- Types of volcanic eruptions
In this article types of eruption will be used to refer to the mechanism causing the eruption. The style of eruption will be used to describe subcategories of eruption, which have the same eruptive mechanism. For example strombolian and vulcanian are two eruptive styles, but they are the same type.The styles of volcanic eruption are often named after famous
volcanoeswhere characteristic behavior 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.
There are three mechanisms of ash formation: gas release under decompression causing magmatic eruptions; thermal contraction from chilling on contact with water causing phreatomagmatic eruptions and ejection entrained particles during steam eruptions causing phreatic eruptions (Heiken & Wholtez 1985.)
Magmatic Eruptions produce juvenile clasts during explosive decompression from gas release. They range in size from the relatively small fire fountains on Hawaii to >30 km Ultra Plinian eruption columns, bigger than the eruption that buried Pompeii. (Heiken & Wohletz 1985)
Strombolian eruptions are named because of activity of Stromboliin Sicily. They are characterised by huge clots of molten lavabursting from the summit crater to form luminous arcs through the sky. Collecting on the flanks of the cone, lava clots combine to stream down the slopes in fiery rivulets. The explosions are driven by bursts of gas slugs that rise faster than surrounding magma. [ [http://www.sciencemag.org/cgi/content/abstract/317/5835/227] ]
Vulcanian eruptions are named after Vulcano, following Giuseppe Mercalli's observations of its 1888-1890 eruptions. Another example was the eruption of Parícutinin 1947. They are characterised by a dense cloud of ash-laden gas exploding from the crater and rising high above the peak. Steaming ash forms a whitish cloud near the upper level of the cone
Peléan eruptionor nuée ardente (glowing cloud) eruption, such as occurred on the Mayon Volcanoin the Philippinesin 1968, a large amount of gas, smoke, dust, ash, and lava fragments are blown out of a central crater, fall back, and form avalanches that move downslope at speeds as great as 160 km per hour. Such eruptive activity can cause great destruction and loss of life if it occurs in populated areas, as demonstrated by the devastation of Saint-Pierre during the 1902 eruption of Mont Peléeon Martinique, Lesser Antilles.
Hawaiian eruptions may occur along fissures or fractures that serve as vents, such as during the eruption of Mauna Loa Volcanoin Hawaii in 1950. Also, they can occur at a central vent, such as during the 1959 eruption in Kilauea Iki Crater of KilaueaVolcano, Hawaii. In fissure-type eruptions, lava shoots from a fissure on the volcano's rift zoneand feeds lava streams that flow downslope. In central-vent eruptions, a fountain of lava is erupted to a height of several hundred metres or more. Such lava may collect in old pit craters to form lava lakes, or form cones, or feed radiating flows.
Plinian eruptions are usually the most powerful, and involve the explosive ejection of relatively viscous lava. Large plinian eruptions — such as during 18 May 1980at Mount St. Helensor, more recently, during 15 June 1991at Pinatuboin the Philippines— can send ash and volcanic gas tens of kilometres into the atmosphere. The resulting ash fallout can affect large areas hundreds of miles downwind. Fast-moving pyroclastic surges and pyroclastic flowstogether with “nuées ardentes,” are often associated with plinian eruptions.
Phreatomagmatic eruptions are the result of thermal contraction from chilling on contact with water. The products of phreatomagmatic eruptions are beliave to have more regular shard shapes and be finer grained than the products of magmatic eruptions because of the different eruptive mechanism.There is debate about the exact nature of the eruptive style. Fuel-coolant reactions may be more critical to the explosive nature than thermal contraction (Starostin et al 2004). . Fuel coolant reactions fragment the material in contact with a coolant by propagating stress waves widening cracks and increasing surface area leading to rapid cooling rates and explosive thermal contraction (Heiken & Wohletz 1985).
Subglacial eruptions are named because of activity under ice, or under a glacier. They can cause dangerous floods, lahars, and create hyaloclastiteand pillow lava. Only five of these types of eruptions have occurred in the present day.
On January, 2008, the
British Antarctic Survey(Bas) scientists led by Hugh Corr and David Vaughan, reported (in the journal Nature Geoscience) that 2,200 years ago, a volcanoerupted under Antarcticaice sheet (based on airbornesurvey with radar images). The biggest eruption in the last 10,000 years, the volcanic ash was found deposited on the ice surface under the Hudson Mountains, close to Pine Island Glacier. [ [http://news.bbc.co.uk/2/hi/science/nature/7194579.stm BBC NEWS, Ancient Antarctic eruption noted] ]
Phreatic eruptions (or steam-blast eruptions) are driven by explosive expanding steam resulting from cold ground or surface water coming into contact with hot rock or magma. The distinguishing feature of phreatic explosions is that they only blast out fragments of pre-existing solid rock from the volcanic conduit; no new magma is erupted. Phreatic activity is generally weak, but has been known to be strong, such as the 1965 eruption of Taal Volcano, Philippines, and the 1975-1976 activity at La Soufrière, Guadeloupe( Lesser Antilles).
* [http://www.edu4hazards.org/volcano.html Volcanic Eruption] Guide for children and youth
* [http://hvo.wr.usgs.gov/ USGS Hawaiian Volcano Observatory (HVO)] accessed June 19, 2007.
* [http://www.inglaner.com/volcan_chaiten.htm] Images of the eruption of Chaitén volcano, 2008
Heiken, G. & Wohletz, K. 1985. Volcanic Ash. University of California Press, Berkeley.
Starostin, A. B., Barmin, A. A. & Melnik, O.E. 2005. A transient model for explosive and phreatomagmatic eruptions. Journal of Volcanology and Geothermal Research, 143, 133-151.
Pyle, D. M. 1989. The thickness, volume and grainsize of tephra fall deposits. Bulletin of Volcanology, 51, 1-15.
Riley, C. M., Rose, W. I. & Bluth, G.J.S. 2003. Quantitive shape measurements of distal volcanic ash. Journal of Geophysical Research, 108, B10, 2504.
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