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basalt
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{{Other uses}}{{pp-semi-indef}}{{short description|A magnesium- and iron-rich extrusive igneous rock}}{{more footnotes|date=August 2014}}







factoids
Basalt ({{IPAc-en|US|b|ə|ˈ|s|ɔː|l|t|,_|ˈ|b|eɪ|s|ɒ|l|t}}, {{IPAc-en|UK|ˈ|b|æ|s|ɔː|l|t|,_|ˈ|b|æ|s|əl|t}})American Heritage DictionaryMerriam-Webster DictionaryCollins English DictionaryOxford Living Dictionaries is a mafic extrusive igneous rock formed from the rapid cooling of magnesium-rich and iron-rich lavaWEB,weblink Basalt, USGS, USGS Volcano Hazards program – Glossary, 8 April 2015, 27 July 2018, exposed at or very near the surface of a terrestrial planet or a moon. More than 90% of all volcanic rock on Earth is basalt.WEB,weblink Basalt, The University of Auckland, Geology: rocks and minerals, 2005, 27 July 2018, Basalt lava has a low viscosity, due to its low silica content, resulting in rapid lava flows that can spread over great areas before cooling and solidification. Flood basalt describes the formation in a series of lava basalt flows.

Definition

File:Basalt columns in yellowstone 2.jpg|Columnar basalt flows in thumbBy definition, basalt is an aphanitic (fine-grained) igneous rock with generally 45–53% silica (SiO2)WEB,weblink USGS: Volcano Hazards Program, 2018, U.S. Geological Survey, 8 February 2018, and less than 10% feldspathoid by volume, and where at least 65% of the rock is feldspar in the form of plagioclase. This is as per definition of the International Union of Geological Sciences (IUGS) classification scheme.JOURNAL, LE BAS, M. J., STRECKEISEN, A. L., The IUGS systematics of igneous rocks, Journal of the Geological Society, 148, 5, 825–833, 10.1144/gsjgs.148.5.0825, 1991JGSoc.148..825L, 1991, 10.1.1.692.4446, JOURNAL, 1999, Rock Classification Scheme - Vol 1 - Igneous,weblink British Geological Survey: Rock Classification Scheme, 1, 1–52, WEB,weblink CLASSIFICATION OF IGNEOUS ROCKS,weblink" title="web.archive.org/web/20110930102012weblink">weblink 30 September 2011, dead, It is the most common volcanic rock type on Earth, being a key component of oceanic crust as well as the principal volcanic rock in many mid-oceanic islands, including Iceland, the Faroe Islands, Réunion and the islands of HawaiÊ»i. Basalt commonly features a very fine-grained or glassy matrix interspersed with visible mineral grains. The average density is 3.0 g/cm3.Basalt is defined by its mineral content and texture, and physical descriptions without mineralogical context may be unreliable in some circumstances. Basalt is usually grey to black in colour, but rapidly weathers to brown or rust-red due to oxidation of its mafic (iron-rich) minerals into hematite and other iron oxides and hydroxides. Although usually characterized as "dark", basaltic rocks exhibit a wide range of shading due to regional geochemical processes. Due to weathering or high concentrations of plagioclase, some basalts can be quite light-coloured, superficially resembling andesite to untrained eyes. Basalt has a fine-grained mineral texture due to the molten rock cooling too quickly for large mineral crystals to grow; it is often porphyritic, containing larger crystals (phenocrysts) formed prior to the extrusion that brought the magma to the surface, embedded in a finer-grained matrix. These phenocrysts usually are of olivine or a calcium-rich plagioclase, which have the highest melting temperatures of the typical minerals that can crystallize from the melt.Basalt with a vesicular texture is called vesicular basalt, when the bulk of the rock is mostly solid; when the vesicles are over half the volume of a specimen, it is called scoria. This texture forms when dissolved gases come out of solution and form bubbles as the magma decompresses as it reaches the surface, yet are trapped as the erupted lava hardens before the gases can escape.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 (coarser) groundmass are generally referred to as diabase (also called dolerite) or, when more coarse-grained (crystals over 2 mm across), as gabbro. Gabbro is often marketed commercially as "black granite."(File:Szentgyörgyhegy03.jpg|thumb|upright|Columnar basalt at Szent György Hill, Hungary)File:VessicularBasalt1.JPG|thumb|Vesicular basalt at Sunset Crater, Arizona. US quarterUS quarterIn the Hadean, Archean, and early Proterozoic eras of Earth's history, the chemistry of erupted magmas was significantly different from today's, due to immature crustal and asthenosphere differentiation. These ultramafic volcanic rocks, with silica (SiO2) contents below 45% are usually classified as komatiites.

Etymology

The word "basalt" is ultimately derived from Late Latin basaltes, a misspelling of Latin basanites "very hard stone", which was imported from Ancient Greek βασανίτης (basanites), from βάσανος (basanos, "touchstone") and perhaps originated in Egyptian bauhun "slate".ENCYCLOPEDIA, basalt (n.), Online Etymology Dictionary, Douglas, Harper,weblink 4 November 2015, The modern petrological term basalt describing a particular composition of lava-derived rock originates from its use by Georgius Agricola in 1556 in his famous work of mining and mineralogy De re metallica, libri XII. Agricola applied "basalt" to the volcanic black rock of the Schloßberg (local castle hill) at Stolpen, believing it to be the same as the "very hard stone" described by Pliny the Elder in Naturalis Historiae.Pliny the Elder, Naturalis Historiae. Book 36, section 11 (Loeb Classical Library): "The Egyptians also discovered in Ethiopia what is called basanites, a stone which in colour and hardness resembles iron: hence the name they have given it." This stone is now believed to have been greywacke, a sedimentary rock unrelated to basalt.

Types

File:Giants causeway closeup.jpg|thumb|Large masses must cool slowly to form a polygonal joint pattern, as here at the Giant's CausewayGiant's CausewayFile:Базальтове.jpg|thumb|right|Near Bazaltove, UkraineUkraine
  • Tholeiitic basalt is relatively rich in silica and poor in sodium. Included in this category are most basalts of the ocean floor, most large oceanic islands, and continental flood basalts such as the Columbia River Plateau.
    • High and low titanium basalts. Basalt rocks are in some cases classified after their titanium (Ti) content in High-Ti and Low-Ti varieties. High-Ti and Low-Ti basalts have been distinguished in the Paraná and Etendeka trapsJOURNAL, Gibson, S. A., Thompson, R. N., Dickin, A. P., & Leonardos, O. H., 1995, Earth and Planetary Science Letters, 136, 3, 149–165, High-Ti and low-Ti mafic potassic magmas: Key to plume-lithosphere interactions and continental flood-basalt genesis, 10.1016/0012-821X(95)00179-G, 1995E&PSL.136..149G, and the Emeishan Traps.JOURNAL,weblink Hou, T., Zhang, Z., Kusky, T., Du, Y., Liu, J., & Zhao, Z., 2011, Ore Geology Reviews, 41, 1, 133–143, A reappraisal of the high-Ti and low-Ti classification of basalts and petrogenetic linkage between basalts and mafic–ultramafic intrusions in the Emeishan Large Igneous Province, SW China, 2016-09-18, 10.1016/j.oregeorev.2011.07.005,
    • Mid-ocean ridge basalt (MORB) is a tholeiitic basalt commonly erupted only at ocean ridges and is characteristically low in incompatible elements.BOOK, Hyndman, Donald W., Petrology of igneous and metamorphic rocks, 2nd, 1985, McGraw-Hill, 978-0-07-031658-4, BOOK, Blatt, Harvey, Robert Tracy, yes, Petrology, 2nd, 1996, Freeman, 978-0-7167-2438-4,
      • E-MORB, enriched MORB
      • N-MORB, normal MORB
      • D-MORB, depleted MORB
  • High-alumina basalt may be silica-undersaturated or -oversaturated (see normative mineralogy). It has greater than 17% alumina (Al2O3) and is intermediate in composition between tholeiitic basalt and alkali basalt; the relatively alumina-rich composition is based on rocks without phenocrysts of plagioclase.
  • Alkali basalt is relatively poor in silica and rich in sodium. It is silica-undersaturated and may contain feldspathoids, alkali feldspar and phlogopite.
  • Boninite is a high-magnesium form of basalt{{Citation needed|date=August 2018}} that is erupted generally in back-arc basins, distinguished by its low titanium content and trace-element composition.
  • Ocean island basalt
  • Lunar basalt

Petrology

File:LvMS-Lvm.jpg|thumb|Photomicrograph of a volcanic (basaltic) sand grainsand grainThe mineralogy of basalt is characterized by a preponderance of calcic plagioclase feldspar and pyroxene. Olivine can also be a significant constituent. Accessory minerals present in relatively minor amounts include iron oxides and iron-titanium oxides, such as magnetite, ulvöspinel, and ilmenite. Because of the presence of such oxide minerals, basalt can acquire strong magnetic signatures as it cools, and 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 tholeiitic basalt has augite and orthopyroxene or pigeonite with abundant olivine, but olivine may have rims of pyroxene and is unlikely to be present in the groundmass. Ocean floor basalts, erupted originally at mid-ocean ridges, are known as MORB (mid-ocean ridge basalt) and are characteristically low in incompatible elements.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 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 Â°C (liquidus) and near or below 1000 Â°C (solidus); these values are higher than those of other common igneous rocks.The majority of tholeiitic basalts 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.BOOK,weblink Plate Tectonics and Crustal Evolution, Condie, Kent C., Butterworth-Heinemann / Elsevier, 1997, 978-0-7506-3386-4, 69, Chapter 3: "Tectonic settings", JOURNAL, KUSHIRO, Ikuo, 2007, Origin of magmas in subduction zones: a review of experimental studies,weblink Proceedings of the Japan Academy, Series B, en, 83, 1, 1–15, 10.2183/pjab.83.1, 24019580, 3756732, 0386-2208, 2007PJAB...83....1K, The origin of high-alumina basalt continues to be controversial, with disagreement over whether it is a primary melt or derived from other basalt types by fractionation.JOURNAL, Ozerov, Alexei Y, 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, January 2000, 95, 1–4, 65–79, 10.1016/S0377-0273(99)00118-3, 2000JVGR...95...65O, {{rp|65}}

Geochemistry

Relative to most common igneous rocks, basalt compositions are rich in MgO and CaO and low in SiO2 and the alkali oxides, i.e., Na2O + K2O, consistent with the TAS classification.Basalt generally has a composition of 45–55 wt% SiO2, 2–6 wt% total alkalis, 0.5–2.0 wt% TiO2, 5–14 wt% FeO and 14 wt% or more Al2O3. 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% Al2O3; boninites have magnesium (MgO) contents of up to 15 percent. Rare feldspathoid-rich mafic rocks, akin to alkali basalts, may have Na2O + K2O contents of 12% or more.The abundances of the lanthanide or rare-earth elements (REE) can be a useful diagnostic tool to help explain the history of mineral crystallisation as the melt cooled. In particular, the relative abundance of europium compared to the other REE is often markedly higher or lower, and called the europium anomaly. It arises because Eu2+ can substitute for Ca2+ in plagioclase feldspar, unlike any of the other lanthanides, which tend to only form 3+ cations.Mid-ocean ridge basalts (MORB) and their intrusive equivalents, gabbros, are the characteristic igneous rocks formed at mid-ocean ridges. They are tholeiitic basalts particularly low in total alkalis and in incompatible trace elements, and they have relatively flat rare-earth element (REE) patterns normalized 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.BOOK, Hofmann, A. W., 3.3 – Sampling Mantle Heterogeneity through Oceanic Basalts: Isotopes and Trace Elements, Richard W., Carlson, The Mantle and Core, Treatise on Geochemistry, 3, 21 October 2014, Elsevier B.V., 67–101, 10.1016/B978-0-08-095975-7.00203-5, 978-0-08-098300-4, Mid-ocean ridge basalts have been subdivided into varieties such as normal (NMORB) and those slightly more enriched in incompatible elements (EMORB).Isotope ratios of elements such as strontium, neodymium, lead, hafnium, and osmium in basalts have been much studied to learn about the evolution of the Earth's mantle. Isotopic ratios of noble gases, such as 3He/4He, are also of great value: for instance, ratios for basalts range from 6 to 10 for mid-ocean ridge tholeiitic basalt (normalized to atmospheric values), but to 15–24 and more for ocean-island basalts thought to be derived from mantle plumes.Source rocks for the partial melts probably include both peridotite and pyroxenite (e.g., Sobolev et al., 2007).

Morphology and textures

(File:20011005-0039 DAS large.jpg|thumb|An active basalt lava flow)The shape, structure and texture of a basalt is diagnostic of how and where it erupted—whether into the sea, in an explosive cinder eruption or as creeping pāhoehoe lava flows, the classic image of Hawaiian basalt eruptions.

Subaerial eruptions

Basalt that 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 cones will often be highly vesiculated, imparting a lightweight "frothy" texture to the rock. Basaltic cinders are often red, coloured by oxidized iron from weathered iron-rich minerals such as pyroxene.ʻAʻā types of blocky, cinder and breccia flows of thick, viscous basaltic lava are common in Hawaiʻi. 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 lakes. Lava tubes are common features of pāhoehoe 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 buildup of volcanic gases. Hawaiʻi'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.Amygdaloidal structure is common in relict vesicles and beautifully crystallized species of zeolites, quartz or calcite are frequently found.

Columnar basalt

File:Causeway-code poet-4.jpg|thumb|The Giant's CausewayGiant's CausewayFile:Boyabat.jpg|thumb|Columnar jointed basalt in TurkeyTurkeyFile:Мыс Столбчатый. После заката.jpg|thumb|Columnar basalt at Cape StolbchatyCape Stolbchaty{{See also|List of places with columnar basalt}}During the cooling of a thick lava flow, contractional joints or fractures form.Smalley, I.J. 1966. Contraction crack networks in basalt flows. Geological Magazine 103, 110-114.weblink If a flow cools relatively rapidly, significant contraction forces build up. While a flow can shrink in the vertical dimension without fracturing, it can't 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 predominantly hexagonal in cross-section, but polygons with three to twelve or more sides can be observed.JOURNAL, Weaire, D., Rivier, N., Soap, cells and statistics—random patterns in two dimensions, Contemporary Physics, 20 August 2006, 25, 1, 59–99, 10.1080/00107518408210979, 1984ConPh..25...59W, The size of the columns depends loosely on the rate of cooling; very rapid cooling may result in very small (

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