Basalt
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225782174
2008-07-15T11:09:16Z
125.24.35.103
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{{Nofootnotes|date=February 2008}}
{{For|the cities|Basalt, Colorado|Basalt, Idaho}}
[[Image:BasaltUSGOV.jpg|thumb|Basalt]]
'''Basalt''' ({{pronEng|ˈbæsɒlt, bəˈsɔːlt}}) is a common [[mafic]] [[extrusive]] [[volcanic rock]]. It is usually gray to black and fine-grained due to rapid cooling of [[lava]] at the surface of a planet. It may be [[porphyritic]] containing larger crystals in a fine [[Matrix (geology)|matrix]], or [[Vesicular texture|vesicular]], or frothy [[scoria]]. Unweathered basalt is black or gray.
On Earth, most basalt magmas have formed by [[Igneous rock#Decompression|decompression melting]] of the [[mantle]]. Basalt has also formed on Earth's [[Moon]], [[Mars]], [[Venus]], and even on the asteroid [[4 Vesta|Vesta]]. Source rocks for the partial melts probably include both [[peridotite]] and [[pyroxenite]] (e.g., Sobolev et al., 2007). The [[crust (geology)|crustal]] portions of [[ocean]]ic [[tectonic plate]]s are composed predominantly of basalt, produced from upwelling mantle below [[ocean ridge]]s.
The term basalt is at times applied to shallow [[intrusive]] rocks with a composition typical of basalt, but rocks of this composition with a [[phaneritic]] (coarse) groundmass are generally referred to as [[dolerite]] (also called [[diabase]]) or [[gabbro]].
[[Image:Columnar basalt at Sheepeater Cliff in Yellowstone-closeup-300px.JPG|thumb|right|250px|Columnar basalt at Sheepeater Cliff in Yellowstone]]
{{DEFAULTSORT:}}==Types of basalt==
[[Image:Giants causeway closeup.jpg|thumb|right|200px|Large masses must cool slowly to form a polygonal join pattern]]
* [[Tholeiitic basalt]] is relatively poor in [[silica]] and poor in [[sodium]]. Included in this category are most basalts of the [[ocean]] floor, most large oceanic islands, and continental [[flood basalt]]s such as the [[Columbia River Basalt Group|Columbia River Plateau]].
** ''MORB'' ([[Mid-ocean ridge|Mid Ocean Ridge Basalt]]), is characteristically low in [[incompatible element]]s. MORB is commonly erupted only at ocean ridges. MORB itself has been subdivided into varieties such as ''NMORB'' and ''EMORB'' (slightly more enriched in incompatible elements).<ref>[http://www.petdb.org/ See the PETDB database].{{cite book | author=Hyndman, Donald W. | title=Petrology of igneous and metamorphic rocks |edition=2nd ed.|year=1985|publisher=McGraw-Hill|id=ISBN 0-07-031658-9}}</ref> <ref>{{cite book | author=Blatt, Harvey and Robert Tracy | title=Petrology | edition=2nd ed. | year=1996|publisher=Freeman|id=ISBN 0-7167-2438-3}}</ref>
*''High alumina basalt'' may be silica-undersaturated or -oversaturated (see [[normative mineralogy]]). It has greater than 17% [[alumina]] (Al<sub>2</sub>O<sub>3</sub>) and is intermediate in composition between tholeiite and alkali basalt; the relatively alumina-rich composition is based on rocks without [[phenocryst]]s of [[plagioclase]].
*''Alkali basalt'' is relatively poor in silica and rich in sodium. It is [[normative mineralogy|silica-undersaturated]] and may contain [[feldspathoid]]s, [[alkali]] [[feldspar]] and [[phlogopite]].
* [[Boninite]] is a high-magnesium form of basalt or [[andesite]] that is erupted generally in [[back-arc basin]]s, distinguished by its low [[titanium]] content and trace element composition.
== Petrology ==
The mineralogy of basalt is characterized by a preponderance of calcic [[plagioclase]] [[feldspar]] and [[pyroxene]]. [[Olivine]] can also be a significant constituent. Accessory [[mineral]]s present in relatively minor amounts include [[iron oxide]]s and iron-titanium oxides, such as [[magnetite]], [[ulvospinel]], and [[ilmenite]]. Because of the presence of such [[oxide]] minerals, basalt can acquire strong [[magnetic]] signatures as it cools, and [[paleomagnetism|paleomagnetic]] studies have made extensive use of basalt.
In tholeiitic basalt, [[pyroxene]] ([[augite]] and [[orthopyroxene]] or [[pigeonite]]) and [[calcium]]-rich [[plagioclase]] are common [[phenocryst]] minerals. Olivine may also be a phenocryst, and when present, may have rims of [[pigeonite]]. The [[groundmass]] contains interstitial [[quartz]] or [[tridymite]] or [[cristobalite]]. ''Olivine tholeiite'' has augite and orthopyroxene or pigeonite with abundant olivine, but olivine may have rims of pyroxene and is unlikely to be present in the [[matrix (geology)|groundmass]].
Alkali basalts typically have mineral assemblages that lack [[orthopyroxene]] but contain [[olivine]]. Feldspar phenocrysts typically are [[labradorite]] to [[andesine]] in composition. [[Augite]] is rich in titanium compared to augite in tholeiitic basalt. Minerals such as [[feldspar|alkali feldspar]], [[leucite]], [[nepheline]], [[sodalite]], [[phlogopite]] mica, and [[apatite]] may be present in the groundmass.
Basalt has high [[liquidus]] and [[solidus]] temperatures -- values at the Earth's surface are near or above 1200 °C (liquidus) and near or below 1000 °C (solidus); these values are higher than those of other common [[igneous rock]]s.
The majority of tholeiites are formed at approximately 50-100 km depth within the mantle. Many alkali basalts may be formed at greater depths, perhaps as deep as 150-200 km. The origin of high-alumina basalt continues to be controversial, with interpretations that it is a [[magma|primary melt]] and that instead it is derived from other basalt types (e.g., Ozerov, 2000).
==Geochemistry==
[[Image:Basalt columns in yellowstone 2.jpg|Columnar basalt flows of the [[Columbia River Basalt]] in [[Yellowstone National Park]]|thumb|300px]]
Basalt compositions are rich in [[magnesium oxide|MgO]] and [[calcium oxide|CaO]] and low in [[silicon dioxide|SiO<sub>2</sub>]] and [[sodium oxide|Na<sub>2</sub>O]] plus [[potassium oxide|K<sub>2</sub>O]] relative to most common [[igneous rocks]], consistent with the [[TAS classification]].
Basalt generally has a composition of 45-55 wt% SiO<sub>2</sub>, 2-6 wt% total alkalis, 0.5-2.0 wt% [[titanium dioxide|TiO<sub>2</sub>]], 5-14 wt% [[iron(II) oxide|FeO]] and 14 wt% or more [[alumina|Al<sub>2</sub>O<sub>3</sub>]]. Contents of CaO are commonly near 10 wt%, those of MgO commonly in the range 5 to 12 wt%.
High alumina basalts have aluminium contents of 17-19 wt% Al<sub>2</sub>O<sub>3</sub>; boninites have magnesium contents of up to 15% MgO. Rare [[feldspathoid]]-rich [[mafic]] rocks, akin to alkali basalts, may have Na<sub>2</sub>O plus K<sub>2</sub>O contents of 12% or more.
MORB basalts and their intrusive equivalents, [[gabbro]]s, are the characteristic igneous rocks formed at [[mid-ocean ridge]]s. They are [[tholeiite]]s particularly low in total alkalis and in [[incompatible]] trace elements, and they have relatively flat [[rare earth elements|REE]] patterns normalised to mantle or [[chondrite]] values. In contrast, alkali basalts have normalized patterns highly enriched in the light REE, and with greater abundances of the REE and of other [[incompatible]] elements. Because MORB basalt is considered a key to understanding [[plate tectonics]], its compositions have been much studied. Although MORB compositions are distinctive relative to average compositions of basalts erupted in other environments, they are not uniform. For instance, compositions change with position along the [[Mid-Atlantic ridge]], and the compositions also define different ranges in different ocean basins (Hofmann, 2003).
[[Isotope]] ratios of [[chemical element|element]]s such as [[strontium]], [[neodymium]], [[lead]], [[hafnium]], and [[osmium]] in basalts have been much-studied, so as to learn about evolution of the [[Earth's mantle]]. Isotopic ratios of [[noble gases]], such as <sup>3</sup>[[Helium|He]]/<sup>4</sup>He, are also of great value: for instance, ratios for basalts range from 6 to 10 for mid-ocean ridge tholeiite (normalized to atmospheric values), but to 15-24+ for ocean island basalts thought to be derived from [[mantle plume]]s.
==Morphology and textures==
The shape, structure and [[rock microstructure|texture]] of a basalt is diagnostic of how and where it erupted - whether into the sea, in an explosive [[cinder]] eruption or as creeping [[pahoehoe]] lava flows, the classical image of [[Hawaii]]an basalt eruptions.
[[Image:Boyabat.jpg |thumb|Columnar [[Joint (geology)|jointed]] basalt in Turkey]]
===Subaerial eruptions===
Basalt which erupts under open air (that is, [[subaerial]]ly) forms three distinct types of lava or volcanic deposits: scoria, [[volcanic ash|ash]] or cinder; [[breccia]] and lava flows.
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. Basaltic cinders are often red, coloured by oxidised [[iron]] from weathered iron-rich minerals such as [[pyroxene]].
[[‘A‘a]] types of blocky, cinder and breccia flows of thick, viscous basaltic [[lava]] are common in Hawaii. [[Pahoehoe]] 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 pahoehoe eruptions.
Basaltic [[tuff]] or [[pyroclastic]] rocks are rare but not unknown. 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 build up of [[volcanic gas]]es. Hawaii's [[Mauna Loa]] volcano erupted in this way in the 19th century, as did [[Mount Tarawera]], New Zealand in its violent 1886 eruption.
[[Image:Giants-causeway-in-ireland.jpg|thumb|Columnar [[Joint (geology)|jointing]] in the basalt of the [[Giant's Causeway]] in [[Northern Ireland]]]]
[[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.
Amygdaloidal structure is common in relict [[vesicle (geology)|vesicle]]s and beautifully [[crystal]]lized species of [[zeolite]]s, [[quartz]] or [[calcite]] are frequently found.
====Columnar basalt====
During the cooling of a thick lava flow, contractional [[Joint (geology)|joints]] or fractures form. If a flow cools relatively rapidly, significant [[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 columns. The topology of the lateral shapes of these columns can broadly be classed as a [[random cellular network]]. These structures are often erroneously described as being predominantly hexagonal. In reality, the [[mean]] number of sides of all the columns in such a structure is indeed six (by geometrical definition), but polygons with three to twelve or more sides can be observed<ref>D. Weaire and N. Rivier. ''Contemporary Physics'' 25 1 (1984), pp. 55–99</ref>. Note that the size of the columns depends loosely on the rate of cooling; very rapid cooling may result in very small (<1 [[centimeter|cm]] diameter) columns, while slow cooling is more likely to produce large columns.
Perhaps the most famous basalt flow in the world is the [[Giant's Causeway]] on the northern coast of [[Ireland]], in which the vertical joints form [[hexagonal]] columns and give the impression of having been artificially constructed.
An ancient 13th century religious complex, called [[Nan Madol]], was built on the Pacific island of [[Pohnpei]], using columnar basalt quarried from various locations on the island. The massive ruins remain to this day.
[[Image:DSC00478 - Basalt Columns in Hong Kong - near Basalt Island and High Island Reservoir - basalt rock with polygonal fracture pattern formation 2004Aug08.jpg|thumb|Columnar jointed basalt in Hong Kong - near Basalt Island and High Island Reservoir areas]]
* Notable columnar basalts:
** [[Giant's Causeway]], Northern Ireland
** [[Borgarvirki|Borgarvirki Fortress]], Iceland
** [[Devil's Postpile]], California
** Narooma Basalt, [[Narooma, New South Wales]], [[Australia]]
** [[Samson's ribs (Edinburgh)|Samson's ribs]], Scotland
** [[Staffa]], Scotland
** [[Pwisehn Malek]], [[Pohnpei]], [[Federated States of Micronesia]] <ref>http://www.pohnpeiheaven.com/pwisehn_malek.htm Alex Zuccarelli, 2003, ''Pohnpei-Between Time & Tide . Pwisehn Malek''</ref>
** Basalt Island area, Hong Kong; including High Island Reservoir area, Hong Kong SAR, China
** [[Reynisdrangar]], [[Vík í Mýrdal]], Iceland
<!-- ** [[Devils Tower National Monument|Devil's Tower]], [[Wyoming]] is phonolite [[Porphyry (geology)|porphyry]]-->
** [[Thunderstruck Rocks (Detunatele)]], Romania
===Submarine eruptions===
[[Image:Pillow basalt crop l.jpg|thumb|Pillow basalts on the south Pacific seafloor]]
====Pillow basalts====
When basalt erupts underwater or flows into the sea, the cold 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 cm up to several metres.
When ''pahoehoe'' lava enters the sea it usually forms pillow basalts. However when a'a enters the ocean it forms a [[littoral cone]], a small cone-shaped accumulation of tuffaceous debris formed when the blocky a'a lava enters the water and explodes from built-up steam.
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 wet, 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 [[pahoehoe]] type behaviour.
[[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.
====Life on basaltic rocks====
The common corrosion features of underwater volcanic basalt suggest that microbial activity may play a significant role in the chemical exchange between basaltic rocks and seawater. The significant amounts of reduced iron, Fe(II), and manganese, Mn(II), present in basaltic rocks provide potential energy sources for bacteria. Recent research has shown that some Fe(II)-oxidizing bacteria cultured from iron-sulfide surfaces are also able to grow with basaltic rock as a source of Fe(II).<ref>Katrina J. Edwards, Wolfgang Bach and Daniel R. Rogers, ''Geomicrobiology of the Ocean Crust: A Role for Chemoautotrophic Fe-Bacteria,'' Biol. Bull. 204: 180-185. (April 2003) http://www.biolbull.org/cgi/content/full/204/2/180 </ref> In recent work at [[Loihi Seamount]], Fe- and Mn- oxidizing bacteria have been cultured from weathered basalts.<ref>Templeton, A.S., Staudigel, H., Tebo, B.M. (2005). Diverse Mn(II)-oxidizing bacteria isolated from submarine basalts at Loihi Seamount, ''Geomicrobiology Journal'', v. 22, 129-137. http://www.ebs.ogi.edu/tebob/pdfs/Templeton%20GeomicroJ.pdf </ref> The impact of bacteria on altering the chemical composition of basaltic glass (and thus, the [[oceanic crust]]) and seawater suggest that these interactions may lead to an application of [[hydrothermal vents]] to the [[origin of life]].
==Distribution==
[[Image:Parana traps.JPG|thumb|Paraná Traps, Brazil]]
The [[lava]] flows of the [[Deccan Traps]] in [[India]], the [[Chilcotin Plateau Basalts]] in [[British Columbia]], [[Canada]], the [[Paraná Traps]] in Brazil, the [[Siberian Traps]] in [[Russia]], the [[Columbia River Plateau]] of [[Washington]] and [[Oregon]], as well as parts of the [[California]] [[inner coastal ranges]] in the [[United States]], as well as the [[Triassic]] lavas of eastern [[North America]] are basalts. Other famous accumulations of basalts include [[Iceland]], the [[Karoo]] [[flood basalt]] province in South Africa and the islands of the [[Hawaii]] volcanic chain, forming above a [[mantle plume]]. Basalt is the rock most typical of [[large igneous province]]s.
Ancient [[Precambrian]] basalts are usually only found in fold and thrust belts, and are often heavily metamorphosed. These are known as [[greenstone belt]]s, because low-grade [[metamorphism]] of basalt produces [[Chlorite group|chlorite]], [[actinolite]], [[epidote]] and other green minerals.
==Lunar and Martian basalt==
The dark areas visible on Earth's [[moon]], the [[lunar mare|lunar maria]], are plains of [[flood basalt]]ic lava flows. These rocks were sampled by the manned American [[Apollo program]], the robotic Russian [[Luna program]], and are represented among the [[lunar meteorites]].
Lunar basalts differ from their terrestrial counterparts principally in their high iron contents, which typically range from about 17 to 22 wt% FeO. They also possess a stunning range of titanium concentrations (present in the mineral [[ilmenite]]), ranging from less than 1 wt% TiO<sub>2</sub>, to about 13 wt.%. Traditionally, lunar basalts have been classified according to their titanium content, with classes being named high-Ti, low-Ti, and very-low-Ti. Nevertheless, global geochemical maps of titanium obtained from the [[Clementine mission]] demonstrate that the lunar maria possess a continuum of titanium concentrations, and that the highest concentrations are the least abundant.
Lunar basalts show exotic textures and mineralogy, particularly shock [[metamorphism]], lack of the [[redox|oxidation]] typical of terrestrial basalts, and a complete lack of [[mineral hydration|hydration]]. While most of the [[geology of the Moon|Moon]]'s basalts erupted between about 3 and 3.5 billion years ago, the oldest samples are 4.2 billion years old, and the youngest flows, based on the age dating method of "crater counting," are estimated to have erupted only 1.2 billion years ago.
Basalt is also a common rock on the surface of [[Mars]], as determined by data sent back from the surface of Mars and by [[Martian meteorite]]s.
==Metamorphism==
Basalts are important rocks within [[metamorphic]] belts, as they can provide vital information on the conditions of metamorphism within the belt. Various metamorphic [[facies]] are named after the mineral assemblages and rock types formed by subjecting basalts to the temperatures and pressures of the metamorphic event. These are;
* [[Greenschist]] facies
* [[Blueschist]] facies
* [[Zeolite]] facies
* [[Granulite]] facies
* [[Eclogite]] facies
Metamorphosed basalts are important hosts for a variety of [[hydrothermal]] [[ore]] deposits, including [[gold]] deposits, [[copper]] deposits, [[volcanogenic massive sulfide ore deposit]]s and others.
==See also==
{{commonscat|Basalt}}
* [[Basalt fiber]]
* [[Mafic|Mafic rocks]]
* [[Volcano|Volcanoes]]
* [[Igneous rocks]]
* [[Flood basalt]]
==References==
{{Reflist}}
{{Refbegin}}
* A. Y. Ozerov, ''The evolution of high-alumina basalts of the Klyuchevskoy volcano, Kamchatka, Russia, based on microprobe analyses of mineral inclusions.'' Journal of Volcanology and Geothermal Research, v. 95, p. 65-79 (2000).
* A. W. Hofmann, ''Sampling mantle heterogeneity through oceanic basalts: isotopes and trace elements.'' Treatise on Geochemistry Volume 2, pages 61-101 Elsevier Ltd. (2003). ISBN 0-08-044337-0 In March, 2007, the article was available on the web at http://www1.mpch-mainz.mpg.de/~geo/hofmann/Hofmann.mantle_heterogen1.pdf.
* A. V. Sobolev and others, ''The amount of recycled crust in sources of mantle-derived melts.'' Science, v. 316, p. 412-417 (2007). http://www.sciencemag.org/cgi/content/abstract/316/5823/412
{{Refend}}
==External links==
*[http://www.geology.sdsu.edu/how_volcanoes_work/lava_water.html Lava - water interface]
*[http://volcanoes.usgs.gov/Products/Pglossary/PillowLava.html Pillow lava USGS]
*[http://www.union.edu/PUBLIC/GEODEPT/COURSES/petrology/moon_rocks/ Petrology of Lunar Rocks and Mare Basalts]
*[http://geographyinaction.co.uk//Geology%20files/Basalt.html Basalt in Northern Ireland]
*[http://giantcrystals.strahlen.org/europe/basalt.htm Basalt Columns]
[[Category:Igneous rocks]]
[[Category:Volcanology]]
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