Editing Magma (section)

=== 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>