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Eruption Styles eerruuppttiioonn ssttyylleess OIKOS > volcano > mechanism >eruption styles z z z Why do some volcanoes erupt violently and others do not? The z z viscosity of magma and lava determines how a volcano will behave. z z Volcanoes that are highly explosive and therefore dangerous to z z people are built-up as highly viscous magma that forces its way to z z the surface, often exploding violently as trapped gasses try to z z escape. Volcanoes that are less explosive and less of a threat to z z people are made up of very fluid magmas. Fluid magma allows z z gasses to escape and therefore are less explosive. There are three z z factors that determine the viscosity of a magma and its resulting z z z lava: z z a) chemical composition of magma: which means silica content z z (amount of SiO2). Magma with a high silica content are sticky and z more viscous. b) magma temperature e viscosity: usually lava is between 600 and 1200 degrees Celcius. Basaltic lavas have the highest temperatures up to 1200 C. Rhyolitic and andesitic lavas are cooler when they erupt. The higher the temperature, the less vacuous magma will be. c) volume of dissolved gasses in magma. As gasses escape from magma in the vent as pressure is released, they propel material out in an explosive way. As magma moves to the surface through a vent, the confining pressure of the surrounding environment decreases rapidly. This reduction of the confining pressure allows the dissolved gases to be released suddenly. The escaping gases in the magma expand and may occupy hundreds of times their original volume. If this occurs rapidly or after a large amount of gasses are trapped the eruption can be explosive, even catastrophic. Every year about 60 volcanoes erupt, but most of the activity is pretty weak. How do volcanologists measure how big an eruption is? There is not any single feature that determines the "bigness", but the following eruption magnitude scale - called the Volcanic Explosivity Index or VEI - is based on a number of things that can be observed during an eruption. VEI Description Plume Height Volume Classification How often Example 0 non-explosive < 100 m 1000s m3 Hawaiian daily Kilauea 1 gentle 100-1000 m 10,000s m3 Haw/Strombolian daily Stromboli 2 explosive 1-5 km 1,000,000s m3 Strom/Vulcanian weekly Galeras, 1992 3 severe 3-15 km 10,000,000s m3 Vulcanian yearly Ruiz, 1985 4 cataclysmic 10-25 km 100,000,000s m3 Vulc/Plinian 10's of years Galunggung, 1982 5 paroxysmal >25 km 1 km3 Plinian 100's of years St. Helens, 1980 6 colossal >25 km 10s km3 Plin/Ultra-Plinian 100's of years Krakatau, 1883 7 super-colossal >25 km 100s km3 Ultra-Plinian 1000's of years Tambora, 1815 8 mega-colossal >25 km 1,000s km3 Ultra-Plinian 10,000's of years Yellowstone, 2 Ma VEI Scale. Source: http://volcano.und.edu/vwdocs/eruption_scale.html Classification of eruption styles The types of volcanic eruption are often named after famous volcanoes where characteristic behaviour has been observed. Some volcanoes may exhibit only one characteristic type of eruption during an interval of activity—others may display an entire sequence of types. Type of volcanic eruptions based on geographic examples Type of volcanic eruptions based on geographic examples. http://cache.eb.com/eb/image?id=4948&rendTypeId=4 http://volcano.gl.ntu.edu.tw/images/class/volcano_eruption01.gif Types of volcanic eruptions Icelandic eruption Icelandic eruptions involve the quiet outpouring of lava from long fissures. Eruptions of the Icelandic type during the Tertiary Period built up vast basalt plateaus in many parts of the World. One of the largest is the Columbia Plateau, more than 20,000 sq mi in the north- western United States. Hawaiian eruption Type of volcanic eruption where lava flows from the vent in a relative gentle, low level eruption, so called because it is characteristic of Hawaiian volcanoes. Typically they are effusive eruptions, with basaltic magmas of low viscosity, low content of gases, and high temperature at Icelandic eruption the vent. Very little amount of volcanic ash is produced. http://geo.web.ru/Lectures/Plechov/lecture-1/icelandic-rift-eruption.jpg This type of eruption occurs most often on hotspot volcanoes such as Kilauea, though it can occur near subduction zones. Hawaiian eruptions are usually the most attractive to tourists and are the safest because there is little danger from ash. Hawaiian eruptions may occur along fissure vents, wgere lava spurts from a fissure on the volcano's rift zone and feeds lava streams that flow downslope. In central-vent eruptions, a fountain of lava can spurt to a height of 300 meters or more (heights of 1600 meters were reported for the 1986 eruption of Mount Mihara on Izu Oshima, Japan). Hawaiian eruptions usually start by formation of a crack in the ground from which a curtain of incandescent magma or several closely spaced magma fountains appear. The lava can overflow the fissure and form aa or pahoehoe style of flows. Eruptions from a central cone can form small lightly sloped shield volcanoes, for example the Mauna Ulu. Hawaiian eruption http://www.classzone.com/books/earth_science/terc/conte nt/investigations/es0801/images/es0801_p1_eruption_a.jpg Strombolian eruption Strombolian eruptions are relatively low-level volcanic eruptions, named after the Italian volcano Stromboli, where such eruptions consist of ejection of incandescent cinder, lapilli and lava bombs to altitudes of tens to hundreds of meters. They are small to medium in volume, with sporadic violence. The tephra typically glows red when leaving the vent, but its surface cools and assumes a dark to black color and may significantly solidify before impact. The tephra accumulates in the vicinity of the vent, forming a cinder cone. The lava flows are more viscous, and therefore shorter and thicker, than the corresponding Hawaiian eruptions; it may or may not be accompanied by production of pyroclastic rock. Tipical Strombolian eruption http://volcanoes.usgs.gov/Imgs/Jpg/Stromboli/30423808-002_large.jpg Vulcanian eruption It is a violent eruption caused by the removal of a plug as gasses and pressures build up. The eruption interval can be very long (tens to hundreds of years). The main type of activity includes large pyroclastic flows, tuff and ash falls, also production of lapilli and bombs. This type of eruption usually occurs in volcanos created of andesitic magmas and lavas. This eruptive style builds a large stratovolcano. The type example is Vulcano in Italy. Vesuvian eruption Vulcanian eruption http://www-b.unipg.it/denz/pecce/vf.html Vesuvian eruptions are named after the eruption of Vesuvius in Italy in the year 79, great quantities of ash-laden gas are violently discharged to form a cauliflower- shaped cloud high above the volcano. Mount Vesuvius Volcano, Italy, 1944. http://pubs.usgs.gov/gip/volc/eruptions.html http://www.ov.ingv.it Pelean eruption They can occur when viscous magma, typically of rhyolitic or andesitic type, is involved, and share some similarities with Vulcanian eruptions. The most important characteristics of a Pelean eruption is the presence of a glowing avalanche of hot volcanic ash, a pyroclastic flow and a eruption cloud. Formation of lava domes is another characteristic feature. Short flows of ash or creation of pumice cones may be observed as well. The initial phases of eruption are characterized by pyroclastic flows. The tephra deposits have lower volume and range than the corresponding Plinian and Vulcanian eruptions. The viscous magma then forms a steep-sided dome in the volcano's vent. The dome may later collapse, resulting on flows of ash and hot blocks. The eruption cycle is usually completed in few years, but in some cases may continue for decades. The 1902 explosion of Mount Pelee is the first described case of a Pelean eruption, and gave it its name. Plinian eruption Volcanic eruptions marked by their similarity to the eruption of Mt. Vesuvius in 79 AD (as described in a letter written by Pliny the Younger) that killed Pliny the Elder. Plinian eruptions are marked by columns of smoke and ash extending high into the Pelean eruption http://volcanoes.usgs.gov/Imgs/Jpg/Mayon/32923351-018_large.jpg stratosphere. The key characteristics are ejection of large amount of pumice and very powerful continuous gas blast eruptions. Short eruptions can end in less than a day. Longer events can take several days to months. The longer eruptions begin with production of clouds of volcanic ash, optionally with pyroclastic flows. The amount of magma erupted can be so large the top of the volcano may collapse, resulting in a caldera. Fine ash can deposit over large areas. The examples of large Plinian eruptions resulting in formation of a caldera are the 1883 Krakatoa eruption, the 1980 eruption of Mount St. Helens, and of course Vesuvius in 79 A.D, which was the prototypical Plinian Eruption. Eruption of Vesuvius in 1822. The eruption of AD 79 would have appeared very similar http://upload.wikimedia.org/wikipedia/en/f/f4/Vesuvius1822scrope.jpg Violent (VEI=4) plinian eruption of Galunggung stratovolcano in Indonesia. Photo by Jack Lockwood, U.S. Geological Survey, August 16, 1982. http://volcano.und.edu/vwdocs/vwlessons/landforms/part5.html External links Volcano World http://volcano.und.edu Collection of photos pf eruptions http://www.volcanodiscovery.com/volcano-tours/photos/volcanoes.html?&L=%255 main contribution to this article from .
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