PHYSICOCHEMICALCONTROLS on ERUPTION STYLEThere is a great range in theexplosivity of volcanic eruptions. Numerous eruptions room relativelyquiescent and are characterized by the calm, nonviolent extrusionof lava operation on the earth"s surface. Other eruptions, however,are extremely explosive and also are characterized by the violent ejectionof fragmentised volcanic debris, called tephra, i beg your pardon can prolong tens that kilometers into the atmosphereabove the volcano. Nonexplosive eruption through effusive lava operation Explosive eruption with voluminous plume of tephra even if it is or no an eruption fallsinto one of these end-member types depends on a variety of factors,which room ultimately connected to the ingredient of the magma(molten rock) basic the volcano. Magma ingredient is discussedbelow, followed by a description of the regulating factors onexplosivity -- viscosity, temperature,and the amount of liquified gases in the magma.MAGMACOMPOSITION and also ROCK TYPESOnly ten elements make up thebulk of many magmas: oxygen (O), silicon (Si), aluminum (Al),iron (Fe), magnesium (Mg), titanium (Ti) calcium (Ca), sodium(Na), potassium (K), and phosphorous (P). Since oxygen and siliconare by far the two most abundant elements in magma, the is convenientto describe the different magma species in regards to their silicacontent (SiO2). The magma species vary indigenous maficmagmas, i m sorry haverelatively short silica and also high Fe and also Mg contents, come felsicmagmas, i m sorry haverelatively high silica and also low Fe and Mg contents. Mafic magmawill cool and also crystallize to produce the volcanic absent basalt,whereas felsic magma will crystallize to create daciteand rhyolite. Intermediate-composition magmas willcrystallize to develop the rock andesite. Becausethe mafic rocks are enriched in Fe and also Mg, they tend to be darkercolored than the felsic absent types. SiO2 content MAGMA type VOLCANIC rock ~50% Mafic Basalt ~60% intermediate Andesite ~65% Felsic (low Si) Dacite ~70% Felsic (high Si) Rhyolite There also exists an ext unusualmagmas the erupt less typically on the Earth"s surface ar as ultramafic, carbonatite, and also stronglyalkaline lavas.For an historic note top top rockterminology see: Basic/Acidicvs. Mafic/FelsicMAGMAVISCOSITY, TEMPERATURE, and also GAS CONTENTThe viscosityof a problem is a measure up of that consistency. Viscosity is definedas the ability of a substance to withstand flow. In a sense, viscosityis the station of fluidity. Cold molasses, for example,has a higher viscosity 보다 water due to the fact that it is less fluid. Amagma"s viscosity is largely managed by that temperature, composition,and gas content (see downloadable program at the bottom that thispage). The impact of temperature on viscosity is intuitive. Likemost liquids, the higher the temperature, the more fluid a substancebecomes, for this reason lowering that is viscosity.Composition plays an even greater function in determininga magma"s viscosity. A magma"s resistance to circulation is a functionof that is "internal friction" obtained from the generationof chemical bonds in ~ the liquid. Chemical bonds are createdbetween negatively charged and positively charged ion (anionsand cations, respectively). The the ten many abundant elementsfound in magmas (see above), oxygen is the only anion. Silicon,on the various other hand, is the most abundant cation. Thus, the Si-Obond is the single most important factor in identify the degreeof a magma"s viscosity. These two elements bond with each other to form"floating radicals" in the magma, while it is stillin its fluid state (i.e., Si-O bonds start to type well abovethe crystallization temperature of magma). This floating radicalscontain a small silicon atom surrounding by four larger oxygenatoms (SiO4). This atom configurationis in the shape of a tetrahedron. The radicals are thus calledsilicon-oxygen tetrahedra, as displayed here. this floating tetrahedra are electrically chargedcompounds. As such, lock they are electrically attractive to otherSi-O tetrahedra. The external oxygen atom in every tetrahedron canshare electrons through the external oxygen atom of various other tetrahedra.The share of electron in this manner results in the developmentof covalent bonds in between tetrahedra. In this waySi-O tetrahedra can attach together to form a range shapes: doubletetrahedra (shown here, C), chain of tetrahedra, double chainsof tetrahedra, and facility networks the tetrahedra. As themagma cools, an ext and more bonds space created, i beg your pardon eventuallyleads come the breakthrough of crystals in ~ the fluid medium.Thus, the Si-O tetrahedra type the structure blocks to the commonsilicate minerals found in all igneous rocks. However, while stillin the fluid state, the bonding that tetrahedra outcomes in thepolymerization that the liquid, which increases the"internal friction" of the magma, so the it much more readilyresists flow. Magmas that have a high silica content will certainly thereforeexhibit greater degrees of polymerization, and have greater viscosities,than those v low-silica contents.The amount ofdissolvedgasesin the magma deserve to also impact it"s viscosity, yet in a an ext ambiguousway 보다 temperature and silica content. Once gases begin to escape(exsolve) indigenous the magma, the impact of gas bubbles on the bulkviscosity is variable. Return the farming gas bubbles will certainly exhibitlow viscosity, the viscosity of the residual liquid will increaseas gas escapes. The as whole bulk viscosity of the bubble-liquidmixture depends on both the size and also distribution that the bubbles.Although gas bubbles do have an result on the viscosity, the moreimportant duty of this exsolving volatiles is that they providethe driving force for the eruption. This is disputed in moredetail below.VESICULATIONAs liquified gases are releasedfrom the magma, bubbles will start to form. Balloon frozen ina porous or frothy volcanic absent are dubbed vesicles,and the process of bubble development is referred to as vesiculationor gas exsolution. The liquified gases canescape only when the vapor press of the magmais greater than the confining push of the surroundingrocks. The vapor press is mainly dependent ~ above the lot ofdissolved gases and the temperature the the magma. Gas escape through vertical vesicle cylinders Vesicle-rich circulation top Explosiveeruptions room initiated by vesiculation, i beg your pardon in turn, deserve to bepromoted in two ways: (1) by decompression, i m sorry lowersthe confining pressure, and (2) by crystallization, whichincreases the vapor pressure. In the an initial case, magma increase canlead come decompression and also the formation of bubbles, much likethe decompression that soda and the formation of CO2bubbles as soon as the lid is removed. This is sometimes referred toas the very first boiling. Alternatively, together magma coolsand anhydrous minerals start to crystallize the end of the magma,the residual fluid will end up being increasingly enriched in gas.In this case, the boosted vapor push in the residual liquidcan also lead to gas exsolution. This is sometimes referred toas 2nd (or retrograde) boiling. Both mechanismscan create an explosive volcanic eruption.CONTROLSON EXPLOSIVITYThe amount ofdissolved gas in the magma gives the driving force for explosiveeruptions. The viscosity of the magma, however, is also an importantfactor in determining even if it is an eruption will be explosive ornonexplosive. A low-viscosity magma, choose basalt, will permit theescaping gases to move rapidly with the magma and escapeto the surface. However, if the magma is viscous, like rhyolite,its high polymerization will certainly impede the increase mobility of thegas bubbles. Together gas continues to exsolve indigenous the viscous melt,the bubbles will be prevent from quick escape, for this reason increasingthe in its entirety pressure on the magma obelisk until the gas ejectsexplosively from the volcano. As a general rule, therefore, nonexplosiveeruptions are usual of basaltic-to-andesiticmagmas which have low viscosities and also low gas contents, whereasexplosive eruptions are typical of andesitic-to-rhyoliticmagmas which have high viscosities and high gas contents. SiO2 MAGMA type TEMPERATURE (centigrade) VISCOSITY GAS contents ERUPTION layout ~50% mafic ~1100 low short nonexplosive ~60% intermediary ~1000 intermediary intermediate intermediary ~70% felsic ~800 high high explosive over there are, however,two exceptions to this generalrule. Andesitic-to-rhyolitic lavas that have actually been degassed oftenerupt at the surface nonexplosively together viscous lavadomes or obsidianflows. Similarly, many of the so-called hydrovolcanic eruptions show off basaltic-to-andesitic magmas that eruptexplosively in the existence of underground water or surface water. Formore info on the variability that explosivity, check out the Volcano Explosivity Index.Intermediate toadvanced users might be interested in the complying with programs forcalculating viscosity. Click picture to download.Viscosity because that Windows. Developedby Dr. Jon Dehn, this regime calculates the viscosity that silicatemagmas native the magma composition and temperature.Magma because that Windows.


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Emerged by Dr. Ken Wohletz, this programcalculates the density and viscosity that silicate magmas native themagma composition and temperature.