Magma 37508 226181609 2008-07-17T05:23:41Z El C 92203 Reverted edits by [[Special:Contributions/122.162.106.199|122.162.106.199]] ([[User talk:122.162.106.199|talk]]) to last version by 194.203.31.210 {{Nofootnotes|article|date=February 2008}} {{otheruses}} '''Magma''' (Plurals: '''magmas''' and '''magmata''') is molten [[Rock (geology)|rock]] that sometimes forms beneath the surface of the [[earth]] (or any other [[terrestrial planet]]) that often collects in a [[magma chamber]] inside a volcano. Magma may contain suspended crystals and gas bubbles. By definition, all [[igneous rock]] is formed from magma. [[Image:magma.jpg|thumb|right|Hawaiian lava flow (lava is the [[extrusive]] equivalent of magma *Pahoehoe)]] Magma is a complex high-temperature fluid substance. Temperatures of most magmas are in the range 700°C to 1300°C, but very rare [[carbonatite]] melts may be as cool as 600°C, and [[komatiite]] melts may have been as hot at 1600°C. Most are [[silicate]] [[solution]]s. Magma is capable of intrusion into adjacent rocks, extrusion onto the surface as [[lava]], and explosive ejection as [[tephra]] to form [[pyroclastic rock]]. Environments of magma formation and compositions are commonly correlated. Environments include [[subduction zone]]s, continental [[rift (geology)|rift zone]]s, [[mid-oceanic ridge]]s, and [[Hotspot (geology)|hotspot]]s, some of which are interpreted as [[mantle plume]]s. Environments are discussed in the entry on [[igneous rock]]. Magma compositions may evolve after formation by [[Fractional crystallization (geology)|fractional crystallization]], contamination, and magma mixing. Contrary to some impressions,{{Clarifyme|date=March 2008}} the bulk of the [[Earth's crust]] and [[mantle (geology)|mantle]] is not molten. Rather, the bulk of the Earth takes the form of a [[rheid]], a form of solid that can move or deform under pressure. Magma, as liquid, preferentally forms in high temperature, low pressure environments within several kilometers of the Earth's surface. == Melting of solid rock == Melting of solid rock to form magma is controlled by three physical parameters: its temperature, pressure, and composition. Mechanisms are discussed in the entry for [[igneous rock]]. ===Temperature=== At any given pressure and for any given composition of rock, a rise in temperature past the [[solidus (chemistry)|solidus]] will cause melting. Within the solid earth, the temperature of a rock is controlled by the [[geothermal gradient]] and the [[radioactive decay]] within the rock. The geothermal gradient averages about 25°C/km with a wide range from a low of 5-10°C/km within oceanic trenches and subduction zones to 30-80°C/km under mid-ocean ridges and volcanic arc environments. ===Pressure=== As magma buoyantly rises it will cross the [[solidus-liquidus]] and its temperature will reduce by [[adiabatic]] cooling. At this point it will liquify and if erupted onto the surface will form lava. Melting can also occur due to a reduction in pressure by a process known as decompression melting.<ref>Geological Society of America, ''Plates, Plumes, And Paradigms'', p. 590 ff., 2005, ISBN 0813723884 </ref> ===Composition=== It is usually very difficult to change the bulk composition of a large mass of rock, so composition is the basic control on whether a rock will melt at any given temperature and pressure. The composition of a rock may also be considered to include ''volatile'' phases such as [[water]] and [[carbon dioxide]]. The presence of volatile phases in a rock under pressure can stabilize a melt fraction. The presence of even 0.8% water may reduce the temperature of melting by as much as 100°C. Conversely, the loss of water and volatiles from a magma may cause it to essentially freeze or solidify. ==Partial melting== When rocks melt they do so incrementally and gradually; most rocks are made of several minerals, all of which have different melting points, and the [[phase diagram]]s that control melting commonly are complex. As a rock melts, its volume changes. When enough rock is melted, the small globules of melt (generally occurring in between mineral grains) link up and soften the rock. Under pressure within the earth, as little as a fraction of a percent partial melting may be sufficient to cause melt to be squeezed from its source. Melts can stay in place long enough to melt to 20% or even 35%, but rocks are rarely melted in excess of 50%, because eventually the melted rock mass becomes a crystal and melt mush that can then ascend ''en masse'' as a [[diapir]], which may then cause further decompression melting. ===Primary melts=== When a rock melts, the liquid is known as a ''primary melt''. Primary melts have not undergone any differentiation and represent the starting composition of a magma. In nature it is rare to find primary melts. The leucosomes of [[migmatite]]s are examples of primary melts. Primary melts derived from the mantle are especially important, and are known as ''primitive melts'' or primitive magmas. By finding the primitive magma composition of a magma series it is possible to model the composition of the mantle from which a melt was formed, which is important in understanding evolution of the [[Earth's mantle|mantle]]. ===Parental melts=== Where it is impossible to find the primitive or primary magma composition, it is often useful to attempt to identify a parental melt. A parental melt is a magma composition from which the observed range of magma chemistries has been derived by the processes of igneous differentiation. It need not be a primitive melt. For instance, a series of basalt flows are assumed to be related to one another. A composition from which they could reasonably be produced by fractional crystallization is termed a ''parental melt''. Fractional crystallization models would be produced to test the hypothesis that they share a common parental melt. ===Geochemical implications of partial melting=== The degree of partial melting is critical for determining what type of magma is produced. The degree of partial melting required to form a melt can be estimated by considering the relative enrichment of incompatible elements versus compatible elements. [[Compatibility (geochemistry)|Incompatible elements]] commonly include [[potassium]], [[barium]], [[caesium]], [[rubidium]]. Rock types produced by small degrees of partial melting in the [[Earth's mantle]] are typically alkaline ([[Calcium|Ca]], [[Sodium|Na]]), potassic ([[Potassium|K]]) and/or peralkaline (high aluminium to silica ratio). Typically, primitive melts of this composition form [[lamprophyre]], [[lamproite]], [[kimberlite]] and sometimes [[nepheline]]-bearing mafic rocks such as [[alkali]] [[basalt]]s and [[essexite]] gabbros or even [[carbonatite]]. [[Pegmatite]] may be produced by low degrees of partial melting of the crust. Some [[granite]]-composition magmas are [[eutectic]] (or cotectic) melts, and they may be produced by low to high degrees of partial melting of the crust, as well as by fractional crystallization. At high degrees of partial melting of the crust, granitoids such as [[tonalite]], [[granodiorite]] and [[monzonite]] can be produced, but other mechanisms are typically important in producing them. At high degrees of partial melting of the mantle, [[komatiite]] and [[picrite]] are produced. so == Composition and melt structure and properties == Silicate melts are composed mainly of [[silicon]], [[oxygen]], [[aluminium]], alkalis ([[sodium]], [[potassium]], [[calcium]]), [[magnesium]] and [[iron]]. Silicon atoms are in tetrahedral coordination with oxygen, as in almost all [[silicate minerals]], but in melts atomic order is preserved only over short distances. The physical behaviours of melts depend upon their atomic structures as well as upon temperature and pressure and composition.<ref>E. B. Watson, M. F. Hochella, and I. Parsons (editors), ''Glasses and Melts: Linking Geochemistry and Materials Science'', [http://www.elementsmagazine.org/ ''Elements'', volume 2, number 5, (October 2006)] pages 259-297</ref> [[Viscosity]] is a key melt property in understanding the behaviour of magmas. More silica-rich melts are typically more polymerized, with more linkage of silica tetrahedra, and so are more viscous. Dissolution of water drastically reduces melt viscosity. Higher-temperature melts are less viscous. Generally speaking, more mafic magmas, such as those that form [[basalt]], are hotter and less viscous than more silica-rich magmas, such as those that form [[rhyolite]]. Low viscosity leads to gentler, less explosive eruptions. {{main|Igneous differentiation}} Characteristics of several different magma types are as follows: :[[Ultramafic]] ([[picrite|picritic]]) ::SiO<sub>2</sub> < 45% ::Fe-Mg >8% up to 32%MgO ::Temperature: up to 1500°C ::Viscosity: Very Low ::Eruptive behavior: gentle or very explosive (kimberilites) ::Distribution: divergent plate boundaries, hot spots, convergent plate boundaries; [[komatiite]] and other ultramafic lavas are mostly [[Archean]] and were formed from a higher [[geothermal gradient]] and are unknown in the present :[[Mafic]] ([[basalt]]ic) ::SiO<sub>2</sub> < 50% ::FeO and MgO typically < 10 wt% ::Temperature: up to ~1300°C ::Viscosity: Low ::Eruptive behavior: gentle ::Distribution: divergent plate boundaries, hot spots, convergent plate boundaries :Intermediate ([[andesite|andesitic]]) ::SiO<sub>2</sub> ~ 60% ::Fe-Mg: ~ 3% ::Temperature: ~1000°C ::Viscosity: Intermediate ::Eruptive behavior: explosive ::Distribution: convergent plate boundaries :[[Felsic]] (rhyolitic) ::SiO<sub>2</sub> >70% ::Fe-Mg: ~ 2% ::Temp: < 900°C ::Viscosity: High ::Eruptive behavior: explosive ::Distribution: hot spots in continental crust ([[Yellowstone National Park]]), continental rifts, island arcs ==See also== * [[Igneous differentiation]] * [[Cumulate rocks]] * [[Fractional crystallization (geology)]] * [[Igneous rocks]] * [[Lava]] * [[Intrusion]] * [[Ultramafic to mafic layered intrusions]] ==References== <references/> [[Category:Petrology]] [[Category:Volcanology]] [[Category:Igneous rocks]] [[af:Magma]] [[ar:ماغما]] [[ast:Magma]] [[bs:Magma]] [[bg:Магма (геология)]] [[ca:Magma (vulcanisme)]] [[cs:Magma]] [[da:Magma]] [[de:Magma]] [[et:Magma]] [[el:Μάγμα]] [[es:Magma]] [[eo:Magmo]] [[fa:تفتال (زمین‌شناسی)]] [[fr:Magma (géologie)]] [[fur:Magma]] [[gl:Magma]] [[ko:마그마]] [[hr:Magma]] [[id:Magma]] [[is:Bergkvika]] [[it:Magma (geologia)]] [[he:מאגמה]] [[sw:Magma]] [[lv:Magma]] [[lb:Magma]] [[lt:Magma]] [[hu:Magma]] [[ms:Magma]] [[nl:Magma (gesteente)]] [[new:माग्मा]] [[ja:マグマ]] [[no:Magma]] [[nds:Magma (Vulkan)]] [[pl:Magma]] [[pt:Magma]] [[ro:Magmă]] [[ru:Магма]] [[simple:Magma]] [[sk:Magma]] [[sl:Magma]] [[sr:Магма]] [[su:Magma]] [[fi:Magma]] [[sv:Magma]] [[ta:மக்மா]] [[th:หินหนืด]] [[vi:Macma]] [[tg:Магма]] [[tr:Magma]] [[uk:Магма]] [[zh:岩漿]]