Editing Granite (section)

==Ascent and emplacement==
Granite magmas have a density of 2.4 Mg/m<sup>3</sup>, much less than the 2.8 Mg/m<sup>3</sup> of high-grade metamorphic rock. This gives them tremendous buoyancy, so that ascent of the magma is inevitable once enough magma has accumulated. However, the question of precisely how such large quantities of magma are able to shove aside [[country rock (geology)|country rock]] to make room for themselves (the ''room problem'') is still a matter of research.{{sfn|Philpotts|Ague|2009|p=80}}

Two main mechanisms are thought to be important:
* Stokes [[diapir]]
* [[Fracture (geology)|Fracture propagation]]
[[File:Magma ascent mechanism.png|thumb|300px|Schematic diagram illustrating the ascent and emplacement of magmas]]
Of these two mechanisms, Stokes diapirism has been favoured for many years in the absence of a reasonable alternative. The basic idea is that magma will rise through the crust as a single mass through [[buoyancy]]. As it rises, it heats the [[Country rock (geology)|wall rocks]], causing them to behave as a [[power-law fluid]] and thus flow around the [[intrusion]] allowing it to pass without major heat loss.<ref>{{Cite journal | doi = 10.1029/93JB03461| title = Diapiric ascent of magmas through power law crust and mantle| journal = Journal of Geophysical Research| volume = 99| issue = B5| page = 9543| year = 1994| last1 = Weinberg | first1 = R. F. | last2 = Podladchikov | first2 = Y. | s2cid = 19470906| bibcode=1994JGR....99.9543W}}</ref> This is entirely feasible in the warm, [[ductility|ductile]] lower crust where rocks are easily deformed, but runs into problems in the upper crust which is far colder and more brittle. Rocks there do not deform so easily: for magma to rise as a diapir it would expend far too much energy in heating wall rocks, thus cooling and solidifying before reaching higher levels within the crust.

[[Fracture (geology)|Fracture]] propagation is the mechanism preferred by many geologists as it largely eliminates the major problems of moving a huge mass of magma through cold brittle crust. Magma rises instead in small channels along self-propagating [[Dike (geology)|dykes]] which form along new or pre-existing fracture or [[fault (geology)|fault]] systems and networks of active shear zones.<ref>{{cite journal|title=Observations on the origins and ascent mechanisms of granitic magmas|journal=Journal of the Geological Society of London|year=1998|first=John|last=Clemens|volume=155|issue=Part 5|pages=843–51 | doi = 10.1144/gsjgs.155.5.0843 |url=http://eprints.kingston.ac.uk/2904/|bibcode=1998JGSoc.155..843C|s2cid=129958999}}</ref> As these narrow conduits open, the first magma to enter solidifies and provides a form of insulation for later magma.

These mechanisms can operate in tandem. For example, diapirs may continue to rise through the brittle upper crust through [[Stoping (geology)|stoping]], where the granite cracks the roof rocks, removing blocks of the overlying crust which then sink to the bottom of the diapir while the magma rises to take their place. This can occur as piecemeal stopping (stoping of small blocks of chamber roof), as cauldron subsidence (collapse of large blocks of chamber roof), or as roof foundering (complete collapse of the roof of a shallow magma chamber accompanied by a [[caldera]] eruption.) There is evidence for cauldron subsidence at the Mt. Ascutney intrusion in eastern Vermont.{{sfn|Blatt|Tracy|1996|pp=21–22}} Evidence for piecemeal stoping is found in intrusions that are rimmed with ''igneous breccia'' containing fragments of country rock.{{sfn|Philpotts|Ague|2009|p=80}}

Assimilation is another mechanism of ascent, where the granite melts its way up into the crust and removes overlying material in this way. This is limited by the amount of thermal energy available, which must be replenished by crystallization of higher-melting minerals in the magma. Thus, the magma is melting crustal rock at its roof while simultaneously crystallizing at its base. This results in steady contamination with crustal material as the magma rises. This may not be evident in the major and minor element chemistry, since the minerals most likely to crystallize at the base of the chamber are the same ones that would crystallize anyway, but crustal assimilation is detectable in isotope ratios.{{sfn|Philpotts|Ague|2009|pp=347–350}} Heat loss to the country rock means that ascent by assimilation is limited to distance similar to the height of the magma chamber.<ref>{{cite journal |title=Physical constraints on magma contamination in the continental crust: an example, the Adamello complex |journal=Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences |date=27 April 1984 |volume=310 |issue=1514 |pages=457–472 |doi=10.1098/rsta.1984.0004|bibcode=1984RSPTA.310..457O |last1=Oxburgh |first1=E. R. |last2=McRae |first2=Tessa |s2cid=120776326 }}</ref>