Pillow lava

File:Pillow lava Boatman's Harbour (Oamaru, New Zealand).jpg

Pillow lavas are commonly of basaltic composition, although pillows formed of komatiite, picrite, boninite, basaltic andesite, andesite, dacite or even rhyolite are known.{{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 }}{{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 }}{{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 }}{{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 }}{{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}} 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.

They occur wherever lava is extruded underwater, such as along marine hotspot volcano chains and the constructive plate boundaries of mid-ocean ridges. As new oceanic crust is formed, thick sequences of pillow lavas are erupted at the spreading center fed by dykes from the underlying magma chamber. Pillow lavas and the related sheeted dyke complexes form part of a classic ophiolite sequence (when a segment of oceanic crust is 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 and Barberton greenstone belts, 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 metamorphic belts.

Pillow lavas are also found associated with some subglacial volcanoes at an early stage of an eruption.[http://www.norvol.hi.is/html/geol/intro/introduction.pdf Geology and geodynamics of Iceland, R.G. Trønnes, Nordic volcanological Institute, University of Iceland]{{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}}

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.[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.] 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.

Pillow lavas can be used as a way-up indicator in geology;{{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}} 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:

1. Vesicles are found towards the top of a pillow (because the gas trapped as part of the rock is less dense than its solid surroundings).
2. The pillow structures show a convex (rounded) upper surface.
3. The pillows might have a tapered base downwards, as they may have molded themselves to any underlying pillows during their formation.

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 Province, Philippines

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

{{reflist}}

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

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Category:Structural geology

Category:Volcanology

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