View source for Extrusive rock

{{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]].

== Texture ==
The texture of extrusive rocks is characterized by fine-grained crystals indistinguishable to the human eye, described as [[Aphanite|aphantic]]. Crystals in aphantic rocks are small in size due to their rapid formation during eruption.<ref name=":0">{{Cite book|title=An Introduction to Igneous and Metamorphic Petrology|last=Winter|first=John DuNann|publisher=Prentice-Hall|year=2001|isbn=0132403420|location=Upper Saddle River, New Jersey}}</ref>  Any larger crystals visible to the human eye, called [[phenocryst]]s, form earlier while slowly cooling in the magma reservoir.<ref>{{Cite book|title=Volcanism|last=Schmincke|first=Hans-Ulrich|publisher=Springer-Verlag|year=2004|isbn=3540436502|location=New York City, New York}}</ref> When igneous rocks contain two distinct grain sizes, the texture is [[porphyritic]], and the finer crystals are called the [[Matrix (geology)|groundmass]].<ref name=":0" /> The extrusive rocks scoria and pumice have a vesicular, bubble-like, texture due to the presence of vapor bubbles trapped in the magma.<ref name=":1">{{Cite book|title=Practical Volcanology: Lecture Notes for Understanding Volcanic Rocks from Field Based Studies|last=Németh, Károly|first=Martin, Ulrike|publisher=Geological Institute of Hungary|year=2007}}</ref>

== Extrusive bodies and rock types ==
[[Shield volcano]]es are large, slow forming volcanoes<ref name=":2">{{Cite book|title=Petrology : Principles and Practice|last=Sen|first=Gautam|publisher=Springer|year=2014|isbn=9783642388002|location=Berlin|oclc=864593152}}</ref> that erupt fluid basaltic magma that cools to form the extrusive rock [[basalt]]. Basalt is composed of minerals readily available in the planet's crust, including [[feldspar]]s and [[pyroxene]]s.<ref name=":3" />

Fissure volcanoes pour out low viscosity basaltic magma from [[fissure vent]]s to form the extrusive rock basalt.<ref name=":3" />

Composite or [[Stratovolcano|stratovolcanoes]] often have andesitic magma and typically form the extrusive rock andesite. Andesitic magma is composed of many gases and melted [[Mantle (geology)|mantle]] rocks.<ref name=":3" />

[[Cinder cone|Cinder]] or scoria cones violently expel lava with high gas content,<ref name=":3" /> and due to the vapor bubbles in this [[mafic]] lava, the extrusive basalt scoria is formed.<ref name=":2" />

[[Lava dome]]s are formed by high viscosity lava that piles up, forming a dome shape. Domes typically solidify to form the rich in silica extrusive rock [[obsidian]] and sometimes [[dacite]] domes form the extrusive rock dacite, like in the case of [[Mount St. Helens]].<ref name=":3" />

[[Caldera]]s are volcanic depressions formed after an erupted volcano collapses. Resurgent calderas can refill with an eruption of rhyolitic magma to form the extrusive rock rhyolite like the [[Yellowstone Caldera]].<ref name=":3" />

[[Submarine volcano]]es erupt on the ocean floor and produce the extrusive rock pumice.<ref name=":3" /> Pumice is a light-weight glass with a vesicular texture that differs from scoria in its silicic composition and therefore floats.<ref name=":1" />

==See also==

* [[Intrusive suite]]
* [[Seamount]]

==References==
<references/>

==External links==
{{wikt|extrusive}}
*[http://www.pitt.edu/~cejones/GeoImages/2IgneousRocks/IgneousTextures.html Igneous and volcanic textures - images]
*[http://www.pitt.edu/~cejones/GeoImages/2IgneousRocks/IgneousTextures/7VesicularAmygdaloidal.html Vesicular and Amygdaloidal Textures]

{{Geologic Principles}}

[[Category:Igneous petrology]] 
[[Category:Igneous rocks]]


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