A volcano is a geological landform (usually a Mountain) where a substance, usually magma (rock of the Earth's interior made molten or liquid by extremely high temperatures along with a reduction in pressure and/or the introduction of water or other volatiles) erupts through the surface of a Planet. Although there are numerous volcanoes (some very active) on the Solar system's rocky planets and moons, on Earth at least, this phenomenon tends to occur near the boundaries of the continental plates. However, important exceptions exist in.. hotspot volcanoes. [image]
The name "volcano" originates from the name of Vulcan, a god of fire in Roman mythology. The study of volcanoes is called vulcanology (or volcanology in some spellings).
If, on the other hand, the magma contains relatively small amounts (<52%) of silica, the lava is called Mafic or "Basic" and will be very fluid as it erupts, capable of flowing for long distances. A good example of a mafic lava flow is the Great Þjórsárhraun (Thjórsárhraun) flow produced by an eruptive fissure almost in the geographical center of Iceland roughly 8,000 years ago; it flowed all the way to the sea, a distance of 130 kilometers, and covered an area of 800 square km. Note that the terms felsic and mafic are sometimes substituted by the older chemistry terms "acidic" and "basic", respectively. The latter were thought to be a little misleading, however, and are slowly falling into disuse.
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Volcanoes are usually situated either at the boundaries between tectonic plates or over hotspots. Volcanoes may be either dormant (having no activity) or active (near constant expulsion and occasional eruptions), and change state unpredictably.
Volcanoes on land often take the form of flat cones, as the expulsions build up over the years, or in short-lived cinder cones. Under Water, volcanoes often form rather steep pillars and in due time break the Ocean surface in new islands.
Volcanic activity is often accompanied by earthquakes, hot springs, fumaroles, mud pots and geysers. Low-magnitude earthquakes often precede eruptions.
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. It is important to note that the span of recorded history differs from region to region; in the Mediterranean, recorded history reaches back more than 3,000 years but in the Pacific Northwest of the United States, it reaches back less than 300 years, and in Hawaii, little more than 200 years.
Dormant volcanoes are those that are not currently active (as defined above), but could become restless or erupt again.
Extinct volcanoes are those that scientists consider unlikely to erupt again. Whether a volcano is truly extinct is often difficult to determine. Since calderas have lifespans sometimes measured in millions of years, a caldera that has not produced an eruption in tens of thousands of years is likely to be considered dormant instead of extinct.
For example, the Yellowstone Caldera (considered a Supervolcano) 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 as relatively recent as 70,000 years ago. For this reason, scientists do not consider the Yellowstone Caldera as extinct. 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 a very active volcano.
The Earth's Moon has no large volcanoes, but does have many volcanic features such as rilles and domes.
The planet Venus is believed to be volcanically active, and its surface is 90% Basalt, indicating that volcanism plays a major role in shaping its surface. Lava flows are widespread and many of its surface features are attributed to exotic forms of volcanism not present on Earth. Other Venusian phenomena, such as changes in the planet's atmosphere and observations of lightning, have been attributed to ongoing volcanic eruptions.
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.
Jupiter's moon Io is the most volcanic object in the solar system, due to tidal interaction with Jupiter. It is covered with volcanoes that erupt Sulfur, Sulfur dioxide and Silicate rock, with the result that the moon is constantly being resurfaced. Its lavas are the hottest known anywhere in the solar system, with temperatures exceeding 1800 K (1500 °C). In February 2001, the largest recorded volcanic eruptions in the solar system occurred on Io . See the list of geological features on Io for a list of named volcanoes on the moon.
The Cassini-Huygens mission has found evidence of a methane-spewing cryovolcano on the Saturnian moon Titan. This volcanism is believed to be a significant source of the methane found in its atmosphere.
Many 'ice volcanoes' have been found on Triton, a moon of Neptune; they are believed to eject liquid Nitrogen, dust, or Methane compounds.
The Kuiper Belt Object Quaoar is also suspected of having ice volcanoes.
Most volcanoes on the land are formed at destructive plate margins: where oceanic crust is forced below the continental crust because oceanic crust is denser than continental crust. Friction between these moving plates will cause the oceanic crust to melt, and reduced density will force the newly formed magma to rise. As the magma rises through weak areas in the continental crust it may eventually erupt as one or more volcanoes. For example, Mount St. Helens is found inland from the margin between the oceanic Juan de Fuca Plate and the continental North American Plate.
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A volcano generally presents itself to the imagination as a mountain sending forth from its summit great clouds of smoke with vast sheets of flame. The truth is that a volcano seldom emits either Smoke or Flame, although various combinations of Hydrogen, Carbon, Oxygen, and Sulfur do sometimes ignite. What is mistaken for smoke consists of vast volumes of fine dust, mingled with steam and other vapors, chiefly sulfurous. Most of what appears to be flames is the glare from the erupting materials, glowing because of their high temperature; this glare reflects off the clouds of dust and steam, resembling fire.
Perhaps the most conspicuous part of a volcano is the crater, a basin of a roughly circular form within which occurs a vent (or vents) from which magma erupts as gases, lava, and ejecta. A crater can be of large dimensions, and sometimes of vast depth. Very large features of this sort are termed calderas. Some volcanoes consist of a crater alone, with scarcely any Mountain at all; but in the majority of cases the crater is situated on top of a mountain (the volcano), which can tower to an enormous height. Volcanoes that terminate in a principal crater are usually of a conical form.
Volcanic cones are usually smaller features composed of loose ash and cinder, with occasional masses of stone which have been tossed violently into the air by the eruptive forces (and are thus called ejecta). Within the crater of a volcano there may be numerous cones from which vapours are continually issuing, with occasional volleys of ashes and stones. In some volcanoes these cones form lower down the mountain, along rift zones or fractures. When the cone is eroded these rifts or lava filled fractures remain as radial near vertical dikes of volcanic rock. For example the radiating dikes at Shiprock in NW New Mexico.
It could be argued that the volcanoes of the Great Rift Valley system of East Africa are modified constructive margin volcanoes. However the modifications caused by the presence of thick continental crust are very substantial, and the magmas produced are very different from the typically very homogenous MORB (Mid-Ocean Ridge Basalt) that makes up the huge majority of constructive margin volcanoes.
Hotspots were originally a catch-all for volcanoes that didn't fit into one of the above two categories, but these days this refers to a more specific circumstance - where an isolated plume of hot mantle material intersects the underside of crust (oceanic or continental), leading to a volcanic center that is not obviously connected with a plate margin. The classic example is the Hawaiian chain of volcanoes and seamounts; Yellowstone is cited as another classic example, in this case the intersection is with the underside of continental crust. Iceland is sometimes cited as yet a third classical example, but complicated by the coincidence of a hotspot intersecting an Oceanic ridge constructive margin.
There are debates about the simple "hotspot" concept, since theorists cannot agree on whether the "hot mantle plumes" originate in the upper mantle or in the lower mantle. Meanwhile, field geologists and petrologists see considerable variation in the detailed chemistry of one hotspot's magmas versus a second hotspot's magmas. On the third hand, high-resolution Seismology of different hotspots is yielding different pictures of the deep sub-structure of Hawaii versus Iceland. There is no detailed consensus about how to interpret these varied results, and it seems plausible that eventually several different sub-types of hotspots will be identified.
Patterns of seismicity are complex and often difficult to interpret. However, increasing activity is very worrisome, especially if long-period events become dominant and episodes of harmonic tremor appear.
In December 2000, scientists at the National Center for Prevention of Disasters in Mexico City predicted an eruption within two days from Popocatépetl, on the outskirts of Mexico City. Their prediction used research done by Dr. Bernard Chouet, a Swiss vulacanologist working at the United States Geological Survey, into increasing long-period oscillations as an indicator of an imminent eruption. The government evacuated tens of thousands of people. Forty eight hours later, bang on time, the volcano erupted spectacularly. It was Popocatépetl's largest eruption for a thousand years and yet no one was hurt.
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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. The most significant impacts from these injections come from the conversion of sulfur dioxide to sulfuric acid (H2SO4), which condenses rapidly in the stratosphere to form fine sulfate aerosols. The aerosols increase the reflection of radiation from the Sun back into space and thus cool the Earth's lower atmosphere or troposphere; however, they also absorb heat radiated up from the Earth, thereby warming the stratosphere. 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. The sulfate aerosols also promote complex chemical reactions on their surfaces that alter chlorine and nitrogen chemical species in the stratosphere. This effect, together with increased stratospheric chlorine levels from chlorofluorocarbon pollution, generates chlorine monoxide (ClO), which destroys Ozone (O3). As the aerosols grow and coagulate, they settle down into the upper troposphere where they serve as nuclei for cirrus clouds and further modify the Earth's radiation balance. Most of the hydrogen chloride (HCl) and hydrogen fluoride (HF) are dissolved in water droplets in the eruption cloud and quickly fall to the ground as acid rain. The injected ash also falls rapidly from the stratosphere; most of it is removed within several days to a few weeks. Finally, explosive volcanic eruptions release the greenhouse gas carbon dioxide and thus provide a deep source of Carbon for biogeochemical cycles.
Gas emissions from volcanoes are a natural contributor to Acid rain.
Volcanic activity now releases about 130 to 230 teragrams (145 million to 255 million short tons) of Carbon dioxide each year.
Volcanic eruptions may inject an Aerosol of particles and chemicals in the Earth's atmosphere. Large injections may have visual effects and affect global climate through climate forcing.
Jesuit Athanasius Kircher (1602-1680), 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.