Editing Basalt (section)

==== Submarine eruptions ====
{{Main|Submarine eruption}}
[[File:Pillow basalt crop l.jpg|thumb|Pillow basalts on the Pacific seafloor]]

The character of submarine basalt eruptions is largely determined by depth of water, since increased pressure restricts the release of volatile gases and results in effusive eruptions.<ref name="francis">Francis, P. (1993) ''Volcanoes: A Planetary Perspective'', Oxford University Press.</ref> It has been estimated that at depths greater than {{convert|500|m||}}, explosive activity associated with basaltic magma is suppressed.{{sfn|Parfitt|Parfitt|Wilson|2008|p={{pn|date=June 2021}}}} Above this depth, submarine eruptions are often explosive, tending to produce [[pyroclastic rock]] rather than basalt flows.<ref name="head and wilson">{{cite journal |last1=Head |first1=James W. |last2=Wilson |first2=Lionel |title=Deep submarine pyroclastic eruptions: theory and predicted landforms and deposits |journal=Journal of Volcanology and Geothermal Research |date=2003 |volume=121 |issue=3–4 |pages=155–193 |doi=10.1016/S0377-0273(02)00425-0 |bibcode=2003JVGR..121..155H }}</ref> These eruptions, described as Surtseyan, are characterised by large quantities of steam and gas and the creation of large amounts of [[pumice]].<ref name="Smithson">[http://www.volcano.si.edu/galleries.cfm?p=11], Smithsonian Institution National Museum of Natural History Global Volcanism Program (2013).</ref> 

===== Pillow basalts =====
{{Main|Pillow lava}}
When basalt erupts underwater or flows into the sea, contact with the water quenches the surface and the lava forms a distinctive ''pillow'' shape, through which the hot lava breaks to form another pillow. This "pillow" texture is very common in underwater basaltic flows and is diagnostic of an underwater eruption environment when found in ancient rocks. Pillows typically consist of a fine-grained core with a glassy crust and have radial jointing. The size of individual pillows varies from 10&nbsp;cm up to several metres.{{sfn|Schmincke|2003|p=64}}

When ''[[pahoehoe|pāhoehoe]]'' lava enters the sea it usually forms pillow basalts. However, when ''{{okina}}a{{okina}}ā'' enters the ocean it forms a [[littoral cone]], a small cone-shaped accumulation of tuffaceous debris formed when the blocky ''{{okina}}a{{okina}}ā'' lava enters the water and explodes from built-up steam.{{sfn|Macdonald|Abbott|Peterson|1983|p={{pn|date=June 2021}}}}

The island of [[Surtsey]] in the [[Atlantic Ocean]] is a basalt volcano which breached the ocean surface in 1963. The initial phase of Surtsey's eruption was highly explosive, as the magma was quite fluid, causing the rock to be blown apart by the boiling steam to form a tuff and cinder cone. This has subsequently moved to a typical pāhoehoe-type behaviour.<ref name="kikelaar-durant-1983">{{cite journal |last1=Kokelaar |first1=B.Peter |last2=Durant |first2=Graham P. |title=The submarine eruption and erosion of Surtla (Surtsey), Iceland |journal=Journal of Volcanology and Geothermal Research |date=December 1983 |volume=19 |issue=3–4 |pages=239–246 |doi=10.1016/0377-0273(83)90112-9|bibcode=1983JVGR...19..239K }}</ref><ref name="moore-1985">{{cite journal |last1=Moore |first1=James G. |title=Structure and eruptive mechanisms at Surtsey Volcano, Iceland |journal=Geological Magazine |date=November 1985 |volume=122 |issue=6 |pages=649–661 |doi=10.1017/S0016756800032052 |bibcode=1985GeoM..122..649M |s2cid=129242411 }}</ref>

[[Volcanic glass]] may be present, particularly as rinds on rapidly chilled surfaces of lava flows, and is commonly (but not exclusively) associated with underwater eruptions.{{sfn|Blatt|Tracy|1996|pp=24–25}}

Pillow basalt is also produced by some [[Subglacial eruption|subglacial]] volcanic eruptions.{{sfn|Blatt|Tracy|1996|pp=24–25}}