Editing Basalt (section)

==== 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>