Editing Basalt (section)

=== Morphology and textures ===
[[File:20011005-0039 DAS large.jpg|thumb|An active basalt lava flow]]
The shape, structure and [[rock microstructure|texture]] of a basalt is diagnostic of how and where it erupted—for example, whether into the sea, in an explosive [[Scoria|cinder]] eruption or as creeping [[pāhoehoe]] lava flows, the classic image of [[Hawaii]]an basalt eruptions.{{sfn|Schmincke|2003|p={{pn|date=June 2021}}}}

==== Subaerial eruptions ====
{{Main|Subaerial eruption}}
Basalt that erupts under open air (that is, [[subaerial]]ly) forms three distinct types of lava or volcanic deposits: [[scoria]]; [[volcanic ash|ash]] or cinder ([[breccia]]);{{sfn|Blatt|Tracy|1996|pp=27–28}} and lava flows.{{sfn|Blatt|Tracy|1996|pp=22–23}}

Basalt in the tops of subaerial lava flows and [[cinder cone]]s will often be highly [[Vesicular texture|vesiculated]], imparting a lightweight "frothy" texture to the rock.{{sfn|Blatt|Tracy|1996|pp=43–44}} Basaltic cinders are often red, coloured by oxidized [[iron]] from weathered iron-rich minerals such as [[pyroxene]].{{sfn|Lillie|2005|p=41}}

[[Lava#{{okina}}A{{okina}}ā|{{okina}}A{{okina}}ā]] types of blocky cinder and breccia flows of thick, viscous basaltic [[lava]] are common in Hawai{{okina}}i. Pāhoehoe is a highly fluid, hot form of basalt which tends to form thin aprons of molten lava which fill up hollows and sometimes forms [[lava lake]]s. [[Lava tube]]s are common features of pāhoehoe eruptions.{{sfn|Blatt|Tracy|1996|pp=22–23}}

Basaltic [[tuff]] or [[Pyroclastic rock|pyroclastic]] rocks are less common than basaltic lava flows. Usually basalt is too hot and fluid to build up sufficient pressure to form explosive lava eruptions but occasionally this will happen by trapping of the lava within the volcanic throat and buildup of [[volcanic gas]]es. Hawai{{okina}}i's [[Mauna Loa]] volcano erupted in this way in the 19th century, as did [[Mount Tarawera]], New Zealand in its violent 1886 eruption. [[Maar]] volcanoes are typical of small basalt tuffs, formed by explosive eruption of basalt through the crust, forming an apron of mixed basalt and wall rock breccia and a fan of basalt tuff further out from the volcano.{{sfn|Schmincke|2003|loc=Chapter 12}}

Amygdaloidal structure is common in relict [[vesicle (geology)|vesicles]] and beautifully [[crystal]]lized species of [[zeolite]]s, [[quartz]] or [[calcite]] are frequently found.{{sfn|Philpotts|Ague|2009|p=64}}

===== Columnar basalt =====
{{Main|Columnar jointing}}
{{See also|List of places with columnar basalt}}
[[File:Causeway-code poet-4.jpg|thumb|The [[Giant's Causeway]] in Northern Ireland]]
[[File:Boyabat.jpg|thumb|Columnar [[Joint (geology)|jointed]] basalt in [[Turkey]]]]
[[File:Мыс Столбчатый. После заката.jpg|thumb|Columnar basalt at [[Cape Stolbchaty]], Russia]]
During the cooling of a thick lava flow, contractional [[Joint (geology)|joints]] or fractures form.<ref>{{cite journal |last1=Smalley |first1=I. J. |title=Contraction Crack Networks in Basalt Flows |journal=Geological Magazine |date=April 1966 |volume=103 |issue=2 |pages=110–114 |doi=10.1017/S0016756800050482 |bibcode=1966GeoM..103..110S |s2cid=131237003 }}</ref> If a flow cools relatively rapidly, significant [[Thermal expansion#Contraction effects (negative expansion)|contraction]] forces build up. While a flow can shrink in the vertical dimension without fracturing, it cannot easily accommodate shrinking in the horizontal direction unless cracks form; the extensive fracture network that develops results in the formation of [[Columnar jointing|column]]s. These structures, or [[basalt prism]]s, are predominantly hexagonal in cross-section, but polygons with three to twelve or more sides can be observed.<ref>{{cite journal |last1=Weaire |first1=D. |last2=Rivier |first2=N. |title=Soap, cells and statistics—random patterns in two dimensions |journal=Contemporary Physics |date=January 1984 |volume=25 |issue=1 |pages=59–99 |doi=10.1080/00107518408210979 |bibcode=1984ConPh..25...59W }}</ref> The size of the columns depends loosely on the rate of cooling; very rapid cooling may result in very small (<1&nbsp;cm diameter) columns, while slow cooling is more likely to produce large columns.<ref name="spry-1962">{{cite journal |last1=Spry |first1=Alan |title=The origin of columnar jointing, particularly in basalt flows |journal=Journal of the Geological Society of Australia |date=January 1962 |volume=8 |issue=2 |pages=191–216 |doi=10.1080/14400956208527873 |bibcode=1962AuJES...8..191S }}</ref>

==== 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}}