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VOLCANOESWork made by: -Javier Aparicio -Adrián García -Joaquín Lucas -Jesús Tomé -Alejandro Pérez
DefinitionA volcano is an opening, or rupture, ina planets surface or crust, which allowshot magma, volcanic ash and gases toescape from below the surface.Volcanoes are generally found wheretectonic plates are diverging orconverging. A mid-oceanic ridge, forexample the Mid-Atlantic Ridge, hasexamples of volcanoes caused bydivergent tectonic plates pulling apart;the Pacific Ring of Fire has examples ofvolcanoes caused by convergenttectonic plates coming together.
DefinitionBy contrast, volcanoes are usually not created wheretwo tectonic plates slide past one another.Volcanoes can also form where there is stretchingand thinning of the Earths crust in the interiors ofplates, e.g., in the East African Rift, the Wells Gray-Clearwater volcanic field and the Rio Grande Rift inNorth America. This type of volcanism falls underthe umbrella of "Plate hypothesis” volcanism.Volcanism away from plate boundaries has alsobeen explained as mantle plumes. These so-called"hotspots", for example Hawaii, are postulated toarise from upwelling diapirs with magma from thecore-mantle boundary, 3,000 km deep in the Earth.
DefinitionErupting volcanoes can pose many hazards,not only in the immediate vicinity of theeruption. Volcanic ash can be a threat toaircraft, in particular those with jet engineswhere ash particles can be melted by the highoperating temperature. Large eruptions canaffect temperature as ash and droplets ofsulfuric acid obscure the sun and cool theEarths lower atmosphere or troposphere;however, they also absorb heat radiated upfrom the Earth, thereby warming thestratosphere. Historically, so-called volcanicwinters have caused catastrophic famines.
EtymologyThe word volcano is derived fromthe name of Vulcano, a volcanicisland in the Aeolian Islands of Italywhose name in turn originates fromVulcan, the name of a god of fire inRoman mythology. The study ofvolcanoes is called volcanology,sometimes spelled vulcanology.
Convergent plate boundariesSubduction zones are places where two plates,usually an oceanic plate and a continental plate,collide. In this case, the oceanic plate subducts, orsubmerges under the continental plate forming adeep ocean trench just offshore. Water releasedfrom the subducting plate lowers the meltingtemperature of the overlying mantle wedge, creatingmagma. This magma tends to be very viscous due toits high silica content, so often does not reach thesurface and cools at depth. When it does reach thesurface, a volcano is formed. Typical examples forthis kind of volcano are Mount Etna and thevolcanoes in the Pacific Ring of Fire.
Divergent plate boundariesAt the mid-oceanic ridges, two tectonic platesdiverge from one another. New oceanic crust isbeing formed by hot molten rock slowly cooling andsolidifying. The crust is very thin at mid-oceanicridges due to the pull of the tectonic plates. Therelease of pressure due to the thinning of the crustleads to adiabatic expansion, and the partialmelting of the mantle causing volcanism andcreating new oceanic crust. Most divergent plateboundaries are at the bottom of the oceans,therefore most volcanic activity is submarine,forming new seafloor. Black smokers or deep seavents are an example of this kind of volcanicactivity. Where the mid-oceanic ridge is above sea-level, volcanic islands are formed, for example,Iceland.
Hotspots"Hotspots" is the name given to volcanic provincespostulated to be formed by mantle plumes. Theseare postulated to comprise columns of hotmaterial that rise from the core-mantle boundary.They are suggested to be hot, causing large-volume melting, and to be fixed in space. Becausethe tectonic plates move across them, eachvolcano becomes dormant after a while and a newvolcano is then formed as the plate shifts over thepostulated plume. The Hawaiian Islands have beensuggested to have been formed in such a manner,as well as the Snake River Plain, with theYellowstone Caldera being the part of the NorthAmerican plate currently above the hot spot. Thistheory is currently under criticism, however.
Fissure Vents Volcanic fissure vents are flat, linear cracks through which lava emerges.
Shield Volcanoes Shield volcanoes, are formed by the eruption of low-viscosity lava that can flow a great distance from a vent, but not generally explode catastrophically. Since low-viscosity magma is typically low in silica, shield volcanoes are more common in oceanic than continental settings. The Hawaiian volcanic chain is a series of shield cones, and they are common in Iceland, as well.
Lava Domes Lava domes are built by slow eruptions of highly viscous lavas. They are sometimes formed within the crater of a previous volcanic eruption (as in Mount Saint Helens), but can also form independently, as in the case of Lassen Peak. Like stratovolcanoes, they can produce violent, explosive eruptions, but their lavas generally do not flow far from the originating vent.
Stratovolcanoes Stratovolcanoes or composite volcanoes 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, created from several structures during different kinds of eruptions.
Cryptodomes Cryptodomes are formed when viscouslava forces its way up and causes abulge. The 1980 eruption of Mount St.Helens was an example. Lava wasunder great pressure and forced abulge in the mountain, which wasunstable and slid down the north side.
Volcanic cones Volcanic cones or cinder cones are the result from eruptions that erupt 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 meters high. Most cinder cones erupt only once.
Lava composition If the erupted magma contains a high percentage (>63%) of silica, the lava is called felsic. Felsic lavas (dacites 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.
Lava composition If the erupted magma contains 52–63% silica, the lava is of intermediate composition. 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
Popular classification ofvolcanoes Active Extinct Dormant
ActiveA popular way of classifying magmaticvolcanoes is by their frequency oferuption, with those that eruptregularly called active.
ExtinctExtinct volcanoes are those thatscientists consider unlikely to eruptagain, because the volcano no longerhas a lava supply.
DormantIt is difficult to distinguish an extinctvolcano from a dormant (inactive) one.Volcanoes are often considered to beextinct if there are no written recordsof its activity. Nevertheless, volcanoesmay remain dormant for a long periodof time.
Effects of volcanoes There are many different types of volcanic eruptions and associated activity: phreatic eruptions (steam- generated eruptions), explosive eruption of high-silica lava effusive eruption of low-silica lava, pyroclastic flows, lahars (debris flow) and carbon dioxide emission. All of these activities can pose a hazard to humans. Earthquakes, hot springs, fumaroles, mud pots and geysers often accompany volcanic activity.
Volcanoes on other planetary bodies The Earths Moon has no large volcanoes and no current 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.
Volcanoes on other planetary bodies 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.
Volcanoes on other planetary bodiesThere are several extinct volcanoes onMars, four of which are vast shieldvolcanoes far bigger than any on Earth.They include Arsia Mons, AscraeusMons, Hecates Tholus, Olympus Mons,and Pavonis Mons. These volcanoeshave been extinct for many millions ofyears, but the European Mars Expressspacecraft has found evidence thatvolcanic activity may have occurred onMars in the recent past as well.
Tradicional beliefs about volcanoes Many ancient accounts ascribe volcaniceruptions to supernatural causes, such as theactions of gods or demigods. To the ancientGreeks, volcanoes capricious power couldonly be explained as acts of the gods, while16th/17th-century German astronomerJohannes Kepler believed they were ducts forthe Earths tears. One early idea counter tothis was proposed by Jesuit AthanasiusKircher (1602–1680), who witnessederuptions of Mount Etna and Stromboli, thenvisited the crater of Vesuvius and publishedhis view of an Earth with a central fireconnected to numerous others caused by theburning of sulfur, bitumen and coal.
Technical classification of volcanoes Volcanic-alert level Volcano warning schemes of United States
Volcanic-alert level The three common popular classifications ofvolcanoes can be subjective and somevolcanoes thought to have been extinct haveannounced to the world they were justpretending. To help prevent citizens fromfalsely believing they are not at risk whenliving on or near a volcano, countries haveadopted new classifications to describe thevarious levels and stages of volcanic activity.Some alert systems use different numbers orcolors to designate the different stages.Other systems use colors and words. Somesystems use a combination of both.
Volcano warning schemes of United StatesThe United States Geological Survey(USGS) has adopted a common systemnationwide for characterizing the levelof unrest and eruptive activity atvolcanoes. The new volcano alert-levelsystem classifies volcanoes now asbeing in a normal, advisory, watch orwarning stage. Additionally, colors areused to denote the amount of ashproduced. Details of the US system canbe found at Volcano warning schemesof the United States.