Basalt

Basalt

For the World War II raid, see Operation Basalt. For thecities, see Basalt, Colorado and Basalt, Idaho.

Basalt (/bəˈsɔːlt/, /ˈbæsɒlt/, /ˈbæsɔːlt/, or/ˈbeɪsɔːlt/)[1][2][3] is a common extrusive igneous(volcanic) rock formed from the rapid cooling of basalticlava exposed at or very near the surface of a planet ormoon. Flood basalt describes the formation in a seriesof lava basalt flows.

1 Definition

Columnar basalt flows in Yellowstone National Park, USA.

By definition, basalt is an aphanitic igneous rock withless than 20% quartz and less than 10% feldspathoid byvolume, and where at least 65% of the feldspar is inthe form of plagioclase. Basalt features a glassy matrixinterspersed with minerals. The average density is 3.0gm/cm3.Basalt is defined by its mineral content and texture, andphysical descriptions without mineralogical context maybe unreliable in some circumstances. Basalt is usuallygrey to black in colour, but rapidly weathers to brown orrust-red due to oxidation of its mafic (iron-rich) miner-als into rust. Although usually characterized as “dark”,basaltic rocks exhibit a wide range of shading due to re-gional geochemical processes. Due to weathering or highconcentrations of plagioclase, some basalts are quite lightcoloured, superficially resembling rhyolite to untrainedeyes. Basalt has a fine-grained mineral texture due tothe molten rock cooling too quickly for large mineral

crystals to grow, although it is often porphyritic, con-taining the larger crystals formed prior to the extrusionthat brought the lava to the surface, embedded in a finer-grained matrix.Basalt with a vesicular or frothy texture is called scoria,and forms when dissolved gases are forced out of solutionand form vesicles as the lava decompresses as it reachesthe surface.The term basalt is at times applied to shallow intrusiverocks with a composition typical of basalt, but rocks ofthis composition with a phaneritic (coarse) groundmassare generally referred to as diabase (also called dolerite)or gabbro.

Columnar basalt at Szent György Hill, Hungary

In the Hadean and Archean (and the early Precambrian)eras of Earth’s history the chemistry of erupted basaltswas significantly different from today’s, due to crustaland asthenosphere differentiation issues—so much sothat there is an alternate (but less well known) name forthis kind of basalt.

1

2 4 PETROLOGY

Vesicular basalt at Sunset Crater, Arizona. US quarter for scale.

1.1 Etymology

The word “basalt” is ultimately derived from Late Latinbasaltes, misspelling of L. basanites “very hard stone,”which was imported from Ancient Greek βασανίτης(basanites), from βάσανος (basanos, “touchstone”) andoriginated in Egyptian bauhun “slate”.[4] The modernpetrological term basalt describing a particular compo-sition of lava-derived rock originates from its use byGeorgius Agricola in 1556 in his famous work of min-ing and mineralogyDe re metallica, libri XII. Agricola ap-plied “basalt” to the volcanic black rock of the Schloßberg(local castle hill) at Stolpen, believing it to be the same asPliny the Elder's “very hard stone”.

1.2 Types

Large masses must cool slowly to form a polygonal joint pattern,as here at the Giant’s Causeway in Northern Ireland.

• Tholeiitic basalt is relatively rich in silica and poor insodium. Included in this category are most basaltsof the ocean floor, most large oceanic islands, andcontinental flood basalts such as the Columbia RiverPlateau.

• MORB (Mid-Ocean Ridge Basalt) is characteristi-cally low in incompatible elements. MORB is com-monly erupted only at ocean ridges. MORB itselfhas been subdivided into varieties such as NMORBand EMORB (slightly more enriched in incompati-ble elements).[5][6]

• High-alumina basalt may be silica-undersaturated or-oversaturated (see normative mineralogy). It hasgreater than 17% alumina (Al2O3) and is interme-diate in composition between tholeiite and alkalibasalt; the relatively alumina-rich composition isbased on rocks without phenocrysts of plagioclase.

• Alkali basalt is relatively poor in silica and rich insodium. It is silica-undersaturated and may containfeldspathoids, alkali feldspar and phlogopite.

• Boninite is a high-magnesium form of basalt that iserupted generally in back-arc basins, distinguishedby its low titanium content and trace element com-position.

2 Occurrence

On Earth, most basalt magmas have formed bydecompression melting of the mantle. Basalt commonlyerupts on Io, the third largest moon of Jupiter, and hasalso formed on Earth’s Moon, Mars, Venus, and the as-teroid Vesta.The crustal portions of oceanic tectonic plates are com-posed predominantly of basalt, produced from upwellingmantle below, the ocean ridges.

3 Uses

Basalt is used in construction (e.g. as building blocks orin the groundwork), making cobblestones (from colum-nar basalt) and in making statues. Heating and extrudingbasalt yields stone wool, said to be an excellent thermalinsulator.

4 Petrology

Themineralogy of basalt is characterized by a preponder-ance of calcic plagioclase feldspar and pyroxene. Olivinecan also be a significant constituent. Accessory mineralspresent in relatively minor amounts include iron oxidesand iron-titanium oxides, such as magnetite, ulvospinel,and ilmenite. Because of the presence of such oxide min-erals, basalt can acquire strong magnetic signatures as itcools, and paleomagnetic studies havemade extensive useof basalt.

4.1 Geochemistry 3

Photomicrograph of a volcanic (basaltic) sand grain; upper pic-ture is plane-polarized light, bottom picture is cross-polarizedlight, scale box at left-center is 0.25 millimeter. Note white pla-gioclase 'microlites’ in cross-polarized light picture, surroundedby very fine grained volcanic glass.

In tholeiitic basalt, pyroxene (augite and orthopyroxeneor pigeonite) and calcium-rich plagioclase are commonphenocryst minerals. Olivine may also be a phenocryst,and when present, may have rims of pigeonite. Thegroundmass contains interstitial quartz or tridymite orcristobalite. Olivine tholeiite has augite and orthopyrox-ene or pigeonite with abundant olivine, but olivine mayhave rims of pyroxene and is unlikely to be present in thegroundmass.Alkali basalts typically have mineral assemblages thatlack orthopyroxene but contain olivine. Feldspar phe-nocrysts typically are labradorite to andesine in compo-sition. Augite is rich in titanium compared to augite intholeiitic basalt. Minerals such as alkali feldspar, leucite,nepheline, sodalite, phlogopite mica, and apatite may bepresent in the groundmass.Basalt has high liquidus and solidus temperatures—valuesat the Earth’s surface are near or above 1200 °C (liquidus)and near or below 1000 °C (solidus); these values arehigher than those of other common igneous rocks.The majority of tholeiites are formed at approximately50–100 km depth within the mantle. Many alkali basaltsmay be formed at greater depths, perhaps as deep as 150–200 km. The origin of high-alumina basalt continues tobe controversial, with interpretations that it is a primarymelt and that instead it is derived from other basalt types

(e.g., Ozerov, 2000).

4.1 Geochemistry

Relative to most common igneous rocks, basalt composi-tions are rich in MgO and CaO and low in SiO2 and thealkali oxides, i.e., Na2O + K2O, consistent with the TASclassification.Basalt generally has a composition of 45–55 wt% SiO2,2–6 wt% total alkalis, 0.5–2.0 wt% TiO2, 5–14 wt% FeOand 14 wt% or more Al2O3. Contents of CaO are com-monly near 10wt%, those ofMgO commonly in the range5 to 12 wt%.High-alumina basalts have aluminium contents of 17–19wt% Al2O3; boninites have magnesium contents of upto 15 percent MgO. Rare feldspathoid-rich mafic rocks,akin to alkali basalts, may have Na2O + K2O contents of12% or more.The abundances of the lanthanide or rare-earth elements(REE) can be a useful diagnostic tool to help explain thehistory of mineral crystallisation as the melt cooled. Inparticular, the relative abundance of europium comparedto the other REE is often markedly higher or lower, andcalled the europium anomaly. It arises because Eu2+ cansubstitute for Ca2+ in plagioclase feldspar, unlike any ofthe other lanthanides, which tend to only form 3+ cations.MORB basalts and their intrusive equivalents, gabbros,are the characteristic igneous rocks formed at mid-oceanridges. They are tholeiites particularly low in total alkalisand in incompatible trace elements, and they have rela-tively flat REE patterns normalized to mantle or chondritevalues. In contrast, alkali basalts have normalized pat-terns highly enriched in the light REE, and with greaterabundances of the REE and of other incompatible el-ements. Because MORB basalt is considered a keyto understanding plate tectonics, its compositions havebeen much studied. Although MORB compositions aredistinctive relative to average compositions of basaltserupted in other environments, they are not uniform. Forinstance, compositions change with position along theMid-Atlantic ridge, and the compositions also define dif-ferent ranges in different ocean basins (Hofmann, 2003).Isotope ratios of elements such as strontium, neodymium,lead, hafnium, and osmium in basalts have been muchstudied to learn about the evolution of the Earth’s man-tle. Isotopic ratios of noble gases, such as 3He/4He, arealso of great value: for instance, ratios for basalts rangefrom 6 to 10 for mid-ocean ridge tholeiite (normalizedto atmospheric values), but to 15-24+ for ocean islandbasalts thought to be derived from mantle plumes.Source rocks for the partial melts probably include bothperidotite and pyroxenite (e.g., Sobolev et al., 2007).

4 4 PETROLOGY

4.2 Morphology and textures

An active basalt lava flow

The shape, structure and texture of a basalt is diagnosticof how and where it erupted—whether into the sea, in anexplosive cinder eruption or as creeping pahoehoe lavaflows, the classic image of Hawaiian basalt eruptions.

4.2.1 Subaerial eruptions

Basalt which erupts under open air (that is, subaerially)forms three distinct types of lava or volcanic deposits:scoria; ash or cinder (breccia); and lava flows.Basalt in the tops of subaerial lava flows and cinder coneswill often be highly vesiculated, imparting a lightweight“frothy” texture to the rock. Basaltic cinders are oftenred, coloured by oxidized iron from weathered iron-richminerals such as pyroxene.ʻAʻā types of blocky, cinder and breccia flows of thick,viscous basaltic lava are common in Hawaii. Pāhoehoeis a highly fluid, hot form of basalt which tends to formthin aprons ofmolten lava which fill up hollows and some-times forms lava lakes. Lava tubes are common featuresof pahoehoe eruptions.Basaltic tuff or pyroclastic rocks are rare but not un-known. Usually basalt is too hot and fluid to build up suf-ficient pressure to form explosive lava eruptions but occa-sionally this will happen by trapping of the lava within thevolcanic throat and buildup of volcanic gases. Hawaii’sMauna Loa volcano erupted in this way in the 19th cen-tury, as did Mount Tarawera, New Zealand in its violent1886 eruption. Maar volcanoes are typical of small basalttuffs, formed by explosive eruption of basalt through thecrust, forming an apron of mixed basalt and wall rockbreccia and a fan of basalt tuff further out from the vol-cano.Amygdaloidal structure is common in relict vesicles andbeautifully crystallized species of zeolites, quartz orcalcite are frequently found.

Columnar basalt See also: List of places withcolumnar basalt

Columnar jointed basalt in Turkey

During the cooling of a thick lava flow, contractionaljoints or fractures form. If a flow cools relatively rapidly,significant contraction forces build up. While a flowcan shrink in the vertical dimension without fracturing,it can't easily accommodate shrinking in the horizontaldirection unless cracks form; the extensive fracture net-work that develops results in the formation of columns.The topology of the lateral shapes of these columns canbroadly be classed as a random cellular network. Thesestructures are predominantly hexagonal in cross-section,but polygons with three to twelve or more sides can beobserved.[7] The size of the columns depends loosely onthe rate of cooling; very rapid cooling may result in verysmall (<1 cm diameter) columns, while slow cooling ismore likely to produce large columns.

4.2.2 Submarine eruptions

Pillow basalts on the south Pacific seafloor

Pillow basalts See also: Pillow lava

When basalt erupts underwater or flows into the sea, con-tact with the water quenches the surface and the lava

5

Outcrop of a pillow basalt, Italy

forms a distinctive pillow shape, through which the hotlava breaks to form another pillow. This “pillow” tex-ture is very common in underwater basaltic flows and isdiagnostic of an underwater eruption environment whenfound 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 toseveral meters.When pahoehoe lava enters the sea it usually forms pil-low basalts. However when a'a enters the ocean it formsa littoral cone, a small cone-shaped accumulation of tuffa-ceous debris formed when the blocky a'a lava enters thewater and explodes from built-up steam.The island of Surtsey in the Atlantic Ocean is a basaltvolcano which breached the ocean surface in 1963. Theinitial phase of Surtsey’s eruption was highly explosive, asthe magma was quite wet, causing the rock to be blownapart by the boiling steam to form a tuff and cinder cone.This has subsequently moved to a typical pahoehoe-typebehaviour.Volcanic glass may be present, particularly as rinds onrapidly chilled surfaces of lava flows, and is commonly(but not exclusively) associated with underwater erup-tions.

5 Life on basaltic rocks

The common corrosion features of underwater volcanicbasalt suggest that microbial activity may play a signifi-cant role in the chemical exchange between basaltic rocksand seawater. The significant amounts of reduced iron,Fe(II), and manganese, Mn(II), present in basaltic rocksprovide potential energy sources for bacteria. Recent re-search has shown that some Fe(II)-oxidizing bacteria cul-tured from iron-sulfide surfaces are also able to grow withbasaltic rock as a source of Fe(II).[8] In recent work atLoihi Seamount, Fe- and Mn- oxidizing bacteria havebeen cultured from weathered basalts.[9] The impact ofbacteria on altering the chemical composition of basaltic

glass (and thus, the oceanic crust) and seawater sug-gest that these interactions may lead to an application ofhydrothermal vents to the origin of life.

6 Distribution

Paraná Traps, Brazil

Basalt is one of the most common rock types in theworld. Basalt is the rock most typical of large igneousprovinces. The largest occurrences of basalt are in theocean floor that is almost completely made up by basalt.Above sea level basalt is common in hotspot islands andaround volcanic arcs, specially those on thin crust. How-ever, the largest volumes of basalt on land correspondto continental flood basalts. Continental flood basaltsare known to exist in the Deccan Traps in India, theChilcotin Group in British Columbia, Canada, the ParanáTraps in Brazil, the Siberian Traps in Russia, the Karooflood basalt province in South Africa, the Columbia RiverPlateau of Washington and Oregon.Many archipelagoes and island nations have an over-whelming majority of its exposed bedrock made up bybasalt due to being above hotspots, for example, Icelandand Hawaii.Ancient Precambrian basalts are usually only found infold and thrust belts, and are often heavily metamor-phosed. These are known as greenstone belts, becauselow-grade metamorphism of basalt produces chlorite,actinolite, epidote and other green minerals.

7 Lunar and Martian basalt

The dark areas visible on Earth’s moon, the lunar maria,are plains of flood basaltic lava flows. These rockswere sampled by the manned American Apollo program,the robotic Russian Luna program, and are representedamong the lunar meteorites.Lunar basalts differ from their terrestrial counterpartsprincipally in their high iron contents, which typically

6 9 SEE ALSO

Lunar olivine basalt collected by Apollo 15.

range from about 17 to 22 wt% FeO. They also possess astunning range of titanium concentrations (present in themineral ilmenite), ranging from less than 1 wt% TiO2,to about 13 wt.%. Traditionally, lunar basalts have beenclassified according to their titanium content, with classesbeing named high-Ti, low-Ti, and very-low-Ti. Never-theless, global geochemical maps of titanium obtainedfrom the Clementine mission demonstrate that the lunarmaria possess a continuum of titanium concentrations,and that the highest concentrations are the least abundant.Lunar basalts show exotic textures and mineralogy, par-ticularly shock metamorphism, lack of the oxidation typi-cal of terrestrial basalts, and a complete lack of hydration.While most of the Moon's basalts erupted between about3 and 3.5 billion years ago, the oldest samples are 4.2 bil-lion years old, and the youngest flows, based on the agedating method of “crater counting,” are estimated to haveerupted only 1.2 billion years ago.Basalt is also a common rock on the surface of Mars, asdetermined by data sent back from the planet’s surface[10]and by Martian meteorites.

8 Alteration of basalt

8.1 Metamorphism

Basalts are important rocks within metamorphic belts,as they can provide vital information on the conditionsof metamorphism within the belt. Various metamorphicfacies are named after the mineral assemblages and rocktypes formed by subjecting basalts to the temperaturesand pressures of the metamorphic event. These are:

• Blueschist facies

• Eclogite facies

• Granulite facies

Basalt structures in Namibia

• Greenschist facies

• Zeolite facies

Metamorphosed basalts are important hosts for a vari-ety of hydrothermal ore deposits, including gold deposits,copper deposits, volcanogenic massive sulfide ore de-posits and others.

8.2 Weathering

Main article: Weathering

Compared to other rocks found on Earth’s surface, basaltsweather relatively fast. The typically iron-rich mineralsoxidise rapidly in water and air, staining the rock a brownto red colour due to iron oxide (rust). Chemical weath-ering also releases readily water-soluble cations such ascalcium, sodium and magnesium, which give basaltic ar-eas a strong buffer capacity against acidification. Calciumreleased by basalts binds up CO2 from the atmosphereforming CaCO3 acting thus as a CO2 trap. To this it mustbe added that the eruption of basalt itself is often associ-ated with the release of large quantities of CO2 into theatmosphere from volcanic gases.Carbon sequestration in basalt has been studied as ameans of removing carbon dioxide, produced by hu-man industrialization, from the atmosphere. Underwaterbasalt deposits, scattered in seas around the globe, havethe added benefit of the water serving as a barrier to there-release of CO2 into the atmosphere.[11]

9 See also• Basalt fiber

• Flood basalt

• Igneous rocks

• Mafic rocks

• Volcanoes

7

10 References[1] Oxford English Dictionary: basalt

[2] basalt definition – Dictionary – MSN Encarta. Archived2009-10-31.

[3] Yourdictionary.com

[4] Etymonline.com

[5] See the PETDB database.Hyndman, Donald W. (1985).Petrology of igneous and metamorphic rocks (2nd ed. ed.).McGraw-Hill. ISBN 0-07-031658-9.

[6] Blatt, Harvey and Robert Tracy (1996). Petrology (2nded. ed.). Freeman. ISBN 0-7167-2438-3.

[7] D. Weaire and N. Rivier. Contemporary Physics 25 1(1984), pp. 55–99

[8] Katrina J. Edwards, Wolfgang Bach and Daniel R. Rogers,Geomicrobiology of the Ocean Crust: A Role for Chemoau-totrophic Fe-Bacteria, Biol. Bull. 204: 180–185. (April2003) Biolbull.org

[9] Templeton, A.S., Staudigel, H., Tebo, B.M. (2005). Di-verse Mn(II)-oxidizing bacteria isolated from submarinebasalts at Loihi Seamount, Geomicrobiology Journal, v.22, 129–137. OGI.edu

[10] “MSL ChemCam Science Reports”. SPACE-FLIGHT101. Retrieved 2013-04-22.

[11] Mongabay.com

• A. Y. Ozerov, The evolution of high-alumina basaltsof the Klyuchevskoy volcano, Kamchatka, Russia,based on microprobe analyses of mineral inclusions.Journal of Volcanology and Geothermal Research,v. 95, pp. 65–79 (2000).

• A. W. Hofmann, Sampling mantle heterogeneitythrough oceanic basalts: isotopes and trace elements.Treatise on Geochemistry Volume 2, pages 61–101Elsevier Ltd. (2003). ISBN 0-08-044337-0 InMarch 2007, the article was available on the webat MPG.de.

• A. V. Sobolev and others, The amount of recycledcrust in sources of mantle-derived melts. Science, v.316, pp. 412–417 (2007). Sciencemag.org

• AblesimovN.E., ZemtsovA.N. Relaxation effects innon-equilibrium condense systems. Basalts : fromeruption up to a fiber. Moskow: ITiG FEB RAS,2010. 400 p.

11 External links• Basalt Columns

• Basalt in Northern Ireland

• Lava–water interface

• Petrology of Lunar Rocks and Mare Basalts

• Pillow lava USGS

8 12 TEXT AND IMAGE SOURCES, CONTRIBUTORS, AND LICENSES

12 Text and image sources, contributors, and licenses

12.1 Text• Basalt Source: http://en.wikipedia.org/wiki/Basalt?oldid=648617478 Contributors: AxelBoldt, Magnus Manske, Mav, Bryan Derksen,

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