Editing Extrusive rock (section)

{{Short description|Mode of igneous volcanic rock formation}}
[[File:Classification extrusive rocks EN.svg|thumb|upright=1.3|[[IUGS]] classification of [[aphanitic]] [[extrusive]] [[igneous rocks]]  to their relative alkali (Na<sub>2</sub>O + K<sub>2</sub>O) and silica (SiO<sub>2</sub>) weight contents. Blue area is roughly where alkaline rocks plot; yellow area where subalkaline rocks plot.

Original source:
*{{aut|Le Maitre, R.W.}} (''ed.''); '''1989''': ''A classification of igneous rocks and glossary of terms'', Blackwell Science, Oxford. ]]
[[File:Haüyne crystal.JPG|thumb|A volcanic rock from Italy with a relatively large six-sided [[phenocryst]] (diameter about 1 mm) surrounded by a fine-grained [[groundmass]], as seen in [[thin section]] under a [[petrographic microscope]]]]
'''Extrusive rock''' refers to the mode of [[igneous]] [[volcanic rock]] formation in which hot [[magma]] from inside the [[Earth]] flows out (extrudes) onto the surface as [[lava]] or explodes violently into the [[Earth's atmosphere|atmosphere]] to fall back as [[Pyroclastic rock|pyroclastic]]s or [[tuff]].<ref>{{cite web|url=https://www.britannica.com/science/extrusive-rock|title=Extrusive rock - geology|access-date=21 October 2018}}</ref> In contrast, [[intrusive rock]] refers to rocks formed by magma which cools below the surface.<ref name=":3">Jain, Sreepat (2014). ''Fundamentals of Physical Geology''. New Delhi, India: Springer. {{ISBN|9788132215394}}.</ref>

The main effect of extrusion is that the magma can cool much more quickly in the open air or under [[seawater]], and there is little time for the growth of [[crystal]]s.<ref name=":0" /> Sometimes, a residual portion of the [[matrix (geology)|matrix]] fails to [[crystallize]] at all, instead becoming a natural glass like [[obsidian]].   

If the magma contains abundant [[Volatile (astrogeology)|volatile components]] which are released as free gas, then it may cool with large or small vesicles (bubble-shaped cavities) such as in [[pumice]], [[scoria]], or [[vesicular texture|vesicular]] [[basalt]]. Other examples of extrusive rocks are [[rhyolite]] and [[andesite]].