be columns of hot material rising from the core-mantle boundary. As with mid-ocean ridges, the rising mantle rock experiences decompression melting which generates large volumes of magma. Because tectonic plates move across mantle plumes, each volcano becomes inactive as it drifts off the plume, and new volcanoes are created where the plate advances over the plume. The Hawaiian Islands are thought to have been formed in such a manner, as has the Snake River Plain, with the Yellowstone Caldera being part of the North American plate currently above the Yellowstone hotspot. However, the mantle plume hypothesis has been questioned.Continental rifting
Sustained upwelling of hot mantle rock can develop under the interior of a continent and lead to rifting. Early stages of rifting are characterized by flood basalts and may progress to the point where a tectonic plate is completely split. A divergent plate boundary then develops between the two halves of the split plate. However, rifting often fails to completely split the continental lithosphere, and failed rifts are characterized by volcanoes that erupt unusual alkali lava or carbonatites. Examples include the volcanoes of the East African Rift.Volcanic features
A volcano needs a reservoir of molten magma, a conduit to allow magma to rise through the crust, and a vent to allow the magma to escape above the surface as lava. The erupted volcanic material that is deposited around the vent is known as a , typically a volcanic cone or mountain.The most common perception of a volcano is of a conical mountain, spewing lava and poisonous gases from a crater at its summit; however, this describes just one of the many types of volcano. The features of volcanoes are varied. The structure and behaviour of volcanoes depend on several factors. Some volcanoes have rugged peaks formed by lava domes rather than a summit crater while others have landscape features such as massive plateaus. Vents that issue volcanic material and gases can develop anywhere on the landform and may give rise to smaller cones such as Puu Ōō on a flank of Kīlauea in Hawaii. Volcanic craters are not always at the top of a mountain or hill and may be filled with lakes such as with Lake Taupō in New Zealand. Some volcanoes can be low-relief landform features, with the potential to be hard to recognize as such and be obscured by geological processes.
Other types of volcano include mud volcanoes, which are structures often not associated with known magmatic activity; and cryovolcanoes, particularly on some moons of Jupiter, Saturn, and Neptune. Active mud volcanoes tend to involve temperatures much lower than those of igneous volcanoes except when the mud volcano is actually a vent of an igneous volcano.