|Mafic: plagioclase, amphibole, and pyroxene, sometimes feldspathoids, and/or olivine.|
Basalt (, ) is a mafic extrusive igneous rock formed from the rapid cooling of lava rich in magnesium and iron 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, and the eruption of basalt lava is observed by geologists at about 20 volcanoes per year.
Molten basalt lava has a low viscosity due to its relatively low silica 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 kilometers and constitute the most voluminous of all volcanic formations.
Basalt is also an important rock type on other planetary bodies in the Solar System; for example, the lunar maria are plains of flood basaltic lava flows, and basalt is a common rock on the surface of Mars.
Basalt is an aphanitic (fine-grained) igneous rock that is relatively low in silica and alkali metals. It has less than 10% feldspathoid by volume, with at least 65% of the rock consisting of feldspar in the form of plagioclase. This places basalt in the basalt/andesite field of the QAPF diagram. Basalt is further distinguished from andesite by its silica content of less than 52%. However, it is often not possible to determine the mineral composition of volcanic rocks, due to their very fine grain size, and basalt is then defined chemically as volcanic rock with a content of 45% to 52% silica and not more than 5% alkali metal oxides. This places basalt in the B field of the TAS classification. The average density of basalt is 2.9 g/cm3.
Basalt is usually dark grey to black in colour, due to a high content of augite or other pyroxene minerals, but can exhibit a wide range of shading. Some basalts are quite light-coloured due to a high content of plagioclase, and these are sometimes described as leucobasalts. Lighter basalt can be difficult to distinguish from andesite, but a common rule of thumb, used away from the laboratory, is that basalt has a color index of 35 or greater.
Basalt 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 augite, olivine, or a calcium-rich plagioclase, which have the highest melting temperatures of the typical minerals that can crystallize from the melt and are therefore the first to form solid crystals.
Basalt often contains vesicles, formed when dissolved gases bubble out of the magma as it decompresses during its approach to the surface, and 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 as scoria.
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 more properly referred to as diabase (also called dolerite) or, when more coarse-grained (crystals over 2 mm across), as gabbro. Diabase and gabbro are thus the hypabyssal and plutonic equivalents of basalt.
In the Hadean, Archean, and early Proterozoic eons 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.
The word "basalt" is ultimately derived from Late Latin basaltes, a misspelling of Latin basanites "very hard stone", which was imported from Ancient Greek (basanites), from ? (basanos, "touchstone") and perhaps originated in Egyptian bauhun "slate".[better source needed] The modern petrological term basalt describing a particular composition of lava-derived rock, originates from its use by Georgius Agricola in 1546 in his work De Natura Fossilium. Agricola applied "basalt" to the volcanic black rock beneath the Bishop of Meißen's Stolpen castle, believing it to be the same as the "basaniten" described by Pliny the Elder in AD 77 in Naturalis Historiae.
The 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 °C (liquidus) and near or below 1000 °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. 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.:65
Basalt generally has a composition of 45-52 wt% SiO2, 2-5 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 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. 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.
The shape, structure and texture of a basalt is diagnostic of how and where it erupted--for example, whether into the sea, in an explosive cinder eruption or as creeping p?hoehoe lava flows, the classic image of Hawaiian basalt eruptions.
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.
?A types of blocky, cinder and breccia flows of thick, viscous basaltic lava are common in Hawai?i. P?hoehoe 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 p?hoehoe eruptions.
Basaltic tuff or 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 gases. Hawai?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.
During the cooling of a thick lava flow, contractional 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. These structures are predominantly hexagonal in cross-section, but polygons with three to twelve or more sides can be observed. 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.
When basalt erupts underwater or flows into the sea, contact with the 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 meters.
When p?hoehoe lava enters the sea it usually forms pillow basalts. However, when ?a enters the ocean it forms a littoral cone, a small cone-shaped accumulation of tuffaceous debris formed when the blocky ?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 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 p?hoehoe-type behaviour.
As well as forming large parts of the Earth's crust, basalt also occurs in other parts of the Solar System.
Basalt is the most common volcanic rock type on Earth. The crustal portions of oceanic tectonic plates are composed predominantly of basalt, produced from upwelling mantle below the ocean ridges. Basalt is also the principal volcanic rock in many oceanic islands, including the islands of Hawai?i, the Faroe Islands, and Réunion.
Basalt is the rock most typical of large igneous provinces. These include continental flood basalts, the most voluminous basalts found on land. Examples of continental flood basalts included the Deccan Traps in India, the Chilcotin Group in British Columbia, Canada, the Paraná Traps in Brazil, the Siberian Traps in Russia, the Karoo flood basalt province in South Africa, and the Columbia River Plateau of Washington and Oregon.
Ancient Precambrian basalts are usually only found in fold and thrust belts, and are often heavily metamorphosed. These are known as greenstone belts, because low-grade metamorphism of basalt produces chlorite, actinolite, epidote and other green minerals.
The dark areas visible on Earth's moon, the lunar maria, are plains of flood basaltic 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 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 mineral ilmenite), 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 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 oxidation typical of terrestrial basalts, and a complete lack of hydration. Most of the Moon'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 of crater counting, are estimated to have erupted only 1.2 billion years ago.
From 1972 to 1985, five Venera and two VEGA 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.
Analysis of Hubble Space Telescope images of Vesta suggests this asteroid has a basaltic crust covered with a brecciated regolith derived from the crust. Evidence from Earth-based telescopes and the Dawn mission suggest that Vesta is the source of the HED meteorites, which have basaltic characteristics.
Lava flows represent a major volcanic terrain on Io. Analysis of the Voyager images led scientists to believe that these flows were composed mostly of various compounds of molten sulfur. However, subsequent Earth-based infrared studies and measurements from the Galileo spacecraft indicate that these flows are composed of basaltic lava with mafic to ultramafic compositions. 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. Initial estimates suggesting eruption temperatures approaching 2,000 K have since proven to be overestimates because the wrong thermal models were used to model the temperatures.
Compared to other rocks found on Earth's surface, exposed surfaces of basalt outcrops weather relatively rapidly in water and air due to oxidation of iron-rich minerals into hematite or other iron oxides and hydroxides, staining the rock a brown to rust-red colour.
Chemical weathering also releases readily water-soluble cations such as calcium, sodium and magnesium, which give basaltic areas a strong buffer capacity against acidification. Calcium released by basalts binds CO2 from the atmosphere forming CaCO3 acting thus as a CO2 trap.
Intense heat or great pressure transforms basalt into its metamorphic rock equivalents. Basalts are important rocks within metamorphic regions because they can provide vital information on the conditions of metamorphism that have affected the region.
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. Some Fe(II)-oxidizing bacteria cultured from iron-sulfide surfaces are also able to grow with basaltic rock as a source of Fe(II). Fe- and Mn- oxidizing bacteria have been cultured from weathered submarine basalts of Loihi Seamount. 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.
Basalt is used in construction (e.g. as building blocks or in the groundwork), making cobblestones (from columnar basalt) and in making statues. Heating and extruding basalt yields stone wool, which as potential to be an excellent thermal insulator.
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.