Editing Pillow lava

{{Short description|Lava containing characteristic pillow-shaped structures due to subaqueous extrusion}}
[[Image:Nur05018-Pillow lavas off Hawaii.jpg|thumb|Pillow lava on the ocean floor of Hawaii]]

'''Pillow lavas''' are [[lava]]s that contain characteristic pillow-shaped structures that are attributed to the extrusion of the lava underwater, or ''subaqueous extrusion''. Pillow lavas in [[volcanic rock]] are characterized by thick sequences of discontinuous pillow-shaped masses, commonly up to one meter in diameter. They form the upper part of [[Oceanic_crust#Composition|Layer 2 of normal oceanic crust]].

==Composition==
[[File:Pillow lava Boatman's Harbour (Oamaru, New Zealand).jpg|thumb|Pillow lava at Boatman's Harbour. Oamaru, New Zealand. ]]
Pillow lavas are commonly of [[basalt]]ic composition, although pillows formed of [[komatiite]], [[picrite]], [[boninite]], [[basaltic andesite]], [[andesite]], [[dacite]] or even [[rhyolite]] are known.<ref>{{cite web|url=http://web.wits.ac.za/NR/rdonlyres/47A7F879-4CC4-43ED-B69A-CC12ABDBDD8A/0/058_091_Chapter3.pdf |title=McCarthy, T. & Rubidge, B. 2008. The story of earth and life, Chapter 3, The first continent. 60-91, Struik Publishers |publisher=Web.wits.ac.za |access-date=2014-03-10 |url-status=dead |archive-url=https://web.archive.org/web/20090407121522/http://web.wits.ac.za/NR/rdonlyres/47A7F879-4CC4-43ED-B69A-CC12ABDBDD8A/0/058_091_Chapter3.pdf |archive-date=2009-04-07 }}</ref><ref name="FangNiu2003">{{cite journal | title=Late Palaeozoic Ultramafic Lavas in Yunnan, SW China | last1 =Fang | first1=N. | last2=Niu | first2=Y. | journal=Journal of Petrology | year=2003 | volume=44 | issue=1 | pages=141–158 | doi=10.1093/petrology/44.1.141| bibcode=2003JPet...44..141F | doi-access=free }}</ref><ref>{{cite journal|title=Kuroda, N., Shiraki, K. & Urano, H. 1988. Ferropigeonite quartz dacites from Chichi-jima, Bonin Islands: Latest differentiates from boninite-forming magma |date=1988-10-01 |doi=10.1007/BF00373580 |bibcode=1988CoMP..100..129K |volume=100 |issue=2 |journal=Contributions to Mineralogy and Petrology |pages=129–138|last1=Kuroda |first1=N. |last2=Shiraki |first2=K. |last3=Urano |first3=H. |s2cid=128613176 }}</ref><ref>{{cite journal|title=Walker, G.P.L. 1992. Morphometric study of pillow-size spectrum among pillow lavas |date=1992-08-01 |doi=10.1007/BF00301392 |bibcode=1992BVol...54..459W |volume=54 |issue=6 |journal=Bulletin of Volcanology |pages=459–474|last1=Walker |first1=George P L. |s2cid=129797887 }}</ref><ref>{{cite book|last1=Harmon|first1=Russel S.|last2=Rapela|first2=Carlos W.|title=Andean Magmatism and Its Tectonic Setting|year=1991|page=24|publisher=[[Geological Society of America]]|isbn=978-0-8137-2265-8}}</ref> In general, the more [[felsic]] the composition (richer in silica - resulting in an [[Intermediate composition]]), the larger the pillows, due to the increase in [[viscosity]] of the erupting lava.

==Occurrence==
They occur wherever lava is extruded underwater, such as along marine [[hotspot (geology)|hotspot]] [[volcano]] chains and the [[Divergent boundary|constructive plate boundaries]] of [[mid-ocean ridge]]s. As new oceanic crust is formed, thick sequences of pillow lavas are erupted at the [[spreading center]] fed by [[dike (geology)|dykes]] from the underlying [[magma chamber]]. Pillow lavas and the related [[sheeted dyke complex]]es form part of a classic [[ophiolite]] sequence (when a segment of oceanic crust is [[obduction|thrust over]] the [[continental crust]], thus exposing the oceanic segment above sea level).

The presence of pillow lavas in the oldest preserved volcanic sequences on the planet, the [[Isua greenstone belt|Isua]] and [[Barberton belt|Barberton]] [[greenstone belt]]s, confirms the presence of large bodies of water on the Earth's surface early in the [[Archean]] Eon. Pillow lavas are used generally to confirm subaqueous volcanism in [[metamorphism|metamorphic]] belts.

Pillow lavas are also found associated with some [[subglacial volcano]]es at an early stage of an eruption.<ref>[http://www.norvol.hi.is/html/geol/intro/introduction.pdf Geology and geodynamics of Iceland, R.G. Trønnes, Nordic volcanological Institute, University of Iceland]</ref><ref>{{cite web|url=https://www.sciencedaily.com/releases/2010/04/100426092807.htm |title=Scientists Study 'Glaciovolcanoes,' Mountains of Fire and Ice, in Iceland, British Columbia, US ScienceDaily, Apr. 23, 2010 |publisher=Sciencedaily.com |access-date=2014-03-10}}</ref>

==Formation==
They are created when [[magma]] reaches the surface but, as there is a large difference in temperature between the lava and the water, the surface of the emergent tongue cools very quickly, forming a skin. The tongue continues to lengthen and inflate with more lava, forming a lobe, until the pressure of the magma becomes sufficient to rupture the skin and start the formation of a new eruption point nearer the vent. This process produces a series of interconnecting lobate shapes that are pillow-like in cross-section.<ref>[https://books.google.com/books?id=4LswmjBnlJMC&dq=volcanoes+and+the+environment+pillow+lava+formation&pg=PA30 2005. Volcanoes and the environment by Joan Martí, Gerald Ernst, Cambridge University Press, 488 pp.]</ref> The skin cools much faster than the inside of the pillow, so it is very fine-grained, with a glassy texture. The magma inside the pillow cools slowly, so it is slightly coarser-grained than the skin, but it is still classified as [[fine grained]].

==Use as a way-up criterion==
Pillow lavas can be used as a [[Way up structure|way-up]] indicator in geology;<ref>{{cite web|author=H. Furnes and F. J. Skjerlie |url=http://geolmag.geoscienceworld.org/cgi/content/abstract/109/4/315 |title=Furnes, H. & Skjerlie, F.J. 1972. The significance of primary structures in the Ordovician pillow lava sequence of Western Norway in an understanding of major fold pattern. Geological Magazine, 109, 315-322 |publisher=Geolmag.geoscienceworld.org |date=1972-07-01 |access-date=2014-03-10}}</ref> that is, study of their shape reveals the attitude, or position, in which they were originally formed. Pillow lava shows it is still in its original orientation when:
# [[Vesicular texture|Vesicle]]s are found towards the top of a pillow (because the gas trapped as part of the rock is less dense than its solid surroundings).
# The pillow structures show a convex (rounded) upper surface.
# The pillows might have a tapered base downwards, as they may have molded themselves to any underlying pillows during their formation.

==Gallery==

<gallery mode="packed">
File:Pillow lava at Oamaru.jpg|Pillow lava near [[Oamaru]], New Zealand
File:Temagami greenstone belt pillow lava.jpg|Weathered Archean pillow lava in the [[Temagami Greenstone Belt]] of the [[Canadian Shield]]
File:ItalyPillowBasalt.jpg|Pillow lava formations from an [[ophiolite]] sequence, Northern [[Apennines]], Italy
File:Pillow basalt outcrop from Baras, Rizal, Philippines (closer view).jpg|Pillow basalt from [[Baras, Rizal|Baras]], Rizal Province, Philippines
</gallery>

==See also==

* [[Pillow basalt]]
* [[Spilite]], a fine-grained igneous rock, resulting particularly from the alteration of oceanic basalt

==References==
{{reflist}}

== External links ==
* [http://www.pmel.noaa.gov/vents/nemo/explorer/concepts/pillows.html NeMO Explorer, NOAA - Contains link to video of pillows being formed]

{{DEFAULTSORT:Pillow Lava}}
[[Category:Structural geology]]
[[Category:Volcanology]]

[[sv:Lava#Kuddlava]]