Basalt (UK:/ˈbæsɒlt,-ɔːlt,-əlt/;[1][2]US:/bəˈsɔːlt,ˈbeɪsɔːlt/)[3] is anaphanitic (fine-grained)extrusiveigneous rock formed from the rapid cooling of low-viscositylava rich inmagnesium andiron (mafic lava) exposed at or very near thesurface of arocky planet ormoon. More than 90% of allvolcanic rock on Earth is basalt. Rapid-cooling, fine-grained basalt has the same chemical composition and mineralogy as slow-cooling, coarse-grainedgabbro. The eruption of basalt lava is observed by geologists at about 20 volcanoes per year. Basalt is also an important rock type on other planetary bodies in theSolar System. For example, the bulk of the plains ofVenus, which cover ~80% of the surface, are basaltic; thelunar maria are plains of flood-basalticlava flows; and basalt is a common rock on the surface ofMars.
Molten basalt lava has a low viscosity due to its relatively lowsilica content (between 45% and 52%), resulting in rapidly moving lava flows that can spread over great areas before cooling and solidifying.Flood basalts are thick sequences of many such flows that can cover hundreds of thousands of square kilometres and constitute the most voluminous of all volcanic formations.
Basalticmagmas within Earth are thought to originate from theupper mantle. The chemistry of basalts thus provides clues to processes deep inEarth's interior.
Basalt is composed mostly of oxides of silicon, iron, magnesium, potassium, aluminum, titanium, and calcium.Geologists classifyigneous rock by its mineral content whenever possible; the relative volume percentages ofquartz (crystallinesilica (SiO2)),alkali feldspar,plagioclase, andfeldspathoid (QAPF) are particularly important. Anaphanitic (fine-grained) igneous rock is classified as basalt when its QAPF fraction is composed of less than 10% feldspathoid and less than 20% quartz, and plagioclase makes up at least 65% of its feldspar content. This places basalt in the basalt/andesite field of the QAPF diagram. Basalt is further distinguished from andesite by its silica content of under 52%.[4][5][6][7]
It is often not practical to determine the mineral composition of volcanic rocks, due to their very small grain size, in which case geologists instead classify the rocks chemically, with particular emphasis on the total content of alkali metal oxides and silica (TAS); in that context, basalt is defined as volcanic rock with a content of between 45% and 52% silica and no more than 5% alkali metal oxides. This places basalt in the B field of the TAS diagram.[4][5][7] Such a composition is described asmafic.[8]
Basalt is usually dark grey to black in colour, due to a high content ofaugite or other dark-colouredpyroxene minerals,[9][10][11] but can exhibit a wide range of shading. Some basalts are quite light-coloured due to a high content of plagioclase; these are sometimes described asleucobasalts.[12][13] It can be difficult to distinguish between lighter-colored basalt andandesite, sofield researchers commonly use arule of thumb for this purpose, classifying it as basalt if it has acolor index of 35 or greater.[14]
The physical properties of basalt result from its relatively low silica content and typically high iron and magnesium content.[15] The average density of basalt is 2.9 g/cm3, compared, for example, togranite’s typical density of 2.7 g/cm3.[16] The viscosity of basaltic magma is relatively low—around 104 to 105cP—similar to the viscosity ofketchup, but that is still several orders of magnitude higher than the viscosity of water, which is about 1 cP).[17]
Basalt is oftenporphyritic, containing larger crystals (phenocrysts) that formed before the extrusion event that brought the magma to the surface, embedded in a finer-grainedmatrix. These phenocrysts are usually made of augite,olivine, or a calcium-rich plagioclase,[10] which havethe highest melting temperatures of any of theminerals that can typically crystallize from the melt, and which are therefore the first to form solid crystals.[18][19]
Basalt often containsvesicles; they are formed when dissolved gases bubble out of the magma as it decompresses during its approach to the surface; the erupted lava then solidifies before the gases can escape. When vesicles make up a substantial fraction of the volume of the rock, the rock is described asscoria.[20][21]
The termbasalt is at times applied to shallowintrusive rocks with a composition typical of basalt, but rocks of this composition with aphaneritic (coarser) groundmass are more properly referred to either asdiabase (also called dolerite) or—when they are more coarse-grained (having crystals over 2 mm across)—asgabbro. Diabase and gabbro are thus thehypabyssal andplutonic equivalents of basalt.[5][22]
Columnar basalt at Szent György Hill, Hungary
During theHadean,Archean, and earlyProterozoiceons of Earth's history, the chemistry of erupted magmas was significantly different from what it is today, due to immature crustal andasthenosphere differentiation. The resultingultramafic volcanic rocks, with silica (SiO2) contents below 45% and high magnesium oxide (MgO) content, are usually classified askomatiites.[23][24]
Etymology
The word "basalt" is ultimately derived fromLate Latinbasaltes, a misspelling of Latinbasanites "veryhard stone", which was imported fromAncient Greekβασανίτης (basanites), fromβάσανος (basanos, "touchstone").[25] The modern petrological termbasalt, describing a particular composition oflava-derived rock, became standard because of its use byGeorgius Agricola in 1546, in his workDe Natura Fossilium. Agricola applied the term "basalt" to the volcanic black rock beneath theBishop of Meissen'sStolpen castle, believing it to be the same as the "basaniten" described byPliny the Elder in AD 77 inNaturalis Historiae.[26]
Types
Large masses must cool slowly to form a polygonal joint pattern, as here at theGiant's Causeway in Northern IrelandColumns of basalt nearBazaltove, Ukraine
On Earth, most basalt is formed bydecompression melting of themantle.[27] The high pressure in the upper mantle (due tothe weight of the overlying rock) raises the melting point of mantle rock, so that almost all of the upper mantle is solid. However, mantle rock isductile (the solid rock slowly deforms under high stress). Whentectonic forces cause hot mantle rock to creep upwards, pressure on the ascending rock decreases, and this can lower its melting point enough for the rock topartially melt, producing basaltic magma.[28]
Decompression melting can occur in a variety oftectonic settings, including in continentalrift zones, atmid-ocean ridges, abovegeological hotspots,[29][30] and inback-arc basins.[31] Basalt also forms insubduction zones, where mantle rock rises into amantle wedge above the descending slab. The slab releases water vapor and other volatiles as it descends, which further lowers the melting point, further increasing the amount of decompression melting.[32] Each tectonic setting produces basalt with its own distinctive characteristics.[33]
High- and low-titanium basalt rocks, which are sometimes classified based on theirtitanium (Ti) content in High-Ti and Low-Ti varieties. High-Ti and Low-Ti basalt have been distinguished from each other in theParaná and Etendeka traps[37] and theEmeishan Traps.[38]
Mid-ocean ridge basalt (MORB) is a tholeiitic basalt that has almost exclusively erupted at ocean ridges; it is characteristically low inincompatible elements.[39][9] Although all MORBs are chemically similar, geologists recognize that they vary significantly in how depleted they are in incompatible elements. When they are present in close proximity along mid-ocean ridges, that is seen as evidence for mantle inhomogeneity.[40]
Enriched MORB (E-MORB) is defined as MORB that is relatively undepleted in incompatible elements. It was once thought to be mostly located in hot spots along mid-ocean ridges, such as Iceland, but it is now known to be located in many other places along those ridges.[41]
Normal MORB (N-MORB) is defined as MORB that has an average amount of incompatible elements.
D-MORB, depleted MORB, is defined as MORB that is highly depleted in incompatible elements.
High-alumina basalt has greater than 17%alumina (Al2O3) and is intermediate in composition between tholeiitic basalt and alkali basalt. Its relatively alumina-rich composition is based on rocks without phenocrysts ofplagioclase. These represent the low-silica end of thecalc-alkaline magma series and are characteristic ofvolcanic arcs above subduction zones.[44]
Boninite is a high-magnesium form of basalt that is erupted generally inback-arc basins; it is distinguished by its low titanium content and trace-element composition.[45]
Ocean island basalts include both tholeiites and alkali basalts; the tholeiites predominate early in the eruptive history of the island. These basalts are characterized by elevated concentrations of incompatible elements, which suggests that their source mantle rock has produced little magma in the past (it isundepleted).[46]
The mineralogy of basalt is characterized by a preponderance of calcic plagioclasefeldspar andpyroxene.Olivine can also be a significant constituent.[47] Accessoryminerals present in relatively minor amounts includeiron oxides and iron-titanium oxides, such asmagnetite,ulvöspinel, andilmenite.[42] Because of the presence of suchoxide minerals, basalt can acquire strongmagnetic signatures as it cools, andpaleomagnetic studies have made extensive use of basalt.[48]
Intholeiitic basalt, pyroxene (augite andorthopyroxene orpigeonite) andcalcium-rich plagioclase are common phenocryst minerals. Olivine may also be a phenocryst, and when present, may have rims of pigeonite. Thegroundmass contains interstitial quartz ortridymite orcristobalite.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 thegroundmass.[42]
Basalt has highliquidus andsolidus temperatures—values at the Earth's surface are near or above 1200 °C (liquidus)[49] and near or below 1000 °C (solidus); these values are higher than those of other common igneous rocks.[50]
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.[51][52] The origin of high-alumina basalt continues to be controversial, with disagreement over whether it is aprimary melt or derived from other basalt types by fractionation.[53]: 65
Geochemistry
Relative to most common igneous rocks, basalt compositions are rich inMgO andCaO and low inSiO2 and the alkali oxides, i.e.,Na2O +K2O, consistent with theirTAS classification. Basalt contains more silica thanpicrobasalt and mostbasanites andtephrites but less thanbasaltic andesite. Basalt has a lower total content of alkali oxides thantrachybasalt and most basanites and tephrites.[7]
Basalt generally has a composition of 45–52wt% SiO2, 2–5 wt% total alkalis,[7] 0.5–2.0 wt%TiO2, 5–14 wt%FeO and 14 wt% or moreAl2O3. Contents of CaO are commonly near 10 wt%, those of MgO commonly in the range 5 to 12 wt%.[54]
High-alumina basalts have aluminium contents of 17–19 wt% Al2O3;boninites havemagnesium (MgO) contents of up to 15 percent. Rarefeldspathoid-richmafic rocks, akin to alkali basalts, may have Na2O + K2O contents of 12% or more.[55]
The abundances of thelanthanide orrare-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 theeuropium anomaly. It arises because Eu2+ can substitute for Ca2+ in plagioclase feldspar, unlike any of the other lanthanides, which tend to only form3+cations.[56]
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 inincompatible trace elements, and they have relatively flat REE patterns normalized to mantle orchondrite 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 understandingplate 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 theMid-Atlantic Ridge, and the compositions also define different ranges in different ocean basins.[57] Mid-ocean ridge basalts have been subdivided into varieties such as normal (NMORB) and those slightly more enriched in incompatible elements (EMORB).[58]
Isotope ratios ofelements such asstrontium,neodymium,lead,hafnium, andosmium in basalts have been much studied to learn about the evolution of theEarth's mantle.[59] Isotopic ratios ofnoble gases, such as3He/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 frommantle plumes.[60]
Source rocks for the partial melts that produce basaltic magma probably include bothperidotite andpyroxenite.[61]
Morphology and textures
An active basalt lava flow
The shape, structure andtexture of a basalt is diagnostic of how and where it erupted—for example, whether into the sea, in an explosivecinder eruption or as creepingpāhoehoe lava flows, the classic image ofHawaiian basalt eruptions.[62]
Basalt that erupts under open air (that is,subaerially) forms three distinct types of lava or volcanic deposits:scoria;ash or cinder (breccia);[63] and lava flows.[64]
Basalt in the tops of subaerial lava flows andcinder cones will often be highlyvesiculated, imparting a lightweight "frothy" texture to the rock.[65] Basaltic cinders are often red, coloured by oxidizediron from weathered iron-rich minerals such aspyroxene.[66]
ʻAʻā types of blocky cinder and breccia flows of thick, viscous basalticlava 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 formslava lakes.Lava tubes are common features of pāhoehoe eruptions.[64]
Basaltictuff orpyroclastic 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 ofvolcanic gases. Hawaiʻi'sMauna Loa volcano erupted in this way in the 19th century, as didMount 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.[67]
During the cooling of a thick lava flow, contractionaljoints or fractures form.[69] If a flow cools relatively rapidly, significantcontraction 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 ofcolumns. These structures, orbasalt prisms, are predominantly hexagonal in cross-section, but polygons with three to twelve or more sides can be observed.[70] The size of the columns depends loosely on the rate of cooling; very rapid cooling may result in very small (<1 cm diameter) columns, while slow cooling is more likely to produce large columns.[71]
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.[72] It has been estimated that at depths greater than 500 metres (1,600 ft), explosive activity associated with basaltic magma is suppressed.[73] Above this depth, submarine eruptions are often explosive, tending to producepyroclastic rock rather than basalt flows.[74] These eruptions, described as Surtseyan, are characterised by large quantities of steam and gas and the creation of large amounts ofpumice.[75]
When basalt erupts underwater or flows into the sea, contact with the water quenches the surface and the lava forms a distinctivepillow 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.[76]
Whenpāhoehoe lava enters the sea it usually forms pillow basalts. However, whenʻaʻā enters the ocean it forms alittoral cone, a small cone-shaped accumulation of tuffaceous debris formed when the blockyʻaʻā lava enters the water and explodes from built-up steam.[77]
The island ofSurtsey in theAtlantic 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.[78][79]
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.[80]
Pillow basalt is also produced by somesubglacial volcanic eruptions.[80]
Distribution
Earth
Basalt is the most common volcanic rock type on Earth, making up over 90% of all volcanic rock on the planet.[81] Thecrustal portions ofoceanictectonic plates are composed predominantly of basalt, produced from upwelling mantle below theocean ridges.[82] Basalt is also the principal volcanic rock in manyoceanic islands, including the islands ofHawaiʻi,[35] theFaroe Islands,[83] andRéunion.[84] The eruption of basalt lava is observed by geologists at about 20 volcanoes per year.[85]
As well as forming large parts of the Earth's crust, basalt also occurs in other parts of the Solar System. Basalt commonly erupts onIo (the third largest moon ofJupiter),[97] and has also formed on theMoon,Mars,Venus, and the asteroidVesta.
Lunar basalts differ from their Earth counterparts principally in their high iron contents, which typically range from about 17 to 22 wt% FeO. They also possess a wide range of titanium concentrations (present in the mineralilmenite),[99][100] ranging from less than 1 wt% TiO2, 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 theClementine mission demonstrate that the lunar maria possess a continuum of titanium concentrations, and that the highest concentrations are the least abundant.[101]
Lunar basalts show exotic textures and mineralogy, particularlyshock metamorphism, lack of theoxidation typical of terrestrial basalts, and a complete lack ofhydration.[102] Most of theMoon's basalts erupted between about 3 and 3.5 billion years ago, but the oldest samples are 4.2 billion years old, and the youngest flows, based on the age dating method ofcrater counting, are estimated to have erupted only 1.2 billion years ago.[103]
Venus
From 1972 to 1985, fiveVenera and twoVEGA landers successfully reached the surface of Venus and carried out geochemical measurements using X-ray fluorescence and gamma-ray analysis. These returned results consistent with the rock at the landing sites being basalts, including both tholeiitic and highly alkaline basalts. The landers are thought to have landed on plains whose radar signature is that of basaltic lava flows. These constitute about 80% of the surface of Venus. Some locations show high reflectivity consistent with unweathered basalt, indicating basaltic volcanism within the last 2.5 million years.[104]
Mars
Basalt is also a common rock on the surface ofMars, as determined by data sent back from the planet's surface,[105] and byMartian meteorites.[106][107]
Vesta
Analysis ofHubble Space Telescope images of Vesta suggests thisasteroid has a basaltic crust covered with a brecciatedregolith derived from the crust.[108] Evidence from Earth-based telescopes and theDawn mission suggest that Vesta is the source of theHED meteorites, which have basaltic characteristics.[109] Vesta is the main contributor to the inventory of basaltic asteroids of the main Asteroid Belt.[110]
Io
Lava flows represent a major volcanic terrain onIo.[111] Analysis of theVoyager images led scientists to believe that these flows were composed mostly of various compounds of molten sulfur. However, subsequent Earth-basedinfrared studies and measurements from theGalileo spacecraft indicate that these flows are composed of basaltic lava with mafic to ultramafic compositions.[112] This conclusion is based on temperature measurements of Io's "hotspots", or thermal-emission locations, which suggest temperatures of at least 1,300 K and some as high as 1,600 K.[113] Initial estimates suggesting eruption temperatures approaching 2,000 K[114] have since proven to be overestimates because the wrong thermal models were used to model the temperatures.[113][115]
Kaolinized basalt near Hungen, Vogelsberg, Germany
Compared to granitic rocks exposed at the Earth's surface, basaltoutcrops weather relatively rapidly. This reflects their content of minerals that crystallized at higher temperatures and in an environment poorer in water vapor than granite. These minerals are less stable in the colder, wetter environment at the Earth's surface. The finer grain size of basalt and thevolcanic glass sometimes found between the grains also hasten weathering. The high iron content of basalt causes weathered surfaces in humid climates to accumulate a thick crust ofhematite or other iron oxides and hydroxides, staining the rock a brown to rust-red colour.[116][117][118][119] Because of the low potassium content of most basalts, weathering converts the basalt to calcium-richclay (montmorillonite) rather than potassium-rich clay (illite). Further weathering, particularly in tropical climates, converts the montmorillonite tokaolinite orgibbsite. This produces the distinctive tropicalsoil known aslaterite.[116] The ultimate weathering product isbauxite, the principal ore of aluminium.[120]
Chemical weathering also releases readily water-soluble cations such ascalcium,sodium andmagnesium, which give basaltic areas a strongbuffer capacity againstacidification.[121] Calcium released by basalts bindsCO2 from the atmosphere formingCaCO3 acting thus as a CO2 trap.[122]
Metamorphism
Metamorphosed basalt from anArcheangreenstone belt in Michigan, US. The minerals that gave the original basalt its black colour have been metamorphosed into green minerals.
Intense heat or great pressure transforms basalt into itsmetamorphic rock equivalents. Depending on the temperature and pressure of metamorphism, these may includegreenschist,amphibolite, oreclogite. Basalts are important rocks within metamorphic regions because they can provide vital information on the conditions ofmetamorphism that have affected the region.[123]
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 forbacteria. Some Fe(II)-oxidizing bacteria cultured from iron-sulfide surfaces are also able to grow with basaltic rock as a source of Fe(II).[125] Fe- and Mn- oxidizing bacteria have been cultured from weathered submarine basalts ofKamaʻehuakanaloa Seamount (formerly Loihi).[126] The impact of bacteria on altering the chemical composition of basaltic glass (and thus, theoceanic crust) and seawater suggest that these interactions may lead to an application ofhydrothermal vents to theorigin of life.[127]
Uses
TheCode of Hammurabi was engraved on a 2.25 m (7 ft4+1⁄2 in) tall basaltstele in around 1750 BC.
Carbon sequestration in basalt has been studied as a means of removing carbon dioxide, produced by human industrialization, from the atmosphere. Underwater basalt deposits, scattered in seas around the globe, have the added benefit of the water serving as a barrier to the re-release of CO2 into the atmosphere.[136][137]
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