Editing Porphyritic (section)

== Formation ==
[[Image:olearyandesite.jpg|thumb|left|[[Andesite]] porphyry from summit of [[O'Leary Peak]].  This is an [[extrusive rock|extrusive]] porphyritic rock, as the pink (and black) [[phenocrysts]] are clearly visible, in contrast to the grey groundmass with its microscopic crystals.]]

Porphyritic rocks are a product of [[igneous differentiation]], and are generally formed when a column of rising [[magma]] is cooled in two stages: In the first stage, the magma is cooled slowly deep in the crust, creating the large crystal grains, with a diameter of 2mm or more. In the final stage, the magma is cooled rapidly at relatively shallow depth or as it erupts from a [[volcano]], creating small grains that are usually invisible to the unaided eye, typically referred to as the matrix or groundmass.<ref name=":0">{{Cite journal|last=Wilson|first=Majorie|date=1993|title=Magmatic differentiation|journal=Journal of the Geological Society|location=London|volume=150|issue=4 |pages=611–624|doi=10.1144/gsjgs.150.4.0611 |bibcode=1993JGSoc.150..611W |s2cid=219542287 }}</ref>

The formation of large phenocrysts is due to [[Fractional crystallization (geology)|fractional crystallization]]. As the melt cools, it begins crystallizing the highest melting point minerals closest to the overall composition first. This forms large, well-shaped [[Euhedral and anhedral|euhedral]] phenocrysts. If these phenocrysts are different in density to the remaining melt, they usually settle out of solution, eventually creating [[Cumulate rock|cumulates]]. However, when this is interrupted by sudden eruption of the melt as lava, or when the density of the crystals and remaining melt remains similar, they become entrapped in the final rock.<ref name=":0" />

This can also occur when the [[chemical composition]] of the remaining melt is close to the [[eutectic point]] as it cools, resulting in multiple different minerals solidifying at once and filling the remaining space simultaneously, limiting their size and shape.<ref name=":0" />
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