A vent or fissure in the Earth's crust where molten lava is erupted onto the surface. The shape of a volcano depends on the composition of the lava. Lower-temperature, viscous, silica-rich lava forms steep-sided cones interbedded with ash, such as Mt Fuji, Japan. Less viscous, silica-poor, basaltic lavas form gentle slopes, as found in Iceland. Most volcanoes are confined to the zones along boundaries between crustal plates, and are closely associated with earthquakes, as in the circum-Pacific ring of fire; but there are notable exceptions, such as the Hawaiian Is, which have formed on a hot spot in the Earth's crust. The scientific study of volcanoes is known as vulcanology.
A volcano is a rupture in the Earth's surface or crust, allowing hot, usually molten rock, ash, and gases originating deep below the surface to periodically escape.
Volcanoes are generally found where two to three tectonic plates diverge or converge. The mid-oceanic ridges, like the Mid-Atlantic Ridge, are typical examples of divergent tectonic plates where volcanoes are formed, whereas the Pacific Ring of Fire is a typical example of volcanic activity on convergent tectonic plates. In zones of prolonged crustal extension and thinning within crustal plates, non-hotspot intraplate volcanism can be caused by decompression of the upper mantle without either of the above processes acting (like in the African Rift Valley, or the European Rhine Graben with its Eifel volcanoes).
Volcanic activity can also occur from mantle plumes, the so-called hotspots, which occur at locations far from plate boundaries; hotspot volcanoes are also found elsewhere in the solar system, especially on its rocky planets and moons.
Locations
Divergent plate boundaries
At the mid-oceanic ridges, two tectonic plates diverge from one another. Where the mid-oceanic ridge comes above sea-level, volcanoes like the Hekla on Iceland are formed.
Convergent plate boundaries
In places where one tectonic plate submerges beneath another, the crust melts and becomes magma. This surplus amount of magma generated in one location causes the formation of the volcano. Typical examples for this kind of volcano are the volcanoes in the Pacific Ring of Fire, and also Mount Etna and Mount Vesuvius.
Hotspots
Hotspots are not located on the ridges of tectonic plates, but on top of mantle plumes, where the convection of Earth's mantle creates a column of hot material that rises until it reaches the crust. Because the tectonic plates move whereas the mantle plume remains in the same place, each volcano becomes extinct after a while and a new volcano is then being formed as the plate shifts over the hotspot.
Petitspots
In July 2006, volcanoes were discovered that did not fit in any of the above-mentioned categories, since they are located far from the plate boundary, but are too small to be the result of a mantle plume.
Shape
The most common perception of a volcano is of a conical mountain, spewing lava and poisonous gases from a crater in its top. This describes just one of many types of volcano and the features of volcanoes are much more complicated. The structure and behaviour of volcanoes depends on a number of factors. Some volcanoes have rugged peaks formed by lava domes rather than a summit crater, whereas others present landscape features such as massive plateaus. Vents that issue volcanic material (lava, which is what magma is called once it has broken the surface, and ash) and gases (mainly steam and magmatic gases) can be located anywhere on the landform.
Other types of volcanoes include cryovolcanos (or ice volcanoes), particularly on some moons of Jupiter, Saturn and Neptune; and mud volcanoes, which are formations often not associated with known magmatic activity. Active mud volcanoes tend to involve temperatures much lower than those of igneous volcanoes, except when a mud volcano is actually a vent of an igneous volcano.
Shield volcanoes
Hawaiʻi and Iceland are examples of places where volcanoes extrude huge quantities of basaltic lava that gradually build a wide mountain with a shield-like profile. Olympus Mons is the largest shield volcano on Mars, and is the tallest known mountain in the solar system. Smaller versions of shield volcanoes include lava cones, and lava mounds.
Quiet eruptions spread out basaltic lava in flat layers. The buildup of these layers form a broad volcano with gently sloping sides called a shield volcano. Examples of shield volcanoes are the Hawaiian Islands.
Cinder cones
Volcanic cones or cinder cones result from eruptions that throw out mostly small pieces of scoria and pyroclastics (both resemble cinders, hence the name of this volcano type) that build up around the vent. Cinder cones may form as flank vents on larger volcanoes, or occur on their own.
Stratovolcanoes
Stratovolcanoes are tall conical mountains composed of lava flows and other ejecta in alternate layers, the strata that give rise to the name.
Super volcanoes
Super volcano is the popular term for large volcanoes that usually have a large caldera and can potentially produce devastation on an enormous, sometimes continental, scale. Such eruptions would be able to cause severe cooling of global temperatures for many years afterwards because of the huge volumes of sulfur and ash erupted. They can be the most dangerous type of volcano. Large igneous provinces are also considered supervolcanoes because of the vast amount of basalt lava erupted.
Submarine volcanoes
Submarine volcanoes are common features on the ocean floor. Because of the rapid cooling effect of water as compared to air, and increased buoyancy, submarine volcanoes often form rather steep pillars over their volcanic vents as compared to above-surface volcanos. Pillow lava is a common eruptive product of submarine volcanoes.
Subglacial volcanoes
Subglacial volcanoes develop underneath icecaps. They are made up of flat lava flows atop extensive pillow lavas and palagonite. These volcanoes are also called table volcanoes, tuyas or (uncommonly) mobergs.
Erupted material
Lava composition
Another way of classifying volcanoes is by the composition of material erupted (lava), since this affects the shape of the volcano. Wright, 1987):
If the erupted magma contains a high percentage (>63%) of silica, the lava is called felsic. Felsic lavas (or rhyolites) tend to be highly viscous (not very fluid) and are erupted as domes or short, stubby flows. Viscous lavas tend to form stratovolcanoes or lava domes. Lassen Peak in California is an example of a volcano formed from felsic lava and is actually a large lava dome. Because siliceous magmas are so viscous, they tend to trap volatiles (gases) that are present, which cause the magma to erupt catastrophically, eventually forming stratovolcanoes. Pyroclastic flows (ignimbrites) are highly hazardous products of such volcanoes, since they are composed of molten volcanic ash too heavy to go up into the atmosphere, so they hug the volcano's slopes and travel far from their vents during large eruptions. If the erupted magma contains 52-63% silica, the lava is of intermediate composition. These "andesitic" volcanoes generally only occur above subduction zones (e.g. If the erupted magma contains <52% and >45% silica, the lava is called mafic (because it contains higher percentages of magnesium (Mg) and iron (Fe)) or basaltic. These lavas are usually much less viscous than rhyolitic lavas, depending on their eruption temperature; Mafic lavas occur in a wide range of settings: At mid-ocean ridges, where two oceanic plates are pulling apart, basaltic lava erupts as pillows to fill the gap; Shield volcanoes (e.g. If the erupted magma contains <=45% silica, the lava is called ultramafic. They are (or were) the hottest lavas, and probably more fluid than common mafic lavas.Lava texture
Two types of lava are erupted according to the surface texture: ʻAʻa (pronounced IPA [ʔaʔa]) and pāhoehoe (pronounced [paːho͡eːho͡eː]), both words having Hawaiian origins. Usually, only mafic flows will erupt as pāhoehoe, since they often erupt at higher temperatures or have the proper chemical make-up to allow them to flow at a higher fluidity.
Volcanic activity
A popular way of classifying magmatic volcanoes goes by their frequency of eruption, with those that erupt regularly called active, those that have erupted in historical times but are now quiet called dormant, and those that have not erupted in historical times called extinct. They use classifications which refer to a particular volcano's formative and eruptive processes and resulting shapes, which was explained above.
There is no real consensus among volcanologists on how to define an "active" volcano. The lifespan of a volcano can vary from months to several million years, making such a distinction sometimes meaningless when compared to the lifespans of humans or even civilizations. For example, many of Earth's volcanoes have erupted dozens of times in the past few thousand years but are not currently showing signs of eruption. Given the long lifespan of such volcanoes, they are very active. Complicating the definition are volcanoes that become restless (producing earthquakes, venting gasses, or other non-eruptive activities) but do not actually erupt.
Scientists usually consider a volcano active if it is currently erupting or showing signs of unrest, such as unusual earthquake activity or significant new gas emissions. Many scientists also consider a volcano active if it has erupted in historic time.
Dormant volcanoes are those that are not currently active (as defined above), but could become restless or erupt again. Confusion however, can arise because many volcanoes which scientists consider to be active are referred to as dormant by laypersons or in the media.
Extinct volcanoes are those that scientists consider unlikely to erupt again. Whether a volcano is truly extinct is often difficult to determine.
For example, the Yellowstone Caldera in Yellowstone National Park is at least 2 million years old and hasn't erupted violently for approximately 640,000 years, although there has been some minor activity relatively recently, with hydrothermal eruptions less than 10,000 years ago and lava flows about 70,000 years ago. In fact, because the caldera has frequent earthquakes, a very active geothermal system (i.e., the entirety of the geothermal activity found in Yellowstone National Park), and rapid rates of ground uplift, many scientists consider it to be an active volcano.
Notable volcanoes
On Earth
The 16 current Decade Volcanoes are:
| Avachinsky-Koryaksky, Kamchatka, Russia Colima, Mexico Mount Etna, Italy Galeras, Colombia Mauna Loa, Hawaiʻi, USA Merapi, Indonesia Nyiragongo, Democratic Republic of the Congo Mount Rainier, Washington, USA | Sakurajima, Japan Santamaria/Santiaguito, Guatemala Santorini, Greece Taal Volcano, Philippines Teide, Canary Islands, Spain Ulawun, Papua New Guinea Mount Unzen, Japan Vesuvius, Italy |
Elsewhere in the solar system
The Earth's Moon has no large volcanoes and no volcanic activity, although recent evidence suggests it may still possess a partially molten core.
There are several extinct volcanoes on Mars, four of which are vast shield volcanoes far bigger than any on Earth. These volcanoes have been extinct for many millions of years, but the European Mars Express spacecraft has found evidence that volcanic activity may have occurred on Mars in the recent past as well. It is covered with volcanoes that erupt sulfur, sulfur dioxide and silicate rock, and as a result, Io is constantly being resurfaced. Europa, the smallest of Jupiter's Galilean moons, also appears to have an active volcanic system, except that its volcanic activity is entirely in the form of water, which freezes into ice on the frigid surface.
In 1989 the Voyager 2 spacecraft observed cryovolcanos (ice volcanoes) on Triton, a moon of Neptune, and in 2005 the Cassini-Huygens probe photographed fountains of frozen particles erupting from Enceladus, a moon of Saturn.
Effects of volcanoes
There are many different kinds of volcanic activity and eruptions: phreatic eruptions (steam-generated eruptions), explosive eruption of high-silica lava (e.g., rhyolite), effusive eruption of low-silica lava (e.g., basalt), pyroclastic flows, lahars (debris flow) and carbon dioxide emission.
The concentrations of different volcanic gases can vary considerably from one volcano to the next.
Large, explosive volcanic eruptions inject water vapor (H2O), carbon dioxide (CO2), sulfur dioxide (SO2), hydrogen chloride (HCl), hydrogen fluoride (HF) and ash (pulverized rock and pumice) into the stratosphere to heights of 10-20 miles above the Earth's surface. Several eruptions during the past century have caused a decline in the average temperature at the Earth's surface of up to half a degree (Fahrenheit scale) for periods of one to three years.
Gas emissions from volcanoes are a natural contributor to acid rain. Volcanic eruptions also provide the benefit of adding nutrients to soil through the weathering process of volcanic rocks. Volcanic eruptions can also create new islands, as the magma dries on the water.
Etymology
Volcano is thought to derive from Vulcano, a volcanic island in the Aeolian Islands of Italy whose name in turn originates from Vulcan, the name of a god of fire in Roman mythology. The study of volcanoes is called volcanology, sometimes spelled vulcanology.
Past beliefs
Before it was understood that most of the Earth's interior is molten, various explanations existed for volcano behavior.
One early idea counter to this, however, was Jesuit Athanasius Kircher (1602-1680), who witnessed eruptions of Aetna and Stromboli, then visited the crater of Vesuvius and published his view of an Earth with a central fire connected to numerous others caused by the burning of sulfur, bitumen and coal. Volcanoes in the Sea. Volcanoes.
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