   #copyright

Volcano

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics

   Volcano 1. Magma chamber 2. Country rock 3. Conduit (pipe) 4. Base 5.
   Sill 6. Branch pipe 7. Layers of ash emitted by the volcano 8. Flank 9.
   Layers of lava emitted by the volcano 10. Throat 11. Parasitic cone 12.
   Lava flow 13. Vent 14. Crater 15. Ash cloud
   Enlarge
   Volcano
   1. Magma chamber
   2. Country rock
   3. Conduit (pipe)
   4. Base
   5. Sill
   6. Branch pipe
   7. Layers of ash emitted by the volcano
   8. Flank
   9. Layers of lava emitted by the volcano
   10. Throat
   11. Parasitic cone
   12. Lava flow
   13. Vent
   14. Crater
   15. Ash cloud

   A volcano is an opening (or rupture) in the Earth's surface or crust,
   which allows hot, usually molten rock, ash, and gases to escape from
   deep below the surface. Volcanic activity involving the extrusion of
   rock tends to form mountains or features like mountains over a period
   of time.

   Volcanoes are generally found where two to three tectonic plates pull
   apart or are coming together. A mid-oceanic ridge, like the
   Mid-Atlantic Ridge, has examples of volcanoes caused by "divergent
   tectonic plates" pulling apart; the Pacific Ring of Fire has examples
   of volcanoes caused by "convergent tectonic plates" coming together. By
   contrast, volcanoes are usually not created where two tectonic plates
   slide past one another (like the San Andreas fault). Volcanoes can also
   form where there is stretching of the Earth's crust and where the crust
   grows thin (called "non-hotspot intraplate volcanism"), such as in the
   African Rift Valley or the European Rhine Graben with its Eifel
   volcanoes).

   Finally, volcanoes can be caused by " mantle plumes," so-called "
   hotspots;" these hotspots can occur far from plate boundaries, such as
   the Hawaiian Islands. Interestingly, hotspot volcanoes are also found
   elsewhere in the solar system, especially on rocky planets and moons.

Locations

Divergent plate boundaries

   At the mid-oceanic ridges, two tectonic plates diverge from one
   another. New oceanic crust is being formed by hot molten rock slowly
   cooling down and solidifying. In these places, the crust is very thin
   and eruptions occur frequently because of the pull by the tectonic
   plates. The main part of the mid-oceanic ridges are at the bottom of
   the ocean, and most volcanic activity is submarine. Black smokers are a
   typical example of this kind of volcanic activity. Where the
   mid-oceanic ridge comes above sea-level, volcanoes like the Hekla on
   Iceland are formed. Divergent plate boundaries create new seafloor and
   volcanic islands.

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. The
   temperature of the plume causes the crust to melt and form pipes, which
   can vent magma. 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. The Hawaiian Islands are thought to be formed in such a
   manner, as well as the Snake River Plain, with the Yellowstone Caldera
   being the current part of the North American plate 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. A new
   theory suggests that submergence of tectonic plates causes stress all
   over the plate, which causes the plate to crack in some places.
   However, other scientists believe the mantle plume theory to be
   incorrect, and consider this discovery a confirmation of their ideas.

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. Many of these vents
   give rise to smaller cones such as Puʻu ʻŌʻō on a flank of Hawaiʻi's
   Kīlauea.

   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

   Toes of a pāhoehoe advance across a road in Kalapana on the east rift
   zone of Kīlauea Volcano in Hawaiʻi.
   Enlarge
   Toes of a pāhoehoe advance across a road in Kalapana on the east rift
   zone of Kīlauea Volcano in Hawaiʻi.

   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. Their lava flows are generally very hot and very
   fluid, contributing to long flows. The largest lava shield on Earth,
   Mauna Loa, rises over 9,000 m from the ocean floor, is 120 km in
   diameter and forms part of the Big Island of Hawaiʻi. 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.
   These can be relatively short-lived eruptions that produce a
   cone-shaped hill perhaps 30 to 400 m high. Most cinder cones erupt only
   once. Cinder cones may form as flank vents on larger volcanoes, or
   occur on their own. Parícutin in Mexico and Sunset Crater in Arizona
   are examples of cinder cones.

Stratovolcanoes

   In difference to pāhoehoe, Aa is a term of Polynesian origin,
   pronounced Ah-ah, for rough, jagged, spiny lavaflow
   Enlarge
   In difference to pāhoehoe, Aa is a term of Polynesian origin,
   pronounced Ah-ah, for rough, jagged, spiny lavaflow

   Stratovolcanoes are tall conical mountains composed of lava flows and
   other ejecta in alternate layers, the strata that give rise to the
   name. Stratovolcanoes are also known as composite volcanoes. Classic
   examples include Mt. Fuji in Japan, Mount Mayon in the Philippines, and
   Mount Vesuvius and Stromboli in Italy.

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. Examples include Yellowstone Caldera in
   Yellowstone National Park, Lake Taupo in New Zealand and Lake Toba in
   Sumatra, Indonesia. Supervolcanoes are hard to identify centuries
   later, given the enormous areas they cover. Large igneous provinces are
   also considered supervolcanoes because of the vast amount of basalt
   lava erupted.

Submarine volcanoes

   Pillow lava (NOAA)
   Enlarge
   Pillow lava ( NOAA)

   Submarine volcanoes are common features on the ocean floor. Some are
   active and, in shallow water, disclose their presence by blasting steam
   and rocky debris high above the surface of the sea. Many others lie at
   such great depths that the tremendous weight of the water above them
   prevents the explosive release of steam and gases, although they can be
   detected by hydrophones and discoloration of water because of volcanic
   gases. Even large submarine eruptions may not disturb the ocean
   surface. 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. In due time, they may break the ocean surface as new islands.
   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. When the
   icecap melts, the lavas on the top collapse leaving a flat-topped
   mountain. Then, the pillow lavas also collapse, giving an angle of 37.5
   degrees. Very good examples of this can be seen in Iceland. 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. Lava can
   be broadly classified into 4 different compositions (Cas & 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. Temperatures as high as 1,200 °C are known to occur
            in pyroclastic flows, which will incinerate everything
            flammable in their path and thick layers of hot pyroclastic
            flow deposits can be laid down, often up to many meters thick.
            Alaska's Valley of Ten Thousand Smokes, formed by the eruption
            of Novarupta near Katmai in 1912, is an example of a thick
            pyroclastic flow or ignimbrite deposit. Volcanic ash that is
            light enough to be erupted high into the Earth's atmosphere
            may travel many kilometres before it falls back to ground as a
            tuff.
     * 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. Mount Merapi in Indonesia).
     * 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; they also tend to be hotter than felsic lavas. 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. the Hawaiian Islands, including Mauna
            Loa and Kilauea), on both oceanic and continental crust;
          + As continental flood basalts.
     * If the erupted magma contains <=45% silica, the lava is called
       ultramafic. Ultramafic flows are very rare; indeed, it is likely
       that none have been erupted at the Earth's surface since the
       Proterozoic, when the planet's heat flow was higher. 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. ʻAʻa is characterized by a rough,
   clinkery surface and is what most viscous and hot lava flows look like.
   However, even basaltic or mafic flows can be erupted as ʻaʻa flows,
   particularly if the eruption rate is high and the slope is steep.
   Pāhoehoe is characterized by its smooth and often ropey or wrinkly
   surface and is generally formed from more fluid lava flows. 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 volcanic fissure and lava channel.
   Enlarge
   A volcanic fissure and lava channel.
   Mount St. Helens shortly after the eruption of May 18, 1980
   Enlarge
   Mount St. Helens shortly after the eruption of May 18, 1980

   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.
   However, these popular classifications—extinct in particular—are
   practically meaningless to scientists. 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. By our lifespans, however, they are not. 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. 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. The Smithsonian Global Volcanism Program's definition of
   'active' is having erupted within the last 10,000 years.

   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. Since "supervolcano" calderas can have eruptive 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 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. For this
   reason, scientists do not consider the Yellowstone Caldera 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 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

   Olympus Mons (Latin, "Mount Olympus") is the tallest known mountain in
   our solar system, located on the planet Mars.
   Enlarge
   Olympus Mons (Latin, "Mount Olympus") is the tallest known mountain in
   our solar system, located on the planet Mars.

   The Earth's Moon has no large volcanoes and no volcanic activity,
   although recent evidence suggests it may still possess a partially
   molten core. However, the Moon does have many volcanic features such as
   maria (the darker patches seen on the moon), rilles and domes.

   The planet Venus has a surface that is 90% basalt, indicating that
   volcanism played a major role in shaping its surface. The planet may
   have had a major global resurfacing event about 500 million years ago,
   from what scientists can tell from the density of impact craters on the
   surface. Lava flows are widespread and forms of volcanism not present
   on Earth occur as well. Changes in the planet's atmosphere and
   observations of lightning, have been attributed to ongoing volcanic
   eruptions, although there is no confirmation of whether or not Venus is
   still volcanically active.

   There are several extinct volcanoes on Mars, four of which are vast
   shield volcanoes far bigger than any on Earth. They include Arsia Mons,
   Ascraeus Mons, Hecates Tholus, Olympus Mons, and Pavonis Mons. 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.
   Galileo orbiter reveals volcanic activity on Jupiter's moon Io.
   Enlarge
   Galileo orbiter reveals volcanic activity on Jupiter's moon Io.

   Jupiter's moon Io is the most volcanically active object in the solar
   system because of tidal interaction with Jupiter. It is covered with
   volcanoes that erupt sulfur, sulfur dioxide and silicate rock, and as a
   result, Io is constantly being resurfaced. Its lavas are the hottest
   known anywhere in the solar system, with temperatures exceeding 1,800 K
   (1,500 °C). In February 2001, the largest recorded volcanic eruptions
   in the solar system occurred on Io . 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. This process is known as
   cryovolcanism, and is apparently most common on the moons of the outer
   planets of the solar system.

   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. The ejecta may be composed of water, liquid nitrogen,
   dust, or methane compounds. Cassini-Huygens also found evidence of a
   methane-spewing cryovolcano on the Saturnian moon Titan, which is
   believed to be a significant source of the methane found in its
   atmosphere. It is theorized that cryovolcanism may also be present on
   the Kuiper Belt Object Quaoar.

Effects of volcanoes

   Volcanic "injection"
   Enlarge
   Volcanic "injection"
   Solar radiation reduction from volcanic eruptions
   Enlarge
   Solar radiation reduction from volcanic eruptions
   Sulfur dioxide emissions by volcanoes.
   Enlarge
   Sulfur dioxide emissions by volcanoes.
   Average concentration of sulfur dioxide over the Sierra Negra Volcano
   (Galapagos Islands) from October 23-November 1, 2005
   Enlarge
   Average concentration of sulfur dioxide over the Sierra Negra Volcano (
   Galapagos Islands) from October 23-November 1, 2005

   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. All of these activities can pose a hazard to humans.
   Volcanic activity is often accompanied by earthquakes, hot springs,
   fumaroles, mud pots and geysers. Low-magnitude earthquakes often
   precede eruptions.

   The concentrations of different volcanic gases can vary considerably
   from one volcano to the next. Water vapor is typically the most
   abundant volcanic gas, followed by carbon dioxide and sulphur dioxide.
   Other principal volcanic gases include hydrogen sulphide, hydrogen
   chloride, and hydrogen fluoride. A large number of minor and trace
   gases are also found in volcanic emissions, for example hydrogen,
   carbon monoxide, halocarbons, organic compounds, and volatile metal
   chlorides.

   Large, explosive volcanic eruptions inject water vapor (H[2]O), carbon
   dioxide (CO[2]), sulfur dioxide (SO[2]), 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 sulphur dioxide to sulphuric acid (H[2]SO[4]), which condenses
   rapidly in the stratosphere to form fine sulfate aerosols. The aerosols
   increase the Earth's albedo—its 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 sulphate 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 (O[3]). 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 releases about 130 to 230 teragrams (145 million to
   255 million short tons) of carbon dioxide each year. Volcanic eruptions
   may inject aerosols into the Earth's atmosphere. Large injections may
   cause visual effects such as unusually colorful sunsets and affect
   global climate mainly by cooling it. Volcanic eruptions also provide
   the benefit of adding nutrients to soil through the weathering process
   of volcanic rocks. These fertile soils assist the growth of plants and
   various crops. 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.

   The Roman name for the island Vulcano has contributed the word for
   volcano in most modern European languages.

Past beliefs

   Kircher's model of the Earth's internal fires, from Mundus Subterraneus
   Enlarge
   Kircher's model of the Earth's internal fires, from Mundus Subterraneus

   Before it was understood that most of the Earth's interior is molten,
   various explanations existed for volcano behaviour. For decades after
   awareness that compression and radioactive materials may be heat
   sources, their contributions were specifically discounted. Volcanic
   action was often attributed to chemical reactions and a thin layer of
   molten rock near the surface. Many ancient accounts claim that divine
   intervention was the actual cause of volcanic eruptions.

   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.

   Retrieved from " http://en.wikipedia.org/wiki/Volcano"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
