In geology, felsic refers to igneous rocks that are rich in elements that form feldspar and quartz. It is contrasted with mafic rocks, which are richer in magnesium and iron. Felsic refers to silicate minerals and rocks which are enriched in the lighter elements such as silicon, aluminium and potassium. Felsic magma or lava is higher in viscosity than mafic magma/lava. Felsic rocks are light in color and have specific gravities less than 3; the most common felsic rock is granite. Common felsic minerals include quartz, muscovite and the sodium-rich plagioclase feldspars. In modern usage, the term acid rock, although sometimes used as a synonym now refers to a high-silica-content volcanic rock, such as rhyolite. Older, broader usage is now considered archaic; that usage, with the contrasting term "basic rock", was based on an incorrect idea, dating from the 19th century, that "silicic acid" was the chief form of silicon occurring in rocks. The term "felsic" combines the words "feldspar" and "silica".
The similarity of the resulting term felsic to the German felsig, "rocky", is purely accidental. Feldspar is linked to German, it is a borrowing of Feldspat. The link is therefore to German Feld, meaning "field". In order for a rock to be classified as felsic, it needs to contain more than 75% felsic minerals. Rocks with greater than 90% felsic minerals can be called leucocratic, from the Greek words for white and dominance. Felsite is a petrologic field term used to refer to fine-grained or aphanitic, light-colored volcanic rocks which might be reclassified after a more detailed microscopic or chemical analysis. In some cases, felsic volcanic rocks may contain phenocrysts of mafic minerals hornblende, pyroxene or a feldspar mineral, may need to be named after their phenocryst mineral, such as'hornblende-bearing felsite'; the chemical name of a felsic rock is given according to the TAS classification of Le Maitre. However, this only applies to volcanic rocks. If the rock is analyzed and found to be felsic but is metamorphic and has no definite volcanic protolith, it may be sufficient to call it a'felsic schist'.
There are examples known of sheared granites which can be mistaken for rhyolites. For phaneritic felsic rocks, the QAPF diagram should be used, a name given according to the granite nomenclature; the species of mafic minerals is included in the name, for instance, hornblende-bearing granite, pyroxene tonalite or augite megacrystic monzonite, because the term "granite" assumes content with feldspar and quartz. The rock texture thus determines the basic name of a felsic rock. QAPF diagram List of minerals List of rock types Bowen's reaction series Archean felsic volcanic rocks Le Maitre, L. E. ed. 2002. Igneous Rocks: A Classification and Glossary of Terms 2nd edition, Cambridge
For the extinct cephalopod genus, see Andesites. Andesite is an extrusive igneous, volcanic rock, of intermediate composition, with aphanitic to porphyritic texture. In a general sense, it is the intermediate type between basalt and rhyolite, ranges from 57 to 63% silicon dioxide as illustrated in TAS diagrams; the mineral assemblage is dominated by plagioclase plus pyroxene or hornblende. Magnetite, apatite, ilmenite and garnet are common accessory minerals. Alkali feldspar may be present in minor amounts; the quartz-feldspar abundances in andesite and other volcanic rocks are illustrated in QAPF diagrams. Classification of andesites may be refined according to the most abundant phenocryst. Example: hornblende-phyric andesite, if hornblende is the principal accessory mineral. Andesite can be considered as the extrusive equivalent of plutonic diorite. Characteristic of subduction zones, andesite represents the dominant rock type in island arcs; the average composition of the continental crust is andesitic.
Along with basalts they are a major component of the Martian crust. The name andesite is derived from the Andes mountain range. Magmatism in island arc regions comes from the interplay of the subducting plate and the mantle wedge, the wedge-shaped region between the subducting and overriding plates. During subduction, the subducted oceanic crust is submitted to increasing pressure and temperature, leading to metamorphism. Hydrous minerals such as amphibole, chlorite etc. dehydrate as they change to more stable, anhydrous forms, releasing water and soluble elements into the overlying wedge of mantle. Fluxing water into the wedge lowers the solidus of the mantle material and causes partial melting. Due to the lower density of the molten material, it rises through the wedge until it reaches the lower boundary of the overriding plate. Melts generated in the mantle wedge are of basaltic composition, but they have a distinctive enrichment of soluble elements which are contributed from sediment that lies at the top of the subducting plate.
Although there is evidence to suggest that the subducting oceanic crust may melt during this process, the relative contribution of the three components to the generated basalts is still a matter of debate. Basalt thus formed can contribute to the formation of andesite through fractional crystallization, partial melting of crust, or magma mixing, all of which are discussed next. Andesite is formed at convergent plate margins but may occur in other tectonic settings. Intermediate volcanic rocks are created via several processes: Fractional crystallization of a mafic parent magma. Partial melting of crustal material. Magma mixing between felsic rhyolitic and mafic basaltic magmas in a magma reservoir To achieve andesitic composition via fractional crystallization, a basaltic magma must crystallize specific minerals that are removed from the melt; this removal can take place in a variety of ways, but most this occurs by crystal settling. The first minerals to crystallize and be removed from a basaltic parent are amphiboles.
These mafic minerals settle out of the magma. There is geophysical evidence from several arcs that large layers of mafic cumulates lie at the base of the crust. Once these mafic minerals have been removed, the melt no longer has a basaltic composition; the silica content of the residual melt is enriched relative to the starting composition. The iron and magnesium contents are depleted; as this process continues, the melt becomes more and more evolved becoming andesitic. Without continued addition of mafic material, the melt will reach a rhyolitic composition. Molten basalt in the mantle wedge moves upwards until it reaches the base of the overriding crust. Once there, the basaltic melt can either underplate the crust, creating a layer of molten material at its base, or it can move into the overriding plate in the form of dykes. If it underplates the crust, the basalt can cause partial melting of the lower crust due to the transfer of heat and volatiles. Models of heat transfer, show that arc basalts emplaced at temperatures 1100–1240 °C cannot provide enough heat to melt lower crustal amphibolite.
Basalt can, melt pelitic upper crustal material. Andesitic magmas generated in island arcs, are the result of partial melting of the crust. In continental arcs, such as the Andes, magma pools in the shallow crust creating magma chambers. Magmas in these reservoirs become evolved in composition through both the process of fractional crystallization and partial melting of the surrounding country rock. Over time as crystallization continues and the system loses heat, these reservoirs cool. In order to remain active, magma chambers must have continued recharge of hot basaltic melt into the system; when this basaltic material mixes with the evolved rhyolitic magma, the composition is returned to andesite, its intermediate phase. In 2009, researchers revealed that andesite was found in two meteorites that were discovered in the Graves Nunataks icefield during the US Antarctic Search for Meteorites 2006/2007 field season; this points to a new mechanism to generate andesite crust. Andesite line Basaltic andesite Continental crust – Layer of rock that forms the continents and continental shelves Fractional crystallization – One of the main processes of magmatic differentiation List of rock types – A list of rock types recognized by geologists Metamorphism – The change of minerals in pre-existing rocks w
Temagami Greenstone Belt
The Temagami Greenstone Belt is a small 2.7 billion year old greenstone belt in the Temagami region of Northeastern Ontario, Canada. It represents a feature of the Superior craton, an ancient and stable part of the Earth's lithosphere that forms the core of the North American continent and Canadian Shield; the belt is composed of metamorphosed volcanic rocks that range in composition from basalt to rhyolite. These are overlain by metamorphosed sedimentary rocks, they were created during several volcanic episodes involving a variety of eruptive styles ranging from passive lava eruptions to viscous explosive eruptions. Part of the Canadian Shield, the TGB contains some of the oldest known rocks on Earth; the belt is made up of a number of geologic features such as batholiths, dikes, volcanic complexes, layered intrusions and deformation zones. These are situated in several geographical townships in the municipality of Temagami, including Chambers, Strathcona and Best. Geologists assume that greenstone belts were formed by many geological processes, such as tectonism, magmatism and sedimentation.
They are important economically for large metal deposits, for the insight they provide into crustal evolution and the tectonics of the early Earth. The TGB is 32 km long, it contains the southernmost remnants of Archean intrusive and supracrustal rocks in Eastern Ontario, as well as some of the most ancient felsic magmatic events in this section of the Superior craton. Uranium-lead dating has established that the Iceland Lake Pluton, as well as an adjacent rhyolitic lava flow, is about 2,736 million years old. Therefore, at least some intrusions were formed during the first volcanic phases in the belt and may have been conduits for volcanic eruptions; the variety of volcanic deposits and intrusions in the TGB indicates that magmatic activity played a significant part in its formation. Pillow lava is found throughout the belt, its pyroclastic deposits are remnants of explosive volcanism. The oldest exposed rocks within the belt are fine to medium-grained andesites. Lava flow units range in thickness from 90 m to 1,500 m.
Mafic agglomerate and breccia are abundant, being either massive and undeformed, or sheared. Dacitic lava flows or tuffs overlie these metamorphosed volcanic rocks along with intermediate volcanic breccias, are overlain by rhyolite lava flows and tuffs. Acidic lava flow units range in thickness from 90 m to 900 m and are common in the Vermilion Lake and Link Lake areas; the felsic tuffs are altered and sheared. The most recent intrusive activity in the TGB was the formation of a rhyolite porphyry dike 2687 ± 2 million years ago; this age correlates well with the 2675–2700 million year old intrusions throughout the Abitibi Subprovince, but the 2736 million year old magmatic events in the TGB are older than the closest exposed portion of the Abitibi Subprovince, about 120 km north of Kirkland Lake. Along with nearby granitic intrusions, the TGB is bounded by layers of rock comprising the Huronian Supergroup. Strathy Township is dominated by metamorphosed volcanic rocks of the northeastern portion of the belt.
It is 24 km north of the Grenville Front Tectonic Zone. The volcanic rocks total as much as 6,000 m thick. However, portions of the sequence might have been sheared by one or several local fault zones; every large volcanic event is capped by metamorphosed sedimentary rocks and/or iron formations. The metamorphosed sedimentary units range in thickness from 60 m to 300 m and consist of laminated slate and greywacke with or without volcanogenic tuffs; the iron formations are composed of alternate layers of magnetite, white quartzite, grey cherty quartz, and/or tremolite-chlorite tuff. They are intruded by sills composed of medium-grained, white-weathering, quartz diorite that range in thickness from 100 m to 210 m; these rocks are similar to the coarse thicker parts of lava flows, but are interpreted to be intrusive conduits that produced mafic volcanism. A layered intrusion composed of diorite, pyroxenite and anorthositic gabbro has been found in northwestern Strathy Township. Pyrrhotite is common in associated pyroclastic rock.
Several northeast-trending shear zones less than 5 m wide intersect the edifice, extending along the Net Lake-Vermilion Lake Deformation Zone. This layered intrusion might be similar in age to the Kanichee layered intrusive complex, may represent a magma chamber, the source of tholeiitic volcanic activity; the Kanichee layered intrusive complex known as the Kanichee Intrusion and Ajax Intrusion, is the most voluminous mafic-ultramafic body in metamorphosed felsic and mafic volcanic rocks of the northern TGB. It is an oval-shaped layered intrusion, formed during five phases of magmatic activity. A series of south-southeast dipping cyclic magmatic layers make up the intrusion, similar to those of the surrounding metamorphosed volcanic rocks, indicating that the rocks of the intrusion were formed horizontally and close to the surface. Numerous magmatic events may have breached the surface to produce volcanic eruptions; the overall structure of the intrusion indicates it is cylindrical in shape and has a long axis plunging to the southeast at a somewhat steep angle.
Its steep angled axis was formed by at least one period of deformation that folded and deformed the surrounding volcanic rocks. An intrusion of light-coloured diorite lies at the northern end of the Tetapaga Syncline, along the Milne-Sherman Road, its colouration is from
The chlorites are a group of phyllosilicate minerals. Chlorites can be described by the following four endmembers based on their chemistry via substitution of the following four elements in the silicate lattice. In addition, zinc and calcium species are known; the great range in composition results in considerable variation in physical, X-ray properties. The range of chemical composition allows chlorite group minerals to exist over a wide range of temperature and pressure conditions. For this reason chlorite minerals are ubiquitous minerals within low and medium temperature metamorphic rocks, some igneous rocks, hydrothermal rocks and buried sediments; the name chlorite is in reference to its color. They do not contain the element chlorine named from the same Greek root; the typical general formula is: 34O102 · 36. This formula emphasizes the structure of the group. Chlorites have a 2:1 sandwich structure, this is referred to as a talc layer. Unlike other 2:1 clay minerals, a chlorite's interlayer space is composed of 6.
This 6 unit is more referred to as the brucite-like layer, due to its closer resemblance to the mineral brucite. Therefore, chlorite's structure appears as follows: -t-o-t-brucite-t-o-t-brucite... That's why they are called 2:1:1 minerals. An older classification divided the chlorites into two subgroups: the orthochlorites and leptochlorites; the terms are used and the ortho prefix is somewhat misleading as the chlorite crystal system is monoclinic and not orthorhombic. Chlorite is found in igneous rocks as an alteration product of mafic minerals such as pyroxene and biotite. In this environment chlorite may be a retrograde metamorphic alteration mineral of existing ferromagnesian minerals, or it may be present as a metasomatism product via addition of Fe, Mg, or other compounds into the rock mass. Chlorite is a common mineral associated with hydrothermal ore deposits and occurs with epidote, sericite and sulfide minerals. Chlorite is a common metamorphic mineral indicative of low-grade metamorphism.
It is the diagnostic species of the zeolite facies and of lower greenschist facies. It occurs in the quartz, sericite, garnet assemblage of pelitic schist. Within ultramafic rocks, metamorphism can produce predominantly clinochlore chlorite in association with talc. Experiments indicate that chlorite can be stable in peridotite of the Earth's mantle above the ocean lithosphere carried down by subduction, chlorite may be present in the mantle volume from which island arc magmas are generated. Chlorite occurs in a variety of locations and forms. For example, chlorite is found in certain parts of Wales in mineral schists. Chlorite is found in large boulders scattered on the ground surface on Ring Mountain in Marin County, California. Clinoclore and chamosite are the most common varieties. Several other sub-varieties have been described. A massive compact variety of clinochlore used as a decorative carving stone is referred to by the trade name seraphinite, it occurs in the Korshunovskoye iron skarn deposit in the Irkutsk Oblast of Eastern Siberia.
Chlorite is so soft. The powder generated by scratching is green, it feels oily. The plates are not elastic like mica. Talc feels soapy between fingers; the powder generated by scratching is white. Mica plates are elastic. Various types of chlorite stone have been used as raw material for carving into sculptures and vessels since prehistoric times. List of minerals Thuringite Hurlbut CS, Klein C. Manual of Mineralogy. New York: Wiley & Sons. ISBN 0471805807. Grove TL, Chatterjee N, Parman SW, et al.. "The influence of H2O on mantle wedge melting". Earth Planet. Sci. Lett. 249: 74–89. Bibcode:2006E&PSL.249...74G. Doi:10.1016/j.epsl.2006.06.043. "The Mineral Chlorite". Amethyst Galleries. 1996. Archived from the original on 25 Nov 2004. Retrieved 22 Mar 2019. "Chlorite Group: Mineral information and localities". Mindat.org. Retrieved 22 Mar 2019. "Chlorite". Maricopa.edu. Archived from the original on 12 Nov 2014. Retrieved 22 Mar 2019.]
Blueschist called glaucophane schist, is a metavolcanic rock that forms by the metamorphism of basalt and rocks with similar composition at high pressures and low temperatures corresponding to a depth of 15 to 30 kilometers. The blue color of the rock comes from the presence of the predominant minerals glaucophane and lawsonite. Blueschists are found within orogenic belts as terranes of lithology in faulted contact with greenschist or eclogite facies rocks. Blueschist, as a rock type, is defined by the presence of the minerals glaucophane + +/- jadeite +/- albite or chlorite +/- garnet +/- muscovite in a rock of basaltic composition. Blueschist has a lepidoblastic, nematoblastic or schistose rock microstructure defined by chlorite, phengitic white mica and other minerals with an elongate or platy shape. Grain size is coarse, as mineral growth is retarded by the swiftness of the rock's metamorphic trajectory and more the low temperatures of metamorphism and in many cases the anhydrous state of the basalts.
However, porphyritic varieties do occur. Blueschists may appear blue, gray, or blue-green in outcrop. Blueschist facies is determined by the particular temperature and pressure conditions required to metamorphose basalt to form blueschist. Felsic rocks and pelitic sediments which are subjected to blueschist facies conditions will form different mineral assemblages than metamorphosed basalt. Thereby, these rocks do not appear blue overall in color. Blueschist mineralogy varies by rock composition, but the classic equilibrium assemblages of blueschist facies are: Basalts: glaucophane + lawsonite and/or epidote + albite + titanite +/- garnet +/- quartz jadeite + quartz - diagnostic of pressures ~> 10 kbar Ultramafic rocks: serpentinite/lizardite +/- talc +/- zoisite Pelites: Fe-Mg-carpholite +/- chloritoid +/- kyanite + zoisite +/- pargasite or phengite +/- albite +/- quartz +/- talc +/- garnet Granites: kyanite +/- paragonite +/- chlorite +/- albite +/- quartz +/- pargasite or phengite Calc-silicates: Various Limestones and marble: calcite transforms to aragonite at high pressure, but reverts to calcite when exhumedBlueschist facies is considered to form under pressures of >0.6 GPa, equivalent to depth of burial in excess of 15–18 km, at temperatures of between 200 and 500 °C.
This is a'low temperature, high pressure' prograde metamorphic path and is known as the Franciscan facies series, after the west coast of the United States where these rocks are exposed. Well-exposed blueschists occur in Greece, Japan, New Zealand and New Caledonia. Continued subduction of blueschist facies oceanic crust will produce eclogite facies assemblages in metamorphosed basalt. Rocks which have been subjected to blueschist conditions during a prograde trajectory will gain heat by conduction with hotter lower crustal rocks if they remain at the 15–18 km depth. Blueschist which heats up to greater than 500 °C via this fashion will enter greenschist or eclogite facies temperature-pressure conditions, the mineral assemblages will metamorphose to reflect the new facies conditions, thus in order for blueschist facies assemblages to be seen at the Earth's surface, the rock must be exhumed swiftly enough to prevent total thermal equilibration of the rocks which are under blueschist facies conditions with the typical geothermal gradient.
Blueschists and other high-pressure subduction zone rocks are thought to be exhumed by flow and/or faulting in accretionary wedges or the upper parts of subducted crust, or may return to the Earth's surface in part owing to buoyancy if the metabasaltic rocks are associated with low-density continental crust. It has been held that the absence of blueschist dating to before the Neoproterozoic Era indicates that exhumed rocks never reached blueschist facies at subduction zones before 1,000 million years ago; this assertion is arguably wrong because the earliest oceanic crust would have contained more magnesium than today's crust and, would have formed greenschist-like rocks at blueschist facies. In Minoan Crete blueschist and greenschist was used as to pave floors of streets and courtyards between 1650 and 1600 BC; these rocks were quarried in Agia Pelagia on the north coast of central Crete. In 1962, Edgar Bailey of the U. S. Geological Survey introduced the concept of "blueschist" into the subject of metamorphic geology.
His constructed definition established the pressure and temperature conditions which produce this type of metamorphism. Metamorphism List of rock types List of minerals Blueschist facies - Rock Library Glossary, Imperial College London
Komatiite is a type of ultramafic mantle-derived volcanic rock defined as having crystallised from a lava with ≥ 18 wt% MgO. Komatiites have low silicon and aluminium, high to high magnesium content. Komatiite was named for its type locality along the Komati River in South Africa, displays spinifex texture composed of large dendritic plates of olivine and pyroxene. Komatiites are rare and predominantly found in rocks of Archaean age, with few Proterozoic or Phanerozoic komatiites known; this restriction in age is thought to be due to cooling of the mantle, which may have been 100 – 250°C hotter during the Archaean. The early Earth had much higher heat production, due to the residual heat from planetary accretion, as well as the greater abundance of radioactive elements. Lower temperature mantle melts such as basalt and picrite have replaced komatiites as an eruptive lava on the Earth's surface. Geographically, komatiites are predominantly restricted in distribution to the Archaean shield areas, occur with other ultramafic and high-magnesian mafic volcanic rocks in Archaean greenstone belts.
The youngest komatiites are from the island of Gorgona on the Caribbean oceanic plateau off the Pacific coast of Colombia, a rare example of Proterozoic komatiite is found in the Winnipegosis komatiite belt, Canada. Magmas of komatiitic compositions have a high melting point, with calculated eruption temperatures up to, in excess of 1600 °C. Basaltic lavas have eruption temperatures of about 1100 to 1250 °C; the higher melting temperatures required to produce komatiite have been attributed to the presumed higher geothermal gradients in the Archaean Earth. Komatiitic lava was fluid when it erupted. Compared to the basaltic lava of the Hawaiian plume basalts at ~1200 °C, which flows the way treacle or honey does, the komatiitic lava would have flowed swiftly across the surface, leaving thin lava flows; the major komatiitic sequences preserved in Archaean rocks are thus considered to be lava tubes, ponds of lava etc. where the komatiitic lava accumulated. Komatiite chemistry is different from that of basaltic and other common mantle-produced magmas, because of differences in degrees of partial melting.
Komatiites are considered to have been formed by high degrees of partial melting greater than 50%, hence have high MgO with low K2O and other incompatible elements. There are two geochemical classes of komatiite; these two classes of komatiite are assumed to represent a real petrological source difference between the two types related to depth of melt generation. Al-depleted komatiites have been modeled by melting experiments as being produced by high degrees of partial melting at high pressure where garnet in the source is not melted, whereas Al-undepleted komatiites are produced by high degrees of partial melts at lesser depth. However, recent studies of fluid inclusions in chrome spinels from the cumulate zones of komatiite flows have shown that a single komatiite flow can be derived from the mixing of parental magmas with a range of Al2O3/TiO2 ratios, calling into question this interpretation of the formations of the different komatiite groups. Komatiites form in hot mantle plumes. Boninite magmatism is similar to komatiite magmatism but is produced by fluid-fluxed melting above a subduction zone.
Boninites with 10–18% MgO tend to have higher large-ion lithophile elements than komatiites. The pristine volcanic mineralogy of komatiites is composed of forsteritic olivine and chromian pyroxene and chromite. A considerable population of komatiite examples show a cumulate morphology; the usual cumulate mineralogy is magnesium rich forsterite olivine, though chromian pyroxene cumulates are possible. Volcanic rocks rich in magnesium may be produced by accumulation of olivine phenocrysts in basalt melts of normal chemistry: an example is picrite. Part of the evidence that komatiites are not magnesium-rich because of cumulate olivine is textural: some contain spinifex texture, a texture attributable to rapid crystallization of the olivine in a thermal gradient in the upper part of a lava flow. "Spinifex" texture is named after the common name for the Australian grass Triodia, which grows in clumps with similar shapes. Another line of evidence is that the MgO content of olivines formed in komatiites is toward the nearly pure MgO forsterite composition, which can only be achieved in bulk by crystallisation of olivine from a magnesian melt.
The rarely preserved flow top breccia and pillow margin zones in some komatiite flows are volcanic glass, quenched in contact with overlying water or air. Because they are cooled, they represent the liquid composition of the komatiites, thus record an anhydrous MgO content of up to 32% MgO; some of the highest magnesian komatiites with clear textural preservation are those of the Barberton belt in South Africa, where liquids with up to 34% MgO can be inferred using bulk rock and olivine compositions. The mineralogy of a komatiite varies systematically through the typical stratigraphic section of a komatiite flow and reflects magmatic processes which komatiites are susceptible to during their eruption and cooling; the typical mineralogical variation is from a flow base composed of olivine cumulate, to a spinifex textured zone composed of bladed
Basalt is a mafic extrusive igneous rock formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or near the surface of a terrestrial planet or a moon. More than 90% of all volcanic rock on Earth is basalt. 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. By definition, basalt is an aphanitic igneous rock with 45–53% silica and less than 10% feldspathoid by volume, 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 classification scheme. 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 Hawaiʻi. Basalt features a 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, physical descriptions without mineralogical context may be unreliable in some circumstances. Basalt is grey to black in colour, but weathers to brown or rust-red due to oxidation of its mafic minerals into hematite and other iron oxides and hydroxides. Although 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 for large mineral crystals to grow; these phenocrysts 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 solid; 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 groundmass are referred to as diabase or, when more coarse-grained, as gabbro. Gabbro is marketed commercially as "black granite." In the Hadean and early Proterozoic eras of Earth's history, the chemistry of erupted magmas was different from today's, due to immature crustal and asthenosphere differentiation. These ultramafic volcanic rocks, with silica contents below 45% are classified as komatiites; the word "basalt" is derived from Late Latin basaltes, a misspelling of Latin basanites "very hard stone", imported from Ancient Greek βασανίτης, from βάσανος and originated in Egyptian bauhun "slate". 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 at Stolpen, believing it to be the same as the "very hard stone" described by Pliny the Elder in Naturalis Historiae.
Tholeiitic basalt is rich in silica and poor in sodium. Included in this category are most basalts of the ocean floor, most large oceanic islands, continental flood basalts such as the Columbia River Plateau. High and low titanium basalts. Basalt rocks are in some cases classified after their titanium content in High-Ti and Low-Ti varieties. High-Ti and Low-Ti basalts have been distinguished in the Paraná and Etendeka traps and the Emeishan Traps. Mid-ocean ridge basalt is a tholeiitic basalt erupted only at ocean ridges and is characteristically low in incompatible elements. E-MORB, enriched MORB N-MORB, normal MORB D-MORB, depleted MORB High-alumina basalt may be silica-undersaturated or -oversaturated, it has greater than 17% alumina and is intermediate in composition between tholeiitic basalt and alkali basalt. Alkali basalt is poor in silica and rich in sodium, it may contain feldspathoids, alkali feldspar and phlogopite. Boninite is a high-magnesium form of basalt, erupted in back-arc basins, distinguished by its low titanium content and trace-element composition.
Ocean island basalt Lunar basalt The mineralogy of basalt is characterized by a preponderance of calcic plagioclase feldspar and pyroxene. Olivine can be a significant constituent. Accessory minerals present in minor amounts include iron oxides and iron-titanium oxides, such as magnetite and ilmenite; because of the presence of such oxide minerals, basalt can acquire strong magnetic signatures as it cools, paleomagnetic studies have made extensive use of basalt. In tholeiitic basalt and calcium-rich plagioclase are common phenocryst minerals. Olivine may be a phenocryst, when