Editing Magma (section)

==Physical and chemical properties==
Magma consists of liquid rock that usually contains suspended solid crystals.<ref name="philpotts-ague-phenocrysts">{{cite book |last1=Philpotts |first1=Anthony R. |last2=Ague |first2=Jay J. |title=Principles of igneous and metamorphic petrology |date=2009 |publisher=Cambridge University Press |location=Cambridge, UK |isbn=9780521880060 |pages=19–20 |edition=2nd}}</ref> As magma approaches the surface and the [[overburden pressure]] drops, dissolved gases bubble out of the liquid, so that magma near the surface consists of materials in solid, liquid, and gas [[State of matter|phase]]s.<ref name="schmincke-2003">{{cite book |last1=Schmincke |first1=Hans-Ulrich |title=Volcanism |date=2003 |publisher=Springer |location=Berlin |isbn=9783540436508 |pages=49–50}}</ref>

===Composition===
{{See also|Igneous differentiation}}
Most magma is rich in [[Silicon dioxide|silica]].<ref name=":0" /> Rare nonsilicate magma can form by local melting of nonsilicate mineral deposits<ref name="ChileIronOxideLava">{{cite journal | url=https://www.researchgate.net/publication/241044499 | title=Geological, Geographical and Legal Considerations for the Conservation of Unique Iron Oxide and Sulphur Flows at El Laco and Lastarria Volcanic Complexes, Central Andes, Northern Chile | author=Guijón, R. |author2=Henríquez, F. |author3=Naranjo, J.A. | journal=Geoheritage | year=2011 | volume=3 | issue=4 | pages=99–315 | doi=10.1007/s12371-011-0045-x| bibcode=2011Geohe...3..299G | s2cid=129179725 }}</ref> or by separation of a magma into separate [[immiscible]] silicate and nonsilicate liquid phases.<ref name="IronOxidelava">{{cite journal | url=https://www.researchgate.net/publication/235632335 | title=Apatite–monazite relations in the Kiirunavaara magnetite–apatite ore, northern Sweden | author=Harlov, D.E. | journal=Chemical Geology | year=2002 | volume=191 | issue=1–3 | pages=47–72 | doi=10.1016/s0009-2541(02)00148-1| bibcode=2002ChGeo.191...47H |display-authors=etal}}</ref>

Silicate magmas are molten mixtures dominated by [[oxygen]] and [[silicon]], the most abundant [[chemical element]]s in the Earth's crust, with smaller quantities of [[aluminium]], [[calcium]], [[magnesium]], [[iron]], [[sodium]], and [[potassium]], and minor amounts of many other elements.{{sfn|Philpotts|Ague|2009|pp=19, 131}} [[Petrologist]]s routinely express the composition of a silicate magma in terms of the weight or [[molar mass]] fraction of the oxides of the major elements (other than oxygen) present in the magma.{{sfn|Philpotts|Ague|2009|pp=132-133}}

Because many of the properties of a magma (such as its viscosity and temperature) are observed to correlate with silica content, silicate magmas are divided into four chemical types based on silica content: [[#Felsic magmas|''felsic'']], [[#Intermediate magmas|''intermediate'']], [[#Mafic magmas|''mafic'']], and [[#Ultramafic magmas|''ultramafic'']].<ref>{{cite book|last1=Casq|first1=R.A.F.|last2=Wright|first2=J.V.|title=Volcanic Successions|date=1987|publisher=Unwin Hyman Inc|isbn=978-0-04-552022-0|page=528}}</ref>

==== Felsic magmas ====
''Felsic'' or [[Silicon dioxide|silicic]] magmas have a silica content greater than 63%. They include [[rhyolite]] and [[dacite]] magmas. With such a high silica content, these magmas are extremely viscous, ranging from 10<sup>8</sup> [[centipoise|cP]] (10<sup>5</sup> Pa⋅s) for hot rhyolite magma at {{cvt|1200|C||}} to 10<sup>11</sup> cP (10<sup>8</sup> Pa⋅s) for cool rhyolite magma at {{cvt|800|C||}}.{{sfn|Philpotts|Ague|2009|p=23}} For comparison, water has a viscosity of about 1 cP (0.001 Pa⋅s). Because of this very high viscosity, felsic lavas usually erupt explosively to produce [[Pyroclastic rock|pyroclastic]] (fragmental) deposits. However, rhyolite lavas occasionally erupt effusively to form [[lava spine]]s, [[lava dome]]s or "coulees" (which are thick, short lava flows).{{sfn|Philpotts|Ague|2009|pp=70-77}}  The lavas typically fragment as they extrude, producing [[Lava#Block lava flows|block lava flow]]s. These often contain [[obsidian]].{{sfn|Schmincke|2003|p=132}}

Felsic lavas can erupt at temperatures as low as {{convert|800|°C}}.{{sfn|Philpotts|Ague|2009|p=20}} Unusually hot (>950&nbsp;°C; >1,740&nbsp;°F) rhyolite lavas, however, may flow for distances of many tens of kilometres, such as in the [[Snake River Plain]] of the northwestern United States.<ref>{{cite journal |last1=Bonnichsen |first1=B. |last2=Kauffman, D.F. |year=1987 |title=Physical features of rhyolite lava flows in the Snake River Plain volcanic province, southwestern Idaho |journal=Geological Society of America Special Paper |series=Geological Society of America Special Papers |volume=212 |pages=119–145|doi=10.1130/SPE212-p119 |isbn=0-8137-2212-8 }}</ref><!-- Rather an old source; are the Snake River flows still regarded as fluid flows and not ash flows?-->

==== Intermediate magmas ====
''Intermediate'' or [[Andesite|andesitic]] magmas contain 52% to 63% silica, and are lower in aluminium and usually somewhat richer in [[magnesium]] and [[iron]] than felsic magmas. Intermediate lavas form andesite domes and block lavas, and may occur on steep [[composite volcano]]es, such as in the [[Andes]].{{sfn|Schmincke|2003|pp=21-24,132,143}} They are also commonly hotter, in the range of {{convert|850|to|1100|°C}}). Because of their lower silica content and higher eruptive temperatures, they tend to be much less viscous, with a typical viscosity of 3.5 × 10<sup>6</sup> cP (3,500 Pa⋅s) at {{cvt|1200|C||}}. This is slightly greater than the viscosity of smooth [[peanut butter]].{{sfn|Philpotts|Ague|2009|pp=23-611}} Intermediate magmas show a greater tendency to form [[phenocrysts]].<ref>{{cite journal |last1=Takeuchi |first1=Shingo |title=Preeruptive magma viscosity: An important measure of magma eruptibility |journal=Journal of Geophysical Research |date=5 October 2011 |volume=116 |issue=B10|pages=B10201 |doi=10.1029/2011JB008243|bibcode=2011JGRB..11610201T |doi-access=free }}</ref> Higher iron and magnesium tends to manifest as a darker [[groundmass]], including amphibole or pyroxene phenocrysts.{{sfn|Philpotts|Ague|2009|pp=1376-377}}

==== Mafic magmas ====
''Mafic'' or [[basalt]]ic magmas have a silica content of 52% to 45%. They are typified by their high ferromagnesian content, and generally erupt at temperatures of {{convert|1100 to 1200|°C}}. Viscosities can be relatively low, around 10<sup>4</sup> to 10<sup>5</sup> cP (10 to 100 Pa⋅s), although this is still many orders of magnitude higher than water. This viscosity is similar to that of [[ketchup]].{{sfn|Philpotts|Ague|2009|pp=23-25}} Basalt lavas tend to produce low-profile [[shield volcano]]es or [[flood basalt]]s, because the fluidal lava flows for long distances from the vent. The thickness of a basalt lava, particularly on a low slope, may be much greater than the thickness of the moving lava flow at any one time, because basalt lavas may "inflate" by supply of lava beneath a solidified crust.{{sfn|Philpotts|Ague|2009|p=53-55, 59-64}} Most basalt lavas are of ''[[ʻAʻā]]'' or ''[[pāhoehoe]]'' types, rather than block lavas. Underwater, they can form [[pillow lavas]], which are rather similar to entrail-type pahoehoe lavas on land.{{sfn|Schmincke|2003|pp=128-132}}

==== Ultramafic magmas ====
''Ultramafic'' magmas, such as [[picrite|picritic]] basalt, [[komatiite]], and highly magnesian magmas that form [[boninite]], take the composition and temperatures to the extreme. All have a silica content under 45%. Komatiites contain over 18% magnesium oxide, and are thought to have erupted at temperatures of {{convert|1600|°C}}. At this temperature there is practically no polymerization of the mineral compounds, creating a highly mobile liquid.<ref name="Condie1994">{{cite book | title=Archean Crustal Evolution | publisher=Elsevier | location=Amsterdam | editor=Condie, K.C.| year=1994 | chapter=Archean komatiites | author=Arndt, N.T. | page=19 | isbn=978-0-444-81621-4}}</ref> Viscosities of komatiite magmas are thought to have been as low as 100 to 1000 cP (0.1 to 1 Pa⋅s), similar to that of light motor oil.{{sfn|Philpotts|Ague|2009|p=23}} Most ultramafic lavas are no younger than the [[Proterozoic]], with a few ultramafic magmas known from the [[Phanerozoic]] in Central America that are attributed to a hot [[mantle plume]]. No modern komatiite lavas are known, as the Earth's mantle has cooled too much to produce highly magnesian magmas.{{sfn|Philpotts|Ague|2009|pp=399-400}}

==== Alkaline magmas ====
{{See also|Alkaline magma series}}
Some silicic magmas have an elevated content of [[alkali metal oxide]]s (sodium and potassium), particularly in regions of [[continental rifting]], areas overlying deeply [[subduction|subducted]] [[Tectonic plate|plates]], or at intraplate [[Hotspot (geology)|hotspots]].{{sfn|Philpotts|Ague|2009|pp=139-148}} Their silica content can range from ultramafic ([[nephelinite]]s, [[basanite]]s and [[tephrite]]s) to felsic ([[trachyte]]s). They are more likely to be generated at greater depths in the mantle than subalkaline magmas.{{sfn|Philpotts|Ague|2009|pp=606-607}} Olivine [[nephelinite]] magmas are both ultramafic and highly alkaline, and are thought to have come from much deeper in the [[mantle (geology)|mantle]] of the [[Earth]] than other magmas.<ref>{{cite web|url=http://gsc.nrcan.gc.ca/volcanoes/cat/feature_volcano_e.php|work=Catalogue of Canadian volcanoes|title=Stikine Volcanic Belt: Volcano Mountain|archive-url=https://web.archive.org/web/20090307140121/http://gsc.nrcan.gc.ca/volcanoes/cat/feature_volcano_e.php |archive-date=2009-03-07|access-date=23 November 2007}}</ref>
{| style="border:0px solid black;"
|+
!valign="top" |
{| class="wikitable" style="text-align: left;"
|+Examples of magma compositions (wt%){{sfn|Philpotts|Ague|2009|p=145}}
! Component
! Nephelinite
! Tholeiitic picrite
! Tholeiitic basalt
! Andesite
! Rhyolite
|-
| Silica |SiO<sub>2</sub>
| 39.7
| 46.4
| 53.8
| 60.0
| 73.2
|-
| TiO<sub>2</sub>
| 2.8
| 2.0
| 2.0
| 1.0
| 0.2
|-
| Al<sub>2</sub>O<sub>3</sub>
| 11.4
| 8.5
| 13.9
| 16.0
| 14.0
|- 
| Fe<sub>2</sub>O<sub>3</sub>
| 5.3
| 2.5
| 2.6
| 1.9
| 0.6
|- 
| FeO
| 8.2
| 9.8
| 9.3
| 6.2
| 1.7
|- 
| MnO
| 0.2
| 0.2
| 0.2
| 0.2
| 0.0
|- 
| MgO
| 12.1
| 20.8
| 4.1
| 3.9
| 0.4
|- 
| CaO
| 12.8
| 7.4
| 7.9
| 5.9
| 1.3
|- 
| Na<sub>2</sub>O
| 3.8
| 1.6
| 3.0
| 3.9
| 3.9
|-
| K<sub>2</sub>O
| 1.2
| 0.3
| 1.5
| 0.9
| 4.1
|-
| P<sub>2</sub>O<sub>5</sub>
| 0.9
| 0.2
| 0.4
| 0.2
| 0.0
|}
! valign="top" | {{Pie chart
| radius = 100
| thumb = 
| caption = Tholeiitic basalt magma
| other = 
| label1 = SiO<sub>2</sub> 
| value1 = 53.8
| label2 = Al<sub>2</sub>O<sub>3</sub>
| value2 =13.9
| label3 =FeO
| value3 =9.3
| label4 =CaO
| value4 =7.9
| label5 =MgO
| value5 =4.1
| label6 =Na<sub>2</sub>O
| value6 =3.0
| label7 = Fe<sub>2</sub>O<sub>3</sub>
| value7=2.6
| label8 = TiO<sub>2</sub>
| value8=2.0
| label9 = K<sub>2</sub>O
| value9 = 1.5
| label10 = P<sub>2</sub>O<sub>5</sub>
| value10 = 0.4
| label11 = MnO
| value11 = 0.2
}}
! valign="top" |{{Pie chart
| radius = 100
| thumb = 
| caption = Rhyolite magma
| other = 
| label1 = SiO<sub>2</sub> 
| value1 = 73.2
| label2 = Al<sub>2</sub>O<sub>3</sub>
| value2 = 14
| label9 =K<sub>2</sub>O
| value9 = 4.1
| label6 =Na<sub>2</sub>O
| value6 = 3.9
| label3 =FeO
| value3 = 1.7
| label4 = CaO
| value4 = 1.3
| label7 = Fe<sub>2</sub>O<sub>3</sub>
| value7= 0.6
| label5 = MgO
| value5= 0.4
| label8 = TiO<sub>2</sub>
| value8 = 0.2
| label10 = P<sub>2</sub>O<sub>5</sub>
| value10 = 0.
| label11 = MnO
| value11 = 0.
}}
|}

==== Non-silicate magmas ====
Some lavas of unusual composition have erupted onto the surface of the Earth. These include:
* [[Carbonatite]] and [[natrocarbonatite]] lavas are known from [[Ol Doinyo Lengai]] volcano in [[Tanzania]], which is the sole example of an active carbonatite volcano.<ref>[http://www.geology.sdsu.edu/how_volcanoes_work/Unusual%20lava.html Vic Camp, ''How volcanoes work'', Unusual Lava Types] {{Webarchive|url=https://web.archive.org/web/20171023185741/http://www.geology.sdsu.edu/how_volcanoes_work/Unusual%20lava.html |date=2017-10-23 }}, [[San Diego State University]], Geology</ref> Carbonatites in the geologic record are typically 75% carbonate minerals, with lesser amounts of silica-undersaturated silicate minerals (such as [[mica]]s and olivine), [[apatite]], [[magnetite]], and [[pyrochlore]]. This may not reflect the original composition of the lava, which may have included [[sodium carbonate]] that was subsequently removed by hydrothermal activity, though laboratory experiments show that a calcite-rich magma is possible. Carbonatite lavas show [[stable isotope ratio]]s indicating they are derived from the highly alkaline silicic lavas with which they are always associated, probably by separation of an immiscible phase.{{sfn|Philpotts|Ague|2009|pp=396-397}} Natrocarbonatite lavas of Ol Doinyo Lengai are composed mostly of sodium carbonate, with about half as much calcium carbonate and half again as much potassium carbonate, and minor amounts of halides, fluorides, and sulphates. The lavas are extremely fluid, with viscosities only slightly greater than water, and are very cool, with measured temperatures of {{cvt|491|to|544|C}}.<ref>{{cite journal |last1=Keller |first1=Jörg |last2=Krafft |first2=Maurice |title=Effusive natrocarbonatite activity of Oldoinyo Lengai, June 1988 |journal=Bulletin of Volcanology |date=November 1990 |volume=52 |issue=8 |pages=629–645 |doi=10.1007/BF00301213|bibcode=1990BVol...52..629K |s2cid=129106033 }}</ref>
* [[Iron oxide]] magmas are thought to be the source of the [[iron ore]] at [[Kiruna]], [[Sweden]] which formed during the [[Proterozoic]].<ref name="IronOxidelava"/> Iron oxide lavas of [[Pliocene]] age occur at the [[El Laco (volcano)|El Laco]] volcanic complex on the Chile-Argentina border.<ref name="ChileIronOxideLava"/> Iron oxide lavas are thought to be the result of [[Miscibility|immiscible]] separation of iron oxide magma from a parental magma of [[Calc-alkaline magma series|calc-alkaline]] or alkaline composition.<ref name="IronOxidelava"/> When erupted, the temperature of the molten iron oxide magma is about {{cvt|700|to|800|C}}.<ref name="Jonsson_etal_2013">{{cite journal | title=Magmatic origin of giant 'Kiruna-type' apatite-iron-oxide ores in Central Sweden | first1=E. | last1=Jonsson | first2=V.R. | last2=Troll | first3=K. | last3=Högdahl | first4=C. | last4=Harris | first5=F. | last5=Weis | first6=K.P. | last6=Nilsson | first7=A. | last7=Skelton | journal=Scientific Reports | date=2013 | volume=3 | page=1644 | doi=10.1038/srep01644| pmid=23571605 | pmc=3622134 | bibcode=2013NatSR...3.1644J }}</ref>
* [[Sulfur]] lava flows up to {{convert|250|m|0|abbr=off}} long and {{convert|10|m|0|abbr=off}} wide occur at [[Lastarria]] volcano, Chile. They were formed by the melting of sulfur deposits at temperatures as low as {{convert|113|°C}}.<ref name="ChileIronOxideLava"/>

=== Magmatic gases ===
{{Main|Volcanic gas}}
The concentrations of different [[volcanic gas|gas]]es can vary considerably. [[Water vapor]] is typically the most abundant magmatic gas, followed by [[carbon dioxide]]<ref>{{cite journal
| last1 = Pedone  | first1 = M. | last2 = Aiuppa | first2 = A.| last3 = Giudice | first3 = G. | last4 = Grassa | first4 = F.| last5 = Francofonte | first5 = V. | last6 = Bergsson | first6 = B.| last7 = Ilyinskaya | first7 = E.| title = Tunable diode laser measurements of hydrothermal/volcanic CO2 and implications for the global CO2 budget| journal = Solid Earth | date = 2014| volume = 5 | issue = 2 | pages = 1209–1221| doi = 10.5194/se-5-1209-2014| bibcode = 2014SolE....5.1209P | doi-access = free }}</ref> and [[sulfur dioxide]]. Other principal magmatic gases include [[hydrogen sulfide]], [[hydrogen chloride]], and [[hydrogen fluoride]].{{sfn|Schmincke|2003|p=42}}

The solubility of magmatic gases in magma depends on pressure, magma composition, and temperature. Magma that is extruded as lava is extremely dry, but magma at depth and under great pressure can contain a dissolved water content in excess of 10%. Water is somewhat less soluble in low-silica magma than high-silica magma, so that at 1,100&nbsp;°C and 0.5 [[GPa]], a basaltic magma can dissolve 8% {{chem2|H2O}} while a granite pegmatite magma can dissolve 11% {{chem2|H2O}}.{{sfn|Philpotts|Ague|2009|pp=244-250}} However, magmas are not necessarily saturated under typical conditions.

{| style="border:0px solid black; text-align: left;"
|+
!valign="top" |
{| class="wikitable"
|+Water concentrations in magmas (wt%){{sfn|Schmincke|2003|p=44}}
! Magma composition
! {{chem2|H2O}} concentration<br/>wt %
|-
| [[MORB]] ([[tholeiite]]s)
| style="text-align: center;" |  0.1 &ndash; 0.2
|-
| Island tholeiite
|style="text-align: center;" |   0.3 &ndash; 0.6
|-
| Alkali basalts
|style="text-align: center;" |   0.8 &ndash; 1.5
|- 
| Volcanic arc basalts
| style="text-align: center;" |  2&ndash;4
|- 
| [[Basanite]]s and [[nephelinite]]s
|style="text-align: center;" |   1.5&ndash;2
|- 
| Island arc andesites and dacites
|style="text-align: center;" |   1&ndash;3
|- 
| Continental margin andesites and dacites
|style="text-align: center;" |   2&ndash;5
|- 
| Rhyolites
| style="text-align: center;" |  up to 7
|}
|}

Carbon dioxide is much less soluble in magmas than water, and frequently separates into a distinct fluid phase even at great depth. This explains the presence of carbon dioxide fluid inclusions in crystals formed in magmas at great depth.{{sfn|Schmincke|2003|p=44}}

=== Rheology ===
[[File:Viscosity of magma EN.svg|thumb|Graph showing [[logarithm]]ic variation of magma viscosity (η) with silica content for three temperatures]]
[[Viscosity]] is a key melt property in understanding the behaviour of magmas. Whereas temperatures in common silicate lavas range from about {{cvt|800|C}} for felsic lavas to {{cvt|1200|C}} for mafic lavas,{{sfn|Philpotts|Ague|2009|p=20}} the viscosity of the same lavas ranges over seven orders of magnitude, from 10<sup>4</sup> cP (10 Pa⋅s) for mafic lava to 10<sup>11</sup> cP (10<sup>8</sup> Pa⋅s) for felsic magmas.{{sfn|Philpotts|Ague|2009|p=20}} The viscosity is mostly determined by composition but is also dependent on temperature.{{sfn|Philpotts|Ague|2009|p=23}} The tendency of felsic lava to be cooler than mafic lava increases the viscosity difference.

The silicon ion is small and highly charged, and so it has a strong tendency to [[Coordination (chemistry)|coordinate]] with four oxygen ions, which form a tetrahedral arrangement around the much smaller silicon ion. This is called a ''silica tetrahedron''. In a magma that is low in silicon, these silica tetrahedra are isolated, but as the silicon content increases, silica tetrahedra begin to partially polymerize, forming chains, sheets, and clumps of silica tetrahedra linked by bridging oxygen ions. These greatly increase the viscosity of the magma.{{sfn|Schmincke|2003|pp=38-41}}

<gallery>
File:Single tet.png|A single silica tetrahedron
File:Double tet.png|Two silica tetrahedra joined by a bridging oxygen ion (tinted pink)
</gallery>

The tendency towards polymerization is expressed as NBO/T, where NBO is the number of non-bridging oxygen ions and T is the number of network-forming ions. Silicon is the main network-forming ion, but in magmas high in sodium, aluminium also acts as a network former, and ferric iron can act as a network former when other network formers are lacking. Most other metallic ions reduce the tendency to polymerize and are described as network modifiers. In a hypothetical magma formed entirely from melted silica, NBO/T would be 0, while in a hypothetical magma so low in network formers that no polymerization takes place, NBO/T would be 4. Neither extreme is common in nature, but basalt magmas typically have NBO/T between 0.6 and 0.9, andesitic magmas have NBO/T of 0.3 to 0.5, and rhyolitic magmas have NBO/T of 0.02 to 0.2. Water acts as a network modifier, and dissolved water drastically reduces melt viscosity. Carbon dioxide neutralizes network modifiers, so dissolved carbon dioxide increases the viscosity. Higher-temperature melts are less viscous, since more thermal energy is available to break bonds between oxygen and network formers.<ref name="schmincke-2003"/>

Most magmas contain solid crystals of various minerals, fragments of exotic rocks known as [[xenolith]]s and fragments of previously solidified magma. The crystal content of most magmas gives them [[thixotropy|thixotropic]] and [[shear thinning]] properties.<ref>{{cite journal |title=Transient phenomena in vesicular lava flows based on laboratory experiments with analogue materials |journal=Journal of Volcanology and Geothermal Research|volume=132 |issue=2–3 |pages=115–136 |doi=10.1016/s0377-0273(03)00341-x  |last1=Pinkerton |first1=H. |first2=N. |last2=Bagdassarov |year=2004 |bibcode=2004JVGR..132..115B}}</ref> In other words, most magmas do not behave like Newtonian fluids, in which the rate of flow is proportional to the [[shear stress]]. Instead, a typical magma is a [[Bingham fluid]], which shows considerable resistance to flow until a stress threshold, called the yield stress, is crossed.{{sfn|Schmincke|2003|pp=39-40}} This results in [[plug flow]] of partially crystalline magma. A familiar example of plug flow is toothpaste squeezed out of a toothpaste tube. The toothpaste comes out as a semisolid plug, because shear is concentrated in a thin layer in the toothpaste next to the tube, and only here does the toothpaste behave as a fluid. Thixotropic behavior also hinders crystals from settling out of the magma.{{sfn|Philpotts|Ague|2009|p=40}} Once the crystal content reaches about 60%, the magma ceases to behave like a fluid and begins to behave like a solid. Such a mixture of crystals with melted rock is sometimes described as ''crystal mush''.{{sfn|Philpotts|Ague|2009|p=16}}

Magma is typically also [[Viscoelasticity|viscoelastic]], meaning it flows like a liquid under low stresses, but once the applied stress exceeds a critical value, the melt cannot dissipate the stress fast enough through relaxation alone, resulting in transient fracture propagation. Once stresses are reduced below the critical threshold, the melt viscously relaxes once more and heals the fracture.<ref>{{Cite journal|last1=Wadsworth|first1=Fabian B.|last2=Witcher|first2=Taylor|last3=Vossen|first3=Caron E. J.|last4=Hess|first4=Kai-Uwe|last5=Unwin|first5=Holly E.|last6=Scheu|first6=Bettina|last7=Castro|first7=Jonathan M.|last8=Dingwell|first8=Donald B.|date=December 2018|title=Combined effusive-explosive silicic volcanism straddles the multiphase viscous-to-brittle transition|journal=Nature Communications|volume=9|issue=1|pages=4696|doi=10.1038/s41467-018-07187-w|issn=2041-1723|pmc=6224499|pmid=30409969|bibcode=2018NatCo...9.4696W}}</ref>

=== Temperature ===
Temperatures of molten lava, which is magma extruded onto the surface, are almost all in the range {{convert|700|to|1400|C|F|sigfig=2}}, but very rare [[carbonatite]] magmas may be as cool as {{convert|490|C|F|sigfig=2}},<ref name="Weidendorfer2017">{{cite journal|last1=Weidendorfer|first1=D.|last2=Schmidt|first2=M.W.|last3=Mattsson|first3=H.B.|year=2017|title=A common origin of carbonatite magmas|journal=Geology|volume=45|issue=6|pages=507–510|doi=10.1130/G38801.1|bibcode=2017Geo....45..507W|doi-access=free|hdl=20.500.11850/190852|hdl-access=free}}</ref> and [[komatiite]] magmas may have been as hot as {{convert|1600|C|F|sigfig=2}}.<ref>{{Cite journal|last1=Herzberg|first1=C.|last2=Asimow|first2=P. D.|last3=Arndt|first3=N.|last4=Niu|first4=Y.|last5=Lesher|first5=C. M.|last6=Fitton|first6=J. G.|last7=Cheadle|first7=M. J.|last8=Saunders|first8=A. D.|date=2007|title=Temperatures in ambient mantle and plumes: Constraints from basalts, picrites, and komatiites|journal=Geochemistry, Geophysics, Geosystems|volume=8|issue=2|pages=n/a|doi=10.1029/2006gc001390|bibcode=2007GGG.....8.2006H|hdl=20.500.11919/1080|s2cid=14145886 |issn=1525-2027|url=http://repository.uwyo.edu/cgi/viewcontent.cgi?article=1003&context=geology_facpub|hdl-access=free|access-date=2019-12-07|archive-date=2019-04-27|archive-url=https://web.archive.org/web/20190427092642/https://repository.uwyo.edu/cgi/viewcontent.cgi?article=1003&context=geology_facpub|url-status=dead}}</ref> Magma has occasionally been encountered during drilling in geothermal fields, including drilling in Hawaii that penetrated a dacitic magma body at a depth of {{cvt|2488|m}}. The temperature of this magma was estimated at {{convert|1050|C|F|sigfig=3}}. Temperatures of deeper magmas must be inferred from theoretical computations and the geothermal gradient.<ref name="hawaii-wellfield-dacite"/>

Most magmas contain some solid crystals suspended in the liquid phase. This indicates that the temperature of the magma lies between the [[solidus (chemistry)|solidus]], which is defined as the temperature at which the magma completely solidifies, and the [[liquidus]], defined as the temperature at which the magma is completely liquid.<ref name="philpotts-ague-phenocrysts"/> Calculations of solidus temperatures at likely depths suggests that magma generated beneath areas of rifting starts at a temperature of about {{convert|1300|to|1500|C|F|sigfig=2}}. Magma generated from mantle plumes may be as hot as {{convert|1600|C|F|sigfig=2}}. The temperature of magma generated in subduction zones, where water vapor lowers the melting temperature, may be as low as {{convert|1060|C|F|sigfig=3}}.{{sfn|Philpotts|Ague|2009|pp=593-597}}

=== Density ===
Magma densities depend mostly on composition, iron content being the most important parameter.<ref name="mg">[http://www.usu.edu/geo/shervais/G4500_PDF/45Week1B_Magmas.pdf usu.edu - ''Geology'' 326, "Properties of Magmas"], 2005-02-11</ref>
{| class="wikitable"
! Type !! Density (kg/m{{sup|3}})
|-
|[[Basalt]]ic magma || 2650–2800
|-
|[[Andesite|Andesitic]] magma || 2450–2500
|-
|[[Rhyolite|Rhyolitic]] magma || 2180–2250
|}

Magma expands slightly at lower pressure or higher temperature.<ref name="mg" /> When magma approaches the surface, its dissolved gases begin to bubble out of the liquid. These bubbles had significantly reduced the density of the magma at depth and helped drive it toward the surface in the first place.{{sfn|Schmincke|2003|p=50}}